Streptococcus thermophilus

This organism is a gram-positive, catalase-negative anaerobic coccus and it is largely used in the manufacture of hard cheese varieties, mozzarella, and yogurt. It does not grow at 10°C but grows well at 45°C. Most strains can survive 60°C for 30 min (22). It is very sensitive to antibiotics. Penicillin (0.005 IU/mL) can interfere with milk acidification (20). It grows well in milk and ferments lactose and sucrose. Two percent sodium chloride may prevent growth of many strains. These streptococci possess a weak proteolytic system. It is often combined with the more proteolytic lactobacilli in starter cultures. Most ST grow more readily in milk than lactococci and produce acid faster. These streptococci strains possess h-galactosidase (h-gal) and utilize only the glucose moiety of lactose and leave galactose in the medium (23).

The proteolytic activities of nine strains of ST and nine strains of LB cultures incubated in pasteurized reconstituted NFDM at 42 °C as single and mixed cultures were studied (24). Lactobacilli were highly proteolytic (61.0 to 144.6 Ag of tyrosine/mL of milk)

Table 3 Optional Cultures for Addition to Yogurt

Lactobacillus acidophilus Lactobacillus rhamnosus Lactobacillus reuteri Lactobacillus casei Lactobacillus gasseri Lactobacillus johnsonii Lactobacillus plantarum Bifidobacterium bifidum Bifidobacterium breve Bifidobacterium adolescentis Bifidobacterium infantis Bifidobacterium lactis Bifidobacterium longum Enterococcus faecalis Enterococcus faecium Sacchromyces boulardi Pediococcus acidilactici Propionibacterium freudenreichii

Source: From Refs. 18 and 19.

Table 4 Selected Characteristics of Organisms Found in Yogurt

Growth Carbohydrates fermented % Lactic acid Lactic - -

in milk acid isomer 15°C 45°C Glucose Inulin Galactose Fructose Lactose Sucrose Ribose Raffinose

Table 4 Selected Characteristics of Organisms Found in Yogurt

Lb. delbrueckii sp.

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