Culture activity in refrigerated, frozen, and soft serve yogurt
Kong, Jean Qin
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Yogurt as a nutrient snack has been accepted by many people and its nutritional benefits have led to increased yogurt sales in the U.S. over the last few decades (Harper et al., 1991 ). Since product qua 1 i ty and consumer sat i sf action are fund am enta 1 factors to the successful and repeated sale of dairy products, the evaluation of yogurt qua 1 i ty becomes very important. According to the Food and Agriculture Organization of the United Nations, yogurt should contain abundant and viable organisms (FAO/WHO, 1972). Some studies have shown that there is wide variation in composition and cell number among yogurt brands, so the dairy product industry has a product 1 dent i ty prob 1 em that must be resolved. Further, a standard method for examination of viable organisms does not exist and the establishment of a standard method is necessary for evaluation of yogurt quality and development of a standard. The objectives of this research were: ( 1) to develop an activity test to determine the viability of yogurt culture; (2) to compare the differences in activity between refrigerated, frozen and soft serve yogurt by conducting a commercial survey; (3) to compare two media used for the enumeration of lactobacilli; and (4) to determine the relationship between B-galactosidase activity and culture activity. A survey of commercially prepared yogurts showed that in refrigerated strawberry lowfat yogurts, there were significant differences in culture activity between different brands (P<.OS), but no brand differences existed for strawberry nonfat yogurts. For other parameters of evaluation of yogurt quality, there were also variation among some brands. In vanilla yogurts, there were no significant differences in culture activity among refrigerated, frozen and soft serve yogurt. The two agars used to enumerate Lactobacilli gave significantly different results, even though they should enumerate similar types of bacteria (P<.001 ). Results from the B-galactosidase activity test and the culture activity test developed in this study were significantly correlated in refrigerated and soft serve yogurt <r=-.42 and r=.53), although the correlation between B-ga 1 actos i dase activity and the activity test was nonsignificant for frozen yogurt.