Review Dietary fiber in extruded cereals: Limitations and opportunities Fr ed eric Robin a,b, * , Heike P. Schuchmann b and Stefan Palzer c a Department of Food Science & Technology, Nestl e Research Center, PO Box 44, 1000 Lausanne 26, Switzerland b Institute of Process Engineering in Life Sciences, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76131 Karlsruhe, Germany (Tel.: D41 21 785 9573; fax: D41 21 785 8554; e-mail: frederic.robin@ rdls.nestle.com) c Nestl e Product Technology Centre for Confectionery, PO Box 204, Haxby Road, York YO91 1XY, United Kingdom Incorporation of dietary fiber in extruded products often leads to reduced expansion volumes and textures that are less pre- ferred by consumers. Nevertheless, soluble fiber provides higher expansion volumes while they affect less the bulk den- sity of extruded products than insoluble fiber. The difference in expansion behavior and related textural properties between soluble and insoluble fiber can be explained by their interac- tions with starch, differences in water sorption and plasticiza- tion behavior but also by the physicochemical transformations they undergo during extrusion. Treatments of insoluble fiber, prior to extrusion, can significantly improve their expansion and textural properties. Introduction Extrusion-cooked products such as savory snacks or break- fast cereals contain significant calorie amounts. One of the strategic avenues followed by the food industry to reduce energy density of foods is to increase their content of die- tary fiber. In addition to its lower calorie content and the regulation of digestion, dietary fiber has positive effects on cardiovascular health, diabetes, weight management and on the immune system (Anderson et al., 2009). The Codex Alimentarius recommends that any product claiming to be a “source‟ of fiber should contain 3 g of fiber per 100 g of serving or 1.5 g of fiber per 100 kcal of serving or 10% of daily reference value per serving. To claim that a food is “high” in fiber, the product must contain at least 6 g of fiber per 100 g of serving or 3 g of fiber per 100 kcal of serving or 20% of daily reference value per serving (Codex, 2009a). Increasing dietary fiber content to achieve these content claims in extrusion-cooked directly expanded cereals is challenging. Indeed, addition of dietary fiber most often leads to reduced expansion volumes, higher densities, harder textures, less crispy and thus less preferred products (e.g. Lue, Hsieh, & Huff, 1991). Nevertheless, soluble fibers such as inulin deliver a higher expansion and more favorable textures compared to insoluble fibers such as e.g. cereal bran fiber (Blake, 2006). The difference in expansion properties between soluble and insoluble fiber may be explained by their effects on the mechanistic steps of expansion. The fiber properties are also modified during extrusion. This will affect the expansion properties and the content of remaining dietary fiber after extrusion. Insoluble fiber is less expensive than soluble fibers, more readily available and still undervalued in human nutrition (Blake, 2006). Modifications of insoluble dietary fiber prior to ex- trusion may lead to significant improvement in its expan- sion properties and thus allow increasing applications in human nutrition. The present work aims at reviewing the current state of the art in the field of extruded cereals enriched with dietary fiber. The effect of the different types of fiber on expansion volumes, mechanism of expansion and associated texture of extruded cereals is reviewed. The impact of extrusion on fi- ber properties as well as newly developed technologies to modify insoluble fiber and improve their properties in ex- truded products is also provided. Definition and classification of dietary fiber The definition of dietary fiber is related to the macromo- lecular structure of fiber which may be modified during ex- trusion. Dietary fiber is defined as “edible carbohydrate * Corresponding author. 0924-2244/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tifs.2012.06.008 Trends in Food Science & Technology 28 (2012) 23e32