Textural and rheological properties of soybean oil organogels structured with polyglycerol and propylene glycol esters during storage

2.1. Materials

Refined soybean oil (SBO) was bought at a local supermarket (Monterrey, N.L., Mexico). The structurants used were: 1) A mixture of mono-diglycerides and polyglycerol esters (Polyglycerol Esters of Fatty Acids) or PGE (Fusion point = 55-61 °C) (Admulse MSPG-40) and 2) Propylene Glycol Esters of Fatty Acids or PPGE (Fusion point = 55-60 °C) (Admulse MEPG-AL). The structurants were kindly provided by ADIPLEX, S.A. de C.V (Monterrey, Nuevo León, Mexico).

2.2. Preparation of organogels

The preparation of organogels consisted of a standard methodology described by Rocha et al. (2013)Rocha JCB, Lopes JD, Mascarenhas MCN, Arellano DB, Guerreiro LMR, da Cunha RL. 2013. Thermal and rheological properties of organogels formed by sugarcane or candelilla wax in soybean oil. Food Res. Int. 50, 318-323. https://doi.org/10.1016/j.foodres.2012.10.043 , in which samples were prepared by heating soybean oil to 80 °C under continuous stirring. When the temperature of 80 °C was reached, the structurant was added at different concentrations (0.5, 1.0, 2.0, 3.0, or 4.0% w/w) and mixed until complete dissolution. The mixture was kept under agitation for about 3 min to assure complete melting. The resulting blends of soybean oil with PEG or PPGE were stored at 20 °C for 24 h to enhance the formation of gels and kept at this temperature for two months. The organogels were identified as SBO/PEG (soybean oil with polyglycerol esters) and SBO/PPGE (soybean oil with propylene glycol esters).

2.3. Visual appearance

After 24 hours of storage of the organogels, a visual assessment was performed. Organogels were subjectively classified into five types: 1, 2, 3, 4, and 5, corresponding to liquid, viscous liquid, high-flowing semisolid gel, low flowing semisolid gel, and a totally solid gel which maintained its structure and hardness, respectively (García et al., 2013Garcia RDKDA, Granda KMB, Arellano DB. 2013. Development of a zero trans margarine from soybean-based interesterified fats formulated using artificial neural networks. Grasas Aceites 64, 521-530. https://doi.org/10.3989/gya.049113 ).

2.4. Thermal stability

The thermal stability of organogels was determined by a cyclization process described by Garcia et al. (2013)Garcia RDKDA, Granda KMB, Arellano DB. 2013. Development of a zero trans margarine from soybean-based interesterified fats formulated using artificial neural networks. Grasas Aceites 64, 521-530. https://doi.org/10.3989/gya.049113 . Briefly, samples of 30 mL of each organogel were placed in 50-mL beakers and then subjected to sequential temperature variations according to the following conditions: 25 °C for 24 h for complete crystallization, followed subsequently by 5 °C for 24 h, 25 °C for 24 h, 5 °C for 48 h, 35 °C for 24 h, 25 °C for 24 h, 35 °C for 48 h and 5 °C for 48 h. Finally, the sample was stored at 25 °C for 24 h according to conditions specified in Table 1. After each storage condition, samples were immediately tested and classified according to the visual subjective appearance (type 1, 2, 3, 4, or 5). According to results of thermal stability, type-5 organogels stable under the cyclization conditions tested herein were stored at 25 ºC for 2 months. Both the visual appearance and thermal stability of the organogels were used as the main criteria to select the optimal concentration of SBO/PEG or SBO/PPGE.

