This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Hydrogen-ion concentration. The reaction of the milk affects the rapidity of coagulation and the character of the curd formed. Ordinarily when the reaction of the milk is alkaline coagulation does not occur. This is shown by the addition of a small amount of soda to milk before the addition of junket. The optimum hydrogen-ion concentration for rennin activity has been reported to lie in the zone between pH 5.99 and 6.40.
Character of cations. In addition to rennin, cations are necessary to bring about coagulation of milk. Because casein and calcium are so closely involved in milk, the cation calcium is important in bringing about coagulation. Hence, Rogers states that it is to be expected that the concentration of both casein and calcium markedly affect both the rate of coagulation and the character of the clot. If milk is diluted with sufficient water, clotting is both delayed and incomplete, the clot being soft. If calcium chloride is added to the water, diluted milk clotting properties are restored, which suggests that the concentration of calcium ions is more important than that of the casein ions.
Rogers states that any metallic ion can replace the calcium in coagulation. However, it is generally accepted that the sodium and potassium salts of paracasein are soluble. Monovalent ions are less effective than divalent ones in replacing the calcium. Rogers reports that all monovalent ions did not bring about coagulation in some instances. The divalent ions were not all equally effective, calcium and barium being more efficient than magnesium.
Sugar. Sugar tends to prevent the coagulation of milk by rennin.
Coagulation of milk by acid. Kruyt states that there are some proteins that are not sufficiently hydrated to be stable by hydration alone. He cites casein as an example of a protein "which can exist either in acid or in an alkaline solution, but does not dissolve in water, with the consequence that the sol ordinarily flocculates when neutralized." Either the acid produced during fermentation or acids added to milk precipitate the casein. The casein is least soluble at its isoelectric point pH 4.6. If enough acid is added to lower the pH below 4.6, casein salts, such as casein chloride or casein lactate, are formed. If these salts are soluble, the casein goes into solution. Hence the largest yield of precipitated casein is near the isoelectric point.
Fermentation of milk. Fermentation, or the production of lactic acid from lactose by bacteria, takes place in milk that is allowed to stand under favorable conditions. Rogers states that true lactic acid fermentation is brought about by the Streptococcus lactis and certain other organisms, lactic acid being the principal end-product, other products being present in only small amounts. In mixed lactic acid fermentation, or when other organisms in addition to S. lactis are present, the end-products may include acetic, propionic, lactic, succinic, formic, and butyric acids, carbon dioxide, hydrogen, acetone, and ethanol. As fermentation increases, an acidity is reached at which the action of most bacteria is suppressed. When fermentation is checked at pH 4.8 to 5.0, the bacteria consists chiefly of Streptococcus lactis.
The rate of fermentation depends chiefly upon the temperature at which the milk is held. At low temperatures, on account of retardation of bacterial action, it takes place slowly. Rogers states that fermented milk, allowed to stand at a fairly high temperature, undergoes a second lactic acid fermentation brought about by the Lactobacillus bulgaricus organisms. Some of these types of bacteria form a high percentage of acid and the hydrogen-ion concentration may reach pH 3.23.
Changes occurring during acid precipitation. During fermentation chemical and physical changes occur in the milk. The flavor becomes acid. The calcium caseinate is changed to casein. During this process calcium is split off and forms soluble calcium lactate. In addition some dicalcium phosphate is converted into monocalcium phosphate. Curdling or clotting occurs when the acidity reaches about pH 5.3. During the clotting process the hydrogen-ion concentration does not increase. Milk clotted by fermentation is often called clabbered milk. Its flavor and aroma may vary, depending upon the types of bacteria producing the fermentation. Fermented milk may be used for drinking, for cooking, and for cottage cheese.
Cheese, such as cottage cheese, when clotted by acid coagulation, loses a large proportion of its calcium. The calcium salts become soluble more rapidly than the phosphorus; hence a larger proportion of the calcium than of the phosphorus is lost in the whey. Casein precipitated by rennin retains most of its insoluble salts, hence has a larger proportion of calcium than the acid precipitated casein.
Heat Coagulation. The term heat coagulation refers to the so-called "denaturation" of the protein, by which it is rendered insoluble.
Lactalbumin. Lactalbumin has temperatures for heat coagulation similar to that of egg albumin. The lactalbumin forms a flocculent precipitate, whereas egg albumin forms a firm coagulum. Rupp has reported the following amount of lactalbumin coagulated when heated for 30 minutes.
Albumin rendered insoluble, per cent
Casein. Casein is not coagulated by heat at ordinary temperatures or when heated for short periods, though the heating may alter the casein. Rogers states that it is necessary to heat milk about 12 hours at 100°C. to bring about coagulation. It takes approximately 1 hour at 135°C. and approximately 3 minutes at 155°C. The time and temperature vary somewhat with different milks.
The rate of coagulation depends upon the concentration of the casein as well as the time and the temperature of heating. Rogers states that evaporated milk, containing twice the concentration of solids-not-fat in normal milk, and thus a higher concentration of casein, requires about 60 minutes for coagulation at 114.5°C, 10 minutes at 131 °C, and 7500 minutes at 80°C.