Changes in eggs with deterioration. The most important changes
occurring in eggs during deterioration are : ( 1 ) the thick white becomes
less viscous and jelly-like, gradually changing to a thin watery white. (2)
Water passes from the white to the yolk increasing the size and fluid
content of the yolk, thus decreasing the yolk solids. In addition the yolk
membrane weakens and, if the weakening has progressed far enough, breaks
when the shell is opened. (3) Loss of moisture usually occurs. (4) The
egg may absorb foreign or off odors. (5) With continuous loss of carbon
dioxide the alkalinity of the egg increases.
Properties of Egg Proteins
The extensive use of eggs in cookery is made possible by their protein
content. The protein coagulates during heating, thus bringing about
thickening as in custards or the binding of pieces of food together as in
croquettes. The proteins of the egg are good emulsifying agents. The pro-
teins form elastic films when beaten, thus incorporating air, which is used
as leavening in such products as angel cakes and souffles. The elasticity of
the egg protein is also important in products such as popovers where the
egg stretches with expansion of steam, and later coagulates to aid in form-
ing the framework of the popover.
The proteins of the egg. The proteins of the white are ovoalbumin,
ovoglobulin, and ovomucin. There may be small amounts of other proteins
and it is also possible that each protein is made up of component fractions.
Hughes and Scott give the relative proportions of the proteins in the three
portions of the white as shown in Table 41.
The principal protein of the yolk is ovovitellin. Sell, Olsen, and Kremers
ISOELECTRIC POINT OF EGG PROTEINS
331
TABLE 41
Percentage of the Total Nitrogen Contributed by Each of the Three
Protein Fractions {Hughes and Scott)
Outer thin layer
of egg white
Thick layer
of white
Inner layer of
thin white
Ovomucin . .
Ovoglobulin
Ovoalbumin
1-91
3.66
94.43
5.11
5.59
89.18
1.10
9.89
89.29
separated salted egg yolk into a soluble lipoid fraction and an insoluble
residue. The latter consisted of sodium chloride and the protein-like mate-
rial of the yolk. This residue they called lecitho-protein. It composed about
32.5 per cent of the yolk. This protein fraction contained nearly one-half
the total lecithin of the yolk.
Solubility o£ the proteins. The albumin of egg forms a sol with
water and dilute salt solutions. The globulin forms a sol in dilute salt solu-
tions, but not in pure water. The globulin composes about 6.5 per cent of
the total proteins of the egg.
Egg-white proteins belong to the group of hydrophilic colloids. Egg
white and water are mutually soluble. Usually the addition of 1 table-
spoon of water to an egg white, unless it is very watery, increases its ex-
tensibility, and when the egg white is whipped a larger volume is obtained.
But with increasing quantities of water a stage is reached at which the
egg white loses too much of its rigidity and will no longer retain air in
small bubbles, the bubbles being large and floating on the more liquid
part.
The ovovitellin of the egg yolk is combined with phosphorus and be-
longs to the phosphoprotein group. It is insoluble in water but is soluble
in dilute salt solutions and in dilute alkalies.
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Changes in eggs with deterioration. The most important changes
occurring in eggs during deterioration are : ( 1 ) the thick white becomes
less viscous and jelly-like, gradually changing to a thin watery white. (2)
Water passes from the white to the yolk increasing the size and fluid
content of the yolk, thus decreasing the yolk solids. In addition the yolk
membrane weakens and, if the weakening has progressed far enough, breaks
when the shell is opened. (3) Loss of moisture usually occurs. (4) The
egg may absorb foreign or off odors. (5) With continuous loss of carbon
dioxide the alkalinity of the egg increases.
Properties of Egg Proteins
The extensive use of eggs in cookery is made possible by their protein
content. The protein coagulates during heating, thus bringing about
thickening as in custards or the binding of pieces of food together as in
croquettes. The proteins of the egg are good emulsifying agents. The pro-
teins form elastic films when beaten, thus incorporating air, which is used
as leavening in such products as angel cakes and souffles. The elasticity of
the egg protein is also important in products such as popovers where the
egg stretches with expansion of steam, and later coagulates to aid in form-
ing the framework of the popover.
The proteins of the egg. The proteins of the white are ovoalbumin,
ovoglobulin, and ovomucin. There may be small amounts of other proteins
and it is also possible that each protein is made up of component fractions.
Hughes and Scott give the relative proportions of the proteins in the three
portions of the white as shown in Table 41.
The principal protein of the yolk is ovovitellin. Sell, Olsen, and Kremers
ISOELECTRIC POINT OF EGG PROTEINS
331
TABLE 41
Percentage of the Total Nitrogen Contributed by Each of the Three
Protein Fractions {Hughes and Scott)
Outer thin layer
of egg white
Thick layer
of white
Inner layer of
thin white
Ovomucin . .
Ovoglobulin
Ovoalbumin
1-91
3.66
94.43
5.11
5.59
89.18
1.10
9.89
89.29
separated salted egg yolk into a soluble lipoid fraction and an insoluble
residue. The latter consisted of sodium chloride and the protein-like mate-
rial of the yolk. This residue they called lecitho-protein. It composed about
32.5 per cent of the yolk. This protein fraction contained nearly one-half
the total lecithin of the yolk.
Solubility o£ the proteins. The albumin of egg forms a sol with
water and dilute salt solutions. The globulin forms a sol in dilute salt solu-
tions, but not in pure water. The globulin composes about 6.5 per cent of
the total proteins of the egg.
Egg-white proteins belong to the group of hydrophilic colloids. Egg
white and water are mutually soluble. Usually the addition of 1 table-
spoon of water to an egg white, unless it is very watery, increases its ex-
tensibility, and when the egg white is whipped a larger volume is obtained.
But with increasing quantities of water a stage is reached at which the
egg white loses too much of its rigidity and will no longer retain air in
small bubbles, the bubbles being large and floating on the more liquid
part.
The ovovitellin of the egg yolk is combined with phosphorus and be-
longs to the phosphoprotein group. It is insoluble in water but is soluble
in dilute salt solutions and in dilute alkalies.