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xData obtained from multiple sources for the given year xData obtained from multiple sources for the given year as probiotics, organisms that confer a health benefit to the host (Box 4-1).

For example, Lactobacillus acidophilus and various species of Bifidobacterium are commonly added to yogurt as probiotics, even though these organisms are not part of the yogurt starter culture, nor are they involved in any way in the actual yogurt fermentation. Their only contribution is as a culture adjunct intended to promote good health. Still, according to Dairy Management Inc., about 80% of the yogurt products produced in the United States contain L. acidophilus. The mechanisms by which these suggested health benefits actually occur and the evidence to support these claims will be discussed later in this chapter.

In addition to probiotic bacteria, a group of compounds called prebiotics are also now being added to cultured milks—mainly yogurt and kefir in the United States, but many more products in Europe, Japan, and Korea. Prebiotics are oligosaccharide-containing materials that are neither degraded nor absorbed during transit through the stomach or small intestine (Box 4-1).They end up in the colon, where they are preferentially fermented by intestinal strains of lactobacilli and bifidobacteria.Thus, they enrich the population of those bacteria thought to contribute to gastrointestinal health.

Probiotics can now be found not only in yogurt, but also in a variety of other cultured and non-cultured dairy products (Table 4-2). In some of these products, as noted above, the probiotic bacteria are not involved in the fermentation. However, some new products have been developed to take advantage of their fermentative ability as well as their probiotic activity. Examples include Yakult, a Japanese product made using a special strain of Lacto-bacillus casei (strain Shirota) that was originally isolated from the human intestinal tract, and Cultura, a European "bioyogurt" made with L. casei F19 (also a human isolate).

At the same time, there are also dairy products containing probiotic lactic acid bacteria that are not fermented but serve strictly as a carrier. The product known as Sweet Aci-dophilus Milk, for example, is simply fluid milk supplemented with L. acidophilus (added after pasteurization). Similar unfermented products may also contain other probiotic bacteria. Maintaining the product at low refrigeration temperatures (the same as for normal fluid milk products) prevents the bacteria from fermenting and souring the milk.

Fermentation Principles

The principles involved in the manufacture of specific cultured dairy products, which lactic acid bacteria are used for these products, and the attributes and properties desired by these bacteria are the focus of this chapter. Fermented dairy products from around the world, including India,Africa, Iceland, and other countries, will be described, but the emphasis will be on those products most widely consumed in the United States. In theory (and practice), some of these products can be made in the absence of a starter culture simply by adding food-grade acidulants to the milk mixture. Indeed, there are manufacturing advantages for following this practice; however, these acid-set products are not fermented—in fact,they must

110 Microbiology and Technology of Fermented Foods Box 4—1. Probiotics and Prebiotics

History

Humans began consuming cultured dairy products thousands of years ago.These yogurt-like products were easy to produce, had good shelf-lives (so to speak), were free of harmful substances, and had a pleasant sensory appeal.At some time, it is theorized, they were also regarded as having therapeutic value, even though there was no scientific basis for this notion (Shortt, 1999).After all,the existence of bacteria and their role in fermentation weren't even recognized until Pasteur's research in the 1860s.Then, at the beginning of the twentieth century, the Russian scientist Elie Metchnikoff (who was working at the Pasteur Institute in Paris) suggested that the health benefits of fermented milk were due to the bacteria involved in the fermentation.

Specifically, Metchnikoff argued that these bacteria (which may have been the yogurt bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus or other lac-tobacilli) inhibited putrefactive bacteria in the intestinal tract, thereby influencing the intestinal microflora such that overall health and longevity could be enhanced. At around the same time and shortly thereafter, other scientists (as recounted by Shortt, 1999) reported that consumption of other lactobacilli (including Lactobacillus acidophilus) and bifidobacteria also had positive health effects, including reducing the rate of infant diarrhea.

Interestingly, the observation a full century ago that bifidobacteria were present in the fecal contents of breast-fed infants suggested that these bacteria were associated with good intestinal health and possibly foreshadowed the concept of prebiotics (see below).These early reports, by highly regarded scientists and research institutes, attracted the attention of the medical community, and by the 1920s, studies using bacteria therapy (with milk as the carrier vehicle) had begun. Unfortunately, many of these subsequent studies suffered from the absence of established measurement criteria, the use of mis-identified strains, and other design flaws (Shortt, 1999).As described later in this chapter, in more recent years, these experimental limitations have been recognized and addressed and now rigorous and appropriate methodologies are being used (Reid et al., 2003).

Definitions

The term "probiotics,"as originally used in 1965,referred to the "growth-promoting factors"pro-duced by one microorganism that stimulated growth of another (i.e., the opposite of antibi-otics).This definition went through several permutations and by 1989, probiotics were defined as a "live microbial feed supplement which beneficially affects the host animal by improving its intestinal balance"(Fuller, 1989).The current definition, adopted by the World Health Organization (part of the United Nations' Food and Agriculture Organization), defines probiotics as "live microorganisms which when administered in adequate amounts confer a health benefit on the host."This latest derivation is important because it recognizes both the relevance of a live and sufficient dose as well as the many reports that indicate probiotics may have health benefits that extend beyond the gastrointestinal tract.Whether all of the benefits ascribed to probiotics (Table 1) can be validated is another matter, however, as discussed later in this chapter.

Prebiotics are defined as "non-digestible food ingredient(s) that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and thus improves host health" (Gibson and Roberfroid, 1995).To re-phrase this definition, but with more detail, prebiotics are carbohydrate substances that escape digestion and adsorption in the stomach and small intestine and instead reach the colon.There, they are selectively metabolized by specific members of the colonic microflora. Prebiotics, therefore, enrich the population of those generally desirable commensal organisms such as lactobacilli and bifidobacteria at the expense (theoretically) of their less desirable competition.

Box 4—1. Probiotics and Prebiotics (Continued)

Table 1. Suggested health benefits of probiotic bacteria.

Reduce blood cholesterol

Maintain intestinal health

Alleviate intestinal bowel diseases

Modulate immune system

Reduce incidence of gastrointestinal infections

Reduce incidence of urinary and vaginal infections

Alleviate lactose intolerance

Anti-carcinogenic and anti-tumorogenic

Reduce incidence and severity of diarrheal diseases

Most of the prebiotics that are used commercially and that have received the most research attention are either polysaccharides or oligosaccharides.They are either derived from plant materials or are synthesized from natural disaccharide precursors (Figure 1). For example, inulin is ch2oh

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