CHEMICAL AHD PHYSICAL FRO PARTIES AFFECTING
THE COOKING OF POTATOES

By
MAHENDRA KUMAR 3HARMA

AN ABSTRACT
Submitted to the School for Advanced Graduate Studies of
Michigan State University of Agriculture and
Applied Science in partial fulfillment of
the requirements for the degree of

DOCTOR OF PHILOSOPHY

Department of Farm Crops
Year 1958

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MAHENDRA KUMAR SKARMA

ABSTRACT

Katahdin potatoes of three specific gravity groups 1.065 - 1.070,
1.074 - 1.076, and 1.060 - 1.065 were stored at 40° and 70° F. to deter­
mine the effect of specific gravity and storage temperatures on the
hardness of the tubers after cooking as measured by mechanical pressure
tester.

Fourteen varieties in 1956 and 15 varieties in 1957 were used

to examine the specific gravity and cooking characteristics of different
parts of tuber before storage, and analysis of pectic compounds and hemicelluloses of hard-cooking Cnaway tubers was correlated with hardness
both before and after storage.
(1) Initially, the internal region (pith) had more resistance to
pressure after cooking than surface region (cortex).
increased both surface and internal areas cooked soft.

As storage period
Changes In

pressure resistance after cooking were greater in the internal area
than in the surface area.

The change from hard to soft was accelerated

by increasing temperature.
(2) Within single tubers of any variety the specific gravity of
the pith was characteristically lower than that of the cortex and
outer medulla, wtiile cortex and outer medulla differed only slightly
in specific gravity.
specific gravity.

These differences were greatest in tubers of high

Such high specific gravity potatoes also exhibited

marked differences in resistance to pressure between surface and internal

- I -

MAHENDRA KUMAR SHARMA

ABSTRACT

areas at eight weeks of storage at 70° F.
exhibited less pronounced differences.

Low specific gravity potatoes

Differences in specific gravity

between variety were not related to the resistance to pressure after
cooking of each variety.
(3)

Excessive deposit of calcium pectate, protopectin and herni-

celluloses were the cause of hardness in the internal area of cooked
tubers.

Upon reduction in the amount of these substances after five to

six weeks of storage at 70° F., these tubers cooked soft.

With the fall

of pectic substances and hemicelluloses to levels which no longer caused
hard cooking, the penetration resistance of tuber tissue was dependent
on starch content.

- II -

CHEMICAL AND PHYSICAL PROPERTIES AFFECTING
THE COOKING OF POTATOES

By
MAHENDRA KUMAR SHARMA

A THESIS
Submitted to the School for Advanced Graduate Studies of
Michigan State University of Agriculture and
Applied Science in partial fulfillment of
the requirements for the degree of

DOCTOR OF PHILOSOPHY

Department of Farm Crops
Year 1956

ACKNOWLEDGMENTS

The author expresses his heartful thanks and deep sense of grati­
tude to Dr. S. T. Dexter, Professor of Farm Crops and Dr. D. R. Isleib,
Assistant Professor of Farm Crops under whose inspiration, able guid­
ance and unfailing interest this investigation was undertaken.
The writer deeply appreciates the financial support of a Graduate
Assistantship provided by the Agricultural Experiment Station, Michigan
State University, which enabled him to undertake this investigation.

TABLE OF CONTENTS
Page
■INTRODUCTION . . .

......... . .

1

REVIEW OF LITERATURE.............

3

EXPERIMENTAL PROCEDURE

..................

15

EXPERIMENTAL RESULTS ...............................

24

SUMMARY AND DISCUSSION...............................56
C O N C L U S I O N S .............
LITERATURE C I T E D

^..........
. . .

.

62
64

INTRODUCTION

Culinary quality of potatoes has been related to the appearance,
texture and taste of cooked tubers.

For highest quality it is gener­

ally agreed that boiled or baked potatoes should be creamy white in
color, have a moderately dry to dry and mealy texture, and a good
natural potato flavor.

An additional quality characteristic is the

time required to cook potatoes to "done-ness" by boiling.

Frequent

complaints are made where tubers do not cook uniformly and quickly, and
it was the purpose of this study to examine the reasons for this lack
of uniformity in cooking.
This difficulty is not invariably associated with any one variety,
but occurs at times in at least several of our standard varieties.

In

1955 1 "the variety Sebago furnished our best experimental material.

In

1956, Katahdin exhibited this characteristic, while in 1957 an early
variety, Onaway, harvested when tops were dry, was found to contain about
80% of tubers that cooked very slowly.

Fifteen other standard varieties,

both early and late, were grown in the same trial and no other variety
was found which cooked particularly hard.

This year 10 to 80 percent of

tubers in samples of Onaway from several locations were found to be
unacceptable.
hard.

A few years ago the variety Menominee was found to cook

In 1957 it cooked without difficulty, which indicated that one

variety which may cook hard one year may cook differently another year.
No theories were found in the literature to explain the occurrence of
this phenomenon.

- 1 -

- 2 -

Practically no work has been reported on the physical and chemical
properties which influence the time required to cook potatoes.
cooking of potatoes two obvious changes take place.

During

Starch is gelatin­

ized, and owing to a marked decrease in cell adhesion, the tissue is
softened and cells separate so that the tuber can be readily mashed.
Upon completion of these changes the tuber tissue is judged to be cooked.
The role of pectic substances in the architecture of plant cell,
walls has been well demonstrated.

It has been shown that pectic sub­

stances are present in the primary cell wall.

They are also found as

a cementing material in the middle lamella between adjoining cells.

It

is assumed that they are made up principally of calcium salts of pectic
acid and pectinic acid, insoluble "glues" which bind the cells together.
The cellulose is so intermingled with pectic substances, heniicelluloses
and other materials in the walls that when these substances are removed
or modified, the walls are weakened and become fragmented and lamellate.
A study of changes in cooking properties with increasing storage
was conducted.

Differences in response to cooking of various tuber

tissue areas were investigated, as were physical differences between the
areas.

Finally, the association of various pectic constituents with

cooking characteristics, and the relationship of changes of pectic
materials to changes in cooking properties were studied.

REVIEW OF LITERATURE

There are many aspects of potato quality which have been studied
in the past.

Among those receiving most attention have been the problems

of post-cooking darkening, the development of color in potato chips, the
mechanism of cooking, and the relationship between specific gravity and
mealiness of cooked tubers.

Less work has been reported on the role of

pectic compounds in regard to cooking.
The work on darkening was reviewed recently by Smith and Muneta (1954)
whose theory related the presence of iron and o-diphenols to the occurence
of dark pigments in tubers after boiling.

The chemistry of chip color

and the occurrence of the reactants in raw tubers was discussed by
Shallenberger and Smith (1955) who postulated that products of condensa­
tion between amino acids and reducing sugars were responsible for the
brown pigment of fried potato slices.

These considerations constitute

major areas of investigation, each with voluminous literature.

The

literature pertaining to the phenomena of texture and hardness in cooking
is reported in the following review.
The mechanism of cooking.

Knowledge of the changes which occur during

cooking has been incomplete and results as interpreted have often been
contradictory.

There has been a general misconception of the physical

changes developed within a potato tuber during cooking.

Atwater (1895)

thought that on boiling the starch swelled and the cells burst open.
Gilmore (1905) and East (1908) believed that mealy potatoes were those
- 3 -

- 4 -

containing sufficient starch to rupture the cell walls during boiling.
In 1917 Langworthy attributed the bursting of the cell walls to internal
steam pressure.

These investigators overlooked the possibility that

disintegration of the cells m y have been due to the mechanical operation
of mashing rather than to cooking itself.

Day (1909) on examination of

cooked potatoes found no breakage of the cell walls.

She postulated that:

"The middle lamella which holds the cells together had dissolved and the
cells separated from each other".
was associated with mealiness.

She did not find that bursting of cells

Sweetman (1933) agreed with Day, and

conjectured that the cooking process of potatoes was characterized by
such physico-chemical changes as partial gelatinization of the starch,
the solution of some of the pectic substances, the increased digesti­
bility of cellulose, the coagulation of most of the protein and more or
less caramelization of the sugars.
Eeeve (1954) claimed that histologically, tissue softening, starch
gelation and swelling, cell separation and cell wall distention and
rupturing were interdependent changes caused by cooking potatoes.

This

was essentially in agreement with the conclusion of Personius and Sharp
(1938, 1939a, 1939b) that neither failure of cell cohesion nor starch
gelation, alone or in combination, was a casual factor for cooked texture
in potatoes, but rather both were mechanisms of structural changes.
Reeve's histological observations on heated slices of potatoes showed
that cell separation, caused by weakening of the adhesive properties of
the middle lamella and aided by swelling of the gelled starch within the
cells, was not directly responsible for the difference between mealy and
non-mealy cooked potatoes.

Roberts and Proctor (1955) discovered that

in aqueous medium, the middle lamellae of the parenchymatous starch-

- 5 -

containing cells of the potato tuber were altered at 55°-60° C. and
pectic substances in the walls were altered at 70° C.

The cellulose of

the walls was not destroyed at this temperature but was so weakened that
the walls were fractured and no longer retained the starch grains which
had gelatinized and were released as a sticky mass.
A general relationship between extent of sloughing from boiled tubers
and specific gravity of raw tubers was observed by Whittenberger and
Nutting (1950).

Considerable variation both in extent of sloughing and

in specific gravity occurred in different regions of the same tuber, in
different tubers of the same lot, and different lots of the same variety.
These investigators thought that the principal cause of sloughing was
associated with the swelling of starch gelatinization, which developed
pressure within cells sufficient to fracture mechanically the inter­
cellular cement.
It is now generally accepted that cooking of potatoes does not cause
bursting of cell walls but permits ready mechanical separation of cells.
The starch grains swell (55°-65° C.) during cooking to fill the cell
more or less completely and the cell separates more or less readily due
to the dissolution of pectic substances at higher temperatures.

Rup­

tured cell walls are not necessarily characteristic of cooked potatoes.
Rupturing of cells, as distinct from separation of cells, is also caused
by swelling of gelled starch*
Specific gravity as related to mealiness in potatoes.

There are many

factors such as variety, date of planting and of harvest, growing condi­
tions (soil moisture, soil and air temperature) soil type, fertility and
cultural practices which affect mealiness in potatoes.

Numerous

- 6 -

investigators have shown that the relationship between specific gravity of
potatoes and their quality as judged by mealiness is very close.

Coudon

and Bussard in 1897 discovered that tubers of good cooking quality showed
a higher dry matter content than those of inferior quality.

