| Walnut rich in unsaturated fatty acids and high-quality vegetable protein,walnut is one of the world’s four major nuts,has a high research and development and edible value,at the same time walnut fresh fruit than walnut dried fruit more tender taste,fresh sweet flavor,palatability stronger,and fresh walnut(kernel)market demand is higher,better economic benefits.However,in the process of storage and transportation,the fruit of fresh walnut is prone to Browning and corruption,which affects the product quality and sales scope of seeds.Low temperature storage has a positive effect on delaying the Browning and senescence of fruits and vegetables,but there are few reports on the effects of different storage temperatures on the quality of walnut kernel.In this experiment,fresh walnut kernel was used as material and stored at different temperatures(25℃,4℃,-18℃).Sensory quality,physical and chemical indexes and flavor at the end of storage of walnut kernel were determined during storage.The influence of different storage temperatures on storage quality of walnut kernel was explored,and the relationship between flavor difference and physiological metabolism of walnut kernel was clarified.It provides a theoretical basis for the research and development of fresh walnut preservation technology and walnut quality deterioration monitoring technology.The main research is as follows:(1)The sensory quality of walnut kernel under three storage temperatures was evaluated,and the changes of appearance and color difference were detected to investigate the sensory quality of walnut kernel under different storage temperatures.The results showed that the sensory quality of walnut kernel in the-18℃ group was the best during storage,while the Browning of walnut kernel in the 25℃ group was more serious and the sensory quality was worse than that in the 4℃ and-18℃ groups.(2)The effects of storage temperature on physicochemical indexes of walnut kernel were analyzed by investigating the changes of water,soluble protein,polyphenol,acid value(AV),peroxide value(POV),catalase(CAT),peroxidase(POD),polyphenol oxidase(PPO)and superoxide dismutase(SOD)activities of walnut kernel under three storage temperatures.The results showed that low temperature storage could effectively slow down the decrease of water and soluble protein of walnut kernel.At the end of storage,the water and soluble protein contents of walnut kernel in-18℃ and 4℃ groups were close to each other,while those in25℃ group were the lowest,which were 21.03% and 0.45 mg/g,respectively.The AV and AV of walnut kernel increased faster in 25℃ group during storage,but the AV and POV of walnut kernel were not high in the three groups as a whole,AV remained below 0.8 mg/g and POV remained below 0.005 g/100 g,which may be related to sample packaging.The activities of four enzymes in walnut kernel increased first and then decreased at different temperatures,and the activities of four enzymes were higher during 20-35 days of storage.(3)Through the detection of free fatty acids,free amino acids and volatile flavor substances of walnut kernel at the end of storage at three temperatures,the differences in flavor characteristics of walnut kernel among different groups were investigated under large temperature differences,and metabolomics technology was further used to detect metabolites,so as to reveal the mechanism behind these differences.The results showed that at the end of storage,the contents of free amino acids and free fatty acids in walnut kernel in 25℃ group were the highest,the contents of fatty acids and amino acids in walnut kernel in 4℃ group were relatively higher,and the contents of fatty acids and amino acids in walnut kernel in-18℃ group were between the other two groups,and the contents of sour and bitter amino acids in 25℃ group were higher.There were 54 kinds of volatile flavor substances detected in walnut kernel at 3temperatures,including 16 esters,15 aldehydes,11 alcohols,7 ketones,2 acids,2pyrazines and 1 furan.On the whole,there were some differences in the volatile flavor compounds of walnut kernel at 25℃ compared with those at 4℃ and-18℃.These differences may be related to the dynamic changes of esters and alcohols.The metabolomics technology was used to explore the causes of these differences.The results showed that 233 kinds of differential metabolites were screened for walnut kernel at 3 temperatures,and these metabolites were divided into two categories in hierarchical clustering.The differential metabolites of walnut kernel at-18℃ were mainly concentrated in the first category,and those at 25℃ and 4℃ were mainly concentrated in the second category.KEGG metabolic pathway analysis showed that233 different metabolites were enriched in 209 metabolic pathways,and these differences were mainly caused by biosynthesis of secondary metabolites,ABC transporters,amino acid biosynthesis,and protein digestion and absorption.In conclusion,the sensory,physical and chemical qualities of fresh walnut kernel were different under the three storage temperatures,and the quality of walnut kernel decreased the fastest under 25℃.According to the analysis of various indexes,the temperature of-18℃ is suitable for long-term storage,and 4℃ is suitable for shortterm storage.Further analysis of the different metabolites showed that biosynthesis of plant secondary metabolites,ABC transporter,amino acid biosynthesis and protein digestion and absorption were the key factors affecting the degenerate flavor of walnut kernel at the end of storage. |
PDF Full Text Request