MAIN INSTABILITIES FOUND IN WINES
The culmination of all the hard work since harvest is without a doubt, wine bottling or wine packaging. This is a crucial step and improper preparation will have serious consequences on wine quality. Pre-bottling stability testing aim to assess potential problems and correct them in order to ensure enduring wine stability. In this article, we will focus on the different disorders we often see in the laboratory.
Wine filtration alone does not guarantee that a crystal-clear wine will be stable. Conversely, gum arabic addition during fining may stabilize the wine but does not clarify it. Laboratory stability testing, in partnership with your oenologist, can address these issues through fining trials and pre-bottling tests. These analyses will help you manage the risk of precipitation. In this article, we will focus on the several disorders we often encounter at OENOSCIENCE.
During wine making, many particles are found in suspension. These come from yeast lees, grape or plant residues that will settle over time. Some particles may remain in colloidal form which might affect wine quality later on by inducing instabilities like protein or metal hazes.
Oxidation is chemically defined as a loss of electron. In wines, oxidation may be caused by enzymes (enzymatic oxidation) or by chemical phenomena (chemical oxidation). Production of the enzyme laccase is a consequence of an infection by Botritis cinerea fungus. This enzyme acts as an oxidase and is the main cause of enzymatic oxidation in red wines. Laccase induced oxidation may be attenuated by adding sulphites or ascorbic acid. When added directly to the must, sulphites will slow down laccase activity while ascorbic acid will have a protective effect through its antioxidant activity. Ascorbic acid acts as an “electron-acceptor” therefore protecting polyphenols and tannins from oxidation. Chemical oxidation is important in wine maturation and aging as it helps stabilize color and soften tannins. However, significant exposure of wine to oxygen will cause loss of varietal aromas and appearance of undesirable volatile molecules such as ethanal. Non-sulphited wine is particularly sensitive to chemical oxidation during bottling. Adding ascorbic acid to wine and using an inert gas to drive the oxygen out of the empty bottle during this step helps manage this risk.
There are two main hazes caused by metals: iron casse and cupric casse. The iron ion exists in two oxidation states in wine.Ferric iron represented by Fe3+ and ferrous iron described by Fe2+. These two forms are in balance. In presence of oxygen, ferrous iron Fe2+ will be oxidized to ferric iron Fe3+ and it is the latter which is responsible for ferric casse, particularly in white wines where it forms a white precipitate in the presence of phosphoric acid. This phenomenon is called the white (ferric phosphate) casse. In red wines, ferric iron (Fe3+) associates with polyphenols causing a black color accompanied by a deposit containing iron. This is referred to the blue (ferric tannate) casse.
Although present in large quantities in the must due to treatments against mildew, the majority of copper will be eliminated during fermentation through its insoluble complexes with yeasts and lees. On the other hand, if the wine comes into contact with copper containing material during storage, there is a risk of cupric casse if the quantity of copper exceeds 1 mg/L. In the presence of oxygen, copper has the form Cu2+. Unlike iron, cupric casse appears after a long period during which the wine is without oxygen, exposed to light and if the copper, in its reduced form Cu+, is present at a concentration close to or greater than 1 mg/L. White wines are more sensitive to cupric casse because the phenolic compounds present in red wines will have a protective effect by acting as a buffer.
Proteins are molecules of different sizes made up of a series of amino acids. This sequence is called the “primary sequence”. Through intramolecular amino acid interactions, the primary sequence will influence how proteins adopt an active three-dimensional state. (secondary and tertiary forms). Depending on their environment and whether the medium is acidic or alkaline, proteins will have a positive, neutral or negative charge which will lead to reactions with other components such as metals, polyphenols and tannins. When heated, some proteins lose their shape or “denature” causing them to flocculate and precipitate. Haze may only appear after cooling because heat can increase protein solubility in some cases. The type of protein varies according to grape varieties and winemaking protocol. For example, pellicular maceration or mechanical harvesting can contribute to an enrichment of unstable proteins. The presence of polyphenols and tannins in red wines will ensure a better stability because unstable proteins will have a tendency to associate with these molecules. White and rosé wines are more susceptible to protein instabilities because of the lack of tanins. Heat stability testing prior bottling is a good way to assess the risk of protein precipitation in whites and rosés.
Cold stability testing aims to cause precipitation of tartaric acid salt crystals, coloring matter in red wines and proteins in white wines. Because of its bivalent structure, tartaric acid is able to form complexes with potassium and calcium ions. These complexes will be more or less stable depending on their composition. For instance, when tartaric acid associates with a potassium ion, the resulting salt will be called potassium hydrogen tartrate or KTH. This salt is very soluble in wines. When tartaric acid is in complex with two potassium ions, a neutral salt called potassium bitartrate (K2T) will be formed. Although very soluble in aqueous solutions, potassium bitartrate is sparingly soluble in alcohol. Its solubility will decrease with increasing alcohol content. Tartaric acid can also form a neutral salt with two calcium ions (Ca2T) or one potassium ion and one calcium ion. Finally, tartaric acid and malic acid can form a mixed salt in the presence of calcium, calcium tartromalate which will form insoluble needle-shaped crystals.
The coloring matter refers to anthocyanins present in red wines and to flavones, pigments present in white wines. These molecules are present in grape skins. The precipitation of the coloring matter can lead to a reduction in color and the appearance of a deposit.
Microbiological disorders generally cause adverse organoleptic effects such as changes in taste and turbidity. They can be triggered by different microorganisms such as yeasts, acetic acid bacteria and lactic acid bacteria. In an unfiltered, unstabilized product and in the presence of sugar, yeasts can resume fermentation, thus causing an increase in alcohol level and pressure due to the release of carbon dioxide (CO2). Depending on the amount of sugar present, increase in pressure can lead to exploding cans or bottles.
Acetic bacteria are responsible for ascescence or acetic sting which is a direct translation of “piqûre acétique”. In presence of oxygen, acetic bacteria will oxidize alcohol in acetic acid and acetaldehyde. Under these conditions, volatile acidity will increase and alcohol level will slighlty decrease.
Lactic disease is caused by transformation of malic acid into lactic acid by lactic bacteria. Wines are particularly susceptible to lactic disease during sluggish alcoholic fermentations. This results in an increase in volatile acidity and carbon dioxide as well as changes in taste caused by the increase in lactic acid.
We have identified several disorders frequently encountered in wines. Thanks to the science of oenology and laboratory tests, it is not only possible to detect these disorders but to prevent and correct them through fining and filtration operations. For more information on bottling tests check out our panels on this page. We will see, in the next article “The stabilization of wines”, solutions to reduce instabilities and avoid the risk of precipitation and organoleptic changes.
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