Effect of exopolysaccharide produced by isogenic strains of Lactococcus lactis on half-fat Cheddar cheese

Fat-reduced cheeses often suffer from undesirable texture, flavor, and cooking properties. Exopolysaccharides (EPS) produced by starter strains have been proposed as a mechanism to increase yield and to improve the texture and cooking properties of reduced-fat cheeses. The objective of this work was to assess the influence of an exopolysaccharide on the yield, texture, cooking properties, and quality of half-fat Cheddar cheese. Two pilot-scale half-fat Cheddar cheeses were manufactured using single starters of an isogenic strain of Lactococcus lactis ssp. cremoris (DPC6532 and DPC6533) that differed in their ability to produce exopolysaccharide. Consequently, any differences detected between the cheeses were attributed to the presence of the exopolysaccharide. The results indicated that cheeses made with the exopolysaccharide-producing starter had an 8.17% increase in actual cheese yield (per 100 kg of milk), a 9.49% increase in moisture content, increase in water activity and water desorption rate at relative humidities ≤90%, significant differences in the cheeses microstructure, and a significant improvement in both textural and cooking properties, without negatively affecting the flavor profiles of the cheeses.     

Lactobacillus reuteri DSM 20016: purification and characterization of a cystathionine gamma-lyase and use as adjunct starter in cheesemaking

A homo-tetrameric approximately 160-kDa cystathionine gamma-lyase was purified to homogeneity from Lactobacillus reuteri DSM 20016 by four chromatographic steps. The activity was pyridoxal-5'-phosphate dependent and the enzyme catalyzed the alpha,gamma-elimination reaction of L-cystathionine, producing L-cysteine, ammonia and alpha-ketobutyrate. The enzyme was active towards a range of amino acids and amino acid derivatives, including methionine. The pH and temperature optima were found to be 8.0 and 35 degrees C. respectively. Isoelectric pH (pI) was approximately 5.0 as determined by two-dimensional electrophoresis. Sensitivity to chemical inhibitors was typical of lactococcal cystathionine gamma- and beta-lyases, except it was inhibited by sulphydryl reagents. The N-terminal sequence was MKFNTQLIHGGNSED, which had 100% homology with cystathionine beta-lyase of Lb. reuteri 104R (Accession Number (CAC05298). Lb. reuteri DSM 20016. together with 10 other strains of non-starter lactic acid bacteria, was used as adjunct starter in the production of miniature ('anestrato Pugliese-like cheeses. After 40 d ripening, the water-soluble extract of the cheeses with added Lactobacillus fermentum DT41 and Lb. reuteri DSM 20016 contained the highest enzyme activities on cystathionine and methionine substrates. Determinations of methanethiol, dimethyl sulphide, dimethyl disulphide and dimethyl trisulphide in the miniature cheeses confirmed the findings of enzyme activities.     

Proteolysis during the ripening of goats’ milk cheese made with plant coagulant or calf rennet

Powdered plant coagulant (PPC) obtained from the cardoon (Cynara cardunculus) was compared with calf rennet (CR) for the manufacture of goats’ milk cheese, by determining difference in the proteolysis throughout ripening. There were no substantial differences between the compositions of cheeses made using the two types of coagulants. However, cheeses manufactured with PPC exhibited higher levels of pH 4.6-SN than cheese made using CR. The extent of breakdown of α_s-casein, as measured by urea-PAGE, was greater in cheese made using PPC than cheese made using CR. The formation of hydrophobic peptides and the ratio of hydrophobic/hydrophilic peptides throughout the ripening were higher in cheeses made with PPC than in cheeses made with CR. Principal component analysis (PCA) of peak heights of RP-HPLC peptide profiles of the ethanol-soluble and ethanol-insoluble fractions distributed the samples according to the coagulant used in their manufacture. Quantitative differences in several peptides were evident between the two types of cheese.     

