| Coix seed is a traditional grain with the same origin of medicine and food,which is prone to lipid oxidation during storage.This problem has become the bottleneck in the development and expansion of coix seed industry.Therefore,this paper tried to use the new technology of radio frequency(RF)inactivation of coix seed lipase to improve its storage stability.Meanwhile,the primary and secondary relationship among three main factors that influence lipid oxidation,i.e.lipase activity,alkyl free radical intensity and oxygen,was explored,so as to provide guidance for the research and application of this technology.First of all,the preferred process schemes of RF inactivation of lipase were determined.The RF power,RF temperature,insulation method,plate spacing and material/water ratio were investigated,using the lipase activity as the evaluation index.The results showed that there are three preferred RF schemes,i.e.1.26 k W,115℃,120 mm,insulating by RF for 60 min without water;1.26 k W,115℃,120 mm,immediately insulating by hot air for 20 min without water;1.26 k W,115℃,140 mm,without insulation and water.The inactivation rates of the three schemes were as high as about 75%.Secondly,the storage stability and nutritional quality of coix seeds treated with RF were evaluated.To kinetically evaluate the storage stability,the three RF-treated samples with high lipase inactivation rate and the raw materials were stored at 37℃ for 30 d,and five indexes such as fatty acid value during storage were fitted dynamically and their correlations were analyzed.The results showed that the storage indexes of the sample treated with RF at 115℃without insulation were relatively stable,thereby this sample was used for the following accelerated storage experiments.In addition,the variation of peroxide value and hexanal content of each sample during storage could be fitted by 0-order or 1-order kinetic equation,and the peroxide value was always positively correlated with the hexanal content extremely significantly.To predict the shelf life,the optimal RF sample obtained above and the raw material packed in vaccum were stored at 37℃ and 47℃ for 60 d.The hexanal content was selected as the index to predict the shelf life,and the nutritional quality of the samples after RF treatment was studied.The results showed that the shelf life of raw coix seed and RF treated coix seed packed in vacuum at 20℃ was 50 d and 48 d,respectively.Moreover,the basic nutrients except protein,the active substance coixenolide and the fatty acid composition of the samples before and after RF treatment did not change significantly.Finally,in order to understand the reason why the shelf life of coix seed was not prolonged after RF treatment even with lipase inactivation,the influence degree of three main influencing factors of lipid oxidation on different storage indexes were studied.Three samples with different lipase activity and alkyl free radical intensity were selected for storage experiment.The results showed that the strongest influencing factor on fatty acid value was lipase activity,followed by oxygen and alkyl free radical intensity,while the strongest influencing factor on peroxide value and hexanal content was alkyl free radical intensity,followed by oxygen and lipase activity.Therefore,the main reason that the shelf life is not prolonged after RF treatment is the increase of alkyl free radical intensity.In conclusion,RF is a rapid,mild and effective method for lipase inactivation.RF treatment does not affect the nutritional quality of coix seed,and can significantly inhibit the increase of fatty acid value during storage.However,it is necessary to strictly control the alkyl free radical intensity.Only when the lipase activity,alkyl free radical intensity and oxygen are considered as a whole,can the storage stability of coix seed be completely solved. |
