Effect of Shear on Polymorphic Transitions in Monoglyceride Oleogels

Abstract

Fat polymorphism plays a critical role in the structural and functional properties of fat-based food products. However, research on the polymorphism of monoglyceride oleogels remains limited. Previous work demonstrated the impact of composition and processing on the polymorphic transitions of monoglyceride oleogels, indicating that high shear and cooling rates accelerate β-polymorph formation. However, a detailed understanding on the effect of shear is still lacking. This research extends previous observations by using a CSS450 shear cell, allowing for precise control over cooling and shear rates. Two commercially available food-grade monoglycerides were mixed with rapeseed oil (10% w/w). Crystallization was performed with varying shear rates and analyzed with synchrotron radiation X-ray scattering techniques (SAXS and WAXS), differential scanning calorimetry and microscopy. The results showed that applying a low shear rate did not result in changes in the polymorphic transitions compared to static crystallization for both monoglyceride oleogels. However, increasing the shear rate resulted in the formation of the β-polymorph, even before the formation of the metastable sub-α polymorph. These findings provide new insights into the role of shear in monoglyceride oleogels, allowing for further optimization of fat structuring in food applications.

1. Introduction

The functionality and organoleptic properties of many food products largely depend on the crystallization behavior of fats. Hereby, fat polymorphism plays an important role. While the polymorphic behavior of triglyceride systems has been widely investigated, the polymorphism of monoglycerides remains comparatively less explored.

The use of monoglycerides proved to be a promising approach in formulating stable oleogels that could be used as alternatives to solid-like fats that are rich in saturated fatty acids. Studies on the polymorphism of monoglycerides have been in progress for many years. A review paper of Lutton (1950) briefly described the polymorphism of (pure) saturated even monoglycerides (C10:0 to C18:0) [1]. They reported that the crystallization behavior of monoglycerides was similar compared to that of triglycerides, going from the melt to α-like crystals and slowly to the only stable β-polymorph. Remarkably, a reversible transformation between the two metastable polymorphs α and sub-α was reported, and β’, with a diffraction pattern similar to sub-α, seems to appear only from solvent [1]. Later on, more insights into the transitions between the polymorphs were obtained based on the melting behavior of pure monoglycerides. A reversible sub-α2 ↔ sub-α1 transition was found next to the sub-α ↔ α transition for monoglycerides with a fatty acid chain length of 18 to 22 carbon atoms. These transitions were shown to be sensitive to impurities and were further investigated by Watanabe (1997) by mixing monostearin (C18) and monopalmitin (C16) [2].

The crystallization behavior of monoglycerides in oil was investigated by Chen et al. (2008) [3]. They found that upon cooling, the glycerol heads of the monoglycerides arrange themselves in a closely packed inverse lamellar phase. X-ray diffraction showed two distinct rings at 4.17 Å and 4.11 Å, characterizing the distances between the glycerol heads in a hexagonal packing. This phase is described as “Lα”, since the fatty acid chains are not yet crystalline. Upon further cooling, this lamellar phase transforms into the sub-α crystalline phase with an orthorhombic packing, characterized by d-spacings between 4.27 and 3.62 Å and the remaining peak at 4.17 Å [3]. Building on this, other research analyzed the behavior of both pure and commercial monoglycerides in oil [4,5,6]. Summarizing these findings, it was found that upon cooling, monoglycerides first form a densely packed Lα phase (inverse lamellar phase; hexagonal packing), followed by a transition towards the crystalline sub-α polymorph (orthorhombic packing). For monoglycerides with a longer chain length (≥C18), reversible sub-α1 ↔ sub-α2 transition is also observed. The storage of monoglyceride oleogels results in the presence of a stable β-polymorph [4,7].

Our previous research illustrated the effect of composition and processing on the transition to the β-polymorph. Herein, samples were crystallized statically in a capillary during SAXS-WAXS analysis or produced in a lab-scale crystallizer. Specifically, the transition was delayed in the hydrogenated rapeseed oil-based monoglyceride oleogel compared to the hydrogenated palm oil-based oleogel under static crystallization. This transition was accelerated when applying a high shear and cooling rate in the lab-scale crystallizer [6]. However, research on the effect of controlled shear conditions on monoglyceride crystallization remains limited. In this research, monoglyceride oleogels, originating from fully hydrogenated rapeseed oil and palm oil, are crystallized in a shear cell while being analyzed using SAXS and WAXS. This approach allows us to control the shear and cooling rates and to investigate how these affect the polymorphic transitions.

