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xanthan mixture
Journal of Food Engineering 105 (2011) 233–240
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Physicochemical and rheological characteristics of commercial chili sauces as thickened by modified starch or modified starch/xanthan mixture C. Gamonpilas a,⇑, W. Pongjaruvat a, A. Fuongfuchat a, P. Methacanon a, N. Seetapan a, N. Thamjedsada b a b
National Metal and Materials Technology Center (MTEC), 114 Paholyothin Road, Pathumthani 12120, Thailand Siam Modified Starch Co., Ltd., Ladlumkaew, Pathumthani 12140, Thailand
a r t i c l e
i n f o
Article history: Received 27 October 2010 Received in revised form 7 February 2011 Accepted 12 February 2011 Available online 17 February 2011 Keywords: Chili sauce Rheological properties Hydrocolloids FTIR spectroscopy
a b s t r a c t Physicochemical and rheological properties of various commercial chili sauces were characterised in this study. It was found that all studied sauces were acidic. Fourier transform infrared spectroscopy and colour staining experiments showed that the studied sauce could be categorised into two groups: one containing only starch and the others having both starch and xanthan within their ingredients. The rheological measurements showed that all sauces had a weak gel-like characteristic with strong shear thinning behaviour. In particular, the sauce with the weakest network contained only starch. Flow experimental data, fitted with the Herschel–Bulkley model, also revealed that the sauce with the lowest total solid content had undetectable yield stress, the lowest consistency coefficient, and the highest flow behaviour index, hence the weakest network structure amongst the studied sauces. The xanthan–starch mixture was shown to be beneficial as they could interact synergistically in acidic condition to enhance the gel-like characteristics and shear thinning behaviour of chili sauces. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Chili (Capsicum annuum) is a spice-cum-vegetable of commercial importance. It is cultivated mainly in Southeast Asia, India, China and Mexico, while it is extensively used in cuisines around the world. One of the most popular products processed from chili is known as chili sauce or sometimes called hot sauce. Such a product is frequently consumed with a variety of foods to add piquant taste as well as to give better appearance and texture characteristics (Ahmed et al., 2000; Ikhu-Omoregbe and Bushi, 2008; Martinez-Padilla and Rivera-Vargas, 2006). A large assortment of chilies produced and consumed around the world results in the manufacture of many types and formulations of chili sauces available in the market. This has satisfied the growing demand of the variety of chili sauces. Typically, raw materials used in producing chili sauces include chili, garlic, water, sugar, salt, vinegar, hydrocolloids and preservatives. The chili and garlic used in the recipe can sometimes be in a fermented form. A manufacturing process for chili sauce production normally involves four different stages. Firstly, all raw materials are mixed and cooked under high shear conditions, typically around 3000 rpm, at temperature approximately between 90 and 95 °C. The resulting sauce is subsequently pumped into a reservoir to allow cooling down before being filled into containers. From ⇑ Corresponding author. Fax: +66 25646446. E-mail address: [email protected] (C. Gamonpilas). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.02.024
manufacturers’ perspectives, good understanding of the rheological characteristics of liquid foods is pertinent to optimising flow processes, product quality control and storage stability (Sikora et al., 2003). Hence, the designs of the aforementioned processing conditions for chili sauce require sufficient knowledge of rheology. Specifically, it is known that viscoelastic properties play dominant roles in the handling and quality of sauces (Rao and Steffe, 1992). Such properties are not only dependent on time but also on processing temperature, solid contents and other ingredients used in many types and formulations of chili sauces. Chili sauces are considered as complex multiphase suspensions of deformable chili particles and sometimes liquid deformable particles such as oil droplets. The continuous phase is essentially water or a solution of macromolecules in water which includes hydrocolloids, salt, organic acid or other components to achieve an acidic characteristic and to preserve the product. Addition of hydrocolloids is thought to be necessary in the manufacturing of chili sauce in order to provide sufficient viscosity and to stabilise the suspensions for prolonging shelf-life (Sikora et al., 2003, 2008a). Although numerous studies have been conducted on the rheological properties of sauce varieties; for instance, ketchup, barbecue and Mexican sauces, limited work is available for commercial chili sauces (Alvarez et al., 2004; Bhattacharya et al., 1999; Juszczak et al., 2004; Martinez-Padilla and Rivera-Vargas, 2006). Furthermore, none of the works published has investigated in depth the types of hydrocolloid used in commercial chili sauces.
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Such information is also not clearly labelled for consumers. Therefore, the objectives of this work were to investigate the physicochemical and rheological characteristics of various commercial chili sauces. Types of hydrocolloid added to the sauces were also examined. Moreover, rheological properties of the sauces were discussed in relation to their physicochemical properties and types of hydrocolloid found. 2. Materials and methods Four commercial chili sauces, here called F (Pegasus, Thai Roong Rueng Chili sauce Co., Ltd.), H (Heinz, Win Chance Foods, Ltd., under the licence of H.J. Heinz Co., USA), K (King’s Kitchen, Premier Canning Industry Co., Ltd.) and A (ABC Sambal Asli, Heinz ABC Indonesia), were purchased from a local supermarket. The first three sauces were produced locally whereas the last one was imported from Indonesia. Compositions of these sauces, as shown on the packaging’ labels, are summarised in Table 1. The missing ingredients from the total compositions of sauces F and H are presumed as amounts of water and hydrocolloids. Xanthan gum (CEROGA 80 mesh, C.E. Roeper GmbH, Germany) and crosslinked tapioca starch (Kreation TU10, Siam Modified Starch Co., Ltd., Thailand) were used to examine functional groups and rheological behaviours in comparison to hydrocolloids present in the chili sauces. The analyses performed on these sauces and hydrocolloids are as follows. 2.1. Determination of pH pH of sauce samples was determined using a pH metre (IQ Scientific Instruments, California, USA). Five replicates were performed for each chili sauce. 2.2. Determination of total solid content Total solids content (TSC) of each chili sauce was measured by drying two grams of sauce in an oven at 100 ± 5 °C for 24 h. For each sauce, the experiment was repeated on triplicate samples. 2.3. Microstructural and particle size analyses An optical microscope (Axioshop microscopy, Carl Zeizz Microimaging GmbH, Hamburg, Germany) was used to obtain a microstructure of chili sauces. Images were taken at a magnification of 100. Sizes of chili particles and other constituents were explored. In particular, sizes of the observed chili particles were measured in terms of their particle areas using the UTHSCSA Image analysis tool
Table 1 Properties of each commercial chili sauce. Note that the composition of sauce A was not available from its label.
a–d
Ingredients (%)
F
H
K
A
Chili
28
28
–
Sugar Vinegar
17 16
23 10
Garlic
12
9.5
Salt Preservatives Total pH TSC (%)
10 Yes 83 3.43 ± 0.09c 17.04 ± 0.06d
– No 70.5 3.56 ± 0.12b 30.07 ± 0.63c
60 (fermented) 10 5 (10%vinegar) 25 (fermented) – No 100 3.51 ± 0.06b,c 30.72 ± 0.11b
– – – – – – 3.93 ± 0.01a 37.61 ± 0.03a
Different superscripts in the same row are significantly different (P < 0.05).
