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thesis entitled

POTATO PROTEIN: NUTRITIONAL EVALUATION
AND UTILIZATION

presented by

Hector Herrera

has been accepted towards fulfillment
of the requirements for

Ph.D. degreein Food Science

 

46m Mm 472’an

 

Major professor

Date 11/4/79

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POTATO PROTEIN: NUTRITIONAL EVALUATION AND UTILIZATION

By

Hector Herrera

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

DOCTOR OF PHILOSOPHY

Department of Food Science and Human Nutrition

1979

ABSTRACT
POTATO PROTEIN: NUTRITIONAL EVALUATION AND UTILIZATION
By

Hector Herrera

The purpose of this research was to a) study the nutri-
tional quality of potato protein and b) prepare a high
protein potato product and use it in breadmaking.

When the N distribution in the raw tuber was studied,
it was found that the N content of potatoes is highest in
the skin. The Protein Efficiency Ratio (PER) of the skin,
however, is practically zero (Meister and Thompson, 1976a).
The pith region of the tuber presented the next highest N
content and the outer layer the lowest N content. It was
also observed that the area near the apical end of the
potato had a slightly higher N content than the stem end.

The free amino acids and amides extracted with distilled
water showed variations in the levels among the raw potato
cultivars (cv) analyzed. In general, the most abundant
free amino acids were aspartic and glutamic, and the amides
asparagine and glutamine. Free cystine was present as
traces in cv Atlantic and Superior potatoes and was not
detected in the other varieties. The range in content of
free methionine was 0.8 to 2.5 9/100 9 free amino acids

plus amides.

Hector Herrera

The whole tuber boiled with intact skin lost the lowest
amount of protein, 0.8 g/lOO g of original protein content,
in comparison to tubers boiled after cutting into halves
and quarters either peeled or unpeeled. The highest pro-
tein loss, l0.4%, was observed when whole potatoes were
peeled, cut into halves, and boiled. After boiling the cv
Russet Burbank and Atlantic potatoes with intact skins
either cut or uncut, the predominant amino acids in the
boiled water were aspartic and glutamic. It was found that
the whole tuber boiled with intact skin suffered a minimum
loss of cystine and methionine; the largest loss of cystine
and methionine was observed in the halved and quartered
potatoes.

The protein quality of the potato as determined by the
PER showed that the difference in the nutritional value of
the unsupplemented or supplemented potato protein was
reflected in their varying PERs. Russet Burbank potatoes
chemically and organically fertilized presented close PER
values 1.62 and l.54, respectively, statistically not
different. Also, the limiting amino acids, cystine and
methionine were present in similar concentrations in the
potatoes organically or chemically fertilized.

Dried whole eggs, and whey powder were used for supple-
menting the Russet Burbank potatoes. Commercially dried

potato flour prepared from the cv Russet Burbank and mixed

Hector Herrera
with dried whole eggs in the ratio 65:35 potato protein to
egg protein, had a PER which was 8% higher than the PER of
whole egg powder. This ratio of potato protein to egg
protein yielded a 54% increase in PER when compared to the
potato flour alone. When the same commercially dried
potato flour was mixed with dried whey in the ratio 90:10
potato protein to whey protein, the PER value of the
mixture was similar to that of casein (2.48 versus 2.50)
but 13% higher than the PER of the potato flour alone.

A potato flour fraction rich in protein (36.3%; Nx
6.25) was prepared from potato flour by air classification
in a Walter Lab. Separator. Bread was made after substi-
tuting 5% and l0% of the wheat flour with this fraction
named potato protein flour (PPF). A 32 and 51% decrease in
loaf volume was observed as a result of this substitution,
respectively. There was also a decrease in tenderness
(Alla-Kramer Press) in the bread containing PPF. Taste and
texture of the substituted bread were lower than those of

the control.

ACKNOWLEDGMENTS

The author wishes to express his sincere appreciation
to Dr. Pericles Markakis for his guidance during the course
of this study and for his aid in preparing this manuscript.

Appreciation and thanks are also extended to Drs.

D.D. Harpstead, J.R. Brunner, M.A. Uebersax, and J.N. Cash
for advice in the preparation of this manuscript.

The author especially thanks Dr. N.G. Bergen, and
Miss Doris H. Bauer for the amino acid analysis, and
Dr. J.L. Gill for his aid in the statistical study.

The author feels deeply grateful to the Organization
of American States (OAS), the International Potato Center
(CIP), for the financial assistance granted to him during
his studies at this University. He also feels grateful
to the Instituto Colombiano Agropecuario (ICA) for the
educational opportunity that enabled him to attend Michi-
gan State University.

And finally the author is indebted, as always, to his
wife, Jenny, and his daughters Sandra and Jessica, for
their constant encouragement and support during the course

of this study.

ii

TABLE OF CONTENTS

LIST OF TABLES.

LIST OF FIGURES

INTRODUCTION.

LITERATURE REVIEW .

A. Botany of the potato tuber — physical
structure . . .

B. Nitrogen distribution within the tuber.

C. Protein quality of potato .

l.

2.
3.
4

Nitrogen containing compounds of

potatoes.

Potato proteins and amino acids .

Nutritional characteristics of potato

proteins. .

Evaluation of prOtein quality in .pOtatO

a. Amino acid analysis of the potato
tuber .

b. Streptococcus zymogenes assay and
animal feeding experiments.

c. Human feeding experiments . .

Influence of processing on quality of

potato protein.

The potato protein as a supplement of

other proteins. . . . . . . . .

 

MATERIALS AND METHODS

Preparation of the samples.

A.
B.
C.
D

Nitrogen distribution

Amino acid distribution

Protein and amino acid losses during
boiling . .

Amino acid analysis of potatO tubers.

1. Amino acid distribution .

2. Free amino acid profile . .

3. Available lysine in boiled pOtatoes

—J .—l—-l
0" 0100 0001 U10)“

N—"
woo

N
01

27

Amide analysis of potato tubers.

Protein efficiency ratio (PER)

l. Fertilization. . . .

2. Supplementation. .

3. Preparation of diets .

G. Taste of the Russet Burbank potatoes
chemically or organically fertilized .

H. Preparation of a high protein potato
flour. . . . . . . . . . .

I. Bread. .

Preparation of bread

"I'Il'l'l
o o

Nitrogen Determination - Micro Kjeldahl.

Amino Acid Analysis.
Acid hydrolysis. .
Methionine and cystine analysis.
Free amino acid profile.
Available lysine
Amide analysis

RESULTS AND DISCUSSION

Part I . .
Nutritional Quality of Potato Protein.

A. Nitrogen distribution within the tuber
8. Amino acid distribution within the
tuber. . .
C. Protein and amino acid losses during
boiling. . .
D. Amino acids of potato tubers
l. Amino acid distribution.
2. Free amino acids .
E. Amides, asparagine and glutamine of

potato tubers. .
F. Available lysine by the FDNB method.
G. Protein efficiency ratio (PER)

l. Fertilization. . . . .

2. Supplementation.

H. Taste of Russet Burbank potatoes chemi-o

cally or organically fertilized.

Part II. . .
Preparation of a Potato Protein Flour and Its
Uses in Bread Making .
A. Preparation of a potato protein flour
and bread. . . . . . . . .
B. Bread analysis data.

SUMMARY. . . .
BIBLIOGRAPHY

iv

TOO

Table

10

II

12

LIST OF TABLES

Nitrogen containing compounds (crude protein)
of the potato (Schreiber, 1961). . .
Amino acid composition of potatoes (mg/g total
N . . . . . . . . . . . . . . . . . . . .
Essential amino acid composition of potato
tuber, a heat coagulable fraction of it (HCF),

whole egg protein and the reference pattern
(FAO/WHO, 1973) (g/l6 g N)

Composition of basal diet for the PER.

Ingredients used to produce bread by straight
dough method . . . . . . . . . . . . .

Nitrogen distribution within the potato tuber
in five freeze-dried varieties . . .

Nitrogen distribution within the raw potato
tuber in five freeze-dried varieties

Amino acid distribution in hydrolysates of the
outer and inner layers (see Figure 2) of
boiled freeze- dried potatoes (9 residue/100 .9
amino acid hydrolysate). .

Amino acid distribution in hydrolysates of the
apical and stem sections (see Figure 2) in the
raw Russet Burbank potatoes (9 residue/loo 9
amino acid hydrolysate). . . . .

Percent protein loss (Nx6. 25) in potatoes
boiled in distilled water. . . .

Amino acid distribution in hydrolysates of the
boiled water from the cooked potatoes (9
residue/loo 9 amino acid hydrolysate).

Cystine and methionine loss during the boiling
of potatoes. . . . . . . . . . .

Page

l4

T7
37

4O

52

53

57

59

62

64

67

Table
13

14

15

16

17

18

19

20

21

22

Cystine and methionine content in raw freeze-
dried potatoes (9/16 9 N).

Amino acid distribution in hydrolysates of the
boiled Russet Burbank potatoes chemically or
organically fertilized (g residue/100 9 amino
acid hydro1ysate). . . . . . . . . .

Free amino acid and amide distribution in
water extracts from raw freeze-dried potatoes.

Available bound lysine in boiled potatoes by
the FDNB method. . . . . .

Percent weight loss during boiling of potatoes
in distilled water . . . . . . . .

Percent total solids of raw and boiled pota-
toes . . . . .

Percent of the raw portion of potato removed
by a home peeler; percent total solids content
of the removed portion; and percent N content
of the removed portion on a dry basis.

Feed intake, protein intake, weight gain, feed
conversion ratio and PER of cooked potatoes
fed to rats for 28 days. . . . . .

Physical characteristics of breads containing
potato protein flour

Sensory characteristics of breads containing
potato protein flour .

vi

Page

70

71

73

76

78

79

81

84

91

93

Figure

LIST OF FIGURES

Structural features of potato tuber section.
Sectioning of potato tuber for analysis.

Percentage distribution of N in seven potato
sections (see Fig. 2) for five varieties

Average weight gain of weanling rats fed diets
containing as a sole source of protein: a)
chemically fertilized potatoes (CF); b) organi-
cally fertilized potatoes (0F); c) whey

powder; d) dried whole eggs; e) commercial
potato powder; f) commercial potato powder +
whey powder; 9) commercial potato powder +
dried whole eggs; h) casein. .

PER of diets calculated as percent of casein
PER. .

Taste panel score of breads containing potato
protein flour (PPF). . . . . . . .

vii

Page

31

55

82

87

95

INTRODUCTION

Origin of potatoes and their potential to feed people

The potato (Solanum tuberosum) is a plant of Andean

 

origin. Potatoes were first cultivated on the border of
Peru and Bolivia. The potato is a widely adaptable crop.

For a long time the potato has been a reliable source
of food for man. In its fresh state the potato has, on
the average, 2.1% total protein. However, on a dry weight
basis its total protein content, 10.4%, is not different
from that of wheat grain (10-14%). Per cultivated unit
area the amount of potato protein produced is higher than
that of wheat, corn or rice. A hectare (2.47 acres) pro-
duces 226 Kg of potato protein as a world average; this is
a yield greater than that in wheat grain protein (200 Kg/
ha) or rice grain protein (168 Kg/ha) (FAO, 1972a).

The potato has a good potential to feed people in
relation to other crops. Borgstrom (1969) stated that one
hectare of land under potato cultivation can supply the
protein requirement for 10 people while the protein of
wheat can satisfy only 6 pe0ple.

In some developing areas the cost of protein of

animal origin limits its consumption; many people get their

protein from plants. One example is South America where in
some regions potatoes constitute a valuable protein and
energy source for the human diet. Kies and Fox (1972)
stated that the potato is a very important source of dietary
protein for some groups of high consumption of it.

The objective of this study was a) to evaluate the
nutritional quality of potato protein and b) prepare a high

protein potato product and use it in breadmaking.

LITERATURE REVIEW

A. Botany of thegpotato tuber-physical structure

The potato is an enlarged underground tip of a rhizome
(Fuller, 1963). The tuber is composed of periderm (peel),
cortex, parenchyma and pith (Schwimmer and Burr, 1959)
(Figure 1). A similar structure was presented by Schuphan
(1959, 1960) for Solanum stenotomum Juz et Buk.

Schwimmer and Burr (1959) described the cortex as a
narrow layer of parenchyma tissue, underlying the periderm.
Parenchyma cells, high in starch content, lie on both sides
of the vascular ring. The pith, forming a small central
core with radiating branches to reach the eyes, contains

less starch.

B. Nitrogen distribution within the tuber

According to Neuberger and Sanger (1942) the nitrogen
content is highest in the periderm, and then decreases
sharply in the cortex and rises again towards the pith. A
similar distribution was reported by Monday and Rieley
(1964), who also showed that throughout the tuber the per-
centages of total and soluble nitrogen, on a dry weight

basis, are inversely related to specific gravity. Schuphan

Lateral bud
Periderm

l Cortex
. Parenchyma

Pith

    
 
 

 

STEM END BUD END

‘---~~

‘--“ --— ---
"--~-\
’ .....

--’-—

Apical bud

Figure 1. Structural features of potato tuber section.

(1970) found that the cortex and the area near the apical
and lateral buds have a much higher concentration of essen-
tial amino acids than the inner layers. The better balance
of essential amino acids appears to be associated with cell
layers which are capable of cell division. An interesting
observation is that protein crystals have been found in the
outer layers.

