Variation of Tannin Contents in Selected Agro-Industrial By-products and their Biological Activity in Precipitating Protein

that feeding of black tea grounds at a 5% DM of the total diet to goats showed lower CP digestibility and higher excretion of acid-detergent insoluble proteins; these could be interpreted as tannin-protein complexes (Kondo et al., 2007). In contrast with PPC on a DM basis, two types of barks also showed higher levels of PPC on a TEPH basis among the by-products in the present study. These results indicated that the barks contained low amounts of extractable phenolics, but these compounds could bind more protein than phenolics found in other by-products. According to comparable PPC on a TEPH basis between barks and black tea grounds, pheolics in barks possibly make complexes with proteins and the complexes would be too tight to break in digestive tract in ruminant, similar to those found in black tea grounds. The phenolics in black tea grounds and two kinds of bark could be useful for anthelmintic effect, which requires a strong protein binding capacity in the intestine (Hoste et al., 2006). Similar to the barks, phenolics in the husks from chestnut, peanut, and cacao also showed relatively higher PPC on a TEPH basis as compared with other by-products. Particularly, peanut husk and cacao husk showed higher levels of PPC when it was expressed on a TEPH basis. This result also indicates that the phenolics in these husks can bind so much protein that even the TEPH contents were low. Furthermore, banana peel, grape skin, winery residue, and green tea grounds showed low or moderate PPC on a TEPH basis. Our previous data showed that phenolics in green tea grounds protect proteins from degradation in the rumen, but do not strongly inhibit protein digestion in the post-rumen (Kondo et al., 2014). According to the present data, phenolics in banana peel, grape skin and winery residue may also bind proteins moderately in the rumen, but they may exhibit lower binding activity in post-rumen, as similar to green tea ground. Min et al. (2003) reported that moderate levels of tannins protect protein degradation in the rumen and increase amino acid absorption in the small intestine, thereby improving the growth rate in lambs. Further research is needed to determine whether the by-products with moderate PPC increase the efficiency of protein digestion in ruminants.

Figure 2 shows simple correlations between PPC on a DM basis and TEPH content (left) or with CT content (right). The correlation coefficients of PPC with TEPH and CT were 0.851 (p<0.001) and 0.468 (p=0.078), respectively. These results are similar to those observed for some agro-industrial by-products in India (Makkar et al., 1990). Significant correlation between PPC and TEPH suggests that the PPC is influenced by TEPH and not by CT. Not only content but also structures of tannins vary widely among different plant materials (Naumann et al., 2014). In addition to CT in plants, hydrolysable tannins and simple phenolic compounds such as gallates and flavonoid monomers that can be detected by TEPH assay, may bind to proteins. Parameters such as TEPH would be acceptable in understanding the general characteristics of tannins in agro-industrial by-products.

In conclusion, based on the information of the chemical compositions and PPC (Table 1), the by-products examined in the present study can be classified into 5 groups; group 1 showed high PPC both on DM and TEPH basis (black tea grounds, oolong tea grounds and chestnut husk), group 2 showed low TEPH content, but high PPC (peanut husk, cacao husk, ceder bark and cypress bark), group 3 showed high CP and moderate PPC (green tea grounds), group 4 showed low CP and moderate PPC (banana peel, grape skin and winery residue) and group 5 showed no PPC (orange peel, grape fruit peel, lemon peel and coffee grounds). For livestock production, group 1 by-products may have the potential for use as anthelmintic agents without any

Figure 2: Correlation between protein precipitating capacity with the contents of total extractable phenolics (left) and condensed tannins (right)

Table 1: Chemical compositions of agro-industrial by-products

CP: crude protein; ADF: acid detergent fiber; ADL: acid detergent lignin; TEPH: total extractable phenolics; CT: condensed tannin

pretreatment, whereas group 2 by-products may have the same potential after extraction and condensation of effective fractions. Among these by-products, four by-products in group 3 and 4 showing moderate PPC can be considered as feedstuffs that protect protein degradation in the rumen; particularly green tea grounds would be potentially preferred as protein supplements due to high CP content and moderate PPC. For the future implications of these by-products, inhibitory interactions between tannins and rumen microorganisms should also be investigated since tannins may inhibit the growth of microoganisms due to their PPC (McSweeney et al., 2001). Further researches using animal experiments are necessary to understand both positive and negative the biological activities of tannins not only on protein utilization but also on microbiome in ruminant with related to tannin contents and PPC.

This work was supported by JSPS KAKENHI Grant Number 15K07272.

Makoto Kondo carried out the experiment, and wrote the draft of manuscript. Anuraga Jayanegara performed the statistics and provided technical support. Yutaka Uyeno and Hiroki Matsui checked the experimental design and supported in writing the manuscript. All authors revised the manuscript.

The authors have no conflict of interests.