2.5. Instrumental color measurements

The color of the SBO, PEG, PPGE, and their respective organogels were analyzed after 24 h at 25 °C. The color measurements based on the system CIEL*C*h (L = luminosity from zero (black) to 100 (white); +a = red, -a = green, +b = yellow, and -b = blue) were obtained with a Hunter Lab colorimeter (MiniScan PLUSXE, Hunter Lab, Reston, VA, U.S.A.) (da Silva et al., 2018bda Silva TL, Chaves KF, Fernandes GD, Rodrigues JB, Bolini HM, Arellano DB. 2018b. Sensory and technological evaluation of margarines with reduced saturated fatty acid contents using oleogel technology. J. Am. Oil Chem. Soc. 95, 673-685. https://doi.org/10.1002/aocs.12074 ) with slight modifications. The equipment was calibrated with standards provided by the supplier. E values were calculated by using the following equation: ∆E = (∆L² + ∆a² +∆b²)^1/2.

2.6. Hardness (compression/extrusion)

The hardness of solid-like SBO/PGE and SBO/PPGE organogels structured with 4% concentration (w/w) were evaluated by compression/extrusion measurements using the texture analyzer Stable Micro Systems model TA-XT2i (Godalming-UK). 40 mL of organogels were conditioned in 50-mL glass containers of 35 mm internal diameter and 22 mm height. The glass containers were kept for 24 h at 20 °C for stabilization and after this period compressed with a 15-mm acrylic cylinder (25 mm diameter and 35 mm height) with a head cross speed of 1.0 mm/s (Rocha et al., 2013Rocha JCB, Lopes JD, Mascarenhas MCN, Arellano DB, Guerreiro LMR, da Cunha RL. 2013. Thermal and rheological properties of organogels formed by sugarcane or candelilla wax in soybean oil. Food Res. Int. 50, 318-323. https://doi.org/10.1016/j.foodres.2012.10.043 ). All determinations were performed in triplicate.

2.7. Microstructure

The organogels were placed on a glass slide and covered with a coverslip. Then, organogels were conditioned at 20 °C for 24 h and examined under constant temperature. The morphologies of the crystals of SBO/PGE and SBO/PPGE organogels (4% w/w) were viewed under a polarized light microscope (Olympus System Microscope model BX 50, Olympus America Inc., Center Valley, PA, USA) equipped with the digital camera Olympus EX300 (Olympus America Inc., Center Valley, PA, USA). Photographs were taken in different fields and visuals, and the resulting images were evaluated using the software Image Pro-Plus 7.0.1 for Windows by Media Cybernetics (Bethesda, MD, USA) with a magnification of 4x (Rocha et al., 2013Rocha JCB, Lopes JD, Mascarenhas MCN, Arellano DB, Guerreiro LMR, da Cunha RL. 2013. Thermal and rheological properties of organogels formed by sugarcane or candelilla wax in soybean oil. Food Res. Int. 50, 318-323. https://doi.org/10.1016/j.foodres.2012.10.043 ).

2.8. Rheological properties: Flow curve

The rheological properties were assessed using a Rheometer (Anton Paar, Graz, Austria). The flow curves were acquired with sand-blasted rough plate geometry of 5 cm wide, a roughness of 5-7 μm, and a gap of 300 μm. The temperature throughout the analyses was maintained constant at 25 °C with shear rates ranging from 0 to 300 s⁻¹ (Rocha et al., 2013Rocha JCB, Lopes JD, Mascarenhas MCN, Arellano DB, Guerreiro LMR, da Cunha RL. 2013. Thermal and rheological properties of organogels formed by sugarcane or candelilla wax in soybean oil. Food Res. Int. 50, 318-323. https://doi.org/10.1016/j.foodres.2012.10.043 ). All determinations were made in triplicate and the models adjusted according to the Power Law, which classifies fluids according to their behavioral index (n) into: Newtonian (n = 1 and τ0 = 0), pseudoplastic (0 <n <1) or dilating (1 <n <∞).

2.9. Statistical analysis

The results were evaluated by analysis of variance (ANOVA) and Tukey tests were employed for comparison of means (p < 0.05). Data were reported as means and standard deviations. All statistical analyses were performed using the software JMP 5.0.1 (SAS Institute, Cary, NC, USA).

