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A STUDY OF THE EFFECT OF
COOKING ON THE DIGESTIBILITY
OF WHOLE EGG PROTEJN

Thesis for tha Degree? of M. S.
ROBERTA HERSHEY
‘92?

THES‘S

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A STUDY OF CHE EFFECT OF COOKING
ON THE DIGESTIBILITY OE WHOLE EGG PROTEIN.

A Thesis Presented for the

Degree of Master of Science

By

Roberta Hershey

Mi chig an St ate College

1929

THEE“?

ACKNOWLEDGMEHP

For the kindly advice and
criticisms received during the course
of this study, the writer is grateful
to Dr. marie Dye under whose general
supervision the work was done.

Special appreciation is due
Miss Whittaker for valuable suggestions

and Dean.Krueger for a final review of

the subject matter.

101157

2.

3.

4.

5.

6.

I N D E X

PURPOSE

REVIEW OF LITERASURE

PROCEDURE

DISCUSSION

SUMMARY

BIBLIOGRAPHY

A STUDY OF SHE EFFECT OF COOKING
ON Th5 DIGESTIBILITY OF LEOLE EGG PROTEIN.

1. PURPOSE

The purpose of this study was to
determine the effect of different methods
of preparation upon the rate and
completeness of digestibility and upon
the assimilation and use of the protein
of whole egg.

2.

2. REVIEW OF LITNBATUhE.

Many articles on the digestibility
of egg as influenced by cooking are
included in the literature on the subject.
However, for the most part, the work has
been directed toward the determination of
the digestibility of egg white due, no
doubt, to the wide spread use of raw egg
white in diets for invalids. Most of
the work reported, unless limited to yolk
or white of egg, is confined either to
gastric or to tryptic digestion.

Dr. H. C. Sherman in his "Food Products"
(1) says, "The digestibility of eggs has been
studied experimentally, but not in such
detail as with some other articles of
food.‘ ************* It is probable that
eggs 'soft cooked' at a temperature below
that of boiling water are the most readily
and rapidly digested, but the ultimate

3.

thoroughness of digestion does not
seem to be greatly influenced by
the method of cooking."

Some of the earliest work on
digestibility of egg is reported wiflh
Dr. Beaumont's classic eXperiments
on Alexis St. Martin (2). Through
the permanent opening in the stomach
of his subject, Dr. Beaumont observed

the following:

Method of Cooking Time of Leaving
V Stomach
Hard boiled eggs 3} hours
Fried eggs 5% hours
Soft boiled eggs 3 hours
Raw eggs. 2 hours
Raw (whipped) eggs 1% hours

He also used methods of artificial
' digestion and feund the relation between

types of preparation the same, but the

4.

actual time longer. He omitted
entirely digestion in the intestine.

hubner (2) tested hard boiled
eggs with a healthy man as a subject.
He found 97% of the egg protein to be
digested.

Langworthy in a Farmer's Bulletin
on “Eggs" (2) reviews the results of
some artificial peptic digestion
experiments and nitrogen balance
experiments carried on by the
Minnesota Experiment Station. By
artificial digestion with a pepsin
solution, they found that an egg
boiled 5 minutes and digested for
5 hours left 8.3% undigested protein,
a 20 minute boiled egg left 4.1%
undigested protein. However, eggs
soft cooked for either 5 or 10
minutes were entirely digested in the

same period. In a nitrogen balance

5.

experiment on one man, they found
90% of the total nitrogen of a
diet of cream, milk, potato and
egg to be digested.

Jorissenne (2) concluded from
his work that mastication was of
prime importance in the digestibility
of eggs.

Tikhivinski (2) at St. Petersburg
conducted some more extensive
experiments on,a man, feeding first a
diet of "hard-boiled" eggs, bread and
meat, and then one of "soft-boiled"
eggs, bread and meat. He concluded
that eggs prepared by both methods
were equally digestible. However
he raised the question whether, When
two foods are eaten together, if the
digestibility of each may not be

changed.