The associa­

tion of high specific gravity (or high starch content) with mealiness of
tubers has been noted by Cobb (1935) > Sweetman (1936), Wright et al.
(1936), Bewe11 (1937), Haddock and Blood (1939), Clark et. al. (1940),
Smith and Nash (1940), Prince et. al.. (1940), and Bonde and Covell (1955)
and others.
Comprehensive studies by Kirkpatrick (1953) showed that dry matter,
starch and alcohol insoluble solids content were also highly correlated
with dryness and mealiness in boiled, mashed and baked potatoes.
Bettelheim and Sterling (1955a) pointed out that specific gravity and
starch content of raw tubers were significantly correlated with texture
scores and a high degree of correlation was found between the starch
content of cooked potatoes and organoleptic tests.

Heinze et. al. (1955)

noted considerable variation in cooking quality between tubers of a
single variety grown at different locations.

In many varieties, dif-

f
ferences between scores of samples grown at different localities were
as great as differences among varieties at a single location.

Chemical

composition varied as much with locality as with variety.
These investigations indicate that at present, measurement of
specific gravity is the best means of predicting the mealiness of cooked
potatoes.

However, it can not be concluded that specific gravity may be

used safely as the sole criterion in assuring mealiness.

Sweetman (1936)

pointed out some exceptions to a positive correlation between specific

- 7 -

gravity and mealiness.

Kirkpatrick et al. (1951) working with four early

varieties of potatoes, ranging in specific gravity from 1.06 to 1.08
r

found no significant correlation between specific gravity and mealiness
in boiled, mashed and baked potatoes.

Haddock and Blood (1939) indicated

that potatoes of identical .specific gravities grown at different locations
differed in mealiness.

Greenwood et al. (1952) showed that tubers with

similar specific gravity may not always exhibit the same degree of mealiness.
They demonstrated that association between specific gravity and mealiness
varied depending on the location where the potatoes were grown.

Whitten-

berger (1951) demonstrated that specific gravity did not indicate correctly
the starch content nor by inference the mealiness of potatoes which had
been stored either at very high or very low temperature.

Nylund and

Poivan (1953), working with Irish Cobbler, Waseca and Pontiac potatoes,
concluded that two varieties even though identical in specific gravity may
be quite different in components of quality.

Similarly, tubers of a single

variety having the same specific gravity but produced under different
environmental conditions may be quite different in cooking quality as
indicated by the difference between tubers from different dates of plant­
ing in this study.

Unrau and Nylund (1957) conducted organoleptic tests on

three varieties of potatoes and found that potatoes of different varieties
which were identical in specific gravity differed in mealiness depending
on the varieties being compared.
Although numerous investigators have studied the factors affecting
the cooking quality of potatoes, many contradictory conclusions have
been drawn and still persist.

Some workers are of the opinion that there

is a definite positive correlation between starch content and mealiness,
whereas soias have found evidence to the contrary.

Shewfelt et al. (1955)

- 8 -

examined five varieties of potatoes and found that decreasing mealiness
of cooked potatoes was associated with decreasing size of raw starch
granules, increasing thickness of cell walls after cooking, increased
fragmentation of cells after swelling, and increased gelatinization of
starch.

Tubers of the same variety grown at different places varied in

mealiness and in thickness of cell walls after cooking.

This led to the

belief that starch, or dry matter, alone is not the only factor influ­
encing texture.

Variation in cooking of different tuber areas.

The several zones within

a potato tuber have been reported to cook differently.

Sweetman and

Lancaster (1931) indicated that the texture (firmness) of cooked potatoes
varied between different points on the same slice but that significant
differences between slices were denoted by averages of ten penetrometer
readings.

Bud ends cooked to a much softer stage than stem ends as

indicated by depth of penetration.

Barmore (1937) on plotting penetra­

tion after cooking against starch content of the whole tuber, found
marked inverse relationship between ease in penetration and starch content.
Thus in tubers of high starch content penetration was low and softening
on cooking less pronounced than in tubers of lower starch content.

Since

such a definite relationship between texture (firmness) and starch and
between penetration and starch was found, a high correlation between
texture (mealiness) and penetration was expected, but was not found.
Thiessen (1935) working with potatoes grown upon dry and irrigated land
discovered that dry land tubers showed greater resistance to softening
at the cortex than in the medullary region, whereas these different
regions of tubers from irrigated fields cooked uniformly.

Cells of dry

- 9 -

land potatoes were much larger than those of irrigated tubers of the
same variety, particularly in the medullary area.

The fact that greatest

resistance to softening was found in the area of smallest cells and least
in the area of largest cells afforded a plausible explanation for the
rapid cooking of dry land as compared to irrigated potatoes of the
same variety.
Thiessen (1947) studied the penetrometer readings of cooked Bliss
Tr i u m p h and Cobbler potatoes after they had been stored three, six and
nine months.
area.

The penetrometer reaoings were taken in the outer medullary

The Bliss Triumph potatoes usually softened readily after cooking

late in February and March.

This change was more evident in the bud than

in the stem half of the tuber.

As the storage period continued until

June, old potatoes from both varieties could be penetrated more readily
than those from the same lot earlier in the year.

Thiessen (1947) also

found differences in penetration in a single tuber when two centimeter
thick sections had been cooked for various periods.

In most of the

tubers the cortex was penetrated less easily than the other areas, and
stem end also cooked harder than the bud end.

There were also some

tubers which reacted abnormally in each of the lots tested.

In some, the

pith or inner medullary area resisted, penetration to a greater degree and
for a longer period than the remainder of the tuber.

As cooking progress­

ed, however, easier penetration was usually obtained in the pith than in
the cortex area.

Although the pith or inner medullary area was usually

penetrated more easily than the cortex, it frequently was rawer to the
taste with similar cooking.
Hilton and Evans (1950) could not find any correlation between speci­
fic gravity of raw potatoes and either the force required to penetrate

- 10 -

the raw tubers by a fruit maturity gauge or electrical resistence of
tuber tissue.

The penetration force as determined by their experiment

had no close relationship with tuber density.

They did not state at

what time during the storage season their observations were made.
In general, there seems to be an inverse relationship between starch
content and softness after complete cooking.

The portions of a single

tuber having much starch are harder after cooking than those having less
starch.

Variation in composition of different tuber areas.

If a potato is cut

in two, several zones can be distinguished, each slightly different in
composition.

Coudon and Bussard (1897) reported that percentage of

water increased, percentage of starch decreased and the percentage of
total nitrogen increased from cortex to core (inner medullary area).
They found that the cortex was richer in protein nitrogen than the core.
East (1908) also separated potatoes into three parts.
firmed these findings.

His results con­

Frisbie and Bryant (1897) separated the cortex

from the medullary area and analyzed the two parts separately.

Their

results directly opposed those of Coudon and Bussard, except that they
also found more protein nitrogen in the cortex than in the medullary area.
Johnson and Boyle (1919) noted that the percentage of dry matter in
tubers of a single variety varied from 21.07 to 27.18 and the starch
percentage varied from 14.65 to 20.16.

Glynne and Jackson (1919) reported

that the percentage dry matter in tubers was lowest in the skin, increased
to the inner cortical layer and decreased towards the centre of the tuber.
Goldthwaite (1925) determined the chemical composition of cortex and
medullary area of individual tubers and found that the percentages of dry

- 11 -

matter, starch, total carbohydrate and ash were greater in the cortex than
in the corresponding medullary area.

No two potatoes of identical compo­

sition were found in the same variety, in the same group or in the same hill.
Thiessen (19V$ showed distinct differences in composition of dif­
ferent areas of the same potato tuber.

The stem end of the tuber con­

tained more starch, less water and less crude fiber than the bud half.
The cortex was higher in starch and lower in water than the other zones
of the tuber.

The outer medullary area had a slightly lower percentage

of starch and higher percentage of water than the cortex.

The inner

I

medullary area had a higher proportion of crude fiber, pectin, and nitrogen
than the rest of the tuber.

She also found that no two tubers taken from

the same lot had the same identical chemical composition.

Whittenberger

and Nutting (1950) noticed that generally, tissues of highest specific
gravity occurred on both sides of the vascular ring, and tissues of lowest
specific gravity were in the central pith or medulla.

The starch content

ranged from 10 percent in the central pith to about 25 percent in peri­
pheral zone.
There is general agreement that the distribution of dry matter
increases from the periphery inwards to the inner cortical tissue, thence
decreasing to the centre.

Thus much of the dry matter is contained in

the storage parenchyma associated with the phloem, less in the cortex
and still less in the pith.
Pectic constituents as related to potato texture.

The relation of pectic

materials to texture in plant tissue has received both theoretical and
practical investigation.

Some investigators have looked for a relation­

ship between pectins and potato tissue texture.

- 12 -

Coudon and Bussard (1697)> Sweetman (1936) and Barmore (1937) found
no significant relation between pectic content and mealiness of tubers.
Freeman and Ritchie (1940) offered additional evidence that the solution
and disintegration of pectic material did not determine mealiness in
potatoes.

Bettelheim and Sterling (1955b) observed no direct relationship

between various pectic fractions and texture of different varieties of
potatoes.

Recently Potter and McComb (1957) determined total pectin in

individual potatoes of different specific gravities from a single variety
and in potatoes of different varieties from different lots.

Results

showed very little difference in the pectin content of different varie­
ties,

Pectin content was independent of the specific gravity of the

tubers and varied with different growing locations and cultural condi­
tions.

There was no correlation between subjective appraisal for texture

and the pectin content of the potatoes.

These results appear to discount

the hypothesis that low pectic content is casually related to mealiness.
Personius and Sharp (1939a) explained the decrease in cell adhesion
after cooking by weakening of intercellular cementing material of
potato tissue.

Pyke and Johnson (1940) demonstrated that soluble pectin

readily dissolves in cold water and protopectin becomes rapidly soluble
in water at 65° C. (165° F.).

The calcium pectate content of potato

decreased continuously during boiling in pure water or more rapidly in
salt water indicating that these cementing materials were rapidly dis­
solved, permitting segments of potato tissue to break away.
In an attempt to explain the shrinkage which occurred in storage,
pectic changes in seed potatoes were examined by Dastur and Agnihotri
(1934).

Tubers were stored according to size and size was assumed to

- 13 -

represent advancing age of the tubers.

The results indicated a progres­

sive increase in the soluble pectic fraction while the total pectic sub­
stance also increased indicating continuous formation of pectic substances
during growth.

During storage the free soluble pectin, which previously

was present in small amount, was formed at the expense of protopectin
and middle lamella pectin.
There have been many attempts to relate potato texture with pectic
content.

On the whole, no obvious relationship has been found.

The

observed changes in texture can not be adequately explained in terms of
the changes in pectin alone.

Changes of pectic substances in fruits during ripening and storage.

Sev­

eral investigators have found softening of fruit to be associated with
changes in pectic substances in cell walls and middle lamella.
In the literature dealing with pectic materials in fruits and
vegetables, the following pectic substances have been recognized.

