The different roles of oxygen By Dominique Delteil, consultant This article was published on the internet site of the Istituto Coopèratif du Vin (www.icv.fr) Flash Info Vendages ICV Edition spèciale Entreprises n. 5, August 2000. This article was also published in Italian on the site www.vinidea.net, 2004 n. 9. To stabilise and develop the aromas and the tastes of mature grapes. To manage a complete and regular alcoholic fermentation. To limit the appearance of sulphorous odours (hints of garlic, onion, rubber, metallic, etc.). These are the three key objectives of a well-defined and well-executed vinification in a Mediterranean environment. Oxygen plays a direct role in each of these points, sometimes positive and sometimes negative. Some practical evidence to avoid certain errors in production: Oxygen is soluble in the must and in the wine. It is always present in the air which circumvents the tanks, the tubing, the joints between pipes, and in the presses. Oxygen is always more concentrated in the air than in the must and the wine in the tank: it will as such always have a tenancy to dissolve in them. This also applies to a must saturated with carbonic gas in the midst of fermentation. This dissolution is extremely rapid: the must in the drainage of a pneumatic press is already completely saturated in oxygen. The air in the cellar is always rich in oxygen, even when it is supplied with carbon dioxide. A bubble of air or oxygen which bursts to the surfaces of the must or wine involves the release of gas from the liquid - this bubble does not transfer oxygen to the liquid, or at least it does only a little. The more the must and the wine are cold, the more the oxygen they can dissolve, up to about 10 mg/l. When the must and wine are in movement and in a thin layer, the dissolution of the oxygen is optimized. This is known as the voluntary Venturi effect, when using a steel porous shaft or if the connection between two pipes is slightly unrewed. This involuntary Venturi effect often occurs when the packing gland (stuffing box) of a pump is worn or when the connections do not fit together perfectly. Voluntary or involuntary as it may be, the result is the same. An open jet which falls into a tub during a pump-over dissolves the oxygen well enough, and much better the greater the height of the drop exposed to the air. For about ten years, the ICV has experimented with different techniques for protection against oxygen, or to the contrary, to the technique of contributing oxygen in Mediterranean conditions. Here we will present an overview of these techniques. Protecting the grapes and the must during the pre-fermentative phases Mediterranean grapes have specificities that render them particularly sensitive to oxidation reactions and 119
to browning. These characters are an elevated cellular maturity of the pulp, with a strong concentration of oxidisable phenolic acids and high ph. Until the fermentation is activated, the must and grapes must be protected against oxidations. Oxygen does not directly react with the SO 2, which then fails to react with the dissolution of oxygen in the must. The SO 2 is effective because it blocks from the start chain reactions that lead to the formation of browning compounds and destroy the majority of the varietal aromas. In the cellar, is illusory to wish to completely prevent contact of the grapes and the must with air. Rather, a homogeneous and fractioned sulphitation of the must permanently secures a minimal and efficient presence of SO 2, the so-called internal protection of the must. In the case of mechanical harvest, the contributions of the SO 2 have to commence from the harvest hopper. The addition of carbon dioxide to the grapes and the must, such as the coverage of the juice with CO 2 are adequate operations if carried out complimentary to the addition of SO 2, which increases the efficiency, since it limits contact with the air this is the external protection. The latter cannot substitute the internal protection, but it does complete it. Provision of oxygen to the yeasts Jean Marie Sablayrolles of the INRA in Montpellier has demonstrated that the oxygen contributed to the musts in fermentation is utilised by the yeasts and that diverse œnological yeasts can have different requirements. Carried out at a density of around 1060, this oxygen serves the yeasts to produce survival factors, and to better resist the difficult conditions at the end of the fermentation. Since 1991, the ICV has integrated this addition into its recommendations for fermentation. In perspective, it is a key point for mastering a regular and complete fermentation. This is as true in vinification of white as it is for red. The optimal quantity is between 5 and 10 mg/l. The technical solutions, a pump-over of the entire must through a connection with the porous candle, or a measured injection of pure oxygen with a dispenser, or a pump-over with a strong aeration of all the must in the vat. The last solution, often less effective, needs to be repeated twice both on the day itself, and a pump-over two days following. The mixing of the vat with a pump-over tube does not provide the necessary oxygen. The tube produces large