6.

Linossier (3) found that
gastric digestion lessened with
the cooking of albumin, and
Talarico (4) reported that tryptic
digestion of egg albumin increased
with the temperature and duration
of heating.

Bateman (5) concluded during
his series of experiments on dogs and
rats that raw egg white was only
50 - 707$ utilized, while cooked egg
white was 90%. In every case he
found that the raw whites caused
intestinal disturbances, which were
not observed while cooked whites were
being fed. The egg whites, making
up 20, 40, and 60% of the diets,
were mixed with cracker meal and
lard. The yolk of the egg, Bateman
considers to be well utilized. Data

was also collected from a number of

people who consumed large quantities

of raw and cooked eggs, the results

7.

of which bore out the conclusions
reached on the animal experiments.
However, no information was given in
the paper concerning the amount of egg
eaten, the exact number of people on
the experiment, or the per cent of the
egg utilized.

Hawk and his co-workers (6) at
Jefferson Medical College did a very
comprehensive piece of work on medical
students. They tested eggs prepared in
many ways, analyzing the stomach contents
at definite intervals after the egg had
been eaten. In brief, the following
results were obtained:

1. Raw egg white left stomach

most rapidly.

2. Raw egg yolk required much

longer than White.

5. Bard-cooked eggs took longer

then soft.

4. Scrambled eggs took longer
than.boiled.
5. Poached, shirred, and soft-
cooked eggs were most rapid.

Rose & Mac Leod (7 & 8) carried on
some experiments with raw white of egg
on six human subjects and found raw
whites to be 80% digestible and cooked
whites 86%. They found absorption to
be better When the eggs were beaten light.

Some very recent reports from German
investigators (9) contribute interesting
additions to this problem. Sheunert and
Wagner after experimenting on rats, conclude
that the difference in nutritive value
between raw and hard-boiled egg yolks is
insignificant. Friedberger and Abraham
found that animals on an egg diet suffered
toxic results, which were nullified if the
eggs were boiled 20 to 80 minutes.

Nfltchell & German (10 and 11) report

whole eggs to be 100% digestible and
to have a biological value, according
to Mitchell's formula, of 94. In
their experiments, the eggs were dried
whole and the fat extracted with ether,
they were then fed to rats and complete
metabolism data was obtained.

An analysis of these articles,
shows that more work has been done on
the rate of digestibility than upon its
completeness. The experiments of
Drs. Beaumont and Hawk are definite results
upon the speed with which eggs leave the
stomach, but they give no indication of the
percentage of egg ultimately digested since
peptic digestion is only one step in the
whole process. The artificial digestion
experiments give only percentages of
protein digested in a given period of time,
and, while they are excellent measurements

of the relative ease of digestion, they

10.

again do not show the actual

biological value of the food tested.
Reports on percentages of digestibility
obtained by feeding cooked eggs to
human subjects vary from 90% to 97%
with no very definite control of the
method of cooking mentioned.

This work was planned to study
further the influence of different
methods of preparation upon the ease
and completeness of digestibility and
to, if possible, see if cooking methods

would alter the actual biological value.

ll.

5. PROCEDURE.

The eggs used in these experiments
were of a constant variety, being
supplied, strictly fresh, by the College
Poultry Department. They were selected,
large, white eggs. Four methods of
preparation were chosen for investigation,-
two, supposedly "easy of digestion,"
the raw and soft cooked, and two "difficult
of digestion," the scrambled and hard
boiled. The raw eggs were whipped with a
Dover egg beater, care being taken to
incorporate a minimum amount of air. The
hard-boiled eggs were placed in boiling
water for 10 minutes. The scrambled eggs
were first beaten and then cooked in butter
in the proportion of 5 gm. butter to each
egg. They were stirred continuously and
heated until the protein.was coagulated.
The soft-cooked eggs were kept in water
Just below the boiling point for 10 minutes.

