Mechanistic study of the solid-liquid extraction of phenolics from walnut pellicle fibers enhanced by ultrasound, microwave and mechanical agitation forces.

The molecular diffusion of phenolics inside walnut pellicle fiber particles under solid-liquid extraction enhanced by ultrasound (US), orbital agitation (OA), impeller agitation (IA), and the combined microwave and impeller agitation (MW-IA) were explored. Numerical modeling considering the temperature-dependent diffusivity revealed that the internal diffusivity of phenolics was the highest under MW-IA and the lowest under OA. At 35 °C, IA (126.246 mg/g, t = 10 min) was more effective to strengthen the phenolic diffusivity than the US-39W (95.538 mg/g, t = 10 min). Due to the simultaneous enhancement of internal diffusivity and external dissolution, the MW-IA extraction reached equilibrium within 16 min, reaching the highest yield among all the treatments (176.944 mg/g). The extraction was then divided into the increasing and falling driving force periods. The comparison of phenolic diffusivity among MW-IA and IA roughly indicated that the proportion of non-thermal effect of microwave at 315 W was 889% higher than 189 W at the extraction equilibrium. Moreover, some soluble polyphenols, i.e. quercitrin and syringic acid, could be adsorbed by the cell wall after equilibrium. The correlations between any two soluble phenolics varied with the phenolic type, deducing that soluble phenolics may interact with each other either positively or negatively. Besides, the mining of phenolic data also indicated that intensive impeller agitation is a good substitution for ultrasonication to extract phenolics effectively.