During the growth of potato tubers, the nitrogen con-
tent decreases gradually on a dry matter basis and the
portion of protein is higher in the immature tuber (Schu-
phan, 1970). The same author (1959) stated that small
tubers have a nutritionally better pattern of essential
amino acids. 0f the essential amino acids, lysine and
threonine increase with tuber weight, while most of the
other amino acids show little change or drop slightly
(Schuphan, 1970).

The potato protein has been studied by electrophoresis
and by combined electrofocusing and electrophoresis. Macko
and Stegemann (1969) and Luescher (1972) showed that the
tuber proteins of different potato varieties are present
in different proportions. Results of this kind may help

the potato breeder.

C. Protein quality of potato

 

l. Nitrggen containing compounds of potatoes
The total nitrogen content can be broken down into

a) true protein fractions soluble in various extracting

solutions; b) in an insoluble protein residue and c)

non protein nitrogen (NPN) which includes inorganic nitro-
gen, amide nitrogen, free amino acid nitrogen, and numerous
other constituents such as alkaloids, purines, pyrimidines,
choline, enzymes, certain vitamins, quaternary ammonium
compounds, etc.

The potato tuber contains on the average 2.1% total
protein (%N x 6.25) on a fresh weight, but it can range
from 1.5 to 4.0%. The total protein (2.1%) represents
10.4% of the total solids of the potato, and it can be as
high as 17%. It is important to point out that a new pro-
tein conversion factor (7.5) has been recently proposed by
Desborough and Heiser (1974). Should this factor be proven
correct, old values must be multiplied by 1.2 to obtain
the total protein content of potatoes.

The results presented in Table 1 are based on many
analysis by various authors (Schreiber, 1961). In relation
to Table 1, Markakis (1975) stated that it is doubtful
that there is so much free NH3 (3%) in potatoes; ammonia is
formed from glutamine during hydrolysis.

Schuphan (1958) reported that the total protein is com-
posed of 30 to 50% true protein. Meister (1977) found for
NPN (g/100 g N) values from 38.8 to 54.8, and Luescher
(1972) reported values from 50 to 61 (9/100 9 N) NPN for

three potato cultivars.

Table l. Nitrogen containing compounds (crude protein)
of the potato (Schreiber, 1961)

 

 

N Fraction % of Total N
True protein N 50
Non protein N 50

Inorganic N

Nitrate N l

Nitrite N trace

Ammonia N 3
Amide N

Asparagine N 13

Glutamine N 10

Remaining N
Free amino acid N 15

Basic N 8

 

2. Potatogproteins and amino acids

Lindner gt 31. (1960) measured the relative amounts of
different proteins in the potato and found that of the total
protein, globulin I (tuberin) accounted for 76.4%; globulin
II, 1.4%; albumin (tuberinin), 4%; prolamin, 1.8%; glutelin,
5.5%; and insoluble residue, 10.9%. These protein fractions
have been classified according to the concept of solubility
in various extracting solutions. Tuberinin is soluble in
water; tuberin is soluble in neutral salts (i.e. NaCl);
prolamin is soluble in 70% ethanol (by volume); glutelin
is soluble in a 0.2% NaCl solution made up in 60 volume
percent ethanol; the insoluble protein fraction in the solu-
tions described above, is found mainly in the skin and outer
cortex.

Protein crystals have been observed occasionally in the
protoplasm of the outer layers of potato cells. Depending
upon varieties, these crystals can also be synthesized in
other tissues of the potato tuber (Hoelzl and Bancher,
1959).

Tuberinin is considered a heterogeneous albumin.
Tuberin is a globulin ; it is slightly soluble in water,
but its solubility is greatly enhanced by the addition of
neutral salts, such as 2% sodium chloride; it is hetero-
geneous in composition.

Tuberin and tuberinin are present in a dissolved form

in the cell sap of the potato. As soon as the cell wall is

ruptured these two proteins can be easily extracted (Hoelzl
and Bancher, 1961). At a pH 6.8 both proteins have a nega-
tive isoelectric point (Groot gt 31., 1947). Together,
they account for 30 to 60% of the total protein. Luescher
(1972) isolated the various potato protein fractions from
three cultivars, by the procedure outlined by Lindner gt g1.
(1957). He found that of the total nitrogen, tuberin
(globulin I) accounted for 29 to 35%, globulin II for 0.3
to 0.6%, tuberinin (albumin) for 1.0 to 1.2%, prolamin for
0.6 to 0.8%, glutelin for 0.1%, and the residue for 5 to
10%. Similar results were reported by Meister and Thompson
(1976b) in a study of the Russet Burbank potato, except for
the percentage of tuberin which was a little higher (39.2%).
Luescher (1972) also determined the sulfur amino acids
(methionine and cystine) of tuberin and tuberinin by his
own standardized procedure using the microorganism Strep-
tococcus zymggenes. Tuberin and tuberinin from two of the
cultivars tested contained more than twice as much methio-
nine as cystine: on the average 2.8 g methionine versus
1.2 g cystine per 16 g N. However, this was not true for
the other cultivarswhere methionine was considerably lower.
Prolamin is an excellent source of cystine and the values
ranged from 3.1 to 3.9 9/16 9 N in the three cultivars.
Breeding potatoes high in sulfur amino acids, particularly
in methionine, is a likelihood for the future. The possi-

bilities of genetic improvement of potato protein have been

10

discussed by Schwartze and Sengebush (1937); Siegle (1951);
Reissig (1958); Schuphan (1970); Desborough and Neiser
(1972); and by a planning commission of the International
Potato Center (CIP, 1973). It is known that heavy nitrogen
fertilization increases the percentage of total protein
mainly due to an increase in aspartic and glutamic acids
and the amides asparagine and glutamine. The concentration
of lysine increases proportionally with fertilization, but
the methionine content draps from 2.2 to 1.6% (Hoff gt gt.,
1971). Mulder and Bakema (1956) reported that free methio-
nine dropped from 1.9% when fertilized with 33 Kg N/ha, to
1.0% when 150 Kg N/ha of fertilizer was used. However,
Luescher (1972) observed that a high free methionine content
(2.07 9/16 9 NPN) was obtained with 185 Kg N/ha. The extent
of response in amino acid composition as influenced by
fertilizer may be dependent upon genotype. Schuphan (1970)
reported that when the level of nitrogen fertilizer per
hectare was 120 Kg, the variety Bona had an essential amino
acid Index (EAA-Index) of 64 and the variety Olympia 86.
The EAA-Index among cultivars is influenced primarily by
the ratio of protein N to non protein N (Reissig, 1958).
The results reported by Talley gt_gl, (1970) and Augustin
(1975) confirm varietal differences for total protein,

the ratio of free to total N and methionine, but all three
are subject to modifications by year, location, fertiliza-

tion, and very highly by genotype x environment conditions.

11

Woodward and Talley (1953) and Luescher (1972) found that

no free tryptophan or free cysteine could be detected in the
NPN fraction. Essential amino acids in the NPN fraction are
present at a much lower level than in the protein fractions.

The methionine content of potatoes has been studied
recently by Kaldy and Markakis (1972); Luescher (1971, 1972);
Peare (1973); Desborough and Neiser (1974). According to
Luescher (1972) the methionine content of potato families
varies and free methionine is responsible for 93% of the
variation in available methionine. Free methionine ranged
from 0.34 to 2.07 g/l6 g non-protein N. Free methionine
contributed from 12 to 62% of all methionine present in the
total protein.

Low temperature storage (40°F) does not appreciably
affect the free amino acid composition of potatoes, but
reconditioning to 75°F results in a disappearance or marked
decrease in the content of free amino acids. A complete
loss of the free basic amino acids, arginine, histidine and
lysine, was observed during reconditioning (Habib and Brown,
1957). Fitzpatrick and Porter (1966), however, showed that
low temperature storage (36°F) followed by reconditioning
at room temperature resulted in an increase of the free
amino acids, but no change in the amino acids of the true
protein. However, Desborough and Neiser (1974) stored 12
genotypes at 38-40°F and found an average loss of 3% in

total protein.

12

The free amino acids that are included in the NPN
fraction are subject to variations as a result of storage,

. nutrition of the plant, year, location and treatment with
chemicals such as ethylene chlorohydrin (Mulder and Bakema,
1956; Schwimmer and Burr, 1959; Talley gt_gl., 1970).

The free amino acid composition of potatoes has been studied
by several investigators (Chick and Slack, 1949; Thompson
and Steward, 1952; Mulder and Bakema, 1956; Kaldy, 1971).
The free amino acid pool has been studied recently by
Desborough and Neiser (1974), using dialyzed and non dia-
lyzed half tuber samples of six genotypes. Free amino acids
accounted for about 10% of the total amino acids and non
essential amino acids constituted over 90% of the free pool.
The authors concluded that the essential amino acids occur
mainly as components of the potato protein. Free amino
acids also have been studied in cooked potatoes, chips,
canned, drum dried and french fried potato samples (Hughes,
1958; Jaswal, 1973). It is interesting to point out that,
in general, the most abundant free amino acids are valine,
arginine, aspartic acid, glutamic acid, and the amides
asparagine and glutamine.

Hughes (1958) analyzed the amino acids of the heat
coagulable fraction of potato nitrogen and reported the
following composition in 9 amino acid per 16 9 total N:
arginine 5.5, histidine 2.4, isoleucine 6.8, leucine 11.1,

lysine 8.3, phenylalanine 6.2, methionine 2.8, cystine 1.6,

13

threonine 5.7, tryptophan 1.8, valine 8.0, alanine 4.7,

aspartic acid 13.0, glutamic acid 11.3, glycine 4.9, pro-

line 5.1, serine 5.8, tyrosine 6.1, r-aminobutyric acid 0,
and ammonia 1.7.

The amino acid composition of whole potatoes is shown

in Table 2, which was presented by Markakis (1975) on the
basis of chemical and microbiological analysis by several

researchers.

The FAD report (1970) cites ranges for the methionine

and cystine content of potatoes, ranging from 54 to 125 and

4 to 81 mg/g total N, respectively. These results may
explain the differences in the nutritional value of potato

protein as reported by various investigators (Schuphan and

Postel, 1957; Luescher, 1972; Peare, 1973; Meister, 1977).

Therefore, improvements in the quality and quantity of

tuber protein are possible.

3. Nutritional characteristics ofgpotato proteins

 

The potato makes important contributions to human
nutrition. As an energy source it is considered good.
However, today it is also being recognized for the nutri-
tional quality of its protein.

The evaluation of the protein quality of potatoes has
been tested chemically by amino acid analysis and biologi-

cally by animal feeding experiments, human feeding

14

Table 2. Amino acid composition of potatoes (mg/g total N)1

 

Amino acid (AA) Average % CV

 

Essential AA

Isoleucine 257 27.6
Leucine 362 30.2
Lysine - 342 43.5
Methionine 92 27.9
Cystine 55 77.2
Phenylalanine 280 24.8
Tyrosine 172 38.6
Threonine 233 24.3
Tryptophan 85 33.4
Valine 323 22.0

Non essential AA
Arginine 305 14.9
Histidine 103 29.1
Alanine 297 20.5
Aspartic acid 1138 29.4
Glutamic acid 729 28.7
Glycine 214 18.6
Proline 213 16.1
Serine 236 14.8

Protein Score 70

MEAA Index 72

 

1Notations MEAA: modified essential amino acid Index
% CV: percent coefficient of variation =

Standard deviation x 100

mean , on data

by FAO (1970).

15

experiments, and microbial growth.
4, Evaluation ofggrotein quality in potato
The protein quality of potato will be considered in
relation to:
a. Amino acid analysis

b. Stregtococcus gymogenes assay and animal

 

feeding experiments.

c. Human feeding experiments

a. Amino acid analysis of the potato tuber

Allison (1959), Pike and Brown (1975) relate the qual-
ity of a dietary protein to the amount and kind of its amino
acids. A diet must provide the essential amino acids
required by the human body, but also the nitrogen of non-
essential amino acids is nutritionally important. Snyderman
_t _l. (1962) indicated that a food, for example milk, can
satisfy the requirements of the essential amino acids, but
not the requirements in total utilizable nitrogen for the
human body. Also, a diet adequate for maintenance and
repair may be inadequate for growth (Mitchell, 1947). Not
only the quantity of protein, but also the amino acid
balance of the protein is important. According to several
investigators the potato protein has a good amino acid
composition and balance for maintenance and growth promotion

in humans. Block (1951) stated that tuberin, the main

protein fraction of the potato, contained sulfur amino acids

16

similar to milk, but only about 75% of methionine and 50%
of the cystine present in the protein of the whole egg.

Markakis (1975) using the protein score procedure
developed by FAD/WHO (1965), calculated a protein score
of 70 for the protein of the whole potato, based on the
average of the amino acids present in potatoes (Table 2).
This value compares favourably with other foods which have
the following protein scores: beef 80, fish 75, soyflour
70, milk 60, wheat flour 50, maize 54, rice 65 (FAD/WHO,
1965; Payne, 1976). A protein score of 70 and a modified
essential amino acid index (MEAA-Index) of 98 were calcu-
lated for the "pure" potato protein by Markakis (1975),
based on the amino acid analysis reported by Hughes (1958).