2. Materials and Methods

2.1. Materials

Monoglyceride oleogels were prepared by using two commercially available food-grade monoglycerides, which were kindly provided by Vandemoortele (Izegem, Belgium). Herein, one of the monoglycerides originated from fully hydrogenated rapeseed oil (MAG purity of 97.3% w/w), containing mainly C18:0 (90.5% w/w), while the second one originated from fully hydrogenated palm oil (MAG purity of 96.4% w/w), rich in C18:0 (53.6% w/w) and C16:0 (43.1% w/w). Rapeseed oil was purchased from Ranson (Harelbeke, Belgium).

2.2. Oleogel Production

Monoglyceride oleogels were prepared by adding the monoglyceride to rapeseed oil at a concentration of 10% (w/w), after which the mixture was heated to 80 °C to obtain a homogeneous solution. The heated solution was transferred to a CSS450 shear cell (Linkam; Redhill, UK) equipped with liquid nitrogen to crystallize the fat blends while shearing. The quartz windows were replaced by a thick Kapton film, supported by a trident propeller, to make it suitable for X-ray scattering analysis. The gap was set at 1500 µm, and a seal was placed inside to prevent leakage [8]. After placing the molten fat blends, the temperature was set at 80 °C for 5 min, followed by a cooling ramp at 10 °C/min to 10 °C while shearing and an isothermal time at 10 °C for 10 min (without shear). The shear rate varied between 1, 25 and 50 s⁻¹, based on the maximum shear rate of the shear cell and preliminary results. The resulting samples are abbreviated as MO-C18 and MO-C18/C16 for the oleogels containing monoglycerides originating from the fully hydrogenated rapeseed oil and the fully hydrogenated palm oil, respectively.

2.3. Differential Scanning Calorimetry

The melting behavior of the monoglyceride oleogels was investigated using differential scanning calorimetry (DSC; DSC2500, TA instruments, Newcastle, UK). The DSC operated in manual mode to set the temperature at 10 °C before loading the sample. For each measurement, a new sample was crystallized in the shear cell. After being crystallized, the sample (7–15 mg) was added to precooled (10 °C) Tzero hermetic DSC pans and placed in the precooled cell of the DSC. Starting from 10 °C, the samples were heated to 80 °C at 10 °C/min. Three replicates were used. Additionally, cooling and heating analysis without applying shear (static) for MO-C18 and MO-C18/C16 was applied as a reference. Hereby, molten MO-C18 and MO-C18/C16 were added to the Tzero hermetic pans (5–10 mg) and heated to 80 °C for 5 min, crystallized at 10 °C/min till 10 °C and kept isothermal for 10 min. Subsequently, the samples were heated at 10 °C/min till 80 °C [6]. This was performed in triplicate. The peak temperature was analyzed with TRIOS software (version 5.7.1.74, TA instruments), and the melting temperature was calculated as the temperature at an area percentage of 99%.

2.4. X-Ray Scattering

2.4.1. Synchrotron SAXS/WAXS

Simultaneous SAXS and WAXS analyses during crystallization were performed at the DUBBLE beamline at the European Synchrotron Radiation Facility (ESRF; Grenoble, France). The X-rays with a wavelength of 1.033 Å at 12 keV were generated in a 16-bunch mode. SAXS images were collected by using a Pilatus 1M detector (Dectris), and WAXS images were collected with a 300K-W linear Pilatus detector (Dectris). The shear cell was placed vertically between the incoming X-rays and the detectors, and the oleogel crystallization protocol was started. After 5 min at 80 °C, the SAXS/WAXS acquisitions started simultaneous with the cooling step, with an acquisition time of 3 s. Based on the applied time–temperature profile and the time point of acquisition, the estimated temperature is reported. The SAXS and WAXS spectra were corrected for the Kapton film of the shear cell. One replicate was used. The position of WAXS/SAXS peaks will be reported in terms of d-spacings, which are calculated from the scattering vector q (Å⁻¹) as d = 2π/q. The abbreviation “SR-” is used to indicate synchrotron data.

