# Use of Dairy and Plant-Derived Lactobacillus as Starters for Cherry Juice Fermentation
## 4 Discussion  
_L. plantarum_ is the most employed species for lactic acid fermentation of fruit juices, as these strains isolated from plant environments are better adapted to these matrices [10]. Indeed, chemical characteristics of fruits, such as pH and the concentration of sugars and organic acids, make them a hostile environment for microorganisms [22; 23; 24]. However, a great biodiversity among LAB, species- and strain-dependent, was observed, which allowed them to adopt specific alternative metabolic pathways in adverse conditions using non-conventional carbon sources for the exploitation of alternative substrates or in a global stress response. This adaptation may result in formation of volatile compounds [17; 18; 23], metabolism of phenolic compounds [11; 25], and production of new molecules. Starting from these premises, a large number of dairy strains were used for cherry juice fermentation, and a global view on the metabolism of both dairy and plant-derived microorganisms was proposed. Cherry juice is recognized to be a stressful substrate for microorganisms, due to its low pH, its high sugar and phenolic content, and the presence of malic acid [23]. This unfavorable environment affects the growth and cell viability of most tested strains, with a particular negative effect on _L. casei_ and _L. rhamnus_, which were not able to grow satisfactorily in this medium. On the contrary, all _L. plantarum_ strains, both of dairy and plant origin, showed a good adaptability. Moreover, _L. rhamnus_ 2360 and _L. paracasei_ 4186 were also able to ferment cherry juice components and to survive during refrigerated storage, also offering the possibility of conveying viable cells of non-plant origin in this fruit juice. PCA analysis showed a clear separation between unfermented and fermented juices, on account of the significant modulation of the composition of the juice matrix by lactic acid fermentation. Figure 4: Phenolic compounds. Hierarchical clustering and heat map performed on phenolic compounds detected in fermented juice with _L. plantarum_ (C1, 1 LE1, POM1 and 285), _L. rhamnus_ (2360) and _L. paracasei_ (4186) and unfermented juice (30 \({}^{\circ}\)C and 37 \({}^{\circ}\)C) after 48 h and 14 days (storage). A scale ranging from a maximum of 2 (red) and a minimum of \(-2\) (blue) was used. The adaptation to the acid environment of cherry juice is based on the ability of LAB to metabolize malic acid [24], resulting in its almost complete degradation. Actually, all tested dairy and plant strains, independently of species and with the exception of _L. plantarum_ C1, completely converted malic acid into lactic acid. Indeed, most LAB can convert malic acid into lactic acid thanks to the maloelastic enzyme, decarboxylolating malate to lactate by a NAD\({}^{+}\) and Mn\({}^{2+}\)-dependent maloelastic enzyme [23; 26; 27]. At low pH, the choice of malic acid as the preferred energy source over glucose had already been reported in literature and linked to the increase of intracellular pH and the increase of reducing power [28], with an associated modification of cellular permeability [23]. In the present work, LAB did not consume glucose and fructose during fermentation, whereas sucrose showed a marked decrease. The metabolic activity of the tested strains, especially in _L. rhamns_ and _L. paracasei_, is mainly based on malolactic fermentation, aimed at the maintenance of vitality, rather than on sugars fermentation, which is mainly correlated with growth [23]. The exploitation of an organic carbon source by a microorganism strongly depends on the type of substrate. Two different works on cherry juice fermentation reported the absence of sugars metabolism in one case [23], and a slight consumption after fermentation in the other [2], the latter being in very good agreement with our results. Observed differences may be related to cultivar, to ripening time, and to the applied processing conditions, which could affect the concentration of nutrients and/or acids [1; 29].