| Hami melon is a kind of heat-sensitive fruit,which can produce a lot of odor in traditional thermal processing,and the product quality is not guaranteed.Therefore,this paper mainly uses thermal(microwave treatment,instantaneous heating)and non-thermal treatment(high hydrostatic pressure treatment)of Hami melon juice to study the variation of aroma and the activity and structural changes of key enzymes,and changes of amino acid,fatty acid.The relationship between enzyme activity and precursor(amino acid,fatty acid)and related enzyme activity in Hami melon juice metabolic pathway was analyzed by various analytical methods.The main findings are as follows:1.The total ester content of Hami melon juice was significantly reduced after two heat treatments(microwave treatment 45s/MW,121℃ heating 45s/TP).The change in the type of ester in the TP sample was larger than MW sample,and the amines were produced.The type of ester in the high pressure treatments was close to that of fresh Hami melon juice,but the total ester content decreased with the increase of pressure.The contents of total alcohols and total aldehydes increased in HHP treatments.The ketones did not change significantly.Through the LEfSe analysis of the two heat treatments,in the heating treatment at 121℃,the main aroma substances causing the flavor change were acetate esters,alcohols and aldehydes of C6C9,and one kind of amines.Different changes in flavor substances under different high hydrostatic pressure conditions.The key aroma substances were esters at 350 MPa sample;The main aroma substances of 400 MPa sample were ethyl acetate,ethyl methylthioacetate,propyl acetate and benzaldehyde.2(E),6(Z)-nonadienal was a key substanceat 450 MPa sample;6-(E)-nonenal,3-(Z)-nonenol,2-methylbutyrate,2,7-octanediol and diethyl phthalate were more influential in the 500 MPa treatment.2.The activities of the four enzymes related to aroma metabolism were significantly different after different treatments.The lipoxygenase(LOX)activities were lower than that of the CK group after high pressure and heat treatments.The activity of phospholipase(PLA2)was similar to that of LOX;The activities of alcohol dehydrogenase(ADH)and acyltransferase(AAT)were just opposite to the change of LOX.Through the CCA analysis of four enzymes and aroma of Hami melon juice in heat treatments and high hydrostatic pressure treatments,it was found that LOX enzyme was the most important enzyme in heat treatments,which was mainly related to some esters,aldehydes and ketones.ADH played a key role in the changes of aroma in high hydrostatic pressure treatments,it was most obvious in 400 MPa and450 MPa treated samples.3.The kinds and contents of amino acids varied under different conditions.The total amount of amino acids changed to varying degrees after high hydrostatic pressure treatments and heat treatments,and the amino acid showed a decreasing trend with the increase of pressure in HHP treatments.The partial least square analysis and variable projection importance analysis(PLS-VIP)of aroma and amino acids in heat treatment groups showed that histidine,lysine and isoleucine were mainly related to 3 aldehydes,2 esters and 1 amine;Aspartic acid and glutamate are mainly related to aldehydes of C6 and alcohols of C9;Serine and proline are mainly related to esters and alcohols.Through VIP analysis,it was found that the key amino acids in thermal treatments were aspartic acid,glutamate,histidine,isoleucine and lysine.The PLS-VIP analysis of aroma and amino acid in HHP treatments found that the amino acids were mainly related to acetates,alcohols of C6 and C9.Alanine,aspartic acid,glutamate,lysine,phenylalanine and valine were found to be the more related to aroma in HHP treatments by VIP analysis.4.There are 9 kinds of fatty acids in Hami melon juice.The species did not change much after different high hydrostatic pressure and heat treatments,but the total amount of fatty acids decreased with the increase of pressure,and the total fatty acid content(39.84 g/100g)of TP treatment was the lowest,which was 46.6% lower than that of the CK group.PLS-VIP analysis of aroma and fatty acids in heat treatments showed that linoleic acid,stearic acid,palmitic acid and α-linolenic acid were mainly related to some esters and ketones.Arachidonic acid and lauric acid were mainly related to acetate,alcohols and acid.Oleic acid is mainly related to five esters and two aldehydes,and α-linolenic acid,arachidonic acid and linoleic acid have relatively great influence on aroma in heat treatments.PLS-VIP analysis showed that palmitic acid,heptadecarboxylic acid and α-linolenic acid were mainly correlated with alcohols and aldehydes of C6 and C9 and some methyl esters.5.Through the comprehensive analysis of substrate,key enzyme activities and aroma products in the aroma metabolism of the heat treatments and the ultrahigh pressure treatments,it was found that there were PLA2,isoleucine and α-linolenic acid related to esters in heat treatment group.There were 7 substances related to alcohols,such as LOX,PLA2,histidine,palmitic acid,linoleic acid,α-linolenic acid,and arachidonic acid.The substances associated with aldehydes were LOX,AAT,and arachidonic acid.By Vegan correlation analysis,the correlation between amino acid and aroma substance was stronger than that in fatty acid in heat treatments.In the ultrahigh pressure treatments,there were 3 amino acids,2 enzymes and 1 fatty acid related to esters.There were 8 substances related to alcohols,all of them were positively correlated,including 6 kinds of amino acids,ADH and stearic acid.Therewere 8 kinds of substances related to ketones,all of which were positively correlated.Among them,there are 5 kinds of fatty acids,3 kinds of enzymes,and no amino acids.There were 8 kinds of substances associated with aldehydes,of which there was a positive correlation with only leucine and a negative correlation with seven(three enzymes and four fatty acids).According to the Vegan analysis,the correlation between fatty acids and aroma substances was stronger than that between amino acids and aroma substances in ultrahigh pressure treatments. |
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