Abstract
In the confectionery industry large quantities of palm fat in the fillings of chocolate products are used. Based on today's nutritional science results, it is desirable to replace palm oil with healthier fats. Oleogels can provide a kind of solution for this replacement. In our work the rheological, textural and thermal properties of oleogels containing high oleic sunflower oil, beeswax and monoglycerides were determined. In the samples we examined, the gelator concentrations were: 20% beeswax, 15% beeswax and 5% monoglyceride, 10% beeswax and 10% monoglyceride, 5% beeswax and 15% monoglyceride, and 20% monoglyceride. Based on our results, the oleogel containing 15% beeswax and 5% monoglyceride seems an eutectic crystal of beeswaxes and monoglyceride. It has relative high hardness, high storage modulus and high viscosity therefore it can replace the Chocofill filling fat, which contains mainly palm fat, used in large quantities in sweets.
Introduction
The use of palm oil in the food industry must be reduced to achieve sustainable development and healthy nutrition. Because the use of palm oil causes many harmful processes, starting from the cultivation of oil palms, which reduces the extent of the rainforest, to the fact that palm oil contains a lot of saturated fat, which is unhealthy (Disdier et al., 2013; Ayompe et al., 2020).
All fractions of palm oil are used in large quantities in the confectionery industry. They can also be used to make coating masses, fillings and cakes (Hassim and Dian 2017). Replacing it is not nearly as easy, especially in the case of chocolate fillings, because the sensory properties of fat-based fillings (texture, mouthfeel, flavors, etc.) are related to the crystal structure of the fat component (Marangoni et al., 2012; Espert et al., 2019). Therefore, replacing palm oil with oil-based oleogels can significantly change the properties of the fillings and thus the quality of the final product (Fayaz et al., 2017a).
Oleogelation is a process in which liquid oil with a gelator – a structuring agent – takes on such a semi-solid structure as the easily spreadable fats and this three-dimensional network structure encloses the liquid oil (Terech and Weiss, 1997; Toro-Vazquez et al., 2007; Davidovich-Pinhas, 2018). A variety of gelators (Patel and Dewettinck, 2016) can be used for the preparation of oleogels, among which waxes and monoglycerides are promising, if the goal is to replace a harder fat for the confectionery industry (Doan et al., 2016; Fayaz et al., 2017a). The strength and properties of the gel are related to the ratio and properties of the gelator and the oil (Patel et al., 2015). In liquid oils the crystallization of various waxes containing long-chain fatty acid esters also can result in various oleogels (Doan et al., 2015; Fayaz et al., 2017b). They also contain other components, such as hydrocarbons, fatty acids and fatty alcohols, which can also affect the structure (Davidovich-Pinhas, 2018). Among the various waxes, beeswax can form strong gels even at low concentrations (1.0–1.5 w/w%) by direct dispersion method (Hwang et al., 2011). Monoglycerides also organize into inverse bilayer nanostructures, and then these structures grow into lamellar plate microstructures, which form the three-dimensional structure, thus immobilizing the liquid oil (Da Pieve et al., 2010; Palla et al., 2022).
Oleogels mainly contain unsaturated fatty acids and are heat-stable and do not oxidize easily (Belingheri et al., 2015), so their consumption is healthy and helps prevent the development of cardiovascular diseases (Xu et al., 2020).
The practical applicability of oleogels was analysed in numerous experiments, where the main goal was to replace solid fats (Patel, 2015). Encouraging results were obtained during their use in confectionery products (Pernetti et al., 2007; Do et al., 2010; Stortz and Marangoni, 2011; Zetzl and Marangoni, 2011; Patel et al., 2014), in bakery products (Stortz et al., 2012) and in fat products (Hwang et al., 2013; Jang et al., 2015). The experiments in meat products (Jimenez-Colmenero et al., 2015; Barbut et al., 2016a, 2016b) and dairy products (Bemer et al., 2016) gave also promising results for the use of oleogels. Although the use of oils in chocolate fillings is not beneficial, as oil migration can cause fat blooming, there are nevertheless studies where the effect of oleogels in reducing oil migration has been investigated (Patel and Dewettinck, 2016; Si et al., 2016).
