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KEFIR MILK: FERMENTATION AND SAFETY
by: Desiree Bayliss
CSU Public Health Graduate Student
Kefir milk is a fermented milk product heralded for its nutritional and probiotic benefits and reported to improve immune and digestive function. Kefir originated in the Caucasus Mountains of Eastern Europe where it has been enjoyed for several centuries. It was traditionally fermented in leather sacks hung or placed by a door and visitors were expected to kick or shake the sack as they entered the household. Kefir contains several types of microorganisms but the major ones are Lactobacillus bacteria and lactose fermenting yeasts. These microorganisms produce lactic acid, acetic acid, and alcohol, which give kefir its distinctive properties and flavor. The acidic nature confers an antimicrobial property to the mixture that inhibits the growth of most undesirable microbes. However, mold and harmful bacteria can grow if the milk is not fermented long enough to produce an adequate level of acid. Over-fermenting can produce too much acid, making the product unacceptable and reducing the probiotic benefits.
The starter culture, called kefir grains, resemble cauliflower and consists of bacteria and yeast in a matrix of proteins, lipids and sugars. The starter culture is added to fresh milk which is then allowed to ferment. However, traditional kefir cannot be made from simple mixing of these microbes in part because the exact nature of kefir grain formation is not well understood. Kefir grains can be purchased for home production of kefir. A lid or cloth is traditionally placed on top of the jar during fermentation. Gases are produced during the fermentation process; therefore, if a lid is used, the container must not be filled more than 2/3 full to allow room for gas formation. The mixture is left out at room temperature for 12-48 hours, depending on taste preferences. Shorter fermentation creates sweeter, thinner kefir, while longer fermentation creates a thicker kefir with more sour tones. Kefir can also be fermented in the refrigerator, but will take approximately 5 days to ferment. The mixture should be shaken or stirred regularly to disperse the fermenting microbes throughout the container and ensure that all sugar is fermented, and proper tasting kefir milk is produced.
It is critical to practice good hygiene throughout the process and use clean and sanitized containers and utensils to prevent contamination. Sanitation requires soaking the kefir-making equipment in a solution of one tablespoon of bleach per gallon of water for 20 minutes, rinsing until no bleach odor can be detected, and then air drying. If contamination is present during the first steps of production, mold and harmful bacterial growth can occur. This is due to the initial lower microbial count of the culture and thus, lower acid content of the kefir at this stage. Beneficial microbes also compete for nutritional resources with contaminating bacteria and molds and lower numbers can reduce this effect. Bacteria which cause foodborne illness, such as E. coli, Listeria and Yersinia, have been shown to survive the kefir fermentation process and emphasize the importance of using good hygiene practices during preparation. Contamination issues can also occur during storage so care must be taken to dispose of kefir at any sign of microbe growth, foul smell, or change in color. Home-made kefir can be pasteurized after the fermentation process and is recommended for those with compromised immune systems.
- Fermented Foods: Kefir. National Center for Home Foods Preservation. 2004. Accessed February 2010 from http://www.uga.edu/nchfp/publications/nchfp/factsheets/kefir.html.
- Garrote, Graciela L., Abraham, Analía G.De Antoni, Graciela L. Inhibitory Power of Kefir: The Role of Organic Acids. Journal of Food Protection. 2000. 63: 364-369.
- L.M. Medina and R. Jordano. Growth of fungal contamination in fermented milk containing bifidobacteria and Lactobacillus acidophillus. Journal of Food Quality. 1993. 16:471-477
- M. Gulmez and A. Guven. Survival of Escherichia coli O157:H7, Listeria monocytogenes 4b and Yersinia enterocolitica O3 in different yogurt and kefir combinations as pre-fermentation contaminant. Journal of Applied Microbiology. 2003. 95: 631-636.
- T.-H. Chen, S.-Y. Wang , K.-N. Chen, J.-R. Liu and M.-J. Chen. Microbiological and chemical properties of kefir manufactured by entrapped microorganisms isolated from kefir grains. Journal of Dairy Science. 2009. 92: 3002-3013.
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