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 Breene (1994), Christianson (1982), Gilbert and Spragg (1964) was 20 ^ 0.2%. The moisture Christianson et al. (1981), Liu and Eskin (1998), and Shi content of samples was 11.5 ^ 0.4% (105 8C, 24 h, three and BeMiller (2002) the viscosity of starch increases due to replications), the density of starch powder was synergistic interactions of starch and gums. Christianson 627.33 ^ 7.02 kg/m3 and the ash content 0.13 ^ 0.05% (1982) and Christianson et al. (1981) believe that gums form (450 8C, 3 h). I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201 193

Table 1 determined iodometrically by the method of blue value Size distribution (diameter values) of unheated wheat starch granules (Gilbert & Spragg, 1964). Two replicates for each Average diameter (mm) Percentage of total granules (%) measurement were used.

2.5–10 38.30 2.3. Dimensions of granules and microscopic observations 10–17.5 28.07 17.5–25 23.10 After the heating process and cooling to room tempera- 25–32.5 9.06 ture, suspensions of starch or starch–xanthan samples were 32.5–40 1.17 .40 0.30 put onto object glasses and were stained with lugol solution (1%, mixture of I:KI 1:2), covered with coverglasses, and were observed using LM microscope at 10 £ and 40 £ magnification. Photos were taken using a video camera 2.2. Swelling powder, solubility index and amylose (SONY, CCD-Iris) and stored in a computer. With an image determination analysis system (Image Proplus 1.3, Media Cybernetics, USA), photos were further treated for improving their SP and solubility index (SOL) were determined using a quality. A filter was used to bring out fine details of the modification of the method of Leach, McCowen, and image (Sharpen 5 £ 5) and brightness/contrast was adjusted Schoch (1959). Starch dispersions of 2% were put in accordingly. The average area, the minimum, maximal, centrifugal tubes and heated in a water bath at temperatures average diameter and the aspect ratio (ratio of length of of 60–90 8C for 5 and 30 min. The average heating rate as horizontal to vertical axes, x=y) were calculated after recorded by thermocouples placed in tubes was 18 8C/min. converting pixels to mm with means of known reference During heating, in order to prevent granules’ sedimentation, lengths. Objects with aspect .1.8 were rejected, because middle stirring was applied periodically using glass stirrers. they related to artifacts or disrupted granules parts. For size In order to prevent water loss, all tubes were covered with measurements more than 250 granules were used from plastic covers. After heating, samples were centrifuged different photos and samples. For temperatures greater than (3000 rpm, 15 min). Precipitated paste was separated from 75 8C, the outline of granules was very faint and it was : supernatant and had been weighed ðWpÞ Both phases were difficult to measure granules dimensions. Therefore their dried at 105 8C for 24 h and the dry solids in precipitated measurement was considered inappropriate. paste ðWpsÞ and supernatant ðWsÞ were calculated. SP is the ratio of the weight of swollen starch granules after 2.4. Viscosity measurement centrifugation (g) to their dry mass (g). Starch–xanthan mixtures were prepared using two Wp SP ¼ ðhydrated granule ðgÞ=dry granule in techniques: separate heating of starch dispersions and W ps xanthan solutions, and subsequent mixing (technique (a)), precipitate paste ðgÞÞ ð1Þ or powders mixing, adding water and heating samples thereafter (technique (b)). Heating process was the same as The SOL is the percentage of dry mass of solubles in described above for SP and SOL calculation. Two heating : supernatant to the dry mass of whole starch sample ðW0Þ temperatures were chosen 75 and 90 8C and two heating times of 5 and 30 min accordingly. Viscosity was measured W SOL ¼ S £ 100% ðsoluble solids ðgÞ=dry mass of with a controlled shear stress SR5 rheometer (Universal W0 Stress Rheometer/Rheometrics Scientific CP, USA) with a whole starch sample ðgÞÞ ð2Þ plate–plate geometry at a distance of 0.5 mm. The diameter of the upper plate was 20 mm. The temperature was For determining SP and SOL of samples containing constant (25 ^ 0.2 8C) by circulating water from a constant xanthan, powders of wheat starch and xanthan, were temperature circulator. All samples remained for tempera- mixed. Deionized water was added in order to produce a ture equilibration for 10 min between the plates before solution with starch concentration 2% (w/w) and xanthan starting the experiments. Shear rate applied was in the range 0.09% (w/w). Samples were stirred with a magnetic stirrer of 0.6–1800 (s21). For each measurement 2–4 different gently, in order to keep the granules in suspension. samples were used. Afterwards, with the help of an automatic pipette, 10 ml Experimental data were fitted to Ostwald–de Waele were put in each centrifugal tube. The procedure was model ðs ¼ k·g_nÞ using Excel 2000 software. The par- continued as it was described for pure starch suspensions. It ameters k and n; which are related to consistency and flow was assumed that the whole xanthan amount was remained index, respectively, were used for comparisons among in supernatant and was subtracted in order to calculate SOL samples. Furthermore, the apparent viscosities of different values. For each measurement of SP and SOL 4–8 samples at shear rate 100 s21 were calculated from the replicates were used. Amylose content in supernatant was above power law equation. 194 I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201

