Food Hydrocolloids 18 (2004) 191–201 www.elsevier.com/locate/foodhyd Xanthan effect on swelling, solubility and viscosity of wheat starch dispersions I.G. Mandala*, E. Bayas Laboratory of Engineering, Processing and Preservation of Foods, Department Food Science and Technology, Agricultural University of Athens, 75 Iera Odos, 11855 Votanikos, Athens, Greece Received 30 August 2002; accepted 19 March 2003 Abstract Xanthan effect on swelling power (SP), solubility index (SOL) and granules status of wheat starch dispersions (2% w/w) was investigated. Control samples and samples with xanthan (0.09% w/w at the final solution) were prepared and heated at temperatures from 60 to 90 8C for 5 or 30 min. Additionally, the viscosity of starch–xanthan mixtures was calculated at different shear rates. Regarding these mixtures two preparation techniques were used: separate preparation of starch and xanthan solutions and subsequent mixing (a), or mixture of the powders prior to addition of water (b). Samples were heated at two different temperatures 75, 90 8C and times 5, 30 min. According to SP values and granules dimensions at 75 8C, xanthan addition enhanced swelling. It also enhanced starch polymers leakage at temperatures ,80 8C. At higher temperatures lower SOL values were found than those of control samples. Furthermore, in the presence of xanthan the proportion of large granules was greater than this found in aqueous solution. However, xanthan induced granules folding, which was noticed even at relatively low temperatures (75 8C). With respect to viscosity, all samples showed pseudoplastic behaviour. Experimental values were fitted well by the Ostwald–de Waele model ðs ¼ k·g_nÞ: Consistency values ðkÞ and flow index ðnÞ were mainly influenced by the preparation technique and the heating temperature. Mixtures prepared with technique (a) had increased viscosity, consistency and pseudoplasticity. Mixtures prepared with technique (b) had irregular granules’ shape and size due to xanthan addition and bad adhesion between continuous and dispersed phase, which influence viscosity negatively. q 2003 Elsevier Ltd. All rights reserved. Keywords: Wheat starch; Xanthan; Heating process; Swelling; Solubility; Viscosity 1. Introduction When starch dispersions are heated, granules’ swelling and starch polymers solubilization occur, which influence Gelatinized starch consists of the continuous phase the properties of both continuous and dispersed phases. By (amylose/amylopectin matrix) and the dispersed phase increasing the temperature during heating, granules’ swel- (starch granules). The rheological properties of such a ling increases as well (Li & Yeh, 2001). This is evident till a composite depends on factors influencing both phases. specific temperature. For rice starch granules peak swelling Some of these factors influencing the dispersed phase are values were reached at 75 8C. Above this temperature the granules’ size and its distribution, the granules’ shape granules’ disruption was observed (Yeh & Li, 1996). At and volume occupied, their rigidity and the interactions 75 8C starch swelling can be also related to food quality. among them. Respectively, regarding the continuous phase, Kim and Seib (1993) reported that wheat starches with high the matrix of amylose/amylopectin, the interactions swelling power (SP) at 75 8C yielded better eating quality of between starch polymers and the interactions with the instant fried noodles. Swelling can also be positively related granules are very important factors (Clegg, 1995; Eliasson to the amount of soluble solids leached outside the granules & Gudmundsson, 1996; Hermansson & Svegmark, 1996). during heating. However, for some starch types (potato, tapioca and waxy corn starch) SP decreases when more solids leached out during cooking at higher temperatures (Li * Corresponding author. Tel.: þ30-210-529-4709; fax: þ30-210-529-4697. & Yeh, 2001). The initial granule size influences also the E-mail address: [email protected] (I.G. Mandala). swelling and it determines the onset of gelatinization (large 0268-005X/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0268-005X(03)00064-X 192 I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201 waxy corn starch granules increased gelatinization tem- stable associations with starch solubles, which increase the perature, whereas small barley granules decreased it) viscosity of starch–gum mixtures. Alloncle et al. (1989) (Myllarinen, Autio, Schulman, & Poutanen, 1998; Tsai, assume that the synergistic effect between starch and gums Li, & Lii, 1999). According to Li and Yeh (2001) the was observed because of different polymers phase separ- granule size of several starches had a positive effect on SP, ation, rather than due to intermolecular