No abstract

Yeast cells display an adaptive stress response when exposed to weak organic acids at low pH. This adaptation is important in the spoilage of preserved foods, as it allows growth in the presence of weak acid food preservatives. In Saccharomyces cerevisiae, this stress response leads to strong induction of the Pdr12 ATP‐binding cassette (ABC) transporter, which catalyses the active efflux of weak acid anions from the cytosol of adapted cells. S. cerevisiae cells lacking the Cmk1 isoform of Ca2+–calmodulin‐dependent protein kinase are intrinsically resistant to weak acid stress, in that they do not need to spend a long adaptive period in lag phase before resuming growth after exposure to this stress. This resistance of the cmk1 mutant is Pdr12 dependent and, unlike with wild‐type S. cerevisiae, cmk1 cells are capable of performing Pdr12‐specific functions such as energy‐dependent cellular extrusion of fluorescein and benzoate. However, they have neither higher PDR12 gene promoter activity nor higher Pdr12 protein levels. The increased Pdr12 activity in cmk1 cells is therefore caused by Cmk1 exerting a negative post‐transcriptional influence over the activity of the Pdr12 ABC transporter, a transporter protein that is constitutively expressed in low‐pH yeast cultures. This is the first preliminary evidence that shows a protein kinase, either directly or indirectly, regulating the activity of a yeast ABC transporter.

An experimental design considering thermal treatment of must, yeast strain, prickly pear variety and degree of ripeness was chosen to evaluate the fermentation behavior and generation of volatile compounds, during the elaboration of a distilled beverage from prickly pear. Four Mexican prickly pear varieties were characterized physically and two of them were selected for fermentation studies. The thermal treatment of the must showed the highest statistical influence on fermentation behavior and production of volatile compounds, followed by prickly pear variety, then yeast strain and finally the degree of ripeness was the least statistically significant factor. The growth rate increased when the thermal treatment was applied whereas the ethanol production rate and alcoholic efficiency were unaffected. The results also suggested that thermal treatment was effective for inhibition of microbial contamination. As regards volatile compounds production, acetic acid and methanol decreased while other volatiles increased when the thermal treatment was applied. Despite the influence of thermal treatment, prickly pear variety strongly influences the volatile profile of fermented musts.

The effect of various concentrations of nisin (250, 500 or 750 IU/g) combined with 50 ppm sodium nitrite on the shelf-life of vacuum-packaged bacon was evaluated. Control packages of bacon containing 50 and 150 ppm nitrite were included. Total numbers of lactic acid bacteria (LAB) (as measured on MRS medium) was used as a criterion for shelf-life. Treated bacon samples were stored at 30 and 5°C for 4 d or 6 wk, respectively. Bacon stored at 30°C showed a 1-d extension of shelf-life at nisin levels of 500 and 750 IU/g. Lowest counts at 6 wk were in bacon treated with 750 IU nisin and stored at 5°C. The LAB count was 1.5-log10 CFU/g lower than the controls. A 1-wk extension of storage life was predicted for nisin-treated (750 IU) bacon.

Thirty strains of lactic acid bacteria from different meat sources (bologna, summer sausage, thurlinger sausage, chicken loaf and bacon) were tested for nisin sensitivity. The maximum concentration of nisin permitting growth for 20 strains was 50 IU/ml. Lactobacilli classified as atypical were sensitive to <5 IU nisin/ml. These strains could not be induced to increase resistance by five transfers to media with increased nisin concentrations. The ten strains with the higher resistance to nisin were checked for nisinase activity. One strain, Lactobacillus brevis, showed weak nisinase activity and the rest were negative.