Modification of Apple Pomace by Extrusion Processing: Studies on the Composition, Polymer Structures, and Functional Properties

Abstract

By-products of fruit and vegetable processing are an inexpensive and sustainable source of dietary fiber, potentially offering valuable functional properties such as water binding and thickening. Due to these favorable properties, they can be utilized to reformulate widely-consumed foods, e.g., bakery products or beverages. In this study, apple pomace was used as a model system to study whether extrusion technology affects food by-product functionality and thus has the potential to broaden the application of by-products in foods. The effect of the process parameters and the extent of thermo-mechanical treatment on the structural and functional properties of apple pomace were analyzed after extrusion trials using various screw speeds, water contents, and barrel temperatures. Compared to the raw material, apple pomace extruded at Tbarrel = 100 °C, n = 700 min−1 and mH2O = 17% showed an increased water solubility up to 33%. The water absorption increased from 5 to 19 Pa·s and the paste viscosity from 5 to 339 Pa·s by extrusion processing. Analyses of dietary fiber contents and fiber polysaccharide structures revealed that thermo-mechanical stress (n = 700 min−1, mH2O = 22%) increased the content of soluble dietary fiber from 12.5 to 16.7 g/100 g dry matter, and that the harshest conditions even enabled the formation of low-molecular-weight dietary fiber. Arabinans (as neutral rhamnogalacturonan I side chains) appeared to be most sensitive to thermo-mechanical stress, whereas xylans (i.e., a group of minor polysaccharides) were an example of a more stable fiber polysaccharide. Also, the degree of methylation of the pectic polysaccharides was strongly reduced from 50% to 15% when thermo-mechanical stress was applied. Imaging and pore size analysis showed that extrusion processing could disrupt the rigid cell wall macromolecular structure.

The Influence of Extrusion Processing on the Gelation Properties of Apple Pomace Dispersions: Involved Cell Wall Components and Their Gelation Kinetics

“…In our previous study [21], we showed that the complex viscosity of apple pomace can be increased by extrusion processing. Table 1 gives an overview of the process conditions and the resulting functional properties.…”

“…Table 1. The process parameters (specific mechanical energy (SME), material temperature (T M )) and complex viscosity of raw and extruded apple pomace from extrusion trails at a barrel temperature of 100 • C and a total feed rate of 10 kg•h −1 (published in [21]). To characterize the crucial components for the formation of high viscous dispersions, the impact of each fraction on the viscosity was investigated.…”

“…The main monosaccharide of soluble dietary fibers was arabinose (42.0 mol%), followed by galacturonic acid (32.9 mol%). More detailed information about chemical structures and composition are described in Schmid et al [21]. The used chemicals (ethanol (96%), calcium chloride, sucrose, and citric acid) were of analytical purity grade and purchased from Carl Roth (Karlsruhe, Germany).…”

“…Their influence on the complex viscosity was investigated in the following sections. Figure 4 shows the water solubility index (WSI) of raw material in comparison to extruded samples (data published in [21]). The water-soluble part contains mainly pectin, but also hemicelluloses and several mono-and disaccharides.…”

“…Previous studies have shown that its water solubility, thickening, and gelling properties can significantly be improved [16][17][18][19]. In particular, a change in the viscosity of dispersions of water and extruded by-products of citrus and apple is described [20,21]. However, the reason for the change in viscosity is still under investigation.…”