(1997). Ten measurements were made and an average area was calculated for each chili sauce. 2.4. Determination of hydrocolloids in commercial chili sauce An extraction of hydrocolloids from individual commercial chili sauces was prerequisite to determining the type of hydrocolloid present in each chili sauce. Fifty grams of each chili sauce sample were dialysed against deionised water overnight using dialysis tubing (molecular weight cut off 3500, Spectra/PorÒ, USA). Next, the dialysate was heated up to 80 °C for the complete dissolution of hydrocolloids. The sample was then centrifuged at 2500g for 5 min at 23 °C. The obtained residue was discarded whereas the supernatant was added with petroleum ether for removal of chili oil. The obtained aqueous phase was concentrated at 40 °C under vacuum condition for approximately 30 min using a rotary evaporator. The hydrocolloids were subsequently precipitated with four volumes of absolute ethanol. The precipitate was then filtered and dried in a vacuum oven at 25 °C prior to further analyses. After the extraction of hydrocolloids, two different analytical techniques were employed. Firstly, Fourier transform infrared spectroscopy (FTIR) was performed to analyse for functional groups of the extracted hydrocolloids using a Perkin–Elmer System 2000 FT-IR spectrophotometer (Perkin–Elmer, Inc., Massachusetts, USA) in the range of 400–4000 cm1. The sample pellets were prepared by mixing the fine hydrocolloid powder with KBr (Sigma–Aldrich, FTIR grade). The results obtained were then compared with the spectra of pure hydrocolloids such as xanthan gum, CMC, guar gum, native tapioca starch and its derivative (crosslinked starch). Secondly, the staining characteristics of extracted hydrocolloids were investigated in order to validate the FTIR results. Toluidine blue solution (0.1%, Fluka), Lugol’s solution (1%, Sigma–Aldrich) (Mandala et al., 2004) and the 2,7-napthalenediol method (Graham, 1971) were used for detection of starch, xanthan and carboxymethyl cellulose (CMC), respectively. 2.5. Rheological measurement Rheological experiments were performed using a controlledstress Gemini HRnano rheometer (Malvern Instruments Ltd., UK). The instrument was equipped with a cone and plate geometry (55 mm cone diameter and 2° Cone angle). The gap between cone and plate was 70 lm. Firstly, dynamic stress sweep experiments from 0.1–100 Pa at a constant frequency of 1 Hz were conducted to determine the linear viscoelastic region (LVE). Dynamic frequency sweep experiments between 0.01–10 Hz were subsequently performed at a fixed stress within the LVE range, i.e. at 1 Pa. Furthermore, steady shear tests were carried out at a range of shear rate from 0.0003–100 1/s. All measurements were performed at 25 °C. Triplicate samples were used for each sauce. It is pertinent to note that all reported data were not corrected for slip effects. In the preliminary analyses, slip effects were negligible as the rheological experiments using both cone and plate and serrated plate geometries were very similar. Furthermore, only ramping-up viscosity is presented for the results of steady shear tests. The rheological properties of food materials can be described by a number rheological models amongst which is the Herschel–Bulkley model that is used extensively in many non-Newtonian liquid foods (Alvarez et al., 2004; Juszczak et al., 2004). Such a model can be expressed as
s ¼ s0 þ K c_ n
ð1Þ
where s is shear stress (Pa), s0 is yield stress (Pa), c_ is shear rate (1/ s), K is consistency coefficient (Pasn) and n is flow behaviour index. In this work, the model was used to fit the results obtained from the
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steady shear experiments. The yield stress was obtained directly from the flow curve (s vs c_ ). The value was then used to calculate K and n by plotting log (s s0) against log (c_ ) where K and n were obtained directly from the y-intercept and slope of the graph, respectively. Note that the range of shear rate used in fitting the Herschel–Bulkley equation is between 1–100 1/s. 2.6. Determination of synergy between xanthan and modified tapioca starch A mixture of xanthan (0.1%w/w) and crosslinked tapioca starch (Kreation TU10, 3.9%w/w) suspension was prepared in pH buffer 3.37. Such level of pH was chosen to mimic the typical acidic characteristic of commercial chili sauces. The suspension was prepared by first heating the starch suspension at 90–95 °C for 20 min. The gelatinised starch suspension was allowed to cool down prior to the addition of xanthan. Density and swelling ability, i.e. weight and volume swelling ratios, of the crosslinked starch were also measured using the Ultrapycnometer 1000 (Quantachrome, Boynton Beach, Florida, USA) and the method described in Çaykara et al. (2003), respectively. Values of the density, weight and volume swelling ratios of the modified starch in aqueous solution at pH 3.37 were found to be equal to 1.51 g/cm3, 13.20 ± 0.41 and 20.79 ± 0.62, respectively. In order to determine the synergy between modified starch and xanthan, an assumption was made based on the work of Mandala and Bayas (2004) that the rheological property enhancement of starch-hydrocolloid systems was due to the excluded volume effect of starch granule. Therefore, in the mixture of xanthan (0.1%w/w) and crosslinked tapioca starch (Kreation TU10, 3.9%w/w) suspension, the amount of water needed for the modified starch to swell to its maximum capability was calculated to be only 50.70 cm3, thus 46.30 cm3 of water left for xanthan to dissolve. As such, the actual concentration of xanthan in the system was found to be equal to 0.22%w/w. Hence, xanthan (0.22%w/ w) solution was also prepared. Rheological measurements, as described earlier, were then performed on the mixture suspension, xanthan (0.22%w/w) solution and Kreation TU10 (4%w/w) suspension for comparison purposes. 2.7. Statistical analyses Experimental data obtained from pH and total solid content measurements and constants in the Herschel–Bulkley model were subjected to statistical analyses using the commercial SPSS 11.5 computer programme. Data were averaged and mean comparisons were performed using ANOVA and a least significant difference (LSD) technique at 95% confidence.
3. Results and discussions 3.1. Physicochemical properties Results of pH and total solid content (TSC) measurements on each chili sauce are given in Table 1. It can be seen that pH values of all chili sauces were in the range of 3.43–3.93, indicating an acidic characteristic. Such characteristic was almost certainly resulted from some vinegar within the recipe as shown in Table 1. Sauce F was found to be the most acidic sauce compared to the others as it contained the highest amount of vinegar. On the contrary, sauce A was observed as the least acidic sauce. TSC was significantly different amongst four sauce samples. Particularly, sauce A was found to possess the highest TSC (Table 1). This was consistent with the appearance of the sauce that appeared the grainiest compared to other sauces. On the other hand,
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sauce F had the lowest TSC value and revealed much smoother appearance under direct observation. 3.2. Optical microstructure and particle size A typical microstructure of chili sauce is shown in Fig. 1a. It is evident from here that the sauce composed of thin platelet chili particles and oil droplets suspended in a soluble matrix. Chili particle sizes were measured and results are presented in terms of the mean area in Fig. 1b. It was found that the average area of chili particles for sauce A was much larger compared to other sauces. This was consistent with the results of TSC and the direct observation that the sauce A was lumpiest amongst all four sauces studied in this work. The particle sizes of sauces F, H and K were very similar. 3.3. Determination of hydrocolloids in commercial chili sauces FTIR experiments were performed on hydrocolloids extracted from all commercial chili sauces. Peak assignments of the hydrocolloids extracted from each chili sauce are summarised in Table 2, together with those obtained from xanthan gum, CMC, guar gum, native and crosslinked tapioca starches. It was found that the spectra of hydrocolloids from sauce H, K and A were comparable (spectra not shown). Moreover, a distinctive band at around 1747 cm1, owing to the C@O stretching of esters, was observed in the spectra of hydrocolloids from all sauces except for sauce F. This peak was designated as the C@O stretching of pyruvate which is typically observed in xanthan. In addition, a broad peak at 1651 cm1, corresponding to bending of water overlapped with the asymmetric stretching of carboxylated anion (COO) of xanthan, was observed in the spectra of the hydrocolloids extracted from sauces H, K and A. It can also be seen that the hydrocolloids extracted from all sauces exhibited characteristic IR peaks similar to those of native or crosslinked tapioca starches. Existence of all aforementioned IR peaks indicated the presence of starch, most likely a modified starch resistant to acid, i.e. crosslinked type, in all studied sauces. Therefore, it could be stated that sauce F was likely to contain only a crosslinked starch, whereas other sauces may potentially include a mixture of xanthan and crosslinked starch in their ingredients. In order to validate the FTIR results, staining of the extracted hydrocolloids from the sauce samples with Lugol’s and Toluidine blue solutions was carried out. Lugol’s solution, a mixture of iodine and potassium iodide, was used as an indicator for the presence of starch. According to staining observations (Table 3), all extracted hydrocolloids from each chili sauce consisted of some kinds of starch since Lugol’s solution converted from yellow to either blue or red. Particularly, the red colour was observed in the hydrocolloid extracted from sauce K, signifying the presence of low amylose starch, i.e. waxy-type starch. It is worth noting here that Lugol’s solution contains polyiodide ions which can form a colour starchiodine complex. Although this reaction is not fully understood, it is thought that iodine (I 3 and I5 ions) fits inside the coils of amylose and the charge consequently transfers between the iodine and the starch. The strength of the resulting blue colour depends on the amount of amylose present. Waxy starch with little or no amylose will be stained red. Moreover, the detection of xanthan using the Toluidine blue solution revealed that the extracted hydrocolloids from all chili sauces, with the exception of sauce F, changed the stained solution colour to purple, suggesting the existence of xanthan gum in the chili sauces. It is worth pointing out that traces for CMC were also examined by the 2,7-napthalenediol method (Graham, 1971) but none was found present in the extracted hydrocolloids.
