Research On Mechanism Of Browning Reactions In Apple Slices Dried By Instant Controlled Pressure Drop Drying(DIC)

Effects of instant controlled pressure drop drying(abbr.D.I.C.)on the colour of dehydrated apple slices were investigated in this study.Change of browning ratio(BR)apple slices at different stages of D.I.C.was evaluated by image analysis,and the contribution of browning-related components to BR was investigated.The results showed the principal reason for the non-enzymatic browning reaction in apple slices dried by D.I.C.is Maillard reaction.And phenolic compounds could accelerate a*value(red-green value)of the models solutions of Lysine-Fructose.The main results and conclusions were summarized as follows:(1)D.I.C.had a significant effect on the colour change of apple slices.The L*and h values of apple slices were descreased,but a*,b*,△E and C*values were increased at different stages of D.I.C..The BR of dehydrated apple slices was regarded as the proportion of featured colour information in the original image during different stages of D.I.C..The varieties had a significant effect on the BR of apple slices dried by D.I.C.,and the largest BR is found in wenhong and the least value is found in changfuerhao.The result indicated that D.I.C.process was a key step for the formation of BR of apple slices.The concentration of 5-hydroxymethylfurfura(5-HMF)and fluorescence intermediary compounds intensity(FIC)in apple slices increased slightly during AD and ME periods,and sharply increased after the D.I.C.period.Furthermore,positive and significant quadratic correlations(R2>0.99)were found between the content of 5-HMF and FIC intensity with BR.(2)The moisture content,water activity and mobile water ratio was decreased along the drying process,and only the hydration water was obsevered in final dehydrated apple slices.The residual activity of polyphenol oxidase retained 15%after moisture equilibrium process,and was compeletly inactivated in the apple slices after D.I.C.period.The concentration of glucose,fructose,amino acids,ascorbic acids and poluphenol compounds were significantly idecreased during D.I.C.period.The results showed that non-enzymatic browning reaction was mainly contributed to the formation of BR of apple slices during D.I.C.process.(3)The lower concentrations of fructose could(<150 g/kg)contribute to the formation of BR,while higher concentration(>150 g/kg)inhibited the formation of BR.In contrast,the lower concentrations of glucose could(<150 g/kg)decreased the formation of BR,while higher concentration(>150 g/kg)increased the formation of BR.L-lysine,L-arginine and L-cysteine showed positive effects on the formation of BR in models based on apple slices.However,the formation of BR of the models based on apple slices gradually increased,and then decreased with increment of the concentration of L-glutamic acid,L-histidine,L-arginine,L-methionine and L-tyrosine(0.1-2 g/kg).Dehydroascorbic acid contributed to the formation of BR.Reducing ascorbic acid(>0.01 g/kg)could inhibit the formation of BR.Compared to the solution containing reducing sugar and amino acids alone,the results showed that the reducing sugar and amino acids solution with the addition of(+)-catechin,L-epicatechin and phlorizin resulted in a remarkable increment of BR in the apple slices.(4)The effect of four structurally variant phenolic compounds,chlorogenic acid,(+)-catechin,L-epicatechin and phlorizin were examined as inhibitors and/or stimulators of apple slices browning.The L*value of CAT-Lys-Fru solution decreased with the incresment of heat temperature and pH value,while a*and b*values showed an increasing trend.The result showed that(+)-catechin could increase a higher a*value of Lys-Fru solutions than other treatments.Chlorogenic acid promoted the formation of 5-HMF in Lys-Fru model solution at 90 ℃.However,(+)-catechin,L-epicatechin and phlorizin significantly inhibited the formation of 5-HMF under same conditons.The results between Lys-Fru and CAT-Lys-Fru were similar,but the compounds(m/z 400-620)of Lys-Fru and CAT-Lys-Fru were siginificant different.The formation of five compounds(C27H37N3O10,C21H23NO8,C21H28N2O8,C21H26O12 and C27H40N2O14)was possibly the main reason of the red formation.