The viability was studied for four L. acidophilus strains (B, V-74, CH-2 and CH-5) and their mixtures in fermented milk products under refrigeration at 7°C for 35 days. L. acidophilus В exhibited no significant loss in viability after 35 days and maintained above 107 cfu/mL. The viability of strains V-74 and CH-2 declined but was higher than 106 cfu/mL of live cell numbers following 35 days. Strain CH-5 showed the poorest survival and its viable counts at the end-point of the test were around 104 cfu/mL. The mixtures of L. acidophilus strains had a lower loss in viability than single-strain cultures. Their viable cell counts were over 107 cfu/mL, which was more than the desired therapeutic-minimum dose. Regarding the viability displayed by the 4 strains, L. acidophilus strain В is suggested to be the best culture for further use. Moreover, use of a mixed-strain culture of L. acidophilus as a starter presents a good way to improve the viability of the organism during storage.
Lactobacillus plantarum SKT109 was isolated and identified from Tibet Kefir, and the exopolysaccharride (EPS)-producing properties of the strain were evaluated. Growth of strain SKT109 in a semi-defined medium at 37°C increased the viscosity of the medium, corresponding to production of an EPS (58.66 mg/L). The EPS was isolated and purified, and it was shown to consist of fructose and glucose in an approximate molar ratio of 3:1, with an average molecular weight of 2.1x106 Da. The aqueous solution of EPS at 1% (w/v) exhibited shear thinning behavior. Microstructural studies of the EPS demonstrated a highly compact structure with a smooth surface, facilitating formation of film by the polymer; the EPS was composed of many different sizes of spherical lumps with tendency to form molecular aggregates. Studies on the milk fermentation characteristics of L. plantarum SKT109 showed that the strain survived well in fermented milk with counts about 8.0 log cfu/g during 21 days of storage at 4°C. The use of the EPS-producing strain improved the rheology of the fermented milk without causing post-acidification during storage. Particularly, L. plantarum SKT109 improved the fermented milk flavor by increasing the concentration of characteristic flavor compounds and eliminating those with disgusting flavors. The results of the present study indicated that EPS-producing L. plantarum SKT109 could serve as a promising candidate for further exploitation in fermented foods.
The aim of the present study was to test the possibility of enhancing the synthesis of flavor compounds with a traditional yogurt culture and a culture containing microflora composed of: S. thermophilus : L. delbrueckii ssp. bulgaricus : Bifidobacterium (TBB yogurt) and Str. thermophilus : L. acidophilus : Bifidobacterium (TAB yogurt). Milk was enriched with lactose (10 g/L), glucose (0.7 g/L), sodium proteinate (25 g/L), sodium citrate (3 g/L), a mixture of citric acid (1 g/L) and sodium citrate (3 g/L), and threonine (1 and 3 g/L). The following determinations were made in fresh yogurts (ripened for 24 h) and yogurts stored for 7 and 14 days: active acidity, lactic acid – according to Lunder, acetaldehyde, diacetyl, acetoin, ethanol, and volatile free fatty acids by gas chromatography. All products were evaluated organoleptically. It was found that from among the substances whose effects on the synthesis of flavor compounds were studied in this experiment particular attention should be paid to threonine. The addition of this amino acid significantly affected the rate of acetaldehyde formation (an increase by 10.24-22.56 mg/L) in all kinds of yogurt. The modification of milk composition had a profound influence on the levels (increase or decrease) of all flavor compounds in yogurt samples, as well as on their sensoric attractiveness.
This study investigated the influence of four Lactobacillus acidophilus strains (B, V-74, CH-2 and CH-5) and their mixtures on the viability of Escherichia coli NCTC 8196 and Staphylococcus aureus NCTC 4163 in fermented milk stored at 7°C for 21 days. With no L. acidophilus present, both E. coli and S. aureus remained viable with more than 108 cfu/mL. In the presence of L. acidophilus, no detectable cells of E. coli were recovered after 7 days. Compared to those of its pure culture, the live cell counts of S. aureus were reduced by about 3- to 6-log following 21 days due to the presence of L. acidophilus in the refrigerated fermented milk. The viability loss of E. coli and S. aureus depended strongly upon the strains of L. acidophilus used. L. acidophilus В exhibited the best inhibitory effect on the growth and survival of both cultures tested, followed by L. acidophilus strains CH-2, CH-5 and V-74. The mixtures of four strains of L. acidophilus were more effective in reducing the viability of E. coli and iS. aureus than its single-strain cultures. In the presence of L. acidophilus mixed-strain cultures, E. coli generally could not be detected at the starting-point of this storage trial; and the viable cell numbers of aureus were less than 20 cfu/mL after 21 days. Use of L. acidophilus as a biological control agent to retard the enteropathogenic bacteria from surviving in dairy products would improve the storage stability of the foods.