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Fig. 1. (a) A typical microstructure of chili sauce as observed through an optical microscope and (b) average size of chili particles in each chili sauce. Error bars shown in (b) represent standard deviations.
Table 2 FTIR spectra characteristics of hydrocolloids extracted from various chili sauces, native or crosslinked tapioca starches, xanthan gum, guar gum and carboxymethyl cellulose. Wavenumber (cm1)
Band assignment
Hydrocolloids from sauce F
Hydrocolloids from sauce A, H, and K
Native or Crosslinked tapioca starches
Xanthan gum
Guar gum
CMC
3366 2929
3322 2932
3298 2913 1731
3320 2926
3273 2921
1641
3350–3375 2930–2933 1732–1742 1643–1653
1418
1413–1426
1421
1614 1424
1424
1241 1152 1076
1241–1243 1154–1157 1079–1080
1244 1157 1080
1257 1155 1052
1152 1076
1020 928
1015–1022 929–934
1015 928
1026
1024
895
869
855
858–861
858
1650
1658
815
3.4. Rheological properties Stress sweep experiment was initially performed in order to determine the linear viscoelastic region. Such experiment can also provide the information on critical stress (sc) that is the maximum stress up to which G0 and G00 remain constant. G0 is proportional to the extent of the elastic component due to crosslinking, entanglement, or aggregation in the system. G00 is rational to the extent of the viscous (liquid-like) characteristic of the system. The sc therefore indicates the onset of non-linear region where the system’s structure starts to deform under applied stress. A wider LVE region, thus a larger sc, indicates the system stability to better resist the external stress. Stress sweep experimental results showed that sauce F had the shortest LVE region (Fig. 2) and, thus, the lowest value of sc (Table 4). These results indicated that such sauce had the lowest stability and required the minimum stress to cause the structure deformation. On the contrary, sauces K and A had the highest sc, thus, requiring the highest stress to deform their original structures. Dynamic frequency sweep experiments were performed at shear stress of 1 Pa, i.e. within the LVE range, and the obtained results are shown in Fig. 3. Results showed that all chili sauces had G0
1598 1426 1377 1330 1267 1155 1062 1015 911 897
OH stretching CH stretching C@O stretching (ester) Water associated in biopolymers COO stretching (asymmetry) CH2 in pyranose ring bending COO stretching (symmetry) C–O stretching O–H bending C–O–C asymmetric stretching C–O–C bending (glycosidic linkage) C–O valence vibration Pyranose ring vibration b-conformer a-conformer Characteristic peak of guar gum
higher than G00 (Fig. 3a), hence a dominant elastic behaviour as compared to the viscous behaviour, typically observed in suspensions with network-like structure. In addition, a power law relationship of frequency-dependence of moduli (G0 > G00 / xn ) was observed. In particular, at frequency less than 0.1 Hz, G00 was independent on the frequency typifying a characteristic of the dispersed particle system. Furthermore, the network structure in sauce F sample was the weakest owing to the most frequency-sensitive G0 and the highest magnitude of tand (Fig. 3b). Results on shear viscosity as a function of shear rate were shown in Fig. 4. It is apparent that sauce F has the lowest viscosity across the shear rate sweep, while other sauces show comparable viscosity levels. It is worth mentioning that viscosity profiles of sauce H, K and A showed two distinct shear thinning, similar to the observations found in the concentrated suspensions with transient network structure. The change in slope had been postulated to arise from various reasons such as a wall slip at low shear rate of flow-induced particle migration (Den Ouden and Van Vliet, 2002), a breakage of agglomerates (De Kee et al., 1983; Tiziani and Vodovotz, 2005) or hydrodynamic stresses dominant (Grizzuti et al., 1990; Moan et al., 2003). Herschel–Bulkley model, shown in Eq. (1), was used to fit the data at high shear rate. Apparent yield
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C. Gamonpilas et al. / Journal of Food Engineering 105 (2011) 233–240 Table 3 Staining observations of hydrocolloids extracted from each chili sauce. Samples
Detection of starch
Detection of xanthan
Control
F
K
H
A
stress (s0), i.e. the stress at peak viscosity, of all sauce samples, except sauce F, was observed with sauce K showing the highest yield stress (Table 4). In particular, sauce A had the highest K value, indicating the most consistency; i.e., the highest viscosity, compared to other sauces. The magnitude of n, which represents the proximity of Newtonian flow, showed that all studied sauces behaved more like a shear-thinning fluid; i.e., n < 1. In addition, an undetectable yield stress and the lowest K value in sauce F might be attributed to its lowest TSC and its weakest network structure amongst all four sauces studied in this work. It is also worth pointing out that thixotropy is not presented in all the studied sauces. The Cox–Merz rule can be used to characterise the rheological properties of materials with network-like structures and can indicate the shear/strain sensitivity of the structure. The relationship is given as;
jg ðxÞj ¼ gðc_ Þjc_ ¼x
ð2Þ
where g⁄ is the complex viscosity (Pas), g is the shear viscosity (Pas), x is frequency of oscillation (rad/s) and c_ is shear rate (1/s). The above relation has been shown to be applicable to several synthetic polymers in both melts and solutions and for several
solutions of random coil polysaccharides. However, it has been shown that shear/strain-sensitive network-like structure exists when complex viscosity plotted against angular frequency (in rad/ s) is consistently higher than the shear viscosity plotted against shear rate (in 1/s) on the same graph (Han et al., 2002; Silva et al., 1997). There have been many attempts to modify the Cox– Merz rule to be applicable for other complex systems. For example, a generalised Cox–Merz rule was proposed by introducing two constants, k and a, to Eq. (2) as follows;
jg ðxÞj ¼ kgðc_ Þa c_ ¼x
ð3Þ
The above equation was originally derived from ten fluid and semisolid food products (Bistany and Kokini, 1983). Moreover, an extended Cox–Merz rule, also known as ‘‘Delaware-Rutgers’’ rule, defined as in Eq. (4), was proposed by Doraiswamy et al. (1991) for concentrated suspensions and materials with yield stress. In this case, the effective steady shear rate which is the product between angular frequency and amplitude of oscillation, i.e. maximum strain (cm), was used in the equation as;
jg ðcm xÞj ¼ gðc_ Þjc_ ¼cm x
ð4Þ
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Fig. 2. Amplitude sweep profiles for various commercial chili sauces obtained at constant frequency of 1 Hz (a) sauce F, (b) sauce H, (c) sauce K, and (d) sauce A.
Table 4 Parameters determined from amplitude sweep and steady shear flow experiments. Sauce F H K A
sc (Pa) *
0.20 0.90* 1.50* 1.50*
s0 (Pa)
K (Pasn)
n
– 0.93 ± 0.13b 3.01 ± 0.57a 2.62 ± 0.29a
4.53 ± 0.44d 10.89 ± 0.21c 18.90 ± 0.14b 22.82 ± 2.30a
0.42 ± 0.01a 0.31 ± 0.01c 0.32 ± 0.01c 0.35 ± 0.02b
– for non-detectable result. a–d Different superscripts in the same row are significantly different (P < 0.05). * standard deviation was close to zero.
