| Scallions are not only essential spices in daily life,but also important vegetables for exporting foreign exchange in China.After the scallion is harvested,it is bundled and stored after a period of drying.However,during this processing,scallions are easily spoiled and deteriorated storage.In order to extend the shelf life of scallions,the drying approach has become the best choice.At present,the traditional method of drying fruits and vegetables is hot air drying,nevertheless,is a long process,as well as it is subjected to weather restrictions and contamination by dust and fungi.Furthermore,the products generated have poor quality and energy loss.Therefore,the scallion drying industry needs a new drying process with high efficiency,green,quality and energy saves.In this paper,the scallion stems and leaves were cut and dried,and the infrared hot air drying with short drying time and high product quality was selected.Because the nutrients of the scallions are mostly distributed in the scallion stems,the dryness of the scallion stem is deeply appreciated by consumers.Nonetheless,the scallion stems are more difficult to dry with scallion leaves,therefore,they were selected as raw materials.The infrared hot air drying was combined with different pretreatments(vacuum ethanol permeation,ultrasonic ethanol permeation,ultrasonic combined with vacuum ethanol permeation)to improve the drying rate and the quality of scallion stems as new drying process with high efficiency,green,quality and energy saving.(1)The drying methods of hot air drying,vacuum drying,vacuum freeze drying and infrared hot air drying are applied to the drying of scallion,and the drying characteristics of scallion stems and leaves are studied.The water ratio,drying rate,rehydration ratio,surface color and odor change of scallion under different drying methods were discussed.The results showed that the difference in tissue structure between scallion stems and leaves made the drying time and drying rate different.When the drying temperature was 60°C,the drying time of scallion leaves in infrared hot air drying,hot air drying and vacuum drying was about 70,110 and At 130 min,the scallion stems were about 80,150 and 180 min,and similar results were obtained at temperatures of 70°C and 80°C.Infrared hot air drying whitening rehydration,color and odor changes are significantly better than a single hot air drying,but the quality and vacuum drying and vacuum freeze drying there is still a gap,so the choice of infrared hot air drying method for subsequent research,and pretreatment To further improve its quality.(2)The effect of vacuum ethanol pretreatment on the drying characteristics and quality of infrared hot air drying scallion stems was studied.The pretreatment includes four pretreatment conditions: 1)control,soaking the scallion stems in distilled water under normal pressure conditions,2)soaking the scallion stems in 75% ethanol under normal pressure conditions,3)soaking the scallion stems under vacuum conditions of 0.6 bar In distilled water,4)soak the scallion stems in 75% ethanol at a vacuum of 0.6 bar.The pretreatment time was 5,10,20 and 30 min and drying process was carried out in an infrared hot air drying oven at 60 °C with a wind speed of 2 m/s.The effects of different pretreatment methods on the re-watering,color,drying characteristics,vitamin C retention rate,three-dimensional morphology and bactericidal effect of scallion stems white were discussed.The results showed that the ethanol pretreated samples showed better rehydration,odor,vitamin C retention,bactericidal effect and significantly shortened drying time.The vacuum pretreated samples exhibited better scallion stems and color.The samples obtained by vacuum ethanol pretreatment combined the advantages of ethanol pretreatment and vacuum pretreatment.Therefore,vacuum ethanol pretreatment can significantly improve the drying rate and quality of scallion stems,and might be used in the drying process of scallion.(3)The effects of ultrasonic ethanol pretreatment on the drying characteristics and quality of infrared hot air drying scallion stems were studied.The effects of different pretreatment methods on the drying characteristics,rehydration,color,odor,microstructure and bactericidal effect of scallion stems were discussed.The results showed that the water loss and dry matter loss of scallion stems after ultrasonic ethanol pretreatment were significantly greater than other pretreatment methods.The drying time of scallion stems after ultrasonic ethanol pretreatment was significantly shortened,which was reduced by 33.34%,50% and 83.34% compared with ethanol,ultrasound and control samples.The cavitation and mechanical effects of ultrasound caused the tissue structure of the scallion stems to collapse and the rehydration decreased,but the ultrasonic pretreatment was superior to the control sample in the color and odor change of the scallion stems.Ultrasonic pretreatment has a part of the bactericidal effect due to its action of exhaust and mechanical effects.Both ethanol and ultrasonic ethanol pretreatment had significant bactericidal effects.There was no microbial residue after pretreatment.The total number of colonies of scallion stem slices after drying was(3.18-log)and(3.13-log),respectively,which was lower than the national standard.In summary,ultrasonic ethanol pretreatment combined with infrared hot air drying can be used as a new drying process in the whitening dehydration process.(4)The effects of ultrasonic combined with vacuum ethanol pretreatment on the drying characteristics and quality of infrared hot air drying scallion stems were studied.The effects of different pretreatment methods on the drying characteristics,rehydration,color,odor,microstructure and bactericidal effect of scallion stems were discussed.The results showed that the ultrasonic vacuum ethanol pretreated samples showed the largest moisture loss(26.49%)and dry matter reduction(4.72%).Ultrasound combined with vacuum showed a higher scallion stems drying rate than a single vacuum and ultrasound.There was no significant difference between the rehydration and ultrasonic pretreatment of the ultrasonic and vacuum combined samples,which was lower than the vacuum pretreated samples.The color and odor changes in the combination of ultrasound and vacuum are between the ultrasonic and vacuum samples.The vacuum could better prevent the whitening browning than the ultrasound,however the ultrasonic is superior to the vacuum in the scent after retaining the scallion.The vacuum,ultrasound,and ultrasonic vacuum all have bactericidal effects,but the difference between them is not significant.Therefore,in summary,ultrasonic vacuum ethanol pretreatment can be applied to a whitening drying process as a new drying process. |
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