Pectin,

which is water soluble and probably is derived from protopectin by
enzyme action; protopectin or pectose, which is water insoluble; and the
pectic substance of middle lamella which is also water insoluble.

The

total pectic substance includes all three of these.
The development, maturation and senescence of fruit involves striking
changes in the amount and nature of pectic materials contained in the
fruit cell walls, a fact first appreciated by Fremy in 1640.

He observed

that the softening of the flesh of apple was at times closely associated
with changes in pectic constituents.

Carre (1922, 1925) made extensive

studies of pectic materials of apples during storage.

She associated

ripening in storage with an increase in pectin and corresponding decrease

- 14 -

in protopectin (pectose).

The protopectin also decreased.

were accelerated by increasing temperature.

These changes

The protopectin decreased

in the apples during storage, first rapidly and later slowly.

Similarly

good correlation between changes in firmness and increase in soluble
pectin substances in apple was found by Haller (1929, 1941), Smock and
Allen (193S), Fisher (1943), Phillips and Poapst (1950) and others.
In studies of pectic changes in peaches, Appleman and Conard (1926),
Conard (1930) and Fisher, Britton and O'Reilly (1943) found decrease in
the protopectin and a corresponding increase in pectin with ripening in
storage.
ture.

The rate of these changes increased with increasing tempera­

Working with pears, Emmett (1926, 1929), Gerhard and Ezell (1938),

Smock and Allen (1938) and Hansen (1939) observed similar changes in
pectic constituents.

The transformation of insoluble pectic substances

into soluble forms during storage and ripening was observed in banana by
Conard (1930).

Barnell (1943), and Von Loesecke (1950); in plums by

Bondet (1952); in citrus by Gaddum (1934) and in grapes by Hopkins and
Gourley (1930).
These investigations suggest that the cell walls are thickened and
cemented together by insoluble pectic materials.

As these insoluble

pectic fractions change to a soluble form softening takes place, thus
permitting the cells to separate readily and decreasing the thickness
of the cell walls.
temperature.

These changes are accelerated by an increase in

EXPERIMENTAL PROCEDURE
Specific Gravity Separation, Storage, Cooking
and Pressure Testing of Katahdin Potatoes
For the study of the effect of specific gravity and two storage
temperatures, 40° and 70° F., on the cooking time of potatoes, Katahdin
tubers were taken from cold storage in December and separated into three
specific gravity groups (l) 1.065 - 1.070, (2) 1.074 - 1.076, and (3)
1.080 - 1.085 by immersing in brine solutions of known specific gravity.
About 250 medium sized tubers in each group were obtained in this way.
They were washed with water and half of the tubers of each specific
gravity group were stored in cold storage at 40°. The other halves were
stored at 70° F. under conditions simulating kitchen storage.

At weekly

intervals for eight weeks three potatoes of each specific gravity group
and from each storage were cooked in boiling water for 15, 20, 25 and 30
minutes.

After cooking they were peeled and tested for done-ness by the

pressure resistance tester.
One time-tested method of determining whether or not potatoes are
thoroughly cooked is to pierce them with some pointed instrument to gauge
the softness or hardness of the tuber tissue.

From this grew the use of

a penetrometer to test the degree of done-ness of potatoes, introduced and
used by Landreth (1929) followed by Sweetman and Lancaster (1931),
Thiessen (1935) ar*d Barmore (1937)-

Since in this experiment it was

desired to measure the characteristics of the tuber as a unit, and since
several investigations have found differences in the rates of cooking of

- 15 -

- 16 -

the different parts within a tuber, it was decided to make penetration
measurements on the surface (cortex) and internal area (inner medulla) of
each tuber.
face

Five pressure resistance measurements were made on both sur­

and internal areas. The tester recorded the pressure

to force

a plunger 0.1596 cm*

tissue to a distance of 1.2 cm.

in diameter (0.02 cm. )
(Figure 1.)

in grams necessary
intothe potato

All pressure readings in

this study refer to resistance of tuber tissue after cooking.

Specific Gravity Determination of
Three Tuber Zones and Gooking Test
In the 1956 study of the differences in cooking characteristics
between tuber areas, the specific gravity of ten individual tubers of
14 varieties was calculated from the weight of the tubers in air and in
water.

The specific gravity was calculated by the following formula:
c

weight
in air_
peci i c gravi y - weight in air — weight in water

The

specific gravity of three different zones of each tuber was

also determined.

An outer layer of each tuber about one centimeter

thick from skin to vascular ring (cortex) was removed and the weight
of the remaining tuber in air and in water was determined; again a layer
was removed up to the inner medullary area and the remaining portion
(pith) of the tuber weighed in air and in water.

All the weighing was

done on a triple beam balance, with accuracy of 0.01 gram.
In 1957, 25 varieties were planted to provide material for examina­
tion of the specific gravity of entire tubers, and of the different
zones; and the differences in cooking both within and between varieties.
Varieties of different characteristics, early and late maturing varieties,

- 17 -

r-5 0 0

i-30

1-050
1-200
|-I5 0

Hoc

— aff>°
c
!

Figure 1*

r°

Penetrometer used to test the resistance to pressure of the
tuber tissue after cooking.

- 18 -

varieties high and low in specific gravity, and of varied cooking char­
acteristics were included.
As the tops of a variety dried it was harvested and the specific
gravity of ten large whole tubers and of the three different zones of
each tuber was determined in the same way as described previously.

Fif­

teen medium sized tubers of each variety were cooked in boiling water
for 25 minutes and five pressure tests were made on the surface area
and five on the internal area of each tuber after cutting the tubers in
half.

Fifteen varieties were chosen from the 25 as representative of

the greatest extremes in earliness, specific gravity and cooking char­
acteristics.

Among these 15 varieties only one variety, Onaway, was

found to be a very hard cooker.

The inside portion of about 80% of

all Onaway tubers did not cook to done-ness when boiled in water for
25 minutes.

To study the cause of this hardness, cellulose, hemicellu-

loses, and different pectic fractions of the hard and soft cookers
we re analyze d .
Sample Preparation for the Determination of Cellulose,
HemiceHuloses and Pectic Fractions
A large number of Onaway tubers were cut into halves and numbered.
One half of each tuber was cooked in boiling water for 25 minutes and
tested with the pressure tester.

Twenty-five uncooked halves of soft

cooking tubers and 25 halves of hard cookers (indicated by pressure resis2
tance of about 300 grams per 0.02 cm. after cooking) were used for
chemical analysis.
indicated done-ness.

Ordinarily a pressure reading of about 80 or below
The centermost portion of each raw tuber was

removed, cut into very small pieces and the pieces composited.
samples weighing 20 grams each were drawn from the composite.

Twelve
Twelve

- 19 -

similar samples from soft cookers were prepared.

Six samples from each

group were used for the analysis of cellulose and hemicelluloses and the
remaining six from each group were used for estimation of water soluble
pectin, ammonium oxalate soluble pectin (middle lamella pectin) and acid
soluble pectin (protopectin).
It had been observed that hard cookers would cook easily after
storage at room temperature for some time.

A number of large Onaway

tubers were cut into pieces and each piece was numbered.

One piece from

each tuber was cooked in boiling water for 25 minutes and was tested for
pressure resistance.

The remaining pieces from tubers which cooked

very hard were stored at 70° F, in cellophane bags for further cooking
tests.

Every week for six weeks 20 pieces were cooked for 25 minutes in

boiling water and tested for pressure resistance.

When

these pieces

cooked soft, samples of raw tuber tissue were analyzed for three pectic
fractions, cellulose and hemicelluloses.

The samples for analysis were

prepared the same way as described above.
Analytical Procedure
Determination of cellulose and hemicelluloses.

The work of Norris and

Preece (1930) was the basis on which the folowing method was developed.
Samples for determination of cellulose and hemicelluloses were blended
in a Waring blender with distilled water for one minute, transferred to
a 250 ml. beaker.
100 ml.

Water was added to make the final volume approximately

They were heated at about 80° C. for three minutes (with constant

stirring to prevent scorching the starch) to gelatinize the starch.

To

avoid erroneously high values due to presence of starch, the starch was
hydrolyzed by enzymatic hydrolysis with a solution of equal amounts of

- 20 -

alpha-and beta-amylase.
vitro.

The amylases attack raw starch very slowly in

Rapid digestion under these conditions occurs only after the starch

has been heated in the presence of water or the starch granule structure
has been destroyed by heating.
After gelatinization and cooling the starch solution to room temper­
ature, the pH was adjusted to 5*7 and enzyme solution composed of equal
amounts of alpha-and beta-amylase were added to bring the concentration
of the total enzyme to 0 ,05^ in the final solution (l ml. of lp alpha-and
beta-amylase mixture in water was added for 20 ml. of liquid) .

The sur­

face of the mixture was covered with toluene to prevent spilage and the
enzyme was permitted to act for 12 hours at 37.5

o

C.

After 12 hours, the

samples were again heated at 8Q°-85° C. for three minutes, cooled to room
temperature and fresh enzyme solution added as before.

After the second

12 hours of digestion an iodine test was made to verify completeness of
hydrolysis ^
After the starch was completely hydrolyzed, steps were taken to
remove sugars and water soluble pectins from the samples.
were centrifuged for 25 minutes (2400 x gravity).

The samples

The tubers were al­

lowed to stand for about five minutes after removal from the centrifuge
and the supernatent liquid was removed by pipette.

The residue was

washed with distilled water in an Erlenmeyer flask, shaken on a mechanical

Two ml. of the hydrolyzed solution was placed in a 3 ml. tube. Five
drops of 0.1 N iodine solution (1.26 grams of iodine and 2.5 grams of
potassium iodide in 100 ml. water) were added. Absence of blue color
indicated complete conversion of starch.
The starch test was also performed by putting a small amount of
hydrolyzed solution on a porcelain spot plate and adding two drops of
iodine-potassiura iodide solution. Absence of blue color indicated
complete conversion of starch.

- 21 -

shaker for two hours, and centrifuged as before.
was discarded.

The supernatant liquid

The residue was extracted once more by the same procedure

for the final removal of sugars and water soluble pectin.
To remove ammonium oxalate soluble pectin, the residue was again
washed and transferred to an Erlenmeyer flask with 0,5% ammonium oxalate
solution, and was extracted for two hours on the mechanical shaker.
was centrifuged and the supernatant liquid discarded.'

It

Again the residue

was extracted with 0,5% ammonium oxalate solution for two hours, centri­
fuged, and the supernatant liquid discarded.

The residue was then washed

in hot distilled water to remove all the ammonium oxalate.
fuged and wash water was discarded.

It was centri­

The residue now contained cellulose

and hemicellulose, which was dried to constant weight at 90° C. in the
centrifuge tubes which had been previously weighed.

