Pressing is now acknowledged as a crucial phase in the elaboration of white wines and rosés.

The aim is to extract the best from the grape and most particularly from the skin. Compounds sought are not only the plant aromas in their « free » or « precursor » form but also those compounds able to preserve them (oxidation inhibitors).  Furthermore, pressing must be able to limit the extraction or formation of compounds harmful to the quality and stability of the future wine.

A critical phaseDuring the must extraction phase and more exactly after the first crumbling operation, the juices extracted from the grape cells are placed in the presence of a large quantity of air and oxidases. Oxidation enzymes such as tyrosinase, a natural grape enzyme and laccase, an enzyme borne by Botrytis cinerea, the grey-mold rot, are at maximum activity during pressing. As oxygen is not a limiting factor, the conditions are ideal for the oxidation of oxidizable compounds such as the polyphenols and indirectly glutathione. During that phase oxidation kinetics is very fast finally causing browning of the juice and a total disappearance of grape oxidation inhibitors (glutathione).  The consumption of oxygen is particularly high on must, 10 to 40 times faster than in wine, estimated to be more than 7 mg.L-1.min-1. Rate of consumption and the total quantity of oxygen consumed depend on the grape variety, the maturity of the grape and its sanitary state (the type and quantity of polyphenols will vary considerably according to the grape variety).

Scientific findings point to glutathione being an increasingly important constituent in wine. It limits the effects of oxidation whether enzymatical or chemical. This oxidation inhibitor is one of the first compounds to be involved in the mechanism of oxidation.  During fermentation this compound is assimilated by the yeast then salted out at the end of alcoholic fermentation.  Its content in wine depends on the residual concentration in the must (Dubourdieu and Lavigne, 2003). 

Browning and the elimination of oxidation inhibitors from the must are not the only effects of oxidases.  A third type, the lipoxygenase, oxididizes the unsaturated fatty acids of the skin and generates herbaceous type compounds. The activity is all the more important that grapes crop are grinded.

Traditional pressing is therefore a phase during which the intensity of oxidation is not controlled. Oxidation above all becomes apparent by the browning of the juice which is one of the final phases in the mechanism of oxidation.  But the consequence of traditional pressing is above all to eliminate the oxidation inhibition capacities of the must and ultimately of the wine.

Juice extraction is THE critical phase to be controlled to obtain greater coherence in the prevention of oxidation in the production chain.

Unsatisfactory preventionMethods for limiting oxidation in the prefermentation phase are limited to the addition of sulfur dioxide and inert gas injection on grapes.

Grapes sulphiting protects the free-run juice but the sulfur dioxide is rapidly combined thus limiting its action on the grape juice. Although widely used, bisulphite has the major disadvantage of favouring the extraction of polyphenols and skin proteins.  The inert gas injection of manually or mechanically harvested grapes is never total and implementation is still complicated and costly. The two solutions, alone or together, greatly reduce the mechanisms of free-run must oxidation but not that of grape must.

To avoid those oxidation phenomena during pressing the air contained in the grape press tank can be replaced by an inert gas, e.g.: nitrogen. The system is simple but requires a new injection of inert gas for every crumbling. Bearing in mind the number of such operations per pressing, large quantities of gas are consumed, so it is significantly costly. Furthermore, the size of any nitrogen supply system aligned with instantaneous flow rates required by inert gas injections leads to unreasonable costs.  Indeed, that rate depends directly on the press’s vacuum pump rate, i.e.: 250 m3/h for a 80 hl press, for instance.

To mitigate uncontrolled oxidation during the pressing phase and so offer coherence to the preventive chain of operations, Bucher Vaslin, world pressing specialist for 150 years, designed Bucher Inertys®, the first pressing process under inert gas with gas recycling. This innovative process provides control at an acceptable cost.

For further information, see item Inertys functioning