Kefir is a fermented dairy product that is described in many reference texts and has long been of academic interest to microbiologists, but was one for which few U.S. students (or consumers for that matter) had any first-hand knowledge. Although several brands of kefir are now available throughout the United States, it is still a mostly unknown product.This is despite the popularity of this product throughout a large part of the world. In the

Middle East, Eastern Europe, and Central Asia, especially Turkey, Russia, Ukraine, Poland, and the Czech Republic, kefir is one of the most widely consumed cultured dairy products, accounting for as much as 70% of all cultured milk products consumed. It is interesting that yogurt manufacturers have begun to introduce fluid or pourable yogurt-like products that are only slightly different than traditional kefir.

Kefir originated in the Caucasus Mountain in Russia. The traditional kefir manufacturing process, which is still widely practiced, relies on a mixed assortment of bacteria and yeast to initiate the fermentation (Figure 4-8).The kefir fermentation is unique among all other dairy fermentations in that the culture organisms are added to the milk in the form of insoluble particles called kefir grains (Box 4-6). Moreover, once the fermentation is complete, the kefir grains can be retrieved from the fermented milk by filtration and reused again and again.

The flavor of plain kefir is primarily due to lactic and acetic acids, diacetyl, and acetalde-hyde, produced by homofermentative and het-erofermentative lactic acid bacteria. However, because kefir grains also contain yeast, in addition to lactic acid bacteria, other end-products are formed that make the finished product quite different from other cultured dairy products. This is because ethanol is produced when the yeasts ferment lactose, such that kefir can contain as much as 2% ethanol (accounting, perhaps, for its appeal). In the United States, this much ethanol would trigger action from the regulatory agencies (i.e., Alcohol and Tobacco Tax and Trade Bureau).Thus, if yeast are present in the culture (most kefir products made in the United States claim yeasts on their labels), they must be low- or non-ethanol producers. Kefir grains can also contain acetic acid-producing bacteria, such as Acetobacter aceti.

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