Bioactive compounds from pomegranate by-products increase the in vitro ruminal accumulation of potentially health promoting fatty acids

Increasing demand and production of pomegranate has led to a large amount of by-products that might be used in ruminant feeding. Inclusion of pomegranate by-products in the ovine diet has recently been shown to enrich meat and milk with potentially health-promoting fatty acids (FA). However, it remains unclear whether this effect is due to the action of the bioactive conjugated linolenic acids (CLnA) or of the tannins present in the pomegranate, or perhaps to their interaction. To fill this gap, two in vitro experiments were conducted: the first one tested the effects of pomegranate oil and tannins, alone or in combination, on the biohydrogenation process, and the second one compared the ruminal responses to by-products rich in CLnA (pomegranate seeds, PS), in tannins (pomegranate peels and pulp, PPP) or in both bioactive components (i.e., the whole pomegranate by-product; WPB). Three cannulated ewes were used as donors of inocula for batch cultures of rumen microorganisms. Incubations lasted for 12 and 24 h and were repeated on 3 different days (runs). In both experiments, digesta FA profile was examined by gas chromatography. Results from both trials support that pomegranate tannins and CLnA played different roles in modulating ruminal FA composition. Specifically, tannins would favour the accumulation of potentially health-promoting FA present in dietary lipids (e.g., 18:2n-6 or 18:3n-3) and cis-9 trans-11 conjugated linoleic acid (CLA), whereas the observed increase in trans-11 18:1 would mainly derive from the biohydrogenation of CLnA isomers. Results from the second experiment included evident shifts in some minor FA that would support not only direct saturation steps (e.g., increases in trans-11 cis-13 and trans-11 trans-13 CLA, and in trans-9, trans-11, cis-13 and trans-13 18:1) but also a putative isomerisation by rumen bacteria (e.g., increases in trans-10 cis-12 CLA and trans-10, cis-15, trans-15 and trans-16 18:1). Changes in ruminal fermentation parameters (i.e., reductions in ammonia concentration and in the proportions of minor volatile FA) showed that pomegranate tannins protected dietary protein from degradation. Nevertheless, a negative impact on in vitro ruminal fermentation (i.e., reductions in DM disappearance, gas production and total volatile FA concentrations) was observed when 20 % of by-products were included in the diet. Finally, there seem to be no evident synergistic but additive effects between pomegranate bioactive compounds (i.e., tannins and CLnA) on ruminal biohydrogenation or fermentation.