Researches On The Quality And Antioxidant Activities Of Pineapple Soy Yogurt

As a functional beverage, soy yogurt has gained wide attention from researchers working in the food area and become a hot spot. Because of the defects of the taste and texture properties, the application of soy yogurt is limited. In order to improve the quality of the product, the effect of bromelain and pineapple juice on the titratable acidity, holding water capability, rheological properties, sensory evaluation and the metabolic of carbohydrate in soy yogurt was investigated in this thesis. Then a strain of Streptococcus thermophilus ST3, which grow well in the soy milk, was screened and its application on soy yogurt was studied. The soy yogurt with good flavor and high antioxidant activity was obtained through the use of the selected starter combination. The main contents and results are as follows.1. Effect of bromelain and pineapple juice on the physical properties and the metabolic of cabonhydrate of soy yogurt, with soy as material and Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis, Leuconostoc mesenteorides subsp. cremoris, Lactococcus lactis subsp. lactisbiovar diacetylactis, Streptococcus thermophilus as starter culture, were investigated. Results showed that most of the 7S subunit and nearly half of the 11 S subunit was hydrolyzed when treated by bromelain. The viscoelasticity and yield stress of soy yogurt were decreased significantly, the apparent viscosity and flow behavior were improved. And texture has been significantly enhanced. When adding pineapple juice, the acidity of the soy yogurt was increased, and the smell, taste and overall acceptability on sensory evaluation were improved obviously, which was correlated to the rich carbohydrate, aroma compounds and organic acids in pineapple juice. The soy yogurt sample with the hydrolysis of bromelain and pineapple juice was the best, but there was still a defect in the utilization of soybean oligosaccharides.2. Effect of the combined starter culture, which include a selected Streptococcus thermophilus strain ST3 after acclimatization in soy milk, and lactobacillus helveticus and lactobacillus rhamnosus on pineapple soy yogurt quality, was studied. The results showed that ST3 could improve the utilization of soy oligosaccharides to more than 30%, but the titratable acidity is only 56 oT. However, The titratable acidity of the soy yogurt sample increased to 70 oT when ST3 was used with Lactobacillus rhamnosus and Lactobacillus helveticus, and the rheological properties were also obviously improved. When ST3 and Lactobacillus rhamnose were used in fermenting soy milk, antioxidant activity and aglycone isoflavone of pineapple soy yogurt were the highest, but the sensory evaluation was the worst. The pineapple soy yogurt fermented by ST3 and Lactobacillus helveticus had a good smell, taste and overall acceptability on sensory evaluation. However, the antioxidant activity is not high.3. Effect of cabonhydrate and combined starter culture on the quality and antioxidant activities was further explored. Results indicated that the titrable acidity and water holding capacity of pineapple soy yogurt fermented by ST3 and Lactobacillus rhamnose increased slightly when glucose and fructose was added in soy milk. Meanwhile, the smell, taste and overall acceptability were improved. And the effect of glucose is better than fructose. Then Lactobacillus rhamnose was replaced by Lactobacillus casei, the pineapple soy yogurt fermented by ST3 and lactobacillus casei were the best in the sensory flavor and antioxidant activity.