Carbohydrate Polymers 89 (2012) 810–820 Contents lists available at SciVerse ScienceDirect Carbohydrate Polymers j ourna l ho me pag e: www.elsevier.com/locate/carbpol Role of vacuum steps added before and after steaming treatment of maize starch. Impact on pasting, morphological and rheological properties Seyed Amir Bahrani a , Catherine Loisel b , Sid-Ahmed Rezzoug a , Jean-Louis Doublier c , Zoulikha Maache-Rezzoug a,∗ a LaSIE, FRE-CNRS 3474, La Rochelle University, Avenue Michel Crépeau, 17042 La Rochelle, France b GEPEA, UMR CNRS 6144, ONIRIS, Rue de la Géraudière, 44322 Nantes, France c INRA, UR1268 Biopolymères, Interactions, Assemblages, 44000 Nantes, France a r t i c l e i n f o Article history: Received 15 January 2012 Received in revised form 13 March 2012 Accepted 5 April 2012 Available online 13 April 2012 Keywords: Maize starch Hydrothermal treatments Vacuum Granular characteristics Rheological properties a b s t r a c t Standard maize starch (SMS) was hydrothermally treated by three processes; DV-HMT (Direct Vapor- Heat Moisture Treatment), RP-HMT (Reduced-Pressurized Heat Moisture Treatment) and DIC (in French: Détente Instantanée Contrôlée, instantaneous controlled pressure drop). Impact of processes were studied in order to determine the role of added steps of vacuum before and after treatment of starch by live steam at different pressures (1, 1.5, 2, 2.5 and 3 bar) on morphological, pasting (Brabender) and rheological properties of SMS suspension. The three treatments tend to modify the physical properties of starch on the same way, but the extent of these modifications depends on the process and on the level presure applied. The intensity of modifications follows this order: DIC > RP-HMT > DV-HMT. This was attributed to the presence of the initial vacuum step (DIC and RP-HMT) which intensified the transfer phenomena and to the mechanical effect induced by the abrupt decompression towards vacuum (DIC) leading to weakness of starch granules. The treatments changed the shape and distribution size of granules. The sizes were shifted towards higher values after the treatment due both to the improvement of swelling capacity of granules and to the presence of agglomerates particles of different sizes as confirmed by scanning electron microscopy. The agglomerates glued together by gelatinized granules were favored by the direct contact of starch with steam during the treatments. The results showed for all treatments, a reduction of the consistency coefficient (k) and of the yield stress ( 0 ) of starch suspensions with increasing of process intensity. For severe conditions (3 bar), no difference between the treatments was observed; a complete fluidization of starch suspensions (the consistencies were too weak to be detected),  0 vanished and the rheological behavior tended to a Newtonian type. Elastic modulus (G ′ ), measured during gelation at 25 ◦ C, decreased dramatically (G ′ < 1 Pa), that revealed the loss of rigidity and disappearance of granular integrity of starch. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Starch is a renewable biopolymer and one of the most abundant carbohydrates reserve. This biopolymer is used in various indus- tries fields e.g. pharmaceutical, textile, biomass energy, chemical process engineering and especially in food industry. The specific characteristics of starch are of interest for its great nutritional value and thickening properties. Most of applications need hydrothermal transformation to be applied first. The two hydrothermal treatments largely studied are annealing and Heat-Moisture Treatment (HMT) which modify the physic- ochemical properties of starch without destroying the granular ∗ Corresponding author. Tel.: +33 05 46 45 86 15; fax: +33 05 46 45 86 16. E-mail address: zrezzoug@univ-lr.fr (Z. Maache-Rezzoug). structure. Annealing involves incubation of starch granules in excess or intermediate water content at a temperature below the onset of gelatinization ranging from 40 to 60 ◦ C (Jacobs & Delcour, 1998). For HMT treatment, starch is exposed to temperatures com- monly above the gelatinization temperature (≈120 ◦ C), at very restricted moisture content (18–30%) during 16 hours (Hoover & Manuel, 1996; Lorenz & Kulp, 1981) or shorter (Collado & Corke, 1999; Lim, Chang, & Chung, 2001). HMT starches have generally been performed at the laboratory scale and many authors have reported that such conditions produce inhomogeneous samples. Maruta et al. (1994) observed, for HMT treatment, that pres- sure is required to ensure an homogeneous diffusion of steam and an effective heat transfer surrounding starch granules. The authors improved the conventional HMT method by introduc- tion of reduced pressure in order to satisfy practical requirements for industrial production. They observed that the combination of 0144-8617/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbpol.2012.04.015