3.1. Visual appearance and thermal stability

The SBO/PGE and SBO/PPGE organogels formulated with different concentrations and kept at different storage temperatures showed changes in stability. Their visual appearance, gel consistency, and stability were affected by the concentration of structurants (Table 1). As expected, the stability and consistency increased with a higher concentration of structurants for both the SBO/PGE and SBO/PPGE organogels. Gels formulated with 0.5% showed a total liquid consistency. However, both structurants supplemented at 2% (w/w) yielded high flowing organogels with semisolid features (type 3).

Among the experimental organogels, the SBO/PGE and SBO/PPGE gels formulated with 4% (w/w) were entirely solid and stable (Type 5) at 25 °C compared to their counterparts prepared with lower concentrations of structurants. Interestingly, both structurants kept forming solid-like organogels when added at 4% and stored at a higher temperature (35 °C). These organogels had a slightly lower consistency (type 3) and did not show any indication of liquid phase separation. The consistency of all organogels decreased 1-point unit after 24 h of storage. As the concentration of structurant decreased, the consistency/stability of organogels also decreased, particularly those formulated with concentrations of 0.5 and 1.0%. These organogels yielded liquid gels rated as Type 1. The higher stability in SBO/PGE and SBO/PPGE organogels formulated with a concentration of 4% (w/w) were attributed to the chemical composition of these pure structurants (Polyglycerol Esters or PEG and propylene glycol esters or PPGE). These structurants can potentially mimic triacylglyceride crystallization through molecular self-assembly, leading to the formation of the more stable three-dimensional gel network. Similarly, Öǧütcü and Yilmaz (2014)Öǧütcü M, Yılmaz E. 2014. Oleogels of virgin olive oil with carnauba wax and monoglyceride as spreadable products. Grasas Aceites. 65 (3), 040. https://doi.org/10.3989/gya.0349141 reported that a 3% addition of monoglycerides in olive oil-based organogels yielded stable gels, whereas the candelilla wax could not create stable gels at the same concentration. Therefore, the results herein clearly indicate the potential for the use of SBO/PGE and SBO/PPGE for the production of gels with semi-liquid consistency at concentrations lower than 3% (w/w). PEG and PPGE structurants at concentrations of 0.5, 1.0, 2.0, or 3.0% w/w, showed total liquid and semi-liquid consistencies and negatively affected the thermal stability of organogels (Tables 1 and 2). Furthermore, a preliminary study was carried out to select the optimal storage of organogels. Organogels stored for two months showed particle aggregation and thus negatively affected the consistency. The best organogels were produced for the assessment of the microstructure, hardness, and rheological properties after 24 hours and two months of storage.

3.2. Color parameters

The color parameters of the organogels (SBO/PGE and SBO/PPGE) formulated with 4% PGE or PPGE are summarized in Table 2. The results indicate that the luminosity and yellowish coloration were not significantly affected by the two different polyglycerols. However, a significant difference in ΔE values was obtained in the organogels formulated with SBO/PPGE. The ΔE values of SBO/PPGE organogels indicated a greater color difference caused by the structurant type.

The luminosity, greenness and yellowish values in organogels formulated with 4% SBO/PGE were L= 24.49; a = 0.70, b = 3.94; whereas in SBO/PPGE counterparts L= 24.48; a = 0.72., b = 3.79 (Table 2). The observed differences can be attributed to the addition of higher proportions of soybean oil. The values of b parameter (b=) (yellowish) in soybean oil can be attributed to the presence of chlorophyll derivatives (pheophytin A) which are responsible for imparting green-yellow colorations (Fraser and Frankl, 1985Fraser MS, Frankl G. 1985. Detection of chlorophyll derivatives in soybean oil by HPLC. J. Am. Oil Chem. Soc. 62, 113-121. https://doi.org/10.1016/j.foodres.2017.12.020 ). On the other hand, the SBO-free PGE presented higher luminosity values (L=27.64). The structurants showed higher luminosity because the PGE was devoid of chlorophyll. Among all organogels, the SBO/PGE containing 2, 3 or 4% presented the lowest color values (data not shown). Thus, the addition of PGE for the production of SBO/PGE organogels enhanced luminosity values.

