Effects of exopolysaccharide-producing strains of Streptococcus thermophilus on technological and rheological properties of set-type yoghurt

Identification of lactic acid bacteria in traditional fermented yak milk and evaluation of their application in fermented milk products

In this study, 53 strains of lactic acid bacteria (LAB) isolated from Xueo, a traditional fermented yak milk in the western Sichuan Plateau of China, were identified and their use in fermented milk was evaluated. All gram-positive and catalase-negative strains were divided into 6 groups at the level of 87% similarity using amplified ribosomal DNA restriction analysis. These groups were identified as 6 species using 16S rDNA sequence analysis and atpA gene analysis. The dominant LAB strains in Xueo were Enterococcus durans, Lactobacillus fermentum, and Lactobacillus paracasei, accounting for 45.3, 22.6, and 17.0% of isolates, respectively. Milk fermented with most of the representative strains was high in quality, exhibiting relatively high viscosity, moderate acidity, good sensory quality, and high counts of viable LAB. Fermented milk of E. durans SCA16 and L. fermentum SCA52 achieved the highest scores for overall sensory quality. Most strains displayed antimicrobial activity against at least 1 of 9 spoilage microorganisms. Lactic acid was the main factor inhibiting the growth of spoilage bacteria, and H(2)O(2) was also inhibitory to some extent. Excluding the influence of acid and H(2)O(2), strains SCA52 (L. fermentum) and SCA7 (Lactobacillus plantarum) were antagonistic against some of the indicators, suggesting that the 2 strains may produce a bacteriocin-like substance. Therefore, the development of superior strains isolated from Xueo to ferment milk with similar flavor and texture to Xueo is expected.

Quality indices of the set-yoghurt prepared from bovine milk treated with horseradish peroxidase

Horseradish peroxidase (HRP, EC 1.11.1.7) was applied to treat whole bovine milk in the presence or absence of ferulic acid (FA). The treated milk exhibited different rheological properties from the control milk, and was used to prepare set-yoghurt with commercial direct vat set starter. Some chemical, textural and rheological properties of the yoghurt prepared were measured and compared. Compared to that prepared with the control milk, the yoghurt prepared with the HRP- or HRP and FA-treated bovine milk exhibited an increased hardness and adhesiveness, lower syneresis extent, higher apparent viscosity, and higher storage modulus and viscous modulus. Observation of the microstructure of the yoghurt samples under scanning electron microscopy illustrated that HRP treatment of bovine milk led to the prepared yoghurt a more compact and uniform structure. The results in the present work stated that treatment of bovine milk with HRP in the presence of ferulic acid could be applied to improve the quality of set-yoghurt.

Gelling properties of microparticulated whey proteins

Subjecting whey proteins to high-pressure shearing with or without heating, commonly termed microparticulation, results in novel ingredients with modulated functionalities. Gelling properties of microparticulated whey proteins (MWP) were specifically assessed in this study. MWP powders were produced from commercial cheese whey retentate, standardized to 10% (w/w) protein, and subjected to microfluidization (MFZ) at 140 MPa either with or without prior heat-induced denaturation, followed by spray-drying. Gels were created from aqueous MWP dispersions either by heating at 90 degrees C for 20 min or by allowing gels to form at ambient temperature through addition of glucano-delta-lactone and/or NaCl. MWP powders produced from unheated WP dispersions created firm gels upon heating, whereas those produced from denatured WP gave only cold-set gels. Covalent and noncovalent protein-protein interactions were involved during both heat- and cold-induced gelation. Hydrophobic interactions were more pronounced during aggregation of bovine serum albumin. In conclusion, microparticulation of WP resulted in heat- and cold-set gels with different molecular and physical characteristics from those of untreated controls.

Structural analysis of the exopolysaccharide produced by Streptococcus thermophilus ST1 solely by NMR spectroscopy

The use of lactic acid bacteria in fermentation of milk results in favorable physical and rheological properties due to in situ exopolysaccharide (EPS) production. The EPS from S. thermophilus ST1 produces highly viscous aqueous solutions and its structure has been investigated by NMR spectroscopy. Notably, all aspects of the elucidation of its primary structure including component analysis and absolute configuration of the constituent monosaccharides were carried out by NMR spectroscopy. An array of techniques was utilized including, inter alia, PANSY and NOESY-HSQC TILT experiments. The EPS is composed of hexasaccharide repeating units with the following structure: --> 3)[alpha-D-Glcp-(1 --> 4)]-beta-D-Galp-(1 --> 4)-beta-D-Glcp-(1 --> 4)[beta-D-Galf-(1 --> 6)]-beta-D-Glcp-(1 --> 6)-beta-D-Glcp-(1 -->, in which the residues in square brackets are terminal groups substituting backbone sugar residues that consequently are branch-points in the repeating unit of the polymer. Thus, the EPS consists of a backbone of four sugar residues with two terminal sugar residues making up two side-chains of the repeating unit. The molecular mass of the polymer was determined using translational diffusion experiments which resulted in Mw = 62 kDa, corresponding to 64 repeating units in the EPS.

Antibacterial activity of some lactic acid bacteria isolated from an Algerian dairy product

In the present study, the antibacterial effect of 20 lactic acid bacteria isolates from a traditional cheese was investigated. 6 isolates showed antibacterial effect against Gram positive bacteria. Streptococcus thermophilus T2 strain showed the wide inhibitory spectrum against the Gram positive bacteria. Growth and bacteriocin production profiles showed that the maximal bacteriocin production, by S. thermophilus T2 cells, was measured by the end of the late-log phase (90 AU ml(-1)) with a bacteriocine production rate of 9.3 (AU ml(-1)) h(-1). In addition, our findings showed that the bacteriocin, produced by S. thermophilus T2, was stable over a wide pH range (4-8); this indicates that such bacteriocin may be useful in acidic as well as nonacidic food. This preliminarily work shows the potential application of autochthonous lactic acid bacteria to improve safety of traditional fermented food.

Effect of exopolysaccharides on the proteolytic and angiotensin-I converting enzyme-inhibitory activities and textural and rheological properties of low-fat yogurt during refrigerated storage

The aim of this study was to examine the influence of using exopolysaccharide (EPS) producing strain of Streptococcus thermophilus on the viability of yogurt starters, their proteolytic and angiotensin-I converting enzyme-inhibitory activities, and on the textural and rheological properties of the low-fat yogurt during storage at 4 degrees C for 28 d. The use of an EPS-producing strain of S. thermophilus did not have influence on pH, lactic acid content, or the angiotensin-I converting enzyme-inhibition activity of low-fat yogurt. However, EPS showed a protective effect on the survival of Lactobacillus delbrueckii ssp. bulgaricus. Presence of EPS reduced the firmness, spontaneous whey separation, yield stress, and hysteresis loop area but not the consistency and flow behavior index of low-fat yogurt.