I. Technique of Milk Examination. II. Saprophytic Flora of Milk and Milk Products. III. Pasteurisation of Milk. IV. Butter. V. Cheese

THE keeping quality of milk may be defined as a measure of the period during which chemical changes due to the action of micro-organisms remain insufficient to be unpleasant to the sense of taste and/or smell. It is important to know to what extent the practice of hygienic methods, resulting in the exclusion of micro-organisms, affects the rate of deterioration of milk. It may also be asked whether our laboratory methods of growing and counting bacteria measure, in fact, those organisms which cause deterioration in milk. It is also desirable to know whether milk gains in keeping quality as bacteria are excluded to the point when numbers become minimal. A test for keeping quality aims at a measure of the gross effects of the bacterial population at a given temperature, and is therefore likely to prove of assistance in solving such problems. Hoy(i) devised a routine method of estimating the keeping quality of milk. The milk was tasted and simultaneously its heat stability at the boiling-point of water measured.The writer has investigated several aspects of this test and work has also been done in America. Thus in 1927 Barkworth et al. (2) showed that coliform organisms, apart from the total bacterial content, had a deleterious effect on the keeping quality, and Hoy and Newland(3) have indicated that this is equally true of pasteurised milk. In America, Robertson and Frayer(4) made a statistical estimate of the relationship between bacterial content and keeping quality. They came to the conclusion that the plate count (as compared with the methylene blue reductase test, microscopic count and clump microscopic count) was the best interpreter of probable keeping quality in low-count milk. They also tested keeping quality at temperatures other than 60° F.,' and stressed the importance of using the logarithm of the plate count when dealing with relationships. Their work, however, makes no allowance for the presence of coliform organisms. Barkworth (5) has stated that the fat content has no appreciable effect on the keeping quality, but the findings of Thomas and Jones (6) do not agree with this. Barkworth (7) has also shown that the average keeping quality of samples of afternoon milk is 9 hours less than that of samples of morning milk of similar plate count. Samples of morning milk are

Section: Regression Valuesmentioning

confidence: 99%

91. A Statistical Examination of the Interrelationship and Variability of Plate Count, Presumptive Coliform Content and Keeping Quality of Raw Milk

Barkworth¹

1935

“…Heat treatment during reconstitution may render protein insoluble at temperatures above 60° C, so that, for the determination of solubility, temperatures above this should not be used. The work on the general chemistry of the processing of milk products has been dealt with in the preceding sections and the pasteurisation of milk has been fully dealt with in a previous review (199). Lampitt and Bushill (197) have observed that organic solvents extract almost all the fat of roller-dried milk but only from 3 to 14 per cent, of that of spray-dried milk, which is increased to 83 per cent, by grinding.…”

Section: Physical Chemistry Of Milkmentioning

confidence: 99%

Section C. Dairy Chemistry

Davies

1933

THE progress of dairy chemistry during the last two years will be reviewed under the following broad subdivisions of the subject: I, Composition of milk; II, Chemistry of the constituents; III, Physical chemistry; IV, Chemistry and technology of milk products; V, Processing; and VI, The analysis of milk and its products. Some aspects of the variation in the composition of milk have already been reviewed (Mackintosh (1)), and a fuller discussion will follow in a later review on "The Physiology of Dairy Cattle." The chemistry of the vitamins will also be dealt with in a later review on "The Nutritional Value of Milk.

Section B. Bacteriology and mycology applied to dairying

Hiscox¹,

Rowlands²

1947

Self Cite

Reviews of the progress of dairy science 637.0:576.8 SECTION B. BACTERIOLOGY AND MYCOLOGY APPLIED TO DAIRYING PAGE Thom6 (39) has studied the change in flora during the incubation of samples in the methyleneblue test. The proportion of the different types, which consisted mainly of streptococci, micrococci and coliform organisms, changed during incubation. Initially the flora of different samples consisted of 7-100% streptococci, 0-95% micrococci and 0-6% coliform organisms; at the time of reduction the proportions were respectively 20-88, 9 r 65 and 0-24%. The reducing ability of pure cultures of representatives of the three groups, in terms of the time taken for an inoculum of 1 million organisms per ml. to reduce methylene blue in 'sterile' milk, was approximately the same, though at the time of reduction counts were higher for streptococci and coliform organisms than for micrococci. With mixed cultures, the rates of increase of each type were changed with varying proportions of inoculum, although the time for complete reduction was not materially affected. Morris (40) has observed that proliferation of psychrophilic bacteria of the genus Pseudomonas, derived from the water supplies used on the producing farms, may lead to a shortening of the reduction time after overnight storage of milk samples in a refrigerator. Reviews of the progress of dairy scienceMany papers dealing with different aspects of the resazurin test have appeared during the period under review. Resazurin has been synthesized (4J) and standards have been suggested for preparations intended for milk testing; these standards have been adopted for resazurin used for official milk tests in this country (42). According to Johns (43) resazurin tablets approved for use in North America are approximately 60% stronger than those used in this country. The use of the stronger solution made colour differentiation easier, especially when dealing with cream, colostrum, and milks tending to give a grey tinge when mixed with the dye, but it delayed by about 30 mm, the time taken to reach the pink stage, although there was no difference in the time taken to reach the Munsell P7/4 colour standard (about half-way between the initial blue and the full pink stage). Thomas(44), however, found that doubling the concentration of resazurin only slightly delayed the time taken to reach Lovibond disk 4, 2 or 0.The interpretation of results obtained by workers in different countries is complicated by the use of different colour standards for the intermediate stages of reduction. Golding & Jorgensen (45) have compared the colour standards of a Munsell colour grader with those of the Lovibond disk; the average disk values were 6 for Munsell A, 4-95 for B, 3-85 for C and 2-35 for D. In some European countries a colour chart (Dr Kloz) graduated in four classes is used(ll): class I corresponds to the initial colour (blue) of resazurin in milk, class IV to the mixture of resorufin and milk (pink), and classes II and III represent the transitions between these two extremes. A...