12.

In order to secure uniform samples,
the cooked eggs were run through a
fine-meshed sieve several times.
The experimental work was
divided into two parts:
determination of the percentage of
amino nitrogen formed in artificial
digestion experiments, and nitrogen
metabolism experiments on white
rate. The former were time con-
trolled and designed to determine
the relative rate of digestion,
while the latter would, of course,

show completeness of digestion.
THE AMINO NITROG"N METHOD

The andnc nitrogen method was
essentially that of Waterman and Jones
(12). About 1.5 grams of egg was
suspended in 26 cc. of 0.1 N.HCL.
25cc of 0.2% solution of pepsin in
0.1 N.HCL was added and the mixture

13.

digested at 37° C. for 1% hours.
Peptic digestion was brought to an
end by the addition of 5 cc.N. NaCH
and tryptic digestion started
immediately with 5 cc. of a 6.0%
solution of trypsin in 0.1 N.NaOH.
. The mixture was digested at 37°
for 2% hours and then the enzymes were
destroyed by heating to BOO-for
5 minutes. The solution was cooled,
made up to 100 cc., filtered if
necessary, and 10 cc. aliquots taken
for the determination of amino
nitrogen.by Van Slyke's (15) method.
A blank determination was run
with each set of triplicate experiments.
The volume of nitrogen obtained from
this blank was subtracted from that
obtained with the egg. Undoubtedly
self-digestion is greater in a blank
Where no substrate is present, than

it is in digests containing a protein

sample, however, since identical
technique was used in each case,
the relative results would be the
same. ‘
Total nitrogen was determined
by the KJeldahl method and the
percentage of amino nitrogen
calculated. The results of these
experiments are in Table 1. Each
figure is an average of three
KJeldahl determinations or three
amino nitrogen determinations.
Considerable trouble was encountered
in obtaining close checks because of
difficulty in sampling, since it was
necessary to use very small amounts
in order that all the egg be in
contact with the digestive enzymes.
There was quite a wide variation
in the nitrogen content of the eggs
as shown.in Table l. The nitrogen
varied from 2.06% to 2.72%,- a

14.

percentage variation of 27.

These variations, while they are
within the range given in the
bulletin.on the chemical
composition of food materials (14),
are all the more surprising in view
of the fact that the eggs were from
the same flock of hens which were

receiving a constant ration.

15.

16.

B. Metabolism Method.

Four healthy, full-grown, male
rats from the laboratory stock colony
were chosen for the metabolism studies.
Animals which had attained their full
growth were used so that the problem
would not be complicated by varying
needs of protein for growth, but would
be based upon the maintenance requirement.

The basal diet consisted of 96%
cornstarch and 4% McCollum's salt mixture
#185, to which was added daily weighed,
400 mg. portions of dried yeast. The
nitrogen contained in the yeast did not
enter into the calculations since the
amount was exactly the same for each
period, and any slight change it might
cause would make no difference in the
relative results for the five periods.

The eggs were fed in such amounts
as to give an approximate 10% protein

level, Mitchell (15) having shown that

17.

the biological value of a protein
decreases as the percentage of protein
fed increases. The raw, hard boiled
and soft cooked eggs were supplied in
8 gm. portions daily, while the
scrambled egg was given in 8.6 gm.
amounts to make up the protein replaced
by the added fat. The eggs were cooked
and placed in a tightly covered jar and
kept in the ice box. In this way two
samples were sufficient for the six-day
period. Each.sample was analyzed for
total nitrogen and the three-day quota
of nitrogen calculated. The two results
were then added together and an average
taken for the daily nitrogen supplied.
No basal ration.was given in any
case until all the egg had been eaten
and after that, plenty of food was kept
in the cages. It was necessary to keep
the rats in their individual cages on an

egg diet for some time before the

experimental period to accustom them to

18.

the flavor of eggs and to the
somewhat unpalatable basal diet.
Distilled water was given in
inverted tubes ad libitum.

