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.

Ecophysiological characterization of common food-borne fungi in relation to pH and water activity under various atmospheric compositions.

The combined effects of pH, water activity (aw), oxygen (O2) and carbon dioxide (CO2) levels on growth and sporulation of 10 common food-borne fungi were studied. The use of a multivariate statistical method (PLS) for the analysis of data showed that the fungi could be grouped according to their physiological response to changes in the four tested factors. Carbon dioxide, aw and pH were found to be the most significant factors describing differences and similarities among the fungi. Maximal inhibitory effect of elevated levels of CO2 (5-25%) and decreased aw (0.99-0.95) varied among the 10 species from 6 to 77% and from 52 to 100%, respectively. Sporulation of the fungi was sensitive to all tested factors. Furthermore, interaction of CO2 and aw displayed a significant effect on sporulation. It was shown that different fungal species associated with the same ecosystem responded similarly to changes in the tested factors. Thus, fungi which are not phylogenetically related may be physiologically related or show a common strategy of life.

Description of conidia from submerged cultivation of Thermomyces lanuginosus for use as a uniform inoculum.

Conidia produced by submerged cultivation of the thermophilic fungus Thermomyces lanuginosus were superior to conidia from agar plates when used as inoculum, due to a faster and more synchronous germination. With conidia derived from submerged liquid culture at 40-45 degrees C more than 90% germination was achieved at 50 degrees C within 3 h whereas the same percentage germination was only achieved after 5 h incubation of conidia produced on agar plates. The temperature during conidial formation, and conidial age at the time of harvesting, were factors influencing germination of the conidia.