Since oleogels have to replace currently used fats (Palla et al., 2022), it is therefore advisable to create oleogels with mechanical, rheological and thermal properties similar to those of the replaced fats (Glibowski et al., 2008; Pehlivanoglu et al., 2021).
Although there has been research on the use of oleogels in fat-based fillings, the number of attempts to replace commercially available special fats is small. In this work, we investigated the replaceability of a special fat used in the confectionery industry with oleogel, which contained high oleic sunflower oil and a mixture of beeswax and monoglycerides as gelators. We chose sunflower oil because it has a relatively neutral smell and taste, so it is likely that the gelled version can also be used in the filling of confectionery products. Similarly, monoglyceride and beeswax have a neutral taste and smell. Both gelling agents are already widely used in the food industry. Oleogels made with monoglyceride are softer, oleogels made with waxes are harder. That is why we chose a mixture of the two gelling agents as the gelling component of oleogels. Five oleogels were prepared, in each of which the concentration of gelators was 20 w/w%. The relatively high concentration of gelling agent was chosen so that the mechanical and rheological properties of the special confectionery fat and the oleogel we prepared are almost identical.
The aim of our work was to produce oleogels from sunflower oil with mixtures of beeswax and monoglyceride – as gelators – in different proportions and to determine the spreadability, the rheological, and crystallization-melting properties of oleogels. The obtained physical parameters were compared with the parameters of a special fat containing palm oil used in the confectionery industry. We also aimed to determine whether among the oleogels we produced there is one that can replace the special industrial fat containing palm oil.
Materials and methods
Materials
High oleic sunflower oil (HOSO) was supplied by Bunge (Martfű, Hungary). White beeswax (BW) was purchased from a cosmetic company in pharmacopoeia purity (Humanity Áruház Ltd.). Monoglycerides (M) (Danisco, Dimodan HP SG) was provided by KUK Hungária Ltd. (Győr, Hungary). Chocofill (AAK product) confectionery fat was offered by Szerencsi Bonbon Ltd. (Szerencs, Hungary).
Preparation of the oleogels
The oleogels were prepared with the following compositions:
20 w/w% monoglycerides + 80 w/w% HOSO, labelled as M20
5 w/w% beeswax + 15 w/w% monoglycerides + 80 w/w% HOSO, labelled as BW5M15
10 w/w% beeswax + 10 w/w% monoglycerides + 80 w/w% HOSO, labelled as BW10M10
15 w/w% beeswax + 5 w/w% monoglycerides + 80 w/w% HOSO, labelled as BW15M5
20 w/w% beeswax + 80 w/w% HOSO, labelled as BW20
The HOSO (120 g) was weighed and placed in a 250 mL beaker. The beaker was heated on a magnetic stirrer plate and stirred at a low speed. Gelling materials (total 30 g) were added to the oil with temperature 90 °C, then stirred with an Ultra-Turrax T25 type homogenizer (IKA-Werke GmbH & Co. KG, Germany) for 20 min at 10,000 r/min to ensure the complete dissolution. The mixtures were cooled in a cold water bath (∼15 °C) with continuous stirring. The homogeneous mixtures were heated to ambient temperature (22 ± 2 °C). After 24 h the gelling process was fully done. The final samples (Fig. 1) were stored in a plastic cup with a screw cap at room temperature until the measurements. All measurements were taken within 20–30 days.