The viscosity curve calculated from Eq. (4) will superimpose with that obtained from Eq. (3) if a is equal to 1. In this work, the frequency sweep experiment was performed in a stress-controlled mode. Therefore, the strain amplitude of oscillation was not constant. Consequently, the extended Cox–Merz rule was not applicable to the obtained data. However, the curve shifting along angular frequency axis of complex viscosity was carried out to obtain the fitting value of amplitude of oscillation, cm, fitting. Goodness of fit in terms of residue of sum square (RSS) was analysed with the shear viscosity in the shear rate range of 0.1–100 1/s. Values of the constants k and a of the generalised Cox–Merz rule are shown in Table 5 for all studied chili sauces. The cm, fitting obtained from the best fit of complex viscosity curve, were also shown in the table. It can be seen that the complex viscosity of
all chili sauces were greater than the shear viscosity as k values were greater than unity. In addition, values of a for sauce H, K and A were higher than one while that of sauce F was close to 1. This indicates that the extended Cox–Merz rule was not applicable for most of the studied sauces. Differences in structure arrangement or particle orientation at small amplitude oscillation and continuous flow of sauce H, K and A may have caused the value of a to be more than 1. Furthermore, the best fitting of extended Cox-Merz rule by shifting angular frequency with cm, fitting cannot be obtained as RSS was quite large. Deviation from Cox–Merz rule, as k and a were higher than 1, is possibly resulted from the interaction amongst particles and complex network structure that may have been unaffected during small amplitude oscillation measurements. Conversely, the large deformation in steady shear tests resulted in structural network rupture, thus, lower viscosity. For the case of sauce F, the smallest difference between the complex and shear viscosity values compared to other sauces suggests that its network structure is relatively weak and can easily be ruptured even at small deformation. This finding is consistent with the earlier observations from the stress and frequency sweep results. It can be stated from the results shown in this work that rheological characteristics of commercial chili sauces can be influenced by the total solid content, particle size of chili paste, and type of the present hydrocolloid. Generally, increases in total solid content and particle size contribute to higher viscosity of sauces. Such findings agree with the previous work by Tanglertpaibul and Rao
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Table 5 Values of constants calculated from generalised and extended Cox–Merz rules for all chili sauces. Sauce
F H K A
Fig. 3. Frequency sweep profiles of various commercial chili sauces showing (a) G0 and G00 , and (b) tand as a function of frequency within LVE region.
Generalised Cox–Merz rule
Extended Cox–Merz rule
k
a
R2
cm,fitting
RSS
1.43 2.21 2.03 4.56
0.99 1.11 1.19 1.15
0.996 0.999 0.999 0.999
0.57 0.30 0.21 0.08
5 704 20209 11696
is very versatile and is extensively used in many food products (Katzbauer, 1998). This is due to its superior zero shear viscosity and more pronounced shear thinning behaviour compared to other hydrocolloids. The shear thinning characteristic is beneficial to organoleptic qualities, e.g. flavour release and mouth-feel, in food products and promotes mixability, pumpability and flowability characteristics which are important factors for the design of flow systems, product development and for scale-up and mechanisation of the process. Furthermore, an overlap concentration, which is the transition point between dilute and semi-dilute regimes, of xanthan gum is relatively low, making it very advantageous to achieve the desirable viscosity in food products. The unique rigid, rod-like conformation, which can form a three-dimensional network, also makes xanthan gum an efficient stabiliser for suspensions and emulsion. Consequently, the stability of xanthan gum over a broad range of acid pH makes it suitable for chili sauce applications (Katzbauer, 1998). Another advantage that xanthan can offer is that it may act synergistically with many other hydrocolloids such as galactomannans, glucomannans and a range of modified starches (Mandala and Bayas, 2004; Sikora et al., 2008b). Rheological measurements were performed on the mixture of xanthan (0.1%w/w) and crosslinked starch (Kreation TU10, 3.9%w/w) suspension, xanthan (0.22%w/w) solution and Kreation TU10 (4%w/w) suspension in order to illustrate that the enhanced rheological properties of commercial chili sauces could be resulted from the synergy between xanthan gum and modified starch. In Fig. 5, weak shear thinning behaviour was observed for the xanthan solution and Kreation TU10 suspension where the viscosity of xanthan solution was lower than that of the starch suspension for the entire shear rate. However, it can also be seen that when a small amount of xanthan (0.1%w/w) was added to the starch suspension of 3.9%w/w, a marked increase in viscosity was observed across the studied shear rates and the mixture possessed a more
Fig. 4. Shear viscosity as a function of shear rate for various commercial chili sauces.
(1987) and Ahmed et al. (2000). Moreover, additions of xanthan gum and/or modified starch into the recipe can give a significant improvement in the rheological properties, especially providing network-like characteristic of the sauce. The former hydrocolloid
Fig. 5. Shear viscosity as a function of shear rate for a mixture of Kreation TU10 (3.9%w/w) and xanthan (0.1%w/w), Kreation TU10 (4% w/w) suspension and xanthan (0.22%w/w) solution. All samples were prepared in pH buffer 3.37.
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Acknowledgement Financial support of this work has been provided by Siam Modified Starch, Co., Ltd. References
Fig. 6. Storage (G0 ) and loss (G00 ) moduli as a function of the applied frequency for (a) a mixture of Kreation TU10 (3.9%w/w) and xanthan (0.1%w/w) suspension, (b) Kreation TU10 (4%w/w) suspension and (c) xanthan (0.22%w/w) solution. All samples were prepared in pH buffer 3.37.
pronounced shear thinning behaviour. This increase in viscosity was extremely larger than the pure xanthan solution (0.22%w/w). Similar to the steady shear experimental results, frequency sweep experiment showed enhanced elastic property, i.e. higher storage modulus, in the mixed system compared to the pure xanthan solution (Fig. 6). Therefore, this investigation revealed that such viscosity improvement of the mixed system was evidently from the synergy rather than the additive effect. However, in this study, we did not attempt to identify the mechanism of such synergy in acidic condition, yet we simply showed that the use of modified starch as a thickener or stabiliser in chili sauce product is of particular commercial interest as it offers the possibility of novel functionalities on the one hand and of using reduced levels of xanthan gum, thus possibly reducing production cost, on the other. 4. Conclusions Four commercial chili sauces were investigated in terms of their physicohemical, microstructural and rheological properties. These sauces showed acidic characteristic with pH in the range of 3.43– 3.93. Optical microstructure analysis on these sauces showed the distinguished areas of chili particles and oil droplets within soluble matrix. FTIR and colour staining experiments revealed that all studied chili sauces contained starch as one of their ingredients. Furthermore, they could consist of xanthan gum which was most likely added to enhance their flow characteristics. Rheological experiments showed that all studied sauces had a weak gel-like characteristic (G0 > G00 / xn ). Their flow characteristics were nonNewtonian with shear thinning behaviour that could be described effectively by Herschel–Bulkley model with R2 1. The presence of starch/xanthan mixture in the commercial chili sauces promoted their elastic properties. Furthermore, it was found that the sauce with low solid content and without xanthan gum had weak network structure and inferior flow properties.
