# Chemical Composition and Flavor Characteristics of Cider
## 1 Introduction  
Cider is a beverage made from fermented apple juice that contains trace amounts of ethanol and is welcomed in many countries [1]. Cider flavor is an important index for evaluating cider quality [2]. Many factors influence the development of aromas in apple cider, for example, type of apple, maturity, fermentation condition, and yeast strain. Studies have found that yeast strains play an important role in the generation of flavor substances in cider [3]. In past studies, _Saccharomyces cerevisiae_ (_S. cerevisiae_) has been used as the sole starter in cider brewing due to its ability to quickly convert sugar to alcohol. However, the resultingcider has a simple aroma and a sour taste [4]. This causes a reduction in people's desire to purchase cider. Non-_Saccharomyces_ yeast can synthesize a variety of enzymes that produce rich materials such as glycerin, turning raw material into esters and higher alcohols, with a variety of flavors and nutrients. It promotes the production of aroma components and is a good producer of fruit acetates [5]. However, it is intolerant to ethanol and often exists in the initial stage of fermentation, resulting in low fermentation efficiency [6]. A study found that the number of acetate acid, ethyl esters and acetate acid, isoamyl esters increased when _Williopsis saturnus_ and _S. cerevisiae_ were co-inoculated in wine [7]. Furthermore, another study found that the use of _Torulaspora delbrueckii_ in co-inoculated fermentation with _S. cerevisiae_ was an appropriate way to control flavor production during beer fermentation [8]. Therefore, it is of great importance to improve the quality of cider by co-culturing non-_Saccharomyces_ yeast with _S. cerevisiae_. The flavor of cider is an important factor representing its quality. The successful extraction of flavor substances is the key point of related substance research. At present, the main methods, such as headspace solid-phase microextraction [9], dynamic headspace gas chromatography, and solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) [10], are used for the analysis of flavor components in fruit wine. SPME-GC-MS has been widely used to analyze aroma compounds in many types of wine; it is an important research method for food flavor analysis at present. It can capture and detect high content of macromolecular flavor components with more than 10 carbon atoms. It can analyze specific flavor compounds of samples to be tested, with high detection sensitivity, convenient operation, and accurate results [11]. SPME-GC-MS combined with the odor activity value (OAV) can determine the ratio of the concentration of a single volatile on its odor threshold in water or air [12]. It is possible to study the potential key aroma compounds in cider and their contributions to the aroma of cider and to reveal the characteristic aroma patterns of cider. Meanwhile, gas chromatography-ion mobility spectrometry (GC-IMS) has been applied to discriminate subtle differences in the aroma of the product [13]. GC-IMS can capture and detect small molecular volatile components with 10 carbons or less so that substances with smaller molecular weights can be detected in the form of monomers and dimers, which is a modern visualization detection method that does not require pretreatment and is highly sensitive [14]. GC-IMS can be used as a supplement to SPME-GC-MS. To make the aroma analysis of volatile compounds in cider more comprehensive. At present, for SPME-GC-MS and GC-IMS combination, different non-_Saccharomyces_ yeast and _S. cerevisiae_ strains are tested in the fermentation of cider aroma components, and combining this with OVA difference analysis of cider aroma research is less common. Therefore, this study aimed to investigate the chemical composition and flavor characteristics of cider fermented by co-inoculation of several different non-_Saccharomyces_ yeasts with _S. cerevisiae_ and to obtain the important volatile compounds by SPME-GC-MS and GC-IMS, and OVA was used to analyze the aroma components of cider.