Valerie M. Marshall

The relevance of exopolysaccharides to the rheological properties in milk fermented with ropy strains of lactic acid bacteria

The subunit structures from a number of different species of lactic acid bacteria are now known, with many of the structures being published only in the last five years. The nuclear magnetic resonance spectra to date show around twenty different subunits and molecular mass determinations suggest that these exopolysaccharides occur as very large molecules, frequently greater than 1×106 Da. The physical properties of these molecules will be influenced by their mass and also by the subunit structure. Milk however is a complex medium and studies concerned with effects on rheology of milk as a result of in situ production of exopolysaccharides from these bacteria have to consider many factors. Studies have found that it is not only the nature of the exopolysaccharide, but that the amount produced, the acidity of the milk, the composition of the milk and the length of fermentation, also influence final texture.

Yogurt production from reconstituted skim milk powders using different polymer and non-polymer forming starter cultures

Using polymer producing (ropy) strains of lactic acid bacteria it was possible to reduce considerably the syneresis of yogurt, even with 12% total milk solids. The viscosities obtained with these strains were also similar to those obtained using normal strains and milk with 17% total solids content. The concentration of milk and the polymer produced by ropy starters had a synergic effect in increasing viscosity. Polymer production was not affected in most cases by milk concentration. One type of ropy culture (Wiesby) seemed to produce a different kind of polymer as it could not be determined by alcohol precipitation, in spite of being able to reduce syneresis and increase viscosity in yogurt. A limited number of yogurts were evaluated organoleptically, one prepared with a ropy starter strain (NCFB at 12, 14·5 and 17% total solids) and one prepared with a non-ropy strain (LL-I at 17% total solids). The results suggest that the ropy strain yogurts had different mouthfeel from the non-ropy strain yogurts; the most acceptable product overall was the ropy strain made with 12% total solids.

Exopolysaccharide-producing Streptococcus thermophilus strains as functional starter cultures in the production of fermented milks

Relationships between exopolysaccharide (EPS) production (amount, molecular mass and sugar composition of the EPS) by different Streptococcus thermophilus strains as a functional starter culture, and textural characteristics (viscosity) of fermented milk and yoghurt have been studied. Five interesting heteropolysaccharide-producing strains have been tested. Both S. thermophilus LY03 and S. thermophilus CH101 produced the highest amounts of EPS and also displayed the highest apparent viscosities in fermented milk. S. thermophilus ST 111 and S. thermophilus STD differed considerably in EPS yields, but not in apparent viscosities of fermented milk. In addition, S. thermophilus ST 111 displayed a high variability in EPS amounts when cultivated in milk. In milk medium, S. thermophilus LY03 produced two heteropolysaccharides, a high-molecular-mass (HMM) EPS and a low-molecular-mass (LMM) EPS of the same composition (Gal/Glu/GalNAc=3.4:1.4:1.0). S. thermophilus ST 111 produced only a HMM-EPS (Gal/Rha=2.5:1.0), while S. thermophilus CH 101 (Gal/Glu=1.0:1.0), S. thermophilus ST 113 (Gal/Glu/Rha/GalNAc=1.7:3.9:1.5:1.0) and S. thermophilus STD (Gal/Glu/Rha/GalNAc=3.5:6.2:1.2:1.0) produced only LMM-EPS. Both HMM-EPS and LMM-EPS solutions (S. thermophilus LY03) demonstrated a pseudoplastic character; HMM-EPS solutions of 0.2% (m/v) displayed a high consistency as well. Although its production of high EPS amounts, S. thermophilus LY03 resulted in relatively thin yoghurts, so that texture values did not directly correlate with EPS production capacity. Once structure/function relationships are known, one can determine the molecular properties of the isolated and purified EPS (molecular size, structural characteristics) from candidate strains to predict their potential in texture formation. For a final selection of interesting EPS-producing starter strains one should test the EPS production under yoghurt manufacturing conditions.