Ahmed, J., Shivhare, U.S., Raghavan, G.S.V., 2000. Rheological characteristics and kinetics of colour degradation of green chili puree. Journal of Food Engineering 44, 239–244. Alvarez, E., Cancela, M.A., Maceiras, R., 2004. Comparison of rheological behaviour of sweet and salad sauces. International Journal of Food Properties 7, 511–518. Bhattacharya, S., Vasudha, N., Krishna, K., 1999. Rheology of mustard paste: a controlled stress measurement. Journal of Food Engineering 41, 187–191. Bistany, K.L., Kokini, J.L., 1983. Dynamic viscoelastic properties of foods in texture control. Journal of Rheology 27, 605–620. Çaykara, T., Bozkaya, U., Kantog˘lu, Ö., 2003. Network structure and swelling behaviour of poly(acrylamide/crotonic acid) hydrogels in aqueous salt solutions. Journal of Polymer Science Part B: Polymer Physics 41, 1656–1664. De Kee, D., Code, R.K., Turcotte, G., 1983. Flow properties of time dependent foodstuffs. Journal of Rheology 27, 581–604. Den Ouden, F.W.C., Van Vliet, T., 2002. Effect of concentration on the rheology and serum separation of tomato suspensions. Journal of Texture Studies 33, 91–104. Doraiswamy, D., Mujumdar, A.N., Tsao, I., Beris, A.N., Danforth, S.C., Metzner, A.B., 1991. The Cox–Merz rule extended: a rheological model for concentrated suspensions and other materials with a yield stress. Journal of Rheology 35, 647–685. Graham, H.D., 1971. Determination of carboxymethylcellulose in food products. Journal of Food Science 36, 1052–1055. Grizzuti, N., Cavella, S., Cicarelli, P., 1990. Transient and steady-state rheology of a liquid crystalline hydroxypropylcellulose solution. Journal of Rheology 34, 1293–1310. Han, X.Z., Campanella, O.H., Guan, H., Keeling, P.L., Hamaker, B.R., 2002. Influence of maize starch granule-associated protein on the rheological properties of starch pastes. Part II. Dynamic measurements of viscoelastic properties of starch pastes. Carbohydrate Polymers 49, 323–330. Ikhu-Omoregbe, D., Bushi, G.M., 2008. Rheological characteristics of South African commercial sauces. International Journal of Food Science and Technology 43, 2230–2236. Juszczak, L., Witczak, M., Fortuna, T., Banys, A., 2004. Rheological properties of commercial mustards. Journal of Food Engineering 63, 209–217. Katzbauer, B., 1998. Properties and applications of xanthan gum. Polymer Degradation and Stability 59, 81–84. Mandala, I.G., Bayas, E., 2004. Xanthan effect on swelling, solubility and viscosity of wheat starch dispersions. Food Hydrocolloids 18, 191–201. Mandala, I.G., Savvas, T.P., Kostaropoulos, A.E., 2004. Xanthan and locust bean gum influence on the rheology and structure of a white model-sauce. Journal of Food Engineering 64, 335–342. Martinez-Padilla, L.P., Rivera-Vargas, C., 2006. Flow behaviour of Mexican sauces using a vane-in-a-large cup rheometer. Journal of Food Engineering 72, 189– 196. Moan, M., Aubry, T., Bossard, F., 2003. Nonlinear behavior of very concentrated suspensions of plate-like kaolin particles in shear flow. Journal of Rheology 47, 1493–1504. Rao, M.A., Steffe, J.F., 1992. Viscoelastic Properties of Foods. Elsevier Applied Science, London. Sikora, M., Juszczak, L., Sady, M., Krawontka, J., 2003. Use of starch/xanthan gum combinations as thickeners of cocoa syrups. Nahrung/Food 47, 106–113. Sikora, M., Badrie, N., Deisingh, A.K., Kowalski, S., 2008a. Sauces and dressings: a review of properties and applications. Critical Reviews in Food Science and Nutrition 48, 50–77. Sikora, M., Kowalski, S., Tomasik, P., 2008b. Binary hydrocolloids from starches and xanthan gum. Food Hydrocolloids 22, 943–952. Silva, P.M.S., Oliveira, J.C., Rao, M.A., 1997. Granule size distribution and rheological behaviour of heated modified waxy and unmodified maize starch dispersions. Journal of Texture Studies 28, 123–128. Tanglertpaibul, T., Rao, M.A., 1987. Rheological properties of tomato concentrates as affected by particle size and methods of concentration. Journal of Food Science 52, 141–145. Tiziani, S., Vodovotz, Y., 2005. Rheological effects of soy protein addition to tomato juice. Food Hydrocolloids 19, 45–52. UTHSCSA Image Analysis Tool, 1997. User Manual IT Version 2.0.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Physicochemical and rheological characteristics of commercial chili sauces as thickened by modified starch or modified starch/xanthan mixture C. Gamonpilas a,⇑, W. Pongjaruvat a, A. Fuongfuchat a, P. Methacanon a, N. Seetapan a, N. Thamjedsada b a b
National Metal and Materials Technology Center (MTEC), 114 Paholyothin Road, Pathumthani 12120, Thailand Siam Modified Starch Co., Ltd., Ladlumkaew, Pathumthani 12140, Thailand
a r t i c l e
i n f o
Article history: Received 27 October 2010 Received in revised form 7 February 2011 Accepted 12 February 2011 Available online 17 February 2011 Keywords: Chili sauce Rheological properties Hydrocolloids FTIR spectroscopy
a b s t r a c t Physicochemical and rheological properties of various commercial chili sauces were characterised in this study. It was found that all studied sauces were acidic. Fourier transform infrared spectroscopy and colour staining experiments showed that the studied sauce could be categorised into two groups: one containing only starch and the others having both starch and xanthan within their ingredients. The rheological measurements showed that all sauces had a weak gel-like characteristic with strong shear thinning behaviour. In particular, the sauce with the weakest network contained only starch. Flow experimental data, fitted with the Herschel–Bulkley model, also revealed that the sauce with the lowest total solid content had undetectable yield stress, the lowest consistency coefficient, and the highest flow behaviour index, hence the weakest network structure amongst the studied sauces. The xanthan–starch mixture was shown to be beneficial as they could interact synergistically in acidic condition to enhance the gel-like characteristics and shear thinning behaviour of chili sauces. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Chili (Capsicum annuum) is a spice-cum-vegetable of commercial importance. It is cultivated mainly in Southeast Asia, India, China and Mexico, while it is extensively used in cuisines around the world. One of the most popular products processed from chili is known as chili sauce or sometimes called hot sauce. Such a product is frequently consumed with a variety of foods to add piquant taste as well as to give better appearance and texture characteristics (Ahmed et al., 2000; Ikhu-Omoregbe and Bushi, 2008; Martinez-Padilla and Rivera-Vargas, 2006). A large assortment of chilies produced and consumed around the world results in the manufacture of many types and formulations of chili sauces available in the market. This has satisfied the growing demand of the variety of chili sauces. Typically, raw materials used in producing chili sauces include chili, garlic, water, sugar, salt, vinegar, hydrocolloids and preservatives. The chili and garlic used in the recipe can sometimes be in a fermented form. A manufacturing process for chili sauce production normally involves four different stages. Firstly, all raw materials are mixed and cooked under high shear conditions, typically around 3000 rpm, at temperature approximately between 90 and 95 °C. The resulting sauce is subsequently pumped into a reservoir to allow cooling down before being filled into containers. From ⇑ Corresponding author. Fax: +66 25646446. E-mail address: [email protected] (C. Gamonpilas). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.02.024
manufacturers’ perspectives, good understanding of the rheological characteristics of liquid foods is pertinent to optimising flow processes, product quality control and storage stability (Sikora et al., 2003). Hence, the designs of the aforementioned processing conditions for chili sauce require sufficient knowledge of rheology. Specifically, it is known that viscoelastic properties play dominant roles in the handling and quality of sauces (Rao and Steffe, 1992). Such properties are not only dependent on time but also on processing temperature, solid contents and other ingredients used in many types and formulations of chili sauces. Chili sauces are considered as complex multiphase suspensions of deformable chili particles and sometimes liquid deformable particles such as oil droplets. The continuous phase is essentially water or a solution of macromolecules in water which includes hydrocolloids, salt, organic acid or other components to achieve an acidic characteristic and to preserve the product. Addition of hydrocolloids is thought to be necessary in the manufacturing of chili sauce in order to provide sufficient viscosity and to stabilise the suspensions for prolonging shelf-life (Sikora et al., 2003, 2008a). Although numerous studies have been conducted on the rheological properties of sauce varieties; for instance, ketchup, barbecue and Mexican sauces, limited work is available for commercial chili sauces (Alvarez et al., 2004; Bhattacharya et al., 1999; Juszczak et al., 2004; Martinez-Padilla and Rivera-Vargas, 2006). Furthermore, none of the works published has investigated in depth the types of hydrocolloid used in commercial chili sauces.
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Such information is also not clearly labelled for consumers. Therefore, the objectives of this work were to investigate the physicochemical and rheological characteristics of various commercial chili sauces. Types of hydrocolloid added to the sauces were also examined. Moreover, rheological properties of the sauces were discussed in relation to their physicochemical properties and types of hydrocolloid found. 2. Materials and methods Four commercial chili sauces, here called F (Pegasus, Thai Roong Rueng Chili sauce Co., Ltd.), H (Heinz, Win Chance Foods, Ltd., under the licence of H.J. Heinz Co., USA), K (King’s Kitchen, Premier Canning Industry Co., Ltd.) and A (ABC Sambal Asli, Heinz ABC Indonesia), were purchased from a local supermarket. The first three sauces were produced locally whereas the last one was imported from Indonesia. Compositions of these sauces, as shown on the packaging’ labels, are summarised in Table 1. The missing ingredients from the total compositions of sauces F and H are presumed as amounts of water and hydrocolloids. Xanthan gum (CEROGA 80 mesh, C.E. Roeper GmbH, Germany) and crosslinked tapioca starch (Kreation TU10, Siam Modified Starch Co., Ltd., Thailand) were used to examine functional groups and rheological behaviours in comparison to hydrocolloids present in the chili sauces. The analyses performed on these sauces and hydrocolloids are as follows. 2.1. Determination of pH pH of sauce samples was determined using a pH metre (IQ Scientific Instruments, California, USA). Five replicates were performed for each chili sauce. 2.2. Determination of total solid content Total solids content (TSC) of each chili sauce was measured by drying two grams of sauce in an oven at 100 ± 5 °C for 24 h. For each sauce, the experiment was repeated on triplicate samples. 2.3. Microstructural and particle size analyses An optical microscope (Axioshop microscopy, Carl Zeizz Microimaging GmbH, Hamburg, Germany) was used to obtain a microstructure of chili sauces. Images were taken at a magnification of 100. Sizes of chili particles and other constituents were explored. In particular, sizes of the observed chili particles were measured in terms of their particle areas using the UTHSCSA Image analysis tool
Table 1 Properties of each commercial chili sauce. Note that the composition of sauce A was not available from its label.
a–d
Ingredients (%)
F
H
K
A
Chili
28
28
–
Sugar Vinegar
17 16
23 10
Garlic
12
9.5
Salt Preservatives Total pH TSC (%)
10 Yes 83 3.43 ± 0.09c 17.04 ± 0.06d
– No 70.5 3.56 ± 0.12b 30.07 ± 0.63c
60 (fermented) 10 5 (10%vinegar) 25 (fermented) – No 100 3.51 ± 0.06b,c 30.72 ± 0.11b
– – – – – – 3.93 ± 0.01a 37.61 ± 0.03a
Different superscripts in the same row are significantly different (P < 0.05).
