Process protocols based on radio frequency energy to control field and storage pests in in-shell walnuts

Abstract

A practical process protocol was developed to control insect pests in in-shell walnuts using a 27 MHz pilot scale radio frequency (RF) system. Fifth-instars, that had been determined to be the most heat resistant life stage for navel orangeworm (Amyelois transitella [Walker]) using a heating block system, were selected as the targeted insect in the protocol development. RF heating to 55 °C and holding in hot air for at least 5 min resulted in 100% mortality of the fifth-instar navel orangeworm. Rancidity, sensory qualities and shell characteristics were not affected by the treatments. The process slightly reduced the moisture content of the walnut kernels, which could prove an additional benefit by providing even nut moisture content and reducing the growth of microorganisms. If this method can be economically integrated into the handling process, it should have excellent potential as a disinfestation method for in-shell walnuts.

Introduction

Infestation by insect pests is a major problem encountered during the production, storage and marketing of walnuts (Juglans regia L.). The three most economically significant of these pests are codling moth (Cydia pomonella [L.]), navel orangeworm (Amyelois transitella [Walker]), and Indianmeal moth (Plodia interpunctella [Hübner]). Larvae of codling moth and navel orangeworm are field pests and may be present in harvested walnuts. Codling moth is targeted by quarantine regulations in Japan and South Korea, and navel orangeworm is of phytosanitary concern in Australian and European markets. Indianmeal moth is a common pest of stored walnuts and is the insect most often responsible for consumer returns and complaints.

Methyl bromide (MeBr) fumigation is the usual treatment applied to walnuts to meet quarantine and phytosanitary requirements before shipment to domestic and international markets. Under the Montreal Protocol of the United Nations, MeBr will be banned by 2005 from use for purposes other than preshipment or quarantine treatments (USEPA, 2001). Greater regulation and restriction of MeBr use will likely increase the cost of the fumigant, as well as reduce its availability (Mitcham, 2001). There is, therefore, interest in developing an alternative, non-chemical process protocol to control insect pests in walnuts while retaining acceptable product quality.

Conventional hot air is currently used for initial and final drying of in-shell walnuts after bleaching and washing. The final drying process typically requires 4–6 h in 52 °C air. The slow heating is due to the interior air pockets in in-shell walnuts (Tang et al., 2000, Wang et al., 2001b). Radio frequency (RF) treatment has the potential for use as an alternative thermal treatment for quarantine (Wang et al., 2001a) as well as a fast drying method (Jones and Rowley, 1996). This is because RF energy interacts directly with dielectric materials to provide fast heating (Nelson, 1996). Nelson and Payne (1982) used a laboratory RF unit to control pecan weevil in pecans, but the treatments resulted in a reduction in pecan seed germination. Recently, Wang et al. (2001a) developed a RF treatment to control third- and fourth-instar codling moths in in-shell walnuts. This treatment increased walnut core temperature to 53 °C in 3 min. A 5 min holding time at this temperature resulted in 100% kill of insects without causing quality degradation based on peroxide values (PV) and fatty acids (FA). Thus, it would be desirable if a similar treatment could be used to control other postharvest insect pests, such as Indianmeal moth and navel orangeworm.

In developing a disinfestation protocol for walnuts, it was important to determine which of the targeted insects is the most heat resistant, as well as the most heat resistant life stage for each species. To accomplish this, information was required on the minimum time–temperature combinations that result in 100% mortality for each insect over a relatively large range of temperatures. Several researchers have reported mortality of codling moth instars subjected to hot water bath treatments (Yokoyama et al., 1991, Neven, 1994, Neven and Rehfield, 1995). The thermal death kinetics for fifth-instars of Indianmeal moth, codling moth and navel orangeworm have been separately reported (Johnson et al., 2002, Wang et al., 2002a, Wang et al., 2002b). The thermal death time (TDT) curve for each of these insects is summarized in Fig. 1. The results suggest that navel orangeworm is the most heat resistant insect at the fifth-instar life stage. Yokoyama et al. (1991) reported that the fifth-instar larva was the most heat tolerant developmental stage of codling moth, but additional research is needed to determine the most heat tolerant life stage of the navel orangeworm.

To be effective, phytosanitation procedures must also retain product quality. Quality factors for walnuts include crackability, kernel color, moisture content and flavor. Walnuts contain high concentrations of polyunsaturated fatty acids. This makes them susceptible to the development of oxidative and hydrolytic rancidity, especially at high temperatures. The two main parameters indicating walnut oxidative rancidity are peroxide values (PV, meq/kg) and fatty acids (FA, % oleic). According to the industry standard (Diamond Walnut Company, Stockton, CA), good quality walnuts should have a PV<1.0 meq/kg and a FA<0.6%. Buranasompob et al. (2001) reported that heating shelled walnut kernels to 60 °C and hold them for up to 10 min did not increase rancidity compared to untreated walnuts. However, the effect of RF treatments on rancidity needs to be tested.

Objectives of this research were: (1) to determine the most tolerant life stage for the navel orangeworm; (2) to develop a practical process protocol to control the targeted insects using RF energy with hot air; and (3) to study the impact of these treatment protocols on walnut quality.