# Influence of Apple Juice Type on Fermentation Process and Volatile Compounds of Hard Apple Cider
## 1 Introduction  
Hard apple cider is an alcoholic beverage obtained by the fermentation of apple juice. This traditional alcoholic beverage is popular all over the world, with the main producers being placed in the USA and Europe. Countries such as France, Spain, and the United Kingdom are traditional cider countries and, during colonial times, helped spread cider production in the USA. Cider can be made not only from apples, but from any other sugary fruit. However, the traditional beverage began losing its popularity as the brewing industry began to develop and competition from other drinks became serious. In rural areas, ciders were cheaper to obtain and contained higher alcohol content (7% vol) [1]. The main characteristics of hard apple ciders depend on the variety of apples, the degree of ripeness of the fruits used, the applied agronomic conditions during the fruit's growth, the extraction processes of the fruit juice, the fermentation parameters (microorganisms, temperature, duration, original extract content), compounds developed during fermentation, processing, and storage [2, 3, 4]. Even though there are traditional apple varieties that give the specific aroma to hard factory-produced ciders, today's modern cider production does not rely on such varieties, but rather involves culinary and dessert varieties [5]. Hard ciders often evoke the sensation of drinking beer due to the similar CO\({}_{2}\) and lower alcohol content (3-7% vol), and they are almost always placed near beer cans in stores. However, legislation in many European countries classifies them as sparklingwines. This is due to the quantity of residual sugar content, expressed as \"dryness\" [6]. Namely, yeast tends to spend all of the available sugars in the apple juice, leaving the cider \"dry\". There are certain modifications during the fermentation process that can result in sweeter cider with less alcohol (France). Spanish ciders, traditional sidra of Asturias, are served in a different manner, poured into the glass from a height, which results in a frothy-headed drink. Sidras tend to be dry and even vinegary. These modifications are indigenous to different countries [7]. In some countries, it is allowed to use apple juice and concentrate in different ratios, while in some countries, such as the United Kingdom, mostly fresh apples are used. However, in Spain and France, specific apples are used for production, even though apple juice or concentrate can be used. Nevertheless, ciders are usually designated by their quality [7]. According to Le Que're [8], French cider fermentation is usually conducted without yeast, pitching at temperatures 5-20 \({}^{\circ}\)C depending on the season and the equipment of the cider producer. In 2023, the European Commission issued a report in accordance with Regulation (EU) No 1308/2013 [9] on new marketing standards for cider and Perry and for dried leguminous vegetables and soya beans. However, the most active European association regarding ciders is the European Cider and Fruit Wine Association (AICV), which represents the interests of European cider and fruit wine producers. This report regards the need for stricter control of cider producers due to a lack of legislation. This is especially important since some producers use juice with 50% apple juice/concentrate and some use 100% fresh apple juice. Also, regulated labels and limits for designations of cider/perry related to higher quality (craft, made from pure fresh juice, etc.) and sugar content (dry, semi-dry, sweet) are suggested in this report [9]. Volatile compounds, such as alcohols, acids, and esters (1-butanol, 1-hexanol, 3-methylbutyl acetate, 2-phenylethyl) acetate and butyl acetate), are an immensely important component of a cider's aroma [10]. Some compounds identified in cider naturally occur in fruits, and some are produced by microorganisms that are used to ferment cider [3; 11].