WINETECH Technical Yearbook 2019

the colour stability of red wines and softens the harsh tannins in wine. It must however be managed properly, because it can lead to oxidation if excessive oxygen is dosed. Excessive oxygen exposure resulting in an increased dissolved oxygen concentration can lead to oxidation. Enzymatic, chemical and microbiological oxidation are the three types of oxidation, which can influence wine quality negatively. Enzymatic oxidation is the catalytic oxidation of juice by polyphenol oxidase. Some phenol compounds are converted to quinones by the enzyme. These compounds form polymers, which influence the colour and flavour of the resulting wine detrimentally and can also lead to a decrease in the cultivar character. Such wines usually have a brownish colour and sherry-like character. The enzymatic oxidation can be prevented by the use of sulphur dioxide. Enzymatic oxidation can however not occur in wines. Laccase, an enzyme which occur in rotten or infected grapes and is formed by the Botrytis cinerea mould can also form the browning of the resulting wine. It is not so easily controlled by the use of sulphur dioxide and is consequently one of the reasons why only sound grapes should be used for quality wines. Chemical oxidation is the type of oxidation that occurs the most in wines. It comprises the oxidation of polyphenols like catechin,

epi catechin, anthocyanins and other polyphenols, which occur in grapes. During a range of reactions with oxygen, quinones and the by-product hydrogen peroxide are formed. The latter is a very strong oxidant, which can oxidise ethanol to acetaldehyde to impart a sherry character to the wine. The average acetaldehyde concentration in red wines is 30 mg/L, in white wines 80 mg/L and 300 mg/L in sherry ( www.hannainst. com ). If wine is not protected by sufficient sulphur dioxide concentrations and exposure of the wine to oxygen is not limited, chemical oxidation can lead to negative sensory characteristics like browning, pinking, formation of off-flavours and decrease in the general wine quality. Microbiological oxidation can occur if acetic acid bacteria, Brettanomyces and film forming yeasts, which require oxygen to grow, form a variety of off-flavours. It will especially occur if sulphur dioxide is not added prior to alcoholic fermentation and the growth of such micro-organisms are promoted by the presence of oxygen, or when containers are not full after alcoholic fermentation and exposed to air. It is usually accompanied by the formation of undesirable acetic acid, acetaldehyde and acetate esters. If it is thus accepted that oxygen or air contact is usually detrimental, except for the few mentioned advantages, it is essential to identify and implement the

PHOTO 2. Increasing browning is one of oxidation’s results.

oxygen at 6 to 9 mg/L oxygen. If insufficient oxygen is present in the juice it can lead to sluggish or stuck fermentations. This is as result of insufficient sterole formation in the yeasts. Sterole formation is essential for yeast propagation and after various generations of yeast propagation a shortage

of oxygen may develop for it. Dissolved oxygen levels of 4 to 6 mg/L are required at the beginning of alcoholic fermentation to overcome this problem. Micro-oxygenation is the managed dosage of small quantities of oxygen to red wine. It promotes phenol polymerisation, improves