bubbles, made to mix the liquid. The mixing of the yeasts is always interesting in this phase, but does not produce its survival factors. In white and rosé, when realised at the opportune moment, the oxygenation is not contradictory to prefermentative protection. The yeasts, in full activity, almost instantly consume this oxygen, which does not participate in the reactions of oxidation, even when there is no longer active SO 2 in the must in fermentation. Fragile varietal aromas are perfectly respected, indeed they are expressed even better, as this contribution of oxygen limits the nasty sulphurous odours. This injection of oxygen for the yeast is all the more necessary the more the other parameters of fermentation are difficult for them. Among these parameters, some are typical of white and red Mediterranean musts - nitrogen very rarely able to be assimilated and with high concentrations in sugar. Others are owing to technology - very clear musts without any contribution of fine deposits, low temperatures, lack of re-suspension of the yeasts on the bottom of the tank. 120
THE DIFFERENT ROLES OF OXYGEN Since the 1994 vintage, the ICV has highlighted the importance of a supplementary oxygenation, to occur earlier, at around a density of 1080. In very clean musts, very rich in sugars (>13% potential alcohol), or fermented at less than 17 C, this oxygenation helps to limit the risk of volatile acidity with certain yeasts. Providing oxygen during the maceration of reds In the vinification of red wines, yeasts have a particular need for oxygen. The conditions of the must are difficult - low in nitrogen and high in sugar. Peaks in temperature above 28 C are very detrimental to the their survival and the end of fermentation. This is true regardless of at which point the maximum temperature peak occurs. Throughout the maceration, the oxygen enters even in complex reactions. It participates in the stabilization of colour and in the chemical modifications of the tannins, which allow for tannins that are both more stable and more contained. The aeration of the must during the maceration restricts the occurrence of malodorous sulphurated compounds. These compounds also cause the bitterness and aggressiveness of the tannins. For management of the quality of colour, tannins, and the cleansing of the aromas of the grapes, we must find the proper amount and a correct rhythm of the aerations (see the experimental example below). The means of the aeration of a vat of red during the maceration are: pump-over of the entire volume of the must or complete délestage of the vat (with addition of pure oxygen by injection in the separate liquid phase, or return pumpage of the must through a steel sintered sleeve), or even an under the cap injection of a defined quantity of pure oxygen with the dispenser. Punching the cap (mechanical immersion of the cap) is a good means of maceration, but does not oxygenate the must during fermentation. In some vintages, the grapes are rich in tannins, with rather thick and tough skins. This requires that the tannins are stabilised and softened up from the maceration phase, to avoid the hard taste that usually occur with such skins. A proper aeration program starts early, from the formation of the cap. In brief macerations, which last less than six days, it is preferable to aerate each day. In long macerations, it is advisable to aerate every day during the tumultuous fermentation, then adjust the pace in accordance with a tasting. With grapes affected by Botritys, it is opportune to macerate very little (2 or 3 days) with aeration under the marc. These aerations cease upon drawing off and whilst ever the laccase remains active (test for cases of oxidase negative on wine). Whilst there is the presence of the cap and not exceeding four days of maceration, the aerations have only beneficial effects: colour which is more vivacious and more stable, and fewer earthy or mouldy odours. In another section of this study, we will see how to protect white and rosé wines starting from the end of the fermentation, and the opportunity to regularly contribute oxygen to red wines during the refinement. An experimental trial A quantity of Syrah grapes was equally divided into two identical vats. Outline of the vinification: Destemming - Pressing - Enzimation with 1 g/hl of selected enzymes - sulphitation with 5 g/hl of SO 2 121
- Inoculation with 20 g/hl of ICV-GRE yeast - Maximum temperature of 28 C One punching of the cap per day Drawing off after 6 days from the formation of the cap and 6 punchings - Straining and pressing, assembly of the first pressing Racking 24 hours after drawing off - Racking 48 hours after the first Direct inoculation with selected lactic bacteria Racking 24 hours after the end of the malolactic - sulphitation with 3 g/hl of S0 2. Bottling in December. The first tank was oxygenated 5 times in the same day, when it was at a density of 1060. Each addition of oxygen was of 10 mg/l with a dispenser. The second tank was oxygenated once each day with 10 mg/l with the same dispenser. Arbitrary ale of sensorial analysis Arbitrary ale of sensorial analysis ASDQ ICV 52 0 3 2 1 0 5 OXYGENATIONS CONCENTRATED IN 1 DAY 5 OXYGENATIONS IN 5 DAYS 51,5 50 48,7 48,1 46,2 46,1 34,2 35,8 End of FA End of FML After 4 months After 4 years Fig. 1 - Effect of the rhythm of aeration on the polyphenols of a syrah (shiraz) red wine. 5 OXYGENATIONS CONCENTRATED IN 1 DAY 5 OXYGENATIONS IN 5 DAYS Herbaceous Confection In-mouth volume Tannic intensity Bitternes Fig. 2 - Effect of the rhythm of aeration on the sensorial profile of a syrah (shiraz) red wine, after 4 years of in-bottle refinement. 122
