Effects Of Temperature,Copper And Sulfur Dioxide On Kinetics Of Oxygen Consumption By Phenols,and Their Correlation With Fe³⁺/Fe²⁺ Ratios In Model Wine Systems

Mechanisms of wine oxidation in literature show that iron and copper,in particular iron plays central roles in initiating and propagating the reactions of wine non-enzymic browning.In essence,iron speciation and distribution may be of most importance comparing iron concentration.As it is well known that the compositions of wine,such as large molecular compounds（organic acid,phenolic compounds,etc.）and environment factors（temperature,oxygen,p H value,etc.）can affect the speciation of iron,leading to influencing the functions of iron in wine oxidation.In our study,on the one hand,developing a method to directly determine total iron and ferrous in wine;on the other hand,investigation the effects of temperature,copper and sulfur dioxide on kinetics of oxygen consumption by phenols,and their correlations with rations of Fe²⁺/Fe³⁺in model wine systems.The main results achieved are as follows:1.The modified ferrozine assay developed for determination of total iron and Fe²⁺in wine.A rapid method for directly accurate determination of total iron and ferrous ion(Fe²⁺)in wine was developed by optimizing the ferrozine-based assay for total iron detection,and the effects of ferric ion(Fe³⁺)and ferrozine levels on the quantification of Fe²⁺under wine conditions were also investigated.The results showed that the modified ferrozine assay was established for the direct determination of total iron in wine,the standard curve exhibited a good linear relationship between absorbance and iron concentration from 0.25 to 2.00 mg/L（R²=0.9979）and the method detection limit was 0.0115 mg/L,with the recovery rate of 94.31%-104.34%.However,the presence of Fe³⁺seriously interfered with the accurate measurement of Fe²⁺in wine,and the optimized molar ratio of ferrozine/total was 7∶1,with the fitting accuracy ranging from 101.98%to 113.50%for Fe²⁺determination.2.Effects of temperature,copper and sulfur dioxide on kinetics of oxygen consumption by phenols,and relationship with ratios of Fe²⁺/Fe³⁺in model wine systems established.（1）Effects of temperature,copper and sulfur dioxide on kinetics of oxygen consumption by phenolsIn our experimental conditions,the change of oxygen concentration（0-59 days）could be divided into two stages.In the first stage（0-16 days,fast oxygen consumption stage）,the oxygen concentrations of all treatments firstly decreased quickly,then coming to keep stable.The consumption oxygen by phenols follows a first-order rate law in the first stage（0-16 days）.In the second stage（16-59 days）,oxygen contents of treatments without adding sulfur dioxide decreased slowly.However,those of all treatments with sulfur dioxide added rose firstly,although the increase amounts are lower than the original oxygen ones,then keeping stable.The produced oxygen content follows a first-order rate law with time.As a result,temperature and sulfur dioxide affected most,while copper with different concentrations affected least.The results of correlation analysis showed that during the first stage（0-16 days）there were remarkably positive correlations between the variations of oxygen consumed by phenols and total sulfur dioxide consumed（R=0.971,P<0.01）,free sulfur dioxide consumed（R=0.981,P<0.01）,and Fe²⁺/Fe³⁺ration（R=0.861,P<0.05）.There was a negative correlation（R=-0.968,P<0.05）between oxygen increase variation and Fe²⁺/Fe³⁺ratio during 16-59 days.（2）Analysis of the phenolic compounds consumed and oxidative intermediates in iron-catalyzed oxidation of wine in model systems.During the process of oxidation catalyzed by iron in model systems,except acetaldehyde,neither glyoxylic acid nor xanthyulium cation and its esters were detected.However,the contents of acetaldehydes in treatments with no sulfur dioxide added significantly were higher than those with adding sulfur dioxide（P<0.05）,and the higher of temperature,the more of the difference.Interestingly,it is likely to occur that dehydrodicatechin A was formed in the treatment with sulfur dioxide added under30℃.During 0-59,there were significant correlations between Fe²⁺/Fe³⁺ratio and（+）-catechin consumed（R=0.917,P<0.01）,ΔA_380nm（R=-0.813,P<0.05）.It suggests that the production of dehydrodicatechin A is correlated to the Fe²⁺/Fe³⁺ratio.