Analysis and production of two exopolysaccharides from Lactococcus lactis subsp. cremoris LC330

Two polysaccharides produced concurrently by Lactococcus lactis subsp. cremoris strain LC330 have been identified. One had a high molecular mass (> 1 × 10 6 Da) and was neutral. The second was smaller (∼ 10000 Da), charged and had a high phosphorus content. Sugar composition also differed. In chemostat culture the neutral polysaccharide was influenced by temperature and by nitrogen limitation. This polysaccharide was branched with terminal galactose moieties and contained galactose, glucose and glucosamine. The phosphopolysaccharide was more complex with glucose, rhamnose, galactose and glucosamine in an approximate ratio of 6:5:4:1.

Threonine aldolase and alcohol dehydrogenase activities in Lactobacillus bulgaricus and Lactobacillus acidophilus and their contribution to flavour production in fermented milks

Cell-free extracts of both Lactobacillus bulgaricus and L. acidophilus demonstrated threonine aldolase activity, the end product of which was acetaldehyde, the major flavour compound of yoghurt. L. acidophilus also possessed an alcohol dehydrogenase activity capable of reducing acetaldehyde so that little yoghurt flavour was present in milks fermentation with this organism. Addition of threonine to fortified milk before fermentation with L. acidophilus increased acetaldehyde production and resulted in a well flavoured product similar to that of yoghurt made with L. bulgaricus . The contribution of these 2 enzymes to flavour production is discussed.

Exopolysaccharide-producing strains of thermophilic lactic acid bacteria cluster into groups according to their EPS structure.

Aims: To compare galactose‐negative strains of Streptococcus thermophilus and Lactobacillus delbrueckii subspecies bulgaricus isolated from fermented milk products and known to produce exopolysaccharides (EPSs).

Structural characterisation of the exopolysaccharide produced by Streptococcus thermophilus EU20

Streptococcus thermophilus EU20 when grown on skimmed milk secretes a high-molecular-weight exopolysaccharide that is composed of glucose, galactose and rhamnose in a molar ratio of 2:3:2. Using chemical techniques and 1D and 2D-NMR spectroscopy (1H and 13C) the polysaccharide has been shown to possess a heptasaccharide repeating unit having the following structure: [chemical structure: see text]. Treatment of the polysaccharide with mild acid (0.5 M TFA, 100 degrees C for 1 h) liberates two oligosaccharides; the components correspond to the repeating unit and a hexasaccharide equivalent to the repeating unit minus the terminal alpha-L-Rhap.

Ability of some strains of lactic acid bacteria to degrade phytic acid

Twelve strains of lactic acid bacteria were examined for their ability to degrade phytate. In media in which phytic acid was the source of phosphate, phytate degradation was observed. Phytate disappearance may however not only be due to phytase, as phytic acid coprecipitated with protein as a consequence of a fall in pH during fermentation.

Structural characterisation of a perdeuteriomethylated exopolysaccharide by NMR spectroscopy: characterisation of the novel exopolysaccharide produced by Lactobacillus delbrueckii subsp. bulgaricus EU23

The exopolysaccharide (EPS) from Lactobacillus delbrueckii subsp. bulgaricus EU23 was perdeuteriomethylated and the perdeuteriomethylated EPS (pdm-EPS) purified by elution from a C(18) Sep-Pak cartridge. Both 1D and 2D NMR spectra were recorded for the pdm-EPS and these were interpreted to provide assignments for the individual 1H and 13C resonances of the sugar residues of the repeating unit. Using a combination of the results from monomer analysis and linkage analysis of the native EPS and the ROESY and HMBC NMR spectra of the pdm-EPS the following structure has been determined for the repeating unit:A process for characterising polysaccharides having low solubility in aqueous solution is reported.

A yoghurt-like product made by fermenting ultrafiltered milk containing elevated whey proteins with Lactobacillus acidophilus

A fermented milk with typical yoghurt flavour and aroma was prepared using a human strain of Lactobacillus acidophilus . Growth of the lactobacillus was stimulated when the whey proteins of the ultrafiltered skim-milk substrate were increased by addition of ultrafiltered Cheddar cheese whey. Fermentation, which was carried out at 37 °C, was accelerated by addition of lactase. The resulting product had a smooth consistency, a pH of 4⋅2, acetaldehyde levels up to 15 p.p.m. and no off-flavours. L. acidophilus remained viable at a level of approx. 10 8 colony forming units/ml, with no impairment of quality, for up to 14 d at 4 °C.