Evaluation of growth quantification methods for modelling the growth of Penicillium expansum in an apple-based medium

Penicillium expansum isolates were grown on an apple concentrate-based medium modified to different water activity levels (0.83-0.99), and incubated at 3-29 degrees C for up to 14 days. The diameters of the growing colonies were measured and colonies were extracted for determination of ergosterol and N-acetyl-beta-D-glucosaminidase activity. Results showed good correlation of measured diameters in the repeated treatments, but slightly lower repeatability for the ergosterol determination, and poor repeatability for the determination of N-acetyl-beta-D-glucosaminidase activity. Primary and secondary models were built for colony diameter and ergosterol content of the colonies, which were similar for both isolates tested. Growth was prevented at low water activity values (0.83-0.85) outside the range 10-23 degrees C. However, given an unpreserved apple or apple product (a(w) > 0.97) left at room temperature growth may occur in a time as short as I day, probably leading to mycotoxin accumulation in a few days in the event of mycotoxigenic strains. This issue also highlights the importance of shortening the time of transport and storage operations when considering the use of culled apples unfit for other, higher quality and higher profit purposes to obtain apple by-products such as puree, juices or jam. (c) 2006 Society of Chemical Industry.