Effect of repeated cycled crystallization on digestibility and molecular structure of glutinous Bora rice starch Pallab Kumar Borah, Sankar Chandra Deka, Raj Kumar Duary ⇑ Department of Food Engineering and Technology, School of Engineering, Tezpur University, Napaam, Assam 784028, India article info Article history: Received 27 July 2016 Received in revised form 30 November 2016 Accepted 9 December 2016 Available online 10 December 2016 Keywords: Glutinous rice Slow digestible starch Repeated cycled crystallization Molecular structure abstract The effects of repeated cycled crystallization on the digestibility and molecular structure of glutinous Bora rice starch were investigated. Temperature cycle 4/45 °C; cycle duration 5 d; time interval of cycles 24 h; and starch to water ratio 1:2 were found to be optimum for SDS (slow digestible starch) product development. The SDS content increased from 18.01 ± 2.11% to 82.81 ± 2.34%. An increase in the resis- tance to digestion, crystallinity, molecular weight, polydispersity and molecular order was observed in the optimal SDS product. Notably, the FT-IR peak at 947 cm 1 and XRD peaks at 2h  13° and 20° in the optimal SDS product indicated the formation of V-type complexes even without the presence of co-polymers. Birefringence studies showed a loss of typical Maltese cross in the SDS product and revealed a reorientation of crystalline structures within starch granules, suggestive of imperfect crystallite development. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Slow digestible starch (SDS) is believed to provide a slow post- prandial rise in blood glucose level (Lehmann & Robin, 2007). Thus, leading to low glycemic index, and effecting in lowering severity of several diseases such as obesity, diabetes, and cardiovascular dis- ease (Han et al., 2006). Among other applications, SDS products are also widely used in athlete diets for their ability to provide sus- tained energy release (Eliasson, 2004). Due to the associated bene- fits and great potential for application in consumer products, numerous techniques have been developed for their production. Few of these techniques are hydrothermal treatment (Lee & Moon, 2015), modification with citric acid (Mei, Zhou, Jin, Xu, & Chen, 2015), debranching using enzymes (Zeng et al., 2014), and starch retrogradation (Ashwar et al., 2016; Hu et al., 2015). SDS products are typically produced by retrogradation treat- ment due to which the digestibility of the starch by enzymatic activity decreases. The isothermal retrogradation of starches also known to generate perfect starch crystallites. However, application of repeated cycled crystallization through time/temperature- cycled retrogradation is known to increase the propagation of imperfect crystallites. Recently, Zeng et al. (2014) and Vernon- Carter et al. (2016) reported the formation of SDS product from starch using temperature-cycled retrogradation. Earlier, few research groups (Tian et al., 2012; Zeng et al., 2014) investigated the effect of temperature cycled retrogradation on the digestibility of glutinous rice starch. However, the extended effects of these conditions on the molecular weight, crystalline properties, and molecular structure are largely unknown. In this study, glutinous rice starch was isolated from traditional glutinous rice (Oryza glutinosa Lour.; Poaceae) which belongs to the Oryza sativa japonica variety found in Assam region of India and was used for the preparation of the SDS product. The rice variety is known in Assamese as Bora and in Tai language as Khao-Niao. A variety of traditional food items is prepared from Bora rice, like Khao-Nong or Tupula bhat (steamed rice balls). The effect of the conditions, viz. temperature cycles, cycle duration, the time inter- val of cycles, and the starch to water ratio on the hydrolysis rate and slow digestibility of the SDS products were evaluated using an advanced in vitro digestion model and standardized. Effect of these conditions on the molecular weight, crystalline properties, and molecular structure were also studied by high-performance gel permeation chromatography, FT-IR spectroscopy, X-ray diffraction, and polarized light microscopy. Further, the effect of http://dx.doi.org/10.1016/j.foodchem.2016.12.022 0308-8146/Ó 2016 Elsevier Ltd. All rights reserved. Abbreviations: SDS, slow digestible starch; HPGPC, high performance gel permeation chromatography; THF, tetrahydrofuran; SGF, simulated gastric fluid; SIF, simulated intestinal fluid; SEM, scanning electron microscopy; FT-IR, fourier transform infrared spectroscopy; XRD, X-ray diffraction; M w , weight average; M p , peak molecular weight; M z , Z-average molecular weight; M n , number average; M w / M n , polydispersities; h, hour; d, day; m, min. ⇑ Corresponding author. E-mail addresses: duary@tezu.ernet.in, rkduary@gmail.com (R.K. Duary). Food Chemistry 223 (2017) 31–39 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem