|MadSci Network: Microbiology|
The normal shelf life of yogurt is 20 - 40 days from processing. Yogurt has a longer shelf life than milk for two major reasons: pH and numbers of beneficial bacteria present at the end of production. In general, the pH of plain yogurt ranges from 4.4 - 4.2. This pH range is not favorable for growth of many bacteria; however, there are bacteria that can tolerate acidic conditions. Yogurt production uses two main bacterial cultures: Lactobacillus bulgaricus and Streptococcus thermophilus. Both of these bacteria are lactic acid bacteria (LAB); that is, they produce lactic acid during normal growth. During fermentation, lactic acid produced will result in a decrease of pH of the yogurt. LAB can tolerate acidic conditions generated; however, growth of other bacteria may be inhibited. In addition, at low pH, lactic acid will dissociate (RCOOH --> RCOO- and H+). In general, the dissociated form of a weak organic acid acts as an antimicrobial agent. The other factor playing a major role in the shelf stability of yogurt are the high numbers of live LAB (as many as one billion live LAB/ml may be present at the end of yogurt production!). The large numbers of viable LAB will thus act as competitive microflora against other microflora (i.e. spoilage and pathogenic bacteria) not normally present in yogurt. The production of plain yogurt from milk begins with homogenization and pasteurization. Before pasteurization, stabilizers (such as gelatin or modified food starched) may also be added (usually, stabilizers are added for production of Swiss and Sundae style and flavored yogurts). The milk is then cooled to 46.7C and inoculated at 1.25% each of Lb. bulgaricus and S. thermophilus. After inoculation, the mix is poured into yogurt cups and incubated at 46.7C for 3-5 hours. During this time, the pH will decrease due to lactic acid production. Other characteristic compounds (i.e. acetic acie, diacetyl and acetaldehyde) of yogurt will also be formed during fermentation. After incubation, yogurt is then stored at refrigeration temperatures. In addition to Lb. bulgaricus and S. thermophilus, other "probiotic" (beneficial) bacterial cultures may be added. These include Lb. acidophilus, and some strains of Bifidobacteria. Research targeting the therapeutic benefits of these bacteria is a current "hot topic". There are two types of yogurts: Swiss (also known as Continental or stirred-style) and Sundae style. Swiss style is flavored yogurt (either flavors or fruit purees). If flavorings are used, they are added prior to incubation. If fruit purees are used, they are added and blended into plain yogurt and packed. Sundae style yogurt has fruit purees on the bottom of plain or vanilla yogurt. References: Kosikowski, F.V. 1982. Cheese and Fermented Foods. 2nd Ed. Brookrondale, NY.
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