Thus the combined

weight of cellulose and hemicelluloses was determined.
To remove hemicelluloses the dried residue (cellulose and hemicellu­
loses) was treated with hot k% sodium hydroxide, transferred to an
Erlenmeyer flask, and extracted on the mechanical shaker for two hours.
It was centrifuged and the supernatant liquid discarded.

The residue

was extracted and centrifuged three or four times in this manner, until
a portion of the extract no longer gave a precipitate when neutralized
with acetic acid and diluted with an equal volume of acetone.
The final residue was washed with hot distilled water to remove all
the sodium hydroxide.

It was centrifuged and wash water discarded.

residue was washed thrice in this way.

The residue now contained

cellulose, which was dried in the weighed tubes to constant weight at
90° C.

The percent of cellulose was calculated.

Dry weight of

The

- 22 -

hemicelluloses was found by difference, and percentage of hemicelluloses calculated.

Determination of pectic substances. Methods used to isolate the pectins
and separate them into water-soluble, 0,5% ammonium oxalate-soluble, and
hot 0.05 N-. HC1 soluble-fractions were essentially the same as those
I

described by McColloch (1952).
The samples of finely-cut tissue were boiled in 50 nil. 95% ethanol
for ten minutes to inactivate the pectic enzymes and extract the sugars.
They were blended for one minute and transferred into a beaker using 95%
ethanol to a final volume of about 125 ml.
stand overnight.

The samples were allowed to

After extraction with ethanol the residue was removed

from the extract by filtration.

In the first extract, the pulp was added

to twice its weight of distilled water, extracted on the mechanical shaker
for two hours, centrifuged, and the supernatant liquid collected.

The

residue was again extracted in the same way, and both the extracts were
combined and called "water-soluble fraction".

In the second extraction

the pulp was extracted twice as above with 0 .5^ ammonium oxalate and
both the extracts were combined and called "calcium pectate filtrate".
The last extraction was performed with 0.05 N. HC1 at 85° C. for two
hours.

Then the material was centrifuged and the supernatant liquid

collected.

It was again extracted and both extracts combined and desig­

nated as "HCl-scluble fraction".
In the process of filtrating small amounts of starch often contami­
nates the pectic fractions hence they must be freed from starch before
precipitation by ethanol.

The starch was hydrolyzed by alpha-and beta-

amylase as described for the determination of cellulose and hemicelluloses

- 23 -

except that the enzymes were permitted to act for 12 hours only once.

The

pectic substances in each fraction were precipitated by adding two volumes
of 95% ethanol and enough concentrated HC1 to make the solution 0,05 N.
in HG1.
paper.

The precipitated pectins were removed on weighed and dried filter
After the filtration the filter papers were dried to constant

weight and the percentage fresh weight of each pectic fraction was
calculated.

EXPERIMENTAL RESULTS

Effect of Storage at Two Temperatures, 40°and 70° F., on the Cooking
of Katahdin Potatoes of Three Specific Gravity Groups
This experiment was performed to assess the changes in cooking of
Katahdin potatoes of different specific gravities, stored at 40° and
70° F.

Three tubers from each of the three specific gravity groups

(1.065 - 1 .070, 1074 - 1.076 and 1.080 - 1.085) and both storages were
cooked for 15, 20, 25 and 30 minutes at weekly intervals and pressure
tested on surface and internal areas.

The data from the potatoes

cooked for 25 minutes is presented because the differences between
penetrometer readings were most obvious when the tubers were boiled
until not quite cooked to done-ness.

If the potatoes were completely

cooked, differential measurements could not be obtained.

A similar

technique was employed by Thiessen (1947)*
Tables I, II and III indicate the resistance to pressure of sur­
face and internal regions of these potatoes after cooking.

The data show

that as the weeks in storage at either temperature increased, the resis­
tance to pressure of surface and internal areas fell in each of the three
specific gravity groups.

The results from potatoes stored at 70° F. are

illustrated in Figures 2, 3, and 4*

It is evident from these figures

that resistance to pressure of internal area was greater than that of
surface area at 0 weeks of storage in all the specific gravity groups.

As

the storage period increased, resistance to pressure after cooking of both

- 25 -

Table I.

Penetration resistance in grams per 0.02 cm.2 of Katahdin
potato tubers (specific gravity 1.065 - 1.070) from 40°
and 70° F. storage, after cooking at weekly intervals.*

Weeks in
Storage

Stored at 40° F.
Surface Area
Internal Area

Stored at 70° F.
Internal Area
Surface Area

0

72.6

90.3

72.6

90.3

1

76.8

95.2

61.3

74.5

2

74.3

87.4

63.2

71.5

3

78.5

90.2

55.3

65.0

4

80.1

89.1

57.6

63.3

5

70.4

89.1

53.2

60.4

6

55-3

68.3

42.1

46.6

7

52.2

58.7

39.4

40.6

8

49.3

53.4

32.4

34.3

* Means of 15 penetrometer measurements, five from surface and
five from internal areas of each of three tubers.

- 26 -

Taole II.

Weeks in
Storage

Penetration resistance in grams per 0.02 cm. of Katahdin
potato tubers (specific gravity 1.074 - 1.076) from 40°
and 70° F. storage, after cooking at weekly intervals.*

Stored at 40° F.
Surface Area
Internal Area

Stored at 70° F.
Internal Area
Surface Area

0

S2 .S

106.4

82.8

106.4

1

85.2

108.7

74.4

83.2

2

88,2

111.3

66.8

79.4

3

85.4

106.1

58.4

78.7

4

85.4

110.1

60.4

74.8

5

78.4

95.4

61.3

66.3

6

64.2

79.6

55.0

58.7

7

60.3

70.8

45.2

43.1

a

58.2

67.3

38.7

21.2

* Means of 15 penetrometer measurements, five from surface and
five from internal areas of each of three tubers.

- 27 -

Table III,

Weeks in
Storage

Penetration resistance in grams per 0.02 cm. of Katahdin
potato tubers (specific gravity 1.080 - 1.085) from 40°
and 70° F, storage, after cooking at weekly intervals.*

Stored at 40° F.
Surface Area
Internal Area

Stored at 70° F.
Internal Area
Surface Area

0

83.2

106.9

83.2

106.9

1

80.4

110.0

64.8

87.4

2

89.1

109.1

68.6

81.4

3

81.6

110.4

57.0

82.8

4

77.6

112.0

62.8

77.4

5

72.6

98.3

59.3

55.8

6

52.1

70.0

52.6

39.5

7

$1.8

64.3

50.1

25.5

8

46.1

63.2

47.3

17.0

* Means of 15 penetrometer measurements, five from surface and
five from internal areas of each of three tubers.

- 28 -

110

Penetration

resistance

(grams per 0,02 cm.

100-

80-

605040

Internal
Area
Surface
Area

20-

10-

0

1

2

3

4

5

6

7

8

V/eeks in Storage at 70° F.
Figure 2.

Effect of pre-cooking storage at 70° F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers (specific gravity 1.065 - 1,070)
boiled 25 minutes.

- 29 -

1009080-

70-

60-

Penetration

resistance

(grams per 0.02 cm.

C \l

Surface
# Area
30Internal
Area

10-

Weeks in Storage at 70° F.
Figure 3*

Effect of pre-cooking storage at 70° F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers (specific gravity 1.074 - 1.076)
boiled 23 minutes.

- 30 -

110

Penetration

resistance

(grams per 0.02 cm

100

-

80 “

70-

60 50-

—#

Surface
Area

40 30-

20

Internal
Area

-

10-

Weeks in Storage at 70° F.
Figure 4.

Effect of pre-cooking storage at 70° F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers (specific gravity 1.080 - 1*085)
boiled 25 minutes.

- 31 -

surface arid internal areas went down.

In every case the change in

resistance to pressure of the internal area was more rapid than that
of the surface area.
After seven weeks of storage the resistance of surface and internal
area to pressure after cooking was almost the same in potatoes of specific
gravity 1.065 - 1.070 (Figure 2 .)

While the pressure resistance of sur­

face and internal area of tubers of 1.074 - I.O76 was approximately equal
at five, six and seven weeks of storage, at eight weeks of storage pres­
sure resistance of internal area was much lower than that of surface area
(Figure 3*)

Thus after eight weeks of storage the inside cooked softer

than the outside.

Resistance to pressure of internal area fell rapidly

in potatoes of specific gravity 1.080 - 1.085 after four weeks of stor­
age, and at five v^eeks of storage, both surface and internal area were
equally hard.

At six weeks the internal area cooked softer than the

surface area and at the end of eight weeks the penetration resistance of
internal area was much lower than that of surface area (Figure 4.)«
Table IV indicates the change in resistance to pressure of the
internal and surface areas (means of all tubers from the three specific
gravity groups) stored at 70° and 40° F.

These data show that resistance

of the surface and internal area to pressure decreased in potatoes stored
at 70° F.

Figure 5 illustrates the results from potatoes stored at 70° F.

and indicates that the resistance to pressure of the internal area was
more than that of surface area at 0 weeks but that it fell more rapidly
than the resistance of surface area.

After five and six weeks of storage

both surface and internal areas were equally hard, and after seven weeks
of storage, the internal area cooked softer than surface area.

- 32 -

Table IV.

Penetration resistance in grams per 0.02 cin.^ of Katahdin
potato tubers (means of three specific gravity groups) from
4CP and 7^P F. storage, after cooking at weekly intervals.*

V/eeks in
Storage

Stored at 40° F.
Internal Area
Surface Area

Stored at 70° F.
Internal Area
Surface Area

0

79.5

101.2

79.5

101.2

1

80.8

104.6

66.8

81,7

2

S3.9

102.6

66.2

77.4

3

81.8

102.2

56.9

75.5

4

81.0

103.7

60.3

71.8

5

73.8

94.3

57.9

60.8

6

57.2

72.6

49.9

48.3

7

54.8

64.6

44.9

36.4

8

51.2

61.3

39.5

24.2

* Means of 45 penetration measurements, five from surface and
five from internal area of nine tubers.

- 33 -

110

100
9080 " ^
70 -

resistance

60 50-

Penetration

(grains per 0.02 cm.

CM

30-

Surface

Internal
Area

20-

10 -

Weeks in Storage at 70
Figure 5.

F.

Effect of pre-cooking storage at 70 F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers boiled 25 minutes (means of three
specific gravity groups).

- 34 -

The changes in resistance to pressure of the internal and surface
areas of the potatoes stored at 40° F. in three specific gravity groups,
1.065 - 1.070, 1.074 - 1.076 and 1.080 - 1.085* are shown in Figures 6,
7 and 8 , respectively.

The tendencies in the changes are similar in all

the three groups, unlike the potatoes stored at 70° F.

The results of

all three specific gravity groups have been combined and are shown in
Table IV and in Figure 9.

