Effects Of Temperature And Harvesting Period On The Quality Of Fresh Walnuts During Frozen Storage

In recent years,fresh walnuts have been favored by consumers for their high water content,unique flavor and health benefits.However,there is a serious phenomenon of early picking of fresh walnuts.The current market supply period of fresh walnuts is mainly limited to the harvesting season.The postharvest storage and preservation period of fresh walnuts is short,and the quality deterioration problems such as browning,mildew,water loss,softening and blackening of the nucleolus are prone to occur.In order to prolong the fresh-keeping period of fresh walnuts,this paper selected fresh walnuts of"Qingxiang"variety with green peel removed as the test material to study the effects of different temperature and harvesting period on the sensory quality,nutrient quality,antioxidant content and oil quality of fresh walnut during frozen storage,to determine the optimum temperature and the harvest time for long-term storage of fresh walnut.At the same time,to study the effects of different thawing temperatures on the re-fresh quality of fresh walnuts after frozen storage,and the optimum re-fresh temperature is selected.The freezing-thawing process of fresh walnut is established.The results showed that:1.The quality of the'Qingxiang'fresh walnuts are decreased during frozen storage at different low temperatures?0?^-2??-2?^-4??-7?^-5??-12?^-10??.The fresh walnuts began to mildew after 2 months of storage at 0^-2?,and mildew became serious after 4 months of storage;the mildew spots began to appear in frozen storage at-2?^-4?after 4 months,and could not be eaten after 6 months.They can be frozen for more than 10months at-7?^-5?and-10?^-12?.At this time,their fresh flavors are close to before storage,and the moisture contents of the nucleolus are still greater than 23%,and the total fat contents are above 53%,and the contents of unsaturated fatty acids are as high as 91%,the activity of lipase was low.The total phenolic and total flavonoids contents are comparable to that before storage,and the DPPH·clearance rates are over 83%,the contents of vitamin E are above 92 mg·kg^-1.The acid value,peroxide value and saponification value of nucleolus oil are higher than harvested,and the iodine value decreased,but all the four indexes are kept within the range specified in GB/T22327-2008?Walnut Oil?.2.The frozen reservoir quality of fresh walnuts harvested in suitable period?40%green skin separation on September 6?and late harvest?more than 75%green skin separation on September 13?are better than those in early harvest?green peel did not crack on August 30?.The water contents of nucleolus decreased and the total fat contents increased with the delay of harvest time.After 10 months of frozen storage,the sensory quality of fresh walnuts decreased during each harvest period,and the flavor of fresh walnut kernels in suitable and late harvesting are better than that in early harvesting;the moisture contents of fresh walnuts in each harvesting period are above 21%,and the contents of soluble sugar and soluble protein are not significantly different from that before storage;and the total fat contents are49.81%,53.23%and 54.79%,and the suitable and late harvesting are significantly higher than that in the early harvesting.The contents of unsaturated fatty acids are not much different from that before storage,and remained at about 90%.The DPPH·clearance rates of fresh walnut in each harvest period are more than 83%.The total content of vitamin E in fresh walnut collected in suitable period is 95.17 mg kg^-1,which is significantly higher than that in early harvesting and late harvesting.The quality indexes of the fresh walnut oil in each production period are kept in the range specified by GB/T22327-2008?Walnut Oil?.3.The effects of different thawing temperature on the quality of fresh walnut after frozen storage were studied,the results showed that the thawing time at room temperature?20?²2??is the shortest,but the flavor of kernels are poor,and the phenomenon of mildew and germination began to appear in a short period of time.After thawing fresh walnut flavor at 0?²?and gradient heating?Before-4?^-2?cold storage for 3 days,and then transferred to 0?²?for storage?,the fresh walnut flavor are close to that before storage,the moisture contents are above 27%,the nutritional qualities and antioxidant contents and oil quality are good.4.Preliminary determination of fresh walnut postharvest annual supply storage technology is as follows:fresh walnuts are collected when the green skin was cracked and separated from 40%to 75%,post-harvest peeling,washing,and stored at-12?^-5?after drying,and thawed at 0?²?before sale.The sensory quality and nucleolus quality of the fresh walnuts remained good after re-freshing.