THE DIFFERENT ROLES OF OXYGEN Comments The two vats received strictly the same quantity of oxygen. In this vinification, the mature grapes, the form of the vats and the complete punching down have optimised the diffusion of colour and stable tannins. The regular breakdown of additions of oxygen has allowed a better stability of the polyphenolic system, in comparison with the intensive contribution in the same day, as shown by the data reported in Figure 1, which was further confirmed over time. After 4 years, this better chemical stability renders great sensorial differences, as illustrated in Figure 2. The wine elaborated with regular additions of oxygen in maceration is fruity and intense in the mouth, with few aggressive tannins, presenting strong aromatic intensity of red fruit jam, impressive in-mouth volume, strong tannic intensity without bitter sensations. It is the typical profile of a red wine well positioned commercially in the heart of the ale. It is a wine realised with a unique addition of oxygen presenting a less Mediterranean profile, with earthy characters and a contemporaneously more diluted and more aggressive mouth. Trial conducted by the department of R&D ICV. Co-financed by ICV, ONIVINS, Conseil Ragionai Rhòne Alpes 123 cs
The role of oxygen in the post-fermentation phase By Dominique Delteil, consultant This article was published on the internet site of the the Istituto Coopératif du Vin ww.icv.fr - Flash Info Vendages ICV - Edition spéciale Entreprises n 7, September 2000. The article was published in the Italian version on the internet site of the technical magazine, ww.vinidea.net, 2003, n 10. Protecting white and rosé wines in the last quarter of the alcoholic fermentation The musts and white wines of the Mediterranean region are very sensitive to the phenomenon of pinking. This is pinking due to oxidation of small size polyphenols of white wines, naturally colourless and soluble in the must and in the wine. If the protection is done properly, they remain colourless during the entire life of the wine. The best prevention is to avoid their involvement in reactions of oxidation, for it is as a consequence that they would colour. Continuous protection of musts in fermentation with CO 2 is the first preventive actione. The yeasts continually produce CO 2 and in large quantities when the fermentation is well underway. From the last quarter of the alcoholic fermentation (from 1030-1020 density), it requires vigilance. It it necessary to verify that the head space above the wine must in fermentation is well saturated with CO 2. In the case of clarified musts, rich in sugars and fermented at low temperatures, the risks of slowdown at the end of fermentation are more elevated, as is the risk of pinking. To simply evaluate the risk, starting with a density of 1020, a sample of 0.25 litres is taken in a white glass bottle. When the risk is high in a few hours you will notice a turning of colour in comparison with compared the must still in the vat. In this situation, vigilance of external protection should be doubled whilst ever the fermentation continues. A sulphitation in the presence of active yeasts would be a grave error - it would be totally ineffective against pinking in this phase. The yeasts would react, producing acetaldehyde and other compounds capable of combining the SO 2. The small-size polyphenols that provoke pinking in practice are not eliminated by bentonite or by casein when they are colourless a must impoverished by these treatments during the clarification or fermentation still remains sueptible to pinking. Conversely, treatment during fermentation with PVPP is the only effective preventative action to complete the protection against the oxidations. (Note well: to ask the supplier for a certificate of conformity of the PVPP in force, in Italy this is specifically Reg. 1622/2000). 124
THE ROLE OF OXYGEN IN THE POST-FERMENTATION PHASE The internal protection within the wine needs to be initiated as soon as the alcoholic fermentation ceases. This is clearly when we do not want the successive malolactic fermentation. The first protective sulphitation is effectuated in the fermentation vat, as soon as the sugars have been terminated. The doses have to be sufficient to ensure that there is free SO 2 for some weeks. With ph's typical of Mediterranean wines, this first sulphiting is around 5-7 g/hl, to be adjusted on the basis of other analytical parameters of the wine. The addition of aorbic acid may add to the effectiveness on certain masses. When all other parameters are well managed, a dose of 5 g/hl fully responds to the demand. The homogenization after this sulphitation is a key element of this phase of vinification. It is essential