The difference in resistance to pressure of

surface and internal areas of potatoes stored at 40° F. was great at
0 weeks of

storage (Figure 9-)*

There was practically no

resistance

of either area up to four weeks but after this period as the

storage period increased the resistance

to pressure

internal area it fell more rapidly than

in surface area.

change in

went down.

In the

At seven or

eight weeks of storage the differences were not very great, whereas in
potatoes stored at 70° F. the inside cooked softer after the same period
of storage (Figure 5*).
Penetration resistance of the surface and internal areas are shown
in Figures 10 and 11 respectively, comparing the potatoes stored at

40° and 70° F.

The change in resistance to pressure of the surface was

greater in potatoes stored at 70° F. than in those stored
(Figure 10).

The changes in resistance

were also greater in potatoes at 70° F.

at 40° F.

to pressureof internal area
than at 40° F. (Figure 11) and

were more marked in the internal area than in surface area.
Specific Gravity of Different Farts of the Tubers and
Cooking Tests of Different Varieties Before Storage
1956.
In 1956 the specific gravity of three different portions, cortex,
outer medulla and pith, of the tubers of 14 varieties was determined as

- 35 -

110-,

10090-

Penetration

resistance

(grams per 0.02 cm.

CNi

80\

70-

\

\

60-

\

50-

Surface
Area

4030
2010-

0-7

0
Weeks in Storage at 40

Figure 6 .

Internal
Area

F.

Effect of pre-cooking storage at 40° F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers (specific gravity 1.065 - 1.070)
boiled 25 minutes.

- 36 -

110

Penetration

resistance

(grams per 0.02 cm.

100

-

BO
70 -

60

Internal
Area

-

— — • Surface
Area

40 -

30 -

10

-

Weeks in Storage at 40° F
Figure 7.

Effect of pre-cooking storage at 40 F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers (specific gravity 1,074 - 1.076)
boiled 25 minutes.

- 37 -

110

100-

80-

70Internal
Area

resistance

6050-

Penetration

(grams per 0.02 cm.

90-

30-

Surface
Area

10-

Weeks in Storage at 40° F.
Figure 8 .

Effect of pre-cooking storage at 40° F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers (specific gravity 1.080 - 1.08$)
boiled 2$ minutes.

- 38 -

110

100 -

Penetration resistance

(grams per 0.02 cm.

90-

80s
70Internal
Area

60-

# Surface
Area

5040-

20 10

-

00

1

2

3

4

5

6

7

6

Weeks in Storage at 40° F.
Figure 9.

Effect of pre-cooking storage at 40° F. on post-cooking
penetration resistance of surface and internal areas of
Katahdin potato tubers boiled 25 minutes (means of three
specific gravity groups).

- 39 -

110

100 90 -

Penetration

resistance

(grams per 0.02 cm.

C\J

80-

70-

60-

50-

40-

2010-

Weeks in Storage at 40° and 70° F.
Figure 10.

Effect of pre-cooking storage at 40° and 70° F. on post­
cooking penetration resistance of surface area of Katahdin
potato tubers boiled 25 minutes (means of three specific
gravity groups).

- 40 -

100
90-

Penetration

resistance

(grams per 0.02 cm.

CM

80-

70-

50-

4030-

20 -

10

0

1

2

3

4

5

6

7

8

Weeks in Storage at 40° and 70° F.
Figure 11.

Effect of pre-cooking storage at 40° and 70° F. on post­
cooking penetration resistance of internal area of Katahdin
potato tubers boiled 25 minutes (means of the specific
gravity groups)

- 41 -

shown in Table V.

Table VT indicates the same data, calculated as per­

cent after deleting the constant 1.000 from each determination and taking
the entire tuber as 100.

Ten tubers of each variety were used.

In most

cases the outer medullary area had the highest specific gravity.

In gen­

eral the tubers of varieties high in specific gravity showed wide differ­
ences in specific gravity between cortex and pith.

The varieties whose

tubers were low in specific gravity showed little difference in specific
gravity between these two zones.

Means of the varieties revealed a great

difference between cortex and pith area.

There was almost no difference

between cortex and outer medullary area.
In December 15 tubers of each variety were cooked in boiling water
for 25 minutes and resistance to pressure of the surface and internal area
was noted.

These results are shown in Table VII.

From these results it

is evident that there was very little difference between varieties in
softening after cooking.

However, there were some differences in penetra­

tion between cortex and pith area within tubers of a single variety.

The

pith cooked slightly harder than the cortex although these differences
were not great.

m iTable VIII shows the specific gravity of three different portions
of the tubers of 15 potato varieties grown in 1957.

Each of these

figures represents the mean of ten tubers of each variety.

The same

data calculated as percentage after deleting the constant 1.000 from
each determination, and taking the entire tuber as 100, is shown in
Table IX.

These results are almost identical to those of 1957.

In most

cases the outer rredulla had highest specific gravity of the three tuber

- 42 -

Table V.

Specific gravity of whole and different portions of potato
tubers of 14 varieties grown during 1956 (means of ten tubers).

Whole
Tuber

Cortex

Outer
Medulla

Pith

Red LaSoda

1.0731

1.0727

1.0836

1.0623

2.

Waseca

1.0734

1.0723

1.0813

1.0616

3.

Onaway

1.0745

1.0713

1.0805

1.0628

4.

Chippewa

1.0795

1.0807

1.0771

1.0726

5*

Se bago

1.0807

1.0855

1.0847

1.0654

6.

Pontiac

1.0815

1.0814

1.0878

1.0675

7.

Katahdin

1.0826

1.0845

1.0910

1.0689

8.

Osage

1.0860

1.0894

1.0909

1.0697

9.

Russet Rural

1.0870

1,0862

1.0901

1.0674

10.

Tawa

1.0891

1.0946

1.0952

1.0678

11.

Delus

1.0948

1.1001

1.0949

1.0631

12.

Saco

1.0954

1.1014

1.0939

1.0677

13.

Green Mountain

1.0970

1.1032

1.0950

1.0751

14.

Merrimack

1.1029

1.1089

1.0982

1.0717

Mean

1.0855

1.0880

1.0889

1.0674

No.

Variety

1.

- 43 -

Table VI.

Specific gravity of different tuber portions from 14 varieties
grown in 1956, expressed as percentage of entire tuber speci­
fic gravity.*

Who le
Tuber

Cortex

Outer
Medulla

Pith

Red LaSoda

100

99.45

114.36

85.22

2.

Waseca

100

98.50

110.76

83.92

3.

Onaway

100

95.70

108.05

84.29

4•

Chippewa

100

101.50

96.98

91.32

5.

Sebago

100

105.94

104.95

81.04

6.

Pontiac

100

99.87

107.73

82.82

7.

Katahdin

100

102.30

110.16

83.41

8.

Osage

100

103.95

105.69

81.04

9.

Russet Rural

100

99.08

103.56

77.47

10.

Tawa

100

106.17

106.84

76.09

11.

Delus

100

105.59

110.10

66.56

12.

Saco

100

106.28

98.42

70.96

13.

Green Mountain

100

106.39

97.93

77.42

14.

Merrimack

100

105.83

95.43

69.67

Mean

100

102.61

104.35

79-37

No.

Variety

1.

* Percentage calculated after deleting the constant 1.000 from
each determination.

- 44 -

Table VII.

Resistance to pressure in grams per 0.02 c m . 2 of surface and
internal areas of cooked potato tubers from 14 varieties
grown in 1956 (means of 15 tubers).

No.

Variety

1.

Surface Area

Internal Area

Red LaSoda

77.6

94.6

2.

Waseca■

71.6

87.5

3.

Onaway

84-3

96.6

4•

Chippewa

64.5

78.9

5.

Sebago

82.3

100.2

6.

Pontiac

69.3

81.4

7.

Katahdin

78.6

97.4

8.

Osage

74.3

93.2

9.

Russet Rural

70.2

88.9

10.

Tawa

88.0

99.7

11.

Delus

68.5

86.4

12.

Saco

77.9

96.3

13.

Green Mountain

77.6

92.8

14.

Merrimack

78.8

96.3

Mean

75.9

91.4

- 45 -

Table VIII,

No,

1.
2.
3.

Specific gravity of whole and different portions of potato
tubers of 15 varieties grown during 1957 (means of ten tubers).

Variety

— -.Sebago
Pontiac
__Qnaway

Whole
Tuber

Cortex

Outer
Medulla

Pith

1.0611

1.0608

1.0663

1.0508

1.0615

1.0584

1.0688

1.0587

1.0627

1.0636

1.0614

1.0626

4•

Chippewa

1.0654

1.0675

1.0686

1.0634

5.

Ontario

1.0668

1.0710

1.0676

1.0503

6.

Tawa

1.0672

1.0632

1.0760

1.0597

7.

Osage

1.0712

1.0650

1.0842

1.0685

8,

■
---Katahdin

1.0723

1.0750

1.0752

1.0565

Irish Cobbler

1.0732

1.0705

1.0795

1.0678

10.

Cherokee

1.0758

1.0789

1.0756

1.0593

11.

Menominee

1.0767

1.0807

1.0782

1.0543

12.

Russet Rural

1.0775

1.0791

1.0831

1.0670

13.

Delus

1.0808

1.0869

1.0810

1.0593

14.

Merrimack

1.0822 .

1.0804

1.0891

1.0710

15.

Green Mountain

1.0930

1.0957

1.0916

1.0713

Mean

1.0725

1.0731

1.0764

1.0614

9.

- 46 -

Table IX.

No.

Specific gravity of different tuber portions from 15 varieties
grown in 1957* expressed as percentage of entire tuber speci­
fic gravity.*

Variety

Whole
Tuber

Cortex

Outer
Medulla

Pith

1.

Sebago

100

99.51

108.51

83.14

2.

Pontiac

100

94.96

111.87

95.77

3.

Onaway

100

101.43

97.92

99.82

4•

Chippewa

100

103.21

104.89

96.94

5.

Ontario

100

106.29

101.19

75.30

6.

Tawa

100

94.05

113.09

88.84

7.

Osage

100

91.29

118.26

96.21

8.

Katahdin

100

103.71

104.01

78.14

9.

Irish Cobbler

100

96.31

108.61

92.62

10.

Cherokee

100

104.09

99.73

78.23

11.

Menominee

100

105.21

101.95

70.79

12.

Russet Rural

100

100.13

107.23

86.45

13*

Delus

100

107.55

100.25

73.39

14.

Merrimack

100

97.81

108.39

86.37

15.

Green Mountain

100

102.90

98.49

76.67

Mean

100

100.56

105.63

85.24

* Percentage calculated after deleting the constant 1.000 from
each determination.

- 47 -

areas.

Here again varieties of high specific gravity had greater differ­

ences between cortex and pith than varieties which were low in specific
gravity.
Means of varieties indicated very little difference between specific
gravity of cortex and outer medulla.

