The fermentation is considered complete when the target acidity is reached and the yogurt is cooled quickly to below 4°C. In fact, for all practical purposes, cooling is really the only way to arrest the fermentation and stop further acid production. Cup-set yogurt must be very carefully moved to coolers (0°C to 4°C) to avoid agitation which may disturb the gel, resulting in syneresis. For Swiss style yogurt, where the fermentation occurs in a vat, the yogurt is typically stirred and cooled in the vat, then mixed with fruit or other flavoring, and filled into cups or containers. It is important to recognize that during the cooling period the pH may continue to drop by an additional 0.2 to 0.3 pH units, so initiating the cooling step even when the pH is 4.8 to 4.9 may be warranted. In addition, some culture strains may continue to produce acid during refrigerated storage, albeit slowly. Over-acidification and post-fermentation acidification are major problems, since U.S. consumers generally prefer less acidic yogurts (see below).
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