Comparative Studies on the Qualities of Commercialized Yoghurt in Kumasi and the Effect of Natamycin on Yoghurt during Storage

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Yoghurt is a fermented dairy product which is consumed as a dessert, snack or as a pro-biotic food drink and has been one of the dairy products patronized by consumers in the Kumasi metropolis. The study was undertaken to assess consumer preference for yoghurt and to compare the nutritional, microbial, physicochemical and sensory qualities of seven freshly prepared brands of vanilla-flavoured stirred yoghurts obtained from the manufacturers in Kumasi, Ghana. The preservative effect of natamycin on yoghurt during storage was also studied. These products were coded as Y1, Y2, Y3 up to Y7. 284 consumers of yoghurt in Kumasi gave many reasons for their preference of the various brands of yoghurt and 26.40% thought the most important nutrients obtainable from yoghurt were fats and proteins, and 26.10% believed vitamins were more readily available in this product. However 31.8% of consumers had no idea about which vitamin(s) were most available in the product. Some consumers indicated that an improvement of the shelf life of the products would be necessary and appreciated. Apart from the fat content which did not vary significantly, there were significant differences (P<0.05) in the nutritional composition and physicochemical properties of the seven commercialized brands of yoghurt. The protein content ranged from 2.08 ± 0% to 3.10 ± 0.19%, while fat contents ranged from 0.24 ± 0.19% to 0.59 ± 0.41% and energy values from 229 to 338KJ per100ml of yoghurt. Four out of seven brands had protein contents lower than the minimum permitted value (2.7%) set by the Codex Standards for such products. The total coliform (0 to 9.30±0 ×102 cfu/ml) and yeast counts (8.40±0.35×105 to 14.00±0.39×105cfu/ml) did not meet Codex Allimentarius and Ghana standards for fermented milk products, which require that these microbial contaminants should not be present at all. In terms of consumer sensory preferences, Y3 was the least accepted product while Y6 was the most preferred product. Preservation of samples of yoghurt with 5 to 10 ppm of natamycin resulted in 8 ppm of natamycin being the most appropriate concentration for improving the keeping quality of the product. This concentration of the preservative gave the highest percentage decrease (69.36%) of yeast loads in yoghurt during storage and also resulted in relatively minimal changes in important physicochemical properties such as pH, titratable acidity and total soluble solids than lower concentrations of 5 to 7 ppm. There were significant differences (P<0.05) in pH, titratable acidity, total soluble sugars, total coliform and yeast counts of all commercialized yoghurts with and without natamycin throughout the 35 days of storage at 5 ± 1 degrees Celsius.
A Thesis submitted to the School of Graduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfillment of the requirements for the degree of Master of Science in Food Science and Technology, March-2011