, 2010). So, the productions of both diacetyl and acetoin by S. thermophilus, for which α-acetolactate synthase and decarboxylase activities are well documented ( Monnet & Corrieu, 2007), can be ascribed in the present work to lactose metabolization. On the other hand, it has been reported that, through citrate permease induction

by citrate, several LAB species are able to metabolize citrate (Mayo et al., 2010) producing 4-carbon compounds such as diacetyl and acetoin. In addition, L. rhamnosus was shown to co-metabolize citrate at low or intermediate levels only in the presence of a fermentable sugar such as lactose ( Jyoti, Suresh, & Venkatesh, 2003). Additional pyruvate is formed during citrate metabolism, so that most of it becomes available Depsipeptide concentration when required to oxidize the NADH released during selleckchem sugar fermentation ( Axelsson, 1998). Since citrate is present in significant amounts in milk of many animals, like cows and goats (1.5 g/L) ( Linzell, Mepham, & Peakert, 1976), the presence of the above flavoring compounds in our fermented products was supposed to be the likely result of citrate fermentation. The highest values of diacetyl (18.4 mg/L) and acetoin (0.8 mg/L) were obtained at the end of the St–Lr co-culture (Fig. 2), which suggests the occurrence of a

synergism between St and Lr, leading to great advantages in the manufacture of dairy products because of their characteristic flavors. According to Oliveira, Perego, Oliveira, and

Converti Etofibrate (2011), the increased presence of these flavoring end-products in co-cultures could be ascribed to substantial metabolic changes. These results taken together demonstrate that L. rhamnosus could be a possible candidate to industrially synthesize diacetyl and acetoin. Another synergistic effect is evidenced in Fig. 2 by the higher increase in biomass concentration in the co-culture compared with pure cultures. St and Lr did in fact exhibit maximum final cell concentrations 15.5 and 44% lower than in St–Lr, respectively. One hypothesis to explain such an effect is that S. thermophilus produces small amounts of formic acid and CO2 ( Mayo et al., 2010) that can stimulate the growth of other LABs, while L. rhamnosus is able to release peptides by a serine protease of the subtilisin family (known as PrtR) that stimulate the growth of S. thermophilus ( Siezen, 1999). As shown in Fig. 3, the presence of inulin enhanced, in general, the levels of all main metabolic end-products. In particular, at the end of fermentation, the concentration of lactic acid in the St pure culture, Lr pure culture and St–Lr co-culture was 1.2, 10.9 and 26.1% higher than without inulin (Fig. 2). By the same reasoning, the acetic acid concentration increased by 21.5% in the Lr pure culture and 33.5% in the St–Lr co-culture, and that of ethanol by no less than 300% in the Lr pure culture and 241% in the St–Lr co-culture.