| High moisture content and various oxidases in fresh lettuce can promote tissue browning and decaying,thus causing obstacles to the production of further processed lettuce products.Blanching is an important step in processing fruits and vegetables,which can effectively inactivate oxidases such as peroxidase(POD),and extend the shelf life of products.Conventional hot water and steam blanching provide poor heating uniformity,decreased product quality,serious loss of nutrients and waste of water resources.Radiofrequency(RF)heating,serves as an electromagnetic field dry heating technology,could avoid the contact of materials and hot water/steam.Hence,RF heating is a potential method for blanching fruit and vegetable.At present,heating uniformity,enzyme inactivation efficiency and quality of fruits and vegetables after blanching are the focus of RF blanching of fruits and vegetables.However,little research reported the mechanisms of RF blanching,which need to be further explored.Stem lettuce was used in this study for RF blanching.The effects of sample height and electrode gap on the heating rate and heating uniformity were studied.Horseradish peroxidase(HRP)was selected to investigate the effects of RF heating temperature on the activity and conformation of HRP;we also investigated the effects of RF heating temperature on POD activity and physicochemical properties of stem lettuce,and analyzed the underlying cell-morphological mechanism.The effects of RF heating rate on physicochemical properties and cell morphology of stem lettuce were evaluated.A novel steam-assisted RF heating equipment was designed in this study.Furthermore,the effects of RF preheating temperature and the steam blanching time on the heating uniformity,the inactivation efficiency of POD and the physicochemical properties were analyzed.All results are as follows:(1)The electrode gap and sample height had significant effects on the RF heating rate and heating uniformity of stem lettuce(P<0.05).Better heating uniformity and faster heating rate were obtained when the electrode gap was 100 mm and the sample height was 55 mm.After RF heating,the center temperature of samples was higher than that of edge.(2)RF heating temperature had a significant effect on HRP activity(P<0.05).The change of the secondary and tertiary structure of HRP induced by RF heating leaded to the change of HRP activity.RF heating temperature significant effected(P<0.05)POD activity,the weight loss,texture,color,vitamin C retention rate and relative electrolyte leakage of stem lettuce.The samples treated by RF heating showed better texture and color,lower nutrient loss and less microstructure destruction than conventional hot water blanching at the same enzyme activity level.The changes in physiochemical properties could be attributed to the destruction of cell membranes,loss of cell turgor,reduced rigidity of cell walls,and loose adhesion between adjacent cells.(3)The rate of POD inactivation increased with the decrease of electrode gap.Electrode gap had significant effects on texture,color and relative electrolyte leakage rate of stem lettuce(P<0.05).RF heating at a fast heating rate may be more suitable for dry and quick-frozen products,because this method can maintain good texture and nutritional value when inactivating the enzyme.However,prolonged blanching at a slow heating rate may be a more favorable blanching way when producing juice and sauce of fruits and vegetables,by which the degradation of cell structure promoted the dissolution of nutrients in further processing for the juice and sauce.(4)Compared with RF heating,steam(ST)-assisted RF blanching significantly improved(P<0.05)the inactivation efficiency of POD and heating uniformity of stem lettuces;The stem lettuces treated by RF preheating to 80 oC combined with stem blanching for 1 min presented less damage of cell membrane,and maintained the best texture,color,vitamin C retention rate. |
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