Acid-Producing Activity of Lyophilized Lactic Streptococcal Cheese Starter Concentrates

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Abstract
Freeze-dried concentrates of lactic streptococcal starter bacteria [Streptococcus lactis, S. cremoris] (single and mixed strains) were prepared in different lyophilization media and stored under vacuum and several gases. The concentrates were tested for survival and acid-producing activity after storage for up to a year by rehydration with water to the original prelyophilization volume. Acid production was measured during incubation at 30.degree. C for up to 6 h and 22.degree. C for up to 18 h. Lyophilization media included 11% nonfat milk, nonfat milk containing 15% sodium glycerophosphate, nonfat milk containing 10% maltose and sucrose-peptone (10-5%) solution. Storage conditions included vacuum-sealing and storage under atmospheres of air, argon, CO2 and N2. Best results were with commercial mixed strain starter cultures of unknown strain composition which were purchased as direct vat set Cheddar cheese starters. Freeze dried concentrates of these cultures were active and stable at ambient temperatures (25.degree. C) for several months when vacuum-sealed and stored in amber containers filled with argon and when lyophilized in 11% nonfat containing 15% sodium glycerophosphate. For example, freeze dried concentrates of 10 such cultures when stored for 5-8 mo. at 25.degree.C produced an average percent titratable acidity of 0.97 (range 0.86-1.03) at 22.degree. C in 18 h when inoculum was made into steamed (95.degree. C for 40 min) nonfat milk; when incubatd at 30.degree. C for 6 h, the average was 0.60% (range 0.35-0.80%) for freeze dried concentrates stored 3-4 mo. The apparent enhanced acid production by rehydrated concentrates incubated at 22.degree. C was compared to 30.degree. C suggests that such preparations may have greater application for direct inoculation of bulk starter milk than for cheese vat milk.