(1997). Ten measurements were made and an average area was calculated for each chili sauce. 2.4. Determination of hydrocolloids in commercial chili sauce An extraction of hydrocolloids from individual commercial chili sauces was prerequisite to determining the type of hydrocolloid present in each chili sauce. Fifty grams of each chili sauce sample were dialysed against deionised water overnight using dialysis tubing (molecular weight cut off 3500, Spectra/PorÒ, USA). Next, the dialysate was heated up to 80 °C for the complete dissolution of hydrocolloids. The sample was then centrifuged at 2500g for 5 min at 23 °C. The obtained residue was discarded whereas the supernatant was added with petroleum ether for removal of chili oil. The obtained aqueous phase was concentrated at 40 °C under vacuum condition for approximately 30 min using a rotary evaporator. The hydrocolloids were subsequently precipitated with four volumes of absolute ethanol. The precipitate was then filtered and dried in a vacuum oven at 25 °C prior to further analyses. After the extraction of hydrocolloids, two different analytical techniques were employed. Firstly, Fourier transform infrared spectroscopy (FTIR) was performed to analyse for functional groups of the extracted hydrocolloids using a Perkin–Elmer System 2000 FT-IR spectrophotometer (Perkin–Elmer, Inc., Massachusetts, USA) in the range of 400–4000 cm1. The sample pellets were prepared by mixing the fine hydrocolloid powder with KBr (Sigma–Aldrich, FTIR grade). The results obtained were then compared with the spectra of pure hydrocolloids such as xanthan gum, CMC, guar gum, native tapioca starch and its derivative (crosslinked starch). Secondly, the staining characteristics of extracted hydrocolloids were investigated in order to validate the FTIR results. Toluidine blue solution (0.1%, Fluka), Lugol’s solution (1%, Sigma–Aldrich) (Mandala et al., 2004) and the 2,7-napthalenediol method (Graham, 1971) were used for detection of starch, xanthan and carboxymethyl cellulose (CMC), respectively. 2.5. Rheological measurement Rheological experiments were performed using a controlledstress Gemini HRnano rheometer (Malvern Instruments Ltd., UK). The instrument was equipped with a cone and plate geometry (55 mm cone diameter and 2° Cone angle). The gap between cone and plate was 70 lm. Firstly, dynamic stress sweep experiments from 0.1–100 Pa at a constant frequency of 1 Hz were conducted to determine the linear viscoelastic region (LVE). Dynamic frequency sweep experiments between 0.01–10 Hz were subsequently performed at a fixed stress within the LVE range, i.e. at 1 Pa. Furthermore, steady shear tests were carried out at a range of shear rate from 0.0003–100 1/s. All measurements were performed at 25 °C. Triplicate samples were used for each sauce. It is pertinent to note that all reported data were not corrected for slip effects. In the preliminary analyses, slip effects were negligible as the rheological experiments using both cone and plate and serrated plate geometries were very similar. Furthermore, only ramping-up viscosity is presented for the results of steady shear tests. The rheological properties of food materials can be described by a number rheological models amongst which is the Herschel–Bulkley model that is used extensively in many non-Newtonian liquid foods (Alvarez et al., 2004; Juszczak et al., 2004). Such a model can be expressed as
s ¼ s0 þ K c_ n
ð1Þ
where s is shear stress (Pa), s0 is yield stress (Pa), c_ is shear rate (1/ s), K is consistency coefficient (Pasn) and n is flow behaviour index. In this work, the model was used to fit the results obtained from the
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steady shear experiments. The yield stress was obtained directly from the flow curve (s vs c_ ). The value was then used to calculate K and n by plotting log (s s0) against log (c_ ) where K and n were obtained directly from the y-intercept and slope of the graph, respectively. Note that the range of shear rate used in fitting the Herschel–Bulkley equation is between 1–100 1/s. 2.6. Determination of synergy between xanthan and modified tapioca starch A mixture of xanthan (0.1%w/w) and crosslinked tapioca starch (Kreation TU10, 3.9%w/w) suspension was prepared in pH buffer 3.37. Such level of pH was chosen to mimic the typical acidic characteristic of commercial chili sauces. The suspension was prepared by first heating the starch suspension at 90–95 °C for 20 min. The gelatinised starch suspension was allowed to cool down prior to the addition of xanthan. Density and swelling ability, i.e. weight and volume swelling ratios, of the crosslinked starch were also measured using the Ultrapycnometer 1000 (Quantachrome, Boynton Beach, Florida, USA) and the method described in Çaykara et al. (2003), respectively. Values of the density, weight and volume swelling ratios of the modified starch in aqueous solution at pH 3.37 were found to be equal to 1.51 g/cm3, 13.20 ± 0.41 and 20.79 ± 0.62, respectively. In order to determine the synergy between modified starch and xanthan, an assumption was made based on the work of Mandala and Bayas (2004) that the rheological property enhancement of starch-hydrocolloid systems was due to the excluded volume effect of starch granule. Therefore, in the mixture of xanthan (0.1%w/w) and crosslinked tapioca starch (Kreation TU10, 3.9%w/w) suspension, the amount of water needed for the modified starch to swell to its maximum capability was calculated to be only 50.70 cm3, thus 46.30 cm3 of water left for xanthan to dissolve. As such, the actual concentration of xanthan in the system was found to be equal to 0.22%w/w. Hence, xanthan (0.22%w/ w) solution was also prepared. Rheological measurements, as described earlier, were then performed on the mixture suspension, xanthan (0.22%w/w) solution and Kreation TU10 (4%w/w) suspension for comparison purposes. 2.7. Statistical analyses Experimental data obtained from pH and total solid content measurements and constants in the Herschel–Bulkley model were subjected to statistical analyses using the commercial SPSS 11.5 computer programme. Data were averaged and mean comparisons were performed using ANOVA and a least significant difference (LSD) technique at 95% confidence.
