Experimental cookery

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 con- sequence 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 /)H 4.6. If enough acid is added to lower the /)H 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 fermenta- tion is checked at />H 4.8 to 5.0, the bacteria consists chiefly of Strepto- coccus lactis. The rate of fermentation depends chiefly upon the temperature at which the milk is held. At low temperatures, on account of retardation of bac- terial action, it takes place slowly. Rogers states that fermented milk, allow^ed to stand at a fairly high temperature, undergoes a second lactic acid fermentation brought about by the Lactobacillus bulgaricus organ- isms. Some of these types of bacteria form a high percentage of acid and the hydrogen-ion concentration may reach /)H 3.23. Changes occurring during acid precipitation. During fermentation chem- ical 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 phos- phate 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 fermenta- tion is often called clabbered milk. Its flavor and aroma may vary, depend- ing 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 HEAT COAGULATION 305 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.