## 3 Results and Discussion
### Analysis of Volatile Compounds
SBC had the highest concentration (253.45 \(\pm\) 3.78 mg/kg), which can be related to the longer fermentation of sourdough. Isoamyl alcohol is a product of the fermentation of leucine also in the yeast cell [20; 24; 31] and presented higher values in sourdough samples (with and without Cava lees). In addition, isoamyl alcohol can also be produced by LAB such as _Lactiplantiobacillus plantarum_ (formerly _Lactobacillus plantarum_) [24; 32]. Aldehydes are formed during lipid oxidation and decarboxylation of unsaturated fatty acids as well as from amino acid degradation by the Ehrlich pathway [20; 33]. The most prevalent aldehydes found in the studied breads were benzaldehyde, hexanal, (E)-2-nonenal and nonanal which are commonly reported in both sourdough and bread [20; 24; 33]. The addition of Cava lees increased the concentration of benzaldehyde, especially in yeast leavened bread (B+L, 88.66 \(\pm\) 11.41 mg/kg). This compound is the result of both fermentative reactions and lipid oxidation and has been found in bread produced with and without sourdough [20; 24; 34]. Benzaldehyde has been reported to be produced by yeast as well as _L. plantarum_ and _L. heleveticus_ via amino acid (phenylalanine) conversion [24; 35]. Moreover, benzaldehyde has also been found in sparkling wines [36; 37; 38], which might explain the increase in its concentration in breads with Cava lees since sparkling wine lees can retain aldehydes in their surface (Table S1) [17]. Nonanal, another compound derived from fermentation and lipid oxidation [20] has also been identified in the surface of sparkling wine lees [17]. In this study, SB+L and B+L showed significantly higher amounts of this compound, that was, indeed, also identified in Cava lees surface (Table S1). In fact, in SBC, nonanal was not detected. It must be taken into account that some heterofermatative LAB strains are able to reduce aldehydes to other compounds, which may explain the lower concentration of certain volatiles in samples with 5% Cava lees [32; 35]. Regarding ketones, BC samples presented a greater variety of those compounds. Those volatile compounds are influenced by LAB in dough fermentation, and only certain homoformertative and facultatively heterofermatative bacteria are able to produce them [32]. Acetoin is a distinct aroma in bread produced during fermentation related to consumer acceptance [23]. In this study it was found that the addition of Cava lees increased its production, especially in B+L where it reached similar values to those of sourdough bread. Esters are characterized by a fruity odor resulting from a direct esterification between ethanol and acetyl co-A derivatives of fatty acids during fermentation mainly due to heterofermatative LAB [32; 35; 39]. In fact, it has been observed that fermentations with LAB produce more esters than those with yeast [24]. In this study, the addition of 5% Cava lees increased the production of esters, especially in bread samples (B+L) in which 13 esters were identified, which is in accordance with higher LAB populations (Figure 2). Ethyl decanoate was the dominant ester in SB+L (127.37 \(\pm\) 13.90 mg/kg), while in B+L it was ethyl octanoate (965.31 \(\pm\) 167.18 mg/kg). As previously mentioned, sparkling wine lees also retain esters such as ethyl hexanoate, ethyl octanoate and isoamyl octanoate [17]. Infact, most of the esters only identified in B+L samples have been reported in sparkling wine [36; 37; 40] and sparkling wine lees [17], and were also found in the Cava lees analyzed (Table S1). Terpenes are generally characterized by a floral odor and commonly found in sparkling wine [37; 38]. Furthermore, vitispirane and nerolidol, identified in this study, have also been found in sparkling wine lees [17]. Overall, it was found that the addition of Cava lees increased terpen concentration and variability in both sourdough bread and, especially, yeast leavened bread. TDN (1,1,6-trimethyl-1,2-dihydronaphthalene) is a C13-norisoprenoid usually found in sparkling wine [36; 37; 38].