Although lactobacilli grow best under slightly acidic conditions, other starter culture microorganisms prefer conditions near neutrality. For example, the optimal pH for growth of S. thermophilus is 6.5, whereas for Lb. delbrueckii subsp. bul-garicus, it is 5.8 (Beal et al., 1989). The optimal pH for growth of lactococci ranges from 6.0 to 6.5 (Mayra-Makinen and Bigret, 1993). As the pH decreases below the optimal range, growth slows, and as the pH continues to decrease, cells become susceptible to sublethal acid injury and gradually lose their activity. The greater the loss of activity, the longer the ripening time required before rennet addition when making Cheddar cheese. Therefore, maintaining the pH of culture
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