Growth and viability of lactobacillus acidophilus in soy milk

Abstract

Lactobacillus acidophilus which has long been used for the manufacture of acidophilus milk was tried to add into Soy milk whose market in Korea is growing explosively. When the fresh cells grown in MRS medium were inoculated in the unformulated Soy milk at the level of $8.0\times10^6$ cells/ml and were incubated at $37\,^\circ\!C$ only a slight growth was observed within the initial 4 hours and followed by a gradual decrease in viable counts. However, the acid production by these organisms in the Soy milk active and continued throughout the 16 hour period of incubation. Supplement of Soy milk with carbohydrates such as glucose, lactose, and sucrose did not increase the cell number. Supplement with protein sources such as peptone, tryptone, proteose peptone, and amino acid, however, could enhance both growth and acid formation of the organism. But casein hydrolysate neither stimulated the growth nor the acid production. Among the nitrogen sources proteose peptone peptone at 4g/1 level should highest effect. The less growth of L. acidophilus in Soy milk, therefore, may be explained by the lack of nitrogen compounds which are needed for growth. Acetate as lipid precursor and Tween 80, supported the growth and helped retain the cell viability. But the degree of the effect was slight. The cells added to the Soy milk could survive long when the product is stored at $4\,^\circ\!C$ whereas, storage at $15\,^\circ\!C$ and at room temperature resulted in rapid loss of viable cells specially after 4 days of storage. The results suggest a possibility of consuming living cells are prepared separately and added to the Soy milk before consumption.