Effect of weak acid preservatives on growth of bakery product spoilage fungi at different water activities and pH values.

Inhibition of spoilage organisms from bakery products by weak acid preservatives in concentrations of 0%, 0.003%, 0.03% and 0.3% (w/v) was investigated experimentally on a substrate media with water activity (a(w)) and pH ranging from sourdough-fermented acidic rye bread to alkaline intermediate moisture sponge cake types (a(w) 0.80-0.95, pH 4.7-7.4). Initially, rye bread conditions (a(w) 0.94-0.97 and pH 4.4-4.8) in combination with calcium propionate were investigated. Results showed that the highest concentration of propionate (0.3%) at all conditions apart from high a(w) (0.97) and high pH (4.8) totally inhibited fungal growth for a 2-week period, with the exception of Penicillium roqueforti, Penicillium commune and Eurotium rubrum. Characteristically for the major spoiler of rye bread, P. roqueforti, all three isolates tested were stimulated by propionate and the stimulation was significantly enhanced at high water activity levels. The effect of propionate on production of secondary metabolites (mycophenolic acid, rugulovasine, echinulin, flavoglaucin) was also studied, and variable or isolate dependent results were found. Subsequently, a screening experiment representing a wider range of bakery products was conducted using calcium propionate, potassium sorbate and sodium benzoate. The obtained data was modelled using survival analysis to determine 'spoilage-free time' for the fungi. At the low a(w) level (0.80) only Eurotium species grew within the test period of 30 days. Higher water activity levels as well as higher pH values decreased spoilage-free times of the fungi. The preservative calcium propionate was less effective than potassium sorbate and sodium benzoate.

Antifungal activity of essential oils evaluated by two different application techniques against rye bread spoilage fungi.

AIMS: To study how antifungal activity of natural essential oils depends on the assay method used. METHODS AND RESULTS: Oils of bay, cinnamon leaf, clove, lemongrass, mustard, orange, sage, thyme and two rosemary oils were tested by two methods: (1) a rye bread-based agar medium was supplemented with 100 and 250 microl l-1 essential oil and (2) real rye bread was exposed to 136 and 272 microl l-1 volatile oil in air. Rye bread spoilage fungi were used for testing. Method 1 proved thyme oil to be the overall best growth inhibitor, followed by clove and cinnamon. On the contrary, orange, sage and rosemary oils had very limited effects. Mustard and lemongrass were the most effective oils by the volatile method, and orange, sage and one rosemary showed some effects. Oil compositions were analysed by gas chromatography-mass spectrography. CONCLUSIONS: Antifungal effects of the essential oils depended on the application method. Larger phenolic compounds such as thymol and eugenol (thyme, cinnamon and clove) had best effect applied directly to medium, whereas smaller compounds such as allyl isothiocyanate and citral (mustard and lemongrass) were most efficient when added as volatiles. SIGNIFICANCE AND IMPACT OF THE STUDY: This study proves that the method used for screening essential oils as potential antimicrobials should correspond with the application sought.

Factors affecting growth and pigmentation of Penicillium caseifulvum.

Color formation, metabolite production and growth of Penicillium caseifulvum were studied in order to elucidate factors contributing to yellow discoloration of Blue Cheese caused by the mold. A screening experiment was set up to study the effect of pH, concentration of salt (NaCl), P, K, N, S, Mg and the trace metals Fe, Cu, Zn, Mn on yellow color formation, metabolite production and mold growth. Multivariate statistical analysis showed that the most important factor affecting yellow color formation was pH. The most pronounced formation of yellow color, supported by highest amount of colored metabolites, appeared at low pH (pH 4). Mold growth was not correlated to the yellow color formation. Salt concentration was the most important factor affecting mold growth and length of lag phase. Production of secondary metabolites was strongly influenced by both pH and salt concentration. The screening results were used to divide the metabolites into the following three groups: 1) correlated to growth, 2) correlated to color formation, and 3) formed at high pH. Subsequently, a full factorial experiment with factors P, Mg and Cu, showed that low P concentrations (2,000 mg/kg) induced yellow color formation. Among the factors contributing to yellow color formation, pH and salt concentration are easy to control for the cheesemaker, while the third factor, P-concentration, is not. Naturally occurring variations in the P-concentration in milk delivered to Blue Cheese plants, could be responsible for the yellow discoloration phenomenon observed in the dairy industry.