stances that lower surface tension become concentrated in the liquid/air
interface. Proteins lower the surface tension of aqueous sols, hence accu-
mulate in the surface. When milk is heated in an open pan, a scum or skin
forms over the surface of the milk. At first this skin is rather thin and
mobile but is gradually altered so that it becomes tenacious and tough
enough to be removed with a stirring rod or spoon. This scum has been
said to contain coagulated albumin and globulin. Tinkler and Masters
state that if the scum is removed as formed, the total amount of protein
that can be removed exceeds the total amount of albumin and globulin in
the milk.
When foods are cooked in milk the milk not only foams readily but the
scum tends to hold the steam formed in heating the milk; it is because
of this that the milk ''boils over" so readily.
Sugar Reactions with Proteins o£ Milk
Ramsay, Tracy, and Ruehe investigated the substitution of dextrose for
sucrose in sweetened condensed skim milk. They found the objections to
using dextrose were (1) a brown discoloration, (2) a physical thickening,
and (3) crystallization of the dextrose during storage. The last objection
could be remedied by using 50 per cent dextrose and 50 per cent sucrose.
The progressive thickening during storage at high temperature was caused
by action of the dextrose on the casein and albumin of the milk. During
this investigation they found additional evidence that sugars react with
proteins. When dextrose, lactose, or levulose was heated with skim milk or
freshly precipitated casein, a dark brown color formed In the product.
When the sugars were heated In distilled water solutions to 250°F. for
30 minutes no caramelizatlon occurred. Neither did darkening occur when
albumin or casein was heated in water solution. But when the milk,
albumin, or casein was heated with lactose or dextrose, a brown dis-
coloration occurred. As the temperature was raised the dextrose and
casein became so firmly attached to each other that no amount of washing
could remove the sugar. Most of the biruet action of the skim milk was
lost. The results were explained on the basis that a protein-sugar complex
of glucosidal nature was formed.
On heating amino acids with dextrose highly colored products were
formed, the reaction probably being a condensation of an amino acid with
an aldehyde or ketone group of the sugar. The very stable linkage of the
aldehyde group of the dextrose and the ketone group of the fructose In the
sucrose molecule Is cited to explain the failure of sucrose to form condensa-
tion products with casein, albumin, or amino acids. It was found that as
the reaction became more alkaline the appearance of the brown color was
more rapid. The Increased alkalinity was said to favor the change of the
sugar from a lactone to a free aldehyde form, the free aldehyde acting with
the amino acid or -NHo groups of the protein. If the pH was much
310 MILK AND CHEESE
Page 252
Presented as published in 1932. Historical recipes may not meet modern food-safety standards. Cook from the modern interpretation, not the original instructions.
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stances that lower surface tension become concentrated in the liquid/air
interface. Proteins lower the surface tension of aqueous sols, hence accu-
mulate in the surface. When milk is heated in an open pan, a scum or skin
forms over the surface of the milk. At first this skin is rather thin and
mobile but is gradually altered so that it becomes tenacious and tough
enough to be removed with a stirring rod or spoon. This scum has been
said to contain coagulated albumin and globulin. Tinkler and Masters
state that if the scum is removed as formed, the total amount of protein
that can be removed exceeds the total amount of albumin and globulin in
the milk.
When foods are cooked in milk the milk not only foams readily but the
scum tends to hold the steam formed in heating the milk; it is because
of this that the milk ''boils over" so readily.
Sugar Reactions with Proteins o£ Milk
Ramsay, Tracy, and Ruehe investigated the substitution of dextrose for
sucrose in sweetened condensed skim milk. They found the objections to
using dextrose were (1) a brown discoloration, (2) a physical thickening,
and (3) crystallization of the dextrose during storage. The last objection
could be remedied by using 50 per cent dextrose and 50 per cent sucrose.
The progressive thickening during storage at high temperature was caused
by action of the dextrose on the casein and albumin of the milk. During
this investigation they found additional evidence that sugars react with
proteins. When dextrose, lactose, or levulose was heated with skim milk or
freshly precipitated casein, a dark brown color formed In the product.
When the sugars were heated In distilled water solutions to 250°F. for
30 minutes no caramelizatlon occurred. Neither did darkening occur when
albumin or casein was heated in water solution. But when the milk,
albumin, or casein was heated with lactose or dextrose, a brown dis-
coloration occurred. As the temperature was raised the dextrose and
casein became so firmly attached to each other that no amount of washing
could remove the sugar. Most of the biruet action of the skim milk was
lost. The results were explained on the basis that a protein-sugar complex
of glucosidal nature was formed.
On heating amino acids with dextrose highly colored products were
formed, the reaction probably being a condensation of an amino acid with
an aldehyde or ketone group of the sugar. The very stable linkage of the
aldehyde group of the dextrose and the ketone group of the fructose In the
sucrose molecule Is cited to explain the failure of sucrose to form condensa-
tion products with casein, albumin, or amino acids. It was found that as
the reaction became more alkaline the appearance of the brown color was
more rapid. The Increased alkalinity was said to favor the change of the
sugar from a lactone to a free aldehyde form, the free aldehyde acting with
the amino acid or -NHo groups of the protein. If the pH was much
310 MILK AND CHEESE