Optimisation of Polyphenols Extraction from Chestnut Shell by Response Surface Methodology

Purpose The extraction of polyphenols from a waste product from the food industry, the shell of the chestnut fruit, was examined with the aim of analyzing the potential of the extracts for the formulation of wood adhesives, for leather tanning and as natural antioxidants. Methods Experiments were planned according to a 2(3) factorial design to analyze the influence of temperature and Na2SO3 and NaOH concentrations on extraction yield and on different extract properties: Stiasny number, total phenols, tannins and non-tannins contents, tannin/soluble solids ratio, proanthocyanidins content, antioxidant activity, average molecular weights, surface tension and power law rheological parameters of extract aqueous solutions. Results Extraction yields in the range 26.1-56.4% together with extract Stiasny numbers higher than 84 and total phenols contents and FRAP antioxidant activities in the range 10.86-33.33 g GAE/100 g shell and 92.05-166.32 mmol AAE/100 g shell, respectively, demonstrated the potential of chestnut shell extracts in the formulation of adhesives and as sources of antioxidant compounds. Additionally, the surfactant character and pseudoplastic behaviour of chestnut shell extracts solutions could improve wood wettability when used in adhesive formulation. Under a range of operating conditions, extracts with tannin content higher than 10% and tannin/soluble solids ratio higher than 0.6 were obtained that could be used as tanning agents. Conclusions The experimental design performed demonstrated the capacity of chestnut shell extracts for the applications proposed and allowed selecting the optimal extraction conditions for obtaining not only high quality extracts but also high extraction yields.