Rogers and Palmer both state that, in the evaporated-milk industry, the forewarming of milk prior to processing increases its stability to heat. "Rapid improvement in resistance to heat coagulation results in increase in temperature for prewarming up to 90° to 100°C. Above 90°C. the change is very small, but in some cases can be effected with increases in temperature up to 120°C. for 10-minute periods of forewarming. When time is chosen as the variable, improvement may be noted with increases in the time up to 30 minutes at a temperature of 95°C. At higher temperatures the same improvement may be effected in shorter periods of time."

Fat. Rogers states that fat particles in relatively large aggregates may act as nuclei about which coagulation of the casein can proceed. In un-homogenized milk the fat affects the coagulation time and temperature but slightly. But when a milk of higher fat content is homogenized the fat clumps may act as nuclei about which the casein may gather during heating. With increase in homogenization pressure as well as fat content, other conditions being the same, a marked decrease in stability to heat is noted. In homogenized milk it was found that the maximum stability to heat coagulation occurs if homogenization is carried out at 80°.

Rogers adds that the feathering of some homogenized cream when added to coffee may be caused by using too high homogenization pressure, thus reducing the stability of the cream to heat.

The role of salts in heat coagulation of milk. In heat coagulation of milk, the milk salts play an important role, for the salt equilibrium is altered by heat. When milk is boiled precipitation of part of the calcium phosphate occurs. Sommer and Hart have concluded that salts are the main factor in heat coagulation of fresh milk. Electrolytes have a marked effect upon the stability of colloids. In precipitating a hydrophilic colloid divalent and trivalent ions are generally more effective than monovalent ones. In the milk are found the monovalent cations, sodium and potassium; the monovalent anion, chlorine; the divalent cations, calcium and magnesium; and the trivalent anions, phosphate and citrate. Sommer and Hart concluded that the coagulation of milk on heating may be due to an excess or a deficiency of calcium and magnesium. They explain this as follows. "The casein of the milk is most stable with regard to heat when it is in combination with the calcium. If the calcium combined with the casein is above or below this optimum, the casein is not in its most stable condition. The calcium of the milk distributes itself between the casein, citrates, and phosphates chiefly. If the milk is high in citrate and phosphate content, more calcium is necessary in order that the casein may retain its optimum calcium content after competing with the citrates and phosphates. If the milk is high in calcium there may not be sufficient citrates and phosphates to compete with the casein to lower its calcium content to the optimum. In such cases the addition of citrates or phosphates makes the casein more stable by reducing its calcium content. The magnesium functions by replacing the calcium in the citrates and phosphates."

Heat coagulation of casein endothermic. Leighton and Mudge have shown that an endothermic reaction accompanies the appearance of visible curds when milk is coagulated by heat. This is accompanied by precipitation of calcium and magnesium as phosphate and citrates.

A similar reaction occurs in custard. In cooking custard, the ingredients of which are milk, egg, and sugar, the temperature drops or does not rise for a period of time during coagulation or setting of the custard, a condition particularly noticeable just before curdling takes place.

Coagulation of milk by cooking meat or vegetables in it. Fresh milk is seldom coagulated by heating for home use. The temperature attained in ordinary heating is not great enough to cause coagulation, nor is the milk heated for the long period required for coagulation at boiling temperatures. But with the addition of other foods to milk in food preparation, coagulation often occurs with a very short period of heating. One of the factors in this coagulation is undoubtedly the salt content of the food added to the milk as well as the salt content of the milk. The balance of the milk salts for greatest stability may be upset and coagulation occurs when the food is heated in the milk.

Cooking of meat in milk. Ham is often baked in milk. Sometimes pork chops are floured, seared in fat, and then baked in milk. Other meats and fish are sometimes baked or cooked in milk. Often curdling of the milk occurs, and the appearance of the meat, owing to adherence of curds of milk, is not attractive. Thus it is desirable to prevent curdling. Among the causes of curdling are the temperature at which the meat is cooked, the salt content of the food cooked in the milk, the manner in which the milk is added to the meat, and the reaction of the milk.

