dossier

Sulfur Dioxide: Status Quo and Alternatives

[vc_row full_screen_section_height=”no”][vc_column][vc_column_text]The use of sulphites is one of the stages that allowed modern oenology to become what it is. In fact, if the use of fumigations with sulphur dioxide to clean barrels and rooms is also attested in ancient times, however the continuous use of the same in oenology dates back to the early 800s, while it is only at the end of the century that the use is extended to the practice of the cellar and only in the first decade of the twentieth century that it also takes hold in Italy.

 

 

Today sulfur dioxide and sulphites are the target of an attack of unusual violence. While it is undeniable, on the one hand, that SO2 has a high toxicity (DL50 = 1,5 g / kg of body weight; DGA = 0,7 mg / kg of body weight – FAO-WHO data), on the other hand we see an almost bipolar attitude on the part of the average consumer who repudiates its use in wine but is completely unaware of its presence in almost the entire agri-food chain as a preservative. In foods, sulphites are in fact hidden behind acronyms, such as E220 which indicates precisely SO2 and which is used massively especially for dried fruit ‒ figs, apricots and plums (in primis) which contain up to 2000 mg/Kg ‒ but also in puree preparations, shellfish, mustard.

 

However, winemaking practice is not standing by and watching: the current trend and improved hygienic conditions are, in fact, leading to a progressive reduction of the same, especially in the early stages of winemaking, thanks to the use of alternative technologies. This allows to limit the production of acetaldehyde by yeasts and to determine the consequent increase in the fraction of SO2 free (compared to total), more active during conservation.

 

Antiseptic action

The antiseptic action of SO2, extremely selective, is the one known for the longest time and the one that has determined its widespread use in oenology. In musts and wines, sulphurous heavily limits the development of bacteria (lactic and acetic) and wild yeasts (with low alcohol-producing power or with a predominantly oxidative metabolism - fioretta), while the selected strains of saccharomyces have an excellent tolerance to the additive, as they are selected also on the basis of this characteristic. This action has therefore been considered up to now irreplaceable in order to allow the chokes of fermentation to take control of the medium and obtain a good fermentation process. However, the exasperation of this belief often leads to errors in the management of the fermentation itself. Too much clarification of the must, with the aim of killing the wild flora, followed by excessive sulphiting, in fact determines an imbalance in the sulphur metabolism by the yeasts. In fact, the yeast uses sulphur to produce some amino acids (cysteine ​​and methionine); in the absence of nitrogenous substances, this production is significantly reduced and the cell eliminates the excess sulphur as hydrogen sulphide. From here the consequent formation of mercaptans, and anomalous odours, of which the reducing environment determined by the fermentation and by the sulphiting itself can emphasize the phenomenon.

Another problem is related to the toxicity of SO2 for the cell; in fact, yeasts react to its presence by increasing the release of acetaldehyde into the medium. The latter is a powerful sulphite sequester, so that at the end of the fermentation process a good part of the added sulphur will be present in a combined form. It is not uncommon for an addition of SO2 to the must of the order of 10 g/hL, determines at the first racking a free content that is almost zero, while the total remains at 7 -8 g/hL; the problem therefore remains that of being able to use only a limited quantity of additive (to remain at values ​​below the limits), in the phase in which it is most needed, and with the risk of seeing the subsequent integrations, gradually in a combined form.

Fortunately, the antiseptic action is the most easily replaceable; regardless of practices such as pasteurization, a correct management of the pre-fermentation phases, through an immediate inoculation of the selected starters and the control of the temperature, allows to obtain a fermentation in almost pure form, even in the total absence of sulfur dioxide, with the advantage of being able to better manage the subsequent additions during the conservation phase. Logically, a fermentation in the total absence of SO2 must take into account the health status of the grapes and the hygienic conditions of the cellar.

