This is one of the key operations and should be attended to carefully by trained personnel dedicated to this duty. The starter tank valves and pipes and hatch with gasket should be assembled and sterilized with live steam at low pressure (3 to 5 lb). Keep the bottom valve open for the condensate to drain. Continue to steam the tank for 30 min after the surface temperature in the tank has reached approximately 99 °C. Turn the steam off and close the bottom valve.
Skim milk with total solids raised to 10-12% is either pumped cold and heated to 90 °C and held for 60 min or the starter mix is heated to 90° C in a plate heat exchanger and then held at that temperature in the tank for 60 min. Some plants prefer to use reconstituted nonfat dry milk at 10-12% TS for their bulk starter. In certain operations, where large amounts of starter are used for yogurt inoculation, yogurt base is used for starter culture preparation. This brings ease of operation and eliminates yogurt composition variation. Cool the mix to 43 °C. Close the chilled water valve early enough that the temperature does not fall below 43 °C. For 500 gallons, thaw a can (350 ml) of frozen culture concentrate (1010 cfu/g) in 5 gallons of tepid water containing 100 ppm chlorine. Inoculate the tank, stir it for 5 min. Turn off the agitator and let it incubate quiescently for 6-8 hr to reach 0.9% titratable acidity. Cool the starter to 5°C, using slow agitation. Then turn off agitation. Turn on agitation for a few minutes before the starter is to be pumped. This rate of inoculation yields approximately 106 cfu/g of starter mix. For a healthy culture, it may take 8 to 10 hr to reach 0.9% acidity. Starter has 2-5 x 108 cfu/mL. Time to 0.9% titratable acidity also depends on the strain composition of frozen culture concentrate.
It is advisable to make a Gramstain of the fresh starter and run an activity test as described in Sec. II. B, using 1% inoculum. Such data are invaluable in tracking the performance of culture(s) and in preventing failed yogurt fermentation.
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