Large amounts of residues are produced in the food industries. The waste shells from cocoa processing are usually burnt for fuel or used as a mulch in gardens to add nutrients to soil and to suppress weeds. The objectives of this work were: (a) to separate valuable compounds from cocoa shell by applying sustainable green separation process—subcritical water extraction (SWE); (b) identification and quantification of active compounds, sugars and sugar degradation products in obtained extracts using HPLC; (c) characterization of the antioxidant activity of extracts; (d) optimization of separation process using response surface methodology (RSM). Depending on applied extraction conditions, different concentration of theobromine, caffeine, theophylline, epicatechin, catechin, chlorogenic acid and gallic acid were determined in the extracts obtained by subcritical water. Furthermore, mannose, glucose, xylose, arabinose, rhamnose and fucose were detected as well as their important degradation products such as 5-hydroxymethylfurfural (5-HMF), furfural, levulinic acid, lactic acid and formic acid.

Cellulose was treated with subcritical water in a batch reactor within a temperature range of 200–300 °C and reaction time of 5–60 min. The main phases, such as water-soluble fraction, acetone-soluble fraction and solid residue (remaining cellulose or char), were separated and analyzed. The analysis of water-soluble phase was done by HPLC equipped with UV and RI detector, whereas acetone-soluble phase was analyzed by GC–MS. Total sugar content was determined by the phenol-sulfuric acid colorimetric method. The properties of char such as specific surface area, pore volume, and pore diameter were determined by gas adsorption method. A water-soluble phase mainly consists of sugar monomers and monomer degradation products, while acetone-soluble phase, referred to also as bio-oil, consists of furans, phenols, carboxylic acids, aldehydes, ketones, and high molecular compounds. The reaction mechanism of cellulose in subcritical water has been proposed based on the obtained results.

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Sweet chestnut tannins were treated with subcritical water at temperatures from 120 to 300 °C for reaction times of 15, 30 and 60 min. A great influence of temperature and reaction time on the product yield was noticed. Spectrophotometric methods were used to determine the total tannins, phenols and carbohydrates contents and antioxidant activity. Furthermore, vescalin, castalin, vescalagin, castalagin, 1-O-galloyl castalagin, gallic, ellagic and ferulic acids were analysed by HPLC. The results obtained from hydrothermal hydrolysis were compared to results from acid hydrolysis. Finally, the reaction parameters of the hydrothermal hydrolysis process were optimized aimed at obtaining a product with a high concentration of ellagic acid. The optimal conditions for obtaining the highest concentration of ellagic acid of 29.55 % were 250 °C and 5 min. The concentration of ellagic acid in tannin extract obtained by acid hydrolysis was 8.19 %.

The aim of the work was the optimization of the subcritical water extraction process of chestnut bark using Box–Behnken response surface methodology. The influence of process parameters, such as temperature, extraction time and solvent-solid ratio, on extraction yield, yield of the main compounds, total phenol content, total tannin content and antioxidant activity has been investigated. The identified compounds were ellagic and gallic acids, ellagitannins (vescalagin, castalagin, 1-o-galloyl castalagin, vescalin and castalin), sugars (maltose, glucose, fructose and arabinose) and sugar derivatives (5-HMF, furfural and levulinic acid). Finally, the optimal process conditions for obtaining the bark extract highly rich in ellagic acid and with satisfactory levels of total phenols and total tannins have been determined.