The high MEAA index of this protein indicates that
the potato protein is a better source than whole egg
protein of every essential amino acid, except for the
sulfur containing amino acids that are low compared to egg
protein. But if the sulfur amino acids of the "pure”
potato protein are compared to the FAD/WHO pattern (1973),
the total 4.4 (methionine + cystine) is higher than 3.5 of
the pattern (Table 3). However, the whole tuber has a
lower value (2.9).

Knorr gt g1. (1977) found that the amino acid compo-
sition of a potato protein concentrate (PPC) was equal to
or greater than that reported by FAO (1972b). Also, in the
PPC a high methionine content (2.70 g/16 g N) was obtained.

17

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18

Finally, the potatoes have a high percent of the
amides asparagine and glutamine, 13% and 10% of the total
nitrogen content, respectively (Schreiber, 1961). It has
been suggested that some compounds of the potato, such as
glutamine, have a substantial influence upon the efficiency
of the utilization of the amino nitrogen. It is known that
certain amino acids act as antagonists to each other. If
these amides in the potato prevent this antagonism and
afford efficient use of protein, they may have a good role
in human nutrition (McCay, 1959).

b. Streptococcus zymggenes assay and animal

 

feedingfexperiments

Ford (1960) developed a microbiological assay to test
the quality of food proteins and demonstrated a good cor-
relation between microbiological and biological experiments
with the growing rat. Comparable results were obtained
when the protein quality of potato was determined using
the §. zymogenes assay, the meadow vole, and rat feeding
(Peare, 1973; Meister and Thompson, 1976a).

The proteolytic bacterium g. gymoggnes is strongly

 

proteolytic and has an absolute requirement for exogenous
methionine, tryptophan, arginine, histidine, leucine,
isoleucine, valine and glutamic acid. 0f the essential
amino acids lysine, threonine and phenylalanine are not
indispensable for the organism although the omission of any

one of the culture causes a marked failure in growth rate.

19

The total growth of the organism on a given amount of
sample protein has been expressed in percent of the growth
on the same amount of casein protein.

Luescher (1971, 1972) used this organism to measure
the biological value (BV) and available methione of potato
protein. 0n the basis of many samples analyzed with this
organism, the average for the EV of total potato protein
was 84 and 78 for non-protein nitrogen (NPN). These values
are in the range 61 to 89 reported by Schuphan (1959).
Peare (1973) used the BV with t. zymogenes and the total

 

percent protein to calculate the net protein content (BV
x% total protein) of potatoes. He concluded that potato
varieties having a high BV will not necessarily contain
high amounts of net protein. Meister (1977) stated that
the EV of potatoes depends on genetic features; he also

studied the BV of potatoes using g, zymoggnes and found

 

values from 77 to 82. He observed that raising the pro-
tein content of the tuber may bring about a dr0p in the BV
of the protein. Most of this drop would be due to an in-
crease in the NPN which has a lower BV than the true protein.
Mitchell (1924) reported a BV for the potato protein
of 68.5% when a diet at 5% protein level was fed to young
rats and 66.7% when the diet contained 10% protein. Drum
dried potato flakes of 5.28% protein content in the diet
gave better growth than drum dried beans when the meadow

vole (Microtus pennsylvanicus) was used as an experimental

 

20

animal (Rios gt 31., 1972). Also, the addition of methio-
nine increased the PER values of the potato protein and
improved the digestibility of the protein in the meadow vole
(Rios, 1969).

When the heat coagulable fraction, tuberin, was tested
a BV of 71 was obtained with rats compared to 68 for casein
(Kon, 1928).

It has been demonstrated that the nitrogen of the
intact potato supports growth at least as well as tuberin
alone. But the NPN alone can not promote growth of rats
(Slack, 1948; Chick and Cutting, 1943). Chick and Slack
(1949) stated that the apparent digestibility of the potato
protein was increased from 74 to 79% when the skin and outer
cortex was removed. The peel obtained by the abrasive
method did not support growth of the meadow vole, but
weight loss was not observed (Meister and Thompson, 1976a).
Peare (1973) determined with rats the nitrogen incorporation
efficiency (NIE) of cooked whole potato flour and a high-
protein potato flour according to the procedure of Stucki
and Harper (1962). The NIE values as a percent of casein
were for whole potato flour 74.3% and for the high protein
flour 73.9%. No differences in protein quality or food con—
sumption were noted.

Protein recovered from waste effluent of potato chip
processing gave equal to or slightly higher protein effi-

ciency index with meadow voles than total protein of whole

21

tubers (Meister and Thompson, 1976a).

Baciqalupo (1969) reported in boiled and dried potato
a PER value with rats of 2.5 after adjustment with respect
to casein. Joseph gt gt. (1963), testing different potato
varieties, reported PER values from 0.95 to 1.99. Brune
(1968, 1969), working with pigs, reported the BV for
potatoes grown in the presence of a fertilizer; he observed
that with an increase in the level of N fertilizer the BV
decreased. Different levels of N fertilization resulted
in different biological values for the potato protein.

The biological values were 76, 82, 74 and 71% for zero, 50,
100 and 200 Kg of N fertilizer/ha, respectively. The
results huficate that an increase in protein content in
potatoes through N fertilization is possible only within
certain limits. Similar information has been obtained with
mice, rats and S. zymogenes.

Peare (1973) observed in a preliminary study that
weanling rats, fed the raw concentrate of potato flour high
in protein content at a level of 9% protein during 5 days,
barely maintained their initial weight, but when the con- '
centrate was cooked, weight gain was normal. The explana-
tion to this could be due to adverse effects of feeding
larger sized starch grains (Morrison, 1936) and also the
presence of protease inhibitors such as trypsin and chymo-
trypsin inhibitors which have been found in the raw potato

by Sohonie and Ambe (1955) and Rayan and Balls (1962).

22

Rayan (1966) pointed out that the potent chymotrypsin
inhibitor of the potato is quickly destroyed by heating.
Raw potato starch caused serious impairment of the net
protein utilization (NPU) and the food efficiency values
when fed to growing albino rats. A gross caecal enlarge-
ment was observed (El-Harith gt gl., 1976).

Whittemore gt gt. (1975) compared the nutritive value
of dried cooked potato versus raw potato for pigs. In
relation to the values obtained for cooked potato, raw
potato had lower apparent digestibility coefficients for
nitrogen, 0.76 versus 0.86. Similar results were reported
earlier when the nutritive value of cooked potato flour,
dried potato dice and raw potato mash were evaluated for
pigs. The apparent digestibility coefficients for nitrogen
were respectively, 0.89, 0.80, and 0.69 (Hhittemore gt gl.,
1973).

Felix and Nhittemore (1975) working with young chicks
found that growth rates and efficiency of food conversion
were lower with a diet containing potato flakes than a diet
containing maize. However, in the discussion of the results
the authors stated that the protein quality of potato flakes
is comparable with that of ground maize, for the young

chick. Other unknown factors than the quality of the

potato protein affected the results.

23

c. Human feeding exgeriments

In the case of protein, growth is used as criterion of
adequacy for infants and children, and maintenance of body
N content for adults (Payne, 1976). Hindhede (1913)
maintained nitrogen equilibrium in three men on a potato
diet. The same author claimed that a man was kept in N
equilibrium for more than three months on a potato diet al-
though the protein intake was 11-15 g daily. Similar
results were obtained by Rose and Cooper (1917) with a young
woman who consumed 1500 g potato per day. The experiment
was performed for 7 days and the N balance was kept by an
intake of 0.096 g N from potato/Kg body weight. A man and
a woman were kept in N balance for 167 days on a diet in
which the potato was the only source of nitrogen (Kon and
Klein, 1928). According to this experiment the daily need
for potato protein was 35.6 g for the man and 23.8 g for the
woman on a 70 Kg body weight basis.

In nitrogen balance studies with college students,
potato protein proved to have the best nutritive value in
relation to other plant proteins tested, such as wheat
flour, rice, kidney beans, soybeans, algae (Kofranyi and
Jekat, 1965, 1967). In relation to animal proteins the
nutritional value was almost equal to whole egg protein,
but better than beef, tuna fish and whole milk. These
authors used single foods and pairs of foods mixed at

definite protein ratios and determined the minimum amount

24

of protein from each diet that was necessary for nitrogen
balance maintenance, in three subjects. The average of the
three subjects was 0.545 g N/Kg body weight for the potato
protein and 0.505 g N/Kg body weight for the egg protein.
When the 65:35 potatozegg mixture of proteins was used, the
total protein requirement for N-balance was less than the
requirement in egg or potato protein alone.

Other pairs of proteins were similarly tested, but
none resulted in as low a minimum of protein requirement
as the 65:35 potato:egg protein mixture. Thompson (1977)
stated that he gave to one graduate student a diet in
which the potato was the only source of nutrients. The
student consumed during one month four pounds per day of a
potato high in methionine. The diet was short in 230
calories that were supplemented with oil in the form of
french fries. The subject gained four ounces and felt
good after one month on the diet.

Kies and Fox (1972) showed that the protein quality is
improved by adding purified L-methionine to the diet of
human adults who consumed dehydrated potato flakes. The
same authors demonstrated that neither leucine nor phenyl-
alanine resulted in improvement of N-balance as methionine
did. The digestibility of potato protein tested was 78%.

McCay (1959) stated that potato gruel made from spe-
cially prepared potato meal or the pulp of baked potatoes

has been found by Germans to be of great service in the

25

feeding of infants or invalids. Hegsted (1957), in a theo-
retical comparative study of vegetable proteins, demonstrated
that the protein of potato with a BV of 71 can provide the
required protein for a child 6 months old with 800 calories
intake, or a child one year old with an intake of 975 calor-
ries. Corn grits with a BV of 54 and white bread with a BV
of 47 can not be used as the only source of protein,
although theoretically with these proteins, the protein and
amino acid requirement can be met.

5. Influence of processing on quality of potato protein

As a result of processing by different methods, i.e.
baking, boiling (cooking), chipping, french frying, can-
ning and drum drying, the quality of the protein of potatoes
could be altered. Also, if the product has been prepared
from fresh or stored tubers the protein quality can be dif-
ferent. Burton (1965) reported that during storage of some
British potato varieties considerable sweetening occurs
after more than 10-15 weeks of storage at 50°F. However,
50°F is an ideal storage temperature, because of the sprout
suppression. Protein N decreases in sprouting tubers (P01
and Labib, 1968), thus making it advisable to avoid sprouting
if the potato is going to be stored.

Exposure of tubers to low temperatures results in a
rapid conversion of starch to hexoses due to a rapid in-
crease in invertase activity (Pressey and Shaw, 1966).

However, it is also known that there is a subsequent

26

disappearance of the sugars when the tubers are returned to
higher temperatures.

Carbonyl amine reactions, including the reaction of
reducing sugars and other aldehydes and ketones with amino
acids, peptides, and proteins, may occur during heating,
resulting in browning of the product (Maillard reaction).
The effect of chipping on the nitrogeneous compounds of
potatoes and the dark color in fried potato products has
been studied by Shallenberger (1956), Chawan (1965), Fitz-
patrick and Porter (1966, 1968). Fitzpatrick and Porter
(1966) found that the greater the accumulation of reducing
sugars in stored potatoes, the greater the loss of free
amino acids after chipping. In 1968 the same authors
studied the effect of fat frying versus microwave finishing
on the extractable potato protein. In microwave finished
chips the amino acids were more extractable. The reason
for this is that in microwave finishing the chain on non-
enzymatic browning reactions are interrupted, because
the temperature of the chips never exceeds the boiling point
of water (Fitzpatrick and Porter, 1968). Jaswal (1973)
found the losses of bound and free amino acids in low
specific gravity potatoes after chipping to be 36.9 and
44.8%, respectively. The corresponding losses in high
specific gravity potatoes were 20.2% for bound amino acids
and 33.2% for free amino acids. The same author reported

losses of amino acids occurring during canning, drum drying

27

and french frying of potatoes. He found that canning caused
almost as much destruction to amino acids as chipping, drum
drying resulted in less damage and french frying caused
insignificant loss in amino acids.

Hughes (1958) analyzed the amino acids in pre-peeled
boiled potatoes and found that the amino acid composition is
not very different from that of raw potato. However, Des-
borough and Neiser (1974) reported a 50% loss of total
protein for 12 genotypes after boiling for 30 minutes.

6. Theppotato protein as a supplement of other proteins

Nutritional protein deficiency can be present in two
ways: deficiency of total protein in the diet and deficiency
of amino acids irrespective of total protein intake.

According to Jansen and Howe (1964), in diets consis-
ting of rice or wheat only a deficiency in lysine and threo-
nine is likely. In corn diets, lysine and tryptophan may
be in short supply. In these foods the protein deficiencies,
at least in relation to the amino acid balance can be over—
come by addition of potatoes, because the complementarity
between potatoes and cereals has been proven. Flodin (1953)
and Markakis gt gt. (1962) stated that a combination of
potatoes and cereals, especially corn and wheat, makes a
desirable food mixture, because of the high lysine and tryp-
tophan content in potatoes. Markakis gt 1. (1962) developed

a fried chip based on cereal and potato products.

28

If the whole potato is dried, a very nutritious dry
product is obtained with more than 10% protein in most
cases.

