Sauce elaboration

The sauces were elaborated with commercial habanero chili paste (Brand: PAPIK, INDUSTRIA AGRÍCOLA MAYA, S.A. de C.V.; mixture of ground habanero chili, size of particle 500 mm, with acetic acid and salt), water (1:1; control), and 0.2, 0.5 and 0.7 % of GG or XG. The mixtures were shaken at 3000 rpm, with a homogenizer (IKA, T18; Germany, with a 18N-19G dispersor), approximately 2 min. The chemical compounds used were of reactive grade (Sigma Aldrich Inc; USA).

The sauces prepared (50 mg) were placed in bags (48.0x14.5 cm) resistant to extreme temperatures and high pressures (pouch), sealed at 1.6 s and 40 % vacuum (Koch Equipment, UV250; USA, 2009), heated for 15 min at 121 °C in a vertical autoclave (ECOSHEL, CVQ-B50L; USA), and stored at 25 °C for 24 h before being analyzed. All of the samples were prepared in duplicate.

Analytical determinations

The partial physical and chemical characterization of chili paste and of the sauces before and after the thermal treatment included: pH according to the ^{NMX-F-317-S-1978} norm and with a potentiometer (Hanna Instruments, HI3222; USA); total soluble solids according to the ^{NMX-FF-015-1982} norm and with a refractometer (Atago, NAR-1Tliquid; Japan); and moisture content with the fast method described in the ^{NMX-F-428-1982} norm, with a thermobalance (Ohaus, B45; USA).

Rheological determinations

The rheological properties of the sauces were determined with a controlled effort rheometer (TA Instruments, DHR₂; USA) and the parallel plate geometry (40 mm of diameter) with a gap of 1050 μm. The flow was evaluated with the flow curves in function of the deformation speed (0.2 to 200 s^-1), cutting effort and viscosity. The experimental data were adjusted to the mathematical expression of the ^{Carreau model (Carreau, 1972)}, which describes the pseudoplastic behavior of polymers of a more realist form than the power law, since it adapts better to a broad interval of deformation speeds. The determination of the model of best fit to the flow curves was carried out with the software of the equipment (Trios 3.2.0 3877, DHR TA instruments) with R²=0.999.

where η ₀ is the constant viscosity at very low deformation speeds (close to resting), η _∞ is the Newtonian viscosity for high values of deformation speed, and c and b are the consistency and the index of deformation speed.

The viscoelastic properties were evaluated in the linear viscoelastic region (LVR) determined by assays of deformation amplitude, logarithmic increments (0.02 to 100 %), and frequency of 10 rad s^-1. The mechanical spectra were obtained with angular frequency sweeping assays (0.1-100 rad s^-1), with a deformation of 0.5 % through the analysis of the elastic modulus (G’), and the viscous modulus (G”) in function of the frequency.

Statistical analysis of the results

The experimental design was mixed factorial of three factors (gum, temperature, and concentration) with different levels. The results were analyzed with ANOVA and the means were compared by the Tukey test (p≤0.05), with the statistical software Statgraphics Centurion XVI. I. version 16.1.17 (Stat Point Technologies, 2011). γ

Analysis of the physical-chemical properties

The chili pastes showed in average 84.3 % of humidity, 1.17 % of titrable acidity, 20 °Brix and pH 3.43. The GG and XG and the thermal treatment did not cause significant changes in pH or soluble solids (3.4 and 20.0 °Brix). The humidity (h) of all the sauces tended to decrease with the gums (h_average=70.0 %) in comparison to the control (h_average= 77.8%) and was significantly lower in the sauces with 0.7 % of XG (h_average=68.0 %). This behavior could be due to the combination of the concentration and structural changes of the XG. A higher number of chains of XG could have favored the intramolecular interaction (network formation) and electrostatic interactions, between polar molecules of water and the polyelectrolyte that allowed retaining water and decreasing the global humidity of the system.

Flow properties

All the samples had a non-Newtonian pseudoplastic behavior, a common characteristic in emulsions, suspensions or dispersions, where viscosity is reduced with the increase of the deformation speed (Figure 1). With the gums, the pseudoplastic behavior was prominent. The index of consistency of the sauces after the thermal treatment decreased significantly only in sauces with 0.2 and 0.5 % of GG. The viscosity of the sauces increased with the gum concentration (h _{0 0.2 %}<h _{0 0.5 %}<h _{0 0.7 %}) (Table 1). The viscosity decreased in the sauces with GG and thermal treatment; in contrast, in those that contained XG the values increased significantly. The environmental conditions and the interactions of the hydrocolloid with other molecules (water, soluble solids and salts) could affect the viscous behavior of the matrix. Opposite to the case of sauces with XG, the thermal treatment in sauces with GG could weaken the interactions between the medium and the gum, or the structure of the disperse particles, which decreased the viscosity.