to involve all parts of the vat with the right concentration of sulphur, and in particular the entire mass of the yeast at the bottom of the vat. Next, it would be necessary to rack white wines 24 hours after the sulphitation, absolutely avoiding contact with air. Before pumping the wine, the tubing is filled with CO 2, the recipient vat is saturated for about one meter in height with CO 2, the joints of the pipes and pumps are in good condition - all measures to get the wine into the destination vat without contact with air. Several experiments have demonstrated that the sulphitation of a wine in its fermentation vat causes the production of unpleasant sulphurous odours (rotten eggs, onion, garlic, etc.), true only if you leave the wine immobile on its own yeasts for several days. For white and rosé wines in the Mediterranean area, the sulphitation at the end of alcoholic fermentation and a racking within the 24 hours following, allow: Achievement of effective internal protection during an action at high-risk of an involuntary Venturi effect, the racking. Avoidance of the production of unpleasant sulphur odours putting the yeasts into movement twice, firstly during the sulphitation to homogenise the mass and during the racking, for yeast which have no sediment. In some vintages, these interventions actively participate in the risk management against emergence of bitter flavours in white wines. In rosé wines, the sources of oxidation at the end of fermentation or during the rackings are the same, even if the consequences are diverse - loss of the vivacious red colour, with deviations towards a more yellow hue. The preventive actions are the same as for white wines. Regular contributions of oxygen to red wines during refinement Before and during the malolactic fermentation from the moment of the drawing off, it is necessary to act whilst keeping in mind 5 elements which strongly interfere with each other. 1. the composition of wine in polyphenols and polysaccharides, and the way in which they are extracted from the pomace; 2. the deposits present in the wine: the quantity but above all the composition; 125
3. the movements of wine and the deposits; 4. the wood, when working in barrique; and, of cou; 5. THE OXYGEN. How does one manage all these elements and develop a practical course of action for each type of wine? Certain actions are of priority at an organizational level, in particular the rackings. The racking causes an intake of oxygen and a movement of the yeasts. Once the program of rackings has been defined, the impact on the other elements needs to be analysed - oxygen and movement of the yeasts. If necessary, other actions of oxygenation and movement of the deposits can be programmed. Example 1. Wine with alcoholic fermentation terminated on the pomace Now proceeding to the drawing off, the pressing and the integration of the parts of the press with wine flowers. A program is applied consisting of two rackings with aeration at 24 hours and 72 hours after the drawing off, followed by the inoculation of the lactic acid bacteria so that the malolactic fermentation terminates 10-15 days after drawing off. In the three days that follow the drawing off, leaving the wine to aerate during the pumpage operations, dissolved oxygen is added twice. In this phase of of the vinification, with the elimination of the course sediment and sulphorous particles, normally there is no need to add other oxygen. In cellars where the rackings prove to be simpler in closed circuit, it is necessary to add to each racking 2 mg/l of oxygen, for example with specific diffusers. Subsequently, the rapid launch of the malolactic fermentation maintains the yeasts in movement owing to the CO 2 produced by the bacteria. Thanks to the oxygen received during the maceration and the pre-fml rackings, the polyphenolic system (polyphenols and polysaccharides) is in a state such as to continue its positive evolution during the 8-10 days at 20 C of the FML. Even the polysaccharides bacteria participate positively in this development. As soon as the degradation of malolactic acid is terminated, a racking followed by a homogeneous sulphitation allows for effective management from a microbiological point of view and permits continued aromatic and taste development of the wine. For certain wines with very elevated polyphenolic concentrations, a micro-oxygenation in the vat can be heduled, which can be initiated at the moment of inoculation of the bacteria. Regular movement of the yeast still present (with a submerged pump of good quality) will allow an even better development of the polyphenol/polysaccharides system during the FML in certain wines. Example 2. Brief maceration of 5 days at 26 C Drawing off at a 1020 density, pressing and assembly of the must-wine flowers and the first fractions of the presser. 126
