In vitro growth response of bread-spoilage Bacillus strains to selected natural antimicrobials.

This study assessed the in vitro responses of Bacillus (B.) strains isolated from ropey bread to natural antimicrobials under optimum growth conditions. The responses of six Bacillus strains [B. subtilis (2), B. licheniformis (2) and B. pumilus (2)] to acetic acid (AA), lactic acid (LA), calcium lactate (CL) and a lactate-containing cocktail (LCC), singly and in combinations were determined and compared to calcium propionate (CP). Isolates were each inoculated into flasks containing Nutrient Broth (NB) and the respective antimicrobial treatments and pHs were left unadjusted. A duplicate set of flasks, also containing NB and the respective antimicrobials, but adjusted to pH's corresponding to those of baked brown bread containing the same antimicrobials was also inoculated. Growth curves were obtained spectrophotometrically and used to estimate lag times. The organic acids used in this study [0.1% (v/v) AA and 0.25% (v/v) LA] singly and in combination with each other and with CL, CP or LCC, completely inhibited the growth of all six Bacillus strains, but only at non-adjusted pHs. The efficacies of LA, AA and CL notably decreased when the pH of the test media containing the respective preservatives was adjusted to the corresponding in situ (bread) values. However, the natural antimicrobials were still as effective as CP in retarding growth of the six Bacillus strains at the in situ (bread) pH values.

Effect of selected natural antimicrobials on Baker's yeast activity.

AIMS: To evaluate the responses of Baker's yeast (Saccharomyces cerevisiae) activity to the natural antimicrobials acetic acid, calcium lactate, a lactate-containing cocktail and lactic acid compared to calcium propionate. METHODS AND RESULTS: A dough fermentometer test was used to measure Baker's yeast activity in the presence of these natural antimicrobials and calcium propionate. Yeast activity generally decreased as a function of increasing antimicrobial concentrations, but the lactate-containing cocktail showed no relationship between concentration and yeast activity reduction. At in situ concentrations, calcium propionate resulted in the highest yeast activity reduction, followed by calcium lactate, acetic acid, the lactate-containing cocktail and lactic acid in decreasing order. CONCLUSION: Based on yeast activity reduction, all natural antimicrobials tested showed potential as possible replacements for calcium propionate. SIGNIFICANCE AND IMPACT OF THE STUDY: This has practical implications since calcium propionate inhibits Baker's yeast activity and attracts negative consumer perceptions as a chemical bread preservative.

Microbial populations associated with commercially produced South African sorghum beer as determined by conventional and Petrifilm plating.

Microbial populations of 46 commercially produced sorghum beer samples from retail outlets in Johannesburg, South Africa, were enumerated and characterized. Aerobic plate counts, lactic acid bacteria counts and yeast counts were performed by conventional and Petrifilm plating. Conventional methods yielded yeast counts of 7.84 log CFU/ml, lactic acid bacteria counts of 6.44 log CFU/ml and aerobic plate counts of 5.96 log CFU/ml. In comparison, Petrifilm counts were 7.85 log CFU/ml for yeasts, 5.31 log CFU/ml for lactic acid bacteria and 5.34 log CFU/ml for aerobic bacteria. Characterization of 419 predominant bacterial isolates from Standard One Nutrient Agar, MRS Agar and corresponding Petrifilm plates yielded 88.0% lactic acid bacteria, 8.4% Bacillus species, 2.9% Micrococcus species and 0.7% Gram negative bacteria. Composition of predominant lactic acid bacteria populations from Standard One Nutrient Agar and both types of Petrifilm plates showed marginal differences. Increased proportions of heterofermentative lactic acid bacteria were, however, isolated from conventional MRS Agar compared to the modified Petrifilm product which represented the equivalent to MRS Agar.