Isolation of streptococcus thermophilus strains from plants in Japan and their application to milk fermentation

Abstract

Streptococcus thermophilus is a lactic acid bacterium used along with Lactobacillus delbrueckii subsp. bulgaricus as a starter culture for yogurt production. Thus, there are many reports on the isolation and characterization of S. thermophilus from dairy products. On the other hand, literature on the isolation of S. thermophilus from plants is limited. Here, we investigate the isolation of S. thermophilus from 688 samples of twigs, leaves, stems, flowers, and berries collected from healthy trees (66 species) and grasses (40 species) found in Japan. Eight S. thermophilus strains were obtained from Acer palmatum var. matsumurae, Cornus controversa, C. officinalis, Fagus crenata, Lagerstroemia indica, Magnolia denudata, and Quercus crispula var. crispula. Then, we evaluated the characteristics of milk fermentation by using these eight strains. All eight strains fermented milk, and six of them coagulated 10 % (w/v) skim milk within 6 h at 40 °C. Finally, we characterized susceptibility to syneresis by centrifugation. Milk fermented by the plant strains was more resistant to syneresis than milk fermented by three S. thermophilus strains isolated from commercial yogurts. We found that S. thermophilus can be isolated from some plants found in Japan, and these plants are useful sources of S. thermophilus for milk fermentation.