Protocol Development Of Radio Frequency Drying And Disinfestation For Walnuts

China is the primary producing country of walnuts worldwide,taking up a proportion with over 50%of global production.Fresh walnuts are perishable and infested easily by insect pests because of the high moisture content,reducing the product quality and economic value.Drying and disinfesting treatments could remove moisture from fresh walnuts and improve the safety and stability of walnuts during storage.However,there are several disadvantages of common hot air drying and methyl bromide fumigation techniques,such as long processing duration,deteriorated product quality,environmental pollution and chemical residues.Radio frequency(RF)heating possesses the advantages of large heating and drying rates,and has potential application prospects in the combined drying and disinfesting treatment for post-harvest agricultural products.This thesis took walnuts as the research object,determined the dielectric and thermal properties of walnuts by an open-ended coaxial probe system and a thermal property analyzer;selected the fifth-instar rice moth(Corcyra cephalonica L.)larva as the target insect pest,investigated the effect of frequency on drying characteristics,disinfestation efficacy,heating uniformity,and product quality;then developed a two-stage hot air assisted RF drying and disinfestating treatment,investigated the effect of packaging method on lipid oxidation of walnuts during storage,providing a theoretical basis for the industrial application of the hot air assisted RF drying and disinfesting technologies for walnuts.The main research contents and findings are as follows:(1)Dielectric constant,loss factor,specific heat capacity and thermal conductivity all increased with raised temperature and moisture content,but dielectric properties decreased gradually with increasing frequency.The penetration depths of electromagnetic waves at 27and 40 MHz were much greater than those under 915 and 2450 MHz,suggesting that RF wave is more suitable for heating and drying products with large volumes.The established quadratic and cubic polynomials predicted dielectric and thermal properties accurately(R~2>0.966),providing data and theoretic basis for subsequential experiments of RF drying.(2)The results showed that with the increase of frequency,average walnut temperature rose and heating uniformity dropped significantly(P<0.05),and less time was needed to achieve 100%mortality of pests.The frequency had no significant effects on walnut cracking ratio(CR)and color(L~*),but significant difference in peroxide value(PV)and free fatty acid(FFA),which still met the industrial standard(PV<1.000 meq/kg,FFA<0.600%).Overall,the shortest drying time(150 min)and the highest energy efficiency(21.50%)were obtained when using the 27.12 MHz RF system for drying walnuts to a MC of 8%(d.b.)under the similar heating rate(1.40℃/min)and final temperature.(3)The electrode gap of 160 mm for one-stage drying was selected for further investigations since it can ensure a short processing time(210 min),100%mortality of C.cephalonica larvae,and eligible PV(0.183 meq/kg)and FFA(0.128%).The two-stage strategy with the heating rate of 5.0℃/min and transition temperature of 55℃shortened the drying duration to 170 min by nearly 20%,improved the heating uniformity index from 0.075 to0.064,and reduced CR from 43.75%to 26.04%significantly(P<0.05)as compared to one-stage treatment.PV and FFA of vacuum and normally packaged in-shell walnuts and vacuum-packed shelled walnuts still met the industrial standard after the 20-d accelerated storage.