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.

1. Food fermentations - Part II – Yoghurt
Dr. Dhanya KC
Assistant Professor, Department of
Microbiology

2. Yoghurt
A fermented milk product
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 Lactobacillus bulgaricus 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 bacteria required 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 Yogurt Making
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 Milk Composition & 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 all ingredients 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 cooled to 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