This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
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 caramelization 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 discoloration 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 condensation 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 -NH1 groups of the protein. If the pH was much above 7 the milk was almost black. Hence, the sugar used, the reaction, and temperature all influenced the development of the brown color. The length of heating in connection with the temperature was important as relatively high temperatures for a short period gave only slight development of the brown color. It is almost impossible to retain the natural color of fresh milk in the condensed milk products, for some brown discoloration occurs in the unsweetened and sweetened product whether made from whole or skim milk.
Whittier and Benton had shown that the hydrogen-ion concentration increases at a rate which is the function of the lactose concentration and the time and temperature of heating. Or, in other words, when milk is heated for a sufficient time at high enough temperatures the lactose is decomposed with formation of acid products. Hence, when milk is heated with sucrose the increasing acidity inverts some of the sucrose to dextrose and levulose, with the development of a brownish color. One example of this is in the cooking of caramels, more brown color developing with long slow cooking of the sucrose and milk. Another instance where this is used to advantage is in making caramel pudding by boiling, in the can, sweetened condensed milk for three hours or longer. The can and contents are chilled. On opening the can it is found that the contents have developed the brown color of caramelized products and are thickened to the consistency of a pudding.
The housewife also makes use of the effect of acid on sweetened condensed milk. If about 1/2 cup of lemon juice is stirred into the contents of a can (about 1 1/2 cups) of sweetened condensed milk, the mixture thickens to a consistency that can be used for a pudding or pie filling and may be thinned with water to a desired consistency. The explanation of the thickening lies in the action of the acid on the complex sugar-protein combination.