Preparation of Starters

The propagation and preparation of LAB starter cultures is one of the most important operations in a dairy plant required for manufacture of fermented products. Since the quality of a starter has a direct bearing on the quality of the final product, starter must be pure, active and in good shape. The good quality starter cultures could only be prepared by employing certain essential factors such as selection of good quality milk, necessary heat treatment of milk, well cleaned and sanitized utensils, optimum amount of inoculum, aseptic transfer of culture, incubation at appropriate temperature and time, cooling and properly storing till its use.

i. Selection of milk

The milk selected for preparation of starter must be from healthy animals that are secreting normal and clean milk, free from inhibitory substances. The abnormal milk,mastitic milk and milk with unusually high lipolytic activity and low in total solids should not be used for preparation of starter cultures as well fermented products.

ii. Heat treatment of milk

Milk to be used for starter propagation should be given adequate heat treatment to inactivate the normal germicidal effects and also to destroy the majority of the microorganisms present as normal flora or contaminants. To achieve best results milk should be heated to minimum of 71.1oC for 30 min. Sometimes sterilized milk is recommended for starter propagation. However, more drastic heat treatment results in the inhibition of culture and the texture and body of the curd formed is soft and the scorched flavour developed therein hinders the accelerate assessment flavoring and aroma development. The steaming and boiling of milk for 30-60 min for mother

culture is preferred. After heat treatment, immediately milk is cooled to incubation temperature to minimize the physico-chemical changes in it.

iii. Containers/ Utensils

Glasswares, stainless steel, aluminium, enamelled wares are the most appropriate material for starter propagation. The copper and copper alloys should not be used.The surface of containers should be smooth and free from crevices. The container should be thoroughly cleaned and sterilized by steaming for at least 30 min at 100oC.

iv. Level of Inoculum

The level of inoculum used for propagation of starter cultures may also affect the performance of the starter. This depends upon the characteristics of starter culture,activity and condition of the culture at the time of inoculation, time and temperature of incubation employed. Normally 0.5 to 2.0% inoculum is used for starter preparation. The amount of inoculum may vary according to type of starter culture.

v. Culture Transfer

The starter culture should be properly and aseptically transferred avoiding all the changes of contamination. The strict aseptic conditions are needed to avoid aerial contamination with bacteriophages, molds and other undesirable microorganisms.For this purpose, a separate culturing room should be constructed and adequately equipped with laminar flow, UV lamp and provision for spraying cleaners and sanitizers. For transfer of cultures, all the accessories required should be thoroughly cleaned and sterilized.

vi. Incubation period

Normally, after inoculation the majority of milk starter cultures are incubated at 21.1oC to 30oC and time of incubation is determined depending upon the inoculums size and the activity of individual culture. In general incubation time of 14-16 hours appears to be ideal when the inoculum level used is 1 percent. The maintenance of proper temperature is very important and hence, it should be carefully monitored.

vii. Cooling

After attaining the desirable growth of the starter culture, it should be immediately cooled to stop further growth so that it should be in sound and active condition for further use. The cultures are stored under refrigeration including mother cultures until further use.