Disinfestation And Pasteurizationof Postharvest Chestnuts Using Radio Frequency Heating

Chinese chestnut(Castanea mollissima)has special flavor of sweet,glutinous,and incense,and high nutritional value.It can be eaten by raw,fried,or cooked products.China is the largest producer of chestnuts in the world.The total output of chestnut is 80 % of the world's total output.After harvest,the chestnut has high moisture content and rich starch content,which provides a good living environment and rich food for pest and mould.Infestations and fungus contaminations are two major issues during chestnut storage.It is estimated that the annual losses of chestnuts due to insects and mildew are about 25 % to 50 % in China,resulting in high economic losses for farmers.Therefore,a new non-chemical method based on the radio frequency(RF)heating technique for disinfesting and pasteurizing chestnuts was discussed in this dissertation.To develop RF disinfestation and pasteurization processes of chestnuts,the dissertation studied on the thermal death kinetics of most heat-resistant insect and main fungus in chestnuts,and discussed the thermal kinetics of chestnut kernel color.According to the death conditions of pests and moulds,the control parameters of RF heat treatment were proposed.In order to improve heating uniformity,the thermal treatment protocol was optimized for disinfesting and pasteurizing chestnuts.The reasons of RF heating uniformity were analyzed by finite element analysis software.Using the optimized protocol,effects of disinfestation and pasteurization were validated based on the efficacy studies and the mechanism of disinfestation and pasteurization was explored.Finally,the influence of RF heating on quality of chestnuts was evaluated.The main contents and results were as follows.(1)The heat resistence of yellow peach moth,Conogethes punctiferalis,and chestnut weevil,Curculio elephas,were discussed.And the thermal death kinetics of the most heat resistence insects,C.punctiferalis was studied.Fifth-instar C.punctiferalis was more heat tolerant than chestnut weevil.The relative heat tolerance of C.punctiferalis life stages was found to be fifth instars > pupae > third instars > eggs.Thermal death curves of fifth-instar C.punctiferalis followed a 0th-order kinetic reaction model.At the heating rate of 5 °C/min,the minimum exposure time to achieve 100 % mortality was 54.9,11.6,6.2,and 2.5 min at 44,46,48,and 50 °C,respectively.(2)Thermal death kinetics of the most heat tolerant fungus in chestnuts was developed.The four main funguses in chestnuts dominated(Penicillium crustosum,Penicilluim discolor,Penicillium expansum,and Penicillium glabrum)and were isolated at frequencies of 58.31±2.39 %,17.78±5.00 %,15.66±7.67 %,and 14.24±6.71 %.P.crustosum was more heat tolerant than other funguses in chestnuts.Thermal death curves of P.crustosum followed a Weibull kinetic reaction model.The minimum exposure time to achieve 4-log inactivation rate was 25.0,5.0,2.3,and 1.1min at 56,58,60,and 62 °C,respectively.(3)Thermal kinetics of color degradation of chestnut kernels was investigated.The L* and b* decreased with increasing temperature(from 50 °C to 70 °C every 5 °C)and heating time.The first-order reaction model showed a better fit than zero-order reaction model.According to requirements of color,insect mortality,and the target microorganism's inactivction rate,a potential disinfestation and pasteurization treatment protocol could be developed for chestnuts.(4)RF heating treatment protocol was optimized.According to the required RF heating rate,the appropriate electrode gap(12 cm)was selected.The heating uniformity of RF treatments was improved by hot air,conveyor moving,and sample mixing.The thermal treatment protocol for completely killing insects in chestnuts was developed to combine 0.6 k W RF power with a forced hot air at 55 °C,movement of the conveyor at 0.15 m/min,twice mixings,and holding at 55 °C hot air for 5 min,followed by forced room air cooling through single-layer samples.(5)RF heating uniformity was determined by finite element analysis software.For whole chestnuts in the container,there were clear edge effects,in which the temperatures of chestnuts in the corners and edges were higher than those in the central parts.Highest temperatures were located in sample contact points of top and middle layers at four corners in the container.For a single chestnut,the temperatures of contact points were higher than other parts of chestnuts at the corner of container.Except for chestnuts at the corner of container,the temperature increased gradually from outside to inside and the highest point was in chestnut center.Heating uniformity index was reduced when three-layer chestnuts were separated with two plastic sheets.The better heating uniformity in chestnuts was obtained when they were treated with a single layer,or under mixing conditions.(6)The effectiveness of RF disinfestation and pasteurization was verified.Mortality of fifth-instar C.punctiferalis increased with increasing holding time at 55 °C using hot air at 55 °C assisted for RF heating,the conveyor belt at 0.15 m/min,mixing at every 1.8 min,and reached 100 % while holding in hot air for at least 5 min.Using the RF energy with hot air,heating chestnut to 60 °C or higher resulted in 4-log reduction of P.crustosum spores isolated from chestnuts.(7)Effects of heat treatment on quality of chestnuts were evaluated.The moisture contents of chestnut kernel and shell were both reduced after RF treatment and storage,but no significant difference was found between treated chestnut shell and control.The starch content and soluble sugar increased firstly and then decreased during storage.The value of protein,colony count,and a* increased gradually and other quality parameters decreased with the increased storage time.The starch content,protein,fat,soluble sugar,firmness,and color of chestnuts had no significant change as compared with the control.