Preparation Of Soy-based Yogurt With Different Types Of Soy Proteins

Different types of soy proteins, i.e. soy milk, defatted soy flour and soy protein concentrate were used in this study to prepare soy based yoghurt with the aim to increase the applications of the soy proteins in the dairy products and expand the source material flexibility of the yoghurt making. Soybean yoghurt flavor and finally quality of product construct were analyzed in depth in order to solve the problems presented in the domestic yoghurt of soybean. Image analysis was carried out for the digitalized scanning electron microscope(SEM) using MATLAB software to study the microstructure of yoghurt and the concept of fractal was introduced to provide a new and effective method to resolve and explain problems in the filed of yoghurt studies.Starting with common lactic acid bacterial (Lactobacillus bulgaricus, Streptocoeus theromophilus), four bacterial domestication methods was used in the first part of this study in order to increase the titration acidity and improve the product quality of soybean yoghurt. Results showed that lactic acid bacterial were domesticated by increasing soymilk content of the yogurt base. A soy/cow milk ratio of 8:2(W/W) was finally obtained at which bacteria could grow well. Fermentation experiments using the domesticated bacterial were conducted and the effects of solid content, fermentation time and sterilization temperature for each kind of material on the total acid degree and yogurt texture were studied. Optimized fermentation conditions for three kinds of soy proteins were obtained and the physicochemical property analysis and sensory properties were compared. Organic acids and volatile off-flavor compounds in the soy based yoghurt were isolated and analyzed by ion exclusion chromatography and GC-MS of solid phase micro-extraction (SPME) samples respectively. Changes in acetone acid, citric acid and lactic acid contents in the fermented course for different of soy proteins were analyzed. Major volatile off-flavor compounds for dairy and soy based yoghurt were compared and their influences on the flavor profiles of soy based yoghurt were suggested.Grey level distribution and variance analysis of the digitalized image of SEM micrographs based on the MATLAB programming were carried out, which showed that the consistency for soy based and dairy yoghurt was different. Box counting method was applied to investigate the fractal structure and the fractal dimension for soy based and dairy yoghurt was determined to be 1.9105 and 1.9268 respectively.