Condensed Tannins Extraction From Grape Pomace: Characterization And Utilization As Tannin-based Adhesives

Grape pomace is a by-product of winemaking industry, which retains high level ofcondensed tannins. Until now, most of the grape pomace is not exploited but only used for soilfertilizer or animal feed. Therefore, the purpose of this study is to extract condensed tannins fromgrape pomace, character its composition and structure by modern instruments analyticaltechniques, and use as tannin-based adhesive for particleboard productions in wood industry. Itnot only proposed a feasibility plan for the efficient use of grape pomace, extend the grapeindustry chain; but also provide a theoretical foundation of preparation and applicationtechnology of tannin-based adhesive in wood industry.The extraction of condensed tannins from grape pomace was examined using water mediumin the presence of different bases as catalyst （NaOH, Na₂CO₃, NaHCO₃and Na₂SO₃）. Twodifferent extraction processes and the influence of4parameters （i.e., temperature, reaction time,chemical reagents and concentration of the chemical reagents） on the tannin extracts yield andproperties were studied. The tannin fractions were characterized by Element analysis, Fouriertransform-infrared spectroscopy （FT-IR）, thermogravimetric analysis （TGA）, carbon nuclearmagnetic resonance spectroscopy (¹³C-NMR) and matrix assisted laser desorption ionization timeof flight mass spectrometry （MALDI-TOF）. Condensed tannins extracts from grape pomace wereapplied to prepare different tannins-formaldehyde formulations, which were applied to presswood particle board productions. The purpose is to investigate the possibility of using these grapepomace tannins in the formulation of wood adhesive and the influence of different tanninsextracts and formulations on the properties of particleboard productions.It was demonstrated that it is possible to extract reactive tannins from grape pomace in basicmedium. The tannin extracts obtained by precipitation in acidic conditions display a highphenolic content （Stiasny number higher95%） but low water solubility and low reactivitytoward formaldehyde because of the formation of catechinic acid phlobatannins during theacidification step. The tannins extracts obtained by lyophilization of the liquid, despite of theirlower phenolic contents, displayed promising properties for adhesive applications. The optimumtemperature of the extraction process was100°C, reaction time was120min; the bestconcentration of reagent was10%（w/w）. It was also shown that addition of sulphite ions duringthe extraction step improved the process: the introduction of a sulfonic group through sulfitationincreased both tannin solubility and reactivity as a result from the opening of the heterocyclicring during extraction.The structure of grape pomace sulfited tannin extracts did not present noticeablediscrepancy exception of the ratio of the opening pyran ring which was different as a function ofthe catalyst used. The opening of pyran ring during the tannins extraction seem to be moreimportant by using Na₂CO₃/Na₂SO₃and NaHCO₃/Na₂SO₃than when using NaOH/Na₂SO₃. TheFT-IR bands assigned to aromatic ring vibration and carbonyl groups were stronger and no bands attributed to sulfited groups were detected in acidified tannins compared to lyophilized tannins.The TG results showed that the weight lost of these tannin extracts mainly composed of two steps.The first step starts from room temperature to200°C. It is attributed to the mass loss of waterand some easy-degraded small low molecules. The secondly steps which is the mainlydegradation step of tannin extract samples from200to400°C. It is attributed to the mass loss ofpolyphenol materials in the tannin extracts.The results from¹³C-NMR showed that condensed tannin extracts from grape pomace wereconsistent with dominant procyanidin units with a minor amount of prodelphinidin units that arelinked together by a C4-C8bond; Relatively low carbonhydrate and high catechinc acid contentwas obeserved in acidified tannins compared to lyophilized tannins. It was shown by MADI-TOFexperiments that grape pomace tannin extracts are mainly composed of flavoinoid oligomers upto6repeating units in lyophilized tannins and10repeating units in acidified tannins respectively,with dominant procyanidin units. A small proportion of substitution with glucose and gallic acidwas detected in procyanidin units of polyflavonoid oligomers. The degree of polymerization ofacidified tannins is higher than lyophilized tannins.Two different formulations （nonfortified tannin–formaldehyde adhesive and fortified withaddition of20%of polymeric4,4’-diphenyl methane diisocyanate （p-MDI） tannin–formaldehyde adhesive） were used to press one layer wood particle board. It was shown that thenature and concentration of the extraction reagents greatly impact the properties of the tanninresins MOE value. When the tannin was extracted by sodium hydroxide （NaOH）, the highestMOE value was reached for the concentration of NaOH is10%. When the tannin was extractedby sulfitaion reagents, the nature of the extraction reagents greatly impact the MOE value, thetannin extracted by Na₂CO₃/Na₂SO₃is higher than the tannin extracted by NaOH/Na₂SO₃andNaHCO₃/Na₂SO₃. The addition of p-MDI did not yield an important increase in the MOE value.The tannin properties were affected by the nature and concentration of the extractionreagents, then directly impacted the IB value of the wood particleboard which press by tanninbased adhesive. The addition of p-MDI yielded an obviously improved the IB strength. Whenused the nonfortified tannin adhesive, the best IB values is0.45MPa, used the p-MDI fortifiedtannin adhesive, the best IB value is0.68MPa. The wood particleboards bonded by the tanninsextracted using Na₂CO₃/Na₂SO₃as catalyst give the best performance and were good enough topass relevant international standard specifications for interior grade panels according to EN312and China National Standard GB-T4897.3（IB≥0.35MPa）. Formaldehyde emission of thesepanels was below the European Norm requirements （≤6.5mg/100g panel）. It proves that thetannin extracted from grape pomace can be used as a kind of environmentally friendly naturaladhesive in wood industry.