Determination and removal of gluconic acid in reduced alcohol wine and high acid grape juice

Abstract

A rapid high performance liquid chromatography (HPLC) method incorporating the use of an Aminex HPX-87H organic acid column was developed for the separation, identification and quantitative analysis of gluconic acid and other major acids in grape juice and wine. This method was used to investigate the effectiveness of deacidification treatments for removing gluconic acid from high acid grape juice and reduced alcohol wine produced by using a glucose oxidase-catalase (GOD/CAT) juice treatment.

Müller Thurgau juice was subjected to a GOD/CAT treatment as a means of reducing the concentration of glucose in the grape juice before fermentation to produce a reduced alcohol wine. The enzyme is an aerobic dehydrogenase which catalyses the oxidation of glucose to gluconic acid. The juice was found to contain ~75g/L gluconic acid, and when a portion of this was fermented to dryness and cold stabilised, the gluconic acid had reduced in concentration to ~45g/L and the reduced alcohol wine contained 8.3% alcohol (v/v). The corresponding increase in acidity in both the reduced sugar juice and reduced alcohol wine had to be neutralised to present a palatable product.

Three deacidification treatments were investigated in reduced alcohol wines and high acid grape juice and these included: neutralisation with CaCO₃ (calcium carbonate), seeding with gluconate salts and CaCO₃, and anion exchange with an Amberlite IRA-93 resin. Both the neutralisation and seeding treatments produced disappointing results, with a significant but minimal decrease in gluconic, tartaric and malic acids. Even though the solutions analysed would have been saturated with potassium tartrate and gluconate, there was obviously a stable equilibrium in force, and the addition of seed crystals and chilling to ~2°C had no effect. The anion exchange treatment showed considerable promise though, with reduction in all the three major organic acids. A new technique was investigated, which involved charging the resin with tartaric acid, and then passing the wine/juice through. The weaker gluconic acid in solution exchanged with the stronger tartaric acid. This technique has the potential to selectively remove gluconic acid. A hindrance to this technique is the removal of colour from the wine/juice due to the resin matrix. The effect of resins on colour and flavour warrants further investigation.