Microwave Heating of Tea Residue Yields Polysaccharides, Polyphenols, and Plant Biopolyester

Microwave heating was used to produce aqueous-soluble components from green, oolong, and black tea residues. Heating at 200−230 °C for 2 min extracted 40−50% of polysaccharides and 60−70% of the polyphenols. Solubilization of arabinose and galactose by autohydrolysis occurred with heating above 170 °C, whereas heating above 200 °C was necessary to solubilize xylose. Catechins were soluble in water by heating at low temperature (110 °C); however, new polyphenols having strong antioxidant activity were produced above 200 °C. The amount of solubilized materials and antioxidant activity increased with increased fermentation of harvested tea leaves (green tea < oolong tea < black tea). Cutin, a plant biopolyester, remained in the residue after heating as did cellulose and lignin/tannin. The predominant cutin monomer that was recovered was 9,10-epoxy-18-hydroxyoctadecanoic acid, followed by dihydroxyhexadecanoic acid and 9,10,18-trihydroxyoctadecanoic acid.