Effect of air-drying temperature on physico-chemical properties of dietary fibre and antioxidant capacity of orange (Citrus aurantium v. Canoneta) by-products M. Carme Garau, Susana Simal, Carmen Rossello´ , Antoni Femenia * Department of Chemistry, Universitat de les Illes Balears, Ctra. Valldemossa km 7.5, 07122 Palma de Mallorca, Spain Received 6 March 2006; received in revised form 12 January 2007; accepted 12 January 2007 Abstract Dehydration promoted important modifications affecting both the physico-chemical properties of dietary fibre (DF) and the antiox- idant capacity of orange by-products (peel and pulp remaining after juice extraction). The significance of such changes was largely depen- dent on the air-drying temperature used (from 30 °C to 90 °C). The major modifications on the DF components were observed when either extended drying periods, i.e. at lower temperatures, or elevated drying temperatures were applied. Dehydration around 50–60 °C apparently promoted the minor disruption of cell wall polymers, in particular of pectic substances. Pulp samples exhibited higher values of swelling (SW) and fat adsorption capacity (FAC) than those derived from orange peel. Although, significant decreases in water retention capacity (WRC), FAC and solubility values were detected for both by-products as the air-drying temperature increased. The antioxidant capacity associated to dehydrated citrus by-products was significantly higher for orange peel than for pulp samples. In general, the by-products studied proved to be quite resistant to the different heat treatments applied within the range of 40–70 °C. In overall, the study shows that, in order to preserve either the DF quality and/or the antioxidant capacity, air-drying tem- perature should be controlled since both types of compounds, DF components and antioxidants, might be degraded or modified either when extended drying periods and/or high drying temperatures are applied. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Orange by-products; Citrus aurantium; Air-drying temperature; Dietary fibre; Functional properties; Antioxidant capacity 1. Introduction For the last decades, the demand for appropriate nutri- tional and health standards has increased considerably. This has been characterized by rising costs and often decreasing the availability of raw materials together with much concern about environmental pollution. Conse- quently there is a considerable emphasis on the recovery, recycling and upgrading of wastes. This is particularly valid for the food and food processing industry in which wastes, effluents, residues, and by-products can be recovered and can often be upgraded to higher value and useful products (Laufenberg, Kunz, & Nystroem, 2003; Reddy & Yang, 2005). Spain is one of the major producers and exporters of cit- rus fruits. For some time, the food industry has shown a special interest in finding uses for citrus industry by-prod- ucts (Larrea, Chang, & Martinez-Bustos, 2005a). During the citrus juice extraction process, thousands of tons of by-products are produced. These are mainly used for ani- mal feeds, although, due to their high fibre content, they could represent an interesting source of dietary fibre (DF) (Lario et al., 2004; Mandalari et al., 2006). DF acts as a bulking agent, normalizing intestinal motil- ity and preventing diverticular disease. Considerable atten- tion has also been focused on the incidence of a number of non-infectious diseases common in civilized societies, such 0308-8146/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2007.01.009 * Corresponding author. Tel.: +34 971 17 32 41; fax: +34 971 17 34 84. E-mail address: antoni.femenia@uib.es (A. Femenia). www.elsevier.com/locate/foodchem Food Chemistry 104 (2007) 1014–1024 Food Chemistry