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.
Highlights
By-products of fruit and vegetable processing have been attracting increasing attention in the fields of food research and industry as affordable, sustainable, and natural sources of both dietary fiber [1,2] and low-molecular-weight bioactive compounds [1,3,4]
To increase the amount of dietary fiber without having a negative impact on product’s properties, and to utilize the full potential of by-products, significant research is being undertaken on the modification of functional properties, e.g., water absorption and gelling of dietary fiber by various methods such as chemical, enzymatic, thermal, and mechanical treatment [6,14,18,19]
Reducing the water content from 42% to 22% resulted in a strong increase in specific mechanical energy (SME) due to the higher viscosity of the melt, whereas a further decrease in water content, i.e., to 17%, led to a slight increase only or even to no further increase in SME
Summary
By-products of fruit and vegetable processing have been attracting increasing attention in the fields of food research and industry as affordable, sustainable, and natural sources of both dietary fiber [1,2] and low-molecular-weight bioactive compounds [1,3,4]. They show a wide variety of techno-functional properties, such as water- and oil-absorbing and thickening and gelling properties [5,6,7]. Modification is often realized through a combination of thermal and mechanical stresses at high concentrations without any need for the use of chemicals
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