4mcb 322-yogurt.docx

**DEPARTMENT OF BIOCHEMISTRY AND MICROBIOLOGY** **4MCB 322 BIOTECHNOLOGY** **Preparation of fermented foods** **Objective** To prepare various items of food by microbial fermentation **Overview** Microbial fermentation is a time-honored method for the preparation and preservation of certain fo...

**DEPARTMENT OF BIOCHEMISTRY AND MICROBIOLOGY** **4MCB 322 BIOTECHNOLOGY** **Preparation of fermented foods** **Objective** To prepare various items of food by microbial fermentation **Overview** Microbial fermentation is a time-honored method for the preparation and preservation of certain foods. Sauerkraut, brined pickles, pickles olives, soy sauces, yogurt, and buttered milk are but few of the products of microbial fermentation. In addition to the feature of preservation, the chemical changes that occur during fermentation add flavor and aroma to the final product which has appeal to the consumer. Furthermore, and particularly in the case of buttermilk, acidophilus milk, and yogurt, the ingestion of large numbers of lactic acid bacteria in these products is claimed to have come beneficial effect on one's health. The bacteria responsible for the fermentation of a food, for example cabbage or cucumber, are normally present on these vegetables. The cultural conditions for the fermentation favor the growth of the desired type of pf microorganisms. For production od other fermented foods, such as yogurt, acidophilus milk and some cheese, it is customary to use **stater cultures** to initiate fermentation. The stater culture contains large numbers of the specific microorganisms needed for the fermentation to result in high quality product. Stater cultures can be purchased from companies that specialize in making these products. The bacteria responsible for the fermentation of shredded cabbage to produce sauerkraut are mainly lactobacilli and leuconostocs. These bacteria ferment the sugars in the cabbage juice and produce a variety of products that include lactic acid, acetic acid, ethanol, mannitol, and carbon dioxide. In addition to providing flavor and aroma of the finished product, some of these substances contribute to the keeping quality of the sauerkraut. The fermentation of milk to produce yogurt is brought about mainly by lactic-acid bacteria, particularly streptococci and lactobacilli. The production of lactic acid from the sugar (lactose) in the milk by these bacteria results in the coagulation of milk proteins where a custard like curd is formed. **Yogurt production** **Materials** - Fresh plain yogurt (or commercial stater culture) - Fresh skim milk (100ml) - Powdered skim milk (3g) - LBS (*Lacobacillus* selection) medium - Beaker 250 ml - Plastic cup - Hot plate - Thermometer - Ph meter **Procedure** 1. Add 100 ml of fresh skim milk to a 250 ml beaker. Measure the pH of the milk using the pH meter. With the use of the hot plate, heat this to 85 ℃for 15 min. during this heating period, stir the milk gently and keep close check on the temperature with a non-mercury thermometer placed in a beaker. 2. After 15 min heating at 85 ℃, remove the beaker and add 3g of powdered skim milk and stir to dissolve. 3. Cool the milk to 40℃ to 42℃, and add 2 teaspoons of yogurt stater culture or 2 teaspoons of fresh plain yogurt per 100 ml of milk 4. Pour the contents into the plastic cup, cover with foil and incubate at 40℃ to 42 ℃ for 18 to 24 hours 5. At the end of the incubation period, check the pH of the product and observe it for the appearance, particularly the consistency of the curd, flavor and aroma.