Production of Rubratoxin by Penicillium rubrum in a Soy Whey-Malt Extract Medium

Abstract

Sterile soy whey (1.75% dissolved solids) was fortified with malt extract and inoculated with spore suspensions of Penicillium rubrum strains P-13, 1062, 2120, 2123, and 3290. Samples were incubated quiescently at 28 C from 1 to 28 days and as shake cultures for 3, 5, and 7 days. Rubratoxin was recovered from culture filtrates by alcohol-acetone extraction and resolved by thin-layer chromatography. Toxin was not produced in shake cultures. Rubratoxins A and B were produced in quiescent soy whey cultures of all P. rubrum strains except P. rubrum P-13 which produced only rubratoxin B. Toxin production increased as the concentration of malt extract increased from 0.5 to 10% (w/v). Rubratoxin formation also increased with an increase in incubation time from 3 to 17 days but the amount of toxin in cultures declined rapidly thereafter. Yields of rubratoxin A ranged from 0.83 to 31.53 mg/100 ml in cultures of P. rubrum 1062 and from 1.89 to 22.70, 0.53 to 25.13, and 2.07 to 31.20 mg/100 ml in P. rubrum 2120 2123, and 3290 cultures, respectively. Yields of rubratoxin B ranged from 0.77 to 105.30, 1.03 to 94.83, 2.13 to 91.57, 0.82 to 78.53, and 1.3 to 85.57 mg/100 ml in cultures of P. rubrum 13, 1062, 2120, 2123, and 3290, respectively. After maximum production, toxin content in cultures leveled off and then decreased. Amounts of toxin declined more rapidly than did mold growth (as measured by mycelial dry weight). Although malt extract stimulated fungal growth, toxin production was enhanced more than mold growth.