Microwave-Assisted Autohydrolysis of Prunus mume Stone for Extraction of Polysaccharides and Phenolic Compounds

Stone of Prunus mume (P. mume) is a by-product of pickled P. mume industry. Stones of native and pickled P. mume, mainly composed of holocellulose (83.8 ± 1.8% and 65.1 ± 0.3%, respectively) and acid-insoluble lignin (25.3 ± 2.2% and 30.6 ± 0.9%, respectively), were autohydrolyzed by microwave heating to extract polysaccharides and phenolic compounds. By heating at 200 to 230 °C, 48.0% to 60.8% of polysaccharide and 84.1% to 97.9% of phenolic compound were extracted in water along with partial degradation of hemicelluloses and lignin. The extracted liquors showed antioxidant activity against hydroxyl radical and DPPH radical originated from phenolic compounds. The pickled P. mume stone showed higher autohydrolyzability and microwave absorption capacity than the native stone due to absorbed salts and acids during pickling in fruit juice of P. mume with external addition of sodium chloride. Pickling process in salty and weak acidic juice seemed to be a kind of pretreatment for softening the stones prior to autohydrolysis induced by microwave heating.