This typical taste and smell, also commonly referred to as " Alterston " or "Altl" in dialect, occurs mainly in older white wines. The name is derived from the Old High German "firni" for "previous year" or "old". However, this most common type of ageing hue should not be confused with UTA (atypical ageing hue), Böckser (Lagerböckser) or the related Petrolton. The cause of ageing varnish is primarily oxidation by atmospheric oxygen. This occurs due to excessive oxygen uptake after filtration, as well as high temperatures during storage, exacerbated by high levels of flavonoid phenols in the wine. This is also promoted by faulty corks (closures). In the case of storage in wooden barrels, firns can develop after only one and a half years, whereas in the case of storage in bottles, this usually takes considerably longer.
Firne is caused by caramelisation reactions between amino acids (proteins) and sugars. The typical bitter taste is expressed by the smell and taste of black tea, old straw, honey, resin, nuts, mushrooms and damp earth. The substances responsible for this are higher aldehydes (alcohol compounds). The tone can increase to an unpleasant camphor-like (after the tree) impression. Oxidation leads to colour changes up to high colour. Enthusiasts of older white wines tolerate age firn to a lesser extent and even describe it positively as "noble firn". In its more pronounced form, it is in any case a wine defect. Sulphuric acid firn, which is caused by improper use of sulphur (over-sulphurisation), has a completely different origin.
