Study On Copigmentation Mechanism Of Oak Polyphenols In Red Wine

The compounds of anthocyanins are the major color substance in red wine,whose variety,state and content determine the color and chromatic characteristics and ageing potential of the wine.However,with unstable chemical property,the anthocyanins are prone to the influence of external environment and degradation.For this reason,most of the red wines in China are characterized by light color and poor color stability,especially so in western arid-hot areas,such as the dry and hot producing areas as Xinjiang province and Gansu province with large red wine production scale.It is found through research that the interaction between anthocyanins and copigments,or the copigmentation can enhance the color and chromatic of red wine as well as the color stability,thus being regarded as a natural way of improving the color and chromatic of red wine.Therefore,this paper offers a research into the copigmentation in red wine,especially an in-depth analysis of the mechanism of oak polyphenol as the copigment with a view to providing data support and scientific reference for the enhancement of color characteristics of the red wine and color and chromatic stability in the ageing process.With the measurement of the polyphenols in oak barrel stave,and the results show that it contains the compounds as gallic acid,vanillic acid,syringic acid,ferulic acid,ellagic acid,vanillin,syringaldehyde,sinaldehyde and coniferyl aldehyde.In the meantime,based on the related previous literatures,12 phenolic compounds,namely,p-hydroxybenzoic acid,protocatechuic acid,gallic acid,vanillic acid,syringic acid,p-coumaric acid,caffeic acid,ferulic acid,ellagic acid,vanillin,syringaldehyde and coniferyl aldehyde are applied to the experiment as the copigments to participate in the relevant simulating reaction to measure the corresponding copigmentation effect.According to the primary result,ellagic acid has the best effect among the 12 compounds for test.Besides,the experimental results also reveal that the copigmentation effect of hydroxycinnamic acids are superior than that of the hydroxybenzoic acids while the phenol aldehyde compounds are lower than that of the corresponding phenolic acids.At the same time,the copigmentation effect is also enhanced along with the increase in the number of hydroxy and methoxy groups on the benzene ring.The research into the factors influencing the copigmentation effect shows that the increase of copigment concentration can shore up the copigmentation effect and improve the color and chromatic of the simulated solution,and the copigmentation is accompanied by the best effect with the highest concentration;The different temperature also has different impact on the copigmentation effect.When the temperature is over 20?,as the damage to the anthocyanin-copigment complex,it will cause the chromatic loss of the solution and induce hypsochromic shift.The reaction system with pH of 1.0 is usually accompanied by insignificant copigmentation effect,and as the pH rises,the bathochromic and hyperchromic shifts of the solution have also been strengthened and even to reach the extreme value at the pH of 3.0,a fact testifying to its obvious copigmentation characteristics;The copigmentation effect can be weakened through the addition of ethanol with different expressions of copigments.At the same time,the analysis of the color indicates that the the color differences caused by ethanol are attributable to quantitative changes.With the calculation of thermodynamic parameters in the process of copigmentation,and the results show that the action is conformed to the chemical equilibrium constant(K)relationship of ellagic acid>ferulic acid>coniferyl aldehyde>caffeic acid>syringic acid>syringaldehyde>vanillic acid>vanillin>gallic acid>p-coumaric acid>protocatechuic acid>p-hydroxybenzoic acid.Standard Gibbs free energy of copigmentation(?G°)data show that the copigmentation participated by phenolic substances for trial is an initiative exothermic process while the changes in enthalpy value(?H°)and entropy(?S°)are intimately related to the structure of reaction molecules.The data from the simulation of quantum chemistry further demonstrate the results of pre-stage experiment.At the same time,by optimizing the spatial structure of anthocyanin-copigment complex and calculating the intermolecular bond types,the studies show that the interaction between hydrogen bond and Van der Waals serves as the major driving force for the formation of complex of copigmentation for experiment.By adding the ellagic acid and gallic acid before the wine fermentation,the research reveals that the addition of phenolic compounds before fermentation will effectively reduce the color lightness(L*)of red wine,enhance its red hues(a*)and color saturation(C*ab)and increase the color differences(?E*ab)from the wine samples.At the same time,the processing can enhance copigmentation degree of the anthocyanin in the red wine at the fermentation stage so as to promote the extration of anthocyanins in the wine.At the ageing stage,the wine sample undergoing pre-stage processing is conducive to the maintenance and stability of the red wine's color.What's more,using barrels for ageing,to some extent,can enhance the color and polyphenol quality.By adding the caffeic acid and gallic acid before the ageing,the experimental results exhibit that the addition of phenolic compounds before ageing will effectively delay the enhancement of color lightness(L*)of the red wine,the reduction of red hue(a*)and increase the color differences(?E*ab)with the control wine sample,and the effect is in a proportional relationship with the addition concentration.It is also found from the research that under the same concentration,the effect of the sample processed by caffeic acid is superior to that by gallic acid.Besides,such processing is effective in delaying the decrease of the copigmentation degree of the anthocyanin in the red wine at the ageing stage,advancing the increase of polymerized anthocyanins ratio in the wine,being helpful to maintain and stabilize the anthocyanins in the wine samples and delaying the reduction of non-anthocyanin phenolic compounds content.