3.3. Hardness

The concentration of 4% SBO/PGE or SBO/PPGE was selected as the best to form structured organogels that could withstand prolonged storage at room temperature. These organogels presented characteristics of ideal thermal stability. The organogel texture values measured as hardness (N) at 24 hours and after 2 months of storage at room temperature are summarized in Table 2. These results showed that higher concentrations of SBO/PPGE and SBO/PEG produced organogels with similar maximum force at 24 hours. However, both SBO/PGE and SBO/PPGE organogels stored for 24 hours showed lower mechanical resistance compared to counterparts stored for 2 months. Interestingly, the organogels SBO/PPGE (4%, w/w) after 2 months of storage showed a higher mechanical resistance (0.080 N) compared to counterparts formulated with SBO/PGE (0.065 N). The major textural changes occurred after 24 h of storage. After organogels were prepared, the crystals gradually rearranged, allowing growth formation, which consequently altered the texture. The rearrangement increased oil exudation (apolar liquid phase) from the fat crystals and also enhanced phase separation (post-hardening phenomena) (Hughes et al., 2009Hughes NE, Marangoni AG, Wright AJ, Rogers MA, Rush JWE. 2009. Potential food applications of edible oil organogels. Trends Food Sci. Tech. 20, 470-480. https://doi.org/10.1016/j.tifs.2009.06.002 ). Moreover, the SBO/PPGE organogel formulated with a concentration of 4% showed two different crystal morphologies, consisting of larger fat crystal networks and spherulite crystals which were more evenly distributed compared to crystals formed in the SBO/PGE organogels. Likewise, the presence of two different morphologies of crystal explains the highest hardness and shear stress values observed for the SBO/PPGE organogel (Figure 1 A-B). This behavior is attributed to the different chemical compositions of the structurants. PEG is a mixture of mono-diglyceride and polyglycerol esters whereas PPGE consisted of propylene glycol esters of fatty acids. These results showed that a higher concentration of SBO/PPGE (4%, w/w) than SBO/PEG produced stronger organogels after 2 months of storage. Pernetti et al. (2007)Pernetti M, van Malssen KF, Flöter E, Bot A. 2007. Structuring of edible oils by alternatives to crystalline fat. Curr. Opin. Colloid Int. 12 (4-5), 221-231. https://doi.org/10.1016/j.cocis.2007.07.002 demonstrated that both diacylglycerols and monoacylglycerols were needed to produce organogels with softer textures. Regardless of the type of emulsifier, the longer the chain length the greater the firmness of the gel. However, these SBO/PGE and SBO/PPGE at 4% organogels showed lower hardness compared to organogels prepared with sugarcane wax (4%) and soybean oil (1.65 N) (Rocha et al., 2013Rocha JCB, Lopes JD, Mascarenhas MCN, Arellano DB, Guerreiro LMR, da Cunha RL. 2013. Thermal and rheological properties of organogels formed by sugarcane or candelilla wax in soybean oil. Food Res. Int. 50, 318-323. https://doi.org/10.1016/j.foodres.2012.10.043 ). Limited information exists about the presence of SBO/PGE and SBO/PPGE organogels, and this research contributes to new valuable information on the effects of PEG and PPGE addition and storage time on the hardness and related properties of the structured organogels.

