Yogurt Making

Two microorganisms namely, Lactobacillus bulgaricus and Streptococcus thermophilus, growing together symbiotically, are responsible for the lactic fermentation of the yoghurt. In some countries, apart from these two bacteria, the culture also contains lactose fermenting yeasts, Leuconostoc strains, Lactococcus lactis subsp. lactis biovar diacetylactis and Lactobacillus acidophilus.The method of maintaining purity of the yoghurt culture is very important and it is better to maintain the mother cultures individually rather than in mixed form.

The optimum pH and temperature for growth of Streptococcus thermophilus is 6.8 and 38°C. These strains usually reach acidities of 0.80 to 0.95 per cent. In case of Lactobacillus bulgaricus, the optimum temperature for growth is 42°C and the acidity obtained is 1.25 to 1.50 per cent. The extreme sensitivity of both these microorganisms to antibiotics especially, penicillin, calls for selection of milk intended for yoghurt manufacture to be free from residual penicilin. More over, extra care has to be exercised in maintaining the purity of the yoghurt cultures since they can be easily overgrown by contaminating organisms.

Traditional method of yoghurt making

The milk is evaporated to 1/3 to 1/4 water content so that it attains the required concentration. Alternatively, 4-5% non-fat dried milk (NFDM) can be added to the whole milk.

It is heated to 82-93°C for 30 minutes. Then it is cooled to 42-43°C and inoculated with 2-3% starter culture (Lactobacillus bulgaricus and Streptococcus thermophilus).

Then the milk is incubated at 42-43°C for 3 hours or until a titratable acidity of 0.75% lactic acid or coagulation occurs.

The product is chilled to 5°C. Further acidity of 0.9% lactic acid may develop while the product is being chilled.

The product can be stored satisfactorily for 1-2 weeks at 5°C.

Flavored yoghurt

The flavored yoghurt has gained popularity in recent years. Three types of flavors viz. artificial, natural with artificial flavors mixed and natural fruits are commonly used. These flavors may be added to the milk intended for preparing yoghurt by mix blending, fruit on bottom and bulk mixing.

Flavored yoghurt has definitely an edge over the plain yoghurt in that the high acidity encountered in the product less pronounced and suppression of objectionable off flavors and the need to concentrate the milk is eliminated.