Bacigalupo (1969) stated that a rat diet containing
6% protein from cotton seed flour (CSF) and 4% protein from
potatoes had a PER value equal to 2.54 and a diet containing
5% protein from CSF and 5% protein from potatoes had a PER
of 2.69 versus 2.70 for casein. The same author found that
enrichment of wheat flour with potatoes plus CSF, increased
the nutritive value of the bread about 100%.

If potato protein concentrates from waste effluent of
potato chip processing or a high-protein air classified
fraction of potato flour are available (Shaw and Shevy,
1972; Peare, 1973; Meister and Thompson, 1976b; Knorr
_t _t., 1977), it is possible to use them to enrich the
total protein content of different food mixtures.

Numerous investigations have been conducted on the
incorporation of potato flour in bread (Harris, 1932;
Trogonitz, 1939; Harris gt gl., 1952; Treadway, 1952;
Braden, 1962; Reynoso and Bacigalupo, 1968; Bacigalupo,
1969; Reynoso _t _t., 1972; Jain and Sherman, 1974; Knorr,
1977).

Jain and Sherman (1974) used potato flakes at the
level of 20-30% using the mechanical dough development
method and the bread had good quality in color, texture and

taste. Bacigalupo (1969) reported the use of 25 and 30%

29

potato flour with wheat flour for bread without involving
great change of bread quality. Similar results were ob-
tained by Reynoso gt gt. (1972), using up to 30% cooked
mashed potatoes.

Harris gt gt, (1952) found that replacing l to 4% of
wheat flour with potato flour increased the loaf volume
and caused a slight change in the bread texture.

Substitution of the wheat flour up to 10% with starch—
free potato protein recovered from potato starch waste
effluents produced an acceptable bread with a longer shelf
life than that of 100% wheat bread (Knorr, 1977). Some
decrease in loaf volume and deformation was observed with

increasing potato protein from 15 to 20%.

MATERIALS AND METHODS

Preparation of the Samples

 

For this study the following potato varieties were
chosen:

Atlantic (high solid content)

Denali (high solid content)

Russet Burbank (medium solid content)

Superior (medium-low solid content)

Monona (low solid content)

The potatoes were grown on the Montcalm Experimental
Farm of Michigan State University in 1977. They were fer-
tilized with 217 lbs/A of N, 200 lbs/A of P205 and 72 lbs/A
of K20. A separate plot of Russet Burbank was fertilized
with cattle manure, only. Thereafter these treatments are
referred to as chemical and organic samples, respectively.

After harvest the potatoes were stored at 40°F, and a

relative humidity of 85%.

A. Nitrogen distribution

Six tubers were randomly chosen from each variety,
washed and sliced according to Figure 2. Two raw tubers
were sliced as shown over "Tuber 1", two raw tubers were
sliced as shown over "Tuber 2" and two boiled tubers were

cut as shown over "Tuber 3". The slices and the inner

3O

31

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32

and outer layers were frozen immediately and freeze-dried.
After freeze-drying the samples were ground in an

electric grinder (Chemical Rubber Co.) and stored in

plastic bags at a temperature lower than 14°F until they

were analyzed.

8. Amino acid distribution

 

Amino acid analysis on the following samples were
carried out: outer and inner layers of the potato varieties
Atlantic, Russet Burbank, and Monona, and apical and stem

ends of the variety Russet Burbank.

C. Protein and amino acid losses during boiling

 

From each variety eight tubers were chosen randomly,
washed, dried and weighed before boiling with distilled
water. Two groups of four tubers for each variety were
prepared for boiling. The tubers were cooled to room tem-
perature and weighed again.

The total solids (TS) were determined on boiled and
fresh samples, according to AOAC (1975).

For each variety the following scheme was carried out
for boiling, using the same volume of water (6 cups):

a) Whole tubers with skin

b) Whole tubers with skin cut longitudinally into

halves.

c) Whole tubers with skin cut into quarters

d) Whole tubers peeled with a home peeler

33

e) Whole tubers peeled and cut into halves

f) Whole tubers peeled and cut into quarters

A narrow stainless steel spatula was inserted into the
potatoes to see when they were cooked. After boiling, the
water for all the samples was filtered hot using cheese
cloth. The volumes were measured accurately after cooling
at room temperature. The water of the whole tubers boiled
with skins was concentrated by evaporation.

The boiling water was examined for the presence of
protein and amino acids. The amino acids were analyzed in
the boiled water from the cooked potatoes with intact skins,
halved potatoes with skins and quartered potatoes with
skins. Four m1 of water after boiling the potato with
intact skins were evaporated to dryness on a rotatory evapo-
rator. The same procedure was followed for the halved and
quartered potatoes but 2 ml of water were evaporated. The

dried samples were used for amino acid analysis.

0. Amino acid analysis of potato tubers

 

1. Amino acid distribution
The following scheme was carried out for the amino acid
distribution in whole potatoes.
a) The sulfur amino acid content in the whole raw

Atlantic and Russet Burbank potatoes.

34

b) Amino acid distribution in the Russet Burbank pota-
toes, boiled with intact skins, chemically or organically
fertilized. The samples for analysis were prepared from a
group of four potatoes for each variety.

2. Free amino acid profile

 

Free amino acids were analyzed in the whole raw potato,
varieties Atlantic, Denali, Russet Burbank, Superior and
Monona. The samples for analysis were prepared from a group
of four potatoes for each variety.

3. Available lysine in boiled potatoes

Available lysine was analyzed in the boiled potato,
varieties Atlantic, Russet Burbank, and Monona. The samples
for analysis were prepared from a group of four potatoes

for each variety.

E. Amide analysis of potato tubers

The amides asparagine and glutamine were analyzed in
the whole raw potato, varieties Atlantic, Denali, Russet
Burbank, Superior and Monona. The samples for analysis were

prepared from a group of four potatoes for each variety.

F. Protein efficiency ratio (PER)

1. Fertilization

 

The variety Russet Burbank, grown with chemical and
organic fertilizer, previously stored for 6 months at 40°F,
was selected for this portion of the research. From each

sample, approximately 30 kg of large and small washed tubers

35

were boiled with distilled water for 30 minutes. The tubers
with skins were sliced, freeze-dried, and stored in plastic
bags at 14°F.

2. Supplementation

 

Dried whole eggs, type w-l (Henningsen Foods, Inc.) and
whey powder (from cheddar cheese whey) were used for supple-
menting the commercially dried potato Russet Burbank. The
whey powder was prepared as follows: the sweet-rennet cheese
whey was collected at the time of the cheese-making operation
and maintained at 50°F for 12 hours. The whey was pre-heated
in a tank-jacket with steam at 115°F, fed to a vacuum evapora-
tor, and concentrated to 23.5% total solids (TS), at 106°F.

The condensed whey (23.5% TS) was spray-dried using the
following conditions in the spray-drier:

Inlet air temperature l60°F

Outlet air temperature 150°F

Feed tank pressure 40 psi

Atomizing pressure 30 psi

Total solids in the whey were determined refractometri-
cally.

3. Preparation of diets

Before incorporation of the freeze-dried potato sample
into diets, the material and the commercially dried potato
buds were ground in a Wiley mill to pass through a 0.50 mm

screen.

36

Eight diets were prepared: In six of them the sole
source of protein was a) Russet Burbank potatoes, chemically
fertilized; b) Russet Burbank potatoes, organically fer-
tilized; c) Russet Burbank, commercially dried potato buds;
d) Whey powder; e) Dried whole eggs; f) Casein. The other
two diets were prepared using composite flours in which the
ground commercially dried potatoes were mixed a) with whey
powder in the ratio 90:10 potato protein to whey protein;

b) with dried whole eggs in the ratio 65:35 potato protein
to egg protein.

The composition of the basal diet is shown in Table 4.
Diet c) and the two diets prepared using composite flours
contained 8% protein. Each diet was thoroughly mixed in a
Hobart blender. The diets were adjusted according to the
proximate analysis of test materials so that all diets,
including the reference (casein) diet, had the same compo-
sition, except for the nature of the protein. Male rats of
Sprague-Dawley strain, 21 to 28 days old, were used, ten
for each assay. After a 4-day acclimatization period,
during which the rats were fed a standard rat diet the first
two days and the experimental diet the last two days, they
were randomly divided into groups. Rats were housed indi-
vidually in metal cages with raised wire mesh floors. Water
and diets were fed ad libitum.

The body weight of each rat was recorded on the first

day of the assay period and again weekly up to the 28th day.

37

Table 4. Composition of basal diet for the PER

 

 

Ingredients Amount %
Protein 10

Corn oil 8

Salt mixture1 5
Vitamin mixture2 1
Non-nutritive fiber 1

Corn starch and sugar to complete 100

 

1USPXIX Salt mixture (Teklad Test Diets)

2Vitamin mix AOAC (Teklad Test Diets).

38

The food intake was determined weekly.
PER (weight gain/protein intake) was calculated from
the weight gain and protein consumption of each rat and the
values were adjusted to PER = 2.5 for the reference casein.
The prepared diets were refrigerated during the duration

of the experiments.

G. Taste of the Russet Burbank pptatoes chemically or

 

organically fertilized

 

The triangle test was used to determine the preference
(flavor differences) of the potato variety Russet Burbank,
grown with chemical versus organic fertilizer.

Twenty-three untrained panelists received three samples,
two of which were identical and they were asked to identify
the odd sample.

The potato samples for tasting were chosen randomly
from the batch of boiled potatoes used for the PER experi-
ment. Each individual sample was prepared by cutting a slice

from three different potatoes.

H. Preparation of a high protein potato flour

 

The potato protein flour was prepared as follows: whole
tubers of low to medium solid content were freeze-dried,
ground in a Wiley mill (.32 mm sieve) and subsequently
air-classified in a Walter Lab. separator at the rate of
2.5-3.0 g flour per min. Air classification yielded three

fractions: a) a small quantity of very fine particles; b) a

39

medium weight fraction; and c) a heavy fraction.

The second fraction was used in this part of the study.
This fraction was named according to the new FDA regulation
(1979) "potato protein flour" (PPF). It was used at levels
of 5 and 10% substitution in wheat bread.

Lasso

The basic formula for bread making with and without PPF
are given in Table 5. Three loaves of bread were made with
each flour formula. The straight dough method was used
(HNF (Human Nutrition and Foods)403-404 Laboratory manual,

Michigan State University).

Prgparation of bread

 

The bread was prepared according to the method outlined
in HNF (Human Nutrition and Foods) 403-404 laboratory
manual (Michigan State University). Commercial bakery
flour, bleached and bromated, was obtained from the General
Food Stores, Michigan State University. The following
steps were followed:

1. The yeast was allowed to hydrate in water at 1130F
for 5 minutes in the stainless steel bowl of the
mixer.

2. The sugar, salt, fat and approximately half of the
flour were added. The above ingredients were

mixed for one minute at speed 1.

40

Table 5. Ingredients used to produce bread by the straight
dough method

 

 

Ingredients Control] 5% PPF2 10% PPF2
Bread flour 200 g 190 g 180 9
Potato protein flour 0 g 10 g 20 9
Water, 65% 130 g 130 g 130 g
Shortening, 10% 20 g 20 g 20 9
Salt, 1% 2 9 2 g 2 9
Sugar, 6% 12 g 12 g 12 g
Yeast (dry granular), 3% 6 g 6 g 6 g

 

1Kneading time 6 min.; Fermentation time 30 min; proof 40
min.; bake 400°F(HNF 403-404 p. 66).

2Kneading time 7 min.; Fermentation time 85 min; proof 45
min.; bake 400°F (Knorr, 1977).

41

Half of the remaining flour was added and mixed

for another minute at the same speed.

The additional flour was added gradually with
continuous mixing and the dough was kneaded at
speed 1 during the time registered in Table 5.

The dough was shaped into a ball by folding in

half about five times, put into a greased bowl

and left to ferment during the time registered

in Table 5, in a proofing cabinet at about 100°F.
The dough was punched down, portions weighing 225 g
were removed, shaped into loaves using the sheeter,
put into greased loaf pans of approximately 7.6x
15.2 cm. and left to ferment during the time
registered in Table 5, to double in volume.

The loaves were baked in a pre-heated oven at
400°F until the crust was golden brown.

The bread was removed from the pan and allowed to
cool. Then, it was wrapped with plastic film, and

frozen for later examination.

The bread was thawed approximately four hours before

analysis.

Evaluations were carried out on the physical and

sensory characteristics of the finished bread according to

HNF 403-404 Lab manual. Physical measurements were color,

tenderness, volume, pH, moisture. Sensory characteristics

were carried out with a four member untrained panel. The

panelists rated crust color, characteristics of the crust,

42

grain, crumb color, taste and texture.
The protein and ash content of the PPF were determined

according to the AOAC method (1975).

Nitrogen Determination - Micro Kjeldahl

Samples containing approximately 8.0 mg dried protein
were digested in duplicate for 1 hour, according to AOAC
(1975).

Sulfuric acid of 1.84 specific gravity was used for
digestion and potassium sulfate and mercuric oxide were
added to raise the temperature. After cooling, the insides
of the flasks were rinsed with deionized water and digestion
was continued for an additional hour.