Fig. 1. Swelling power (g/g) at different heating temperatures for two heating times, 5 min (W) and 30 min (S). Close symbols: starch dispersion with added xanthan, open symbols: starch dispersion in aqueous solution (control samples).

2.5. Statistical analysis (Table 2) they were not any statistical differences for average SP values between samples heated for 5 and Analysis of variance for multiple comparisons 30 min. However, when statistical analysis was performed (ANOVA) using Statgraphic 2.1 Statistical Graphics comparing SP values at each temperature from 60 to 90 8C System Software (Statistical Graphics Corp, Rockville, for two heating times (5 and 30 min) differences were found MD, USA) was performed in order to find differences for samples heated for 5 and 30 min at 85 and 90 8C. It among samples tested. Least significant differences (LSD) seems that the heating time was significant at high heating were calculated by the Fisher’s test. Differences were temperatures due to extensive granules swelling. considered significant when p , 0:05: With respect to SP According to Tester and Morrison (1990) the swelling and SOL, comparisons were performed calculating the process is rapid during the first 5–10 min at a certain average values of the variables over the temperature range temperature and continues with further heating. Takahashi previously used (60–90 8C), considering as main effects the and Seib (1988) reported that amylose creates crystals with heating time and the addition of xanthan. Analysis of wheat starch lipids and particularly with lysolecithin, which variance was repeated so as to find the effects of heating are being formed at 50–60 8C, a process that inhibits temperature and time on SP and SOL values for each sample excessive granules’ swelling. At temperatures greater than type (starch dispersions or starch/xanthan mixture). For k 85 8C the crystallites melt therefore swelling is enhanced, a and n values, analysis of variance was performed taking into fact that justifies fast swelling increase above 80 8C. account three different factors (heating temperature, heating By adding xanthan swelling of granules was amplified time and preparation technique) and their interactions. (Fig. 1). Rojas et al. (1999) noticed also increase in wheat

Table 2 3. Results and discussion Average SP and SOL values of starch dispersions for the temperature range of 60–90 8C as influenced by xanthan addition and heating time