The difference between cortex and

pith area were greater, but this difference was more marked in 1956 than
in 1957.
Fifteen tubers of each variety were cooked just after harvest in
boiling water for 25 minutes and resistance to pressure of the surface
and internal areas was noted.

These data are shown in Table X, which

indicates very little difference in penetration resistance except for
Cnaway tubers.

Eighty percent of these were hard in the inside after

being cooked in boiling water for 25 minutes.

There were various degrees

of hardness in different tubers, but the average resistance to pressure
of all the cooked Onaway tubers (including tubers which cooked soft) was
136.7, while the resistance to pressure of a cooked potato should be
approximately 80 grams per 0.02 cm.

2

or belowr. Onaway tubers from this

lot were used for the determination of three different pectic fractions,
cellulose and hemicelluloses, both before and after extended storage.
Determination of Three Pectic Fractions,
Cellulose and Hemicelluloses
Twelve composite samples of the inner portion of the 25 hard cooking
tubers and 12 composite samples of the inner part of 25 soft cooking
tubers of Onaway potatoes were made and analyzed as previously described.
Results of these analysis are shown in Table XI.

It may be seen from the

- 48 -

Table X.

No.

Resistance to pressure in grains per 0.02 cra.^ of surface and
internal areas of cooked potato tubers from 14 varieties
grown in 1957 (means of 15 tubers).

Surface Area

Internal Area

Sebago

72.4

91.0

2.

Pontiac

78.6

90.3

3.

Onaway

64.1

136.7

4.

Chippewa

60.1

71.4

5.

Ontario

75.8

86.3

6.

Tawa

69.5

88.3

7.

Osage

82.8

105.5

8.

Katahdin

78.0

94.6

9.

Irish Cobbler

81.3

101.6

10.

Cherokee

72.0

88.3

11.

Menominee

69.0

86.6

12.

Russet Rural

74.3

96.7

13.

Delus

65.6

80.3

14.

Merrimack

87.2

101.4

15.

Grreen Mountain

70.6

90.3

Mean

73.42

93.95

1.

Variety
»

- 49 -

Table XI.

Three pectic fractions, cellulose and hemicelluloses of hard
and soft cooking Onaway potato tubers, expressed as percentage
fresh weight.*
Hemicel­
luloses
Minus
Acid
Soluble
Pectin

Water
Soluble
Pectin

Ammonium
Oxalate
Soluble
Pectin
(middle
lame11a
pectin)

Acid
Soluble
Pectin
(proto­
pectin

Cellulose

1.

0

0,6102

0.7934

0.4000

2.6920

1.8948

2.

0

0.5949

0.8556

0.4321

2.5142

1.6586

3.

0

0.5079

0.9001

0.3921

2.4069

1.5068

if

0

0.6200

0.8356

0.3887

2.7200

1.8844

5.

0

0.5947

0.8124

0.4589

2.9013

2.0889

6.

0

0.5411

0.8317

0.4399

3.0012

2.1695

1.

0.0589

0.2986

0.3891

0.4902

1.5789

1.1898

2.

0.0484

0.3001

0.3902

0.4833

1.3245

0.9343

3.

0.0570

0.2876

0.4117

0.5210

1.5242

1.1125

4•

0.0678

0.3233,

0.4254

0.5004

1.4788

1.0534

5.

0.0524

0.3838

0.3758

0.4463

1.4524

1.0766

6

0.0624

0.2732

0.4101

0.4536

1.3024

0.8933

Hard

0

0.5781

0.8384

0.4186

2.7056

1.8671

Soft

0.0578

0.3111

0.4003

0.4824

1.4435

1.0431

Sample
No.

Hemicel­
luloses
(includes
Acid
Soluble
Pectin)

Hard

•

Soft

.

Means

* Each of the six samples drawn from a composite sample of 25 tubers.

- 50 -

means that in hard cooking tubers there was no water soluble pectin,
while in soft cookers there was 0.0578 percent of fresh weight.

There

was almost twice as much ammonium oxalate soluble pectin and acid soluble
pectin in hard cookers as in soft cookers.

Again the percentage of

hemicelluloses was much higher in hard cookers than in soft cookers.
cellulose percentage was almost the same in both kinds of tubers.

The

Thus

hard cooking inside was associated with the presence of a high percentage
of water insoluble pectins (ammonium oxalate and acid soluble pectins)
and hemicelluloses.
The hard cooking tubers cooked soft after being stored at 70° F.
Table XII indicates the penetration resistance of the twenty halves taken
from the lot of very hard cooking Onaway tubers stored at 70° F. and
pressure tested after cooking at weekly intervals.
data graphically.

Figure 12 shows this

At 0 days of storage the resistance to pressure was

over 300 grams per 0.02 cm.

2

As the storage period increased the resis­

tance to pressure declined rapidly and after five weeks storage these
tubers cooked soft.
Twenty-five halves of the tubers which were stored at 70° F. for
six weeks were analyzed for three different pectic fractions, cellulose
and hemicelluloses to determine the changes which made these hard cookers
cook soft.-

These data as percentage fresh weight are shown in Table XIII.

Table XIV compares the percentage fresh weight of the three pectic frac­
tions, cellulose and hemicelluloses of hard cookers initially and after
six weeks storage, when they cooked soft.

It is evident from Table XIV

that at the beginning in the hard cookers no water soluble pectin was
present but when they cooked soft .13U5% water soluble pectin was present.

- 51 -

Table XII.

Penetration resistance in grains per 0.02 cm.^ of internal area
of hard cooking Onaway tubers stored at 70° F. and cooked at
weekly intervals for 25 minutes (means of five pressure tests).

Tuber
No.

Weeks in Storage at 70° F.
0

1

2

3

4

5

1.

264.6

265.0

251.6

206.4

138.6

111.6

62.8

2.

295.3

253.8

240.9

188.3

132.6

94.2

73.0

3.

370.1

260.1

228.5

210.5

136.8

98.2

80.6

4.

327.3

278.8

258.4

196.0

118.7

82.8

86.2

5.

296.9

236.6

220.7

180.2

146.5

93.0

64.4

6.

300.5

251.1

238.8

200.5

134.0

92.9

61.3

7.

338.8

270.0

257.9

207.6

130.2

106.6

80.5

8.

310.2

244.5

239.6

214.5

140.6

88.6

79.7

9.

345.5

268.8

240.0

188.7

126.3

116.7

84.6

10.

272.0

250.0

250.5

207.3

148.2

83.4

66.2

11.

285.2

258.8

223.8

204.4

148.8

85.4

91.6

12.

326.1

281.6

220.4

186.4

124.2

99.3

74.2

13.

338.0

294.2

251.1

210.3

106.6

106.2

78.0

14.

274.4

269.5

248.5

215.2

146.2

84.4

63.4

15.

286.4

270.0

260.5

189.0

119.7

90.2

65.4

16.

241.7

281.2

218.2

188.3

122.5

81.3

79.4

17.

278.7

254.0

216.6

194.5

146.7

99.7

70.2

18.

309.3

270.0

230.5

208.4

126.0

86.2

86.6

19.

333.0

267.9

232.6

193.6

102.5

104.6

68.6

20.

356.7

270.6

251.0

198.7

140.0

100.5

96.7

Mean

307.4

264.8

239.0

199.4

131.8

95.3

75.7

6

- 52 -

325

300
275 -

Penetration

resistance

(grams per 0,02 cm.

250

-

225 -

200

-

175 ‘

150
125 -

100

-

75 50 -

25 -

0

1

2

3

4

5

6

Weeks in Storage at 70° F.
Figure 12.

Effect of pre-cooking storage at 70° F. on post-cooking
penetration resistance of Onaway tubers (internal area)
selected for extreme internal "hardness11.

- 53 -

Table XIII.

Percentage fresh weight of three pectic fractions cellulose
and hemicelluloses of hard cooking Onaway tubers analyzed
after being stored until they cooked soft.

Water
Soluble
Pectin

Ammonium
Oxalate
Soluble
Pectin
(middle
lamella
pectin)

Acid
Soluble
Pectin
(protopectin)

Cellulose

1.

0.1266

0.3972

0.4916

0.4412

1.8112

1.3296

2.

O'.1239

0.3511

0.5329

0.4400

1.7034

1.2505

3.

0.1284

0.4016

0.5009

0.4055

1.7380

1.2771

4*

0.1483

0.3624

0.4926

0.4508

2.0021

1.3195

5.

0.1357

0.3500

0.5200

0.4187

1.8222

1.3622

6.

0.1442

0.4084

0.5306

0.4272

1.7236

1.1930

0.1345

0.3784

0.5114

0.4306

1.8001

1.2887

Sample
No.

Mean

Hemicel­
luloses
(includes
Acid
Soluble
Pectin)

Hemicel­
luloses
Minus
Acid
Soluble
Pectin

- 54 -

Table XIV.

Percentage fresh weight of three pectic fractions cellulose
and hemicelluloses of hard cookers before and after being
stored until they cooked soft (means of six samples from a
composite of 25 tubers).

Water
Soluble
Pectin

Before
Storage
(hard)

After
Storage
(soft)

0

0.1345

Ammonium
Oxalate
Soluble
Pectin
(middle
lamella
pectin)

Acid
Soluble
Pectin
(protopectin)

0.5781

0.8384

0.3784

0.5114

Hemicel­
luloses
(includes
Acid
Soluble
Pectin)

Hemicel­
luloses
Minus
Acid
Soluble
Pectin

0.4186

2.7056

1.8671

o .4306

1.8001

1.2887

Cellulose

- 55 -

The ammonium oxalate soluble pectin and acid soluble pectin had decreased
to a marked degree.

The cellulose percentage remained the same, but the

percentage of hemicelluloses was much reduced after storage.
These results clearly indicate that due to the presence of a high
amount of ammonium oxalate and acid soluble pectins and of hemicelluloses
these potatoes cooked hard.

After storage at 70° F. for five to six

weeks they cooked soft as the amount of these substances had been
considerably reduced.

I

SUMMARY AND DISCUSSION
The internal portions of a fraction of the potatoes in a given lot
sometimes cook much more slowly than the outer portions.

This leads to

considerable inconvenience and loss since the over-cooked portions slough
off, and the rapid cookers are badly over—cooked and water soaked in the
attempt to complete the cooking of the slow cookers.

The previously

described investigation attempted to clarify the causes and discover
the remedy for this condition.
The firmness of a potato after cooking can readily be determined by
the use of a penetrometer that measures the resistance, in grams, to
penetration by a small rod.

The cooked and cooled potato starch forms

a relatively stiff mass, depending upon the proportions of starch and
water, and thus gives a relatively low resistance to penetration when
uncomplicated by other factors.

However, when cooked starch is held

inside cell walls that do not readily separate on the application of
pressure, the resistance to penetration may be many times that of the
starch gel, itself.

