99D-23

The objective of multifactor preservation is to select and combine antimicrobial factors in such way that microbial stability can be assured. Information gathered when several factors are applied in a multifactorial way could be useful to generate microbial predictive models.

Predictive models of Aspergillus parasiticus growth responses (germination time and radial growth rate, RGR) were generated throughout the evaluation of combined effects of water activity (a_w, 0.90 - 0.99, adjusted with sucrose), pH (2.0 - 5.0, adjusted with citric acid), and incubation temperature (10 - 35°C) in potato-dextrose agar model systems using a Box-Behnken experimental design.

The a_w-adjusted model systems were sterilized, cooled, and acidified to the desired pH. Triplicate Petri dishes of every system were centrally inoculated with a mold spore suspension and incubated for one month over sodium chloride solutions of the same a_w. The inoculated systems were observed every day and after germination, colony diameter was measured. Second order polynomial equations were fitted to describe and predict mold RGR and germination time.

Increase in colony diameter after the lag period shows a linear increase with incubation time. Calculated RGR varied from 0.031 (pH 3.5, a_w 0.90, 10°C) to 0.567 mm/h (pH 3.5, a_w 0.99, 35°C) with germination times ranging from < 24 to 720 h. RGR increase along with pH, a_w and incubation temperature while germination times increased as pH, a_w and incubation temperature decreased. Analysis of variance of RGR and germination times demonstrated that the effects of a_w, pH and incubation temperature as well as their interactions were significant (p£0.01). Reduced quadratic predictive models effectively (r² >0.9) describe mold growth response and predict with errors < 5% the experimental data.

A. parasiticus is an organism capable of germinate and grow under adverse conditions, significant reduction of RGR and delay of germination time can be obtained when reducing temperature, pH and a_w.