Phase Diagram of Glycerol Monostearate and Sodium Stearoyl Lactylate

Dec 2, 2015 - ABSTRACT: The phase behavior of two solid emulsifiers glycerol monostearate. (GMS) and sodium stearoyl lactylate (SSL) was examined ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/crystal

Phase Diagram of Glycerol Monostearate and Sodium Stearoyl Lactylate Fan C. Wang, Fernanda Peyronel, and Alejandro G. Marangoni* Department of Food Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada ABSTRACT: The phase behavior of two solid emulsifiers glycerol monostearate (GMS) and sodium stearoyl lactylate (SSL) was examined using differential scanning calorimetry (DSC), small and wide-angle X-ray scattering (SAXS and WAXS), and ultra small angle X-ray scattering (USAXS). The phase behavior of GMS−SSL in water (gelled aqueous dispersions) was also examined. For the nonaqueous systems, GMS and SSL were immiscible in the range of 20−80% and exhibited phase separation. On the other hand, systems with higher than 80% GMS or higher than 80% SSL were miscible and remained mixed after 4 weeks. Increasing the amount of SSL in GMS−SSL mixtures caused a reduction in the transition temperature of the sub-α phase and the melting temperature of GMS. Furthermore, adding up to 20% SSL to GMS slowed down the polymorphic transition of GMS from the α phase to the ß phase. Examination of the stability and polymorphic behavior of gelled aqueous dispersions of GMS−SSL mixtures revealed similar structural and thermal properties to their corresponding neat systems. Miller notation. The α phase is known to display d = 4.2 Å in the WAXS region and d = 50 Å in the SAXS region; while the β phase shows several WAXS peaks that give d-spacings between 3.6−4.6 Å and a d001 = 49 Å in the SAXS region.6,7,10,12,13 Distilled MGs molecules alone do not structure water and a co-emulsifier is required in order to form gels.15−21 SSL has previously been used as a co-emulsifier with GMS to form gels and structured emulsion systems. The polymorphic behavior and stability of gels and structured-emulsions formed by GMS and SSL has been well characterized.18,21−25 However, the structures formed and the polymorphic behavior of GMS and SSL neat systems have not been characterized. Understanding the structures formed by GMS and SSL mixed at different ratios could help predicting their behavior in gel and emulsion systems. SSL is commonly used in baked food and meat emulsions. It can interact with other emulsifiers and hydrocolloids to change the textural and rheological properties in meat batters, cookie dough, and frozen foods.26−31 SSL molecules are negatively charged and have an 18-carbon chain attached to a lactic acid headgroup. SSL crystals are naturally stable in the α form with a melting point of 45 °C.6,32 SSL molecules crystallize in a single chain length (SCL) packing configuration that shows a typical d-spacing of 4.1 Å in the WAXS region and d001 = 38 Å in the SAXS region.6 In the presence of water and 5% (w/w) SSL, GMS forms an Lα lamellar phase when heated above their Krafft temperature.33,34 Upon cooling below the Krafft temperature, this hydrated mesophase will crystallize into a hexagonally packed

1. INTRODUCTION Emulsifiers have been widely used as crystallization modifiers in fats and oils because they can affect the polymorphism of triglycerides.1−5 It is common to use two or more emulsifiers in gel and emulsion systems to create stable structures because of their synergistic effect. However, the phase behavior of mixed emulsifier systems has not been characterized. Understanding the structures formed by two lipid-based crystalline emulsifiers could assist in predicting their functionality when used in emulsion systems. This work focuses on the co-crystallization behavior of two lipid-based crystalline emulsifiers, glycerol monostearate (GMS) and sodium stearoyl lactylate (SSL). Monoglyceride (MGs) molecules have a single fatty acid chain attached to a glycerol backbone. MGs are commonly used food emulsifiers in spreads, whipped creams, toppings, and desserts because of their amphiphilic property.6 MGs have polymorphic and mesomorphic properties that affect their behavior in emulsion systems.7−9 The polymorphic behavior of GMS has been extensively studied. 6,10−13 During the crystallization process, GMS crystallizes into the hexagonally packed α form when the temperature drops below 75 °C and can further undergoes a thermally reversible polymorphic phase transition into two different sub-α phases at ∼50 °C and ∼30 °C.10,11,14 The two sub-α phases and the α phase are thermodynamically unstable and gradually transform into the triclinic β phase with a higher melting temperature at 85 °C.11,13 GMS molecules pack into a double chain length (DCL) lamellar structure upon crystallization.6 The sub-α phase is characterized in the wide-angle X-ray diffraction region (WAXS) by Bragg peaks that showed d-spacings of 4.3, 3.9 and 3.7 Å; and in the small-angle X-ray diffraction region (SAXS) by Bragg peaks that showed d = 50 Å. This d = 50 Å is identified with the crystallography (001) plane following the © 2015 American Chemical Society

