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Abstract
Microencapsulation was tested for its protective role against oxygen toxicity in probiotic bacteria. Two strains of Bifidobacterium lactis and one strain of Lactobacillus acidophilus were encapsulated in calcium alginate and grown aerobically in reconstituted skim milk broth for 24 h. Counts of encapsulated cells in all three strains were one log higher than corresponding free cell counts. The encapsulated cell count (log^sub 10^cfu/mL) of B. lactis 920 was 9.12 as against 8.66 of free cells whereas L. acidophilus 2409 when encapsulated, demonstrated a cell count (log^sub 10^cfu/mL) of 7.84 as compared to a free cell count of 6.67. In a follow-up study, the protective effect of microencapsulation was also tested in yogurt. Twelve strains of L. acidophilus and Bifidobacterium spp. were encapsulated and incorporated in yogurt for 24 h maintained aerobicaily at 6[degrees]C. Interestingly, while microencapsulation was found to significantly increase viability in six strains, no significant difference was observed between encapsulated cell counts and free cell counts in the other six strains. Preliminary evidence therefore suggests that although microencapsulation can offer protection to probiotic bacteria against oxygen toxicity in broth culture, further optimisation studies are needed before its application in yogurt.
Introduction
There has been a rapid increase in the probiotic foods market in recent times (Hughes and Hoover 1991; Playne 1997; Kim 1998). Much of this has been stimulated by the reported health benefits of the probiotic bacteria (Tojo et al. 1987; Gilliland 1990). For probiotics to be effective, it has been suggested that there be a minimum level of 10^sup 6^ colony forming units per gram or mL of the product (Kurmann and Rasic 1991). Numerous market surveys however, have revealed a steady decline in the numbers of probiotic bacteria over the shelf life period (Anon. 1992; Iwana et al. 1993; Shah et al. 1995; Anon. 1999).
Among the several factors cited to affect the viability of probiotic cultures in fermented milk products (Lankaputhra and Shah 1995; Rybka and Kailasapathy 1995; Kailasapathy and Supraidi 1996; Dave and Shah 1997), cell death from oxygen toxicity is considered as very critical (Brunner et al. 1993). Lactobacillus acidophilus and Bifidobacterium spp., the two most commonly used probiotic species in yogurts, are generally classified as micro-aerophilic and anaerobic respectively (Hammes and Vogel 1995;...