Study On Non-enzymatic Browning And Changes In Flavor Quality Of Freeze-dried Lemon Slice During Storage

The freeze-dried lemon slices dried by vacuum freezing technology are deeply loved by consumers at home and abroad because it retains the original nutrients of lemon.The main problem in the development of freeze-dried lemon slices is non-enzymatic browning.In this paper,freeze-dried lemon slices produced by Guangdong Zhongxing Lvfeng Agricultural Science and Technology Development Co.,Ltd.were used as raw materials,and the method of physiological and biochemical indicators and nuclear magnetic resonance(NMR)techniques were used to monitor the change of non-enzymatic browning indicators and browning substrates during storage.And the browning products were extracted and identified by infrared spectroscopy to reveal the non-enzymatic browning mechanism of freeze-dried lemon slices.Simultaneously,electronic nose,electronic tongue and GC-MS technology were used to detect and analyze the flavor quality during storage of freeze-dried lemon slices in order to provide a scientific basis and technical support for further research on browning prevention technology of freeze-dried lemon slices.The main findings of this article are as follows:(1)Determination of changes in non-enzymatic browning index of freeze-dried lemon slices during storage.The results are as follows:△E,browning degree and malondialdehyde increased significantly(p<0.01);the content of browning substrates such as polyphenols,ascorbic acid,and amino acid nitrogen decreased significantly in the first20 days(p<0.05)then change gently;the content of browning substrates such as reducing sugars,total sugars,and proteins increased significantly(p<0.01)at 10 days before storage,and decreased at the end of 40 days;5-hydroxymethyl furfural(5-HMF)and other intermediate browning products rose first and then decreased,p H basically stable at about3.0,and the total acid content decreased first and then rose.By correlation analysis and path analysis,the three main factors affecting the color difference of freeze-dried lemon slices were the interaction between polyphenols and proteins(d=-1.6459),the interaction between polyphenols and ascorbic acid(d=1.4958)and the effects of polyphenols(d=0.9692);the three main factors affecting the browning degree were the interaction of polyphenols and proteins(d=-1.7303),the interaction of polyphenols and ascorbic acid(d=1.3179)and the interaction of polyphenols with reducing sugars(d=-1.2019).In summary,the reason that the non-enzymatic browning occurred during the storage of freeze-dried lemon slices was the result of oxidative polymerization,ascorbic acid oxidation and Maillard reaction.(2)A total of 19 primary metabolites and secondary metabolites of the freeze-dried lemon slices extract during storage were obtained by ~1H NMR and 2D NMR(COSY、JRES、HSQC、HMBC)analysis.The PCA analysis showed that the PC1 was glutamic acid,γ-aminobutyric acid,alanine,limonin,citric acid,glutamine,phosphoric acid,α-aminobutyric acid,and the PC2 was fructose and ethanol.By semi-quantitative analysis of NMR,fructose is more involved in the Maillard reaction than glucose activity.The free amino acids of freeze-dried lemon slices are mainly hydrophilic amino acids,about 59.5%,of which the highest serine content and the strongest activity.By HPLC analysis,it was found that the reducing sugar of the freeze-dried lemon slices was mainly fructose and glucose.(3)Using browning index as index,the optimal extraction scheme for browning products of freeze-dried lemon slices was obtained through single factor and orthogonal test:the ratio of material to liquid 1:30 g/m L,ethanol concentration 70%,extraction time 2 h,extraction temperature 60℃.The best macroporous adsorption resin D4020 was obtained by static adsorption test.The adsorption rate was 74.04%and the desorption rate was86.94%,which was significantly different from other resins(p<0.05).The browning product extract was purified and separated using Sephadex G-50 to obtain three components M1,M2,M3.Infrared spectrum shows that the three components contain alkyl,alcohol hydroxyl,ketone,carboxylic acid,amide,olefins,amines,phenolic hydroxyl and other functional groups,containing pyranoside bonds,and may Amadori compound(C=O)and pyrazine(C-N)which contain carbonyl compounds and amino compounds such as sugars,proteins,and Maillard reaction intermediates.(4)Through the analysis of PCA and LDA of electronic nose and electronic tongue,the volatile components and taste of freeze-dried lemon slices changed during storage.A total of 39 aroma components were detected from freeze-dried lemon slices by GC-MS.Among of them,14 were olefin compounds,which the content was 64.998%in the early storage period.The relative contents of characteristic aroma components such as d-limonene,geranial,β-myrcene,geranyl acetate and neroliacetate showed a decreasing trend,especially the d-limonene and geranial content decreased significantly,which the decrease was 11.185%and 3.175%.The relative content of undesirable flavor substances such as furfural increased by 11.262%during storage.In summary,the flavor quality of freeze-dried lemon slices gradually decreased during storage.