Formula optimization and verification of mould inhibitor for southern rice by Design-Expert response surface method

【Objective】The mould inhibitor formula for southern rice during storage was selected so as to provide technical support for the rice storage.【Method】The main rice mold contaminants were determined from two indica rice varieties stored for 3-6 months and 12-24 months. The total mold counts and the mildew proof effects of five plant extracts, including leaf extracts from ampelopsis, tea, and mulberry, essential oils of orange peel and clove, on Aspergillus flavus growth in stored rice were analyzed. Finally, inhibition zone diameters achieved against A. flavus was used as an index and combined with the Design Expert response surface method to obtain an optimized compound formulation of the plant extracts.【Result】The microbial presence in grain stored for 3-6 months was 1.1×10³-9.2×10³ CFU/g, while that in rice stored for 12-24 months was significantly greater at 8.1×10³-2.2×10⁵ CFU/g. Aspergillus and Penicillium were the main contaminating molds in southern rice, followed by Mucor and Fusarium. The inhibition zone diameter of A. flavus is between 8.4-11.1 mm, and the minimum inhibitory(%) and bactericidal concentrations were between 0.0125%-0.0500% and 0.0500%-0.1000%, respectively. All five plant extracts were effective as independent fungicides, the most effective of which was the leaf extract of ampelopsis. The prediction and simulation equation of mold inhibitor is as follows:Y=10.34A+10.06B+7.18C+8126.84D-10096.90E+11.97AB+ 12.18AC-8236.89AD+10212.4AE+10.74BC-8245.77BD+10084.50BE-8149.38CD+10357.06CE+6202.14DE. Where Y represents the diameter of the antifungal zone and A, B, C, D and E represent the proportions of extracts of ampelopsis, tea, mulberry, orange peel and clove in the mold inhibitor formulation respectively. The optimal proportion of leaf extracts and essential oils was as follows:55.00% ampelopsis, 22.12% tea, 22.56% mulberry, 0.10% orange peel and 0.21% clove. The inhibition of mildew development by the compound fungicidal formula was clearly more effective than individual extracts or essential oils and the inhibition zone diameter of 25 combinations against A. flavus was between 11.30-13.90 mm. The safe grain storage time after spraying with the best compound fungicide on rice could be extended for about 60-120 d.【Conclusion】The compound fungicide composed of five natural plant extracts described above could inhibit the reproduction of microorganisms and extend the safe storage period for grain.