Formulations and Processing of Yogurt Affect the Microbial Quality of Carbonated Yogurt

¹ Department of Food Science and Technology, Mississippi Agricultural and Forestry Experiment Station, Southeast Dairy Foods Research Center, Mississippi State University, Mississippi State 39762

Carbonation, flavor, culture type, pH, and storage time were varied to investigate the effects of these variables and their interactions on the growth of both typical and nontypical yogurt cultures and some contaminating bacteria. Two types of yogurt cultures (YC-470 and YC-180) were used as the source of typical yogurt bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. In addition, Lactobacillus acidophilus (LA-K) and Bifidobacterium longum ATCC 15707 were added as nontypical yogurt cultures to make sweetened low fat (1%) Swiss-style plain, strawberry, and lemon yogurts. Samples were incubated at 43°C until pH values of 5.0 or 4.2 were reached. Strawberry yogurts at low (4.2) and high (5.0) pHs were divided into three portions, which were separately inoculated with contaminating bacteria, Bacillus licheniformis ATCC 14580, Escherichia coli ATCC 11775, and Listeria monocytogenes Scott A. After incorporation of carbon dioxide (1.10 to 1.27 volume of CO₂ gas dissolved in water), the yogurt was stored at 4°C for a 90-d period. Carbon dioxide did not affect the growth of typical or nontypical yogurt bacteria. Also, CO₂ did not inhibit the growth of undesirable microorganisms. In general, low levels of CO₂ did not affect the bacterial population in yogurt. The microflora of yogurt were influenced by culture type, pH, flavor type, and storage time or their interactions.

Key Words: carbonation • yogurt • culture • pH

Submitted on May 16, 2000
Accepted on October 25, 2000

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