3.1. Swelling power and solubility index Sample Time of heating SP valuesa SOL valuesa (min) (g/g) (%) Wheat starch granules swelling is confirmed to be a two stage process (Blanshard, 1979; Kokkini, Lai, & Chedid, Without xanthan 5 10.33a 5.01a ab a 1992). An initial low swelling was noticed till 75 8C and a 30 11.37 6.57 great one thereafter (Fig. 1). Experimental values were fitted With xanthan 5 12.88bc 5.44a well using polynomial equations. SP values were calculated 30 14.18c 7.51a for 30 min heating and were similar to those referred by Data in the same column with different letter differ significantly ðp , Eliasson (1986) and Eliasson and Gudmundsson (1996) for 0:05Þ: the same heating time. According to statistical analysis a LS means. I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201 195 granules’ swelling by xanthan addition. Swelling was not a 30 min. However, relatively to the heating time (5 and two-stage process, as it was for control samples, but a 30 min), the average values of soluble solids did not differ gradual swelling increase was noticed. Statistical differ- significantly (Table 2), as it was also found for SP values. ences between the average SP at 5 and 30 min heating were By adding xanthan the amount of soluble solids in the not noticed, as it was also found in control samples. continuous phase were initially increased, compared to the Differences between the SP values at 5 and 30 min were respective amount found in samples without xanthan. After found only in samples heated at 75, 85 and 90 8C. Although a certain temperature (.80 8C), control samples showed samples with xanthan showed greater values of SP, the rate greater values of soluble solids. In the presence of xanthan of swelling increase was lower from 85 to 90 8C during the amount of polymers leaked out from the granules 30 min heating, compared to control samples. It is suggested increased slightly from 75 to 90 8C compared to the that at high heating temperature and time the presence of continuous increase of polymers released in aqueous xanthan can decrease granules’ swelling capacity. solution. At 90 8C and 30 min heating the soluble solids The effect of xanthan addition in starch dispersions was by xanthan addition were the 60% of the respective amount greater by comparing the soluble solids leaked out from in aqueous solution. granules, regarding samples with and without xanthan According to statistical analysis the average values of (Fig. 2). The values of soluble solids found in control soluble solids in two heating times (5 and 30 min) did not samples were similar to those reported by Eliasson (1986) differ significantly (Table 2). However, when statistical and Eliasson and Gudmundsson (1996) during 30 min analysis was performed for soluble solids in every heating. By increasing the temperature, the amount of temperature varying from 60 to 90 8C, differences were soluble solids in the continuous phase increased gradually, found between the two heating times (5 and 30 min) for compared to SP increase, which was a two stage process. samples heated at 75, 80 and 85 8C, suggesting that there is a According to preliminary experiments the amount of temperature range, where the leakage of starch polymers is soluble solids was affected by heating rate in greater extent enhanced. than SP was. Thebaudin, Lefebvre, and Doublier (1998), who Generally the amount of exudates was small; conse- used for the preparation of starch dispersions great heating quently it is suggested that the main component leaked out rate and a strong agitation (750 rpm), found greater values of from the granules was amylose. In order to prove this wheat soluble solids (6.7 and 34.9% at 75 and 85 8C, suggestion, amylose amount was calculated (Table 3) and respectively, whereas for the same temperatures the values similar values to the total amount of solids leaked outside obtained at this study were 2.5 and 9.3%). Takahashi and the granules, were obtained. Some variations compared with Seib (1988) found also greater values (3.5 and 11.9% at 75 the soluble solids values were noticed because the and 85 8C correspondingly) than the values mentioned in this assumption that the whole amount of xanthan is remained study. It should be mentioned that the whole heating process in the continuous phase is not completely true; a small had a duration of 40 min, whereas in our research the values amount of xanthan can sediment with the granules. This mentioned were obtained after heating starch dispersions for overestimation of xanthan amount in the continuous phase

Fig. 2. Soluble solids (%) at different heating temperatures for two heating times, 5 min (K) and 30 min (A). Close symbols: starch dispersion with added xanthan, open symbols: starch dispersion in aqueous solution (control samples). 196 I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201

Table 3 and polymers leaking out. The inner pressure increases, Effect of temperature and time on amylose amount leached out from starch which, at higher temperatures can facilitate the disinte- granules heated in the presence of xanthan gration of granules compared to granules’ status in aqueous Temperature (8C) Heating time (min) Amylose percentagea (%) solution. The creation of amylose film around the granules is also referred by Langton and Hermansson (1989), who 60 5 2.09 claim that this film can influence the behaviour of granules 30 2.98 on rehydration and reheating and was also observed by 65 5 2.44 Mandala, Palogou, and Kostaropoulos (2002) in potato 30 3.55 starch–xanthan mixtures. 70 5 4.17 30 6.94 3.2. Dimensions of granules and microscopic observations