Figure 1 Flow curves of habanero chili sauces, with 0.2, 0.5 and 0.7 % of gums and thermal treatment. 

The viscosity in emulsions with heated GG or XG depended on XG (^{Desplanques et al., 2012}). ^{Naji et al. (2012)} obtained opposite results and observed that dispersions of XG, heated up to 121 °C for 15 min, decreased significantly the viscosity, and they studied the gum without taking into consideration other components, such as salts. The sauces, due to the number of components such as salt, plant particles, water, gum or others, are more complex systems than a binary solution. XG, in addition to having a higher molecular weight than GG, has anionic polyelectrolyte nature, which is why the rheological behavior could be impacted by the presence of salts in chili pastes. Sodium or potassium chloride in solutions with xanthan gum help to maintain the ordered structure and increase the thermal stability (^{Katzbauer, 1998}). The instability of the system formed with GG could be due to the increase in temperature and decrease of pH (higher dissociation of H⁺ ions from heat). Both extrinsic factors, in high levels, could affect the hydration speed of the gum and decrease the viscosity (^{Carlson and Ziegenfuss, 1965}; ^{BeMiller and Whistler, 1996}; ^{Srichamroen, 2007}).

Viscoelastic properties

The dynamic modulus of storage (G’) and loss (G’’), the viscoelastic region, and the critical deformation (point at which the dynamic modulus cease to be constant), represented by the horizontal lines in a specific deformation (vertical lines), increased in sauces with GG in relation to the increase in concentration (0.2 % < 0.5 % <0.7 %). However, in the sauces with XG the differences occurred only between the lowest and the highest concentrations; it seems that the lowest concentration didn’t impact the viscoelastic properties (Figures 2A and 3A). The loss of linearity in presence of XG took place in higher deformations (g≤1), and the fall in the dynamic modulus was more drastic in comparison to the samples with GG. This behavior could be because the structure of the system formed with GG was more altered or because the structural reordering was greater (Figures 2B and 3B).

Figure 2 Elastic modulus (G’, dark symbols) and viscous modulus (G’’, grey symbols) as function of the deformation (γ) of habanero chili sauces added with 0.2 (▲), 0.5 (■) and 0.7 % (♦) of guar gum without thermal treatment (A) and with it (B) 

Figure 3 Elastic modulus (G’, dark symbols) and viscous modulus (G’’, grey symbols) as function of the deformation (γ) of habanero chili sauces added with 0.2 (▲), 0.5 (■) and 0.7 % (♦) of xanthan gum without thermal treatment (A) and with it (B). 

Frequency sweeps

In all the samples the elastic response predominated over the viscous character during the interval of frequencies studied (G’>G”). The dynamic modulus (G’ and G’’) of the sauces supplemented with gums increased significantly with the increase of frequency in comparison with the control sample (Figures 4 and 5). This behavior indicated the formation of a network of a slightly structured, weak type of gel, typical of disperse viscoelastic systems. The positive slope of the lines of behavior showed the contribution of the solid phase in the structure of the system, without reaching the formation of a tridimensional network (^{Rao, 2013}).

Figure 4 Elastic (G’) and storage (G’’) modulus in function of the angular frequency of habanero chili sauces with different concentrations of guar gum without thermal treatment (A) and with it (B). 

Figure 5 Elastic (G’) and storage (G’’) modulus in function of the angular frequency of habanero chili sauces with different concentrations of xanthan gum without thermal treatment (A) and with it (B). 

The dynamic modulus (G’, G”) of the sauces with GG increased with the concentration (G’, G”_{0.2 %} < G’, G”_{0.5 %} < G’, G”_{0.7 %}); however, for sauces with XG there were no significant differences between 0.5 and 0.7 % (G’, G”_{0.2 %}<G’, G”_{0.5 %}=G’, G”_{0.7 %}). This could be due to the higher concentration of XG (0.7 %), the molecular interaction and because the ionic force favored the repulsive forces or the shielding of charges, decreasing the polymer association and impeding the formation of a structured and rigid network. The effective critical concentration that was observed for XG was lower than for GG (0.5 %), since there was no significance in the flow and viscoelasticity at greater concentrations.

The G’ and G’’ values of all the treatments decreased slightly with the thermal treatment; but for sauces with GG the separation between G’ and G’’ was higher than with XG (Figure 4B and 5B). This could be because of the degradation of the structure of GG from temperature, which affected the molecular interactions with the aqueous medium and between its chains. In the XG molecules, the temperature favored the hydration speed and, therefore, the formation of a stable network (^{Sworn, 2000}).

The rheological properties of habanero chili sauce improved with the three concentrations of gums. In addition, the temperature affected positively the sauce with XG, since it improved the viscosity and the dynamic modulus (elastic and loss) as compared to GG.