Oleogel sample
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
Spreadability test
Stable Micro Systems (UK) TA-XTPlus texture analyser was used to test the spreadability (Steffe, 1996) of oleogels and Chocofill. The measurement was carried out with the help of a built-in program for testing the spreadability of margarines. A conical measuring head with a 90° angle was descended into a conical base containing the sample. As a result of the indentation, the material was squeezed out between the two conical units. The penetration distance was 23 mm. Speed of the measuring head in the sample was 3 mm s−1. Five measurements were taken. From the spreadability curve (Fig. 2) the hardness – the force at maximal penetration –, the work of shear – area under the penetrating curve –, the stickiness – minimal value of force at the initial moment of lifting the measuring head –, and the work of adhesion – area under the lifting curve – were determined. Calculating the work, we used the distance traveled by the measuring head instead of the elapsed time.
A typical spreadability curve with determined parameters
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
Rheology
All the rheological measurements were carried out with an MCR302 oscillating rheometer (Anton Paar, Austria). The device, equipped with a Peltier system and water bath (Julabo, Seelbach, Germany), provides temperature control with high accuracy. Parallel-plate (PP50 measuring head; diameter, ϕ = 50 mm; gap = 1 mm) geometry was used at 25 °C for both flow curve measurements and amplitude and frequency sweep measurements.
At 1 Hz constant frequency, an oscillatory amplitude sweep (0.01−100 %) was carried out. The elastic (G′) and storage (G″) modules were determined as a function of amplitude and from these curves the linear viscoelastic region (LVE) and yield point – cross-section of G′ and G″ curves were determined. Both elastic and storage modulus were also determined with oscillation frequency sweep method between 0.1 and 50 Hz at a strain value of 0.02%.
Thermal analysis
The thermal properties were determined with a Differential Scanning Calorimetry device (DSC 3500 NETZSCH, Germany). The samples of 10–15 mg were weighed into aluminium pans, then the pans were sealed and their tops were punched. The fats were first heated from 25 °C to 90 °C, at which temperature the sample is definitely liquid and does not contain crystals. After that, the sample was cooled from 90 °C to −20 °C and the crystallization process of the sample followed, then the sample was heated again to 100 °C and the melting process of the sample followed. Cooling/heating was carried out at a rate of 10 °C min−1. An empty aluminium pan was used as a reference (Lim et al., 2016). In each case, three repetitions were performed. The temperature of onset and temperature at maximal heat flow were determined and the crystallization and melting heat were calculated with the Proteus Analysis program.
Statistical analysis
Analysis of standard deviation with ANOVA was performed (P < 0.05) on the spreadability test results to determine whether there was a significant difference between the groups. Multiple comparison was performed using Tukey's post hoc test (P < 0.05).
Results
The oleogels produced for our measurements were white, easily spreadable fat-like substances (Fig. 1). They looked very similar to the oleogels with monoglyceride (Palla et al., 2022) or with beeswax (Martins et al., 2016) described in the scientific literature.
The crystal structure of the oleogels determines its physical properties – as viscosity, rheological and textural parameters, melting temperature, melting heat – of oleogel. However, what kind of crystal structure is created is determined by the conditions of gel production – the type of gelator or gelators and the type of oil, processing temperature, stirring rate, shear rate during cooling and the storage time and temperature (Palla et al., 2022). The purity and fatty acid composition of monoglycerid determine its emulsifying characteristics and melting point (Palla et al., 2022).
The crystal structure of oleogels containing monoglyceride is created during a multi-step process. A monoglyceride molecule has a polar head and an apolar hydrocarbon chain. At temperatures above the melting point, it forms inverse double layers and micelles in oil – apolar solvents. As the temperature decreases, the hydrophilic heads are compressed in a hexagonal shape between the layer of hydrophobic chains (Chen and Terentjev, 2009). This structure can be stabilized by OH groups. Subsequently, the inverse bilayers grow and organize into lamellar platelet microstructures (da Pieve et al., 2010). Below the crystallization point the lamellar phase through some metastable phases is transformed to the so-called β-crystalline phase (Chen and Terentjev, 2009). The crystals grow and get branched out and these microparticles are connected to each other by Van der Waals forces and hydrogen bonds and this system of interactions determines the macroscopic properties of the oleogel – viscosity, spreadability, storage and loss moduli (Mandu et al., 2020).
