# Aroma Profile of Monovarietal _Pet-Nat_ Ciders: The Role of Croatian Traditional Apple Varieties
## 3 Results and Discussion
### Volatile Compounds in Sparkling Pet-Nat Ciders
_C13-norisoprenoids_ are a diverse class of aromatic compounds, originating from carotenoids, that contribute to the varietal character of many grapes and wines. They are detectable at very low levels, sensory thresholds have been reported as low as 2 ng\(\cdot\)L\({}^{-1}\), although it is more likely that they begin to impact wine character at levels between 2 and 10 \(\mu\)g\(\cdot\)L\({}^{-1}\). \(\beta\)-Ionone plays an important role in the floral, violet character of some red wines and vitispirane has an aroma that has been described as floral, fruity, woody, or reminiscent of eucalyptus. Microbial biotransformation of \(\alpha\)- and \(\beta\)-ionones to a number of hydroxy and keto derivatives has been reported for several fungi [62]. In our case, the highest significant C13-norisoprenoids concentration was detected in 'Crvenka' cider, strongly influencing its sensory profile. _Esters_ are one of the most important volatile byproducts of alcoholic fermentation in cider, after ethanol [63]. Most esters are responsible for fruity, floral and sweet notes [64]. Although some of the esters are present in apples juice, they are primarily synthetized by yeast and lactic bacteria, or by chemical esterification during ageing or lees contact. Esters are subjected to continuous changes regarding the complexity of cider [59], and the smallest variation in concentration may have a significant impact on wine quality [65]. Modifications in ester profiles are influenced by ageing temperature, and pH, as well as by the alcoholic level of the final products [66]. In this study, total esters concentration was, significantly, the highest in 'Bozicnica' cider. In total, 27 esters were detected, while ethyl hexanoate was the most representative compound, with the highest concentration in 'Bobovac' cider, with 1231 ug\(\cdot\)L\({}^{-1}\), which is above the range reported in monovarietal 'Ilzer rose' ciders (275-980 ug\(\cdot\)L\({}^{-1}\)) [50]. According to the high OAV value, it significantly contributed 'fruity', 'green apple' and 'banana' scents to the cider aroma, together with other esters with OAV > 1 (Table 5). Previous research noted that ethyl hexanoate and ethyl octanoate are influenced by the time of contact with lees and are often referred to as the \"apple esters\" [18; 28], while acetate esters production is influenced by non-_Saccharomyces_ activity [67]. _Volatile phenols (VPs)_ contribute characteristic odors to various foods and fermenting alcoholic beverages, including ciders. They impart odors reminiscent of smoke, medicinal/cleaning products, vanilla, spice and leather [44] or organoleptic defects ( 'Brett character') associated with animal (e.g., horse-like) or phenolic aromatic notes [68]. The 'Brett character' occurs in ciders rich in polyphenols, caused by some types of _Brettanomyces_ yeast and _Lactobacillus_ bacteria. In this study, four VPs were detected, eugenol, 4-ethylphenol (4-EP), 4-vinylbaaicol, and 4-ethylgaiaicol. Only eugenol was detected in all ciders with OAV > 1 (cinnam, clove). The lowest total VPs were detected in sparkling cider 'Crvenka' (Table 2). The most the most abundant compound was 4-EP (126-3151 ug\(\cdot\)L\({}^{-1}\)), with the the highest significant concentration and OAV > 1 found in 'Bozicnica' sparkling cider (Table 5). According to published data, the 4-EP detection threshold is influenced by product complexity (0.6-4.0 mg\(\cdot\)L\({}^{-1}\)) [68]. 4-Ethylcatechol (4-EC) and 4-EP are the main volatile phenols in French ciders and phenolic off-flavor defects were found in ciders containing up to 12 mg\(\cdot\)L\({}^{-1}\) 4-EC and 3.0 mg\(\cdot\)L\({}^{-1}\) 4-EP [69]. The 4-EP concentration in Spanish cider presenting a significant 'Brett character' reaching up to 0.7 mg\(\cdot\)L\({}^{-1}\) [70]. According to [PERSON] et al. [28], levels of VPs in Asturias cider range between 100 ug\(\cdot\)L\({}^{-1}\) and 14 mg\(\cdot\)L\({}^{-1}\), similar to French ciders, with a significant increase in 4-EP after the lees contact. [PERSON] et al.