75 5 6.50 30 8.55 The area distribution of starch granules heated at 75 8Cat 5 and 30 min is illustrated in Fig. 3 for both control samples 80 5 5.50 and samples with xanthan. 30 8.53 It is known that there are two wheat granules’ categories, 90 5 6.13 the small, spherical B-granules, and the large, lenticular 30 10.09 A-granules. Although A-granules represent the 90% of a Coefficient of variation: 2–15%. the total weight of granules, as a number are ,20% (Hermansson & Svegmark, 1996; Hoseney, 1998). Since the swelling of lenticular wheat starch granules involves an can cause an underestimation of starch solids leaked out the initial radial expansion at lower temperatures, followed by granules. tangential expansion at higher temperatures (Bowler, About the role of xanthan on starch systems the Williams, & Angold, 1980), then the use of area change following assumptions can be mentioned. According to was considered more representative instead of mean Abdulmola, Hember, Richardson, and Morris (1996), starch diameter. molecules can interact and a network can be created at According to area classification the first category is concentrations well below ‘close-packing’. Xanthan pro- corresponded to small B-granules with area lower or equal motes adhesive interactions among the gelatinized granules. to 75 mm2. These granules are not gelatinized and keep their It is assumed that it can entrap them keeping them closely. original dimensions. Concerning control samples heated at This can enhance the forces applied to them, facilitating 75 8C for 5 min, it can be noticed that ,50% of the total water entering (increasing swelling), amylose solubilization number of granules keep their original dimensions. Assum- and its exudation. By increasing further temperature the ing that the small B-granules represent more than 80% of leaked amylose and the xanthan in the continuous phase total granules number, at least 60% of their population create a film around the granules and inhibit further swelling remain intact. This proportion was reduced to 45% by

Fig. 3. Average area distribution of granules heated at 75 8C for 5 and 30 min in aqueous and xanthan solutions. I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201 197

Table 4 Mean values and standard deviation of starch granules’ area and diameter heated at 75 8C for 5 and 30 min. Control samples and samples with xanthan

Sample Temperature (8C)/time Mean area (mm2) Standard deviation (mm2) Mean diameter (mm) Standard deviation (mm) of heating (min)

Without xanthan 75/5 242 409 13.12 10.40 75/30 331 540 15.85 12.09 With xanthan 75/5 445 706 18.29 13.92 75/30 594 743 21.50 15.05 increasing heating time to 30 min. In the presence of According to Gallant, Bouchet, and Baldwin (1997) starch xanthan for 30 min heating, the portion of small granules granules are organized in a manner that they have radial decreased further to ,38% of their initial number. ‘channels’ to be predominantly composed of semi-crystal- If we consider that the last two categories include the line or amorphous material. Through these channels originally large A-granules, a slight increase can be amylose can leak out from the granules during gelatiniza- noticed from 5 to 30 min heating for control samples. By tion. Fannon, Hauber, and BeMiller (1992a,b) and Fannon, xanthan addition, the percentage of large granules was Shull, and BeMiller (1993) have observed, that along the considerably greater and reached 30 and 40% of the total equatorial groove, there are surface pores. These pores are population for heating time of 5 and 30 min, respectively. the external openings of the interior channels and permit Compared to the respective control sample, this is the amylose leakage. This is in agreement with the assumption double amount. of Langton and Hermansson (1989), who claimed that It is clear that by xanthan addition there is a gradual solubilized amylose is transported outside the granules granules’ population shift from all categories to greater through openings in the equatorial groove. In the presence dimensions. Considering that gelatinization temperature for of xanthan, at higher temperatures, amylose leakage is wheat granules is in the range of 50–60 8C(Addo, Xiong, hindered. That suggests that except of the film created & Blanchard, 2001; Lorenz & Kulp, 1978; Noel et al., around the granules, the excessive granules folding could 1993) and that for all granules population this could be also deform granules surface openings and could hinder varied from 10 to 15 8C(Eliasson & Gudmundsson, 1996) amylose leakage. there are some granules at 75 8C that have just begun or have not yet begun to gelatinize. Granules at 75 8Cof control samples can be classified from small to medium size, whereas granules in xanthan solution from medium to large size (Table 4). However, there are not very large granules with diameter greater than 40 mm, which indicates that at this temperature even the large granules have not reached their maximum swelling. By adding xanthan or by increasing heating time the variability in values is also increased. In Fig. 4 stained wheat granules are presented, heated at 75 and 85 8C for 30 min. As it can be seen, and it is already mentioned, at 75 8C there are many small granules, which have not been gelatinized. There are also swollen granules without loosing their original shape and some of them have a lighter blue colour in the center because of the amylose solubilization. By increasing the heating temperature granules swell further, but they continue keeping their shape. By adding xanthan, more granules are folded even at 75 8C, confirming the above assumption that xanthan increases the forces exerted to granules, making them to be deformed. At 85 8C even more granules are folded, but they are not disrupted. Solubilization of amylose seems to be more extensive by Fig. 4. LM photos of starch granules stained with lugol solution heating for 30 min at different temperatures: (a,b) control samples heated at 75 and adding xanthan, according to the lighter blue colour in the 85 8C, respectively; (c,d) samples with xanthan heated at 75 and 85 8C, center of granules compared with control samples. respectively. Original magnification of the photos 400 £ , magnification Solubilization is the first step for amylose leakage. used for these photos 540 £ . 198 I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201

