Ultrasound Assisted Extraction Of Phenolics And Their Stability From Citrus Peel

Phenolic acids and flavonoids are two main types of polyphenols, with many health benefits, such as antioxidant activity, anti-inflammatory activity and anti-tumor activity. They are ubiquitously presenting in fruits and vegetables, among which citrus fruits is one of the superstars containing large amount of polyphenol, especially in non-eatable parts like peel. Many researches demonstrated polyphenols extracted could serve as ingredients for functional food, which makes citrus by-products a good source of polyphenols. Nowadays, a few researches found the commonly used ultrasound assisted extraction (UAE) had diverse effect on bioactive components. Therefore, it is necessary to investigate the stability of phenolic acids and flavonoids during sonication treatment.This dissertation was to evaluate the stability of representative phenolic acids and flavonoids subjected to ultrasound, and further explored their sonochemical degradation mechanism. Moreover, this research focused on the extraction principles of phenolics acids, flavonoids, total pehnolics and total flavonoids from citrus peel (Ponkan) during UAE. The main results and conclusions were listed as follows:1. The stability of 7 phenolic acids (protocatechuic, p-hydrobenzoic, vanillic, p-coumaric, ferulic, caffeic and sinapic) and 14 flavonoids (eriocitrin, narirutin, neohesperidin, quercitrin, eriodictyol, didymin, quercetin, naringenin, luteolin, sinensetin, nobiletin, tangeretin, naringin and hesperidin) treated by ultrasound were evaluated in model systems. Amongst the detected polyphenols,5 phenolic acids and 13 flavonids were stable, only caffeic and sinapic along with quercetin were degraded by sonication. Solvent and temperature were important factors, others including ultrasound intensity, duty cycle, duration and liquid height only changed degradation rate instead of degradation nature. The worst sonochemical degradation of sinapic happened in water, while for caffeic and quercetin occurred in 80% ethanol aqueous. The higher the temperature was applied, the lower degradation rate was obtained, which was contrary to the Arrhenius theory.2. The corresponding degradation products were tentatively identified by FTIR and HPLC-ESI-MS. During -5-25℃, the degradation kinetics of caffeic and sinapic under UAE fitted zero-order reaction. Caffeic and sinapic were speculated to decompose and polymerize during UAE based on their dimmers and decarboxylation products. While for quercetin, there were a total of four types of reactions occurred simultaneously, including decomposition, polymerization, oxidation and addition during sonication.3. The ultrasound extraction principles of phenolic acids, flavonoids, total phenolics and total flavanoids were comprehensively investigated. The majority of phenolics were hydrocinamic acids, while hydrobenzoic acids were a little. In terms of flavonids, three-quarters were glycoside type, another one quarter were polymethoxyflavones. No matter for individual phenolic acids/flavonids or for total phenolics/total flavonids, the optimized ultrasound extraction condition could be unified using methanol with small amount of water at relatively higher temperature in short sonication duration (methanol/80% methanol at 45℃ in 10-20min). Although most of the time, ultrasound promoted polyphenols extraction, while it had side effect to some specific polyphenols stability sometime. Longer ultrasound treatment in mixture of water alcohol reducted caffeic acid and sinapic acid yield, and 25℃ was against their stability. Similar phenomenon was observed for hesperidin, the lower temperature was applied, the samller yiled was obtained. Additionly, using ethanol at 45℃ was not suitable for extraction polymethoxyflavones such as nobiletin and tangeretin.