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PHYSICS OF THE HEN'S EGG

II. THE BURSTING STRENGTH OF THE VITELLINE MEMBRANE

BY T . MORAN.

(Received May 18, 1935.)

(With Three Text-figures.)

THE strength of this membrane is an important factor in the commercial storage of eggs, but it is also of fundamental interest as an index to the complex changes, chemical, physical and bacteriological, taking place in the egg. This paper describes a method for measuring the bursting strength together with a number of observa-tions on the effects of different storage condiobserva-tions.

EXPERIMENTAL.

It is almost certain that the strength of the membrane in an egg varies with the physiological factors of breed and diet of the hen. In an attempt to make this factor as constant as possible eggs from one farm and one breed of hen (White Leghorn) were used. Further, in order to counter obvious criticism, it was considered desir-able to work with the intact membrane rather than with pieces of membrane.

The method chosen was a hydrostatic one and the apparatus is shown diagram-matically in Fig. 1. The yolk, freed from white, is placed in an isotonic solution of sucrose with the surface remote from the germinal disc uppermost. The tube A (open at both ends) is then placed on the yolk and gradually lowered into the solu-tion. In some of the experiments tube A was attached to a rachet; in others tube A was fixed and the sugar solution raised by means of a worm-gear. On top of the yolk, inside tube A, is placed a little sucrose solution to keep the exposed surface of the yolk moist. Tube A is lowered at a constant rate, approximately 1 cm. per min., and measurements made of the heights shown in Fig. 1. If 7\ and Tt are the tensions

in the yolk membrane at the highest and lowest points, rx and r% are the radii of

curvature at these points and p, and p, are the densities of yolk and sugar respectively then it can readily be shown that the following equation should apply:

2 ^ 2 ^ (1).

The radius of the tube A used in most of these experiments was 4-175 mm. (= r1),

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T T

counts, as a first approximation, * can be neglected in comparison with -1. Equation (i) therefore reduces to

The density of the sugar solution was i -037 and that of the yolk varied from 1 -023 to 1-030. As a further approximation therefore equation (2) can be written

Fig. 1.

As tube A is lowered a point is eventually reached when the membrane bursts; this always occurs at the topmost point of the yolk in tube A. The values of hx, Aj and

hs at this point, when substituted in equation (2), give the bursting strength of the

[image:2.451.59.412.58.518.2]

membrane.

Table I shows a selection from a typical set of readings. With one batch of eggs the values for the bursting strength in a sample of six eggs varied from 4310 to

1 This equation applies rigidly to the simple case when, instead of using the intact membrane, a

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Physics of the Hen's Egg 43

[image:3.451.85.370.116.272.2]

4870 dynes/cm., with an average of 4450. This agreement, however, was particu-larly good and the range of variation in subsequent experiments was greater.

Table I. Radius of tube A = 4-175 mm.

A,

cm. 4'5 6-45

8-15

10-05 12-55 i4-&5 16-65 17-8 19-95 22-15 24-65 25-5

A,

cm.

3-i 3'2

3 3 3'3 3-35 3-4 3-25 3'4 3-45 3-4 3 45 Burst

h,

cm. o-8 o-8 0-85 0-85

0 8 5

0-85 o-8 0-85 o-8 o-8 o-8

d

cm.

o - i 0 - 2

0-25

0 - 3

o'35

0-4

0-425 o-45

0-5

0-525 o-6

A few experiments were also carried out at the same time with a wider tube (radius 8 mm.) and the values for the bursting strength varied from 4350 to 5300 dynes/cm, (four experiments), with an average of 4940. The average value for the bursting strength of these eggs (laid in June) was 4640 dynes/cm.1

As far as one could judge the values of Z^, Aj and A, in (say) Table I were " equili-brium values ", since no alteration was observed in the values when at a given point the system was left undisturbed for 2 or 3 hours.

ELASTICITY OF MEMBRANE.

At a fixed tension in the membrane the height of the yolk in tube A (d) was greater with stale eggs than with fresh eggs: apparently the membrane becomes more elastic with age.

If at a given tension it is assumed that the same area of membrane contributes to the total area inside tube A then a relative measure of this elasticity can be calcu-lated. Thus, if for two eggs X and Y, the heights of the yolk in tube A are d1 and d^

respectively, then:

Elasticity of membrane X _ zwr (d1 — r) + 2TTT% _ dx

Elasticity of membrane Y imr (dt — r) + ZTTT1 dt (4).

STORAGE AT INCUBATION TEMPERATURE.

Two batches of eggs, one fertile and the other infertile, were incubated at 390 C. and the bursting strength of the membranes tested at intervals. The pH of the white was measured at the same time. The results are shown in Fig. 2. These eggs were laid on October 15th and the initial range of variation was great, the actual values being as shown in the table on p. 44.

1

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44

Fertile eggs (dynes/cm.)

3115 345° 3§SO 4 " 5 425° 4660

Infertile eggs (dyne«/cm.)

3260 33io 4O3S 4090 4640 4960

5000

4000

3000

2000

1000

\ \ .

o ~ \

k\\

y

1

i

a

J

t

r i i

at

X

"***• x

• " • i N F t R T I L t EGOS

\

-8-5

8-7

8-9

9-1

9-3

9-5

40 80 120 160 200 240

Time in hours Fig. 2.

