Masters Theses

Date of Award

8-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

J. L. Collins

Committee Members

J. R. Mount, F. A. Draughon, B. J. Demott

Abstract

The objectives of this study were to establish critical parameters in the processing of yogurt with sweet potato as an ingredient and to determine the effect of sweet potato on sensory, physical, chemical, and nutritional attributes of the yogurt. Sweet potato level, sugar level, and incubation time had a significant effect on the production of lactic acid in the yogurt. When the percentage of sugar was increased at a given level of sweet potato and period of incubation, acidity production in the yogurt was relatively low. An increased in level of sweet potato caused a lowering of acidity for all levels of sugar at a given period of incubation. Percentage of sweet potato and sugar had an inhibitory effect on the activity of the starter culture. The predicted periods of times necessary to reach 0.85% titratable acidity in yogurt with 5% sugar and with 12, 14, 16, and 18% sweet potato were 6.25, 6.42, 6.50, and 7.25 hours, respectively. The amount of gelatin had a significant effect on firmness of the yogurt, but sweet potato had no effect. As gelatin was increased, firmness of the yogurt was increased until the gelatin content was increased to 3 g per 500 g of yogurt. The expert panel found that amount of gelatin and percentage of sweet potato had no effect on the yogurt attributes of flavor, body and texture, and appearance and color. Panelists scored the samples significantly different for each of the attributes. From the hedonic panel, gender of the panelists, the type of yogurt preferred by the panelists, the frequency of buying yogurt by the panelists and the willingness to buy the sweet potato yogurt if made commercially available had significant effects on the scores given by the panel. Panelist preferred yogurt of 16 and 18% sweet potato over the yogurt with 14% sweet potato. As level of sweet potato was increased, the percentages of moisture and fat were decreased and the percentage of carbohydrate was increased. At 14% sweet potato, yogurt had 81.3% moisture, 8.5% gat, and 66.3% carbohydrate. At 18% sweet potato, yogurt had 79.7% moisture, 4.9% fat, and 69.8% carbohydrate (DWB). Sweet potato contributed dietary fiber to the yogurt. Level of sweet ptoato did not affect the percentages of protein (mean 19.0%), ash (mean 3.9%), and total dietary fiber (mean 2.43%) (DWB). Caloric content was decreased as the level of sweet potato was increased. The calculated values indicated that yogurt with 14% sweet potato had 1,726 kjoules energy, and yogurt with 18% sweet potato had 1,651 kjoules energy (DWB). Percentage of sweet potato affected the level of vitamin C and pro-vitamin A of the yogurt. As percentage of sweet potato was increased, the vitamin contents were increased to 0.41 mg for vitamin C and to 1,252 retinol equivalents of vitamin A per 100 g sample (DWB). Level of sweet potato did affect the level of riboflavin (mean 0.41 mg) and niacin (mean 0.62 mg) per 100 g sample (DWB). However, the vitamin level of the experimental samples was higher than that for yogurt reported in the literature, except for vitamin C. As percentage of sweet potato was increased, the amounts of calcium and zinc in the yogurt were decreased. The levels of six other minerals were not affected by level of sweet potato. Percentage of sweet potato affected the levels of fructose, glucose, sucrose, and lactose of the yogurt but did not affect the level of maltose. Levels of sweet potato and sugar affected the Hunter color, L, "a" and "b", values of the yogurt. Time of incubation had only an effect on L and "b" values. As level of sweet potato was increased, the yogurt developed a darker and more red and yellow color. The yogurt with the higher sugar level was darker, less red and more yellow. As time of incubation was increased, the yogurt became lighter, less red, and more yellow. During fermentation, the bacterial culture increased at approximately 0.44 log CFU/g each hour.

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