Influence of temperature, water activity and pH on growth of some xerophilic fungi

The combined effects of water activity (a(w)), pH and temperature on the germination and growth of seven xerophilic fungi important in the spoilage of baked goods and confectionery were examined. Eurotium rubrum, E. repens, Wallemia sebi, Aspergillus penicillioides, Penicillium roqueforti, Chrysosporium xerophilum and Xeromyces bisporus were grown at 25, 30 and 37 degreesC on media with pH values of 4.5, 5.5, 6.5 and 7.5 and a range of water activities (a(w)) from 0.92 to 0.70. The a(w) of the media was controlled with a mixture of equal parts of glucose and fructose. Temperature affected the minimum aw for germination for most species. For example, P. roqueforti germinated at 0.82 a(w) at 25 degreesC, 0.86 a(w) at 30 degreesC and was unable to germinate at 37 degreesC. E. repens germinated at 0.70 a(w) at 30 degreesC, but at 25 and 37 degreesC, its minimum a(w) for germination was 0.74. C xerophilum and X bisporus germinated at 0.70 aw at all three temperatures. The optimum growth occurred at 25 degreesC for R roqueforti and W sebi, at 30 degreesC for Eurotium species, A. penicillioides and X bisporus and at 37 degreesC for C. xerophilum. These fungi all grew faster under acidic than neutral pH conditions. The data presented here provide a matrix that will be used in the development of a mathematical model for the prediction of the shelf life of baked goods and confectionery. (C) 2002 Elsevier Science B.V. All rights reserved.