associations. Shi and till the temperature of 75 8C, since small barley granules BeMiller (2002) suggest that interactions between maize displayed the greatest SP (Tang, Watanabe, & Mitsunaga, starch amylose and carboxymethylcellulose (CMC) are at 2002). However, according to Sasaki and Matsuki (1998) least partially responsible for starch viscosity increase. and Tester and Karkalas (1996) the granule size does not However, wheat, tapioca and potato amylose did not seem influence the SP values. to interact as strongly with CMC as maize amylose did. Rheological behaviour of starch systems is also influ- Although the existing information about starch swelling, the enced by granules swelling and consequently by their volume factors that influence it (temperature, soluble solids, initial fraction and their rigidity. When starch granules are swollen granule size), its relation to rheological properties and the enough and can be deformed under applying shear force, effects of added hydrocolloids on starch systems, there are pseudoplastic behaviour will be observed. On the contrary, still unknown aspects because of the complexity of such when granules are rigid and they are not so readily deformed, systems. Furthermore, according to the above information, dilatancy will be observed (Bagley & Christianson, 1982; there are many contradictory aspects, while the correlation Christianson, Baker, Loffredo, & Bagley, 1982). Rheological of these properties to real food is still limited. behaviour can be related to temperature as well, when The aim of this study was to investigate the effects of observing dilatant behaviour in early stages of gelatinization temperature, heating time, xanthan addition on wheat starch (low swelling) of corn starch (Okechukwu & Rao, 1995; Rao, granules swelling, solubilization and size during heating. Okechukwu, Silva, & Oliveira, 1997), but pseudoplastic at The effects of preparation technique and heating conditions higher heating temperatures (85–90 8C) (Rao et al., 1997). (temperature and time) on starch–xanthan mixture viscosity Pseudoplastic behaviour of gelatinized maize, waxy maize were also investigated. and wheat starch pastes was also reported by Nguyen, Jensen, and Kristensen (1998) and Noel, Ring, and Whittam (1993). : Viscosity can also be related to particle volume fraction w At 2. Materials and methods intermediate volume fractions ð0:1 , w , 0:5Þ—found in starch systems of low concentrations like in our case— suspensions of rigid, spherical particles are usually shear- 2.1. Materials thinning (Dickinson, 1992). By hydrocolloid addition, both structural and rheological Wheat starch from Merck and xanthan gum from Sigma characteristics can be altered. Starch’s gelatinization were used for the experiments. The intrinsic viscosity of temperature (onset, peak and conclusion temperature) is xanthan aqueous solution, according to the Huggins’ not modified by hydrocolloids (Biliaderis, Arvanitoyannis, equation (1942) as it was referred by Launay, Cuvelier, Izydorczyk, & Prokopowich, 1997; Liu & Eskin, 1998; and Martinez-Reyes (1984), was ½h¼18:7 dl/g. Intrinsic Rojas, Rosell, & Benedito de Barber, 1999; Shi & BeMiller, viscosity was measured with a falling ball viscometer 2002). However, swelling of granules can be affected by (HAAKE Mess-Technik GmbH u. Co, Germany boron hydrocolloids addition, suggesting that swelling is enhanced silica glass density ðrÞ¼2:20 g/cm3, diameter in their presence (Rojas et al., 1999; Tecante & Doublier, (d) ¼ 15.643 mm). The moisture content of xanthan powder 1999). According to Christianson (1982) and Christianson, was 11.2 ^ 0.1% (105 8C, 24 h, three replications). Hodge, Osborne, and Detroy (1981) the addition of For unheated starch granules, size distribution according hydrocolloids can increase the shear forces applied on the to their average diameter was calculated. More than 300 granules as regards with the forces applied in starch–water granules were used for this reason. Light microscopy (LM) suspensions. This fact affects significantly the breakdown of pictures obtained, were further treated using an image granules and the amount of material exuded into the process analysis system (Image Proplus 1.3, Media continuous phase. Cybernetics, USA) and six different diameter categories An addition of a given hydrocolloid, depending on its were chosen. The size distribution of granules is shown in thickening capacity, usually results in a noticeable increase Table 1. The first three categories correspond to small to in starch viscosity (Tecante & Doublier, 1999). According medium size and the other three to large size granules. The to Alloncle, Lefebvre, Llamas, and Doublier (1989), amylose content of starch determined by the method of Bahnassey and