Effects of heat pump drying and superfine grinding on the composition of bound phenolics, morphology and microstructure of lychee juice by-products

Abstract

The effects of drying (heat pump and hot air, HPD and HAD) and grinding (superfine and ordinary, SG and OG) on the content and composition of bound phenolic, morphology and microstructure of lychee juice by-production (lychee pulp pomace, LPP) were investigated. It was observed that the bound phenolic content in LPP powder dried by heat pump was higher (44.8%) than powder that was dried with hot air. It was also observed that the bound phenolic content in HPD + SG LPP powder was significantly higher (20.8%) than that of HPD + OG powder Compared with LPP powder, from the analysis of scanning electron microscopy (SEM) imagery, it was found that the LPP had become loose and broken after hydrolyzing the bound phenolic from LPP powder. The Fourier transform infrared (FTIR) spectroscopy measurements confirmed the structure had changed significantly after the loss of bound phenolic. Furthermore, the water holding capacity, water solubility and swelling capacity of HPD + SG powder was found to better than those of the HPD + OG powder. Overall, heat pump drying with superfine grinding was found to have improved the bound phenolic content and change the physical properties of the processed pulp powder, thus, providing a theoretical basis for the product development of LPP.