Defined shear and heat treatment of apple pomace: impact on dietary fiber structures and functional properties

Food by-products can be modified by extrusion processing. However, the impact of thermal and mechanical stress, respectively, on the structure and thus functional properties of dietary fiber-rich food by-products is still unknown. In the extrusion process, thermal and mechanical stress are coupled, not constant, and difficult to measure or calculate. Thus, their influence on structural changes and functional properties cannot be evaluated separately. In this work, a specific shear cell, denoted by closed cavity rheometer, was used to treat apple pomace with defined thermal and/or mechanical stress. Dietary fiber composition and fiber polysaccharide structures appeared to be more susceptible to high temperatures than mechanical stress. With increasing temperature (and mechanical stress) soluble and low-molecular-weight soluble dietary fiber contents increased, whereas insoluble fiber contents decreased. Arabinans as rhamnogalacturonan type I polysaccharides and galacturonic acid containing pectic polysaccharides were identified as being most susceptible to degradation under these conditions. Furthermore, the defined treatment affected the functional properties. Although changes in the water solubility index (WSI) and/or the water absorption index (WAI) were not detected up to 90 degrees C, WSI and WAI decreased significantly at a treatment temperature of 120 degrees C. However, at very high temperatures (160 degrees C), WSI and WAI increased. The application of shear and longer treatment times resulted in higher WSI values and complex viscosities as compared to low shear stress. [GRAPHICS] .

Automated summary

Extrusion processing can modify food by-products, with thermal stress having a greater impact on dietary fiber composition and polysaccharide structures than mechanical stress. High temperatures increase soluble fiber contents while decreasing insoluble fiber contents, leading to changes in functional properties such as water solubility and absorption index. Arabinans and galacturonic acid containing pectic polysaccharides are identified as the most susceptible to degradation under these conditions, indicating the importance of temperature control in food processing.