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Abstract
This study investigated the growth of bifidobacteria, lactococci, leuconostocs and yeast during kefir fermentation, using bifidobacteria and kefir grain as starters. The tolerance of lactic acid bacteria from kefir to HCL and bile salt solution, the influence of bifidobacteria to the quality and the antioxidative of kefir were also investigated.
Results indicated that bifidobacteria could co-cultivate with kefir grain at 21℃ and there’s cells counts were from 107~108 CFU/ml and the higher counts are B. longum and B. adolescentis. B. longum would not affect the original microbial flora and ethanol content of kefir. Furthermore, the fermentation time of bifidobacteria treatments were shorter than the control, which titratable acidity of final fermentation broth were from 1.64 to 1.71%, volatile acidity were from 0.038 to 0.047%. Bifidobacteria grew rapidly at 30 and 37℃; however, the higher growth rate caused off-flavor and whey separation of kefir when it’s pH arrived at 4.7.
Lactococci and leuconostocs from kefir could survive in HCl and bile salt solutions. In pH2 and 0.6% bile salt solutions for 4 to 8 hours, their cells counts were reduced about two log values, and bifidobacteria were reduced about one or two log values.
Kefir grain could produce polysaccharide with a higher yield at 21℃ than at 30 or 37℃. As fermentation time was extended, the cell-counts of microflora, the titratable acidity and polysaccharide yields were also increased while the reducing sugar were decreased. The production of polysaccharide usually raised at the end of kefir fermentation. Because bifidobacteria grew badly at 21℃ and produced slime, B. longum and B. adolesentic treatments showed the highest polysaccharide yield at 21℃. Bifidobacteria treatments produced higher polysaccharide at pH 6 and 7 than at pH5 and 8.
Antioxidative property of kefir indicated that intracellular extracts of kefir flora could inhibit ascorbate autooxidation, metal ion chelation for Fe+2 and Cu+2, and scavenge of free hydroxyl radical. Bifidobacteria inoculation could increase chelation ability for Fe+2 and Cu+2, hydroxyl radical scavenging ability, but couldn’t increase the inhibition to ascorbate autooxidation.
Milk fermentation broth inoculation with bifidobacteria showed higher antioxidative property than MRS broth. Inhibition rates of ascorbate autooxidation, Cu+2 chelation and hydroxyl radical scavenge were also found in milk and MRS fermentation broths. The supernatant obtained from MRS broth didn’t show Fe+2 chelating ability.
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