The collection method was
the one of H. H. Mitchell (16)
modified to suit conditions in our
laboratory. The rats were placed
in inverted circular wire cages,
the t0p of the cage thus forming
a raised screen bottom. The pans
in which the cages were placed were
covered with filter paper to absorb
the urine. The food was placed in
a cup and suspended in a metal ring
fastened in an opening at the side
of the cage. It was not possible to
keep an accurate record of the amount
of the basal diet consumed, because
rats have a habit of eating food with
their fore feet and scatter much of it.
The portion that fell through the

raised screen became contaminated with

urine and therefore had to be
included in the washings later.
Since the ration contained no
nitrogen, this had no effect
upon the determination of the
nitrogen content of the excreta.

The experimental period
consisted of a three-day
preliminary period, since it has
been shown that the endogenous
level of urinary excretion in
rats is reached in from one to
four days on a nitrogen—free
diet (16), and a six—day
collection period. During the
latter, the urine and feces were
collected on alternate days.
The rats were weighed at the
beginning and at the end of each
period.

The dish, paper, and raised
screen bottom were washed wdth

300 - 400 cc. of boiling water

19.

acidified with sulphuric acid
using 10-12 washings. They
were kept in the ice box over
night and made up to 500 cc.
volume the next day at ice box
temperature. Six-day composite
samples of the washings were kept
and analyses for total nitrogen
made by the KJeldahl method.

The feces were preserved in
10% sulphuric acid, digested
until a homogeneous suspension
was obtained and made up to
500 cc. volume._ Aliquots were
taken for nitrogen analysis, which
was also done by the Kieldahl
method.

20.

 

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21.

4. DISCUSSION.
A. Artificial Digestion Experiments.

The data on these eXperiments is
given in Table 1. These figures
represent the checks obtained from some
seventy triplicate determinations of
amino and Kdeldahl nitrogen. It was
especially difficult to obtain checks
on the amino nitrogen determinations
since there were so many variables
outside of the possible variation
in the way the eggs themselves might
react. Enzyme preparations are not
of constant strength and are known to
deteriorate upon aging. Three
separate bottles of powdered trypsin
and two of pepsin.were used during these
experiments, which might account for
some of the day-to-day variations.

A clumping of the egg during the
digestion period was often noted in the

cooked samples. This would obviously

 

22.

lessen the degree of digestion by
reducing the surface of the egg in
contact with the digestive solutions.
It is quite possible that heating for
5 minutes at 80° 0. might not, in
every case, destroy all the enzyme so
that slight digestion might go on
after the time record had been taken.
This explanation is somewhat sub-
stantiated by the fact that several
times the last sample of a group to
be used for analysis in Van Slykeis
apparatus gave the hightest percentage
of amino nitrogen.

The percentages given in the table,
with one exception, are averages of
three determinations of 5% difference or
less. In many cases the percentage
difference is only 1 or 0.8. It would
seem, consequently, that the data given
is indicative of the actual behavior of

egg protein during artificial time-

 

 

 

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Adm
V

controlled digestion.

From these figures one would
conclude that "soft cooked" eggs,
cooked for 10 minutes in‘water;Just
below the boiling point, are more
rapidly (and perhaps more easily)
digested than the others, an average
of 37.7% of the total nitrogen having
been changed to amino nitrOgen during
the four-hour digestion period.

Hard boiled eggs seem to be the next
most rapidly digested, 30.5% amino
nitrogen having been obtained in the
same period of time.

The result with raw eggs is the
surprising factor, since they are
popularly thought to be most easily
and rapidly digested. There was
only 3.3% more amino nitrogen after
four hour's digestion.than was pro-
duced by scrambled eggs, which were

well coated with fat, a substance

 

24.

which has long been known to slow up
processes of digestion.