2.4.2. Lab-Scale SAXS-WAXS

To extend the isothermal follow-up of the shear crystallized oleogels, a Xeuss 3.0 XRS-system was used (Xenocs; Grenoble, France) operating with an Eiger2R 1M detector (Dectris, Baden-Daettwil, Switzerland). The X-ray beam was generated by a Cu-source (Genix 3D) with a wavelength of 1.54 Å at 50 kV and 0.60 mA. WAXS analysis was performed with a sample-to-detector distance of 55 mm and an acquisition time of 60 s. To reduce the time between being crystallized and analyzed, the isothermal time in the shear cell was reduced to 2 min, after which the sample was placed between two (precooled) Kapton layers in a precooled sample holder (10 °C). WAXS analysis of MO-C18 and MO-C18/C16 was performed every 60 s for around 2 h, and for MO-C18/C16, this was extended to more than 12 h, with measurements every 10 min. The temperature was kept constant at 10 °C by using a Peltier system. The intensity was corrected for the scattering of the two layers of Kapton film.

2.5. Microscopy

The morphology of MO-C18 and MO-C18/C16 was visualized using polarized light microscopy and phase contrast microscopy (Leica DM2500 LED; Machelen, Belgium). After being crystallized in the shear cell, the sample was placed between precooled glasses, and the temperature was kept at 10 °C by using a Peltier system (PE120 Peltier stage Linkam). Images were acquired with LAS X software (version 3.9.0.28093, Leica) with a 10× objective (HC PL FLUOTAR 10×/0.32 PH1). After acquisition, the contrast and/or brightness of the images was optimized with ImageJ (version 1.54d) based on the procedure of Campos (2012) [9]. These images are shown in the Supplementary Data (Figures S4 and S5) to complement the dataset; however, no clear differences could be observed.

2.6. Statistics

Statistical analyses were performed in RStudio (Version 2024.12.1). First, the normality and homogeneity of variances were verified with, respectively, the Shapiro–Wilk test and Levene’s test. The means were compared with the non-parametric Kruskal–Wallis test and the Dunnett T3 post hoc test. The tests were performed with a significance level of 0.05.

3. Results and Discussion

3.1. Thermal Properties

Monoglyceride oleogels crystallize in different polymorphs under quiescent conditions depending on the composition. MO-C18 crystallizes in three different polymorphs (Lα, sub-α1 and sub-α2), while only two polymorphs occurred for MO-C18/C16 (Lα and sub-α), as reported in previous research [6]. This static crystallization behavior of MO-C18 (Figure 1A) and MO-C18/C16 (Figure 1B) is visualized in gray as the reference. Since the Lα ↔ sub-α transitions are reversible, the same number of melting peaks can be identified compared to the number of crystallization peaks [4,10]. The final melting temperature of the Lα polymorph of the statically crystallized MO-C18 and MO-C18/C16 is, respectively, 63.6 °C and 57.6 °C (

Table 1. Peak temperature (T_peak) and melting temperature (T_m) for the different events of MO-C18 and MO-C18/C16 upon heating.

	Shear Rate (s−1)	Tpeak,1 (°C)	Tpeak,2 (°C)	Tpeak,3 (°C)	Tm (°C)
MO-C18	0	32.8 ± 0.2 c	40.0 ± 0.1 a	59.6 ± 0.1 a	63.6 ± 0.3 b
	1	33.5 ± 0.3 c	39.8 ± 0.1 a	70.0 ± 0.6 b	74.1 ± 0.5 d
	25	-	-	69.5 ± 0.2 b	73.6 ± 0.5 d
	50	-	-	69.1 ± 0.4 b	72.7 ± 0.5 d
MO-C18/C16	0	16.7 ± 0.2 b	55.4 ± 0.2 b	-	57.6 ± 0.7 a
	1	15.9 ± 0.2 a	62.7 ± 0.3 c	-	67.4 ± 0.5 c
	25	-	63.4 ± 0.4 c	-	67.9 ± 0.4 c
	50	-	63.7 ± 0.1 c	-	67.6 ± 0.6 c

Peaks 1, 2 and 3 correspond to the peaks occurring around the temperature of the different polymorphs present in the reference. Significant differences (p < 0.05) between all samples within the same peak (T_peak,X) and T_m are indicated with the letters a–d.

