Gums tuning the rheological properties of modified maize starch pastes: Differences between guar and xanthan Bart Heyman a, * , Winnok H. De Vos b, c, d , Paul Van der Meeren e , Koen Dewettinck a a Laboratory of Food Technology & Engineering, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000, Ghent, Belgium b Cell Systems and Imaging Research Group, Department of Molecular Biotechnology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000, Ghent, Belgium c NB-Photonics, Ghent University, Coupure Links 653, 9000, Ghent, Belgium d Laboratory of Cell Biology & Histology, Department of Veterinary Sciences, University of Antwerp, Groenenborgerlaan 171, 2020, Antwerp, Belgium e Particle and Interfacial Technology Group, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000, Ghent, Belgium article info Article history: Received 9 July 2013 Accepted 20 December 2013 Keywords: Rheology Xanthan Guar Modified starch abstract The experimental setup aimed at gaining a more fundamental knowledge on the effects of two frequently used food gums, guar and xanthan, on the rheological properties of modified waxy starch systems. With regard to starch/non-starch hydrocolloid combinations, chemically modified starches receive much less attention than their native counterparts, despite their commercial and scientific value. The use of cross-linked waxy starches allows to treat the combined systems as dispersions of undis- rupted, swollen granules, surrounded by the gum solution. This hypothesis was verified by using confocal scanning laser microscopy with fluorescently labeled gums. The average swollen granule sizes were derived by laser light scattering, and the corresponding total volume fraction occupied by the swollen starch (F) was calculated. The specific effects of the gums appeared to strongly depend on the starch content. Both gums, which were effectively increased in concentration due to the starch swelling, dominated the dynamic rheological properties and flow behavior at low F. At higher F, granule in- teractions governed the rheological properties. Whereas both gums could weaken the granule network, they contributed to the elastic behavior by their own entanglements. Moreover, xanthan gum appeared capable of reducing the granule interactions within flow, which results in lower viscosities at high starch volume fractions and high shear rates. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction In nature, starch occurs as granules in cereals (common and durum wheat, maize, rice and rye), in roots and tubers like potato and cassava, and in legumes such as peas. Starch comprises two different glucose polymers: amylose and amylopectin. Amylose is a linear polymer of D-glucopyranose units which have a-(1,4) linkages. Amylopectin has the same backbone but it is highly branched. On the branching points D-glucopyranose has an a-(1,6) linkage. The starch granule consists of alternating amorphous and semi-crystalline growth rings. The amorphous shells are less dense and contain amylose and less ordered amylopectin, whereas the semi-crystalline shells have alternating amorphous and crystalline lamellae (BeMiller & Whistler, 2009; Buleon, Colonna, Planchot, & Ball, 1998; Tester, Karkalas, & Qi, 2004). When heated above the gelatinization temperature in the presence of excess water, the starch granules lose their internal order and absorb the water. This causes a swelling of the granules and a rise of the viscosity. The resulting system is generally denoted as a ‘paste’. For non-waxy starches a significant amount of amylose leaches out of the granules (BeMiller & Whistler, 2009; Hermansson & Svegmark, 1996). For waxy starches e which are essentially free of amylose e amylopectin molecules remain initially inside the granules during swelling, however upon pro- longed heating, the majority of the granules breaks up, and the system is converted to a macromolecular amylopectin solution (Schirmer, Höchstötter, Jekle, Arendt, & Becker, 2013). Chemically cross-linked starches are commercially very popular, because the strengthened granules can much better withstand the elevated temperatures and high shear forces encountered in production processes (Singh, Kaur, & McCarthy, 2007; Tharanathan, 2005). For numerous industrial food applications chemically modified starches * Corresponding author. Laboratory of Food Technology and Engineering (FTE), Department of Food Safety and Food Quality (BW07), Faculty of Bioscience Engi- neering (FBW), Ghent University, Coupure Links 653, B-9000, Gent, Belgium. Tel.: þ32 (0) 9 264 61 98; fax: þ32 (0) 9 264 62 18. E-mail addresses: Bart.Heyman@UGent.be, bart.heyman@gmail.com (B. Heyman). Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodhyd.2013.12.024 Food Hydrocolloids 39 (2014) 85e94