# Use of Dairy and Plant-Derived Lactobacillus as Starters for Cherry Juice Fermentation
## 4 Discussion
In the present work, tartaric acid was also consumed by LAB, especially by _L. plantarum_, in agreement with literature, where _Lactobacillus spp_. were reported to be able to convert it into oxaloacetic acid (and then into lactic acid, acetic acid, and CO\({}_{2}\)), thanks to the tartrate dehydratase enzyme [30; 31; 32]. Nevertheless, the degradation of tartaric acid is not widespread in LAB, and it has been studied especially in wine. Moreover, not all LAB were able to metabolize tartaric acid in the same way; for instance, differently from _L. plantarum_, in _Lactobacillus brevis_, succinic acid can be produced instead of lactic acid [33]. The acetic acid found in several fermented fruit juices [18; 23; 34] is characterized by sharp, pungent and vinegar notes. However, thanks to specific bacterial metabolic traits, acetic acid can be converted to the corresponding esters [35; 36], such as propyl acetate; this was especially observed after fermentation with _L. rhamns_ 2360 and at the end of storage using _L. paracasei_ 4186. The increase of \(\beta\)-linalool, observed in different fermented samples, could be ascribed to glycosyles produced by LAB, involved in the release of aglycones from glycosylated terpen [18]. Acetoin was also affected by fermentation and its bio-synthesis can be derived from citrate metabolism [18; 27]. Phenolic compounds have been reported to exert health benefits in humans [37], to exhibit antimicrobial activity, and to impact the flavor, taste and color [10] of plant products [38]. Their beneficial activities have been partially correlated with microbial metabolism that can occur during fermentation processes [39]. Moreover, the metabolism of phenolics may contribute to bacterial stress response when microorganisms are in hostile conditions [19; 40]. In the present study, hydroxycinnamic acids were transformed by LAB, ideally through phenolic acid decarboxylase or phenolic acid reductases [19]. Caffeic acid was effectively converted into dihydrocaffeic acid, as also already reported in the literature for _L. plantarum_ POM1, [11; 12; 19], with a potential impact on health, due to its more effective capacity to inhibit platelet activation than its phenolic precursor [41] and to its antioxidant effect on endothelial cells [42]. In addition to putative effects on the health-promoting activity of fermentation metabolites, the microbial transformation exerted by LAB may be relevant for other purposes. For example, \(p\)-coumaric acid was decarboxylated into \(p\)-vinylphenol and subsequently reduced, possibly by a phenolic acid reductase, to the phenolic volatile 4-ethylphenol, which contributes to aroma in fermented food [19]. The reduction of \(p\)-vinylphenol was favoured under anaerobic conditions or in absence of electron acceptors, i.e., when fructose is found at high concentration, to increase NAD\({}^{+}\) quantity [19; 43]. Protocatechuic acid was also metabolized [11; 12], in particular by _L. plantarum_ 285 and POM1, reaching complete depletion, but catechol was not detected. Finally, worthy of note is that phenyllactic acids were produced ex-novo by all the tested strains, probably deriving from amino acid metabolism. Phenylalanine can be converted into phenylpyruvate acid by a transamination reaction, and finally metabolized into phenyllactic acid by hydroxyacid dehydrogenase, while _p_-hydroxyphenyllactic acid may originate from tyrosine metabolism [44]. Their antimicrobial activity against pathogenic strains and moulds has been well documented [44; 45], even if the concentrations found in the present study were lower compared to those explaining antimicrobial activity.