). In the literature, the melting temperatures for pure monostearin and monopalmitin in the Lα-polymorph are around of 74 °C and 67 °C, respectively [5,11]. These reported values are 10 °C higher due to the higher concentration and purity.

The melting profiles of the shear crystallized oleogels are visualized in Figure 1. Upon melting the samples crystallized under low-shear conditions (1 s⁻¹), the melting peak corresponding to sub-α2 and sub-α1 for MO-C18 and sub-α for MO-C18/C16 could still be observed. Nevertheless, the subsequent melting peak is shifted towards a higher temperature compared to the peak temperature of the Lα polymorph for both MO-C18 and MO-C18/C16. The melting temperatures of the low-sheared samples are, respectively, 74.1 and 67.4 °C for MO-C18 and MO-C18/C16 compared to 63.6 and 57.6 °C for the static-Lα results (Table 1). This significant increase in melting temperature, by approximately 10 °C, can be attributed to the polymorphic transition towards the β-polymorph. This is confirmed by the literature, where the melting temperature of the β-polymorph is reported to be 10 °C higher compared to the melting temperature of the Lα-polymorph for both monostearin and monopalmitin [5,11]. Additionally, for MO-C18, a small increase in heat flow can be observed after the melting of sub-α1 at around 45 °C, indicating the polymorphic transition to β. By increasing the shear rate, MO-C18 and MO-C18/C16 both showed only one melting peak around 69–70 °C and 63–64 °C, respectively, characteristic for the β-polymorph. As a result, the low shear rate was not sufficient to transform all the crystals into the most stable β polymorph in contrast to the intermediate and high shear rates. The higher shear rates of 25 and 50 s⁻¹ promoted the polymorphic transition, resulting in the absence of the unstable Lα and sub-α polymorphs upon melting. These insights are in line with the literature [12] and will further be investigated with SAXS and WAXS in the following part.

3.2. Polymorphism

A detailed overview of the short spacings during the main events upon crystallization of MO-C18 and MO-C18/C16 is provided in

Table 2. Main changes in short spacings (in Å) during crystallization with shear (1, 25 or 50 s⁻¹) of MO-C18 and MO-C18/C16.

	1 s−1			25 s−1			50 s−1
	Event 1	Event 2	Event 3	Event 1	Event 2	Event 3	Event 1	Event 2	Event 3
MO-C18	4.18	4.18	4.15	4.19	4.19	4.18 (m)	4.20	-
	4.12	4.10 (w)	4.08	4.14	4.10 (m)	4.10 (m)	4.15	-
		+4.28	4.28		+4.54	4.54		+4.53
		+4.07	-		+4.49	4.49		+4.48
		+3.95	3.86		+4.39	4.39		+4.38
		+3.79	3.71		+4.28	4.28		+4.28
		+3.63	3.55		+3.95	3.95		+3.94
					+3.85	3.85		+3.85
					+3.78	3.78		+3.77
					+3.62	3.62		+3.74
						+3.55		+3.61
MO-C18/C16	4.19	4.17 (s)		4.20	4.16 (m)	4.17	4.20	4.16 (w)	4.16
	4.15	-		4.16	4.10 (w)	-	4.16	4.09 (w)	-
		+4.25			+4.54	4.53		+4.53	4.52
		+4.07			+3.90	3.94		+3.90	3.93
		+3.93			+3.74	3.73		+3.73	3.73
		+3.79				+4.08			+4.08
		+3.65				+3.64			+3.65

“+” indicates the occurrence of a new peak, while “-“ indicates that the peak from the previous event has disappeared. The abbreviations w, m and s correspond to clear changes in the peak intensity to weak, medium and strong peak intensities.

. Additionally, the SR-time-resolved WAXS profiles are shown in Supplementary Data (Figures S1 and S3). The onset of crystallization of monoglycerides oleogels is characterized by the occurrence of two short spacings around 4.2 Å and 4.1 Å, characteristic for the hexagonal packing of the glycerol (Lα) [13]. These two short spacings were present for all the different conditions in event 1, as shown in Table 2. Going from low to high shear, the onset of crystallization (event 1) was around 2.5, 2.4 and 2.3 min after the start of the cooling ramp for MO-C18 and 2.9, 2.5 and 2.7 min for MO-C18/C16. This corresponds to an onset temperature around 55, 56 and 58 °C for MO-C18 and 51, 55 and 53 °C for MO-C18/C16. Previously, an onset temperature of crystallization of 57.6 °C and 52.2 °C was found for, respectively, MO-C18 and MO-C18/C16 when crystallized statically in a capillary [6]. As a result, there was no major effect of the shear rate on the crystallization of Lα. A similar observation was reported by Mazzanti et al. (2005) for the onset time of crystallization for the α-polymorph in palm oil [14].