Fig. 5. Viscosity-shear rate curves for starch–xanthan mixtures heated at 75 or 90 8C for 30 min prepared with two different techniques (a) and (b).

3.3. Viscosity measurements values increase as well although n values decrease accordingly. k values differ also in relation to the In Fig. 5 the apparent viscosity versus the shear rate is preparation technique. Greater values were obtained by presented for samples prepared at two different temperatures, using technique (a), whereas n values obtained, were lower with different preparation techniques (a) and (b) but using the indicating that samples became more pseudoplastic. same heating time. The two temperatures (75 and 90 8C) According to the F-values calculated for n values, the were chosen according to the results obtained after preparation technique, the heating temperature and their measuring SP and SOL at different temperatures. At 75 8C interactions influence significantly the rheology of samples, swelling is not great but granules remain intact and keep their whereas the time was not significant. However, its shape; at 90 8C granules are folded and soluble amount interaction with heating temperature was significant. Heat- showed great differences according to preparation technique. ing time was not significant neither for average SP nor for All samples showed pseudoplastic behaviour with SOL values. constant viscosity values at high shear rates. Greater h Viscosity and consequently k values in a starch–gum values were obtained by preparing samples separately system depend on the exudates present in the media and (technique (a)). The flow behaviour of all samples was increase in their amount increases also the viscosity examined by using Ostwald–de Waele model ðs ¼ k·g_nÞ: k (Christianson, 1982). As it is known, others factors, such values related to the samples’ consistency, and n values as granules’ size, their size distribution, their volume related to flow index, were calculated (Table 5). Samples fraction and their compressibility determine also the were compared according to heating conditions (tempera- rheology of a gelatinized starch suspension. ture and time) and preparation technique. In regard to the The consistency of a starch suspension, k; increased heating conditions, with temperature increase, generally k exponentially with granule diameter, while the flow

Table 5 k and n coefficients of Ostwald–de Waele equation for starch–xanthan mixtures prepared at different temperatures and time with two different techniques

Heating conditions Preparation technique F values for n values

Temperature (8C) Time (min) Technique (a) Technique (b) Main effects Interactions

n n k (Pa s ) n h100 (mPa s) k (Pa s ) n h100 (mPa s)

75 5 0.64 0.56 84 0.31 0.63 56 A 29.26* AB 9.39* 30 0.68 0.60 100 0.19 0.73 55 B 13.46* AC 1.09 90 5 0.62 0.59 94 0.46 0.61 76 C 3.04 BC 11.96* 30 0.70 0.56 92 0.54 0.60 85