The size of the crystals in our M20 sample may have been similar to the size of the crystals – about 57 µm – described in an oleogel containing monoglycerides by Palla et al. (2019). To prepare M20 sample, we dissolved the monoglyceride in 90 °C HOSO and stirred it at a speed of 1,000 r/min for 20 min, then cooled it at 15 °C with constant stirring. In the work of Palla et al. (2019) samples were stirred at 75 °C with 400 rpm for 30 min and then they were cooled at 17.5 °C without stirring. The temperature was practically the same in both processes. Stirring while cooling generally results in smaller crystals (Palla et al., 2022). The concentration of monoglyceride was 10 w/w% and 20 w/w% in the referred work and in our experiments, respectively. The higher gelator concentration can result in greater number of crystals, which can give a more ordered crystal structure with an effective spatial distribution (Palla et al., 2022).
The natural waxes dissolved in oils during cooling form crystals with a morphology of stems or platelets, which results in a strong self-sustaining crystal structure. This complex structure can be more stabilized by the interaction of polar groups of oils with it (Doan et al., 2017). The beeswaxes gave tiny needle-like crystals in oleogels (Doan et al., 2017) and in our BW20 sample probably such crystals were formed.
In samples BW5M15, BW10M10 and BW15M5 the form and size of crystals are difficult to estimate based on literature data.
Spreadability test
The spreadability test is very important among the mechanical tests because it is close to the sensory evaluation.
For the firmness values, it is likely that almost all gelators – except of BW15M5 – affected the hardness equally. No significant difference was found between M20, BW5M15, BW10M10 and BW20 groups. The hardness of BW15M5 was higher than that of other oleogels, closer to that of Chocofill. Gelators also had a similar effect on shear force and shear work. For stickiness it is beeswax that really determined this property, although the BW15M5 sample showed a higher value here and it was not different from Chocofill. For the work of adhesion, both gelators had roughly the same effect on the values, but the effect of monoglyceride was perhaps stronger for BW15M5. Chocofill also had the highest value in this case. Based on the results of the spreadability test, sample BW15M5 was the closest to the Chocofill sample (Table 1).
Spreadability of oleogels and Chocofill: firmness, work of shear, stickiness and work of adhesion (where the different letters indicate significantly different groups (Tukey, P < 0.05))
Samples | Firmness (N) | Work of Shear (N.mm) | Stickiness (N) | Work of Adhesion (N.mm) |
M20 | 15.33 ± 0.77 a | 43.91 ± 2.79 b | −16.85 ± 0.68 b | −44.53 ± 3.62 b,c |
BW5M15 | 13.33 ± 0.81 a | 39.83 ± 3.40 a,b | −14.28 ± 0.47 c | −38.61 ± 3.17 c,d |
BW10M10 | 13.79 ± 0.84 a | 40.67 ± 3.04 a,b | −14.85 ± 0.76 c | −41.68 ± 3.69 c,d |
BW15M5 | 20.24 ± 1.03 b | 55.08 ± 3.18 c | −23.12 ± 0.58 a | −49.22 ± 3.53 b |
BW20 | 13.84 ± 1.37 a | 33.96 ± 3.83 a | −14.94 ± 1.02 c | −37.09 ± 2.65 d |
Chocofill | 22.57 ± 1.79 c | 78.89 ± 9.09 d | −23.07 ± 1.64 a | −67.55 ± 4.60 a |
Zbikowska et al. (2022) show that as the beeswax concentration in oleogel increases (2–8%), the firmness, the stickiness, the work of shear and the work of adhesion of oleogel also increase. It is interesting that the spreadability parameter values obtained for a sample containing 8% beeswax (Zbikowska et al., 2022) are essentially the same as our values obtained at 20% beeswax content (Table 1, row 5).