The greatest difference between
the various samples tested is shown
by the 7.2% increase in the amounts
of amino nitrogen produced by soft
cooked over hard boiled eggs.

There was a difference of 2.4%
between the hard boiled and raw eggs,
with one of 3.5%, already referred to,
between the raw and scrambled ones.

It is quite probable that in normal
persons these small variations in the
rapidity of digestion would be
insignificant when the eggs were taken

as a part of a mixed diet.

 

Table 1e
Percentage of Protein Digested by
Artificial Digestion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I .
Method gTotal g Amino Nitrogcd
of Lyitrogen I Nitrogen Digested
Cooking {per cent E per cent per cent
Raw [i__;5h1.-0_i_,-_:.718.-_ 28.6
I 2.30 .647 28.1
2.35 .653 27.6
Av. 3 28.1
Hard 3
591155 ! 2.25 .547 50.0
i 2.19 .715 52.5
2.72 .753 27.6
2.13 .678 31.8
‘Av. 50.5
Scrambled 2.31 .685 25.3
2.62 .636 24.2
Av. 24.8
Soft
Cooked 2.24 .817 36.4
2.06 .805 39.0
”av. 37.7

 

 

 

26.

B. Metabolism ExPeriments.

An analysis of Table 2 shows
a remarkable agreement in the
percentage of the different samples
of egg actually digested. Except
for the raw egg period, the percentage
digestibility was calculated after
subtracting the amount of nitrogen
excreted in the feces during a protein-
free period. This procedure does away
with the complicating factor of
endogenous nitrogen. It was impossible
to use this method of calculation for
the raw egg period because there was
less fecal nitrogen excreted during this
period than there was during the non-
protein period.

This occurrence is difficult to
explain.although Sherman (17) in
reviewing the work of Prausnitz on
human beings states that on a liberal
diet consisting entirely of non-

 

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nitrogenous food, the amount of
nitrogen in.the feces was 0.5 to
0.9 gram per day, which is more
than is sometimes found in feces ’
from food furnishing enough protein
to meet all the needs of the body.
Mitchell (16) reports unaccountable
variations in the metabolic
nitrOgen in the feces. It may be
that the fluid nature of the raw
eggs'was the cause, since there was
an increase in the fecal nitrogen
during the cooked egg periods.
Assuming the small amount of
nitrogen in the feces during the raw
egg period to be all metabolic
nitrogen, there would be no nitrogen
from undigested foOd and the
coefficient of digestibility would be
100. This latter figure more likely
represents what really happened during

Period 2. than the average of 92.2%

27.

25

which appears in.the table.
The hard boiled, scrambled,

and soft cooked egg rations

resulted in practically the

same coefficient of digestibility,

the latter being more nearly 100%.
The biological values given in

the last column, table 2, were

calculated after the method of

Mitchell (18). The amount of fecal

nitrogen actually derived from the

food is obtained by subtracting the
fecal nitrogen excreted during a
non-protein period from that of the
protein period. This figure,
subtracted from the total nitrogen
ingested gives the amount of
absorbed nitrogen. To find how
much of the absorbed nitrogen is
wasted in metabolism, the amount of
urinary nitrogen (corrected in the

same manner for the nitrogen of the

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catabolism of the body's own
tissues) is subtracted from the
nitrogen actually absorbed.

This figure divided by the
absorbed nitrogen times 100 is

the biological value of the
protein. The calculation
resolves itself into the following:
formula:

H -(F - F) - (U - U)
D 2 1 2 l

 

X 100 2 Biological Value
n -(F -F)
D 2 1

where, -

P = Fecal N. during non-protein period
F : Fecal N. during protein period

Urinary H. during non-protein period

H
II

U2 2 Urinary :1. during protein period

2
ll

Nitrogen in diet

29.

30.

