Mariano García-Garibay

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.

Enzymes involved in carbohydrate metabolism and their role on exopolysaccharide production in Streptococcus thermophilus

The role of the enzymes uridine‐5′‐diphospho‐(UDP) glucose pyrophosphorylase and UDP galactose 4‐epimerase in exopolysaccharide production of Gal− ropy and non‐ropy strains of Streptococcus thermophilus in a batch culture was investigated. Growth of the ropy and non‐ropy strains was accompanied by total release of the galactose moiety from lactose hydrolysis in modified Bellinker broth with lactose as the only carbon source. This was associated with a greater exopolysaccharide production by the ropy strain. The polymer produced by both strains in cultures with lactose or glucose as carbon sources contained glucose, galactose and rhamnose, indicating that glucose was used as a carbon source for bacterial growth and for exopolysaccharide formation. UDP‐glucose pyrophosphorylase activity was associated with polysaccharide production during the first 12 h in a 20 h culture in the ropy strain, but not in the non‐ropy strain. UDP‐galactose 4‐epimerase was not associated with exopolysaccharide synthesis in any strain. The evidence presented suggests that the glucose moiety from lactose hydrolysis is the source of sugar for heteropolysaccharide synthesis, due to a high UDP‐glucose pyrophosphorylase activity.

Kluyveromyces marxianus CDBB-L-278: A wild inulinase hyperproducing strain

Abstract Kluyveromyces marxianus CDBB-L-278 is an inulinase hyperproducing strain. It was able to grow in a medium containing inulin as the unique carbon source in the presence of 2-deoxyglucose. It produced up to 3.3 times the activity of the control strain K. marxianus NCYC-1429 in an inulin medium, and 3.6 times in a medium with glycerol as the sole carbon source. Although the strain CDBB-L-278 was able to produce inulinase in the presence of 2-deoxyglucose, it was demonstrated that it is not a de-repressed strain since enzyme production was reduced when the concentration of glucose or fructose was increased in the medium. Since inulinase was produced in a glycerol medium without an inducer, it can be considered that the enzyme production was partially constitutive in K. marxianus CDBB-L-278 as well as strain NCYC-1429. The inulinase from K. marxianus CDBB-L-278 was characterized. It had a higher affinity for inulin than for sucrose. Temperature and pH profiles were different for both of these two substrates. The enzyme was stable to high temperatures, with a half-life of 180 min at 50°C.

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.

Evaluation of Kluyveromyces marxianus as baker's yeast

Abstract Two strains of Kluyveromyces marxianus (NRRL-Y-2415 and NRRL-Y-1109) were assessed as bakers yeasts comparing them with two strains of Saccharomyces cerevisiae isolated respectively from compressed yeast and active dry yeast. Strains were tested for dough proofing activity in lean dough and rich doughs (prepared with sucrose, lactose or whey) and sensory evaluation of breads. In rich doughs containing lactose or whey, both strains of K. marxianus had superior proofing activity compared to commercial bakers yeast strains but no difference in bread flavor was detected.

Isolation and identification of killer yeasts from Agave sap ( aguamiel) and pulque

Wild killer yeasts have been identified as inhibitory to strains used as starters in the production of alcoholic beverages such as beer and wine; therefore, killer or killer-resistant strains have been sought for use in alcoholic fermentations. In the current paper a total of 16 strains belonging to six species were isolated. From two samples of Agave sap (aguamiel) the following yeast strains were isolated: Candida lusitaneae (1), Kluyveromyces marxianus var. bulgaricus (2), and Saccharomyces cerevisiae (capensis) (1). Additionally, in seven samples of pulque (the fermented product), the species C. valida (six strains), S. cerevisiae (chevalieri) (4), S. cerevisiae (capensis) (1), and K. marxianus var. lactis (1) were found. The killer strains were C. valida and K. marxianus var. lactis from pulque and K. marxianus var. bulgaricus from aguamiel. One strain of S. cerevisiae (chevalieri) isolated from pulque which did not show killer activity was, on the other hand, resistant to other killer strains and it had a remarkable ethanol tolerance, suggesting that this strain could be used for alcohol production.

Evaluation ofKluyveromyces marxianus for the production of lactase simultaneously to pectinase or inulinase

SummaryFive strains ofK. marxianus were evaluated for the production of intracellular lactase, intra and extracellular pectinase and intra and extracellular inulinase. The strain NRRL-Y-1109 showed the highest lactase activity, but the strain CDBB-L-278 produced notably higher activities of inulinase and pectinase than the rest of the strains tested. The strain CDBB-L-278 was selected for the simultaneous production of two enzymes. Two enzymes fermentations were achieved with productions of 44% lactase and 53% pectinase, or 26% lactase and 47% inulinase compared to the single enzyme levels.

Influence of water activity on the fermentation of yogurt made with extracellular polysaccharide-producing or non-producing starters

Abstract Ropy and non-ropy strains of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus were studied with respect to their resistance to reduction of water activity in milk used to produce yogurt. Glycerol reduced the a w of the milk considerably, but it was less inhibitory to acid production than sucrose. The ropy strains did not show any advantage over nonropy ones when they were used to produce yogurt in milk with reduced a w ; their performance was similar to non-producer strains in terms of lactic acid production.

Isolation and identification of killer yeasts from sugar cane molasses

A total of 13 yeast strains were isolated from 10 samples of sugar cane molasses obtained from sugar mills in Mexico; nine of them were killer strains. There was at least one killer strain in every sample. The resulting species were the following: Torulaspora delbrueckii (2), Schizosaccharomyces pombe (7), Cryptococcus albidus var. albidus (1) and Saccharomyces cerevisiae (3). A new killer species is reported: Schizosaccharomyces pombe.

β-Lactoglobulin's Conformational Requirements for Ligand Binding at the Calyx and the Dimer Interphase: a Flexible Docking Study

β-lactoglobulin (BLG) is an abundant milk protein relevant for industry and biotechnology, due significantly to its ability to bind a wide range of polar and apolar ligands. While hydrophobic ligand sites are known, sites for hydrophilic ligands such as the prevalent milk sugar, lactose, remain undetermined. Through the use of molecular docking we first, analyzed the known fatty acid binding sites in order to dissect their atomistic determinants and second, predicted the interaction sites for lactose with monomeric and dimeric BLG. We validated our approach against BLG structures co-crystallized with ligands and report a computational setup with a reduced number of flexible residues that is able to reproduce experimental results with high precision. Blind dockings with and without flexible side chains on BLG showed that: i) 13 experimentally-determined ligands fit the calyx requiring minimal movement of up to 7 residues out of the 23 that constitute this binding site. ii) Lactose does not bind the calyx despite conformational flexibility, but binds the dimer interface and an alternate Site C. iii) Results point to a probable lactolation site in the BLG dimer interface, at K141, consistent with previous biochemical findings. In contrast, no accessible lysines are found near Site C. iv) lactose forms hydrogen bonds with residues from both monomers stabilizing the dimer through a claw-like structure. Overall, these results improve our understanding of BLGs binding sites, importantly narrowing down the calyx residues that control ligand binding. Moreover, our results emphasize the importance of the dimer interface as an insufficiently explored, biologically relevant binding site of particular importance for hydrophilic ligands. Furthermore our analyses suggest that BLG is a robust scaffold for multiple ligand-binding, suitable for protein design, and advance our molecular understanding of its ligand sites to a point that allows manipulation to control binding.