These values can be expected. On the one hand, in the work of Zbikowska et al. (2022) yellow beeswax and peanut oil were used, while in our experiments the oleogels were prepared from white beeswax and sunflower oil. The composition of the two beeswaxes is different, so the crystalline structure may also be different, the viscosity of the two oils is likewise dissimilar, which could also cause a difference in the spreadability of the oleogels in the two exmeriments. On the other hand, the preparation of the sample was different in the two experiments. In the experiment of Zbikowska et al. (2022), the sample with temperature 80 °C was statically cooled to 20 °C in 24 h, while our samples were cooled from 90 °C to 15 °C under constant stirring. Stirring during cooling results in much smaller crystals (Palla et al., 2022). In our case, at high concentrations, smaller crystals can give smaller parameter values.
Rheology
Flow curves
The flow curve of our samples were similar to flow curves of other oleogels described in scientific literature (Wan Nik et al., 2007; Oh and Lee, 2018). A typical measured flow curve – of M20 sample – with approached Herschel-Bulkley model can be seen in Fig. 3.
Flow curve of M20
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
The flow index, n values were less than 1 for all samples (Table 2), hence the materials were shear thinning, having pseudoplastic properties. In general, materials with higher viscosity have a lower n value and are less similar to Newtonian materials.
Parameters of Herschel-Bulkley model: τ0, the yield stress; K, the consistency; n the flow index
τ0 (Pa) | K (Pasn) | n | |
M20 | 0.00 ± 0.00 | 149.76 ± 10.93 | 0.10 ± 0.01 |
BW5M15 | 0.01 ± 0.00 | 142.65 ± 0.32 | 0.15 ± 0.00 |
BW10M10 | 0.01 ± 0.00 | 85.45 ± 4.85 | 0.28 ± 0.01 |
BW15M5 | 0.01 ± 0.02 | 93.87 ± 1.95 | 0.21 ± 0.00 |
BW20 | 0.00 ± 0.00 | 91.51 ± 3.55 | 0.18 ± 0.01 |
Chocofill | 0.00 ± 0.00 | 98.58 ± 2.79 | 0.13 ± 0.01 |
It is interesting that practically no sample has a yield stress, the τ0 value is near to zero (Table 2). This can imply that both oleogels and Chocofill can be considered as plastic materials and exhibit flow properties even under the lowest force. But it could also be because, although the samples have a yield stress, it is so small that it is lower than the sensitivity limit of our measuring devices. At low shear rates, small shear stresses occur that cannot be accurately measured by the measuring device. Different approximation programs for determining the yield strength can also give uncertain results. Instead of the yield point, a more accurate value for the flow characteristics can be obtained from the amplitude sweep measurements by determining the yield point (see Fig. 5)
The sample, M20, containing only monoglyceride as gelling material showed the highest consistency (Table 2). Sample BW5M15 containing 75% monoglyceride and 25% beeswax as gelling material had also relatively high consistency. The samples containing less amount of monoglyceride and more amount of beeswax showed lower consistency. The consistency values of the BW15M5 oleogel was very close to the value of consistency index of Chocofill. The same was found for the firmness and stickiness values of the spreadability test, so the BW15M5's texture was most like the Chocofill's texture.
It appears that samples containing mainly monoglyceride crystals give a higher consistency parameter than samples containing mainly beeswax crystals. This can be explained by the fact that monoglyceride and beeswax crystallize differently and create crystals with different structures. There can also be a difference in the binding of the oil between the two gel-forming materials.
The viscosity of the oleogel containing only monoglyceride was about 30% higher than the viscosity of the oleogel containing only beeswax (Fig. 4). The viscosity of BW5M15 and BW10M10 sample was practically the same as the viscosity of M20. On the base of the flow curve and viscosity curve it seems that the crystal structure and oil binding capacity of samples M20, BW5M15 and BW10M10 are very similar. When the concentration of BW increased – samples BW15M5 and BW20 – both viscosity and consistency decreased (Table 2, Fig. 4). The nature of crystal structure and oil binding changed at higher BW concentration.