The digests were transferred into the distillation
apparatus by using approximately 10 m1 deionized water.

The digestion mixture was neutralized with 10 ml of a 50%
NaOH solution, containing 5% of sodium thiosulfate.
Released ammonia was steam-distilled into 5 ml of 5% boric
acid solution, containing 4 drops of methyl red-methylene
blue indicator. The distillation was continued until the
volume in the receiving flask reached 25 ml. The ammonium
borate complex was titrated with 0.02 N HCl which had been
accurately standardized against tris-hydroxy-amino methane
as primary standard. Nitrogen was calculated from the

following formula:

43

= (m1 HCl-ml b1ank)(normality of HCl(l4.007) x

% N mg of sample

100

 

Amino Acid Analysis

 

Acid hydrolysis. Amino acid analysis was performed

 

on HCl hydrolysates of protein using the"New Amino Acid
Analyzer Technikont according to the procedure of Moore

_t _t. (1958). Samples consisting of approximately 10 mg
of protein were placed into hydrolysis tubes with screw
caps. Two m1 of 12 N HCl were added to the tubes, followed
by the addition of 2 m1 of norleucine standard solution

(1 mM). Then, ten m1 of 6 N HCl were added to the tubes.

A stream of nitrogen was gently run into each tube for 1
minute. The tubes were capped quickly and screwed tightly.
The tubes were autoclaved for 16 hours at 250°F for hydroly-
sis. After hydrolysis the tubes were cooled to room tem-
perature and opened. The hydrolysate was then quantitati-
vely filtered through Whatman No. 2 filter paper and the
hydrolysate collected into a 100 ml flask. The filter
paper was washed well with five times (5x) deionized water.
Three washings were done but always keeping the volumes
small. The hydrolysate was evaporated to dryness on a
rotatory evaporator using a 131°F water bath. The dried
sample was washed and dissolved with 10 m1 of 5x deionized
water and again taken to dryness. In all samples, three

washings were performed to remove residual HCl.

44

The washed and dried hydrolysate was dissolved in 4 ml
0.01 N HCl and transferred carefully to a small tube and
frozen. Later, the frozen hydrolysate was thawed and 1 m1
of it diluted to a volume of 4 ml with lithium citrate
buffer (0.2 N Li+, pH 2.20 i 0.01) (Pierce Chemical Company,
Rockford, IL.). The solution was then filtered with a
0.20 um Metricel filter using a syringe with a Gelman hypo-
dermic adapter (Gelman Instrument CompanY); 60 ul aliquots
were used for analysis.

The chromatograms were quantitated by the peak area
method. Standard amino acid mixtures were analyzed using

the same ninhydrin solution.

Methionine and Cystine Analysis

 

Since methionine and cystine undergo a variable amount
of oxidation during acid hydrolysis, they must be analyzed
separately. The methods of Schram gt gt. (1954) and Lewis
(1966) were used. These methods involve performic acid
oxidation of methionine and cystine to methionine sulfone
and cysteic acid, respectively.

The performic acid solution was prepared by mixing one
volume of 30% (w/w) hydrogen peroxide with nine volumes of
88% (w/w) formic acid. This mixture was allowed to stand
for one hour at room temperature.

Samples representing 5—9 mg of protein were

weighed into hydrolysis tubes with screw caps. The protein

45

was oxidized for 24 hours with 10 ml of performic acid at
39°F. After oxidation 2 m1 of norleucine (1 mM) and 0.3
m1 of hydrobromic acid were added. The performic acid was
removed in a rotatory evaporator at 100°F.

The dried sample was hydrolyzed as previously dis-
cussed. The sulfur amino acid analysis was performed with
a Beckman Amino Acid Analyzer, model 121, using a tempera-
ture of 134.6°F and a citrate buffer, pH 2.2. Standard
methionine sulfone and cysteic acid mixtures were analyzed.
The chromatograms were quantitated by the height-width
method.

Free Amino Acid Profile

 

Free amino acid analysis was performed on distilled
water extracts of raw, freeze-dried potatoes, using the

"New Amino Acid Analyzer Technikon".

The procedure described by Booth (1971) was adapted
to extract the free amino acids of raw freeze-dried
potatoes. The free amino acids were extracted with dis-
tilled water. To 1 g of freeze-dried potato flour 20 ml of
distilled water were added. The mixture was shaken at room
temperature for four hours in a Junior Orbit Shaker (Lab-

Line Instruments, Inc.) at 200 rpm. Then 20 ml of 5%

trichloroacetic acid were added. The mixture was kept

in the refrigerator overnight at 39°F, centrifuged at 5,100 g

46

for 10 minutes and filtered using Whatman No. 2 filter
paper. Since acid hydrolysis was not employed, the free
sulfur amino acids were not destroyed and could be quan-

titated directly.

The chromatograms were quantitated by the peak area

method. Standard amino acid mixtures were analyzed using

the same ninhydrin solution.

Available Lysine

 

Carpenter's direct l-fluoro 2,4-dinitrobenzene (FDNB)
method as modified by Booth (1971) was used.

The samples were fine enough to pass through a .125 mm
sieve. A 0.8 9 quantity of sample was placed in a 100 m1
round bottom flask, with four antibump glass beads. Ten
ml of NaHCO3 solution (8% w/v) was added, the flask was
gently shaken by hand and sonicated for 7-10 minutes until
the sample was wetted. Care was taken so that the sample
would not be widely scattered in the flask.

Fifteen m1 of ethanol into which 0.5 ml of FDNB was
dissolved (Eastman Kodak Co.) was added to each flask. The
flask was shaken, gently at first, for 4 hours at room tem-
perature in a Junior Orbit Shaker (of the Lab-Line Instru-
ments, Inc.) at 200 rpm. Midway through the shaking period,
the flask was twirled and the rpm increased to 210 to

disperse the sample and to make sure that all particles

47

were wetted by the FDNB solution. After the shaking period,
the ethanol was evaporated in a boiling water bath (it was
checked by making sure the flasks lost 12.5 g of their
weight).

The mixture was cooled, mixed with 30 m1 of 8.1 N HCl
and refluxed for 16 hours.

The heat was turned off, the condenser was washed with
a little water and the flask disconnected. The contents
were filtered, while still hot, through a Whatman No. 2
filter paper and rinsed repeatedly with hot distilled water
until the total filtrate was almost 250 m1. When the
filtrate was cooled it was made to volume and mixed. The
hydrolysates were stored at 39°F, in the dark, until used
for further analysis.

Two m1 of the filtrate were pipetted into each of two
stoppered test tubes, A and B. The contents of tube B were
extracted three times with about 5 m1 of peroxide-free
diethyl ether. Most of the ether was removed with a water
aspirator. The dissolved ether was removed by placing the
tube in a steam bath until effervescence from the residual
ether ceased. A drop of phenolphthalein solution was added
followed by addition of NaOH solution (120 g/liter) until
the first pink appeared. Two ml of carbonate buffer pH 8.5
(19.5 g NaHC03, and 1 g NaZCO3 dissolved in 250 m1 of water
and pH adjusted to 8.5) were added. Under the fume hood

0.06 ml of methyl chloroformate (Eastman Kodak Co.) were

48

added. The tube was firmly stoppered and shaken vigorously.
Internal pressure was released cautiously. After about 8
minutes, 0.75 ml concentrated HCl were added dropwise with
agitation to prevent frothing. The tube was shaken occa-
sionally. The solution was extracted with ether 4 times as
described above and the residual ether was removed by
warming the tube in a steam bath. The tube was cooled and
the contents made up to 10 ml with water. In the meantime,
tube A was extracted three times with peroxide-free diethyl
ether. Residual ether was removed and the contents made up
to 10 ml with 1 N HCl.

The absorbance of both A and B were read at 435 nm
against deionized water. Reading A minus reading B (the
blank) is the net absorbance attributable to e-dinitro-
phenyllysine (DNP-lysine). -

With each set of samples analyzed, the absorbance of
the standard e-DNP-lysine solution, carried through the
same procedure as the samples, was determined.

The standard solution was prepared by dissolving 254
mg of mono-c-N-dinitrophenyllysine hydrochloride (e-DNP-
lysine, mol. wt. 348.8, lysine content 41.9%)(Sigma Chemical
Company), in 200 m1 of 8.1 N HCl. Two tenths of 1 ml of
this solution were used in each run for tubes A and B. This
amount contained 0.106 mg lysine when it was diluted to 10

m1 and gave a net absorbance of .400 at 435 nm.

49

The available lysine content was calculated from the

following formula:

= stAuxVxlOOxlOO
WuxAsxax(CP)

 

content as g lysine/l6 g N

0
ll

NS = weight of standard, expressed as mg lysine in
standard solution used in each run (usually 0.2
m1 of standard solution used containing 0.106 mg
lysine)

Wu = weight of sample hydrolyzed in mg

As = net absorbance (absorbance tube A - Absorbance
tube 8) of standard solution

Au = net absorbance of unknown

total volume of filtrate (usually 250 ml)

V

a aliquot of filtrate (usually 2 m1)

(CP) = crude protein of sample, 6.25ng/100 g potato

A recovery factor was calculated by using e-DNP-lysine
as internal standard. Duplicate samples of each material
were treated as described above as far as the addition of
8.1 N HCl, but approximately only half of the volume of the
HCl was added. Standard efDNP-lysine solution was then
added containing 7.95 mg lysine. More 8.1 N HCl was added
to a total volume of 30 m1, and the procedure was continued

as previously described. From the difference in available

lysine between the two samples and the known amount of

50

lysine added into the second sample, the percent recovery
factor was calculated. A second digestion was performed
with the residue of the variety Russet Burbank, The resi-
due was left in the tip of the filter cones after the last
washing. Using 6 N HCl, the residue was washed, together
with the antibump balls, back into the same round flask.
Replicated residues were pooled. Twenty m1 of 6 N HCl were
used for the second digestion. The solution was refluxed
16 hours, filtered hot, washed, and made up to 100 m1 and
the DNP-lysine determined as described above.

The values of samples for available lysine were cor-
rected only for half of the loss estimated by the internal
standard, since it has been shown by Booth (1971) that the
loss of protein-bound dinitrophenyl lysine (as the lysine
of the sample) is only half of the loss of free c-DNP-

lysine (as that of the internal standard).

Amide Analysis

The amides asparagine and glutamine were analyzed on
distilled water extracts, free of protein, using the "New
Amino Acid Analyzer Technikonfl The procedure used to
extract the free amino acids was adapted to extract the
amides asparagine and glutamine in raw potatoes. Since
acid hydrolysis was not employed, the amides were not

destroyed and could be quantitated directly.

RESULTS AND DISCUSSION

The discussion of the results will be presented in two
parts. Part I discusses the nutritional quality of potato
protein and Part II covers the preparation of an air-clas-
sified high protein potato flour and its use in bread-

making.

Part I

Nutritional Quality of Potato Protein

A. Nitrogen distribution within the tuber

 

In Table 6 the results of an analysis of the nitrogen
content distributed within the raw and boiled potato tuber
of five freeze-dried potato varieties are presented. The
results of Table 6 were obtained with the potato flour free
of skin. The skin of the raw freeze-dried sections was
removed with the tip of a stainless steel spatula. The
skin of the boiled potatoes was removed by hand.

For the statistical study, the data of Table 6 for
potato sections 1, 2, 3 and 4 (see Figure 2) were rearranged
by averaging the N content of sections 2 and 3. The average
of sections 2 and 3 was named 2' as it is shown in Table 7.

These data were analyzed using Dunnett's test (Gill, 1978).

51

Table 6. Nitrogen distribution within the potato tuber in
five freeze-dried varieties.I

52

 

 

 

 

Section % N (9/100 9 sample)
N°° Atlantic Denali Russet Superior Monona
Burbank
1 1.33 1.47 1.41 1.91 2.09
2 1.39 1.59 1.55 1.97 2.21
3 1.65 1.64 1.58 2.10 2.26
4 1.49 1.75 1.53 2.10 2.11
5 1.91 1.85 1.90 2.09 2.35
6 1.65 1.55 1.69 1.72 2.06
7 1.67 1.65 1.76 2.01 2.05
Outer
Tayer2 1.62 1.32 1.45 1.88 1.55
Inner
layer3 1.74 1.47 1.54 2.00 1.57
periderm4
(skin)

 

1Mean based on two different tubers.

2’3The whole potato was boiled with distilled water.

4Periderm removed after boiling and drying in an oven.

53

Table 7. Nitrogen distribution within the raw potato tuber
in five freeze-dried varieties

 

% N (g/100 9 sample)

 

Section No.

 

Atlantic Denali Russet Superior Monona
Burbank
1 1.33 1.47 1.41 1.91 2.09
2 -
>X(2') 1.52 1.52 1.57 2.04 2.24
3
4 1.49 1.75 1.53 2.10 2.11

 

2 Average of the N content of sections 2 and 3.

54

The results showed that there was no significant difference
(P<0.01) between sections 1, 2' and 4, but a slight signifi-
cant difference (P<0.05) was found in section 1 versus 4
(Table 7). Apparently the section near the apical end of
the potato had slightly higher N content than the section
near the stem end.

