INTRODUCTION
Probiotics have been defined as “live microbial feed supplements that have beneficial effects on the host by improving its intestinal microbial balance” (9). Al- though yet to be proven conclusively, it is believed that probiotic organisms compete with and suppress the growth of undesirable microorganisms in the colon and small intestine, and thus help to stabilize the digestive system. Other benefits include prevention of intestinal infections, expression of antitumor activities, and im- provement of lactose utilization in the human gut (12). Several species of Bifidobacterium are considered to be the most important among the probiotic organisms. Although these organisms ferment lactose, they are un- able to generate enough lactic acid for the manufacture of fermented foods with characteristic aroma and flavor. They are normally incorporated as live cultures into fermented dairy products that can serve as carrier foods, such as yogurt (24). Yogurt, which is consumed by a wide cross section of people throughout the world, has an established market as a functional therapeutic food. Incorporation of bifidobacteria has been shown to enhance the therapeutic value of yogurt (12). However, the survival of bifidobacteria in yogurt is quite low be- cause the pH of yogurt ranges from 4.2 to 4.6. Lanka- puthra et al. (16) reported the survival of only three out of nine bifidobacterial strains in the pH range of
3.7 to 4.3. Klaver et al. (14) found that 14 out of 17
strains lost their viability in fermented milks in the first week of storage. Hughes and Hoover (11) reported the same observation. Also, Samona and Robinson (24) noted that the presence of yogurt culture adversely af- fected the growth of bifidobacteria irrespective of their species. Because the minimum suggested level of viable bifidobacterial cells at the time of consumption is ap- proximately 107 cfu/g of product, and the suggested daily intake is approximately 109 viable cells (around
100 g of product per day), this poses a problem for the yogurt industry (15).