Development Of Co-Fermented Milk By Lactic Acid Bacteria And Yeast

Currently, commercial fermented milk are mostly fermented by lactic acid bacteria. The variety of commercial fermented milk is single and the taste and nutritional value are approximately the same, which can’t fully meet consumer demand. This study aims to develop a new alcohol-containing fermented milk with unique taste and flavor combined with some nutritional functions. Firstly, the yeast and lactic acid bacteria that performed excellently were screened and identified from Xinjiang fermented camel milk. Secondly, we determined the optimum fermentation process of co-fermented milk through single factor test and orthogonal experiments. Then the characteristics of quality, flavor component and shelf life were investigated to evaluate the co-fermented product. Finally, the antioxidant capacity, angiotensin converting enzyme(ACE) inhibitory activity, total phenol content of co-fermented milk as well as tolerance tests of lactobacillus were analysed. The results are as follows:(1) One lactic acid bacteria and one yeast was screened out from 6 lactic acid bacterias and 12 yeasts that isolated from Xinjiang fermented camel milk. Then we evaluated the morphological and molecular identification of yeast and lactic acid bacteria. The results showed that: yeast is Kluyveromyces marxianus and lactic acid bacteria is Lactobacillus casei.(2) Based on single factor experiment results, three factor conditions which were the fermentation temperature, fermentation time and inoculation ratio of lactic acid bacteria and yeast were selected.The optimal fermentation conditions were determined by the three factors and three level orthogonal experiments L9(34). The optimal fermentation conditions were as follows: 2:1 of lactic acid bacteria and yeast, fermentation temperature is 35℃, fermentation time is 14 h. The final product has excellent flavor and taste.(3) The physicochemical properties, flavor components and shelf life were investigated to evaluate the co-fermented product. It was indicated that the acidity was 79.4° T and contained 0.52(v/v) alcohol. The concentration of protein and nonfat milk solid were higher than the requirement of GB 19302-2010. 74 flavor compounds including 8 aldehydes, 24 alcohols, 12 esters, 15 ketones, 12 acids, 3 other substances were identified by simultaneously distillation extraction-gas chromatography-mass spectrometry, which were more than that in the unfermented milk and commercial yoghurt. By measuring the changes in pH, alcohol content, the viable count of lactic acid bacteria and yeast and combined with sensory evaluation, product’s shelf life was finally determined to 7 days, at 4℃~5℃ refrigeration.(4) The antioxidant capacity, angiotensin converting enzyme(ACE) inhibitory activity, total phenol content of the co-fermented milk as well as tolerance tests of lactobacillus were analysed and compared with unfermented milk and commercial yoghurt. The results indicated that co-fermented milk has a higher total phenolic content, better antioxidant activity and ACE inhibitory activity. In addition, Lactobacillus casei used in this study also has a strong acid and bile resistant capability.