A, preparation technique; B, temperature; C, time of heating. *differ significantly for p , 0:05: R2 for Oswald–de Waele model were .0.99 for most samples. For samples prepared with technique (a) and (b) at 75 8C for 5 min R2 was 0.988 and 0.975, respectively. I.G. Mandala, E. Bayas / Food Hydrocolloids 18 (2004) 191–201 199 behaviour index n; decreased linearly with increase in Model A as the assumption proposed by Abdulmola et al. the extent of gelatinization, which was accompanied by an (1996): gum molecules interact with amylose molecules increase in standard deviation of the granule size. The leached from granules and adhere partially swollen variation of k with granule size and that of n with SD were granules. found to be independent of temperature over 70–90 8C Model B proposed by them: gum molecules interact (Okechukwu & Rao, 1995; Rao et al., 1997). with leached amylose molecules, produce a viscosity According to Wong and Lelievre (1981) dynamic increase via synergism, and/or prevent retrogradation. viscosity and storage modulus of wheat starch pastes increased with the volume fraction w of the swollen However, opposite assumption has been reported granules as a power law, a fact that was also observed by supposing that incompatibility phenomena unlike poly- Ellis, Ring, and Whittam (1989) at high w: mers, i.e. amylose and xanthan could cause phase Eliasson (1986) observed that more highly swollen separation (Alloncle & Doublier, 1991). Additionally, (and presumably softer) granules lead to a reduction in a the presence of swollen starch granules favours the mutual starch paste rigidity. Carnali and Zhou (1996), who used exclusion of amylose and amylopectin (Conde-Petit & an artificial starch gel (granules of corn starch freed of Escher, 1995), since in binary mixtures like starch– amylose combined with purified amylose solution), found xanthan, an exclusion effect may occur, where interactions that the rigidity of granules influence the rheology of between chain segments of the same type are favoured pastes/gel proportionally to the temperature at which they energetically in comparison with interactions between have been pasted. At 70 8C granules are not fully swollen, different polymer species. Like molecules are concen- they are relative rigid but the reinforcement of paste by trated in local domains in a type of phase separation them is not strongly dependent on their rigidity. At 85 8C, (Eidam, Kulicke, Kuhn & Stute, 1995). however, granules are more swollen, have high volume Xanthan does not seem to act synergistically with starch fraction w and they are less rigid than these swollen at and does not enhance the viscosity of the mixture when 70 8C. At this higher w; the reinforcement attributes are samples are prepared by technique (b). It is more probable very much controlled by granules’ individual rigidity. that due to excluded volume effect of starch granules, local However, in systems investigated, soluble solids and concentration of xanthan in continuous phase increase and swelling cannot be directly correlated to k or n values, phase separation of xanthan–amylose polymers is pro- because xanthan influence their values, since it seems to moted. Phase separation phenomena were also observed by affect the granules’ rigidity and shape but also the adhesion Mandala and Palogou (2003) in potato starch of higher between the continuous phase and granules. concentration and xanthan systems during starch gelation According to the above assumptions, in our research, and in wheat starch–xanthan systems stored for a long samples heated with xanthan (technique (b)) at 90 8C, period at ambient conditions (Mandala, Michon, & Launay, have an increased w—not granules’ rupture observed— 2003). and consequently their viscosity should be lower compar- ing to samples prepared at 75 8C. However, this does not happen and it seems that xanthan makes granules more 4. Conclusions rigid. This increased rigidity is caused due to a film creation around the granules by xanthan and solubilized The addition of xanthan in starch dispersions increases amylose and due to adhesive interactions among the the swelling of granules during heating and many large granules as it was mentioned before. Consequently granules are observed even at relatively low temperature granules cannot act as fillers, that reinforce the continuous (75 8C). Amylose leakage increases also during heating, phase. but at high temperature (.80 8C) it is restricted. Due to Furthermore, continuous phase is affected by xanthan xanthan effects on granules swelling and SOL, one would addition notably. This late assumption issues by the fact that expect that the viscosity and consistency of starch/xanthan samples prepared without xanthan (technique (a)) at 90 8C mixtures heated together at 75 8C would be relatively high have similar viscosity values to those prepared with (technique (b)). However, viscosity values were greater xanthan, although the last ones contain more rigid granules, for samples prepared with technique (a). It seems that have a greater standard deviation of the granule size and many other factors influence the structure of the resulted normally they should have an increased viscosity. Besides, mixture. Xanthan is concentrated around the swollen at 75 8C the amount of amylose present in the continuous granules, increasing the shear forces applied to them and phase is greater in samples prepared by technique (b) but forming with the already leached amylose, a film around their viscosity is significant lower. For the role of xanthan in their surface. The main disadvantage is that the deformed continuous phase several assumptions exist. According to granules cannot enhance the consistency of the continuous Shi and BeMiller (2002) amylose–gum interactions are the phase. They seem to be rather rigid and xanthan main responsible for viscosity increase. 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