THE ROLE OF OXYGEN IN THE POST-FERMENTATION PHASE Implementation of a program that provides for a racking with aeration 24 hours after drawing off. End of fermentation at 24 C. Cleaning and inoculation of lactic bacteria to complete the FML in the 15 days following the drawing off. The two aerations provoked by the drawing off and the first racking will have put the yeasts in motion and contributed two quotas of dissolved oxygen, whilst removing solids of earthy origins in the meantime. These three actions are particularly important in certain vintages to avoid sulphurated and earthy odours. During the 3-5 days necessary for the completion of the alcoholic fermentation, however, the wine does not receive oxygen it is thus necessary each day to add it at a rate of 2 mg/l, utilising for example injectors positioned at the base of the vat to replace the yeasts in suspension in the meantime. As soon as the fermentation is terminated, the wine should be racked. 48 hours after this operation, the racking is repeated again, simultaneously inoculating the lactic acid bacteria. When these rackings are effectuated for convenience in closed circuit, it is necessary to add 2 mg/l of oxygen each time into the reception vat. As in Example 1, the wine continues its process of aromatic and taste evolution during the rapid FML, without necessitating specific interventions of oxygenation or movement of the deposits. As soon as the FML terminates, the wine is racked and sulphated as in example 1 and for the same reasons. After the malolactic fermentation As of this moment, the wines have a charge of very reduced particles - the racking after the drawing off has removed the earthy particles and the two rackings following have removed all parts of the yeasts. The racking after the FML removes a negligible share of malolactic bacteria, which sediments with difficulty but which represents nonetheless a very small mass. The wine has a constant need for oxygen to continue the evolution of polyphenols and polysaccharides, but this oxygen does not need to added with the same intensity as in previous stages. Micro-oxygenation is an interesting tool for this function of regular contributions of small quantities, as shown in the graphs that follow. The variables that govern micro-oxygenation in Mediterranean wines are well known - dosage, height of the vat, temperature. The wines still present yeasts at quite an important level and should thus be put into movement regularly during the micro-oxygenation even the wine must be treated in a homogeneous mode, and the yeast particles are too heavy to be returned into suspension with the weak flux of oxygen from the micro-oxygenation. In barriques, the referenced practice is bâtonnage, whose rhythm needs to be adjusted in accordance with the moment of placement in wood, the amount of yeast in suspension, the polyphenol and polysaccharide structure of the wine, the age and quality of the barriques, and the degree of fusion sought out between the wine and wood. 127
An example with interesting results A wine from perfectly ripe Syrah grapes, obtained with a long maceration was put into new barriques 24 hours after the drawing off and pressing (with all the fermentation yeasts, but without the sedimentary particles). FML in barrique, post-fml sulphating, bâtonnage twice per week for 6 months, then once a week for another 6 months, without racking during the whole year in barrique. A lot of work, a special organization of work (barriques on the same level), but obtaining a wine with great Mediterranean personality. In this example, the five elements cited at the beginning (wine, yeast, wood, movement, oxygen) were combined in a particular way, but with coherence in respect to the market objectives. An experimental example Corbières Trial 1993 This first experimental work in the cellar with micro-oxygenation, followed by the ICV, was effectuated on a red Corbières wine (Carignan, Grenache, Syrah), which was separated into two identical vats at the same temperature of 16 C. The addition of oxygen was 5 ml/l/month [corresponding to 6.5 mg/l/month]. After 3 months the wines were sampled and analysed. Comments Initially, the wine presented no aromatic or tasting defects. The micro-oxygenation allowed the development of an aromatic profile of a more mature and concentrated nature candied fruit, liquorice and Arbitrary ale of sensorial analysis ASDQ ICV 2 1.5 1 0.5 0 CONTROL SAMPLE MICRO-OXYGENATED TESTS Red fruits Candied fruits Plum Liquorice Pepper Animal Fig. 1 - Effect of the micro-oxygenation on the aromatic profile. Corbières 1993. 128
THE ROLE OF OXYGEN IN THE POST-FERMENTATION PHASE Arbitrary ale of sensorial analysis ASDQ ICV 2.5 2 1.5 1 0.5 0 CONTROL SAMPLE Volume Acidity Intensity Tannicity Bitterness Fig. 2 - Effect of micro-oxygenation on the tasting profile. Corbières 1993. pepper. For 3 months, without movement or aeration, the sample wine has developed aromas of evolution and fatigue, with the plum deriptor dominating over the others, and simultaneously the odours of reduction (deriptor animal ). To note is that in this wine the dominant animal character is given by the aromatic closure of the wine and not by development of Brettanomyces. To the palate, the micro-oxygenated wine is greater in volume and in tannic potency, and the tannins are of better quality - astringency and bitter notes are lower. In this trial, micro-oxygenation has proved to be an effective and practical means for the management of èlevage of Mediterranean red wines. MICRO-OXYGENATED TESTS 129