Average viscosity values with standard deviations (at shear rate 30.0 1/s; at temperature = 25 °C)
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
For the viscosity values, similar to the K values, it was stated that the viscosity values of BW15M5 and BW20 oleogels were closer to the Chocofill viscosity values.
Amplitude sweep
For all investigated samples the G′ was higher than G″ at low shearing angle (Fig. 5), that is, all oleogels showed elastic properties at low shear strain and all samples had a flow point. The flow point can be interpreted as a yield point from a certain point of view. In the oscillation mode, we obtained accurate shear stress and G′ and G″ parameters for small shear deformations. For the flow curves measured in the rotation mode, the accuracy of the shear stresses measured at very small deformations was not sufficient for the mathematical approximation methods we used to give correct yield stress.
Amplitude sweep curves with flow points (τf)
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
Sample BW15M5 gave the highest G′ and G″ values (Fig. 5) at the flow point and the rest of the samples are ordered by decreasing G′ and G″ values BW10M10, BW20, BW5M15, M20 and Chocofill. But the deformation at the flow point was the smallest for sample BW15M5. This may mean that the formed crystal structure has a relatively high elasticity – high storage modulus, but it breaks under the influence of a relatively small deformation, and the sample shows flow with relative high viscosity – high loss modulus. Other samples have lower storage and loss moduli and higher shear deformation at their flow point.
The storage and loss moduli at the yield point of the confectionery fat – Chocofill – to be replaced were the smallest compared to the other samples, and the Chocofill began to flow at a relatively high shear deformation.
Frequency sweep
The LVE values obtained from the amplitude sweep were slightly above 0.02% shear deformation, so the frequency scanning was performed at 0.02% shear deformation. In the frequency sweep curves for all samples the G′ and G″ modules slightly increased as the function of frequency (Fig. 6). This may indicate that the oleogel samples we prepared can be stable over a relatively long period of time. By observing the shape of the curves, it was noticed that the BW15M5 oleogel behaved similarly to Chocofill (Fig. 6).
Frequency sweep curves
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
Thermal analysis
Figure 7 shows the heat flow curve of Chocofill and the temperature change as a function of time. For the evaluation of the curves, the second and third stages of the temperature programme were considered. During the first stage the samples were heated up to 90 °C temperature. At this temperature all crystals were melted and the solution of the mixture of monoglyceride and beeswax was only present in oil. In the second stage, the crystallization of the monoglyceride and beeswax and the formation of the gel state were observed. In the third stage the melting of crystals was observed.
Heat flow curve of Chocofill
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
Chocofill was composed of a mixture of palm fat and shea butter. The crystallization curve of Chocofill showed two peaks – Tpeak1 at about 18 °C and at about 2°C and a shoulder at about −10 °C. In the melting curve of it there are three peaks at 8 °C, 25 °C and 35 °C, respectively and two shoulders at −5 °C and 10 °C (Figs. 7 and 9F).
Figure 8 shows the heat flow curves as a function of time of M20, BW20, BW10M10 oleogels. Both monoglyceride and beeswax-containing oleogel samples – M20 and BW20 – showed two crystallization and melting peaks each. For the sample containing 1:1 w/w % of beeswax and monoglyceride – BW10M10 – all four peaks were present at both crystallization and melting. This suggests that both the monoglyceride-containing and the beeswax-containing crystals were formed and were independently present in the sample. The temperature of the peaks shifted to a lower temperature for the BW10M10 sample compared to the BW20 and M20 samples.
