Solubilization of apple pomace by extrusion

Apple pomace (AP), a fibrous by-product of apple juice processing, was solubilized by a twin-screw extruder. AP was extruded under 14 different operating conditions, where the process variables are controlled in the following ranges: screw speed, 250-350 rpm; feed rate, 30-40 kg/h; moisture, 20-30%. Water soluble index (WSI) increased linearly with specific mechanical energy (SME). Soluble dietary fiber of extruded AP also increased with SME, which in turn decreased the content of insoluble dietary fiber (IDF). As a result, total dietary fiber (TDF) remained nearly unchanged. Water holding capacity (WHC) remained nearly constant at low SME, but markedly deceased at high SME due to disintegration of cell wall. Gel filtration of WSP exhibited the structural fragmentation of cell wall by extrusion: the higher SME, the higher fragmentation of SME. Ion exchange chromatography of WSP demonstrated that increasing SME resulted in increasing the extractability of free neutral polysaccharides from the cell wall of AP. It was also found that the degradation of AP cell wall by extrusion preferentially occurred at the arabinogalactan sidechains of pectins.