Call number: LD2668 .T4 1978 S515 / Master of Science

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Thesis (M.Inst.Agrar.)(Food Production)--University of Pretoria, 2006. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

This study investigated the effect of the Lactoperoxidase system (LPS) alone and in combination with pasteurisation, on the growth of Listeria monocytogenes (LM) ATTC 7644 in goat milk and goat milk cottage cheese during a shelf life of 10 days at 4 °C. Goat milk was inoculated with LM ATTC 7644 and divided into two samples, one the control and Lactoperoxidase (LP) was activated in the other sample. Both the control and LP activated samples were kept at ambient temperature for 6h. After 6h the control and LP activated samples were again divided into two and one of the respective samples was pasteurised at 72 °C for 15 s. All the four samples were analysed for LM ATTC 7644 immediately after LP activation at 0h, after 6h of LP activation and after pasteurisation. Goat milk cottage cheese was made with all four samples, i.e. control raw, control pasteurised, LP activated raw and LP activated pasteurised goat milk and analysed for LM ATTC 7644 on days 1, 2, 5, and 10. Six hours after LP activation the mean LM ATTC 7644 count for the LP activated milk decreased by log 0.5 cfu/ml where as the LM ATTC 7644 for the control increased by log 0.5 cfu/ml. The reduction of LM ATTC 7644 count in LP activated milk when compared to the control shows that goat milk lactoperoxidase is capable of reducing L. monocytogenes when stored at ambient temperatures. Furthermore, LM ATTC 7644 count in LP activated pasteurised goat milk decreased by log 1.1 cfu/ml more, compared to the control pasteurised goat milk. Therefore, pasteurisation together with LP activation may be more effective than pasteurisation alone in controlling the growth of L. monocytogenes in goat milk. For the control raw goat milk cottage cheese on day 10, the LM ATTC 7644 count was 90 % less than on first day of storage. The LP activated raw goat milk cottage cheese count followed a similar trend to the control raw goat milk cottage cheese, and reached levels of log 2.9 cfu/g on the last day of storage. The control pasteurised goat milk cottage cheese LM ATTC 7644 count on day 10 was 92 % lower compared to day 1 where as the LP activated pasteurised goat milk cottage cheese LM ATTC 7644 count was 98 % less than on day 1. The results of this study indicate that the activation of the LPS significantly (p≤0.05) decreased the LM ATTC 7644 count in goat milk, during a period of 6h. Combined pasteurisation and LP activation had a synergistic effect on the LM ATTC 7644 count in goat milk. The LM ATTC 7644 count declined in cottage cheese made from both control and LP activated goat milk. A greater decrease was observed in LP activated pasteurised goat milk cottage cheese over the storage period of 10 days at 4 °C. This combination may be used to reduce the multiplication of LM ATTC 7644 for production of safer products like goat milk and goat milk cottage cheese. / Dissertation (M Inst Agrar (Food Production and Processing))--University of Pretoria, 2007. / Food Science / M Inst Agrar / unrestricted

When rennet type cottage curd was made from cheese milk pasteurized at 143°F. for 30 min. the heat labile serum proteins remained with the whey and represented a loss of approximately 15% of the total cheese milk protein. However, when higher heat treatments than pasteurization were applied, up to 65% of the serum proteins of cheese milk were denatured, rendered coagulable with the caseins and incorporated into the cottage curd. Similar results were obtained by blending high- and low- heat cheese milk. Composition and flavor quality of cards containing increased amounts of serum protein were closely comparable to curds made from conventional pasteurized cheese milk. The yield of curd was increased. As the heat treatment of cheese milk was increased from 143 to 180°F., the denaturation of serum protein in drained whey increased from 0.0 to 60.01%, the percent of whey solids decreased 7.1%, pH at the A-C endpoint increased from 4.72 to 5.10, and time from setting to cutting decreased from 4.75 to 2.50 hr. When the percent of high-heat milk in blended cheese milks was increased from 20 to 95%, the percent of denatured serum protein in drained whey increased from 4.8 to 37.2%, the percent of whey solids decreased from 1.1 to 5.3%. pH at the A-C endpoint increased from 4.70 to 5.05, and time from setting to cutting decreased from 5.50 to 2.50 hr. High quality curd comparable to the control was made from cheese milks receiving heat treatments up to 170° F. for 30 min. and from blonde containing up to 95% high·heat milk. Heat treatments over 170°F. slightly lowered the quality score of curd. Pounds of curd per lb. of cheese milk protein was found to be the best procedure for calculating and expressing yield. When curd was made from cheese milk heated at 170°F. and from blends containing 85% high·heat milk, the curd yield was increased 16.22 and 9.61% above the control, respectively. / Master of Science