Heat flow curves of M20, BW20, BW10M10 (red: BW20; grass green: M20; olive green: BW10M10)
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
Sample M20 containing only monoglyceride as gelator showed two peaks during both crystallization – at about 53 °C and 13 °C – and melting – at about 61 °C and 18 °C (Figs 8 and 9A, and Table 3). These peak temperature values are similar to the peak temperatures for monoglyceride containing oleogels described in the literature (Lopez-Martinez et al., 2015; Malvano et al., 2024). The first crystallization peak corresponds to the formation of aliphatic tails of the mixed lamellar structure and the second peak is in connection with the polymorphic transition into the sub-α structure (Lopez-Martinez et al., 2015). The values of the peak temperatures we obtained were higher than the values described in the literature, and the phase transformation enthalpy values we calculated were also higher than the values obtained by other authors. The reason for this is that the monoglyceride concentration in our samples was about 4–5 times higher than in the cited works. The relatively large temperature difference between melting and crystallization for the M20 sample at each peak probably occurred because the heating and cooling rates we used were about twice as high as those reported in the literature.
Heat flow curves as a function of temperature (the figure shows stages 2 (crystallization) and 3 (melting) of the temperature program)
Citation: Progress in Agricultural Engineering Sciences 20, 1; 10.1556/446.2024.00120
Thermal parameters (on-set (Ton), peak (Tpeak) and enthalpy (ΔH)) of oleogels (each number represents a different peak)
Sample | Crystallization | Sample | Melting | ||||
Ton (°C) | Tpeak (°C) | ΔH (J g−1) | Ton (°C) | Tpeak (°C) | ΔH (J g−1) | ||
M20-1 | 55.60 ± 0.12 | 53.20 ± 0.25 | −19.06 ± 0.41 | M20-1 | 54.87 ± 0.21 | 60.90 ± 0.21 | 17.61 ± 0.29 |
M20-2 | 15.77 ± 0.26 | 13.73 ± 0.46 | −5.05 ± 1.73 | M20-2 | 15.93 ± 0.25 | 18.03 ± 0.20 | 4.28 ± 0.40 |
BW5M15-1 | 52.27 ± 0.46 | 49.97 ± 0.38 | −26.86 ± 1.45 | BW5M15-1 | 53.23 ± 0.12 | 59.27 ± 0.20 | 14.77 ± 0.28 |
BW5M15-2 | 15.33 ± 0.06 | 17.50 ± 0.12 | 4.28 ± 0.59 | ||||
BW10M10 | 48.63 ± 0.25 | 47.00 ± 0.10 | −24.93 ± 1.74 | BW10M10 | 13.77 ± 0.31 | 55.80 ± 0.26 | 23.13 ± 1.35 |
BW15M5 | 46.37 ± 0.21 | 26.30 ± 0.20 | −25.56 ± 1.89 | BW15M5 | 39.40 ± 0.10 | 41.77 ± 0.31 | 24.65 ± 1.69 |
BW20 | 27.10 ± 0.10 | 26.30 ± 0.10 | −26.77 ± 0.57 | BW20 | 32.43 ± 0.42 | 46.23 ± 0.06 | 25.31 ± 1.44 |
Chocofill | 18.47 ± 0.32 | 2.17 ± 0.21 | −50.38 ± 0.78 | Chocofill | 1.93 ± 0.35 | 8.80 ± 0.36 | 31.73 ± 1.42 |
The crystallization curve of sample BW20 (Figs 8 and 9E) – which contained only beeswax as a gelling agent – gave two peaks, a smaller one around 45–50 °C and a larger one around 25 °C. The first peak is the crystallization of esters, while the second peak is the crystallization of carbohydrates (Doan et al., 2017; Mandu et al., 2020). These two peaks also appeared in the melting curve shifted towards higher temperatures. Malvano and coworkers found only one peak in BW-based oleogel at around 25 °C (Malvano et al., 2024). It is interesting that in our BW20 sample the ratio of peak intensity at around 45 °C to peak intensity at around 25 °C is much lower for crystallization than for melting (Fig. 9E). The reason of this phenomena can be the relative high heating and cooling rate.
