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Summary
This paper presents an overview of the probiotic organisms used as live supplements, with emphasis on Lactobacillus acidophilus and Bifidobacterium spp., their therapeutic potential and their survival during the manufacture and storage of yogurt. The factors affecting viability of these probiotic bacteria and ways of improving their survival are also discussed. L. acidophilus and Bifidobacterium spp. have been reported to provide excellent therapeutic benefits; their incorporation into yogurt, therefore, improves it as a therapeutic functional food. However, poor viability and survival of these bacteria remain a problem in commercial yogurts. Selecting better functional probiotic strains and adopting improved methods of enhancing survival could increase delivery of viable beneficial bacteria to yogurt consumers.
Introduction
With the emergence of antibiotic-resistant bacteria and natural ways of suppressing pathogens, the concept of 'probiotics' has attracted much attention. Probiotic bacteria not only compete with and suppress "unhealthy fermentation" in the human intestine, but also produce a number of beneficial health effects of their own (Metchnikoff 1908). Fuller (1989) defined probiotics as "a live microbial feed supplement, which has beneficial effects on the host by improving its intestinal microbial balance". Most commonly, Lactobacillus acidophilus, L. casei, Bifidobacterium bifidum, B. longum and the yeast Saccharomyces boulardii have been used as probiotics in humans (Playne 1994).
The contribution of yogurt bacteria to the improvement of intestinal microflora has been widely recognised. Most strains of L. delbrueckii subsp. bulgaricus and Streptococcus thermophilus do not survive in the intestinal tract. The incorporation of L. acidophilus and B. bifidum into the yogurt starter culture produces a milk product of excellent 'therapeutic' value (Tamime and Robinson 1985). Regular consumption of yogurt (400-500g/week) containing 1.0 ? IO6 cfu/ g of Bifidobacterium spp. and L. acidophilus (human origin), which are able to survive the upper regions of the gastrointestinal tract, is essential to achieve potential therapeutic advantages (Tamime et al. 1995).
Some yogurt products have been reformulated to include L. acidophilus and Bifidobacterium species (known as AB-culture). They have captured 4% of the total fresh milk sales in France, and approximately 25% of the fermented milk products' market in Sweden (Hughes and Hoover 1991). Probiotic yogurts (containing AB-culture) are produced in Germany, Japan, Canada, Italy, Poland, Czechoslovakia, England and Brazil (Orihara et al. 1992). In recent...