Environmental factors and weak organic acid interactions have differential effects on control of growth and ochratoxin A production by Penicillium verrucosum isolates in bread

Abstract

Studies were conducted to examine the effect of water activity ( a w, 0.97, 0.95, 0.93), pH (4.5, 6.0) and preservative type and concentration (potassium sorbate (KS), calcium propionate (CaP), 300 and 3000 ppm) on colonisation and ochratoxin A (OTA) production by three strains of Penicillium verrucosum on bread analogues. At 25 °C, colony extension from a point source was similar for all three strains at the different environmental treatments examined, with optimum growth at 0.97 and 0.96 a w and pH 6. Temporal OTA production over 36 days varied significantly with strain, pH and a w level. Maximum OTA was produced at approximately 28–36 days for two of the strains with an optimum at 0.93 a w at pH 6.0. One strain produced 75–80 μg g −1 analogue under these conditions. However, at pH 4.5, optimum OTA was produced at 0.95 a w after a similar time period on bread analogues, with maximum amounts of about 25–35 μg g −1 bread. KS and CaP incorporation inhibited growth completely at pH 4.5 and 3000 ppm at all three a w levels. However, at suboptimal concentration, little growth inhibition occurred. At pH 6, even with 3000 ppm, growth of all three strains occurred. Where effective inhibition of growth was achieved (3000 ppm/pH 4.5, all a w levels), no OTA was detected. However, when 300 ppm was used, up to 7 μg g −1 was detected in bread analogues at pH 4.5. At pH 6, OTA was produced at all treatment conditions by all three strains, although preservative-treated bread had less present. Statistical analysis showed that many of the one-, two- and three-way interactions had a significant effect on growth and OTA production. These results suggest that there is a significant risk from OTA contamination if lower concentrations of existing preservatives are used in wheat-based bakery products contaminated by spoilage mycotoxigenic moulds such as P. verrucosum.