Sales of cottage cheese have been on the decline since 1972. Several factors have contributed towards this decline, including limited shelf-life. Cottage cheese shelf-life is estimated to be 10-21 days, in standard, non-barrier containers held at refrigeration temperatures. Shelf-life is shortened when aerobic, psychrotrophic microorganisms grow at refrigeration temperatures, producing changes which are undesirable. Previous studies have demonstrated that modified atmosphere packaging (MAP) is able to maintain cottage cheese quality and extend shelf-life over air packaging. The objectives of our study were to evaluate the ability of MAP to maintain cottage cheese quality, while establishing the proper atmosphere to be used. Further, we wanted to determine the potential for discoloration and development of undesirable acid flavors in cottage cheese by elevated CO₂ levels. Direct-set cottage cheese was packaged in barrier containers and flushed with 100% CO₂, 75% CO₂:25% N₂, 100% N₂, and air, and stored at 4°C for 28 days. Product quality was assessed by sensory evaluation. Microbiological and chemical tests were conducted to obtain a better understanding of the effects of MAP on cottage cheese. Results obtained demonstrated that there was no change during storage for headspace gas composition. Psychrotrophic and lactic acid bacteria increased for air treated samples. Counts for MAP cottage cheese remained unchanged. In contradiction to previous studies, elevated CO₂ levels did not cause product discoloration. Acidity increased over storage life; however, the increase in acidity was not perceived organoleptically. These results contradicted previous studies which demonstrated that elevated CO₂ levels imparted a sharp acid flavor to the food product. Lactic acid did not contribute towards increased acidity. Sensory evaluation demonstrated that air treatment was inadequate in maintaining product quality past day 19. Cottage cheese packaged under 100% CO₂ was judged most acceptable, followed by 75% CO₂ - 25% N₂, and 100% N₂ treatments. / Master of Science

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Call number: LD2668 .T4 1978 V43 / Master of Science

The study involves two different approaches to increasing cottage cheese yields by the inclusion of whey proteins in the curd. They were the use of high heat treated skim milk and ultrafiltered skim milk retentates for cottage cheese making. Increasing the pasteurization temperature of skim milk from 61.8 to 79.4 C for 30 minutes resulted in 15.6% increase in cultured cottage cheese yields. However, the high heat cheese exhibited variable quality and was generally inferior to the control. Cheese making from high heat skim milk was done by cutting at the A.C. end point and using 30 ml of rennet per 454 kg of skim milk. There appeared to be no relationship between the rate of heating to 79.4 C and the quality of cottage cheese produced. Skim milk was concentrated by ultrafiltration to 12, 16, and 20% solids. The 20% retentate was produced by concentrating skim milk 5:1 and then subjecting it to diafiltration with an equivalent volume of deionized water. The acidification of the retentates was accomplished by metering concentrated hydrochloric acid into the vortex of the centrifugal Culture growth in 20% retentates was inhibited below and soluble phosphate seemed to be an important factor in this inhibition. The yield trials indicated that the increase in yields over cultured cottage cheese made from skim milk was 12.4, 15.3, 5.6 and 1.6% for 16 % cultured, 16 % direct acid, 20% cultured and 20% direct acid cottage cheeses made from retentates. The lower yield increases from 20% retentates was because of the shattering of the curd while cooking, and the diffusion of the whey proteins. While the quality of cultured cottage produced by ultrafiltration was as good as the cultured cottage cheese from skim milk, the direct acid cottage cheeses were much inferior. Considering both high heat treatment of skim milk and ultrafiltration for increasing cottage cheese yields, ultrafiltration seems to be the better method owing to the much better quality of cheese produced. To obtain maximum benefits from ultrafiltration, problems of lactic culture growth in retentates must be solved and non conventional cooking methods have to be developed for handling curds from high solids retentates.

Graduation date: 1994