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a Do not grow in 2.5% salt. b Can grow in 6 to 8% salt. c 11 to 89% of strains are positive. Source: From Refs. 5, 20, and 21.

a Do not grow in 2.5% salt. b Can grow in 6 to 8% salt. c 11 to 89% of strains are positive. Source: From Refs. 5, 20, and 21.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

and ST were less protelytic (2.4 to 14.8 ag of tyrosine/mL of milk). Mixed cultures, with the exception of one combination, liberated more tyrosine (92.6 to 419.9 ag/mL) than the sum of the individual cultures. Mixed cultures also produced more acid (lower pH). Of 81 combinations of LB and ST cultures, only one combination was less proteolytic (92.6 ag tyrosine/mL) than the corresponding LB strain in pure culture (125 ag tyrosine/mL). Streptococcus thermophilus requires fewer amino acids than lactococci and lactobacilli. Only glutamine and glutamic acid, along with sulfur amino acids, are essential for all the strains that have been tested (25,26). Recently it was shown that ST possess a branched-chain amino acid (leucine, isoleucine, and valine) biosynthesis pathway as an essential pathway for optimal growth in milk. This pathway is thought to play a role in maintaining the internal pH of the organism by converting acetolactate to amino acids (26). These organisms also have urease, which produces ammonia from urea in milk, to counter the acid effects. Dairy lactococci do not have this pathway (26).

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