Applying a low shear rate (1 s⁻¹) during the crystallization of MO-C18 resulted in a polymorphic transition from Lα to sub-α1, 3.5 min after the start of the cooling ramp (45 °C), followed by a transition towards sub-α2 at 6 min (21 °C). The Lα → sub-α1 is evidenced by the presence of additional peaks related to the packing of the fatty acid chains at 4.28 Å and three peaks in the range of 3.63–3.95 Å (event 2). Next, sub-α1 → sub-α2 only generated minor shifts in the three peaks at 3.63–3.95 Å towards 3.55–3.86 Å (event 3). Hereby, the low shear rate during crystallization did not alter the crystallization behavior of MO-C18 compared to static crystallization. Increasing the shear rate to an intermediate value of 25 s⁻¹ induced major differences. Starting from Lα, new peaks with d-spacings of 4.54, 4.49 and 4.39 Å occurred during event 2 at 3.3 min (48 °C), while they were absent in the low-shear condition. These d-spacings correspond to the formation of the β-polymorph. Chen et al. (2011) reported short spacings of β-crystals for 10 wt% monoglyceride (C18) in oil at 4.55–4.51 Å; 4.38–4.26 Å; 4.09–3.94 Å and 3.84–3.78 Å [13]. Other research on monostearin found d-spacings at 4.55–4.6 Å; 4.1–4.37 Å; 3.7–3.9 Å and 3.6 Å for β [5,11]. These results confirm the suggestion of López-Martínez et al. (2014) that the β-polymorph can crystallize directly from Lα [4]. However, the temperature at which this occurs is higher than the proposed temperature of 8–8.5 °C, which will be the result of the applied cooling protocol. In the third event at 5.9 min (21 °C), an additional peak at 3.55 Å occurred. Based on the literature and the WAXS profiles obtained at low shear rates, this corresponds to sub-α2. The three events when applying intermediate shear can be presented as Lα → β → β + sub-α2. Nevertheless, some sub-α1 might be present, but these d-spacings overlap with β, and no melting peak related to sub-α1 was found with DSC (Figure 1). Upon storage, the remaining sub-α2 will probably also transform into the stable β-polymorph, as seen in previous research [6]. When applying a high shear rate (50 s⁻¹), only two main events occur, namely the formation of Lα and the transition towards β (3.1 min, 49 °C).

Interestingly, the shear rate did not have a major effect on the onset time of the different events, while the polymorphs being formed within these events largely differed. This indicates that the effect of shear on monoglyceride oleogels is more expressed in the polymorphic transitions compared to the nucleation. Hereby, shear might affect the interactions associated with the different polymorphs. MacMillan et al. (2002) described that applying shear during the crystallization of cocoa butter breaks the van der Waals forces related to the packing of forms III and IV, enabling the packing of the more stable V form [15]. The same principle might occur in the monoglyceride oleogels, resulting in breaking the van der Waals forces within the hexagonal packing of the glycerol heads in Lα to enhance the formation of β. However, cocoa butter is very different in composition compared to the monoglyceride oleogels. Monoglycerides only have one fatty acid esterified to the glycerol molecule, leaving two free hydroxyl (-OH) groups that can form hydrogen bonds. Regarding this, Chen et al. (2009) described a change in hydrogen bonds upon aging of C18-rich monoglyceride oleogels (10%) from sub-α polymorph towards β [16]. Initially, a hydrogen bond between the OH group at the end of the glycerol head (3-OH) and the neighboring C=O group was found to be present for the polymorphs Lα and sub-α. During aging, they observed a shift towards hydrogen bonds between the middle OH of the glycerol head (2-OH) and the C=O group. At first, the 3-OH and 2-OH hydrogen bonds coexist, while later on the 2-OH hydrogen bonds dominates. They concluded that the 3-OH hydrogen bond was not stable, and the 2-OH hydrogen bond forced the reorganization of the MAGs to a more ordered way [16]. Therefore, applying shear might enhance this process by breaking the 3-OH hydrogen bonds, resulting in the formation of β before sub-α2. The intermediate shear rate of 25 s⁻¹ might result in the coexistence of 3-OH and 2-OH hydrogen bonds so that the less stable sub-α can also be formed when reaching its onset temperature. Contrarily, when applying the high shear rate of 50 s⁻¹, the 2-OH hydrogen bonds might be dominant at the onset temperature of sub-α2, preventing its formation. Figure S2 illustrates the WAXS profiles within the temperature range where the formation of sub-α2 is expected for the intermediate and high shear rates.

