Pasteurization Of Postharvest Corn Using Radio Frequency Heating

Corn(Zea mays L.)is ranked third as food crop because of its larger planting area and high total yield production of China.Corn is consumed worldwide for various purposes,except for food,corn production is also going for feed purposes and for processing into various other value added products.Corn kernels are rich sources of starch and protein,and are abundant sources of mineral elements.Mildews are the most main problem for corn,and it often loses its nutritional and economical values owing to serious infections during storage time.Therefore,to effectively weed out the mold in corn,reduce the quality damage by heating process,and improve the international competitiveness of China's food became the huge challenge that we need to solve in food industry.Zheng dan was chosen as one representative of corn variety to develop the radio frequency pasteurization protocol because of its large planting area.Then to evaluate the effect of RF pasteurization on the quality attributes of corns during an accelerated storage.The detailed research contents and results are described as follow:(1)Comparison of RF heating uniformity under different operational conditions.A corn sample of 3.0 kg with moisture content(MC)of 15.0% w.b.was used for full loads in a polypropylene container which was placed on the stationary conveyor belt between the two electrodes gap in the RF heating uniformity heating test.An appropriate electrode gap for RF treatments was determined by changing the distance of two electrodes gap and the suitable heating rate.The sample was placed in the middle of top and bottom electrode gap to compare the temperature profiles of corns between hot air and RF heating,and determine an appropriate cooling method.The corn sample was placed in the middle of two electrode gaps and heated in the RF system to compare the sample temperature distribution under seven conditions: RF heating only;RF heating with hot air assisted;RF heating with conveyor belt movement;RF heating with a single mixing;RF heating with two mixings;RF heating with forced hot air of 70 °C assisted,conveyor belt movement and a single mixing;RF heating with hot air,conveyor belt movement,two mixings and holding in 70 °C hot air for 14 min.The results showed that RF energy sharply reduced the heating time and improve processing efficiency compared with the traditional hot air heating.The RF heating uniformity for corns was both improved under the above different operational conditions.Mixing was the bestmethod to improve heating uniformity as compared to hot air and movement.With holding,the average sample temperatures in the three layers were reduced and the heating uniformity was further improved due to reduced standard deviations of temperature.An effective RF treatment protocol was finally developed to combine 0.8 kW RF power with a forced hot air at 70 °C,conveyor movement at 6.6 m/h,two mixings,and holding at 70 °C hot air for 14 min,followed by forced room air cooling through thin-layer(2 cm)samples.(2)The optimal RF treatment protocol was validated for corns with different MC(9.1%,12.0%,17.9%,and 20.9% w.b.)and containers(polyetherimide and polystyrene).The corn samples at 15.0% w.b.in the polyetherimide container had a better RF heating uniformity compared to samples in the polystyrene and polypropylene containers.For the corn samples with five moisture levels,the smaller heating uniformity index appeared in the sample with low MC.(3)Verifaction of radio frequency pasteurization treatment for controlling Aspergillus parasiticus on corn grains.3.0 kg corn samples with MC of 15.0% w.b.were filled in the polypropylene container,then heated with RF energy together with hot air heating to determine the effects of sample temperature(55 °C,60 °C,65 °C,and 70 °C),holding time(4,8,12 and 16 min)and moisture content(9.1,12.0,15.0 and 17.9% w.b.)on thermal resistance of Aspergillus parasiticus.Results showed that the pasteurizing effect of RF heating on Aspergillus parasiticus increased with increasing heating temperature and holding time,and RF heating at 70 °C holding in hot air for at least 12 min resulted in 5-6 log reduction of Aspergillus parasiticus in corn samples with the moisture content of 15.0% w.b..Furthermore,thermal resistance of Aspergillus parasiticus decreased with increasing moisture content(MC)of corn samples.(4)Quality evaluation after radio frequency treatment.Control and RF treated corn samples were stored at 35 ± 0.5 °C with 68 ± 0.5 % relative humidity(RH)for 17 d to simulate commercial storage at 10 °C for 1 year.After 0,4,8,12 and 17 d of storage,the corn samples were withdrawn from each treatment for quality analysis.The results showed that the MC and water activity decreased slightly with storage time between control and RF treated samples,but the protein,starch,ash and fat content were not affected by RF energy during storage.RF treatment has no effect on the values of fatty acid content of corn samples at 0 day,but with the increasing of storage time,the fatty acid content in control samples increased at a faster speed than that in RF treated corn samples.The color values of corn samples at 15.0%w.b.were no significant differences before and after RF treatment or during storage.The electrical conductivity and germination rates are the most important quality attribute for corn seed.After higher temperature,the electrical conductivity increased with increasing storagetime both for control and RF treated samples.The germination rate of corn sample was sharply declined and had the significant difference in control and RF treated samples.