采后坚果的射频杀虫灭菌机理及方法研究

With concerns of consumers on safety of postharvest agricultural products and environmental pollutions caused by chemical fumigations, and increased international regulations to limit the use of fumigants, it is extremely urgent to study novel and environmentally-friendly physical alternatives for postharvest control of insect pests and pathogens. Radio frequency (RF) treatment has been identified as one novel and physical heating method, providing significant advantages and broad application prospects in postharvest disinfestations and pasteurizations of agricultural products. In the proposed project, we will select walnuts, almonds and pistachio as three representative nuts and measure their thermal and dielectric properties as a function of frequency, moisture content, and temperature to determine the penetration depth, heating uniformity and differential heating in nuts; establish thermal death kinetic models of pathogens and three selected insect pests in nuts under different moisture contents and heating rates to explore the mechanism of controlling pathogens and pests using RF energy; improve heating methods and optimize RF treatment parameters using finite element simulation methods to ensure pest and pathogen controls and achieve marketable quality characteristics of treated nuts. This project mainly combines unique research capacities in three interdisciplinary research directions of food microbiology, economic entomology and food engineering to develop novel and environmentally friendly postharvest RF treatments for insect control and pathogen inactivation in nuts. The project will address critical challenges to the nut industry, significantly improve the food safety in consuming nuts and product quality with extended shelf life, provide reliable mechanisms and new technology for the nut industry, and lay a foundation to establish postharvest insect and pathogen control in other agricultural products.

针对公众对化学熏蒸剂造成的采后农产品安全和环境恶化问题的普遍关注以及国际公约对其使用的不断限制，研究快速高效环保的非化学杀虫灭菌方法显得尤为迫切。射频技术作为一种新型物理加热方法，在采后农产品热处理应用中具有明显优势和广泛应用前景。本项目拟以采后坚果为对象进行射频杀虫灭菌的机理与方法研究，测定害虫与坚果在不同水分与温度下的热特性和介电特性，探明射频波在坚果中的穿透性、均匀性和选择性加热特性；构建不同水分和加热速率下采后坚果中常见致病菌和害虫的热致死动力学模型，明确害虫与微生物的射频波热致死机理；利用有限元计算机模拟改善加热方法，优化调控参数，提高杀虫灭菌的有效性并确保坚果品质。本项目将主要通过食品微生物、经济昆虫和食品工程三大学科的交叉融合，建立新型环保的采后坚果射频杀虫灭菌方法，为保障我国采后坚果的食用安全与产品品质提供可靠的理论与技术支持，并为其它采后农产品杀虫灭菌方法的建立奠定基础。

随着人们对环保和农产品质量安全要求提高，用于采后处理的化学熏蒸剂给生态环境与农产品安全带来的危害引起了人们的广泛关注。不断出台的相关国际公约也逐步限制或淘汰了甲基溴等传统化学熏蒸剂使用。因此，开发高效环保的非化学杀虫灭菌技术极为必要。射频加热作为一种新型物理加热技术，在采后农产品的热处理中具有潜在优势和广泛应用前景。本项目以采后坚果为对象进行射频杀虫灭菌的机理与方法研究，主要内容包括：测定害虫与坚果在不同水分与温度下的热和介电特性，探明射频波在坚果中的穿透性、均匀性和选择性加热特性；建立不同水分和加热速率下采后坚果中常见致病菌和害虫的热致死动力学模型，明确害虫与微生物的射频热致死机理；利用有限元计算机模拟改善加热方法，优化调控参数，提高杀虫灭菌的有效性并确保坚果品质。研究结果表明，射频加热比传统加热快、均匀且效率高，相比微波红外等新型加热技术穿透深度大，更适合于开展大宗农产品物料的热处理；确定了米象、玉米象和桃蛀螟等三种常见仓储害虫中最为耐热的害虫，建立了最耐热害虫的热致死动力学模型。采用自制的TDT加热单元在等温条件下研究了沙门氏菌替代菌ATCC 25922的热致死动力学，为坚果射频加热杀虫灭菌方法提供了基础数据；检测了3种坚果随频率、温度、水分含量与盐含量变化的介电特性，计算了射频在坚果中的穿透深度，明确了射频对坚果与害虫的选择性加热特性；通过有限元模拟，明确了采用相似介电常数材料的筐子、热风表面辅助加热与传送带运动等措施可以改善射频加热均匀性；通过坚果加湿预处理降低微生物耐热性，达到射频快速灭菌要求，然后利用热风辅助射频干燥使坚果水分含量恢复至原始水平以便贮藏。采用连续化处理方式，开展了开心果射频加热均匀性、害虫死亡率、加热效率、处理量等研究，结果显示连续化加热的均匀性优于单批次静止处理，可快速有效杀灭常见仓储害虫印度谷螟。平均加热效率为70%以上，处理量超过250kg/h。