Food Hydrocolloids 48 (2015) 1e7
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Rheological properties and microstructure characterization of normal and waxy corn starch dry heated with soy protein isolate Chao Qiu, Xiaojing Li, Na Ji, Yang Qin, Qingjie Sun*, Liu Xiong School of Food Science and Engineering, Qingdao Agricultural University, Qingdao 266109, Shandong Province, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 November 2014 Received in revised form 25 January 2015 Accepted 29 January 2015 Available online 12 February 2015
Physicochemical and microstructure properties of normal corn starch (CS) and waxy corn starch (WCS) with soy protein isolate (SPI) before and after dry heating were investigated by rapid visco analyser (RVA), rheometer, differential scanning calorimeter (DSC), x-ray diffraction (XRD), and scanning electron microscopy (SEM). Dry heating with SPI significantly increased the paste viscosities of both CS and WCS. Compared with untreated starch, the values of G0 and G00 of all the samples significantly increased, and the tan d values decreased. The SEM images showed that starch accumulation occurred after dry heating with SPI, and WCS/SPI formed larger lump compared with CS/SPI. The starch accumulation suggested that the interactions between starch and SPI took place after dry heating starch/SPI mixture, and the interactions of WCS/SPI were more pronounced than those of CS/SPI. This simple heating process with SPI could be used as a promising modification method for WCS. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Corn starch Rheology Dry heating Soy protein isolate Microstructure
1. Introduction Modified starches are widely used in the food industry because the functional properties of these starches are improved over those of native starches. Starch modifications usually include chemical, physical, and enzymatic methods (Eliasson & Mundungus, 2006; Hashish, 2000; Thomas & Aswell, 1999). However, chemical modifications have potential toxicity because the reactive compounds € m, that remain can migrate to foods during processing (Siljestro € rk, & Westerlund, 1989; Theander & Westerlund, 1987). PhysBjo ical modification involves pregelatinization, heat-moisture treatment, and dry heat treatment of starch, etc. (Astute, 1992). Heating starches under dry conditions is a promising method to produce modified starches, which is simple, safe, and produces no pollution. In the past, much attention has been paid to the various characteristics of starch and polysaccharide mixtures after dry heating. Lim, Han, Lim, and BeMiller (2002) found that anionic food gums reacted with starch during dry heat treatment, producing significant changes in the pasting properties of the starch. Waxy rice starch heated with a mixture of phosphate salts and xanthan has
* Corresponding author. School of Food Science and Engineering, Qingdao Agricultural University, 700 Changcheng Road, Chengyang District, Qingdao 266109, China. Tel.: þ86 532 88030448; fax: þ86 532 88030449. E-mail address:
[email protected] (Q. Sun). http://dx.doi.org/10.1016/j.foodhyd.2015.01.030 0268-005X/© 2015 Elsevier Ltd. All rights reserved.
been shown to have a continuous increase in pasting viscosity (Chung, Min, Kim, & Lim, 2007). Sun, Si, Xiong, and Chu (2013) reported that the pasting and thermal properties of starch and ionic gums experienced significant changes after dry heating. Pramodrao and Riar (2014) reported that the effect of modification with ionic gums and dry heating on the physicochemical characteristic of potato, sweet potato and taro starches. Sun, Gong, Li, & Xiong (2014) found the pasting viscosity of proso millet flour and starch increased during dry heat treatment. Li et al. (2013) reported the pasting viscosity of waxy rice starch substantially increased following dry heat treatment with xanthan. They suggested that the dry heat treatment appeared to crosslink xanthan polymers both intra- and inter-granular to the starch granules. Lim, BeMiller, and Lim (2003) investigated the effect of dry heating with ionic gums at controlled pH on starch paste viscosity. They suggested that the rise in viscosity of the samples might be due to the esterification reaction between the carboxyl groups of ionic gums and the hydroxyl groups of starch. However, the modification of starch with protein by dry heat treatment has rarely been studied. Soy protein isolate (SPI) is an abundant, inexpensive, nutritional, and high-quality vegetable protein (Galus, Mathieu, Lenart, & Debeaufort, 2012). Like food gums, SPI also contains a great deal of hydrophilic groups, such as carboxyl and hydroxy. So, the carboxyl group of SPI can be integrated with starch. Lee, Lee, Kim, Park, and Shim (2004) reported that after dry heating at 130 C, the pasting viscosity of CS/SPI
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increased. Galus, Lenart, Voilley, and Debeaufort (2013) reported that cross-linking between soy proteins and oxidized starch increased tensile strength of SPI films. The addition of SPI will not only improve the properties of starch, but also increase the nutrition. Sun, Xu, and Xiong (2014) studied the effect of microwaveassisted dry heating with xanthan on CS and WCS. They found the properties of WCS were more affected by heating with xanthan than those of CS. However, the rheological properties of the CS/SPI and WCS/SPI mixtures before and after dry heating have not been reported. So, the aim of this research is to study the effect of dry heat treatment with SPI on pasting properties, thermal properties, rheological properties and microstructure of corn starch and explore the reason of change. In this study, CS and WCS were heat-treated at 130 C for 2 h or 4 h in a dry state with SPI (3%). These treatment conditions were selected based on the results of preliminary examinations (treatments at 110e130 C for 1e4 h), so that the most pronounced effects would be obtained. More importantly, DSC results of SPI-alone with and without dry-heat treatment showed the same enthalpy peaks at 74.2 and 93.6 C, indicating that no protein degeneration was found in this condition due to the lower water contents. The effect of dry heating on pasting properties, rheological properties, thermal properties, and microstructure of starch and the starch/SPI before and after dry heating were examined.
starch sample. Parameters including peak viscosity (PV), trough viscosity (TV), final viscosity (FV), breakdown (BD ¼ PV HV), setback (SB ¼ FV TV), and pasting temperature were recorded. 2.4. Rheological tests Steady flow properties of 6% (w/w) freshly prepared starch and the starch/SPI mixed pastes from RVA were determined using a rheometer (MCR102, Anton Paar, Austria). For all rheological tests described as follows, the mixed pastes were added on the peltier plate, and then the upper parallel plate was moved down to provide the 1 mm gap. After removing the excess suspension and applying silica oil to the edge of the plate, the tests were commenced according to the following parameters: (1) The continuous shear tests were performed at 25 C over the shear rate range of 0.01e300 s1 to measure the apparent viscosity. (2) Frequency sweeps were performed at 25 C over the angular frequency range of 0.1e100 rad/s and a constant deformation (0.5% strain) within the linear viscoelastic range. The mechanical spectra were obtained recording storage modulus (G0 ), loss modulus (G00 ), and loss tangent (tan d ¼ G00 /G0 ) as a function of frequency (u).
2. Materials and methods 2.5. Differential scanning calorimeter (DSC) 2.1. Materials CS (amylose content: 28.55%) and WCS (amylose content: 0.54%) was obtained from National Starch Co., Ltd. (Guangdong, China). SPI (protein content: 93%, dbs) materials were purchased from Solae (Solae c/o DuPont China Holding Co., Ltd., Shanghai, China). 2.2. Samples preparation The starches were modified with SPI according to Lim et al. (2002) with minor modifications. SPI (3 g, db) was slowly added to distilled water (170 ml) with vigorous stirring. After the SPI was completely dissolved, CS or WCS (97 g, db) was dispersed into the SPI solution. The pH of the mixture was neutral (pH 6.8). The dispersion was stirred at 35 C for 2 h and transferred to a glass dish and dried at 45 C in a convection oven until the moisture content was