For MO-C18/C16, applying a low shear rate (1 s⁻¹) did not result in differences in polymorphism compared to static crystallization (Figure S3). Upon cooling, a polymorphic transition from Lα to sub-α occurred in event 2 after 6.1 min of cooling (19 °C). Contrarily, three main events could be distinguished when applying an intermediate (25 s⁻¹) or high (50 s⁻¹) shear rate. Upon further cooling of Lα with an intermediate shear rate, peaks at d-spacings of 4.54, 3.90 and 3.74 Å occurred during event 2, indicating the formation of β. These d-spacings are different compared to those of MO-C18. According to Chen et al. (2011), not only sub-α but also the β polymorph occurs in multiple forms depending on the composition [13]. Therefore, the difference in composition between MO-C18 and MO-C18/C16 is substantial enough to form a different β-type. Next, in event 3, these β-peaks remained, with the addition of two more peaks at 4.08 and 3.64 Å. Based on the results of the slow-sheared MO-C18/C16, these indicate the formation of sub-α. A similar result was found for the highly sheared MO-C18/C16, namely Lα→β→β + sub-α. When applying intermediate shear and high shear, the onset times were, respectively, 3.2 min (48 °C) and 3.3 min (47 °C) for the β-polymorph (event 2) and 5.9 min (21 °C) and 6.4 min (16 °C) for sub-α (event 3). The same hypothesis as for MO-C18 can be applied, where shear might impact the hydrogen bonding. However, high shear did not prevent the formation of sub-α. This might be the result of the more heterogeneous composition of the sample due to a change in the relative stability between sub-α and β [17].

Figure 2A illustrates the final WAXS spectra of MO-C18, in which the presence of β is clearly visible once applying an intermediate shear rate, while it is absent for the low shear rate. Similarly, in Figure 2B, the final WAXS profiles of the intermediate- and the highly sheared MO-C18/C16 are identical, while no β peaks were present for the slow-sheared sample. To further investigate the polymorphic transition towards the stable β-polymorph when applying a low shear rate, the isothermal time at 10 °C was extended. Time-resolved WAXS profiles of MO-C18 and MO-C18/C16 upon storage at 10 °C for several hours are shown in Figure 3. The transition of MO-C18/C16 began almost immediately after the start of the WAXS analysis (Figure 3B). This was observed by the slow increase in intensity at a q-value around 1.36 Å⁻¹ (d-spacing of 4.6 Å). Within the timeframe of 140 min, only a very small amount of β-MO-C18 was being formed (Figure 3A). Therefore, the isothermal time was extended up to 800 min. This is in line with our previous publication on statically crystallized MO-C18 and MO-C18/C16, where the transition of MO-C18 was delayed [6].

