## 1 Introduction  
Yeasts are the most important micro-organisms in the process of converting grape must into wine as they carry out alcoholic fermentation. In most cases, _Saccharomyces cerevisiae_ species are responsible for carrying out this process, due to their fermentative capacity and ethanol tolerance [1]. It has long been known that must contains a wide variety of yeasts, mainly composed of _non-Saccharomyces_ species; i.e., wild yeasts, such as _Kloeckera_, _Pichia_, _Candida_, _Metschnikowia_ and _Kluyveromyces_. Occasionally, species of other genera (_Zygosaccharomyces_, _Saccharomyces_, _Saccharomyces_, _Torulaspora_, _Dekera_, _Schizosaccharomyces_, _Rhodotorula_) may also be present [2, 3, 4, 5]. These yeasts may originate from both the microbial communities of grapes and the cell environment. Many of the non-_Saccharomyces_ yeasts, mainly the _Hansenispora_, _Candida_, _Pichia_ and _Kloeckera_ species, spontaneously initiate fermentation of up to 4-5% alcohol. Above this level, they rapidly start to die. _Saccharomyces cerevisiae_ yeast cell numbers increase as wild yeasts decline, dominating the middle and final stages of the alcoholic fermentation process [3, 4]. _Saccharomyces cerevisiae_ strains produce wines with high ethanol concentrations and only small amounts of unfermented sugars remain in the medium [6]. An exception to this is the fermentation of wines with very high sugar content, where fermentation stops at the limit of the strains' alcohol tolerance; this leaves a few percent sugar in the wine. Wine yeasts in higher-sugar environments tend to undergo anaerobic metabolism [7]. In this process, known as glycolysis, the yeast cell obtains energy in the form of ATP (adenosine-5\({}^{\prime}\)-triphosphate) by breaking down the sugar;while the cofactor NAD+ (nicotinamide adenine dinucleotide), which is necessary for enzymatic function, is reduced. The pyruvate synthesised during glycolysis is converted to carbon dioxide and alcohol in the last two steps of alcoholic fermentation in an oxygen-free environment [8]. In addition to ethanol, a significant amount of other primary and secondary metabolites are formed, which are important in shaping the taste and aroma of wines. These include glycerol, succinic acid and other organic acids, esters, aldehydes, ketones, higher alcohols and sulphur-containing organic compounds; the types of which are important not only in their species, but also in their quantitative proportions in the development of harmonious flavor and aroma character [9]. Among the inhibitory metabolites produced by yeasts, sulphur dioxide plays a significant role and is only slightly likely to remain in free form in the medium after fermentation [10]. The rate of sulphur dioxide production by wine yeasts is a strain-dependent property. They can be divided into two groups based on sulphur dioxide production: _Saccharomyces cerevisiae_ strains with low sulphur dioxide synthesis; and _Saccharomyces bayanus_ strains with high sulphur dioxide content, capable of producing up to 300 mg L\({}^{-1}\) of sulphite. The magnitude of sulphite production can be influenced by a number of factors in addition to yeast strains, such as initial pH, must composition, available nitrogen source, fermentation temperature and sulphate concentration [11]. Sulphur dioxide is formed as an intermediate metabolic product in yeast cells by sulphite reductase enzymes and is transferred from there to musts. For commercially available starter cultures, it is important to test this property during selection [12]. The production of free sulphur dioxide by 10-10 yeast strains from two distributors was investigated by [PERSON] [13] in red and white wines. The yeast sulphite synthesis varied between strains; however, in all the tests, the white wines produced significantly more sulphite than the red wines. Moreover, he found that the sulphur dioxide produced by the yeasts was not sufficient to protect the wine and that external supplementation was necessary.