It was found also that the N content of the potato is
higher in the dried peel flour compared to the N content of
the potato flour of the outer and inner layers. The same
results were observed when it was compared to the N content
for potato sections 1, 2, 3, 4, 5, 6, and 7 (Table 6). The
PER of the skin, however, is practically zero, according to
previous work (Meister and Thompson, 1976a).

The varieties Russet Burbank and Monona displayed
higher N contents in sections 2 and 3 than what were found
in sections 1 and 4. In the varieties Atlantic, Denali and
Superior, sections 3 and 4 had a higher N content than did
sections 1 and 2. For the variety Monona, section 3 was
found to have the highest N content. In all the varieties
studied, the sections 2, 3 and 4 had higher N contents than
section 1 (Table 6, Figure 3).

In relation to sections 5, 6, and 7, it was observed
that section 5 had the highest N content in all five potato
varieties (Table 6, Figure 3). However, the difference was
not statistically significant (P<0.05). Section 5 corres—

ponds roughly to the pith (Figure 1). When the skin is not

55

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56

considered, the pith of the potato has the highest N con-
tent. If the outer layer of the potato is cut after
boiling, the inner layer is found to have a higher N con-
tent than outer one (Table 6). This confirms the results
presented above, that the N content tends to rise toward
the pith of the potato. The outer layer was cut after
observing visually two defined areas following the boiling

of the potato.

B. Amino acid distribution within the tuber

The amino acids in the acid hydrolysate were calcu-
1ated in micrograms and expressed as g residue per 100 9
amino acid.hydrolysate.

In Table 8 is presented the amino acid distribution in
acid hydrolysates of the outer and inner layers of the
boiled freeze-dried potatoes. The results showed higher
amounts of aspartic and glutamic acids in the inner layer
compared to the outer layer in the varieties Atlantic,
Russet Burbank and Monona. The amino acid proline tended
to be higher in the outer layer than in the inner layer
(Table 8). 0f the essential amino acids, valine, leucine,
and lysine tended to be higher in the outer layer compared
to the inner layer. 0f the total content of amino acids
in the acid hydrolysate, the essential amino acids threo-
nine, valine, isoleucine, leucine, phenylalanine and lysine

comprised 27.5% in the outer layer and 24.0% in the inner

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58

layer for the variety Atlantic; 26.6% in the outer layer
and 22.3% in the inner layer for the variety Russet Bur-
bank; and 26.8% in the outer layer and 22.8% in the inner
layer for the variety Monona. These data indicate that
there is also variation among varieties in relation to

the distribution of the essential amino acids in the outer
versus inner layers. A higher concentration of essential
amino acids is observed in the outer layer in all the
potato varieties studied. In order to protect methionine
and cystine from destruction during acid hydrolysis, these
amino acids were oxidized before hydrolysis to methionine
sulfone and cysteic acid respectively, by performic acid.
Analysis of the sulfur amino acids, methionine and cystine
in the variety Russet Burbank, showed for cystine 1.30 and
1.20 g/l6 g N for the outer and inner layer, respectively.
For methionine it showed 1.54 and 1.37 g/l6 g N, respec-

tively, for the outer and inner layers.

Table 9 represents the amino acid distribution in acid
hydrolysates of the apical and stem sections of the raw
freeze-dried Russet Burbank potatoes. It was observed

that the predominant amino acids in the apical section
were aspartic and glutamic acids. In the stem section
aspartic acid was the predominant amino acid. The apical

section compared to the stem section had higher amounts of

59

Table 9. Amino acid distribution in hydrolysates of the
apical and stem sections (see Figure 2) in the
raw Russet Burbank potatoes (9 residue/100 9 amino
acid hydrolysate)1

 

Amino acid Russet Burbank

 

 

 

Apical section Stem section

Aspartic acid 18.5 44.0
Threonine 2.9 5.9
Serine 2.6 6.4
Glutamic acid 20.5 3.7
Proline 7.6 5.2
Glycine 4.1 2.0
Alanine 4.4 2.6
Valine 8.8 5.0
Cystine 0.0 0.0
Methionine 1.2 0.8
Isoleucine 4.0 2.4
Leucine 6.1 6.2
Tyrosine 4.5 3.5
Phenylalanine 4.6 2.8
Lysine 6.5 6.4
Histidine 1.4 l l
Arginine 2.3 2.0

Total 100.0 100.0
Cystine2(g/16 g N) 1.10 1.12
Methionine3(g/16 g N) 1.50 1.34

 

1Mean of duplicates
2 3

These amino acids were oxidized before hydrolysis by
performic acid, to protect them during acid hydrolysis.

6O

glutamic acid, glycine, alanine, valine, Isoleucine, tyro-
sine, and phenylalanine. Lower contents of aspartic acid,
threonine and serine were observed in the apical end in
relation to the stem end (Table 9). The content of glutamic
acid of the stem section was 3.7% compared to 20.5% of the
apical end section. The amino acids leucine, lysine,
histidine, and arginine seems to be similarly distributed

in the apical and stem sections (Table 9). It was observed
also that the total distribution of the essential amino
acids threonine, valine, isoleucine, leucine, phenylaline,

and lysine was high in the apical section, 32.9% compared

to the stem section, which has 28.7%.

The sulfur amino acids were oxidized before hydro-

lysis to methionine sulfone and cysteic acid, as described
previously.

The sulfur amino acids, methionine and cystine were
analyzed in the stem and apical sections of the Russet
Burbank potatoes. The values for cystine were 1.12 and
1.10 g/16 g N for the stem and apical sections, respec-
tively. For methionine, 1.34 and 1.50 g/16 g N were
obtained, respectively, for the stem and apical sections.

The N and amino acid distribution within the potato

tuber could be a help to the potato breeder because the

61

biological value of the potato protein can be improved

through plant breeding.

C. Protein and amino acid losses during boiling

Table 10 represents the percent protein loss from boiled
potatoes compared to the original protein content, and raw
sample. The results expressed as percentages of the raw
sample seem insignificant. Therefore, the discussion of
the percent protein loss during boiling will be related to
the percent change from the original protein content, based
at 100%. There are some differences among varieties in the
protein losses during boiling under the types of cutting
indicated in Table 10.

If the data for all varieties and for each type of cutting
is averaged, it is found that the whole tuber boiled with
skin suffered the lowest loss of protein, 0.8%, and the
whole tuber peeled and cut into halves had the highest
protein loss, 10.4%. Comparison of the peeled versus non-
peeled types of cuttings of the potato, showed that the
protein losses are higher in the peeled potatoes: 6.5%
protein loss for the whole tuber when peeled versus 0.8%
for the whole tuber with skin, 10.4% for the whole tuber
when peeled and cut into halves versus 4.3% for the whole
tuber with skin cut into halves, and 9.3% for the whole
tuber when peeled and cut into quarters versus 4.8% for

the whole tuber with skin cut into quarters.

62

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63

Next, when comparing the types of cuttings only in
potatoes with skin, the largest loss of protein was observed
for the quartered tuber. The loss was 4.8% for quarters,
4.3% for halves and 0.8% for the whole tuber. Examining
the area of the potato directly exposed to the water could
explain this difference. The results of this study are
lower compared to 50% loss of total protein reported by
Desborough and Weiser (1974) after boiling halved potatoes
with skin for 30 minutes.

When the peeled potato was boiled, the largest loss,
10.4%, of protein was observed with the whole tuber peeled
and cut into halves; the loss in the whole peeled tuber
was 6.5%.

The difference in protein loss between the peeled halved
and the peeled quartered tuber might be due to the extent
of time for boiling. The peeled quartered potato required
less time for cooking, 10.2 min. versus 15 min for the
peeled halved tuber (Table 10).

After boiling the potatoes, the water was examined for
the presence of amino acids. Table 11 represents the amino
acid distribution in the acid hydrolysates of the boiled
water from the cooked potatoes. The predominant amino
acids in the boiled water of Russet Burbank and Atlantic
potatoes boiled with intact skins were aspartic and glutamic
acids. These non-essential amino acids comprised 68.8% and

70.2% of the total acid hydrolysate for the Russet Burbank

64

Table 11. Amino acid distribution in hydrolysates of the
boiled water from the cooked otatoes (g residue/
100 g amino acid hydrolysate 1

 

 

 

 

 

 

Amino acid a Russethurbankc a Atlagtic c
Aspartic acid 43.3 36.1 39.2 48.1 37.5 42.3
Threonine 1.2 1.1 0.8 2.3 1.2 0.
Serine 2.8 1.2 1.2 1.9 1.4 1.1
Glutamic acid 25.5 38.3 35.4 22.1 30.3 33.9
Proline 2.6 10.4 1.5 8.4 10.2 2.2
Glycine 3.7 1.8 0.7 2.7 1.3 1.4
Alanine 1.9 1 1 1.3 1.3 1.2 2.0
Valine 5.3 3.7 3.8 4.6 4.0 4.0
Isoleucine 2.8 1.2 3.4 1.2 1.3 1.6
Leucine 2.4 1.3 1.2 1.7 1.3 1.3
Tyrosine 3.8 1.0 2.5 1.7 1.8 2.1
Phenylalanine 3.4 1.4 2.4 1.4 2.1 2.4
Lysine 1.3 1.4 2.7 1.3 1.7 3.0
Histidine traces traces 1.4 traces traces 0.7
Arginine traces traces 2.5 1.3 4-7 1.1

Total 100.0 100.o Too.o :cc.( 1o0.o Too o
—TNotations: ”

Means of duplicates
a = whole tuber with skin

b

whole tuber with skin cut longitudinally into halves

c whole tuber with skin cut into quarters

65

and Atlantic potatoes, respectively. 0f the essential amino
acids, valine comprised 5.3%‘fiM‘the Russet Burbank variety
and 4.6% for the Atlantic variety.

The same predominant amino acids were found in the
boiled water from the boiled halved potatoes with skins.
Aspartic and glutamic acids comprised 74.4 and 67.8% of the
total pool in the acid hydrolysate for the Russet Burbank
and Atlantic potatoes, respectively.

Potatoes cut into quarters were observed also to yield
predominantly aspartic and glutamic acids. In the water
from the Russet Burbank variety 74.6% of the total pool
consisted of these acids and 76.2% in the Atlantic variety.
A decrease in threonine and proline was also observed whereas
isoleucine, lysine and histidine increased when compared to
other types of cuttings with the Russet Burbank and Atlantic
potatoes (Table 11).

The sulfur amino acids, cystine and methionine, were
analyzed in the boiled water from the Russet Burbank and
Atlantic potatoes boiled under three cutting conditions.
These conditions were intact skins, halved potatoes with
skins and quartered potatoes with skins. The time for
boiling with intact skins for the Russet Burbank and Atlan-

tic potatoes is registered in Table 10.
In order to protect methionine and cystine from destruc-

tion during acid hydrolysis, these amino acids were oxidized

66

as indicated before.

Table 12 represents the loss of cystine and methio-
nine from boiled potatoes, expressed as a percent (mg/100 g)
from the original content of cystine and methionine, based
at 100%. In the same table appears the losses of each
sulfur amino acid per each 100 g of raw potatoes and protein.
The losses expressed per each 100 g of raw potatoes and
protein seem insignificant. Therefore, the discussion
about the losses of each sulfur amino acid during boiling
will be related to the percent from the original content of

cystine and methionine, based at 100%.

Of the three cuttings, it was found that the whole
tuber boiled with intact skins suffered a lower loss of
cystine and methionine than the halved or quartered tubers.
This result was observed in both Russet Burbank and Atlantic
potatoes boiled with intact skins. In the boiled Russet
Burbank potatoes, 0.24 g and 1.33 g were lost for each
100 g of cystine and methionine present in the raw potatoes.
For the boiled Atlantic variety, losses per 100 g of cystine
and methionine present in the raw potatoes were 0.13 g for
the former amino acid and 0.79 g for the latter one
(Table 12).

The largest loss of sulfur amino acids was observed
for the halved and quartered potatoes boiled with intact

skins. For the halved Russet Burbank potatoes the losses

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68

were 0.39 g of cystine and 1.94 g of methionine per each

100 g of each sulfur amino acid present in the raw potatoes.
For the quartered Russet Burbank potatoes the losses were
0.52g10fcystine and 2.89 g of methionine per each 100 g

of each sulfur amino acid present in the raw potatoes.

For the halved Atlantic potatoes the losses were 0.63 g

of cystine and 2.94 g of methionine per each 100 g of each
sulfur amino acid. For the quartered Atlantic potatoes the
losses were 0.77 g of cystine and 2.72 g of methionine per
100 g of each sulfur amino acid (Table 12).

The loss for methionine is higher than for cystine for
each type of cutting. One explanation of this might be
that the potatoes have higher free methionine content than
free cystine. It was found in this study that free cystine
appears as traces in some varieties and in others it is not
detected (Table 15).

The fact that the loss of sulfur amino acids during
boiling of potatoes increases by cutting the potatoes,
indicates that it is advisable to boil the potatoes with
intact skins. Nutritionally, this is very important.
Methionine and cystine are the limiting amino acids in
potatoes; therefore, avoiding the loss of methionine and
cystine is significant for the biological quality of the

pctato protein.

69

0. Amino acids of potato tubers

 

1. Amino acid distribution

 

Table 13 represents the sulfur amino acid content
in the raw freeze-dried Russet Burbank and Atlantic

potatoes.