3. Results and discussions 3.1. Physicochemical properties Results of pH and total solid content (TSC) measurements on each chili sauce are given in Table 1. It can be seen that pH values of all chili sauces were in the range of 3.43–3.93, indicating an acidic characteristic. Such characteristic was almost certainly resulted from some vinegar within the recipe as shown in Table 1. Sauce F was found to be the most acidic sauce compared to the others as it contained the highest amount of vinegar. On the contrary, sauce A was observed as the least acidic sauce. TSC was significantly different amongst four sauce samples. Particularly, sauce A was found to possess the highest TSC (Table 1). This was consistent with the appearance of the sauce that appeared the grainiest compared to other sauces. On the other hand,
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sauce F had the lowest TSC value and revealed much smoother appearance under direct observation. 3.2. Optical microstructure and particle size A typical microstructure of chili sauce is shown in Fig. 1a. It is evident from here that the sauce composed of thin platelet chili particles and oil droplets suspended in a soluble matrix. Chili particle sizes were measured and results are presented in terms of the mean area in Fig. 1b. It was found that the average area of chili particles for sauce A was much larger compared to other sauces. This was consistent with the results of TSC and the direct observation that the sauce A was lumpiest amongst all four sauces studied in this work. The particle sizes of sauces F, H and K were very similar. 3.3. Determination of hydrocolloids in commercial chili sauces FTIR experiments were performed on hydrocolloids extracted from all commercial chili sauces. Peak assignments of the hydrocolloids extracted from each chili sauce are summarised in Table 2, together with those obtained from xanthan gum, CMC, guar gum, native and crosslinked tapioca starches. It was found that the spectra of hydrocolloids from sauce H, K and A were comparable (spectra not shown). Moreover, a distinctive band at around 1747 cm1, owing to the C@O stretching of esters, was observed in the spectra of hydrocolloids from all sauces except for sauce F. This peak was designated as the C@O stretching of pyruvate which is typically observed in xanthan. In addition, a broad peak at 1651 cm1, corresponding to bending of water overlapped with the asymmetric stretching of carboxylated anion (COO) of xanthan, was observed in the spectra of the hydrocolloids extracted from sauces H, K and A. It can also be seen that the hydrocolloids extracted from all sauces exhibited characteristic IR peaks similar to those of native or crosslinked tapioca starches. Existence of all aforementioned IR peaks indicated the presence of starch, most likely a modified starch resistant to acid, i.e. crosslinked type, in all studied sauces. Therefore, it could be stated that sauce F was likely to contain only a crosslinked starch, whereas other sauces may potentially include a mixture of xanthan and crosslinked starch in their ingredients. In order to validate the FTIR results, staining of the extracted hydrocolloids from the sauce samples with Lugol’s and Toluidine blue solutions was carried out. Lugol’s solution, a mixture of iodine and potassium iodide, was used as an indicator for the presence of starch. According to staining observations (Table 3), all extracted hydrocolloids from each chili sauce consisted of some kinds of starch since Lugol’s solution converted from yellow to either blue or red. Particularly, the red colour was observed in the hydrocolloid extracted from sauce K, signifying the presence of low amylose starch, i.e. waxy-type starch. It is worth noting here that Lugol’s solution contains polyiodide ions which can form a colour starchiodine complex. Although this reaction is not fully understood, it is thought that iodine (I 3 and I5 ions) fits inside the coils of amylose and the charge consequently transfers between the iodine and the starch. The strength of the resulting blue colour depends on the amount of amylose present. Waxy starch with little or no amylose will be stained red. Moreover, the detection of xanthan using the Toluidine blue solution revealed that the extracted hydrocolloids from all chili sauces, with the exception of sauce F, changed the stained solution colour to purple, suggesting the existence of xanthan gum in the chili sauces. It is worth pointing out that traces for CMC were also examined by the 2,7-napthalenediol method (Graham, 1971) but none was found present in the extracted hydrocolloids.
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Fig. 1. (a) A typical microstructure of chili sauce as observed through an optical microscope and (b) average size of chili particles in each chili sauce. Error bars shown in (b) represent standard deviations.
Table 2 FTIR spectra characteristics of hydrocolloids extracted from various chili sauces, native or crosslinked tapioca starches, xanthan gum, guar gum and carboxymethyl cellulose. Wavenumber (cm1)
Band assignment
Hydrocolloids from sauce F
Hydrocolloids from sauce A, H, and K
Native or Crosslinked tapioca starches
Xanthan gum
Guar gum
CMC
3366 2929
3322 2932
3298 2913 1731
3320 2926
3273 2921
1641
3350–3375 2930–2933 1732–1742 1643–1653
1418
1413–1426
1421
1614 1424
1424
1241 1152 1076
1241–1243 1154–1157 1079–1080
1244 1157 1080
1257 1155 1052
1152 1076
1020 928
1015–1022 929–934
1015 928
1026
1024
895
869
855
858–861
858
1650
1658
815
3.4. Rheological properties Stress sweep experiment was initially performed in order to determine the linear viscoelastic region. Such experiment can also provide the information on critical stress (sc) that is the maximum stress up to which G0 and G00 remain constant. G0 is proportional to the extent of the elastic component due to crosslinking, entanglement, or aggregation in the system. G00 is rational to the extent of the viscous (liquid-like) characteristic of the system. The sc therefore indicates the onset of non-linear region where the system’s structure starts to deform under applied stress. A wider LVE region, thus a larger sc, indicates the system stability to better resist the external stress. Stress sweep experimental results showed that sauce F had the shortest LVE region (Fig. 2) and, thus, the lowest value of sc (Table 4). These results indicated that such sauce had the lowest stability and required the minimum stress to cause the structure deformation. On the contrary, sauces K and A had the highest sc, thus, requiring the highest stress to deform their original structures. Dynamic frequency sweep experiments were performed at shear stress of 1 Pa, i.e. within the LVE range, and the obtained results are shown in Fig. 3. Results showed that all chili sauces had G0
1598 1426 1377 1330 1267 1155 1062 1015 911 897
OH stretching CH stretching C@O stretching (ester) Water associated in biopolymers COO stretching (asymmetry) CH2 in pyranose ring bending COO stretching (symmetry) C–O stretching O–H bending C–O–C asymmetric stretching C–O–C bending (glycosidic linkage) C–O valence vibration Pyranose ring vibration b-conformer a-conformer Characteristic peak of guar gum
higher than G00 (Fig. 3a), hence a dominant elastic behaviour as compared to the viscous behaviour, typically observed in suspensions with network-like structure. In addition, a power law relationship of frequency-dependence of moduli (G0 > G00 / xn ) was observed. In particular, at frequency less than 0.1 Hz, G00 was independent on the frequency typifying a characteristic of the dispersed particle system. Furthermore, the network structure in sauce F sample was the weakest owing to the most frequency-sensitive G0 and the highest magnitude of tand (Fig. 3b). Results on shear viscosity as a function of shear rate were shown in Fig. 4. It is apparent that sauce F has the lowest viscosity across the shear rate sweep, while other sauces show comparable viscosity levels. It is worth mentioning that viscosity profiles of sauce H, K and A showed two distinct shear thinning, similar to the observations found in the concentrated suspensions with transient network structure. The change in slope had been postulated to arise from various reasons such as a wall slip at low shear rate of flow-induced particle migration (Den Ouden and Van Vliet, 2002), a breakage of agglomerates (De Kee et al., 1983; Tiziani and Vodovotz, 2005) or hydrodynamic stresses dominant (Grizzuti et al., 1990; Moan et al., 2003). Herschel–Bulkley model, shown in Eq. (1), was used to fit the data at high shear rate. Apparent yield
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C. Gamonpilas et al. / Journal of Food Engineering 105 (2011) 233–240 Table 3 Staining observations of hydrocolloids extracted from each chili sauce. Samples
Detection of starch
Detection of xanthan
Control
F
K
H
A
stress (s0), i.e. the stress at peak viscosity, of all sauce samples, except sauce F, was observed with sauce K showing the highest yield stress (Table 4). In particular, sauce A had the highest K value, indicating the most consistency; i.e., the highest viscosity, compared to other sauces. The magnitude of n, which represents the proximity of Newtonian flow, showed that all studied sauces behaved more like a shear-thinning fluid; i.e., n < 1. In addition, an undetectable yield stress and the lowest K value in sauce F might be attributed to its lowest TSC and its weakest network structure amongst all four sauces studied in this work. It is also worth pointing out that thixotropy is not presented in all the studied sauces. The Cox–Merz rule can be used to characterise the rheological properties of materials with network-like structures and can indicate the shear/strain sensitivity of the structure. The relationship is given as;
jg ðxÞj ¼ gðc_ Þjc_ ¼x
ð2Þ
where g⁄ is the complex viscosity (Pas), g is the shear viscosity (Pas), x is frequency of oscillation (rad/s) and c_ is shear rate (1/s). The above relation has been shown to be applicable to several synthetic polymers in both melts and solutions and for several
solutions of random coil polysaccharides. However, it has been shown that shear/strain-sensitive network-like structure exists when complex viscosity plotted against angular frequency (in rad/ s) is consistently higher than the shear viscosity plotted against shear rate (in 1/s) on the same graph (Han et al., 2002; Silva et al., 1997). There have been many attempts to modify the Cox– Merz rule to be applicable for other complex systems. For example, a generalised Cox–Merz rule was proposed by introducing two constants, k and a, to Eq. (2) as follows;
jg ðxÞj ¼ kgðc_ Þa c_ ¼x
ð3Þ
The above equation was originally derived from ten fluid and semisolid food products (Bistany and Kokini, 1983). Moreover, an extended Cox–Merz rule, also known as ‘‘Delaware-Rutgers’’ rule, defined as in Eq. (4), was proposed by Doraiswamy et al. (1991) for concentrated suspensions and materials with yield stress. In this case, the effective steady shear rate which is the product between angular frequency and amplitude of oscillation, i.e. maximum strain (cm), was used in the equation as;
jg ðcm xÞj ¼ gðc_ Þjc_ ¼cm x
ð4Þ
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Fig. 2. Amplitude sweep profiles for various commercial chili sauces obtained at constant frequency of 1 Hz (a) sauce F, (b) sauce H, (c) sauce K, and (d) sauce A.
