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Fermented Foods: Kefir

Brian A. Nummer, Ph.D.
National Center for Home Food Preservation
November 2004

Kefir is a cultured-milk beverage believed to have originated many centuries ago in the Northern Caucasus Mountains. Kefir has a uniform creamy consistency, a slightly sour taste somewhere between buttermilk and sour cream, and a mild yeasty aroma. Kefir may have small amounts of carbonation and alcohol. It can be enjoyed plain or sweetened to taste. Traditional kefir is prepared by combining fresh milk with the Kefir culture made up of yeasts and lactic acid bacteria. Kefir's live culture has been claimed to have health benefits similar to that of yogurt.

The kefir culture is more commonly referred to as "grains" since it forms grain-like casein-polysaccharide-microorganism particles during fermentation. The exact combination of bacteria and yeasts vary between kefir cultures, and might include: Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. Cremoris, Lactococcus lactis subsp. Diacetylactis, Leuconostoc mesenteroides subsp. Cremoris, Lactobacillus kefyr, Klyveromyces marxianus var. Marxianus, and Saccaromyces unisporus. To ensure consistency and sterility, commercial producers now generally use a powdered starter culture rather than grains. However, such cultures may not form grains or continue to culture indefinitely; making kefir grains the preferred choice for individuals.

Commercial production starts with whole, low-fat or skim milk, adjusted for body with nonfat milk solids. The milk is pasteurized, and then heat-treated at 203F for 10 to 15 minutes denaturing whey proteins. This product is then cooled to 64.4 to 71.6F, 2% to 5% kefir grains or culture are added, and the mixture is incubated at 64.4 to 71.6F for 24 hours. After that time the kefir grains are sieved out and the product is pasteurized, chilled and packaged. The final kefir product can be flavored in a manner similar to yogurt.

Home fermentation of kefir was traditionally a mechanism to preserve milk before the advent of refrigeration. Fermented foods are generally considered to be less likely to cause foodborne illness due to the fermentation process. The competitive activity and metabolites of the culture help to - partially or completely -- kill or inhibit the growth of illness-causing microorganisms. Today however, the preservation of milk is easily accomplished using pasteurization and refrigeration, leaving kefir to be enjoyed for its flavor.

Kefir is generally considered to be safe due to the lack of evidence of foodborne illness events related to it. Properly fermented kefir (pH less than 4.5) inhibits many pathogens, but not for Escherichia coli, Listeria monocytogenes, Salmonella spp., and Yersinia enterocolitica. Care therefore must be taken in the home fermentation of kefir to prevent the access or growth of these microorganisms.

  • Use only pasteurized milk.
  • Use quality kefir grains from a reputable source.
  • Because of the small risk of pathogen growth in home fermented kefir, it is NOT recommended for those with weakened immune systems, e.g. pregnant women, the elderly, the very young and the chronically ill.
  • Pasteurization of kefir before consumption will kill the microorganisms listed above.

Pasteurizing home-made kefir can be accomplished by heating the kefir, after the grains have been filtered off, to 161°F for 15 seconds. Place the open jars of kefir in a hot water bath. Stir the kefir while heating until a temperature of 161°F is reached, using a quality food thermometer. Hold at this temperature, making sure the temperature does not drop below 161°F for at least 15 seconds, and then remove the jars from the bath. Cool them quickly in a cool water bath. Promptly refrigerate the kefir at or below 40°F; it may be stored for 7-10 days. Pasteurized kefir will not have the probiotic effects of a live culture, but will be safer, especially for those with weakened immune systems.

References

Heller, KJ. 2001. Probiotic bacteria in fermented foods: product characteristics and starter organisms. American Journal of Clinical Nutrition (73)2:374S-379s.

Gulmez, M and A Guven. 2003. Survival of Escherichia coli O157:H7, Listeria monocytogenes 4b and Yersinia enterocolitica O3 in different yogurt and kefir combinations as prefermentation contaminants. Journal of Applied Microbiology 95:631-636.

Dom's Kefir-making in-site Website: http://users.chariot.net.au/~dna/Makekefir.html (Accessed 15 Nov 2004).


Brian A. Nummer is Project Coordinator with the National Center for Home Food Preservation, Department of Foods and Nutrition, College of Family and Consumer Sciences and Adjunct Assistant Professor, Department of Food Science and Technology, The University of Georgia, Athens.

This material is based upon work supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 00-51110-9762.

Document Use:

Permission is granted to reproduce these materials in whole or in part for educational purposes only (not for profit beyond the cost of reproduction) provided the authors and the University of Georgia receive acknowledgment and this notice is included:

Reprinted with permission of the University of Georgia. B.A. Nummer. 2004. Home Preservation of Pecans. Athens, GA: The University of Georgia, Cooperative Extension Service.

References to commercials products, services, and information is made with the understanding that no discrimination is intended and no endorsement by the University of Georgia, U.S.

Department of Agriculture and supporting organizations is implied. This information is provided for the educational information and convenience of the reader.

The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service, the University of Georgia College of Agricultural and Environmental Sciences offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, sex or disability. An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force.

Contact:

National Center for Home Food Preservation
208 Hoke Smith Annex
The University of Georgia
Athens, GA 30602-4356

Tel: (706) 542-3773
Fax: (706) 542-1979
Email: nchfp@uga.edu
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