Response Mechanism Of Acetaldehyde To Oxygen Exposure During Aging Process Of Dry Red Wine

After alcoholic fermentation,wine is usually treated with oxygen for a period of time to reach a mature and stable state,which named aging process.The aging process stabilizes the color of the wine,reduces the dry taste and increases the complex aroma,all of them are related to the oxidation of polyphenols.In addition,acetaldehyde,the oxidation product of ethanol with the highest content in wine except water,is involved in almost all the important chemical reactions in wine.For example,the combination of anthocyanin and flavanol,the acetaldehyde-mediated condensation of flavanols and anthocyanins,and the involvement of acetaldehyde in the formation of pyrine and anthocyanin-flavanol conjugates.Finally,the amount of acetaldehyde in the wine is also a marker to judge whether the wine is ripe or not.The purpose of this paper is to investigate the effects of SO_2,total phenol content and metal ions（copper,iron）on the evolution of acetaldehyde and polyphenols during wine oxidation.The main results are as follows:（1）Three oxygen exposure cycles were carried out on wine samples,and it was found that there were stage differences in oxygen consumption.In the initial stage of oxygen exposure,the oxygen consumption rate of wine is the fastest,mainly depending on the presence of certain substances in the wine that can react quickly with oxygen.While these substances consume oxygen rapidly,they are also rapidly reacted,and the oxygen consumption rate decreases and tends to be stable.The average initial oxygen consumption（OICR）of all wine samples was 3.880 mg L^-1day^-1,which was about 3.5 times of the average oxygen consumption（OACR）of all wine samples,and there was no correlation between these two,indicating that the oxidation mechanism was just a general pattern of wine behavior.（2）The relationship between oxygen consumption rate and initial components of wine SO₂,total phenol content and metal ions（copper,iron）during wine aging was studied.Among them,polyphenols have the fastest reaction rate with oxygen under the catalysis of metal ions（copper,iron）,and the total phenol content is negatively correlated with the oxygen consumption rate.The effect of total phenol content on oxygen consumption rate of wine without metal ion and SO₂ is very weak.As a reducing agent,SO₂ recycles the quinone in wine into catechol or forms an admixture with the quinone,or directly reacts with hydrogen peroxide to promote the forward process of oxidation,which improves the oxygen consumption rate.However,the oxygen consumption rate of wine samples with SO₂ added alone is lower than that of wine samples with SO₂ acting together with metal ions.We believe that the function of SO₂ to promote oxygen consumption can be fully played under the catalysis of copper and iron ions.（3）SO₂ is often added as an antioxidant to protect wine from oxidative spoilage.Theoretically,the addition of SO₂ can eliminate hydrogen peroxide and quinone generated by the oxidation of catechol,reduce the generation of hydroxyl radical（OH^·）,and inhibit the generation of acetaldehyde,the oxidation product of wine.However,in the actual experiment,it was found that acetaldehyde level showed significant accumulation with the depletion of free SO₂ in wines with SO₂ added.In contrast,there was little accumulation of acetaldehyde,the oxidized product,in wines without SO₂.This indicates that while SO₂ acts as an antioxidant,there is also some oxidation mechanism that can promote the process of oxidation.On the one hand,the oxidative chain reaction of SO₂ occurs under wine conditions,producing a series of free radicals with strong oxidation ability,such as SO₄^·-and peroxysulfate free radical(SO₅^·-),among which,SO₄^·-can react with hydroxyl or H₂O to obtain OH^·to further promote oxidation.On the other hand,SO₂ is used to reduce enzymatic reaction kinetics and form more complex sulfonates as oxidation-related reserves of aldehydes.Once the free state of SO₂ is exhausted,some aldehydes are released.In addition,adding a few copper and iron ions can significantly increase the oxidation rate of catechol and the yield of acetaldehyde.The lower the acetaldehyde yield,the higher the consumption of free anthocyanin in wines with high total phenol content.In the beginning of the experiment,the anthocyanin content of high-phenolic wine was 72 mg L^-1,nearly 2.1times of that of low-phenolic wine,which decreased to 0.8 times after 100 days,indicating that a large amount of free anthocyanin condensed with polyphenols during the period,forming more complex pigments.In addition,acetaldehyde acts as an ethyl bridge linking anthocyanin adducts,and further cycled to reorder the more stable pyranoid anthocyanins.Free SO₂ can capture acetaldehyde,which binds to anthocyanin,and protect anthocyanin.However,with the depletion of free SO₂,acetaldehyde acts and accelerates the loss of anthocyanin.Considering that the reaction of anthocyanin with acetaldehyde is more sensitive than other polyphenolic compounds with acetaldehyde,the rate of acetaldehyde consumption also slows down as the content of free anthocyanin decreases continuously.