Methods of Propagation and Production of Starters

For the best performance and maintenance milk should be sterilized by heating at 90oC for 1 hour. The breakdown products of milk by employing heat treatment act as bacterial growth factors. This is probably one of the reasons why lactic acid bacteria grow more rapidly in heated than unheated milk. After giving sufficient heat treatment milk is cooled to 22-25oC and inoculated with appropriate inoculum size.After inoculation, the culture is incubated at 22-25oC until clotting takes place, and thereafter it is stored in a refrigerator. Further, a small aliquot of the culture is restarter inoculated into a similar container containing sterilized milk for storage of mother culture. The remaining culture is inoculated into the starter can or vessel containing sterilized milk. The purity of mother culture however is very essential. The inoculums generally is added at the rate of 1 percent from a culture having approximately 0.8 percent acidity. The acidity should not exceed 0.9 percent lactic acid. In case of poor performance including slow activity, any visible abnormality in behaviour, flavour,and colour and appearance of the starter, it should be immediately discarded and fresh starters should be used.

The purity and activity of the starter culture should be maintained by any means to achieve desirable fermentation efficiently in the manufacture of cultured dairy products.The starter culture must contain maximum number of viable organisms and must be very active under production conditions of the plant. For the preparation of fermented dairy products like cheese, dahi, yoghurt, etc. starter cultures are often maintained in milk, heated to 90oC for 1 hour and bulk starters may be grown in milk held at 90oC for 30 min. This heat treatment is adequate to kill phages and all other vegetative bacterial cells.

i. Preparation of Mother Culture

The preparation of mother culture is a very important step in the production of bulk starters for manufacture of fermented milk products at large scale. The mother cultures are generally maintained in narrow neck polyethylene bottles, which are thoroughly cleaned and sterilized using steam jets. Sometimes these bottles are sanitized with 0.1 per cent -hypochlorite solution. The milk is pasteurized in these bottles at 72-73oC for 45 minutes and the bottles then submerged in the cooling water, and thereafter, these milk bottles are stored in a refrigerator for future use. The milk containing bottle is inoculated by injecting few drops of the desired culture by means of a hypodermic needle, which can pierce the seals of caps. Thereafter, the content is incubated at desired temperature in a BOD incubator.

ii. Preparation of Bulk Starters

The vessels of different sizes are used for preparation of bulk starters. These vessels are closed completely air tight after the initial stage of filling with milk. These cans are filled with milk and heat treated by steam at 72-73oC for 45 min and cooled to incubation temperatures. The bulk culture is inoculated and incubated in the same manner as the mother cultures.

iii. Continuous Starter Production

Recently, this culturing technique has stimulated a lot of interest and dairy industry is striving for the introduction of this technique in the cheese plants with the advent of this technology, the manipulation and transfer of large bulk of starters are now avoided.

Many cheese plants today may use a very huge amount of bulk starters every day.This has been possible only through continuous cultivation of the starters. The handling of large size cans for bulk starters is extensively cumbersome and inconvenient.Inspite of convenience and ease, the continuous starter production suffers from a serious problem. During the operation of starters, if the starter gets contaminated(attacked by a phage), the whole bulk and all the equipment get contaminated.Hence, the control of purity and activity is even more important in continuous starter production than in a batch method of production.

iv. Preparation of Master Culture

In case of preparation of master culture, the method of inoculation is the same as for mother culture. For this, litmus milk may be used in the polyethylene tubes. Specific care must be taken to sterilize the chalk before adding to the milk as spores in chalk may be extremely resistant to heat. Litmus milk previously sterilized in glass bottles is filled into the tubes using a hypodermic needle. These are now ready for inoculation with the starter cultures. Such cultures may be maintained indefinitely with careful checking and testing after three months interval. Polyethylene containers are preferred over glass tubes and bottles as these are unbreakable, very light easily portable.