Optimization of microwave-assisted extraction of phenolic compounds from chestnut processing waste using response surface methodology

Portugal is one of the world's top ten chestnut producers. The nut shells generated in the industrial processing have been discarded for use as soil conditioner or energy production. This waste material is rich in polyphenols, which can be used for various purposes, including well-established applications and new usages. Tannin-based coagulants/flocculants for water treatment are one of the emerging applications. Polyphenols extraction from vegetal sources is the initial step of the coagulant synthesis. In this work, tannin extraction from chestnut shell was accomplished in aqueous solution through different methods. After a preliminary assessment, microwaveassisted method (MAE) was selected to be optimized using Response Surface Methodology. The effect of extraction time, liquid:solid ratio, and temperature on the extraction yield and total phenolic content of the extracts was examined. The desirability function approach was applied to find the best extraction conditions leading to a quantitative extract with the highest phenolic content. The optimum conditions were determined as extraction time of 5 min, liquid-to-solid ratio of 50 mL g-1 and temperature 107 degrees C. In these optimized conditions, the expected extraction yield and phenolic content of the extract are 25 +/- 3% and 344 +/- 27 mg-GAE/g, respectively (values confirmed by experimental assays). The optimal extract presented a condensed tannin content of 1296 +/- 68 mg-CE/g. This work showed that polyphenols can be quantitatively extracted from chestnut shells through a green approach and the generated extract has good features to be further used as a raw material for water treatment coagulants.