Graduation date: 1995

Graduation date: 1930

Thesis (M.S.P.H.)--University of Michigan, 1939.

Thesis (M.P.H.)--University of Michigan, 1947.

Thesis (M.P.H.)--University of Michigan, 1947.

Thesis (M.S.P.H.)--University of Michigan, 1939.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1945. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.Sc.Agric.(Agricultural Economics))--University of Pretoria, 2000. / Abstract in English. Includes bibliographical references.

Selected public health criteria of pasteurised milk available to the consumer from milk -shops in a selected area of Pretoria compared with a national distributor's milk were evaluated. Pasteurised milk samples were obtained from five randomly selected milk-shops in the northwestern part of Pretoria over a six-week period from June to August 1998. Milk from a wellknown national distributor was also obtained from three supermarkets in the same area during the same time period to act as the control milk. Mean total aerobic bacterial counts, coliform counts and psychrotrophic bacterial counts were determined. The presence of Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), Salmonella species, staphylococcal enterotoxins, inhibitory substances, alkaline phosphatase and somatic cells in milk were also determined. Of the 135 milk samples purchased from milk-shops, 87% were not fit for human consumption on the basis of the minimum standards prescribed in the Foodstuffs, Cosmetics and Disinfectants Act (Act 54 of 1972). In contrast, 100% of the 79 control milk samples passed all the safety criteria laid down in the Act. x Milk-shop milk quality varied between milk-shops and between sampling days. All milk-shop milk was sold as having been pasteurised, yet 38.5% of milk samples purchased failed the alkaline phosphatase test, indicating that they had not been pasteurised correctly or were contaminated with raw milk. The total aerobic plate counts were generally high for all milkshop milk samples ranging from 1.0 x 102 to 2.7 X 107 CFU/mQ with a median value of 41000 CFU/mQ, whereas for the control milk it ranged from 7.0 x 102 to 8.7 X 103 CFU/mQ, with a median value of 2 200 CFU/mQ. Coliform counts varied from 0 to 3.4 X 104 per mQ in milk-shop milk, with 68% of samples having counts lower than 20 coliforms/mQ, which is the maximum number allowed when the Petrifilm method of counting is used. Coliforms could not be detected in 1 mQ of control milk samples. E. coli was detected in 1 mQ of 17% of milkshop milk, 95% of which originated from milk which was alkaline phosphatase positive. Salmonella spp. could not be detected in 1 mQ in any of the E. coli-positive milk tested. Psychrotrophic bacterial counts done after pre-incubation of milk-shop milk were extremely high, and ranged from 3 x lOS to 2.2 X 108 CFU/mQ, with a median value of 2.4 x 107 CFU/mQ. In Europe the psychrotrophic count may not be greater than 100 000 CFU/mQ. None of the milk-shop milk passed this European standard, whereas 98.7% of the milk obtained from the national distributor fell within the prescribed parameters. S. aureus was isolated from 54 (40%) milk-shop milk samples, and four (7.8%) of 51 isolates tested produced staphylococcal enterotoxins A (SEA), B (SEB), D (SED) or a combination. Control milk did not contain any S. aureus and 15 milk samples tested for the enterotoxin gave a negative result. All control milk was negative for inhibitory substances, but these were detected in 54.1 % of milk-shop milk. Somatic cell counts varied between 1.2 x 104 and 1.6 x 106 cells/mQ in the milk-shop milk, with a median count of 4.2 x 105 cells/mt Only 18.7% of samples had counts above the legal limit of 500 000 cells/mQ. The national distributor's milk always had counts less than 150 000 cells/mQ. xi The results showed that milk-shop milk differed significantly (p < 0.05) from the national distributor's milk, and that the quality of milk purchased from milk-shop outlets was generally of a poor bacteriological quality. The presence of inhibitory substances, and the isolation ofE. coli and S. aureus (some of which were able to produce enterotoxins) indicated potentially unsafe milk and posed a serious public health risk to consumers. / Dissertation (MMed Vet (Hyg))--University of Pretoria, 2000. / Production Animal Studies / unrestricted

Recent studies have shown that when milk fat is heated, a homologous series containing the n-alkyl members of methyl ketones with odd numbers of carbon in their chains are produced (48; 44; 38; and 3). The same series of compounds also is found in evaporated and dried whole milk and in these products the concentration increases during storage (70 and 46). It is believed by some investigators that the methyl ketones play an important role in flavor deterioration of milk fat and in the aforementioned concentrated products. At the present time, however, there is disagreement on the factors influencing methyl ketone production; some workers relate their formation to autoxidation (25), while there are others who report that heat and water are essential in the reaction (48; 38 and 3). Finally, a recent report indicates that anhydrous milk fat will give rise to methyl ketones when heated in the absence of oxygen (44). The purpose of this investigation was to study the effect of various factors on the qualitative and quantitative composition of methyl ketones in heat treated milk fat. It is anticipated that the resulting information will contribute to a more thorough understanding of the reactions leading to ketone production in the fat; hence, to development of suitable processing measures for prevention of this type of deterioration in dairy products. Milk fat was prepared from raw cream two days after milking. It was washed free from phospholipids, centrifuged at 30,000 x G for 20 minutes and degassed at two to five microns pressure for one hour. The fat was then heat treated in sealed vials at various temperatures and time periods under controlled conditions. The samples were quantitatively analyzed for methyl ketones by direct conversion of the ketones to 2, 4-dinitrophenylhydrazine (DNP) derivatives in the intact fat sample. The derivatives were isolated from the fat, separated and identified by a combination of column and paper chromatographic methods and by their absorption spectra. Methyl ketone formation in heated milk fat was shown to be non-oxidative. A plateau in ketone production was approached in the 120°C to 140°C range when the time of heat treatment was 30 minutes. Added water enhanced total methyl ketone production at 140°C but not at 200°C. Air did not hinder ketone production. Maximum ketone production (1.733 mM/kg fat) was noted after three hours of heat treatment at two to five microns pressure, and 140°C. Milk fat centrifuged at 30,000 x G for 20 minutes and degassed at two to five microns pressure for one hour was found to contain 0.27% water. This quantity of water is sufficient for hydrolysis of β-ketoesters assuming them as the precursors of the methyl ketones. Conventional methods of preparing "anhydrous" milk fat were not adequate for removal of trace amounts of water. When milk fat was dried over calcium hydride (35) prior to heat treatment, total ketone formation was significantly reduced indicating that water is a limiting factor in ketone formation. A homologus series of n-alkyl methyl ketones (C₃, C₅, C₇, C₉, C₁₁, C₁₃, C₁₅) was isolated from heat treated samples. The ketones produced in large amounts were the C₃, C₇, and C₁₅. When the heat treatment was for three hours or longer the C₄ ketone was detected and composed approximately 11% of the total. The possible origin of the C₄, ketone is discussed. The reaction of intact fat with DNP-hydrazine and the subsequent isolation and identification of methyl ketones were quantitatively evaluated. / Graduation date: 1964