The biological values show a
wide range among rats even upon the
same diet. The differences in
biological value of the eggs among
the animals were as follows: for the
raw eggs 33.8, the hard boiled 18.9,
the scrambled 15.7, and the soft
cooked 5.8. The metabolism data
for one rat (427 C) on the last
period was omitted because of marked
intestinal disturbance. With the
exception of the raw egg period, the
variations in biological values
between several animals is no greater
than that obtained by Mitchell (15)
for foods at a 10% protein level of
intake. With milk protein he found
a difference of 18, with corn protein
a difference of 23, with cats one of
19, and with potato one of 10. In
the experiments reported here, there

is as great a variation in biological

value of eggs between animals on
the sans diet as there is between
the values obtained for the same
animals on different egg diets.
This would seem to indicate that
the biological value of eggs is
not greatly influenced by the
method of cooking.

31.

32.

TABLE 2.

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cowospfiz
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cooked Egg Ration

         

ncwonpfiz
awcqfihm sawen

Protein-free Ration

 

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'?Eriod 5.

Period 1.

venues Henna

Intestinal Disturbance

 

scenes Heaoaea

 

.oz 96%

 

Metabolic fecal nitrogen

 

1.

2.

3.

S U M M.A.R Y

Artificial digestion experiments
on eggs prepared in different
ways seem to show a slight
variation in the rate of
digestion. In descending order
of speed of digestibility, they
are as follows, - sort cooked,

hard boiled, raw, and scrambled.

Metabolism experiments on rate
show no difference in the
coefficient of digestibility

for the methods of preparation

tastede

Calculation of the biological
values indicates no very marked
difference in the ultimate
assimilation and use of the

different egg diets.

3.

2.

3.
4.
5.

6.

7.

34

BIBLIOGRAPHY

Food Products, 2nd Edition

H. C. Sherman,page 168

Farmers's Bulletin 7128,

U. S. Dep't. Agriculture, page 15.
Compt. rend. soc.biol.Vol.68,page 709.
Compt. rend. soc.biol.Vol.68,page 662.
"Digestibility and Utilization of

Egg Protein." Bateman.

Journal Biological Chemistry. 26,

page 263.

"Digestion in normal Human Stomach

of Eggs Prepared in Different Ways."

Miller, Fowler, Bergeim, hehfuss

& Hawk.

American Journal of Physiology

Vol. 49, page 254.

"Human Digestion Experiments with

Raw White of Egg." Rose & Mac Leod.

Journal of Biological Chemistry 50,

page 83.

8.

9.

10.

11.

12.

35.

"The Digestibility of Unbeaten

in Comparison with Beaten Egg
Whites." Rose and Mac Leod.
Journal of Biological Chemistry,
Vol. 58, p. 369.

Deutsche medizinische Hochenschrift
Vol. 55.p.395 (Reported in Journal
American Medical Association Vol.92
pg. 1719.)

"The Biological Value for
maintenance and Growth of the
Proteins of Whole Wheat, Eggs, and
Pork." Mitchell and Carmen.
Journal Biological Chem.Vol.60

pg. 613.

"Protein Value of Foods in Nutrition."
H. H. Mitchell.

Journal of Home Economics Vol.19,
pg. 122.

"Digestibility of Proteins in Vitro."
Waterman and Jones '

Journal Biological Chemistry 47,p.285.

13.

14.

15.

16.

17.

18.

"Amino Nitrogen Content of Eggs."
van Slyke.

Journal Biol. Chem.9, p.194.
Bulletin #28 - U.S. Dep't.

Agri culture .

"The Biological Value of Proteins
at Different Levels of Intake."
H. R. Mitchell.

Jr. Biol.Chem. 58, p.905.
"A.Method of Determining the
Biological Value of Protein."

H. H. Mitchell.

Jr. Biol. Chem.Vol.58, p.873.

"Chemistry of Food and Nutrition."

3rd Edition. Sherman,page 116.
"Nutritive Value of Proteins."

H. R. Mitchell.

Physiological Review Vol.4.p.424.

36.

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