The onset of crystallization and polymorphic transitions were also characterized by changes in long spacings, analyzed via SAXS (Figure 4 and Figure 5). Starting with MO-C18 crystallized with a low shear rate (Figure 4A), two main events could be distinguished. First, there was an increase in intensity of the peak at 51.5 Å (I). Secondly, this peak intensity decreases while a new peak at 49.2 Å is being formed (II). These events correspond to the transitions melt→Lα and Lα→sub-α1. Additionally, a third event can be identified, in which the peak at 49.2 Å slightly shifts towards 49.5 Å (III). Based on SR-WAXS data, this third event is the sub-α1→sub-α2 transition. These results are in line with statically crystallized MO-C18 [6]. The complexity increases when increasing the shear rate to an intermediate value of 25 s⁻¹ (Figure 4B). During event II, the decrease in peak intensity related to Lα overlaps with the formation of a shoulder at a slightly higher q-value. Upon further cooling, this shoulder evolves towards a distinct peak at 50.1 Å. This peak is broader and located at a lower q-value (higher d-spacing) compared to the peak related to sub-α1 that was being formed in event II of the low-sheared MO-C18 (Figure 4A). Taking into account the SR-WAXS results, event II is most likely the formation of β. This is also in line with the literature, where Lutton (1950) reported a long spacing of 50.1 Å for the β-polymorph of monostearin [1]. Next, in event III, the peak related to Lα keeps decreasing, while a new peak is formed at the right side of β-peak, namely at 49.3 Å. Important to note is that the β-peak remains unchanged during this event. It is, therefore, assumed that the Lα→sub-α transition occurred while β was already present. In event IV, small shifts of both peaks towards each other occur. This might indicate that event III was characterized by Lα→sub-α1 and event IV by sub-α1→sub-α2. Nevertheless, SR-WAXS data presented that sub-α2 and β crystals were present at the end of the crystallization process of MO-C18 when applying a shear rate of 25 s⁻¹. Increasing the shear rate to 50 s⁻¹ decreased the complexity in crystallization behavior exhibiting two main events, namely melt→Lα (51.3 Å) and Lα→β (50.0 Å) (Figure 4C). The latter one is hypothesized based on the shape of the peak being formed in event II and the comparison with the intermediate-sheared MO-C18. Additionally, the increase in peak intensity of β is simultaneous with a decrease in peak intensity of Lα, indicating a polymorphic transition from Lα to β, confirming the SR-WAXS data.

Figure 5 visualizes the SR-SAXS profiles of MO-C18/C16 upon crystallization. Figure 5A, again, clearly illustrates the formation of Lα, followed by a gradual transition towards sub-α. This corresponds to a decrease in the lamellar thickness from 51.9 Å (Lα) to 50.2 Å (sub-α). Increasing the shear rate to 25 or 50 s⁻¹ resulted in the occurrence of three events. In both conditions, event II is characterized by a gradual decrease in the peak intensity of Lα simultaneously with the formation of a new, broader peak at 48.5 Å (Figure 5B,C). López-Martínez et al. (2014) reported a similar long spacing for the β-polymorph of a fat blend containing 37.66% monostearin and 54.02% monopalmitin [4]. Once the peak related to the β-polymorph reached a stable intensity, a new peak in between the decreasing peak intensity of Lα and the stable β occurred. This peak was located at 50.0 Å and 50.2 Å for applying a shear rate of 25 s⁻¹ and 50 s⁻¹, respectively. This, again, confirms the occurrence of β before sub-α and the presence of both β and sub-α at the end of the isothermal time.

4. Conclusions

The impact of applying different shear rates (1, 25 and 50 s⁻¹) on the crystallization behavior of two 10% monoglyceride oleogels (rapeseed oil-based MO-C18 and palm oil-based MO-C18/C16) was investigated. Previous research demonstrated that polymorphic transitions of MO-C18 and MO-C18/C16 under static crystallization occurred as Lα→sub-α1→sub-α2 and Lα→sub-α, respectively. When applying a low shear rate of 1 s⁻¹, the same transitions could be distinguished. Increasing the shear rate towards 25 and 50 s⁻¹ resulted in a different crystallization behavior. The results indicate that Lα can transform directly into the most stable β-polymorph when applying an intermediate (25 s⁻¹) or high (50 s⁻¹) shear rate during crystallization. For MO-C18/C16, the formation of the sub-α polymorph still occurred when the β-polymorph was already present at the intermediate and high shear rates. Contrarily, for MO-C18, the high shear rate fully inhibited the formation of the metastable sub-α2. At an intermediate shear rate, the sub-α2 also coexisted with β for MO-C18 within the timeframe of the experiment. However, the transition of sub-α2 to the stable β-polymorph is expected upon storage. Since no major effect was found on the onset time of the transitions, it was hypothesized that shear affected the hydrogen bonds, enhancing the 2-OH hydrogen bonds that were found to be characteristic for the stable β-polymorph. This research shows that the polymorphic transitions can be controlled by changing the crystallization conditions. The initial inverse lamellar phase (Lα) of monoglycerides can transform to either sub-α or β depending on the composition of the monoglycerides and the applied shearing protocol. Further research is needed to elucidate the effect of these results on the functional properties of oleogels, such as firmness and oil binding capacity.