Treatment design of radio frequency heating based on insect control and product quality

Abstract

Non-uniform heating caused by different size, geometry, and properties of agricultural commodities is a major challenge that needs to be addressed in developing industrial-scale radio frequency (RF) treatments for postharvest phytosanitary and quarantine applications to dry grains or nuts. A mathematical model based on normal distributions of product temperatures and taking into account insect thermal mortality and product quality was developed to predict treatment temperature–time ranges as a function of number of mixing. To demonstrate its applications, this model considered two boundary conditions: complete mortality of fifth-instar navel orangeworm with temperature–time relationships (46, 48, 50, 52, and 54 °C for 140, 50, 15, 6, and 1 min, respectively), and maximum temperature tolerance relationship for walnut quality (53, 69, and 75 °C for 240, 5, and 1 min, respectively). This model was validated by experimental data of the uniformity index and applied to walnut, soybean, lentil, and wheat, each with its own unique RF heating characteristics. The results showed that the operation ranges (temperature–time combinations) for effective control of pests without causing adverse quality changes expanded with increasing mixing number and the improved heating uniformity. This study suggested more flexibility in developing RF treatments of pest control for small-size crops with better heating uniformity, as compared to large-size crops.

Introduction

Agriculture commodities, such as wheat, soybean, lentil, and walnut, are important contributors to exported goods from the United States. Agricultural commodities are, however, natural carriers of exotic insect pests. These pests can cause major local economic losses (Mumford, 2002). To reduce the risk of introducing pests, importing countries or regions impose quarantine or phytosanitary requirements for commodities that host targeted pests. Methyl bromide is an important quarantine and phytosanitary treatment method, but it is a highly toxic gas and listed as an ozone-depleting chemical under the Montreal Protocol of 1992 (UNEP, 1992). Recent regulatory actions have limited its use to quarantine treatments, requiring industries to apply for yearly Critical Use Exemptions for phytosanitary applications through a complicated procedure, thus creating an urgent need to find environmentally friendly and effective alternatives.

Thermal treatments can be non-chemical alternatives for postharvest insect control in agricultural commodities. Radio frequency (RF) energy, proposed as a novel form of delivering thermal energy to commodities, has been widely used as an advanced thermal technology in food processing over conventional hot air and water heating (Tang et al., 2000). Research on applications of RF heating in disinfesting agricultural products has been hindered by the low price of methyl bromide fumigation practices and a lack of heating uniformity in the RF-treated products (Frings, 1952, Hallman and Sharp, 1994, Nelson, 1996). Recent studies on thermal death kinetics for insects and on dielectric properties of the insects and the related agricultural materials (Wang et al., 2002a, Wang et al., 2002b, Wang et al., 2003, Wang et al., 2005, Johnson et al., 2003, Johnson et al., 2004) has made it possible for the development of laboratory and industrial scale RF pest control treatments for walnuts with acceptable product quality (Wang et al., 2001, Wang et al., 2002c, Wang et al., 2006, Wang et al., 2007a, Wang et al., 2007b, Birla et al., 2004, Mitcham et al., 2004).

Since dried materials are more heat tolerant than fresh produce, RF treatments hold greater potential in disinfesting wheat, soybean, lentil, and walnut. Heating non-uniformity is a major problem in RF treatments, which would result in either insect survival or product damage (Birla et al., 2004). Mixing can reduce commodity temperature variations in RF treatments as it eliminates the position effect in a container caused by non-uniform electrical field in RF systems (Wang et al., 2005). A mathematical model based on normal temperature distribution of RF-treated walnuts has been used to determine the intermittent mixing number to meet the required insect mortality. The concept of heating uniformity index was used in this model to quantify the severity of non-uniformity problem (Wang et al., 2007a, Wang et al., 2007b). However, the selection of the product mean temperature as a key operation parameter in that model was based only on insect mortality, with no consideration for product quality. It is desirable to develop a model that takes into account both insect thermal mortality and product quality. It is also important to extend our research from walnuts into similar low-moisture and small particle crops, such as soybeans, lentils, and wheat.

The objectives of this study were to determine RF heating characteristics of walnut, soybean, lentil, and wheat, to develop a mathematical model that takes into account insect mortality and product quality in providing time–temperature parameters for the four crops, to validate the model with experimental data of the uniformity index, and to analyse the influence of mixing numbers and required security levels on time–temperature regimes for the four crops.