Aflatoxin Formation by Aspergillus flavus and Aspergillus parasiticus in a Casein Substrate at Different pH Values

Abstract

Acid curd from skimmilk was mixed with water, blended, and the pH of the mixture adjusted to 1.0 to 11.35 using HCl and NaOH, and to 1.8 to 10.7 with lactic acid and NH4OH. Curd was then inoculated with spore suspensions of Aspergillus flavus or Aspergillus parasiticus and held at ambient temperature for 21 days. After four days, mold growth was evident on curd at pH values between 2.73 and 5.47 (HCl-NaOH series) and between 4.20 and 7.0 (lactic acid-NH4OH series). Six days later, growth of mold was evident at pH values in the range of 1.70 to 9.90 (HCl-NaOH series) and 3.10 to 7.0 (lactic acid-NH4OH series). Growth did not appear at any additional pH values during the subsequent 11 days of incubation. Both A. flavus and A. parasiticus neutralized the strong acid and base so that after ten days the pH values of nearly all samples approached neutrality. This was also true, but in a more limited pH range, when the weak acid and base were used. Aflatoxins B1 and G1 were produced at all pH values where growth was initiated, with highest levels generally present after 21 days in samples which initially were at the extreme ends of the pH scale. © 1968, American Dairy Science Association. All rights reserved.