Effect of using different levels of tannic acid on in vitro ruminal fermentation parameters and fatty acid profile

Meat and dairy products from ruminants are the main source of vaccenic acid (VA) and conjugated linoleic acid (CLA) in human nutrition. In particular, the isomer of cis-9, trans-11 CLA is active in the prevention of cancer and atherosclerosis in human (Belury, 2002). CLA is partially synthesized in the rumen by cellulolytic bacteria, and mainly by Butyrivibrio spp. during the biohydrogenation (BH) of linoleic acid (cis-9, cis-12 C18:2, LA). However, the largest proportion of CLA present in milk is endogenously produced in the mammary gland by the action of the enzyme Δ9-desaturase on vaccenic acid (trans-11 C18:1, VA), which is another product of the ruminal BH (Jenkins et al., 2008; Shingfield et al., 2013). In the last decade several efforts have been made in order to develop efficient enrichment strategies of VA and CLA in ruminant products (Shingfield et al., 2013). The main strategy to enhance these beneficial fatty acids (FAs) content in food is manipulating ruminant feeding and modulating rumen BH. Tannins are phenolic compounds that are chemically classified as hydrolysable (HT) or condensed (CT). Tannins extracted from chestnut and oak are HT and those extracted from quebracho are an example of CT (Waghorn, 2008). Tannic acid (TA) is a commercial HT. Tannins have been reported to modify ruminal fermentation by inhibiting ammonia and methane production (Carulla et al., 2005) partially by their ability to form complexes with dietary protein and fiber (Waghorn, 2008). It is widely known and accepted that tannins are able to bind proteins and inhibit the growth of ruminal bacteria (Min et al., 2003). Concerning lipid metabolism, tannins were shown to inhibit Butyrivibrio fibrisolvens (Jones et al., 1994), one of the bacterial species known to be a major microbial species involved in ruminal BH (Jenkins et al., 2008). In vitro and in vivo studies have suggested that feeding tannins to ruminants can favourably alter ruminal BH of dietary linoleic acid, enhancing accumulation of VA in the rumen and thereby the content of some human health promoting FAs, such as VA and CLA in dairy or meat products. However, reports on impacts of these phenolic compounds are very limited and inconsistent. The main objective of the present study was to verify whether the TA inhibit the ruminal BH of diet containing extruded soybean seeds in vitro and whether there is a dose-dependent effect of TA.

Basal diet formulated to contain high concentrate to forage ratio (58.2:41.8) and extruded soybeans as unsaturated FA source. Experimental treatments consisted of control (C; without additive), 0.1 mg/L of monensin (M), 250 mg/L of tannic acid (TA1), 500 mg/L of tannic acid (TA2) and 750 mg/L of tannic acid (TA3). Sample of basal diet was dried at 60°C in a forced air oven for 48 h, ground to pass through a 1-mm screen using a Wiley mill. The rumen fluid was collected from two Holstein cows fed alfalfa a mixture of hay and wheat straw (700:300 g/kg on a DM basis) ad libitum. Ruminal fluid was filtered through four layers of cheese cloth and transferred quickly to the laboratory in anaerobic condition at 39° C and was continuously purged with oxygen free CO2 to ensure anaerobic conditions. The buffer solution was prepared according to Makkar et al. (1995) procedure and mixed with rumen fluid as 3:1 (v/v). Incubation was carried out in 150 mL bottles containing 200 mg of the basal diet and 40 mL of buffered rumen fluid. Gas production (GP) after 24 h incubation, ruminal volatile FA (VFA), ammonia-N and pH and FA profile were measured.

The results showed that gas production (GP) and ruminal fluid concentration of ammonia-N and volatile fatty acid (VFA) were not affected by additives (P > 0.05). Alipanahi el al. (2019) reported no effect of oak acorn (HT source) on ruminal pH and VFA concentration in lactating does. The capacity and trend of tannins to bind to specific proteins may be dependent on the type of protein (Gonzalez et al., 2002). In our experiment, it was expected to reduce ruminal ammonia-N concentrations by TA. The lack of effect of TA on ruminal ammonia-N may be related to the type of protein source and processing method. In the current experiment, we used extruded soybeans, which may loss its natural structure to bind to TA. Ruminal fluid C10:0 to C14:0 concentrations decreased and C16:0 increased by TA (P < 0.05). The M and TA1 treatments had lower ruminal fluid C18:0 concentration compared to other treatments (P < 0.05). The highest trans-11 C18:1 (VA) and cis-9, trans-11 conjugated linoleic acid (CLA) were observed in M treatment (P < 0.05). Ruminal fluid unsaturated (USFA) and long-chain (LCFA) fatty acid concentrations were increased in TA2 compared to other treatments (P < 0.05). Vasta et al (2009) reported higher VA and lower stearic acid concentrations by different sources of tannin on in vitro study, but concentrations of CLA isomers were not affected. They concluded that tannins prevented BH of USFA by inhibiting the microorganisms. Similar to our results, feeding diet containing oak acorn (as a source of HT) increased and decreased USFA and SFA concentrations in does milk fat, respectively (Alipanahi et al., 2019).

Our results confirmed that addition of TA to a diet containing extruded soybean seeds can modify BH pathways without any negative effect on ruminal fermentation. Consequently, TA has potential to enhance beneficial FA in ruminant products, although more researches need to confirm these results.