Received: August 27, 2015 Revised: November 17, 2015 Published: December 2, 2015 297

DOI: 10.1021/acs.cgd.5b01241 Cryst. Growth Des. 2016, 16, 297−306

Crystal Growth & Design

Article

Figure 1. Melting curves of different GMS-SSL blends: (a) freshly made, stored at 25 °C for (b) 1 week and (c) 4 weeks, and (d) during crystallization.

α-gel phase, specifically a lamellar crystalline hydrated phase when the system contains10−60% wt of GMS with 5−10% wt co-emulsifier in water. This α-gel phase is able to structure a thick layer of water between the bilayers formed by GMS molecules.9,20 The metastable α-gel phase transforms in time into the coagel phase accompanied by water syneresis, which in turn leads to a reduced water layer thickness of the lamellar hydrated crystal.18,35−38 The α-gel phase and the coagel phase can be identified by the position of the Bragg Peaks in the WAXS region. The α-gel phase displays a single d-spacing at 4.15 Å, while the coagel phase displays multiple Bragg Peaks that give d-spacings between 3.6 and 4.6 Å.6 The objective of this work is to examine the solid structure formed by mixtures made with GMS and SSL at various concentrations upon crystallization and to construct a phase diagram of GMS-SSL mixed systems. Understanding the phase diagram will help interpret effects of SSL on the polymorphic transformation from the α-gel phase to the coagel phase formed by GMS, which in turn can help identify optimal ratios of the two emulsifiers for maximum stability of the α-gel phase in hydrogel and emulsion systems.

SSL used was Emplex sodium stearoyl lactylate, also obtained from Caravan Ingredients (Lenexa, Kansas, USA). GMS−SSL solid mixtures were prepared by mixing GMS and SSL powders from 100:0 GMS:SSL to 0:100 GMS:SSL in increments of 10% by weight. The powders were placed on glass microscope slides and heated on a hot plate set at 80 °C. After fully melting the powders, glass slides were incubated at 25 °C to induce crystallization under static conditions. Samples containing 100% GMS, 90% GMS, 80% GMS, 70% GMS, 60% GMS, 50% GMS, 40% GMS, 30% GMS, 20% GMS, 10% GMS, 0% GMS were named as 10M0S, 9M1S, 8M2S, 7M3S, 6M4S, 5M5S, 4M6S, 3M7S, 2M8S, 1M9S, and 0M10S, respectively. Samples that contain 15, 75, and 85% GMS were also prepared and analyzed. Despite the presence of impurities, these GMS−SSL mixtures will be called binary systems. The gelled aqueous dispersion systems were prepared using 20% (w/w) of one of the solid mixtures in 80% (w/w) water. The GMS− SSL solid mixtures were added to water, heated to 70 °C in a hot water bath, and held until the crystals were fully dissolved/melted. The system was then cooled statically to 25 °C. Neat solid mixtures and gels were incubated at 25 and 5 °C for 4 weeks or 2 years. Samples were measured in replicates. 2.2. Differential Scanning Calorimetry (DSC). The melting and crystallization profiles of the solid mixtures were determined with a Mettler Thermal Analysis DSC 1 (Mettler Toledo Canada, Mississauga, ON, Canada). Samples were heated from 1 to 75 °C (or 85 °C for 10M0S), held at 75 °C (or 85 °C) for 5 min, cooled to 1 °C, held at 1 °C for 5 min, and heated to 75 °C again. The heating and cooling rate was 10 °C/min. Peak integrations were performed with

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The GMS used was Alphadim 90 SBK distilled monoglyceride (Caravan Ingredients, Lenexa, KS, USA). The 298