280 320 360 400

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Physics of the Hen's Egg 45

In the case of the fertile eggs it was impossible to measure the strength of the membrane after about 40 hours' incubation as in every case the membrane broke in the region of the germinal disc. Infertile eggs could be tested up to about 15 days' incubation by which time the bursting strength had fallen to less than 1000 dynes/cm. As will be observed the results are very irregular but they can be represented roughly by the curve shown, the equation of which is given by

— * l°gio- (5),

120 160 200

Time in hours

Fig-

3-320

where s is the strength of the membrane in dynes/cm. (s0 initial value; sL value after

time t), t is time in hours, k is a constant equal to 2-27 x io"3.

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of the height of the yolk to its diameter when it is resting on a flat surface. This ratio decreases with time, but satisfactory values can be obtained by taking all measure-ments after a fixed interval, in these experimeasure-ments 5 minutes. The yolk index could not be measured after 140 hours' incubation.

STORAGE AT o°C.

Eggs were stored at o° C. and 80 per cent, relative humidity (within 24 hours of laying) in air and in air enriched with carbon dioxide; a number of eggs were also coated with vaseline and others stored in water glass at the same temperature.

Table II gives the average bursting strengths of samples of four or five eggs after 3, 6 and 8 months' storage. The eggs stored at o° C. for 3 months were stored in air at laboratory temperatures for 48 hours before testing, the 6 and 8 months' eggs for 4 days at laboratory temperatures.1 Also included in the table are (1) the pH values of the white immediately on bringing the eggs out of store, (2) the values of the yolk index after 2 or 4 days at laboratory temperatures, (3) a few values of the water content of the yolks, (4) the pH values of the yolk after 8 months' storage.

[image:6.451.44.413.307.500.2]

The average bursting strength prior to storage was 4640 dynes/cm.

Table II. Time of 8torn(zc months 3 4 6 8 Mcflsurcmcnt

AT X ^ O O V41 V-'l l l v l l V

Bursting strength pH of white Bursting strength

Yolk index Water content, yolk

pH, white Bursting strength Yolk index pH, white pH, yolk Air 4250 8-45 3930 0-40

4 9 3

8-6

3210 0-42

8-6 6-4

Composition of atmosphere: air

5 % CO,

3900

7-45

3420 o-43

4 9 5 7-5 3210 0-46 7-5 6-o enriched with 10 % CO, 3360 7-15 34io 0-47 48-7 7-3 3200 c-45 7-5 6-2

2 O %

CO,

3480

7-1 3 1 1 0

0-47 49-0 7-2 3090 0-46 7-4 6 0 IOO % CO, 3815 6'2 3660 3400 0-47 50-4 6-6 — — — — Vase-lined eggs 3900 8 0 34io 0-38

4 9 5 8-5 2700 o-43 8-4 6-5 Water ffl&SS eggs 3S8o 8-9 2900 0-39 50-7 8-6 — — — — DISCUSSION.

As a round figure the average bursting strength of the vitelline membrane of the fresh eggs examined was 4500 dynes/cm. On storage, under all the conditions in-vestigated the membrane weakens. Excluding the sealed eggs (vaselined and water glass) in which anaerobic spoilage may play a part, there is an indication that at o° C. and up to about 6 months' storage, weakening of the membrane appears most marked when the pH of the white is approximately 7-7-2. The sampling problem is however a difficult one as Fig. 2 shows and more extensive experiments are called for.

1

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Physics of the Hen's Egg 47

Balls and Swenson (1) have shown that there is a proteinase in thick white of egg which is identical with tryptic proteinase. They claim that it is this enzyme which breaks down or hydrolyses the mucin in thick egg white and which is also responsible for the weakening of the yolk membrane. Thin white on the other hand is not only inactive but contains a substance inhibitory to proteolyses.

Ordinary trypsin rapidly weakens the vitelline membrane. Thus in one experi-ment a number of yolks were placed at 33° C. in an isotonic sugar solution (approxi-mate) buffered to pH 9 and containing 0-25 per cent. B.D.H. trypsin; the bursting strengths after different periods in this solution and in a control solution without trypsin are given in the following table:

Time in hours Bursting strength

(with trypsin) Bursting strength

(control)

0 4000 4000

1 1910

at

980

3170

4

445 dynes/cm.

4°io ,.

The optimum pH for the action of tryptic proteinase on mucin and keratin is not known accurately but a value of 7-7-2 is at least a reasonable one. Experiments however are also required to indicate how far proteolytic bacteria in the egg are responsible for the weakening of the yolk membrane although the pH values of the yolks even after 8 months' storage are not favourable for the formation of proteo-lytic enzymes.

It will be observed that whereas in storage in ordinary air a weak yolk membrane is associated with a small yolk index and reduced viscosity of the yolk substance, in storage in atmospheres of carbon dioxide a comparatively weak membrane may be associated with a high yolk index. The two criteria therefore are independent, but it is clear that other things being equal a yolk whose yolk index has decreased will be more liable to break because of the stretching of the membrane due to the greater surface area of the yolk.

The fact that the bursting strength of fertile eggs could not be measured after about 40 hours suggests that the membrane is weakened by an enzyme secreted by the developing embryo. This may possibly have some evolutionary connection with the hatching enzymes of lower vertebrates and invertebrates (3).

SUMMARY.

The average bursting strength of the vitelline membrane in new-laid eggs is approximately 4500 dynes/cm. On storage the membrane weakens but becomes more elastic.

The hatching enzyme in the hen's egg appears after about 40 hours' incubation.

My thanks are due to Mr C. Coulson, Fellow of Trinity College, Cambridge, for his advice on the statics of the experimental method.

REFERENCES.

Figure

Table I shows a selection from a typical set of readings. With one batch of eggsthe values for the bursting strength in a sample of six eggs varied from 4310 to
Table I. Radius of tube A = 4-175 mm.
Table II.Composition of atmosphere: airenriched with

References

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