Thus, minimum pressures for high-starch potatoes

were around 40-60 grams, and for low-starch potatoes were around 20-30
grams.

But resistance pressures after the same period of cooking, but

before cell walls separated readily, were from 100-300 grams pressure.
Any pressure more than about 80 grams would designate the potato as
being somewhat raw or uncooked.
• The condition of.hard cooking at the start of storage of several
specific gravity groups of Katahdin potatoes and the changes during
-

$6

-

- 57 -

storage at 40

and 70° F, (indicated by resistance to pressure after

cooking) may be summarized as follows:
(1) Surface area of all potatoes cooked more rapidly than internal
area at the start of storage•
(2) As the storage period increased the resistance to pressure after
cooking of both surface and internal areas fell.
(3) There was more change in resistance to pressure of the internal
area than surface area as the storage period increased; these changes
were more rapid at 70° storage than at 40° F.
(4) At 70° F, the internal area cooked softer than surface area
after seven weeks of storage.

The internal area cooked much softer

than surface area in high specific gravity potatoes (1.080 - 1.085)
while in low specific gravity potatoes (1.065 - 1 .070) the softness of
thesurface
(5)

and internal areas after cooking was almost the same.
At 40° F. the inside did not cook softer than the surface area

in any of the specific gravity groups.
The condition and changes in Onaway tubers before and after storage
can be summarized:
(1) At the beginning of storage- the center part of about 80% of
the Onaway tubers was hard to cook and calcium pectate, protopectin and
hemicelluloses were present in large amounts.
(2) After 5-6 weeks of storage at 70° F. they cooked soft and the
quantity

of these substances was much reduced.

(3) At the beginning of storage, hard-cooking Onaway tubers were
high and soft-cooking tubers low in insoluble pectic substances and
hemicelluloses.

After six weeks of storage the initially hard-cooking

- 58 -

tubers which now cooked soft had about the same quantity of these
materials as tubers which had cooked soft at the beginning of storage#

Before storage.

In the beginning of storage the internal area cooked

harder than the surface area because of the large amount of insoluble
pectic substances and hemicelluloses present in the cell walls and as
cementing materials between the cells.

Innermost portions of Onaway tubers

cooked very hard and were high in amounts of these substances, while those
which cooked softer did not have so much of these materials#

This is in

accordance with the findings of Thiessen (1947), who reported that in
potato tubers the inner medulla had a higher proportion of crude fiber and
pectin than did the rest of the tuber.
Bonner (1952) mentioned that during the development of fruit, proto­
pectin is laid down in the primary walls and in many cases proto pectin
accumulates to high concentration particularly in apple, pear and citrus
species.

Middle lamella pectate is likewise accumulated in the fruit.

Norman (1937) in his review of cellulose and related cell wall compounds,
stated that middle lamella of plant tissue is believed to be insoluble
calcium pectate while the cell wall substances are protopectin and soluble
pectin.

The protopectin is always found closely associated with other cell

wall constituents.

Ippolito (1928) mentioned that when predominating pectic

compounds are soluble or can be hydrolyzed, the tissue easily disassociates,
becoming soft.

With an excess of cellulose and hemicelluloses the pectic

compounds are not hydrolyzed and vegetables do not cook readily.

An explan­

ation of the observed cooking performance of hard cooking tubers incor­
porates these findings.

- 59 -

Af ter 3torage. Onaway tubers which were initially very hard to cook due
to the presence of large amounts of calcium pectate, protopectin and
hemicelluloses cooked normally after 5 to 6 weeks of storage at 70° F.
These changes may be attributed to the observed decomposition of calcium
pectate, protopectin and hemicelluloses, with a consequent decrease in
cell wall rigidity due to alteration in thickness and firmness of the
cell walls and the cementing substances of middle lamella.

These changes

had a pronounced effect on the force required to push the cells apart, as
measured by firmness after cooking.
At the beginning of storage hardness in cooking was due to the
presence of large amount of cell wall materials.

After the hydrolysis

of these substances starch determined the penetration resistance after
cooking.

It was found in this investigation that surface (cortex) had

more starch as indicated by specific gravity than the internal area
(pith) and thus cooked harder after storage than internal area.

There

were greater differences between specific gravities of surface and internal
areas of high specific gravity potatoes than in low specific gravity
potatoes; this was reflected by greater difference in hardness between
areas in high specific gravity tubers.
Many investigators (Sweetman and Lancaster (1931), Barmore (1937),
Thiessen (1947) and others) have reported that the portions within a
tuber which had high starch content were firmer after cooking than those
which had less starch.
At the end of storage the potatoes stored at 40
at the surface and internal areas.

F . cooked similarly

Apparently the insoluble pectic sub­

stances and hemicelluloses had not yet hydrolyzed to the extent that

- 60 -

starch content alone rather than in combination with cell—wall materials
determined hardness after cooking.

It seems probably that this stage

would have been reached after a few more weeks of storage.

There is

clear indication in the literature that changes in insoluble pectic
fractions are more rapid at high than at low temperature, which was
confirmed by these results.

Specific gravity patterns for regions within tubers of different varieties.
In 15 varieties the specific gravity of cortical, outer and inner medul­
lary regions was determined.
clear pattern emerged.

For the two years of this study, a rather

In varieties where specific gravity of the entire

tuber was high, there was a large specific gravity difference between
outer and inner regions.

In varieties with a lower specific gravity,

the differences between outer and inner portions was comparatively small.
When cooked, before storage, there seerned to be no relationship
between rapid cooking, or resistance to the penetrometer, and specific
gravity.

In the cases where cooking after storage was done, varieties

higher in starch, as indicated by high specific gravity, were higher in
resistance to penetration than tubers low in starch.
Analogous changes in other fruits and vegetables.

Several investigators

have found similar changes in other fruits and vegetables during ripening
and storage.

This has been thoroughly studied in apples by Carre (1922,

1928), Emmett (1925, 1926), Haller (1929) and others who have found that
softening of fruits takes place due to hydrolysis of insoluble pectic
substances of cell wall and middle lamella.

Dastur and Agnihotri (1934)

noticed decomposition of protopectin and middle lamella pectin of cell

- 61 -

walls of potatoes during storage but did not associate these changes
with characteristics of cooked tubers.

Several other investigators

working with pears, peaches, bananas and grapes have found similar
changes.

The rate of such degradation increases with increasing

temperature.
In the present investigation it was found that the amount of starch
present in the cells, the thickness or toughness of cell walls and ease
with which the cells separate as indicated by insoluble pectins and
hemicelluloses play an important role in determining hardness of potatoes
after cooking.

In the beginning of storage the insoluble pectic frac­

tions and hemicelluloses plus the starch determine the hardness after
cooking but after storage these cell-wall materials are hydrolyzed and
become soluble and the amount of starch present in the cells mainly
determines the hardness after cooking.

If the low resistance pressure

after storage is attributed to a starch effect, the decrease during
storage may be called the cell-wall effect.

This cell-wall resistance

decreases as cell-wall materials hydrolyze.

It is thus suggested that

lots of potatoes containing hard cookers be kept in storage until such
time as the hardness has disappeared due to changes in the cell wall
co nstituents,

CONCLUSIONS
Potatoes generally are slower to cook immediately after harvest than
after a period of storage.

Degree of "done-ness" can be measured

with a penetrometer.
In some cases, extremely slow cooking shortly after harvest is en­
countered in a considerable fraction of the tubers in a given lot,
but this does not seem to be an invariable varietal characteristic.
A reduction in hardness after a standard cooking procedure is ac­
complished more rapidly in storage at 70° F. than at 40° F.
Tubers from a given varietal lot may be selected for either "hard"
or "soft” cooking.

Those that are hard to cook are characterized

by a far higher content of insoluble pectins and of hemicelluloses
than those that cook soft.

The cellulose contents are found to be

about the same in soft and hard.
Storage that reduces the hardness of cooking also reduces the content
of insoluble pectins and hemicelluloses to the level of the soft
cookers.
Tubers or parts of tubers with high specific gravity, (and starch)
are firmer after cooking, when cooking "soft", than are those with
a low specific gravity.

This is attributed largely to the relative

concentration and stiffness of the potato-starch gel in either case.
Before storage, or when cooking hard, the effect of the high pectin
and hemicelluloses in the cell walls, which prevents their easy

- 63 -

separation, may completely mask any starch gel effect on hardness.
This cell-wall effect may easily be 10 times the starch-gel effect.
8.

Varieties with a high average specific gravity strongly tended to
vary greatly in the specific gravity between outer .(.cortex) and inner
( pith 0 regions of the tuber, while those with a lower specific
gravity had comparatively little differences between regions.

LITERATURE CITED

1.

2.

Atwater, W. 0. 1895* Methods and results of investigation on the
chemistry and economy of food. U. S. Dept, of Agr. Office
Exp. Sta. 3ul. 21: 88.
Appleman, C. 0. and C. M. Conard. 1926. Pectic constituents of
peaches and their relation to softening of the fruits. Md.
Exp. Sta. Bui. 283*

3.

Barnore, M. A. 1937* Potato mealiness and changes in softness in
cooking. Food Res. 2: 377-385.

4.

Barnell, H. R. 1943. Studies in the tropical fruits. XV. Hemicelluloses metabolism of the banana fruit during storage.
Ann. 3ot. 7: 297-323.

5.

Bettelheim, F. A. and C. Sterling. 1955a. Factors associated with
potato texture.
I.
Specific gravityand starch content.
Food Res. 20: 71-80.

6.

Bettelheim, F. A. and C. Sterling. 1955b. Factors associated with
potato texture.
II. Pectic substances. Food Res. 20: 118-129.

7.

Bewell, E. R. 1937* The determination of the cooking quality of
potatoes. Amer. Potato Jour. 14: 235-242.

8 . Bonde, R. and M. Covell.

1955* Effects of spray treatments on
yield rate and specific gravity of potatoes. Amer. Potato
Jour. 32: 399-406.

9.

10 .

Bondet, V. 1952. Les pectines au cours du sechage de la prune
d' Ente. (Pectins in the course of drying prune plums.)
C. R. Acad. Agric. Fr. 38: 189-192.
Bonner, J. 1952.
Press Inc.

Plant Biochemistry.

New York:

Academic

11 . Carre, M. H.

1922. An investigation of the changes which occur
in the pectic constituents of stored fruits. Biochem.
Jour. 16: 704-712.

12.

Carre, M. H. 1925. Chemical studies on the physiology of apples.
IV. Investigations on the pectic constituents of apples.
Ann. Bot. 39: 811-839.

- 64 -

- 65 -

13.

Clark, C. F., P. M. Lamberd, E. F. Whiteman. 1940.
of the potato as measured by specific gravity.
Jour. 17: 36-45.

14.