The problem, in the case of white wines, remains the management of malolactic fermentation. This secondary fermentation is subject to factors, including a high pH value and SO levels.2. The determining action of the SO2 it can also be replaced, at least in part, through the use of alternative means such as lysozyme which has a specific action on lactic bacteria (Pediococcus spp., Lactobacillus spp., Oenococcus spp), causing lysis by degradation of the cell wall; the action of this enzyme is especially important in wines with high pH in which, while the antibacterial action of SO2 is lower (due to the lower content of free molecular weight), the activity of lysozyme is higher, clearly also linked to the bacterial strain and the activity of the enzyme.

 

Antioxidant action

The antioxidant and antioxidant action of SO2 concern its ability to act on the redox balance of wine. The first appears relatively modest and occurs through the oxidation of sulphites to sulphates, "sequestering" oxygen and avoiding the oxidation of musts and wine over a long period of time, especially if the latter is at a low temperature, a condition favourable to the solubilisation of oxygen in the mass. The antioxidant action of sulphur dioxide can only be partially replaced by the use of other antioxidants naturally present in the must and wine or added during the refinement phase, such as for exampleascorbic acid (vitamin C). This additive is used up to a maximum dose of 15 g/hL, due to its strong antioxidant action, usually in association with sulphites; it is therefore not a true substitute, but rather a strengthening tool. Its action is much faster than that of SO2 but less prolonged over time; it is also useful for maintaining iron in a reduced state, reducing the risk of ferric casse. Its use therefore becomes interesting following sudden oxygenation of wines (e.g. before bottling), rather than to guarantee prolonged conservation of the same. Ascorbic acid also has an indirect action on polyphenol oxidases, even if its use on musts is not permitted by community legislation. Another support for the antioxidant function of SO2 are the oenological tannins; their use in the technology of white wines cannot however be decisive from this point of view, since their use in excessive doses seriously compromises the sensorial characteristics of the wines themselves.citric acid finally, it is used before bottling as an iron chelator (Fe3+), a powerful catalyst of oxidative reactions.

 

Antioxidant action

The action of the SO is more complex.2 against enzymatic oxidations (antioxidase action), as these oxidations are faster than chemical ones. This problem is particularly serious in the case of musts. Polyphenol oxidases (PFO) are powerful catalysts of oxidative reactions; their presence in musts is one of the main factors of instability for white wines. The problem is further aggravated in the case of vinification of grapes strongly affected by Botrytis cinerea, as for grape tyrosinase, inactivated by SO2 and easily broken down by using normal clarifying agents (e.g. bentonite), laccase secreted by the fungus is added and is decidedly more resistant to treatments. This enzyme, moreover, has a much greater activity than the PFO of grapes, determining much more marked negative effects on the characteristics of wines. Tyrosinase in fact acts on ortho-diphenols; this activity is particularly expressed on hydroxycinnamic tartaric acids (ICT), or on the esters of cinnamic acids which are oxidized to quinones. This triggers a series of oxidative polymerizations which determine the browning of the color with negative effects on the organoleptic properties (catecholase oxidase). Tyrosinase is also able to act on monophenols by oxidizing them to ortho-diphenols, which in turn are subject to the catecholase activity of the same enzyme. Laccase, on the other hand, is less specific than grape tyrosinase, as it also acts on meta-diphenols, catechins and anthocyanidins, so its action will be more intense because more compounds are subject to being its substrate of action; furthermore, laccase is also able to inactivate important natural antioxidants such as glutathione and ascorbic acid, which instead counteract the activity of tyrosinase. The action of sulfur dioxide is expressed on the protein structures of PFO, inactivating them; as mentioned, laccase is more resistant to this action; in fact, the decrease in oxygen consumption following sulphitation is slower in musts obtained from botrytized grapes. In addition to this direct action, an indirect action on the activity of PFO is linked to the consumption of oxygen by the additive and to the reducing action on the quinone forms. Precisely for this reason, the antioxidant activity of sulphites has so far been considered almost irreplaceable. In recent years, however, some innovative technologies have proven to be an excellent alternative for the control of PFO in musts; oenological tannin, for example, seems to have knockdown effects on laccase, while the case of technologies based on thehyperoxygenation. This principle, introduced by Müller – Späth in 1977, is based on the saturation of the musts with an excess of oxygen (added as such or in the form of air), which determines the oxidation of everything that is oxidizable starting from the prefermentation phase; the subsequent clarification operations will eliminate the brown polymers formed. In reality, before fermentation the hyperoxygenated musts present a very intense brown color, which will be gradually lost during the fermentation itself; the wines thus obtained will present a stability in the maderization tests superior to the corresponding comparisons, obtained from traditional vinification. This determines a strong reduction in the polyphenol content of the musts already from the first hour of treatment, especially in relation to the most easily oxidizable fractions; the classic analytical parameters, however, do not present appreciable variations. The acidity and sugar content of the musts do not change following hyperoxygenation, while wines from hyperoxygenated musts may present slightly lower dry extract contents. Instead, a decrease in protein nitrogen in the musts themselves is noted, resulting from the precipitation of the brown polymers formed which, flocculating, also incorporate the proteins. The advantage of adding oxygen to the musts is that, in addition to the immediate removal of the PFO substrates, it also determines the denaturation and inactivation of tyrosinase during the oxidative reactions that it catalyzes.

