PHYSICAL AND CHEMICAL PROPERTIES OF TANNINS

Generally, tannins from different plants species have different properties and reactivities. However, CTs and HTs have some native physical and chemical properties, such as they have a three-dimensional structure and noncrys- tallized amorphous and nonnitrogenous plants products. But, in particular, tannins have astringent and bitter taste properties and some of them can be dissolved in water, especially the HTs. Their aqueous solutions show an acidic reaction and a sharp astringent taste. Most of their compounds cause precipitation of proteins, alkaloids, glycosides, gelatin, and heavy metal salts (Frutos et al., 2004; Takuo and Hideyuki, 2011).

Multiple phenolic hydroxyl tannins can create many hydrogen bonding with carbonyl groups of some macromolecules such as proteins. Figure 6.4 shows the sum of the general chemical reactivity of CTs. Nucleophilic substitution reactions occur at the hydroxyl groups introducing either acyl or alkyl groups. Acylation is commonly achieved by reaction with acid chlorides or anhydrides, while the alkylation reaction is commonly achieved through reaction with alkyl halides or epoxides (Warren et al., 2013; Helene et al., 2011).

FIGURE 6.4 Chemistry of tannins.

The chemical reactivity of tannins within the electrophilic reaction is high because of high hydroxyl group content. This functional group is a relativity strong electron donor in basic media, whereas phenoxide salts are formed during this reaction. Two examples of electrophilic reactions are halogena- tions and the reaction with aldehydes; the site of reaction with aldehydes is the A ring and only the В ring in very alkaline conditions. The nucleophilic center on the A rings of flavonoid units tends to be more reactive than that found on the В rings. This reaction is well studied due to its applications in adhesives systems (Pizzi, 2003).

SOURCES AND SIGNIFICANCE OF TANNINS

Essentially, the sources of tannins are numerous, as there are a large number of trees and shrubs that contain tannins. There are many species rich in tannins, which contain both HTs and CTs. Notable for either their present or past economic and industrial importance, tannins resources are black-wattle or black mimosa bark, quebracho wood, oak bark, chestnut wood, mangrove, eucalyptus bark, tara, barks of several pines, and many more. Moreover, tannins can range between 2% and 40% of the diy bark of many trees species; such values may render the product economically viable for industrial utilization. Tannins are most commonly obtained from the wood, bark, and leaves of different plants; the bark of Picea abies can contain up to 15% tannins, some mangrove species can possess up to 35% tannins, and the tannins content varied between 30% and 35% in the case of the quebracho tree (Michel et al., 2011).

Tannins from the bark of the black-wattle and quebracho trees are considered to be the most important sources for CTs when produced at industrial scales. In Brazil, works involved with tannins, including wood gluing, were initiated in the late 1970s and the early 1980s. The authors in the previous reference (i.e., Michel et al., 2011) reported that, according to Sociedade Brasileira de Silvicultura, SBS, approximately 3000 companies in Brazil use pine wood in their production processes, concentrating on sawn wood, cellulose, papers, and panels (plywood, MDF, and OSB); pine and other trees barks are a residue for the majority of the wood industries, presenting an environmental and economic challenges upon disposal.

The international markets for tannin extracts of vegetable origin produce about 160,000 tons per annum (TPA), including approximately 100,000 TPA originating from acacia plantation and the remainder is sourced from other species, such as castanheira (Italy), quebracho (Argentina), and tara (Peruvian) (Michel et al., 2011; Antwi-Boasiako and Animapauh, 2012). Practically, tannins are extractable from wood and bark by soaking or percolating in water or in some aqueous solutions or by organic solvent extraction. Researchers have extracted tannins from the bark of Pin us radiate and Acacia, leaves of sumac, seeds of pigeon pea. flowers and leaves of Crataegus, membrane of pecan nut, bark of mimosa, and wood of quebracho.

In Malaysia, tannins have been extracted from Phyllanthus niruri or known as “dukung anak” and from mangrove bark of Rhizophora opiculata. The bark of mangrove is a waste product from the charcoal industry in Malaysia. By using the solid-liquid extraction process with about 70% aqueous acetone solution, 34.68 wt% of tannins can be isolated from the mangrove bark; therefore, the barks of these trees can be considered a potentially affordable and abundantly available source of tannins (Raquez et al., 2010; Chuan Wei, 2008).

Tannins are extracted using different solvents such as methanol, ethanol, water, ethers, NaOH, sodium carbonate or bisulfite solutions, and other alkali. Many teclmiques such as water extraction with the addition of chemicals, solvent extraction, and fractionation extraction with ultrafiltration have also been developed in an attempt to increase the yield of bark tannins and improve their quality for wood adhesives. However, only the water extraction processes, with or without the addition of chemicals, is relatively economically feasible (Antwi-Boasiako and Animapauh, 2012).

The objectives of study, which is earned out by Michel et al. (2011), were to evaluate the tannin extractions from the bark of Pinus occarpa by using water and water solutions containing sodium carbonate and sodium bisulfate; the results of that study have revealed that it is possible to obtain high yield (~39%) of tannins from the bark by using extraction when coupled with the addition of sodium salts. Generally, the solid-liquid extraction is still the most common procedure to extract and analyze the phenolic fraction of plant sources due to its simplicity, high efficiency, and easy tunability. Eucalyptus bark is separated as a waste product from eucalyptus wood when used as a raw material, mainly to produce cellulose pulp and secondary to produce panels and boards, and is also used as a fuel. So, this bark represents an important and industrial resource for tannin production at industrial scales (Onifade, 2001).