3.4. Rheological properties: Flow curve

The organogels flow curves determined after 24 hours and 2 months of storage at 25 °C are depicted in Figures 2 A and B. The curves can be used for qualitative comparisons among organogels. Both SBO/PGE and SBO/PPGE organogels showed a characteristic thixotropic behavior (Steffe, 1996Steffe JF. 1996. Rheological methods in food process engineering. J. Food Eng. 23, 418.; Rocha et al., 2013Rocha JCB, Lopes JD, Mascarenhas MCN, Arellano DB, Guerreiro LMR, da Cunha RL. 2013. Thermal and rheological properties of organogels formed by sugarcane or candelilla wax in soybean oil. Food Res. Int. 50, 318-323. https://doi.org/10.1016/j.foodres.2012.10.043 ). However, organogels prepared with SBO/PGE stored for 24 h showed considerably lower shear stress values (24.16 Pa at 10.40 s^-1) (Figure 2A). However, a significant portion of the observed changes in shear stress occurred during storage because flow curve values increased with storage time for all treatments. Higher shear stress values were observed in organogels prepared with SBO/PPGE (Figure 2B). Shear stress values increased with the shear rate for the SBO/PPGE (4%) organogels after 2 months of storage (64.63 Pa at 300 s^-1 at 25 °C). Also, the SBO/PGE organogels presented a fast increase and decrease in shear stress at low shear rates which are a consequence of the easier disruption of the structural network (Riscardo et al., 2005Riscardo MA, Moros JE, Franco JM, Gallegos C. 2005. Rheological characterization of salad-dressing-type emulsions stabilised by egg yolk/sucrose distearate blends. Eur. Food Res. Technol. 220, 380-388. https://doi.org/10.1007/s00217-004-1052-9 ; Perrechil et al., 2010Perrechil FDA, Santana RDC, Fasolin LH, Silva CASD, Cunha RLD. 2010. Rheological and structural evaluations of commercial Italian salad dressings. Food Sci. Technol. 30, 477-482. https://doi.org/10.1590/S0101-20612010000200027 ). The shear stress values for SBO/PGE organogels at 2 months storage were 78.50 Pa at 10.40 s^-1. Storage resulted in stronger structural deformational changes in gels kept for 2 months, likely due to the formation of stronger crystal network aggregates.

The observed negative effects of storage for 8-10 weeks on the structure of organogels formulated in combination with monoglycerides and phytosterols have been previously described by Sintang et al. (2017a)Sintang MDB, Rimaux T, Van de Walle D, Dewettinck K, Patel AR. 2017a. Oil structuring properties of monoglycerides and phytosterols mixtures. Eur. J. Lipid Sci. Tech. 119, 1-14. https://doi.org/10.1002/ejlt.201500517 . Similar results related to shear stress (62.2 at 3 s^-1 at 25 °C) were obtained by Rocha et al. (2013)Rocha JCB, Lopes JD, Mascarenhas MCN, Arellano DB, Guerreiro LMR, da Cunha RL. 2013. Thermal and rheological properties of organogels formed by sugarcane or candelilla wax in soybean oil. Food Res. Int. 50, 318-323. https://doi.org/10.1016/j.foodres.2012.10.043 , who used sugarcane wax (4%) to structure organogels. Recently, Buitimea-Cantúa et al. (2020)Buitimea-Cantúa GV, Serna-Saldívar SO, Pérez-Carrillo E, Silva TJ, Barrera-Arellano D, Buitimea-Cantúa NE. 2020. Effect of quality of carnauba wax (Copernica cerífera) on microstructure, textural, and rheological properties of soybean oil-based organogels. LWT 136, 110267. https://doi.org/10.1016/j.lwt.2020.110267 reported that organogels elaborated with refined carnauba wax (5.5%) increased when the shear rate increased (110 at 3 s^-1) from 0 to 50 1/s and this effect was higher in organogels stored for 2 months (300 at 3 s^-1). Results herein demonstrated that the combination of SBO/PGE or SBO/PPGE added at different ratios could provide an array of new organogels with desired rheological properties which can last up to two months in storage at 25 °C. All organogels showed pseudoplastic flow characteristics.

3.5. Microstructure

Polarized light microphotographs of the SBO/PGE and SBO/PPGE organogels at 4% of concentration and stored for 24 h or 2 months are depicted in Figure 1. The micrographs show that the crystal networks of the SBO/PPGE organogels at 24 hours of storage tended to be smaller, more uniform, smoother, and more evenly distributed (Figure 1A) compared to the crystal arrangements of the SBO/PGE counterparts (Figure 1C).

