Lactobacillus fermentation for yogurt making experiment

Summary

Yogurt is a nutritious and unique health drink made from milk as the main raw material and fermented by lactic acid bacteria. Lactic acid bacteria used for yogurt fermentation are mainly Lactobacillus texans Bulgaria subspecies, and Streptococcus salivarius thermophilus subspecies, and some other lactic acid bacteria such as Lactobacillus acidophilus matter Lactobacillus lactis subspecies, (the old name of Streptococcus lactis), etc. are used in some yogurts. In this experiment, fermentation of lactic acid bacteria such as Lactobacillus dextrose Bulgaria subspecies and Streptococcus thermophilus is utilized to make yogurt.

Operation method

fermentation

Principle

The main biological changes in the fermentation of yogurt are: lactic acid bacteria in milk lactose fermentation into lactic acid to make its H drop to the isoelectric point of the casein egg 4.6 near 4.0-4.6 so that the milk to form a gel; secondly, lactic acid bacteria will also promote the degradation of part of the casein protein, the formation of lactic acid and calcium to produce a number of fats, acetaldehyde, bisacetyl acetyl acetate and butanediyl copper and other flavor substances. This is the main reason why yogurt has good health effects and is suitable for the majority of lactose intolerant patients.

Materials and Instruments

Powdered milk Lactobacillus texans Bulgarian subspecies Streptococcus thermophilus
White sugar
Aseptic bottle Incubator Refrigerator

Move

I. Milk distribution

Use milk powder to make reduced milk by adding water in the ratio of 1:7, then add 6%-10% sugar to the milk and stir well, and put it in 85-90 ℃ to sterilize for 15 min;
II. Cooling

Cool the sterilized milk to about 45 °C;
III. Vaccination

Inoculate commercially available high quality yogurt as strain into cooled milk by 5-10% and stir well;
IV. Holding Fermentation

Put the inoculated milk in a warm box at 40-42℃ to keep warm for 3-4 h (the specific time depends on the curdling speed).
V. After-ripening

Keep the yogurt in the gel state at a low temperature of about 4℃ for 12-24 hours for maturing;
VI. Taste

There are two types of physical and chemical indicators and microbiological indicators to assess the quality of yogurt. In this experiment, the quality of the product is based on the good taste and flavor when tasting, while observing the appearance of the production mouth, including the state of the curd, color cleanliness, surface finish, no bubbles, and a pleasing aroma and so on. On the contrary, if there is an odor when tasting, it means that the fermentation is contaminated with stray bacteria.

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