Methionine and cystine were oxidized before hydro-

lysis, as described previously. The sulfur amino acid

content of the Russet Burbank and Atlantic potatoes was:
cystine 1.4 and methionine 1.6 g/l6 g N for Russet Bur-
bank, and cystine 1.3 and methionine 1.7 g/l6 g N for

Atlantic. These values of sulfur amino acids were used
to evaluate the cystine and methionine losses during
boiling of Russet Burbank and Atlantic potatoes (Table 12).

Table 14 represents the amino acid distribution in
acid hydrolysates of the boiled Russet Burbank potatoes
chemically or organically fertilized. The Russet Burbank
potatoes chemically or organically fertilized showed
similar patterns in the amino acid distribution. Appar-
ently, slight variations were observed in serine, glutamic
acid, glycine, isoleucine, and histidine.

In relation to the sulfur amino acids, similar values
were observed in the boiled Russet Burbank potatoes chemi-

cally or organically fertilized (Table 14).

70

Table 13. Cystine and methionine content in raw freeze-
dried potatoes (g/16 g N)1

 

 

Amino acid Atlantic Russet Burbank
Cystine 1.3 1.4
Methionine 1.7 1.6

 

1Mean of duplicates.

71

Table 14. Amino acid distribution in hydrolysates of the
boiled Russet Burbank potatoes chemically or
organically fertilized (g residue/100 9 amino
acid hydrolysate)1

 

Amino acid

Russet Burbank

 

Chemically Fertilized Organically

Fertilized

 

 

Aspartic acid 33.7 34.3
Threonine 2.1 2.1
Serine 2.3 1.5
Glutamic acid 18.0 21.1
Proline 2.9 3.0
Glycine 3.0 2.0
Alanine 2.8 2.8
Valine 5.0 4.4
Cystine traces traces
Methionine 0.9 0.5
Isoleucine 3.6 4.2
Leucine 4.8 5.1
Tyrosine 5.5 4.8
Phenylalanine 4.3 3.6
Lysine 5.0 5.4
Histidine 3.0 2.2
Arginine 3.1 3.0
Total 100.0 100.0
Cystine2(g/l6 g N) 1.37 1.39
Methionine3(g/l6 g N) 1.58 1.60

 

1Mean of duplicates

2’3These amino acids were oxidized before hydrolysis by
performic acid, to protect them during acid hydrolysis.

72

2. Free amino acids

 

The determination of free amino acids in potatoes
was warranted in this study because a) free amino acids are
well utilized by the human body, and b)free amino acids
can be leaked or rinsed during cooking or processing of
potatoes.

Table 15 represents the free amino acid distribution
and amide distribution in distilled water extracts from
raw freeze-dried potatoes. The discussion about amide
distribution in potatoes will be presented later. The
results for free amino acids are expressed as g residue per
100 g of non-protein extract (free amino acids plus amides).
In the distilled water extract the free essential amino
acids threonine, valine, methionine, isoleucine, leucine,
phenylalanine, and lysine comprised summarily the following
percentages of the total pool: 16.0% for Atlantic potatoes;
7.5% for Denali potatoes; 23.3% for Russet Burbank potatoes;
.20.0% for Superior potatoes and 21.1% for Monona potatoes.

In the potato varieties Atlantic and Denali the pre-
dominant free amino acids in distilled water extracts were
aspartic acid, glutamic acid, proline, valine and arginine.
These same predominant amino acids were observed in the
varieties Russet Burbank, Superior and Monona with the
exception of proline that was present in lower percentages
than observed in Atlantic and Denali varieties (Table 15).

In relation to the sulfur free amino acids it was found in this

study that cystine was present as traces in the distilled protein free

Table 15.

73

Free amino acid1 and amide2 distribution in

water extracts from raw freeze-dried potatoes

 

. . Russet .
AtlantTc Denali Burbank SuperTor Monona

 

Amino acids

Aspartic acid
Threonine
Serine
Glutamic acid
Proline
Glycine
Alalanine
Valine
Cystine .
Methionine
Isoleucine
Leucine
Tyrosine
Phenylalanine
Lysine
Histidine

Arginine

Amides

Asparagine
Glutamine

Total

15.
12.
1.
1.

5

traces

1

.4

.7

1.2

17.
16.

100

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2.0 1.8 1.7
2.6 2.2 1.8
6.6 8.7 13.6
1.1 traces 0.5
0.3 0.5 0.3
1.0 0.9 1.4
7.9 3.9 6.1
0.0 traces 0.0
1.1 2.2 0.8
2.7 2.5 1.9
1.4 1.1 1.2
4.0 12.0 4.8
3.5 5.5 4.9
4.7 3.0 4.5
2.0 4.3 2.3
4.7 10.6 6.4
20.1 14.4 16.1
24.4 11.6 22.8
100.0 100.0 100.0

 

1
Mean of duplicates (Grams residue/100

2

amides).

9 free amino acids +

Mean of duplicates (Grams/100 9 free amino acids + amides).

74

water extracts from Atlantic and Superior potatoes. These
potato varieties had higher amounts in free methionine
compared to Denali, Russet Burbank, and Monona potatoes.
The free methionine content, expressed as g residue per

100 g of non-protein extract (free amino acids plus amides),
were: 1.7, 0.7, 1.1, 2.2 and 0.8 for Atlantic, Denali,
Russet Burbank, Superior and Monona potatoes, respectively
(Table 15). The varieties Atlantic and Superior could be
chosen to improve the sulfur containing amino acids in
potatoes through plant breeding. Also, the wide range in
content of free methionine, 0.7 to 2.2 g per 100 g of the
non-protein extract (free amino acids plus amides), indi-
cates that selection of potatoes high in methionine could
be possible. Luescher (1972) suggested that with intensive
”N fertilization the ideal potato to select should be high
in free methionine. Also, the possible presence of free
cysteine should be kept in mind.

The variation of free amino acids among potato varie—
ties is pronounced. Mulder and Bakema (1956) stated that
the composition of the soluble non-protein fraction is much
less constant than that of the true protein fraction. Some
factors such as type of variety, mineral nutrition of the
plant, environmental conditions, etc. might affect the

composition of the nonprotein fractions.

E. Amides,yasparagine and glutamine ofypotato tubers
Table 15 shows the free amide distribution in distilled

water extracts from raw freeze-dried potatoes. The results

75

are expressed as g amide per 100 g of the non-protein
extract (free amino acids plus amides).

In relation to amides it was found that the distribu-
tion of asparagine ranged from 14.4 to 20.1%, g per 100 9
non protein extract (free amino acids plus amides), and glu-
tamine ranged from 11.6 to 24.4% (Table 15). The amides
comprised 34.0, 36.4, 44.5, 26.0 and 38.9% of the total pool
(free amino acids plus amides), for the potato varieties
Atlantic, Denali, Russet Burbank, Superior, and Monona, re-
spectively. The results obtained in this study suggest a
variation in the content of amides among potato varieties
and also that the concentration of amides is high in pota-
toes. A high content of amides in potatoes could be useful
from the nutritional point of view, since these amides
(asparagine and glutamine) may prevent antagonism among

amino acids (McCay, 1959).

F. Available lysine by the FDNB method

Table 16 shows the values of available bound lysine
obtained by the FDNB method, in whole boiled potatoes with
intact skins.

The results were obtained from the study of potato
flour free of the skin which was removed by hand after
boiling the potatoes.

Comparison of the amounts of available bound lysine in
boiled potatoes showed that there were slight differences in

the amounts of available lysine among the varieties analyzed.

76

Table 16. Available bound lysine in boiled potatoes by the
FDNB method

 

 

 

Russet
Atlantic Burbank Monona
Corrected values 4.77 4.54 4.94
9 available 2
lysine/100 g protein (5.08)
1

Mean based on two determinations made on one group of four
freeze-dried boiled potatoes.

2The value in parenthesis belong to raw Russet Burbank
potatoes.

77

The corrected values of available bound lysine (g/Ioofprotein)
were: Atlantic, 4.77, Russet Burbank, 4.54, and Monona, 4.94.

Raw potato flour and cooked potato flour of Russet Bur-
bank potatoes were tested. Available lysine in the raw
freeze-dried material was 5.08 g/100 g protein versus 4.54 g
in the boiled freeze-dried potato. These results demonstrated
that there is a decrease of 10.6% in the availability of the
lysine after boiling the Russet Burbank potatoes. When the
residue left after the HCl hydrolysis was subjected to a
second Hcl hydrolysis, Russet Burbank potatoes were found to
contain 0.17 9 more lysine per 100 g protein.

Table 17 shows the percent weight loss during boiling
of potatoes in distilled water. The results showed a de-
crease in weight of potatoes after boiling compared to raw
weight. However, an increase in weight was observed in
relation to raw potatoes, peeled or unpeeled, when they were
overcooked. Presumably the starch bound more water in the
overcooked, cracked potatoes (Van Den Berg gt gt., 1975).
Therefore, to avoid overcooking, the cooked potatoes were
carefully tested with a thin stainless steel spatula by inser-
ting it into the tuber to determine when the potatoes were cooked.

Table 18 represents the total solids (T5) of raw and
boiled potatoes. The results show that the TS of the

boiled potatoes with intact skins is approximately that
of the raw material. However, if potatoes are peeled or

cut prior to boiling, the TS content increases. One

78

 

 

 

Table 17. Percent weight loss during boiling of potatoes
Tn detTlled water

Variety‘ a b c d e
Atlantic 1.3 l 6 2 2 4.6 4 4
Denali 1.4 4 7 4.9 6 0 4 3
Russet
Burbank 1.3 3 7 4.0 5 0 4 8
Superior 1.5 5 l 4 3 7.4 5 7
Monona 2.1 5 9 4.9 4 2 3 6
1Notations:

a = whole tuber with skin

b = whole tuber with skin cut longitudinally into

halves

c = whole tuber with skin cut into quarters

d = whole tuber peeled with home peeler

e = whole tuber peeled and cut into halves

f = whole tuber peeled and cut into quarters
Mean of duplicates

79

Table 18. Percent total solids of raw and boiled potatoes1

 

 

 

Variety Raw a b c d e f
Atlantic 21.6 22.7 23.6 22.5 24.1 22.9 25.
Denali 23.0 22.7 23.2 24.7 23.7 23.5 25.
Russet

Burbank 21.0 19.3 21.3 20.3 22.2 21.7 23.
Superior 17.5 19.5 18.0 17.1 17.4 19.6 19.
Monona 16.3 14.9 - 15.7 16.5 17.9 20.

 

1Notations:

whole tuber with skin

a

b = whole tuber with skin cut longitudinally into
halves

c = whole tuber with skin cut into quarters
d = whole tuber peeled with a home peeler

e = whole tuber peeled and cut into halves

f = whole tuber peeled and cut into quarters

Mean of duplicates

80

explanation to this could be that there is a loss of the
water present in the potato tissue. This fact could explain
the weight loss during boiling of potatoes, too.

Table 19 shows the percent of the raw portion of potato
removed by a home peeler; also, the total solids and N con-
tents of this portion. An average of 10.1% of the raw
potato was removed by the home peeler. This removed portion

contained 14.9% TS, on the average, and 2.8% N on dry basis.

G. Protein efficiency ratio (PER)

1. Fertilization

2. Supplementation

The growth curves for the 8 groups of the rats fed the
experimental diets are shown in Figure 4. Rats fed the
supplemented diets, potato + dried whole eggs and potato +
whey powder, grew at higher rates 5.07 and 2.71 g/rat/day,
respectively, when compared to rats fed the commercial dried
potato diet that grew at a rate of 2.00 g/rat/day. Those
fed diets containing Russet Burbank potatoes chemically or
organically fertilized (CF, 0F) grew at rates of 1.79 and
1.63 g/rat/day, respectively; these rates were not statis-

tically different.

In general the results indicated that rats fed the
potato diets grew at slower rates than those fed who1e dried

eggs, casein and the supplemented diets.

The average values of food intake, protein intake,

weight gain, food conversion tuitio. and PER are shown in

81

Table 19. Percent of the raw portion of potato removed by
a home peeler; percent total solids content of
the removed portion; and percent N content of
the removed portion on a dry basis.

 

 

 

Variety1 % portion % TS2 % N
Atlantic 8.6 15.0 2.9
Denali 10.5 15.4 2.7
R. Burbank 10.6 16.4 3.0
Superior 10.5 14.2 2.8
Monona 10.2 13.5 2.8

x 10.1 14.9 2.8
1

Mean of duplicates

2Total solids.

82

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Table 20. The food conversion ratio (feed intake, g/weight
gain, 9) was 5.6 for the Russet Burbank potatoes, CF; 5.9
for the Russet Burbank potatoes, OF; 2.7 for dried whole
eggs, 4.5 for potato + whey powder, 5.2 for the Russet
Burbank potatoes, commercially dried; 3.2 for the Russet
Burbank potatoes commercially dried + dried whole eggs;

and 3.6 for casein.

For the whey powder, the PER and food conversion ratio
could not be calcu1ated, since the rats had diarrhea during
the entire experiment, with the exception of two that
apparently tolerated the lactose present in the whey
powder. After one week of the experiment, the enzyme
lactase (Maxilact (R) 20,000-Lab and Plant: Keyport, N.J.)
was added to the water at 0.05 g/150 ml of water. No
improvement in lactose tolerance was observed after the
rats drank the enzyme-fortified water.

