# Functional Characterization of _Saccharomyces_ Yeasts from Cider Produced in Hardanger
## 1 Introduction  
Norwegian cider is becoming more and more popular in Norway in recent years among producers and consumers. Especially in the Southwest part of Norway, in the Hardanger region, there is a long tradition of producing ciders. Available data show that traditional cider from Hardanger is very different from French, English, or Spanish ciders in terms of sensory characteristics, apple cultivars, and fermentation process. A recent comparison of the aromatic component composition of different French and Norwegian ciders, including ciders from Hardanger, has confirmed that Norwegian ciders contain more aromas, which are behind fruity and fresh sensory sensations [1]. Cider from Hardanger is mostly made from desert apples, which have different chemical compositions in comparison to cider apples [2,3]. Ciders in Hardanger were traditionally produced by employing long spontaneous fermentation, even over winter, at low temperatures, often with the addition of sucrose and nothing else to increase the alcoholic strength [2]. Nowadays, spontaneous fermentation is more and more replaced by inoculated commercial yeasts. In our previous study of yeast ecology from ciders produced in the Hardanger area, we had seen thatin ciders, apart from the presence of non-_Saccharomyces_ species at the early stages of fermentation, the most predominant species isolated during fermentation were _S. cerevisiae_ and _S. warrant_[4]. _S. warrant_ is a cryotolerant yeast and belongs to the _Saccharomyces sensu stricto_ clade, being the furthest relative from _S. cerevisiae_, and is now recognized as a pure species, distinct from _S. bayanus_, which is a hybrid of _S. warrant_ and _S. eubayanus_[5, 6]. Due to its problematic taxonomy and incorrect identification, it is difficult to trace it in the scientific literature and obtain data about its origin, diversity, and potential usage in cider and winemaking. _S. uvarum_ is not only related to cider fermentations, but it is also known in white wine production from colder grape-growing regions [7]. _S. uvarum_ in grape juice, in comparison to _S. cerevisiae_, produces less acetic acid, acetaldehyde, and ethanol but more glycerol, succinic acid, and malic acid [7]. It is also capable of producing substantially more 2-phenyl ethanol (rose note), isoamyl alcohol (whisky), iso-butanol (solvent, bitter), and ethyl acetate (pineapple) [8, 9]. There are also reports about the use of _S. uvarum_ in cider production [10, 11]. The phenotypic differences between _S. uvarum_ and _S. cerevisiae_, the primary yeast species used worldwide for wine and cider making, are associated with pronounced proteomic differences [12]. _S. cerevisiae_ gives ciders consistent aroma and taste and less risk of spoilage, however, resulting in less complex ciders [13]. Thus, to modulate the profile of ciders by enhancing microbial diversity seems a rational approach and was shown in recent studies [14, 15]. To better evaluate the potential of different natural _Saccharomyces_ strains, we need to assess the impact of nitrogen sources on fermentative behavior and possible undesirable production of metabolites such as acetic acid, H\({}_{2}\)S, and BAs as well. In spontaneous fermentations, yeasts generally use naturally present amino acids, which results in higher aromatic complexity but also higher production of BAs. The impact of _Saccharomyces_ strains on the aroma profile related to different fermentation conditions is already well described in wine and beer production [7]; however, in recent years, cider production has been supported by research mainly based on non-_Saccharomyces_ yeast strains [14, 15, 16, 17], and not so much on potential alternative _Saccharomyces_ species, such as _S. uvarum_. _S. cerevisiae_ is known to be more controllable in its fermentation output and performance in wine and beer production [7, 18]. Therefore, it is very important to study them in natural media for cider production, namely apple juice. Most of the yeast characterization studies are still performed in synthetic must.