I. GROWTH AND ACID PRODUCTION1

Abstract

Soymilk with a protein content similar to that of cow's milk was prepared from soybeans (variety Chippewa 64). Soybeans were washed, soaked until 1 ml of water per gram of beans was absorbed, comminuted with water equivalent to 7.6 times their dry weight, and the mixture filtered through cheese cloth to obtain an aqueous extract free of large particles. Growth rates of 13 species of lactic-acid bacteria in sterile soymilk were generally greater than or comparable to those in cow's milk or Elliker's broth. Acid production in soymilk was not always directly related to growth rates of the organisms. Substantial formation of acid was limited to those bacteria able to utilize the sugars in soymilk, e.g., Streptococcus thermophilus, Lactobacillus delbrueckii, Lactobacillus pentosus, and Leuconostoc mesenteroides. Sources of readily available nitrogen (e.g., protein digests), when added to soymilk, enhanced acid production by S. thermophilus, the Leuconostoc species, and L. pentosus; appeared inhibitory to L. delbrueckii; and had no apparent effect on the other test cultures. Addition of whey powder, glucose, or lactose to soymilk enhanced acid production by Streptococcus lactis, Streptococcus cremoris, Streptococcus diacetilactis, Lactobacillus casei, and Lactobacillus helveticus; whereas addition of sucrose was without benefit. The presence of 0.23–0.25% titratable acid, corresponding to a pH of 5.7, caused coagulation of the sterilized soymilk.