DOI: 10.1021/acs.cgd.5b01241 Cryst. Growth Des. 2016, 16, 297−306

Crystal Growth & Design

Article

to merged into one peak at ∼50 °C, indicating the components were again miscible in the solid state. The crystallization curves for all the samples are summarized in Figure 1d. All solid mixtures showed similar crystallization profiles with their melting profiles when freshly prepared. Signs of both phase separation and polymorphic phase transition appeared after storing the solid mixtures at 25 °C for 1 week (Figure 1b). In particular, phase transition is dictated by a shift in the phase transition temperature of the pure component, while phase separation is revealed by splitting or broadening of a melting peak. The peak representing the sub-α phase disappeared in the 10M0S mixtures, and only one melting peak at 80 °C was observed, indicating that GMS undergoes polymorphic transformation upon aging, eliminating the sub-α phase and the α phase from the system to form only the ß phase. The 9M1S blend displayed a larger area under the peak representing the transition from the sub-α phase to the α phase compared with freshly prepared one, and 7M4S−5M5S samples showed a better defined SSL melting peak at ∼50 °C. Such changes in the peak shapes reveal enhanced phase separation between the GMS and SSL components in the binary systems upon aging because the sub-α phase is only formed in systems with high GMS as the major component. Upon storage of the mixtures at 25 °C for 4 weeks (Figure 1c), the phase transition temperatures of the sub-α phase to the α phase in 9M1S, 8M2S, and 7M3S samples changed to a similar temperature at ∼35 °C. At the same time, the SSL melting peak at ∼50 °C got better defined in samples containing 10−80% GMS. Therefore, storage for 4 weeks leads to a higher degree of phase separation between GMS and SSL. The peak representing the phase transition between the sub-α phase and the α phase was still present in samples containing 50−90% GMS, indicating that the addition of SSL prevents the GMS to transform to the β phase even though phase separation between the two components took place. Figure 2 summarizes the melting temperatures of freshly prepared GMS−SSL binary systems plotted using either the peak temperatures (Figure 2a) or the onset temperatures (Figure 2b) obtained by DSC. This was done to see if the onset temperature or the peak temperature shows different trends when composing a phase diagram. Samples with >85% GMS

Stare Software equipped with the DSC unit to determine the peak temperatures of melting and crystallization, as well as the enthalpy of melting and crystallization. In particular, the onset temperature of melting was determined at the temperature where the tangent of the peak crosses with the baseline. 2.3. Powder X-ray Diffraction (XRD). A Rigaku Multiflex X-ray diffractometer (RigakuMSC Inc., The Woodlands, TX, USA) was used to study the SAXS and WAXS diffraction patterns to determine the lamellar spacing and the polymorphic forms of the samples. The XRD unit has a copper source (λ = 1.54 Å) set at 44 kV and 40 mA, and the divergence slit, receiving slit, and scattering slit were set at 0.3 mm, 0.5°, and 0.5°, respectively. GMS−SSL mixtures were placed onto a glass sample holder with an area of 20 × 20 mm2 and a depth of 1 mm. Samples were scanned at 1°/min in diffraction angles of 1° < 2θ < 35° at room temperature, 5 or 55 °C. The XRD results were analyzed with Jade 9 (Materials Data Inc., Livermore, CA, USA) and Prism 6 (GraphPad Software Inc., San Diego, CA, USA). 2.4. Synchrotron X-ray Diffraction. Synchrotron X-ray diffraction experiments were preformed at the Advanced Photon Source (APS) at Argonne National Laboratory (Argonne, USA) to study the ultra small angle X-ray scattering (USAXS) patterns. Experiments were complemented by using a PinSAXS camera and a slit WAXS detector. Enough GMS-SSL blend material was placed in a circular silicon isolator (Grace-Bio-Laboratories, Oregon, USA) with 9 mm in diameter and 1 mm in depth, and was closed with a 2 μm thick microscope glass coverslip on each side. Samples were prepared at University of Guelph, mounted on a round sample holder, and shipped to the APS via a ground courier. Samples were measured at room temperature followed the method published by Peyronel et al.39,40 SAXS patterns obtained from the APS were first fitted with a spline in Prism 6 to generate complementary data points in the diffraction pattern. The fitted spline patterns were imported to PeakFit v4.12 (Systat Software Inc., San Jose, CA, USA) and fitted with Voigt function to determine the summit position of the peaks. The summit positions of the peaks were used for structure interpretation. The USAXS patterns were analyzed following the method used by Peyronel et al.39,40