Cobb, J. S. 1935* A study of culinary quality in white potato.
Amer. Potato Jour. 12: 335-346.

15.

Conard, C. M. 1930. A furfurol-yielding substance as a splitting
product of protopectin during the ripening of fruits. Plant
Physiol. 5: 93-103.

16.

Cooking quality
Amer. Potato

Coudon, H. and L. Bussard. 1697. Recherches sur la pomme de terre
alimentaire. Ann. Sci. Agron. 2e ser. 1: 250-291.

17.

Day, E. D. 1909.
Econ.l: 177-

The effect of cooking on cellulose.

Jour. Home

18.

Dastur, R. H. and S. D. Agnihotri. 1934. Study of the pectic changes
in the potato tuber at different stages of growth and in storage.
Indian Jour. Agr. Science. 4: 430-450.

‘19.

East, E. M. 1908. The study of the factors influencing the improve­
ment of the potato. 111. Agr. Exp. Sta. Bui. 127.

20 . Emmett, A. M. 1926. Pectic changes in apples and pears. (Gt.
Britain) Dept. Sci. and Ind. Res. Food Invest. Bd. Rpt. 1925/26:
47-49.

21

.

Emmett, A. M. 1929. An investigation of the changes which take
place in the chemical composition of pears stored at different
temperatures with special reference to pectic changes. Ann.
Bot. 43: 269-308.

22 . Fisher, D. V. 1943. Mealiness and quality of delicious apples as
affected by storage conditons, maturity and storage techniques.
Sci. Agr. 23: 569-588.
23.

Fisher, D. V., J. E. Britton and H. J. 0'Rielly. 1943. Peach har­
vesting and storage investigations. Sci. Agr. 24: 1-16.

24.

Freeman, M. E. and W. S. Ritchie. 1940. Pectins and the texture
of cooked potatoes. Food Res. 5: 167-175*

25.

Fr£my, E. 1840. Premiers essais sur la maturation des fruits. Re­
searchers sur la pectine et 1* acid pectique. J. De. Pharm.

26: 3 68.
26.

Frisbie, A. J. and A. P. Bryant. 1897. The composition of different
parts of the potato and the loss of nutrients during the process
of boiling. U. S. Dept. Agr., O.E.S. Bui. 43: 25-31.

- 66 -

27*

Gaddum, L, */. 1934* The pectic constituents of citrus fruits,
Agr, Exp. Sta. Bui. 268.

Fla,

28.

Gerhardt, F. and B. D. Ezell. 1938. Effect of carbon dioxide storage
on Bartlett pears under simulated transit conditions. Jour, Agr.
Res. 56: 121-136.

29-

Gilmore, J. W. 1905* Quality in potatoes.
Bui. 230: 503-525.

30.

Glynne, M. D. and V. G. Jackson. 1919* The distribution of dry matter
and nitrogen in the potato tuber. Variety, King Edward. Jour.
Agr. Sci, 9: 237-258.

31.

Goldthwaite, N. E. 1925. Variation in composition of Colorado
Colo. Agr. Exp. Sta. Bui. 296.

32.

Greenwood, K. L., M. H. McKendrick and A. Hawkins. 1952. The rela­
tionship of the specific gravity of six varieties of potatoes
to their mealiness as assessed by sensory methods. Amer. Potato
Jour. 29: 192-196..

33.

Haddock, J. L. and P. T. Blood. 1939. Variations in cooking quality
of potatoes as influenced by varieties. Amer. Potato Jour. 16:
126-133.

34.

Haller, M. H. 1929. Changes in the pectic constituents of apples in
relation to softening. Jour. Agr. Res. 39: 739-746.

35.

Haller, M. H. 1941. Fruit pressure tester and their practical appli­
cation. U . 5. Dept. Agr. Circ. 627.

36.

Hansen, E. 1939. Effect of ethylene on certain chemical changes
associated with the ripening of pears. Plant Physiol. 14:
145-161.

37.

Heinze, P. H., M. E. Kirkpatrick and E. F. Dochterman. 1955. Cooking
quality and compositional factors of potatoes of different
varieties from several commercial locations. U. S. Dept, of
Agr. Tech. Bui. 1106.

38.

Hilton, R. J. and V/. D, Evans. 1950. Factors in relation to tuber
quality in potatoes. I. Penetration force and electrical
resistance. Sci. Agr. 30: 343-349.

39.

Hopkins, E. F. and J. H. Gourley. 1930. Nitrogen fertilization and
pectic materials in grapes. Proc. Amer. Soc. Hort. Sci. 27:
164-169.

40.

Ippolito, G. D. 1926. Causes determining cooking power of vegetables.
Rass. Intern. Agr. 10-2, 994* (Chem. Abs. 22: 1413* 1928).

Cornell Agr. Exp. Sta.

potato.

- 67 -

41*

Johnson, T. and C. I^yle. 1919. Observations on the industrial and
nutritive value of the potato in Ireland. Jour, Dept* Agri. Ire.
19: 416-429.

42.

Kirkpatrick, M. E. 1953* Cooking qualities of potatoes: It*s evalu­
ation and relationship to potato characteristics. Amer. Potato
Jour. 30: 52-53.

43.

Kirkpatrick, M. E., B. M, Mountjoy, L, C. Albright and P. H. Heinze.
1951* Cooking quality, specific gravity, and reducing sugar
content of early-crop potatoes. U. S, Dept. Agr. Circ. 872.

44.

Landreth, C. 1929. A method of standardizing ridoneness" incooked
vegetable. Jour. Home Econ. 21: 826-828.

45.

Langworthy, C. F. 1917* Potatoes, sweet potatoes and other starchy
roots as food. U. S. Dept. Agr. Bui. 468.

46.

McCoHoch, R. J. 1952. Determination of pectic substances and pectic
enzymes in citrus and tomato products. U. S. Dept.Agr. Fruit
and Veget. Chem. Lab. Pasadena, Cal., A.I.C. 337.

47*

Nonnan, A. G. 1937. The biochemistry of cellulose the polyuronides
lignin etc. Oxford Univ. Press, Oxford.

48.

Norris, F. W. and I. A. Preece. 1930. Studies on hemicelluloses.
1. The hemicelluloses of wheat bran. Biochem. Jour. 24: 59-66.

49.

Nyland, R. E. and A. J. Poivan. 1953. The influence of variety
and date of planting on the relative cooking quality of potatoes
graded according to specific gravity. Amer. Potato Jour. 30:

107-118.
50.

Personius, C. J. and P. F. Sharp. 1938. Adhesion of potato-tuber
cells as influenced by temperature. Food Res. 3: 513-524.

51.

Personius, C. J. and P. F. Sharp. 1939a. Adhesion of potato tissue
cells as influenced by pectic solvents and precipitants. Food
Res. 4: 299-307.

52.

Personius, C. J. and P. F.
agents, of cooking of

53.

Phillips, W. R. and P. A. Poapst.
1950. Low temperature research.
Soluble pectin trends in cold stored apples. Dom. Canada Dept.
Agr. Exp. Farms Serv., Div. Hort. Cent. Exp. Farm, Ottawa,
Progr. Rep. 1934-1948: 191.

54.

Prince, F. S., P. T. Blood, W. H. Coates, and T. G. Phillips. 1940.
Experiments with potatoes. N. H. Agr. Exp. Sta. Ail. 324*

Sharp. 1939b.
potato tissue.

Stimulation, by chemical
Food Res. 4: 469—473.

- 68 -

55*

Potter, A. L. and E. A. Me Comb.
potatoes. Pectin content.

1957* Carbohydrate composition of
Amer. Potato Jour. 34: 342-346.

56.

Pyke, W. E. and G. Johnson. 1940. The relation of the calcium ion
to the sloughing of potatoes. Amer. Potato Jour. 17: 1-9.

57. Reeve, R. M. 1954. Histological survey of conditions influencing
texture in potatoes. 1. Effects of heat treatment on structure.
Foo,d Res. 19: 323-332.
58. Roberts, E. A. and B. E. Proctor. 1955. The comparative effect of
ionizing radiations and heat upon the starch containing cells of
the potato tuber. Food Res. 20: 254-263.
59. Shallenberger, R. S. and 0. Smith. 1955. The browning reaction in
potato chips (Abs.) Amer. Potato Jour. 32: 428.
60.

Shewfelt, A. L., D. R. Brown and K. D. Troop. 1955* The relationship
of mealiness in cooked potatoes of certain microscopic observa­
tions of the raw and cooked product. Canada Jour. Agr. Sci.
35: 513-517.

61.

Smith, 0. and L. B. Nash. 1940. Fotato quality. I. Relation of
fertilizers and rotation systems to specific gravity and cook­
ing quality. Amer. Potato Jour. 17: 163-169.

62.

Smith, 0. and P. Muneta. 1954. Potato quality VIII. Effect of
foliar applications of sequestering and chelating agents on
after cooking darkening. Amer. Potato Jour. 31: 404-409.

63.

Smock, R. M. and F. W. Allen. 1938. Soluble pectin changes in gas
stored fruit. Proc. Amer. Soc* Hort. Sci. 35: 184-187.

64.

Sweetman, M. D. 1933. The physico-chemical changes produced by
the cooking of potatoes. Arrter* Potato Jour. 10: 169-173.

65.

Sweetman, M. D. 1936, Factors affecting the cooking quality of
potatoes. Maine Agr. Exp. Sta. Bui. 383: 297-387.

66.

Sweetman, M, D. and M. C. Lancaster. 1931. Penetrometer measure­
ment of "doneness’1 in cooked vegetables. Jour. Home Econ. 23:
565-567.

67.

Thiessen, E. J, 1935. Penetrometer and photomicrographic studies
of two varieties of potatoes grown upon dry and irrigated land.
Jour, of Home Econ. 27: 539*

68.

Thiessen, M. J. 1947. The culinary qualities and nutritive values
of potatoes grown upon dry and irrigated land. Wyom. Agr. Exp.
Sta. Bui. 280.

- 69 -

69.

Unrau, A. M. and R. E. Nylund.
1957. The relation of physical pro­
perties and chemical composition to mealiness in the potato. I*
Physical properties. Amsr. Potato Jour. 34s 245-253*

70.

Von Loesecke, K. W.

1950. Bananas. Interscience.

71-

Whittenberger, R. T. 1951- Changes in specific gravity, starch
content, and slaughing of potatoes during storage. Amer.
Potato Jour. 28: 738-747.

72. Whittenberger, R. T. and G. C. Nutting. 1950.
ing of potatoes. Food Res. 15: 331-339.
73*

New York.

Observation on slough­

Wright, R. C., W. M,Peacock, T. M. Whiteman, and E. F. Whiteman.
1936. The cooking quality, palatability, and carbohydrate
composition of potatoes as influenced by storage temperature.
U. S. Dept. Agr. Tech. Bui. 507*