The higher the temperature at which the meat is cooked the greater the tendency to curdle. Larger curds are also formed at higher temperatures. The temperature to which milk must be heated to bring about curdling is high, so this cooking temperature alone is not sufficient to bring about the coagulation. Even when the meat is cooked in an oven at a high temperature, the liquid portion does not reach a higher temperature than boiling. It is possible that the acidity is increased during cooking, but the resulting pH of the meat-milk broth is changed very little from that of the original milk or from that of the meat broth when the meat is cooked in water. Thus it seems that the heating and the acidity developed during cooking do not alone bring about coagulation.

The salt content of the food cooked in the milk probably influences the coagulation, and this combined with the heating, the temperature of heating, the acidity developed, and the altering of the casein by heat are sufficient to cause curdling. Sodium chloride has some effect, for curdling is more likely to occur when the meat cooked in the milk is a cured or salted one than when fresh meat is used.

Curdling may be prevented by the addition of soda, about 1/16 teaspoon per cup of milk. From this it appears that the reaction has some part in the coagulation. The soda may combine with other salts that tend to bring about coagulation or the coagulation may be prevented by the slightly alkaline reaction. A slightly alkaline reaction also prevents coagulation by rennin and by fermentation.

If a portion of the milk is added to the meat when cooking is first started and the rest of the cold milk added gradually to the meat during the cooking period, curdling is less likely to occur. The addition of acid foods, such as prepared mustard, which may contain vinegar, or apples and pears to be baked with the meat, would tend to increase the tendency to curdle. Evaporated milk has less tendency to curdle than fresh milk, which may be due to the previous heating.

Cooking of vegetables in milk. Milk usually does not curdle when cabbage, chard, spinach, or cauliflower is cooked in it. But it is likely to curdle when asparagus, string beans, peas, and carrots are cooked in it. Asparagus usually curdles the milk after a few minutes of cooking. There are several factors that may aid in bringing about coagulation of the milk. The slight acidity of some vegetables combined with the heating of the milk may tend to bring about coagulation, but the acidity is not usually great enough, nor the boiling temperature high enough, nor the boiling long enough continued for these factors to be very important. The salt and the tannin contents of the vegetables are probably the principal causes of coagulation. Some vegetables contain larger amounts of tannin than others. Tannin is a dehydrating agent and brings about denaturation of hydrophilic sols, like gelatin, starch, and agar-agar. After denaturation the hydrophile is sensitive to small amounts of electrolytes and precipitation occurs readily. Kruyt states that tannins do not bring about dehydration of the protein in an alkaline medium. Hence the addition of soda in small amounts to the milk in which the vegetable is cooked prevents coagulation of the milk. Tannins lower the surface tension, which results in foaming of vegetables containing tannin when they are cooked in water. It is rather interesting that the vegetables that usually foam the most when cooked in water are the ones that have the greatest tendency to coagulate milk.

Tomato soup. When tomatoes are combined with milk to make cream of tomato soup, coagulation may occur. The acidity of tomatoes varies somewhat, but is about pH 4.4 to 4.6. If the amount of tomato added to the milk is great enough to lower the pH of the mixed milk and tomato to 4.8 to 4.6, the casein is precipitated without heating. This may happen if the milk is already fairly acid.

Since a longer time of heating milk increases the tendency to curdle it is preferable to heat tomato soup for only a short time. Heating slowly also increases the tendency to curdle, which may be due to the longer time required. In Experiment 55 in the laboratory outline several different methods of combining the tomato juice and milk are given. The tomato is usually added to the milk by stirring, for in this way the milk is diluted with a smaller amount of acid substance during the first part of the mixing. There is less tendency to curdle when the hot tomato is added to cold milk, than when the cold tomato is added to hot milk. Probably the slight denaturation brought about by heating the milk may partially account for this. Occasionally some milk is acid enough and the tomato is acid enough to cause curdling with all methods of combining unless soda is added. At other times curdling does not occur with any method of combining as outlined in this experiment.

Fruits and milk. When cream is added to fruit, clotting often occurs. This is usually due to the acidity of the fruit, but may also be due to an enzyme in it. Raw pineapple contains an enzyme, bromelin, that brings about clotting of milk. However, the pineapple juice not only brings about clotting but also peptization, for after a time the clot formed is less firm and the flavor is similar to that of peptized meat.