1, the strain must have vitality.

Streptococcus thermophilus and Lactobacillus bulgaricus are usually used in yogurt fermentation. After the single liquid strain (or freeze-dried strain) is purchased, cocci and bacilli should be transplanted into test tubes in sterile rooms for single strain propagation and passage. In order to maintain its vitality, the test-tube strains were transplanted 1 time within 7 ~ 10 days. Too long shelf life, poor preservation and too many generations are easy to make the strain aging and reduce its vitality. Generally spread to about 30 generations, or find that yogurt is not solidified well and has a bad flavor, it is necessary to introduce new strains in time. Freeze-dried strains can also be used and inoculated directly in fermentor. Direct-throw starter has been widely used in China.

2. Fermentation temperature must be strictly controlled.

Milk is sterilized at 80℃ ~ 85℃/30min (or 120℃ ~ 140℃/2 ~ 4 seconds), then cooled to 40℃ ~ 45℃ and added with starter. Stir continuously for 5 ~ 10 minute to make the strains evenly distributed, and finally ferment at constant temperature. The fermentation temperature is 43℃ 65438 0℃ and the fermentation time is 2 ~ 3h. During the fermentation process, the fermentation temperature should be observed and adjusted at any time. Too high fermentation temperature leads to too much acid production and a large amount of whey precipitation, which affects the flavor; The fermentation temperature is too low, which leads to slow fermentation.

3. After fermentation, the temperature should be lowered in time.

Yogurt is fermented for 2 ~ 3 hours (dry powder fermentation for 5.5 ~ 6 hours), and when the acidity reaches pH 4.5 ~ 4.6, the fermentation reaches the end point. At this time, the temperature must be immediately reduced to 10℃ ~ 12℃. Timely cooling can control the growth of lactic acid bacteria and reduce the activity of enzymes. Too fast cooling will affect the formation of aromatic substances in yogurt, and too slow cooling will make the acidity of the product too high.

The solidified yogurt can be cooled by direct cooling: the yogurt is moved from the culture room to the outside and cooled by natural wind or strong exhaust air. Then it is transferred to the cold storage and cooked at 2℃ ~ 7℃ for about 10 ~ 12 hours (or 12 ~ 24 hours).

4. The solid content of milk must be stable.

In order to make the product reach a certain consistency, it is necessary to ensure the content of milk solids. When the milk solid content is low, the protein content is also low, and the milk protein has poor coagulability during fermentation. The solid content of yogurt milk should be 12% ~ 14% and not less than 12%.

5. Do not use milk or milk powder containing antibiotics.

When cows get sick, veterinarians often use antibiotics, streptomycin or sulfonamides, so their milk contains antibiotics and drugs, which will seriously inhibit the vitality and reproduction of lactic acid bacteria, make yogurt fermentation fail and cause raw material loss. So it is required to test the antibiotic content in milk and milk powder.

6. Add stabilizer to improve the quality of yogurt.

Yogurt products have some defects, such as unsmooth structure, unsmooth taste, whey precipitation, protein precipitation and oil slick. Appropriate amount of L- milk stabilizer D (special for lactic acid bacteria milk) can be added to make up for it, which can make yogurt smooth and delicate, fragrant and delicious, increase viscosity, strengthen freezing, and eliminate water precipitation, oil precipitation and stratification.

7, must be carried out in strict accordance with the hygiene requirements.

Before yogurt production, all containers, pipes and equipment should be disinfected with 100℃ boiling water for 20 minutes. After the production, all containers, pipelines and related equipment shall be cleaned with clean water → hot water → lye → warm water → hot water above 95℃.