3. RESULTS AND DISCUSSION 3.1. Thermal Properties of GMS-SSL Mixtures. The melting and crystallization curves of GMS-SSL solid mixtures measured with the DSC are summarized in Figure 1. Upon melting freshly prepared samples (Figure 1a), the 10M0S sample displayed two endothermic peaks at 35 and 73 °C, representing the phase transition from the sub-α phase to the α phase and the melting of the α phase, respectively. Samples contain 9M1S to 5M5S also displayed these two peaks, but they are shifted to lower temperatures upon the addition of SSL, while systems with 4M6S or less GMS did not show the endothermic peak at the lower temperature. Such melting behavior indicates that the sub-α phase forms only when GMS is the major component in the system, and that the system does not organize into the sub-α phase when SSL becomes the major component. Additionally, increasing SSL concentration decreased the phase transition temperature from the sub-α phase to the α phase, and lowered the melting temperature of the α phase. A third peak at ∼50 °C is observed for samples containing less than 70% GMS (7M3S), or more than 30% SSL (3M7S). This peak is identified with the melting of SSL, as seen in sample 0M10S. This third peak gets shifted slightly to lower temperature with the increase of the GMS in the mixture. The appearance of this peak at ∼50 °C suggests that GMS and SSL display intersolubility in the solid state, which leads to a phase separation between the two components that shows two independent melting peaks. A further decrease in the GMS concentration causes the melting peaks at ∼50 °C and ∼70 °C

Figure 2. (a) Peak and (b) onset temperatures of melting GMS−SSL solid mixtures. 299

DOI: 10.1021/acs.cgd.5b01241 Cryst. Growth Des. 2016, 16, 297−306

Crystal Growth & Design

Article

Figure 3. XRD patterns of different GMS−SSL blends: (a) freshly made, stored at 25 °C for (b) 1 week and (c) 4 weeks.

melting temperatures (Figure 1). The enhanced stability of the α phase is a result of high SSL concentration in the system because SSL is naturally stable in the α phase and it possibly blocks GMS molecules from organizing into the more densely packed ß phase. The Rigaku XRD unit was not able to provide accurate measurements of SAXS peak positions at the scanning rate of 1° per minute; therefore the SAXS patterns shown in Figure 3 are not discussed in this section. More elaborate discussions of SAXS peak positions, based on the data obtained from the APS, are presented in the following section (Figure 5). The WAXS results indicate GMS and SSL are miscible and they co-crystalize by forming one structure where the minor component incorporate into the structure formed by the major component (i.e., SSL is the minor component in 9M1S). When they co-crystalize with each other, the Bragg Peaks in the SAXS region only reveals one structure; when they are immiscible the systems form two separate crystals that should be seen by Bragg Peaks appearing in the SAXS region. In order to study this, 10M0S, 0M10S and 5M5S samples were examined by X-ray diffraction at 5, 25, and 55 °C (Figure 4). XRD scans were performed at these temperatures because: (i) 5 °C is lower than the phase transition temperatures from the sub-α phase to the α phase in all the three samples; (ii) 25 °C is higher than the phase transition temperature of the sub-α phase of GMS but lower than the melting temperature of SSL for the 5M5S sample; and (iii) 55 °C is higher than the melting temperature of SSL but lower than the melting temperature of GMS. XRD patterns of GMS and SSL at 5, 25, and 5 °C are presented in Figure 4b. GMS is in the sub-α phase at 5 °C given by d-spacings of 4.3 and 3.9−3.7 Å, while it is in the α phase at 55 °C judged by d = 4.2 Å. On the other hand, the SSL at 25 °C displayed d = 4.1 Å representing the α phase but with shorter d-spacings than GMS. Figure 4c shows the XRD patterns of 5M5S solid mixtures at 5, 25, and 55 °C. Two Bragg peaks with corresponding d-spacings at 4.2 and 3.8 Å appear at 5 °C, 4.1 Å at 25 °C, and 4.2 Å at 55 °C, suggesting that the 5M5S crystal is in the sub-α phase, α phase formed by GMS

and