The selective biodegradation of grape pomace tannins was studied using Aspergillus niger strains. The conditions were optimized in terms of monomeric catechin production. The optimal degradation conditions were: 10.0 g·L -1 of tannins in the induced medium, pH 6.5, and cultured on a shaking table at a rate of 120 r·min -1 at 28 °C for 36 h. The biodegraded tannins were used to partially substitute for phenol-formaldehyde in adhesive formulations. The biodegradation enabled the production of adhesives and wood panels with better properties (lower resin viscosity, higher internal bond strength, and lower thickness swelling). A particleboard in which the resin contained 60% biodegraded grape tannins displayed good performances (IBdry = 0.46 MPa, IBwet = 0.15 MPa) and passed the standard specifications for dry and wet conditions.
Keywords
INTRODUCTIONVegetable tannins are polyphenolic substances that are widely present in the plant kingdom (such as bark, wood, fruits, leaves, roots, and needles) as secondary metabolites for protective purposes, with molecular weights between 500 Da and 30,000 Da. Vegetable tannins are important renewable natural resources and have been used industrially in a wide range of applications. On the basis of their chemical structural characteristics, tannins are divided into hydrolysable tannins and condensed tannins (Venter et al. 2012;Radebe et al. 2013;Ricci et al. 2015). Condensed tannins obtained from natural resources have been used for the production of wood adhesives since the 1970s. They can be used to prepare tannin-based adhesives and/or to replace part of the phenol in phenol-formaldehyde adhesives (Pizzi 1994(Pizzi , 2006Lei et al. 2008;Tondi and Pizzi 2009;Kemppainen et al. 2014;Grasel et al. 2016).Grape pomace, consisting of skins, seeds, and stems, is a co-product generated by grape juice and wine-making processes and retains a high level of condensed tannins (20% to 30% of the initial weight) (Llobera and Canellas 2007;Zocca et al. 2007;Rondeau et al. 2013;Bosso et al. 2016). In previous works, the authors described the optimization of the extraction of crude condensed tannin extracts from grape pomace using water as a solvent, in the presence of a base (carbonate or sodium hydroxide) and sodium sulfite. The tannins extracted from grape pomace mainly consist of phloroglucinol A-ring procyanidin polymers with a minor amount of prodelphinidin linked through a C4-C8 linear bond, and a part of procyanidins that contain glucose and gallic acid substituents. It has been demonstrated that these extracts can be successfully employed in the formulation of tannin-PEER-REVIEWED ARTICLE bioresources.com Lan et al. (2018). "Grape tannins in wood adhesive," BioResources 13(1), 894-905. 895formaldehyde-based adhesives usable for the production of wood panels (Ping et al. 2011a(Ping et al. ,b, 2012. However, it was concluded that, compared with tannins extracted from barks, the performance of some of the pomace-based formulations was poor. This was probably due to the large variability...