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Yoghurt fermentation
Yogurt is made by fermenting milk with bacterial cultures such as Lactobacillus bulgaricus and Streptococcus thermophilus. The milk is pasteurized, inoculated with the cultures, held to ferment and thicken, and cooled before optional flavors or fruits are added. During fermentation, the cultures convert milk sugars into lactic acid, which coagulates the milk proteins to produce the yogurt's texture while the acidity prevents spoilage.
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Yoghurt fermentation
- 1. Food fermentations -Part II – Yoghurt Dr. Dhanya KC Assistant Professor, Department of Microbiology
- 2. Yoghurt A fermented milkproduct Either pasteurized or remain live Flavored with fruits Full fat yogurt - contain not less than 3.25% milk fat Low fat yogurt - contains not more than 2% milk fat Nonfat yogurt - contain less than 0.5% milk
- 3. The main ingredient– milk Whole milk for full fat yogurt Low fat milk for low fat yogurt Skim milk for nonfat yogurt Bacterial Culture Lactobacillus bulgaricus Streptococcus thermophilus Stabilizers - Improve the body and texture by Increasing firmness, preventing separation of whey, keeping fruit uniformly mixed Alginates, gelatins, gums, pectins, starch Sweeteners, Flavors or Fruit preparations
- 4. Bacterial cultures Lactobacillusbulgaricus and Streptococcus thermophilus Streptococcus thermophilus Acid from lactose in the milk pH drop to 4-5 from about 7.0 Causes milk coagulation Lactic acid - gives sour taste and limits growth of spoilage bacteria Lactobacillus bulgaricus Organoleptic compounds, acetaldehyde Provide flavor Proteolytic enzymes and extracellular polymers from bacteria - protein-gel formation
- 5. Only 2 bacteriarequired to be present in yogurt Lactobacillus bulgaricus and Streptococcus thermophiles Other bacteria may be added - as probiotic cultures For improving lactose digestion For improving gastrointestinal function Stimulating the immune system Lactobacillus acidophilus Lactobacillus subsp. Casei Bifidobacteria
- 6. Steps in YogurtMaking 1. Adjust Milk Composition & Blend Ingredients 2. Pasteurize Milk 3. Homogenize 4. Cool Milk 5. Inoculate with Starter Cultures 6. Hold 7. Cool 8. Add Flavors & Fruit 9. Package
- 7. 1. Adjust MilkComposition & Blend Ingredients Add dry milk powder - achieve desired fat and solids content stabilizers are added 2. Pasteurize Milk Pasteurize at 85°C for 30 minutes / 95°C for 10 minutes High heat - to denature whey proteins Form more stable gel Prevents separation of water Reduces the number of spoilage organisms in milk
- 8. 3. Homogenize Mix allingredients thoroughly Improve yogurt consistency 4. Cool Milk Milk is cooled to 42°C For ideal growth of starter culture 5. Inoculate with Starter Cultures L bulgaricus & S thermophilus are added 6. Hold Milk held at 42°C until pH 4.5 reached – take few hours Fermentation - formation of a soft gel and characteristic flavor
- 9. 7. Cool Yogurt cooledto 7°C, to stop fermentation 8. Add Fruit & Flavors At different steps depending on type of yogurt Set style yogurt Fruit added in the cup, Inoculated yogurt is poured Yogurt is fermented in the cup Swiss style yogurt Fruit blended with fermented cooled yogurt prior to packaging 9. Package Yogurt is packaged
- 10. References 1. Industrial Microbiology:An Introduction, M J. Waites, N L. Morgan, J S. Rockey, G Higton 2. Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers 3. http://www.milkfacts.info/Milk%20Processing/Yogurt%20Production.htm