PHOTO 1. Rackings are a source of oxygen contact.
“Oxygen can make or break a wine.” – Louis Pasteur. Oxygen is an integral part of life and plays an important role in different biological and chemical reactions.
Dissolved oxygen is the free molecular oxygen in solution and is expressed as milligram per litre (mg/L), parts per million (ppm) or percentage saturation (%sat). The extent of oxygen solution into wine during air contact is influenced by temperature, atmospheric pressure and the pH of the wine. Lower temperature and an increased atmospheric pressure favour the solubility of oxygen and higher pH decreases the percentage molecular sulphur dioxide, which limits the influence of oxygen as an anti-oxidant. At standard temperature and atmospheric pressure wine is saturated with oxygen at a dissolved oxygen concentration of 6 mg/L (www.hannainst.com).
The general perception is that oxygen is detrimental for wines, although certain stages of winemaking exist where it can be beneficial, if managed properly. This includes the hyperoxidation of juice, the role of oxygen during the initiation of alcoholic fermentation and the micro-oxygenation of red wines.
The aim of hyperoxidation prior to alcoholic fermentation is to protect the resulting wine against browning or oxidation during the further winemaking processes. It is also stated that it can improve the shelf life of such wines. It comprises the enzymatic oxidation where oxygen reacts with certain phenol groups in the absence of sulphur dioxide to form yellow quinones. The latter compounds react further with oxygen to form brown coloured products, which precipitate and can be removed by racking. Wines made from such juice are consequently protected against further browning. The uncertain potential advantages of such procedure depend on various factors of which the vineyard and cultivar can play a role. Different opinions also exist regarding the influence of such practice on the sensory quality of wine.
Oxygen is essential for the multiplication of yeasts and formation of flavour profiles in the beginning of alcoholic fermentation. Most of the dissolved oxygen results from the crushing of the grapes, pressing of skins and rackings. Depending on the temperature, equipment and the executed processes it can even lead to the saturation level of 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 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, epicatechin, 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 required preventative actions. It is especially applicable for winemaking processes after alcoholic fermentation up to and during bottling. Sources of air contact and the consequential dissolving of oxygen into the wine must be limited. Actions like rackings, unnecessary ullages in containers, wine transfers, cold stabilisation, filtration and bottling are actions which require the necessary attention. During these actions oxygen uptakes from 0,5 to 8 mg/L oxygen can occur (www.vinlab.com). Depending on the temperature, the dissolved oxygen concentration of wines can vary from 6 to 9 g/L. The concentrations are higher at lower temperatures. During the cold stabilisation of wine, air contact must consequently be limited to prevent potential oxygen uptake. The rate of oxidation reactions is however faster at higher temperatures. Sufficient sulphur dioxide protection is consequently necessary. This depends on the dissolved oxygen concentration seeing that 4 mg/L molecular SO2 is required to bind 1 mg/L oxygen. If wine is transferred between tanks or barrels it is exposed to potential oxygen uptake. Such actions must consequently be limited if possible. Suppliers’ prescriptions must be complied with to ensure that equipment operates optimally. Leaking hose seals, loose pipe connections and sheet filters are potential sources of oxygen dissolution. During rackings all pipelines and receiving containers must be purged with inert gasses like nitrogen, argon or carbon dioxide in advance. Rackings must also be from the bottom valve of the delivery tank to the bottom of the receiving container. When wine is transferred, containers must be kept full to prevent an air space on the surface of the wine. If air space does occur, the surface must be covered with inert gas. During the cold stabilisation of wine, the potential uptake of oxygen is favoured as result of the lower temperature. Sufficient sulphur dioxide concentrations must consequently be maintained and containers kept full (Steiner, 2013).
PHOTO 2. Increasing browning is one of oxidation’s results.
References
Steiner, T.E., 2013. Strategies to manage dissolved oxygen. How to identify practices that prevent unwanted oxygen absorption in wine. Wines & Vines, August 2013.
www.hannainst.com/resources/wine/ebooks/measuring-do-in-wine-hanna-instruments.pdf.
www.vinlab.com/dissolved-oxygen-and-free-SO2.
