Optimization of Storage Conditions of the Medicinal Herb Ilex asprella against the Sterigmatocystin Producer Aspergillus versicolor Using Response Surface Methodology

The root of Ilex asprella is a commonly used herb in Southern China, and also constitutes the main raw material of Canton herbal tea. I. asprella is readily contaminated by mildew because of rich nutrients. Aspergillus versicolor producing sterigmatocystin is one of the most common molds that contaminate foodstuffs and medicinal herbs. Previous study on the evaluation of fungal contamination on samples of I. asprella revealed that A. versicolor was the dominant contaminant. In this study, experiments based on response surface methodology combined with central composite design were carried out to determine the optimal storage conditions of I. asprella to minimize the contamination of sterigmatocystin. The herb, manually innoculated with A. versicolor, was stored under different temperatures (20–40 °C) and humidity (80–95%) for seven days. The effects of temperature and humidity were evaluated using total saponin, polysaccharide and the sterigmatocystin levels as quality indexes. The results showed that A. versicolor grew quickly and produced large amounts of sterigmatocystin on I. asprella, at humidity ranging from 85% to 90% and temperatures above 26 °C. Meanwhile, total saponin and polysaccharide amounts were reduced significantly. These findings suggested that I. asprella samples should be stored in an environment with humidity and temperature below 85% and 26 °C, respectively, to reduce A. versicolor growth and sterigmatocystin production.