Table 4 Parameters determined from amplitude sweep and steady shear flow experiments. Sauce F H K A
sc (Pa) *
0.20 0.90* 1.50* 1.50*
s0 (Pa)
K (Pasn)
n
– 0.93 ± 0.13b 3.01 ± 0.57a 2.62 ± 0.29a
4.53 ± 0.44d 10.89 ± 0.21c 18.90 ± 0.14b 22.82 ± 2.30a
0.42 ± 0.01a 0.31 ± 0.01c 0.32 ± 0.01c 0.35 ± 0.02b
– for non-detectable result. a–d Different superscripts in the same row are significantly different (P < 0.05). * standard deviation was close to zero.
The viscosity curve calculated from Eq. (4) will superimpose with that obtained from Eq. (3) if a is equal to 1. In this work, the frequency sweep experiment was performed in a stress-controlled mode. Therefore, the strain amplitude of oscillation was not constant. Consequently, the extended Cox–Merz rule was not applicable to the obtained data. However, the curve shifting along angular frequency axis of complex viscosity was carried out to obtain the fitting value of amplitude of oscillation, cm, fitting. Goodness of fit in terms of residue of sum square (RSS) was analysed with the shear viscosity in the shear rate range of 0.1–100 1/s. Values of the constants k and a of the generalised Cox–Merz rule are shown in Table 5 for all studied chili sauces. The cm, fitting obtained from the best fit of complex viscosity curve, were also shown in the table. It can be seen that the complex viscosity of
all chili sauces were greater than the shear viscosity as k values were greater than unity. In addition, values of a for sauce H, K and A were higher than one while that of sauce F was close to 1. This indicates that the extended Cox–Merz rule was not applicable for most of the studied sauces. Differences in structure arrangement or particle orientation at small amplitude oscillation and continuous flow of sauce H, K and A may have caused the value of a to be more than 1. Furthermore, the best fitting of extended Cox-Merz rule by shifting angular frequency with cm, fitting cannot be obtained as RSS was quite large. Deviation from Cox–Merz rule, as k and a were higher than 1, is possibly resulted from the interaction amongst particles and complex network structure that may have been unaffected during small amplitude oscillation measurements. Conversely, the large deformation in steady shear tests resulted in structural network rupture, thus, lower viscosity. For the case of sauce F, the smallest difference between the complex and shear viscosity values compared to other sauces suggests that its network structure is relatively weak and can easily be ruptured even at small deformation. This finding is consistent with the earlier observations from the stress and frequency sweep results. It can be stated from the results shown in this work that rheological characteristics of commercial chili sauces can be influenced by the total solid content, particle size of chili paste, and type of the present hydrocolloid. Generally, increases in total solid content and particle size contribute to higher viscosity of sauces. Such findings agree with the previous work by Tanglertpaibul and Rao
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Table 5 Values of constants calculated from generalised and extended Cox–Merz rules for all chili sauces. Sauce
F H K A
Fig. 3. Frequency sweep profiles of various commercial chili sauces showing (a) G0 and G00 , and (b) tand as a function of frequency within LVE region.
Generalised Cox–Merz rule
Extended Cox–Merz rule
k
a
R2
cm,fitting
RSS
1.43 2.21 2.03 4.56
0.99 1.11 1.19 1.15
0.996 0.999 0.999 0.999
0.57 0.30 0.21 0.08
5 704 20209 11696
is very versatile and is extensively used in many food products (Katzbauer, 1998). This is due to its superior zero shear viscosity and more pronounced shear thinning behaviour compared to other hydrocolloids. The shear thinning characteristic is beneficial to organoleptic qualities, e.g. flavour release and mouth-feel, in food products and promotes mixability, pumpability and flowability characteristics which are important factors for the design of flow systems, product development and for scale-up and mechanisation of the process. Furthermore, an overlap concentration, which is the transition point between dilute and semi-dilute regimes, of xanthan gum is relatively low, making it very advantageous to achieve the desirable viscosity in food products. The unique rigid, rod-like conformation, which can form a three-dimensional network, also makes xanthan gum an efficient stabiliser for suspensions and emulsion. Consequently, the stability of xanthan gum over a broad range of acid pH makes it suitable for chili sauce applications (Katzbauer, 1998). Another advantage that xanthan can offer is that it may act synergistically with many other hydrocolloids such as galactomannans, glucomannans and a range of modified starches (Mandala and Bayas, 2004; Sikora et al., 2008b). Rheological measurements were performed on the mixture of xanthan (0.1%w/w) and crosslinked starch (Kreation TU10, 3.9%w/w) suspension, xanthan (0.22%w/w) solution and Kreation TU10 (4%w/w) suspension in order to illustrate that the enhanced rheological properties of commercial chili sauces could be resulted from the synergy between xanthan gum and modified starch. In Fig. 5, weak shear thinning behaviour was observed for the xanthan solution and Kreation TU10 suspension where the viscosity of xanthan solution was lower than that of the starch suspension for the entire shear rate. However, it can also be seen that when a small amount of xanthan (0.1%w/w) was added to the starch suspension of 3.9%w/w, a marked increase in viscosity was observed across the studied shear rates and the mixture possessed a more
Fig. 4. Shear viscosity as a function of shear rate for various commercial chili sauces.
(1987) and Ahmed et al. (2000). Moreover, additions of xanthan gum and/or modified starch into the recipe can give a significant improvement in the rheological properties, especially providing network-like characteristic of the sauce. The former hydrocolloid
Fig. 5. Shear viscosity as a function of shear rate for a mixture of Kreation TU10 (3.9%w/w) and xanthan (0.1%w/w), Kreation TU10 (4% w/w) suspension and xanthan (0.22%w/w) solution. All samples were prepared in pH buffer 3.37.
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Acknowledgement Financial support of this work has been provided by Siam Modified Starch, Co., Ltd. References
Fig. 6. Storage (G0 ) and loss (G00 ) moduli as a function of the applied frequency for (a) a mixture of Kreation TU10 (3.9%w/w) and xanthan (0.1%w/w) suspension, (b) Kreation TU10 (4%w/w) suspension and (c) xanthan (0.22%w/w) solution. All samples were prepared in pH buffer 3.37.
pronounced shear thinning behaviour. This increase in viscosity was extremely larger than the pure xanthan solution (0.22%w/w). Similar to the steady shear experimental results, frequency sweep experiment showed enhanced elastic property, i.e. higher storage modulus, in the mixed system compared to the pure xanthan solution (Fig. 6). Therefore, this investigation revealed that such viscosity improvement of the mixed system was evidently from the synergy rather than the additive effect. However, in this study, we did not attempt to identify the mechanism of such synergy in acidic condition, yet we simply showed that the use of modified starch as a thickener or stabiliser in chili sauce product is of particular commercial interest as it offers the possibility of novel functionalities on the one hand and of using reduced levels of xanthan gum, thus possibly reducing production cost, on the other. 4. Conclusions Four commercial chili sauces were investigated in terms of their physicohemical, microstructural and rheological properties. These sauces showed acidic characteristic with pH in the range of 3.43– 3.93. Optical microstructure analysis on these sauces showed the distinguished areas of chili particles and oil droplets within soluble matrix. FTIR and colour staining experiments revealed that all studied chili sauces contained starch as one of their ingredients. Furthermore, they could consist of xanthan gum which was most likely added to enhance their flow characteristics. Rheological experiments showed that all studied sauces had a weak gel-like characteristic (G0 > G00 / xn ). Their flow characteristics were nonNewtonian with shear thinning behaviour that could be described effectively by Herschel–Bulkley model with R2 1. The presence of starch/xanthan mixture in the commercial chili sauces promoted their elastic properties. Furthermore, it was found that the sauce with low solid content and without xanthan gum had weak network structure and inferior flow properties.
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