The technique becomes particularly interesting for the vinification of grapes affected by Botrytis which with traditional techniques would lead to musts with significant problems in relation to oxidative phenomena. The operating methods of the treatment must consider various variables, such as sulphiting (which determines a greater need for oxygen for hyperoxygenation), temperature (room temperature seems to be optimal), the health status of the grapes (botrytized grapes require more oxygen), clarification (hyperoxygenation works better on cloudy musts, as the proteins facilitate the flocculation of brown polymers). The presence of appropriate adjuvants can be useful for the stabilizing effect of hyperoxygenation, as it facilitates the precipitation of quinone polymers; finally, the technique is particularly interesting when combined with flotation. However, this technology does not find exclusively favorable opinions, especially in relation to the aromatic fraction of wines and in particular to the varietal component. For some authors, the early oxygenation treatment leaves the varietal fraction relatively unaltered, as the aromas are still present in the form of glucosides and therefore less subject to oxidation. According to others, however, the wines obtained based on this practice would be characterized by an aromatic component that is predominantly of a fermentative type and therefore marked by strong similarities; hyperoxygenation would therefore determine a leveling of the varietal characteristics of white wines. It is certain that not all wines are suitable for this technology and the choice of the possibility of using it must be aimed at the product that one wishes to obtain. For example, it is known that 4MMP, a sulfur compound typical of the aroma of Sauvignon, is very sensitive to oxidation and develops better in a reducing environment; for this variety, the possibility of using hyperoxygenation of the must must therefore be appropriately assessed.

 

 

Clarifying and leaching and extracting action

In addition to the effects considered so far, the SO2 It also has must clarification properties and has a leaching and extracting action on phenolic substances. If the clarification action appears practically negligible compared to the action of normal clarifying agents used for must cleaning, the latter is particularly interesting for color extraction in red wine technology.

The limit for the total elimination of the SO2 if it seems possible today, however, it is not yet real and remains tied to the fact that the action of surrogates presents limitations of a different nature, which compromise their technological characteristics. These limitations translate into an effectiveness of action that is often not analogous to that of sulphites or with different durations; some substitute means give only partial coverage compared to SO2, especially in relation to the antiseptic action; the costs of the substitutes are not the most convenient compared to the use of sulphurous; which means that the applicability is not always convenient from either a sensorial or economic point of view, which must be seen in relation to the type of wine to be obtained.

As things stand, the total elimination of the use of the OS2 it appears not practicable without risks for the product. An interesting aspect, however, appears to be the possibility of integrating the use of sulphites with appropriate additives or with appropriate technologies, which will be chosen based on the product to be obtained, the technological phase (e.g. winemaking or refinement) and the type of action of the supplement itself. This will allow a significant reduction in SO levels2 present in wine, aiming its use at the stages in which it becomes a discriminant for quality, or delaying its use in the final stages of the production process; in this perspective, the reduction in use would benefit from a double advantage, also by virtue of a higher content of free sulphur dioxide, more active for all the functions considered.

 

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