A significant portion of the organogel crystal structure changed after 2 months of storage at 25 °C because these gels had larger crystal aggregations (Figures 1B and D). The structure formed by the SBO/PGE organogels (spherulite-crystals) (Figure 1D) is an indication of weaker intermolecular interactions such as Van der Waals interactions and London dispersion forces (Sintang et al., 2017aSintang MDB, Rimaux T, Van de Walle D, Dewettinck K, Patel AR. 2017a. Oil structuring properties of monoglycerides and phytosterols mixtures. Eur. J. Lipid Sci. Tech. 119, 1-14. https://doi.org/10.1002/ejlt.201500517 ). Different complex phenomena took place after two-months’ storage of solid-like organogels produced with 4% of PGE or PPGE. Post-crystallization events included polymorphic transitions from less stable to more stable polymorphs, the appearance of new crystalline particles, sintering, and Ostwald ripening (Johansson and Bergenståhl, 1995Johansson D, Bergenståhl B. 1995. Sintering of fat crystal networks in oil during post-crystallization processes. J. Am. Oil Chem. Soc. 72 (8), 911-920. https://doi.org/10.1007/BF02542069 ; Ojijo et al., 2004Ojijo NK, Kesselman E, Shuster V, Eichler S, Eger S, Neeman I., Shimoni E. 2004. Changes in microstructural, thermal, and rheological properties of olive oil/ monoglyceride networks during storage. Food Res. Int. 37 (4), 385-393. https://doi.org/10.1016/j.foodres.2004.02.003 ). These phenomena yielded the formation of larger crystal clusters instead of smaller counterparts, thereby leading to a weaker gel (Ribeiro et al., 2015Ribeiro APB, Masuchi MH, Miyasaki EK, Domingues MAF, Stroppa VLZ, de Oliveira GM, Kieckbusch TG. 2015. Crystallization modifiers in lipid systems. J. Food Sci. Technol. 52 (7), 3925-3946. https://doi.org/10.1007/s13197-014-1587-0 ; Tanaka et al., 2007Tanaka L, Miura S, Yoshioka T. 2007. Formation of granular crystals in margarine with excess amount of palm oil. J. Am. Oil Chem. Soc. 84 (5), 421-426. https://doi.org/10.1007/s11746-007-1064-2 ). These observations were previously documented by Doan et al. (2017)Doan CD, To CM, De Vrieze M, Lynen F, Danthine S, Brown A, Dewettinck K, Patel AR. 2017. Chemical profiling of the major components in natural waxes to elucidate their role in liquid oil structuring. Food Chem. 214, 717-725. https://doi.org/10.1016/j.foodchem.2016.07.123 and recently by Buitimea-Cantúa et al. (2020)Buitimea-Cantúa GV, Serna-Saldívar SO, Pérez-Carrillo E, Silva TJ, Barrera-Arellano D, Buitimea-Cantúa NE. 2020. Effect of quality of carnauba wax (Copernica cerífera) on microstructure, textural, and rheological properties of soybean oil-based organogels. LWT 136, 110267. https://doi.org/10.1016/j.lwt.2020.110267 in organogels structured with refined carnauba wax.

This might explain the observed higher mechanical resistance at the beginning of the compression/extrusion tests. It is important to visualize the morphology and crystal networks because they reflect the spatial distribution of crystals that influences their rheological properties (Marangoni and Rousseau, 1996Marangoni AG, Rousseau D. 1996. Is plastic fat rheology governed by the fractal nature of the fat crystal network? J. Am. Oil Chem. Soc. 73, 991-994. https://doi.org/10.1007/BF02523406 ; Blake et al., 2014Blake A I, Co ED, Marangoni, AG. 2014. Structure and physical properties of plant wax crystal networks and their relationship to oil binding capacity. J. Am. Oil Chem. Soc. 91, 885-903. https:/doi.org/10.1007/s11746-014-2435-0 ).