# _Torusspora delbrueckii_ Strain Behaviour within Different
## 1 Introduction  
Cider is an alcoholic beverage obtained from the complete or partial fermentation of apple juice. It represents an important segment of the apple fruit industry in Europe, North America, and Australia, where apple cultivation is diffused [1]. The main cider-producing countries (UK, Spain, France, Ireland, and Germany) have long-standing cider styles and precise legislation. In the past, cider consumption was restricted to traditional regions while, nowadays, cider and sparkling ciders are becoming more popular and attractive for many consumers [2]. The increasing demand for ciders is accompanied by a growing request for natural and locally produced beverages with peculiar aromatic characteristics [3]. The use of a selected _Saccharomyces cerevisiae_ starter is widespread in the industrial production to obtain a complete alcoholic fermentation, to avoid sensory deviation and to ensure a consistent and reproducible quality [4]. Nevertheless, in traditional and small-scale cider manufacturers, the fermentation is often spontaneous and carried out by indigenous yeasts [5]. The recognition of 'territoriality' is important for cider appreciation [4] and the choice of selected yeast starters may actively contribute to the expression of cider typicity, Non-_Saccharomyces_ yeasts have often been considered as sources of spoilage and associated with higher levels of residual sugars and unpredictable by-products and off-flavours [6]. More recently, the significant role of non-_Saccharomyces_ yeasts in enhancing the flavour, body, and texture complexity of fermented beverages have been revalued. Indigenous yeasts have been associated with the production of peculiar aroma compounds, such as esters, aldehydes, higher alcohols, acids, terpenes, as well as higher glycerol concentration [7]. The spontaneous fermentation of apple juice has been described as a complex of microbial activities characterized by the sequential enrichment of non-_Saccharomyces_ and _Saccharomyces_ yeasts. In the first phases of the process, _Kloeckera_ spp., _Candida_ spp., _Pichia_ spp., _Hansenula_ spp., _Hanseniaspora_ spp., and _Metschnikowia_ spp. represent the most detected yeasts, while _Saccharomyces cerevisiae_ become dominant later when the ethanol produced by the early fermenting species inhibits their own growth [4]. In the recent years, several studies on the use of non-_Saccharomyces_ yeasts for cider production have been published, suggesting an increasing interest in the search of novel actors to produce ciders with unique aromatic characteristics [8; 9; 10; 11; 12]. The influence of non-_Saccharomyces_ yeasts has been investigated both in pure and in mixed fermentations, and in combination with _S. cerevisiae_[13; 14; 15]. Several studies documented the improvement of the fruity aroma of ciders correlated to the production of esters, higher alcohols, and terpenes with fruity aromas after a metabolomic analysis [16; 17]. Among non-_Saccharomyces_ yeasts, the use of _Torulspora delbrueckii_ is attracting the attention of wine, beer, and cider producers because of its good fermentation performance [18]. In a study on synthetic media, [PERSON] et al. [19] described that _T. delbrueckii_ produced metabolites correlated to odor activity. _T. delbrueckii_ was associated with the valuable production of aromatic compounds such as fruity esters, lactones, thiols, aldehydes, and terpenes, contributing to a fruity and pastry bouquet, and the decreased amount of undesired aromatic compounds. Moreover, it has been documented that _T. delbrueckii_ improves the quality of wine, resulting in a product characterized by a lower volatile acidity and acetaldehyde production, increased amount of glycerol, and greater manoprotein and polysaccharide release in comparison with wine produced with _S. cerevisiae_[20; 21]. Despite non-_Saccharomyces_ yeasts having been investigated as main actors in cider production, only a few publications are available on the application of _T. delbrueckii_ in cider making [9; 11; 19; 22].