Responding changes systematically within sessions during conditioning procedures

When the procedure is held constant within an experimental session, responding often changes systematically within that session. Many of these within-session changes in responding cannot be dismissed as learning curves or by-products of satiation. They have been observed in studies of positive reinforcement, avoidance, punishment, extinction, discrimination, delayed matching to sample, concept formation, maze and alley running, and laboratory analogues of foraging, as well as in the unconditioned substrates of conditioned behavior. When aversive stimuli are used, responding usually increases early in the session. When positive reinforcers are used, responding changes in a variety of ways, including increasing, decreasing, and bitonic functions. Both strong and minimal reinforcement procedures produce within-session decreases in positively reinforced behavior. Within-session changes in responding have substantial theoretical and methodological implications for research in conditioning.     

Nonparametric Post Hoc Comparisons for Trend.

Post hoc procedures using orthogonal polynomials are illustrated for 3 nonparametric tests that are frequently employed by behavioral scientists in the analysis of empirical data. These examples involve null hypotheses which have been rejected by the Kruskal-Wallis 1-way analysis-of-variance test for rank data, by the Friedman 2-way analysis-of-variance test for rank data, and by the Cochran extension of the McNemar test of change for dichotomous data. Finally, the proposed post hoc procedures are compared to planned-comparison methods. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Screening of bacteria from the cattle gastrointestinal tract for inhibitory activity against enterohemorrhagic Escherichia coli O157:H7, O111:H-, and O26:H11

A quick and reproducible microgel plate assay was adapted to screen bacteria from cattle gastrointestinal tracts for production of compounds inhibitory to the growth of three enterohemorrhagic Escherichia coli (EHEC) serotypes: O157:H7, O111:H-, and O26:H11. The inhibitory activity of 309 bacteria, isolated on several agar media, was assessed by a microgel assay performed in 96-well microtiter plates. Fifty-three isolates secreted inhibitory compounds with a molecular weight of less than 1,000. In 12 isolates, the inhibitory activity was attributable to compounds other than lactic or acetic acid. These compounds were highly heat tolerant, with varying sensitivity to digestion by proteolytic enzymes. The inhibitory isolates were identified as lactic acid-producing bacteria on the basis of a combination of analyses, including 16S-rDNA restriction fragment length polymorphisms, 16S-rDNA gene sequences, and fermentation end products. The lactic acid bacteria of ruminants may contain antibacterial compounds not yet described. Naturally occurring populations of lactic acid bacteria may have potential as probiotics, to reduce the carriage of EHEC in the gastrointestinal tract of ruminants.     

Effect of diet on the concentration of complex Shiga toxin-producing Escherichia coli and EHEC virulence genes in bovine faeces, hide and carcass

An experiment was conducted to determine whether diets based on structural carbohydrate and/or simple sugars, as found in roughage and/or molasses-based diets, reduce the bovine faecal populations of Shiga toxin-producing Escherichia coli (STEC) isolates containing the eaeA and ehxA genes, referred to as complex STEC (cSTEC), compared with typical high starch, grain-based feedlot diets. In addition, whether commercial lairage management practices promote or diminish any diet-induced responses on the contamination of carcasses was also investigated. After 13 days on the dietary treatments total faecal E. coli numbers were approximately one log lower in the roughage (R) and roughage +50% molasses (RM) diets compared with grain (G) fed animals, this difference varying between 0.5 and 1 log at lairage. Fermentation patterns were similar in the R and RM diets whereas decreased pH and enhanced butyrate fermentation pathways were associated with the G diet. A significant decrease in the faecal concentration of the eaeA gene occurred when animals were changed from high grain to R and RM diets for 6-13 days, compared with animals maintained on the G diet. Significantly lower concentrations of the ehxA gene were also associated with the R diet. Concentrations of the stx(2) gene however, were unaffected by diet. cSTEC were infrequently isolated, with the faecal concentrations of these organisms being low (<3 log(10) MPN per g faeces). cSTEC were only isolated from animals fed G or RM diets, but were never isolated from cattle fed the roughage-based diet, with this diet-induced effect sustained following lairage. These organisms were not detected on the hide and carcass of animals found to shed cSTEC in their faeces and thus appeared uncontaminated with cSTEC.