736. Physical changes in milk caused by the action of rennet: III. Effects of separation, homogenization and pasteurization and of varying calcium content on the rigidity moduli and viscosities of curd

Abstract

1. Preliminary experiments show that homogenization, separation and pasteurization all tended to delay the onset of rigidity in renneted milk, though it must be borne in mind that the first two of these treatments were accompanied by some heating of the milk.2. Once setting had started, homogenized and separated samples set more quickly, and pasteurized (especially over-heated) milk more slowly, than did raw milk.3. Homogenization would seem to increase, and separation perhaps to decrease the proportion of elastic recovery of the gel which is immediate. The torsiometer, which gives an over-all measure of general consistency, agrees well with the -tube gelometer but when the latter shows an unusual ratio of immediate to slow recovery, the torsiometer follows the former modulus.4. Increasing quantities of CaCl₂ added to reconstituted dried fat-free milk progressively reduce the time needed for rigidity to make its appearance but do not greatly alter the final setting rates. All setting curves approximate to the law that, once rigidity is well established, rate of setting is proportional to the percentage of casein still unset. When the amount of added lactic acid is adjusted so as to prevent the addition of various quantities of CaCl₂ from altering the pH appreciably, no great difference is found in the setting curves.5. The relaxation times, retardation times and their ratio, called ‘the springiness number’, are not greatly affected by the treatments to which the milk samples, either whole or dried, were subjected.6. Appreciably syneresis usually occurs in the -tube gelometer only when the curd has been strained, though very small strains are sufficient to produce it. The site of its occurrence depends on the hydrodynamic distribution of the larger fat globules. In fat-free or homogenized milk, such syneretic phenomena are either absent or very slight.