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.

A method for measuring the increasing rigidity moduli and internal viscosities of milk during rennet setting is described. Curves are given showing the degree of reproducibility on replicate samples of dried and liquid milks, and of the effects of changes in calcium levels in the case of the former.

A tentative interpretation of the shape of the main part of the setting curve is proposed. The upper part of the curve generally becomes erratic with the onset of syneresis, but some dried milks do not show this behaviour.

Some curious phenomena concerning the production of syneretic liquid (whey) are described and methods for obviating the disturbing effects of syneresis are discussed.

1. The sensitivity to calcium of the caseinate-phosphate complex of milk heated at 120°C., increases to an early maximum and thereafter steadily declines, apparently in consequence of parallel changes in the sensitivity of its caseinate constituent.

2. A simultaneously developed capacity of the heated complex to bind additional colloidal phosphate appears to be unrelated to these changes in casein sensitivity.

3. Colloidal phosphate content is confirmed as a factor in the heat coagulation of milk.

4. Heat-developed acidity contributes to the heat coagulation of milk primarily by increasing hydrogen-ion concentration and only slightly, if at all, through release of calcium ions from insoluble combinations.

5. Neither serum proteins nor protein-lactose combinations play any significant part in the heat coagulation of milk.

6. The bearing of these results on the heat coagulation of milk in general and on the operation of certain high-temperature stabilization treatments is discussed.

The rennet-catalysed coagulation of milk casein has been examined calorimetrically. The heat of coagulation and syneresis was found to be less than 0·1 cal.'40 ml. of milk, the limiting sensitivity of the apparatus.

Heats of reaction approaching 0·5 cal. were observed when the milk was acted upon by the enzymes trypsin and papain. It is doubtful whether, at this stage, the results can be correlated with the several phases of the reaction.

Average values for the total phosphorus content of sera from unsalted butter samples and for the phosphorus contents of the various fractions of the sera after partition with trichloroacetic acid were: total P, 83·6 mg./100g.; acid-soluble P, 32·4 mg./100 g.; inorganic P, 28·8 mg/100 g.; acid-insoluble P, 51·2 mg./100 g.; and phospholipid p, 33·6 mg./100 g.

There was no evidence that decomposition of the phospholipids or other organic phosphorus compounds occurred in salted or unsalted sweet-cream butters during storage at –10° C. for 8 months.

1. Four batches of soft (Karish) cheese were made from raw and from heat-treated milk containing 1·5% fat and 7% added salt. The cheeses were sealed in tins in contact with their own whey and examined after storage for 1 month and 2 months at room temperature and at 4—6°C.

2. Cheese stored at room temperature lost weight throughout the 2 months, but cheese stored at the low temperature gained weight after 1 month and lost weight after 2 months. Parallel changes occurred in the moisture content and salt content of the cheese.

3. The fat-in-dry-matter content of the cheese increased with room temperature storage, but at 4—6°C. it decreased after 1 month and increased after 2 months.

4. Pasteurization of the milk increased the yield of cheese and the flavour, body and texture were better than that of raw milk cheese stored at the same temperature. The effect of low-temperature storage was, however, greater than that of pasteurization.

5. The total nitrogen content of the cheese decreased throughout storage, particularly at room temperature. The decrease was smaller with pasteurized milk cheese which ripened more slowly. The total nitrogen content of the whey increased throughout storage at room temperature, but at 4—6°C. it increased after one month and decreased slightly after 2 months. The increases were greatest at room temperature and in wheys from raw milk cheese.

6. The initial acidities and also the subsequent rates of increase were higher in wheys from raw milk cheese.

7. The results are discussed in detail.

A viscometric study of solutions of alkaline (sodium and potassium) caseinate was made to determine the effects of renneting with and without the addition of calcium ions. There was no real viscometric effect of these treatments at high dilutions. At a higher temperature, the viscosity increment of the solutions decreased. It was suggested that particle aggregation occurs more readily at higher temperatures, leading to a greater particle symmetry as well as causing some dehydration.

Fluorescence microscopy was applied to the examination of casein in milk, curd and cheese, to check or support the picture obtained by electron microscopy. The best fluorochroming effect was shown by thioflavine T, acridine orange, coriphosphine and basic fuchsin.

In milk, larger casein particles (0·1–0·8 μ in diameter) were rendered visible as minute nearly spherical corpuscles in lively Brownian movement. The sediment obtained on centrifuging the whole milk at about 900 g contained predominantly large particles (average diameter 0·;8 μ). On acid or rennet coagulation the casein particles aggregated to flocs, which finally formed a three-dimensional framework.

From curd or cheese freezing-microtome sections (10 μ thick) were examined. The curds which did not possess the fibrous (‘chicken-breast’) texture, characteristic for wellcheddared curd, under the fluorescence microscope displayed a granular structure, whereas curds with a pronounced fibrous texture showed a network of thin micro-fibres, particularly in sections cut along the direction of macro-fibres.

The sections (10 μ or less thick) of curd and cheese on the surface of water slowly spread out. The spreading was accompanied either by conversion of the form of casein from granular into fibrous, or by stretching of the micro-fibres in the already existing network.

The grain boundaries in curd and cheese (Cheddar, Emmental, Edam) were almost free of fat. No grain boundaries existed in process cheese.

It has been shown that the degree of homogenization can be tested directly by measuring the transmission of milk samples of thickness ca. 0·1 mm., using wavelengths in the near infra-red region.

The purpose of this paper is to show how the performance of a continuous heat-treatment plant may be predicted from the temperature distribution and the flow time distributions obtained as described in the first paper of this series (1), using thermal death-time characteristics appropriate to the micro-organisms of greatest importance. The calculations are carried out using the data obtained for the plant described in the previous paper, and they provide an estimate of the bactericidal effectiveness to be expected from that plant.

The previous papers of this series (1,2) have shown how the bactericidal efficiency of a flow-type heat exchanger may be predicted from physical measurements, and have given the necessary calculations for a small plate-type ultra-high-temperature sterilizing plant made by the A.P.V. Co. The purpose of this paper is to show the degree of agreement between practice and prediction from theory.