Growth of lactic starter cultures in milk studied by dry weight method

Abstract

In order to measure bacterial cell mass in fermenting milk accurately a new method including processes of dissolving milk curd and measuring dry weight has been developed. Single strains of $\mbox{\underline{Lactobacillus}}$ $\mbox{\underline{bulgaricus}}$ and $\mbox{\underline{Streptococus}}$ $\mbox{\underline{thermophilus}}$ were cultured in 11 percent nonfat dry milk separately. When curd was formed, 1N-KOH solution was added until pH reaches 11.5 and shaked strongly to dissolve curd particles completely. Bacterial cells in the solution were separated by centrifuging three times including two times of washing with distilled water for 30 minutes at 3,000 R.P.M. The lactic acid producing properties of these two strains have been known to be growth associated. This growth associated characteristics was proved better with growth curves determined by the new dry weight method than that of conventional viable count method. Using the new method effects of temperature on the growth of these two lactic starter strains in the milk was examined. Optimum temperatures for the growth of $\mbox{\underline{L}}$. $\mbox{\underline{bulgaricus}}$ and $\mbox{\underline{S}}$. $\mbox{\underline{thermophilus}}$ were found to be 47$^\circ$C and 45$^\circ$C, respectively. $\mbox{\underline{L}}$. $\mbox{\underline{bulgaricus}}$ responded more sensitively to the change of culture temperature than $\mbox{\underline{S}}$. $\mbox{\underline{thermophilus}}$ did. Through these experiments it was confirmed that the newly developed method can be used more conveniently and accurately for the measurement of bacterial cell mass in fermenting milk than any other existing methods.