PER values were calculated from the formula weight
. gain, g/protein consumed, 9. One group of rats was fed a
diet containing casein, and was used as the control group
to calculate the corrected (adjusted) PER values.

The adjusted PER values are presented in Table 20.

The Russet Burbank potatoes chemically or organically

fertilized produced close values of 1.62 and 1.54, respec-
tively. This difference was not statistically significant.
The similarity in the PER values observed with Russet Bur-

bank potatoes, CF or 0F, are in agreement with the sulfur

84

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86

amino acid composition. It was found that the sulfur amino
acid content was similar in Russet Burbank potatoes,CF or
0F (Table 14). Earlier reports indicate that methionine
plus cystine are the amino acids which limit the nutri-
tional value of potato protein.

The cooked Russet Burbank potatoes, CF, showed a lower
PER, 1J2, than the 2.15 value observed for the same variety
which was commercially dried. This difference may be due
to these factors: a) some non-protein N is removed in the
manufacture of potato powder, and b) the skin that was fed With
the cooked potatoes is known to reduce the digestibility of
protein.

The PER of the commercially dried potato flour pre—
pared with the variety Russet Burbank and mixed with dried
whole eggs in the ratio 65:35 potato protein to egg protein
was 8.0% greater than the PER of the dried whole eggs.

This ratio of potato protein to egg protein resulted in a
PER which was 54% higher than that of the commercially
dried potato, and 40% higher than that of casein (Figure
5). These results are in agreement with those of Kofranyi
and Jekat (1965, 1967), who experimented with human volun-
teers.

When the same commercially dried potato flour was
mixed with whey powder in the ratio 90:10, potato protein
to whey protein, a PER value was obtained which was similar

to that of casein (2.48 versus 2.50), but a 13% higher than

 

87

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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F
125 » F—
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1 2 3 4 5 6 7
[)IE'TS
Figure 5. PER of diets calculated as percent of casein PER.

l and 2=chemically and organically fertilized
potatoes, respectively, 3=whey powder (from
Cottom, 1974), 4=dried whole eggs, 5=commercial
potato + whey powders, 6=commercial potato powder,

7=commercia1 potato + dried whole egg powders.

88

that of the commercially dried potato flour alone (Figure
5). This may be attributed to the relatively high content
of whey in methionine plus cystine (4.1 g per 100 g protein
according to Glass and Hedrick, 1976), the amino acids which
limit the nutritional value of potato protein.

Lactose tolerance was observed after the rats ate the
mix prepared with commercially dried potato flour plus
whey powder mixed in the ratio 90:10, potato protein to
wheyprotein. The rats did not have diarrhea. The whey
powder can be a good supplement for potato protein.

Finally, it can be said that potato is an economical
food and these research results suggest a high nutritional
value. However, the low methionine content can limit it.
But solutions to this limitation are on hand. To obtain a
potato diet higher in methionine the following aspects can
be considered (Kies and Fox, 1972): a) genetic selection
of potato tubers having a high methionine level; b) addi-
tion of purified methionine in the industrial processing;
c) education of consumers in usage of desirable food
combinations.

In less developed areas such as South America which
produces large quantities of potatoes, genetic selec-
tion of potato tubers high in methionine and food combi-
nations should be kept in mind in order to have a potato
diet higher in methionine. At present in South America

it is economically unfeasible to consider the fortification

89

of potatoes with purified methionine during the industrial
processing. Also, 111 some areas of South America
potatoes can be an excellent supplement for lysine poor

vegetative proteins such as wheat or other cereals.

H. Taste of Russet Burbank potatoes chemically or organi-

callyifertilized

 

There have been claims that "organically" grown foods
taste better than those grown with chemicals.

In this research, no significant difference (P<0.05)
in taste (flavor) was found for the Russet Burbank potatoes
grown with organic or chemical fertilizer, when boiled

potatoes were tested by 23 untrained panelists.

Part II
Preparation of a Potato Protein Flour and Its Uses

in Bread Making

A. Preparation of a potato protein flour and bread

Three fractions were obtained from the prepared potato
flour by air classification: a very fine fraction repre-
senting 1.9% of the flour, a medium weight fraction repre-
senting 11.2% of the flour, and a heavy fraction (mostly
starch) representing 80.0% of the flour. Approximately
6.9% was lost during the process. This loss is excessive

but it would not occur in a large commercial operation.

90

In this study the medium weight fraction known as
potato protein flour (PPF) was used to prepare bread.
This fraction contained 36.3% protein (Nx6.25) and 10.1%
ash, and it was used as a replacement of 5% and 10% wheat
flour, in preparing two types of "potato bread". No
studies on breads with potato protein obtained by air

classification were reported in the literature reviewed.

8. Bread analysis data

 

In Table 21, the physical characteristics of the
control and PPF substituted bread are presented. The con-
trol had the best volume as expected. The loaf volume of
the bread with 5% PPF was 31.5% lower and that with 10% PPF
was 51.3% lower than the control. In similar experiments
Knorr (1977) found that the loaf volume was influenced by
the potato protein obtained by heat coagulation. Also,

Jain and Sherman (1974) found that addition of potato

flakes to bread decreased dough development time, dough
stability, and dough strength. Poor volume is also found
with excessive salts (Paul and Palmer, 1972). The PPF has

a high ash content (10.1%) that could affect the volume.

Kim and D'Appolonia, 1977, stated that the ratio of starch
to protein is important in the dough because the effect of
the protein in reducing the staling rate was due to dilution
of the starch by the protein. The pH of the bread with PPF,

was lower (5.53) as compared to the control (5.85). This

91

Table 21. Physical characteristics of breads containing
potato protein flour.

 

 

 

Ph sical Variables
tezt 5% Potato 10% Potato
Control Protein Flour Protein Flour

Volume, cc 825 i 13.23 565 t 4.5 402 i 5.0

Tenderness1

(lb/9) 9.6 i 1.2 11.3 t 0.5 12.1 t 0.7

c61or2 L 75.6 t 1.2 67.6 t 0.7 61.5 i 2.0
aL -o.5 : 0.2 0.7 i 0.2 1.7 t 0.5
bL 14.9 t 0.2 16.8 i 0.5 16.5 i 0.8

pH 5.83 t 0.03 5.53 t 0.03 5.53 i 0.03

moisture. % 30.5 t 0.5 35.6 i 0.8 38.4 i 1.2

 

1Allo Kramer shear press (to measure textural characteris-
tics of foods).

2Hunter colorimeter. The L values on this table indicate
lightness. The aL values (negative readings) indicate
greenness. The aL values (positive readings) indicate
redness. The bL values (positive readings) indicate

yellowness.

3Mean of 3 replicates : standard deviation.

92

could be due to the long fermentation time of the dough in
the bread with potato. The percent of water retained in
the bread increased with PPF. The values were 30.5 i 0.5%
for the control versus 35.6 i 0.8% and 38.4 i 1.2% for the
bread with 5 and 10% PPF, respectively. One explanation
could be the presence of potato starch in the potato pro-

t al. (1975) stated that potato starch

tein. Van Den Berg
has the highest water binding capacity of any starch because
it has the lowest degree of association between the starch
molecules. In the tenderness test, the control bread had
the lowest value 9.6 t 1.2 lb/g compared to 11.3 i 0.5 and
12.1 i 0.7 for 5 and 10% PPF bread. This indicates that
bread with PPF was firmer in relation to the control,
because the potato protein was less extensible than gluten.
The control had lighter color in the Hunter L range in
.relation to the bread with PPF (Table 21). In the aL range
the bread with PPF showed that redness increased as the PPF
increased. In the bL range the bread with PPF had higher
yellowness in relation to the control. The sensory charac-

teristics of breads containing PPF are presented in

Table 22. These data were analyzed using f-test for
orthogonal contrasts to compare the control bread versus
5%. 10% Per bread. The panelists found a small difference
in crust color between control and 5% PPF bread and a much
larger difference between control and 10% PPF bread. The
difference was not statistically significant (P<0.05).

93

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94

For texture, bread with potato protein received lower
scores in relation to the control. The results showed
significant difference (P<0.05) between the control bread
and 5% and 10% PPF bread. Grain was also rated lower for
bread with PPF. The difference was not statistically dif-
ferent (P<0.05).

For taste, the bread with 5% PPF was closer to the
control than the bread with 10% PPF that had the lowest
score. The results showed significant difference (P<0.05)
between the control bread and 5% and 10% PPF bread.

In general, the 5% PPF substitution in bread showed
physical and sensory characteristics closer to the control
than the 10% PPF substitution (Table 21, Figure 6). How-
ever, the 1oaf volume was markedly decreased with 5 and 10%
PPF, 31.5% and 51.3%, respectively.

The potato protein flour or the potato flour itself
could be promising substitutes in breadmaking in less

develOped areas where there is an abundance of potatoes.

95

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SUMMARY
POTATO PROTEIN: NUTRITIONAL EVALUATION AND UTILIZATION

The purpose of this research was to a) study the nutri-
tional quality of potato protein and b) prepare a high pro-

tein potato product and use it in breadmaking.

Nutritional Quality of Potato Protein

A. When the N distribution in the raw tuber was stu-
died, it was found that the N content of potatoes is highest
in the skin. The Protein Efficiency Ratio (PER) of the skin,
however, is practically zero, according to previous work
(Meister and Thompson, 1976a). The pith region of the tuber
presented the next highest N content and the outer layer the
lowest N content. It was also observed that the area near
the apical end of the potato had a slightly higher N content
than the stem end. The results showed higher amounts of
aspartic and glutamic acids in the inner layer compared to
the outer layer after boiling. 0f the essential amino
acids, valine, leucine, lysine and methionine, tended to be
higher in the outer layer compared to the inner layer.

B. The free amino acids and amides extracted with
distilled water showed variations in the levels among the
raw cv potato varieties analyzed. In general, the most
abundant free amino acids were aspartic and glutamic, and

the amides asparagine and glutamine.

96

97

The content of the free essential amino acids threonine,
valine, methionine, isoleucine, leucine, phenylalanine, and
lysine, in g residue/100 g of free amino acids plus amides,
was: 16.0% in cv Atlantic, 7.5% in cv Denali, 23.3% in cv
Russet Burbank, 20.0% in cv Superior and 21.1% in cv Monona.
Free cystine was present as traces in Atlantic and Superior
potatoes and was not detected in the other varieties. The
range in content of free methionine was 0.7 to 2.2 g residue
(0.8 to 2.5 9), indicating that selection of potatoes high
in methionine could be possible.

C. The whole tuber boiled with intact skin suffered
the lowest protein loss, 0.8 g/100 g of original protein
content, in comparison to tubers boiled after cutting into
halves and quarters either peeled or unpeeled. The highest
protein loss, 10.4%, was observed when whole potatoes were
peeled, cut into halves and boiled. After boiling the cv
Russet Burbank and Atlantic potatoes with intact skins
either cut or uncut, the predominant amino acids in the
boiled water were aspartic and glutamic. It was found that
the whole tuber boiled with intact skin suffered a minimum
loss of cystine and methionine; the largest loss of cystine
and methionine was observed in the halved and quartered
potatoes. The average values for the halved potatoes were
0.5 g cystine and 2.4 g methionine per 100 g of each of
these amino acids present in the raw potatoes. Russet
Burbank potatoes showed a decrease of 10.6% in the availa-

bility of the lysine after boiling.

98

D. The protein quality of the potato as determined by
the Protein Efficiency Ratio (PER) showed that the difference
in the nutritional value of the unsupplemented or supplemented
potato protein was reflected in their varying PERs. Russet
Burbank potatoes chemically and organically fertilized pre-
sented close PER values 1.62 and 1.54, respectively, statis-
tically not different. Also, the limiting amino acids,
cystine and methionine were present in similar concentrations
in the potatoes organically or chemically fertilized.

Dried whole eggs, and whey powder were used for supple-
menting the Russet Burbank potatoes. Commercially dried
potato flour prepared from the cv Russet Burbank and mixed
with dried whole eggs in the ratio 65:35 potato protein to
egg protein, had a PER which was 8% higher than the PER of
whole egg powder. This ratio of potato protein to egg
protein yielded a 54% increase in PER when compared to the
potato flour alone. When the same commercially dried
potato flour was mixed with dried whey in the ratio 90:10
potato protein to whey protein, the PER value of the mixture
was similar to that of casein (2.48 versus 2.50) but 13%

higher than the PER of the potato flour alone.

Preparation of a Potato Protein Flour and Bread Supplemented
With It
A potato flour fraction rich in protein (36.3%; Nx6.25)

was prepared from potato flour by air classification in a

99

Walter Laboratory Separator. Bread was made after substi-
tuting 5% and 10% of the wheat flour with this fraction
named potato protein flour (PPF). A 32 and 51% decrease in
loaf volume was observed as a result of this substitution,
respectively. There was also a decrease in tenderness
(AllO-Kramer Press) in the bread containing PPF. The
panelists found a small difference in crust color between
control and 5% PPF bread and a much larger difference between
control and 10% PPF bread. Both differences were not sta-
tistically significant. According to an untrained taste
panel the score for taste and texture of the substituted
bread were lower than those of the control, statistically

different.

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