The sample BW5M15 (Fig. 9B) showed crystallization and melting curve like the curves of BW10M10 sample (Fig. 9C), only the peaks characterizing the BW crystals had lower intensity. In this, oleogel crystals characteristic of both beeswax and monoglyceride were also apparently present.
The shape of the crystallization and melting curves of sample BW15M5 (Fig. 9D) is significantly different from the heat flow curves of the other samples. On the one hand, phase transformations appear at lower temperatures, and on the other hand, the heat flow curves consist of several highly overlapping curves. This may indicate that, in addition to BW and M crystals, common crystals of the two gel-forming substances may also be formed.
A similar phenomenon was observed in the case of an oleogel consisting of a 3:1 mixture of candelilla wax and monoglyceride and grape seed oil (Choi et al., 2020). The crystallization and melting temperatures also decreased in this system. Its eutectic behavior was observed. Polarization microscopic images showed that the co-crystallization of the two gel-forming materials produced smaller crystals and a denser structure. On the bases of NMR investigation the composite of monoglyceride – glyceryl 1-monostearate and glyceryl 1,3-distearate – caused the co-crystallization of candelilla wax and monoglyceride.
In the case of oleogels created from two different wax mixtures, the common crystal structure of the two gel-forming waxes was also observed at certain concentration ratios, which created a harder, higher viscosity structure. Microscopic images of these systems also showed a more ordered and solid structure (Winkler-Moser et al., 2019).
In our case the BW15M5 sample showed the highest firmness (Table 1) and highest flow point in amplitude sweep curve (Fig. 5). The viscosity of this sample also was high (Fig. 4). The mechanical characteristics of this oleogel was very similar to the mechanical properties of Chocofill.
Presumably, the cooling rate applied was very fast, resulting in extremely wide peaks, within which we can assume further phase transitions. In the case of oleogels, enthalpy changes of similar amounts were observed in both the crystallization and the melting profiles, which were due to phase changes in wax and monoglycerides, whereas in Chocofill, the enthalpy change during crystallization was much larger than the change during melting. This suggests that the rapid cooling caused an unstable crystal structure to form in the fats (palm and shea), which required a much lower amount of energy to remelt and started melting at a much lower temperature.
The area under the DSC curves is related to the texture of the samples, because it is noticed that during the melting and solidification of the more solid Chocofill, larger enthalpy changes occurred than in the case of the softer textured, high oil content oleogels (Table 3). These changes are proportional to the solid content. The different areas under the curves can be clearly observed in Fig. 9 too. Knowing the thermal properties, it can be stated that oleogels could only be used in praline production by changing the parameters of the technological process (crystallization time, resting time, etc.).
Conclusions
Contrary to the literature, we used very high concentrations of gelling agents in our work, as the reference specialty fat – Chocofill – we chose was also a harder fat. At this high concentration, it is possible that the crystal aggregation in the oleogels was more intense, which was also observed in the sample preparations in the form of a slightly gritty stock. This phenomenon may be responsible for the inhomogeneity and possible higher standard deviations of the physical parameters in our samples.
The hardness of the M20, BW5M15, BW10M10 and BW20 oleogel samples we examined in this work was almost the same – about 15 N – based on the spreadability test measured with the SMS texture analyzer, while the hardness of the BW15M5 oleogel gave a value about 50% higher value as the hardness of Chocofill.
The highest G′ and G″ values were also obtained for the BW15M5 oleogel at the yield point determined by amplitude sweep, while the G′ and G″ values were lower at the yield points of the other samples. Based on these, the BW15M5 sample was proved to be much more flexible, of higher viscosity and harder than the other oleogels.
Based on the thermal analysis of the samples, not only were the monoglyceride and beeswax crystals separately present in the BW15M5 sample, but the two gel-forming substances synergistically created a more flexible and harder structure, unlike the other samples. Similar phenomena, such as eutectic crystallization when two gel-forming materials are used together, have already been described in the literature. Due to its mechanical properties, the resulting BW15M5 oleogel can be used as a substitute for the industrial fat, Chocofill.
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