The physical properties of oils and fats largely determine their uses and applications (Gunstone, 2002). Among the factors determining the melting and crystallization profiles of a fat, the most determinant appear to be the fatty acid composition and the distribution of saturated fatty acids in the triacylglycerol (TAG) molecules. Accordingly, a model of the fatty acid distribution has been proposed for vegetable oils (Van der Wal, 1960) based on the 1,3 random 2-random principle and it is thought to predict the TAG composition from the data available regarding the fatty acids present at the sn-1,3 and sn-2 positions of a given oil.

However, this model is based on an assumption that does not hold true in most species, i.e. that the sn-1 and sn-3 positions of TAGs are occupied by an identical proportion of saturated and unsaturated fatty acids. This principle was shown to be false by stereo-chemical studies carried out on oils from different species (Takagi and Ando, 1995; Santinelli et al., 1992), reflecting the different specificities of the enzymes involved in TAG biosynthesis that carry out the successive acylation of glycerol-3P: glycerol-3P-acyltransferase (GPAT), lysophosphatidate acyltransferase (LPAAT) and diacylglycerol acyltransferase (DAGAT) (Ohlrogge and Browse, 1995). Since each of these enzymes displays a preference for different substrates, certain pairs of fatty acids are favored at the expense of others, according to the joint specificity of GPAT and DAGAT, and asymmetric TAGs with regards to the sn-1, 3 positions which are produced. These deviations from the model of Van der Wal can be quantified using the so called α coefficient, which is defined as a coefficient of asymmetry given by the fatty acid content in the sn-1 position of TAGs in proportion to the total content of the same fatty acid at positions sn-1 and sn-3 (Martínez-Force et al., 2004). Therefore, this coefficient ranges from 0 in the case of totally asymmetric oils to 0.5 in symmetric TAG mixtures that fit Van der Wal's model.

In reference to saturated fatty acids (α_sat), the α coefficient changes from one plant species to another. Likewise, it is also modified in function of the level of saturated fatty acids and other factors, such as the ratio between oleic and linoleic acid in different sunflower fatty acid mutants (Martínez-Force et al., 2009). Here, we studied different collections of oils containing similar fatty acid compositions but different distributions of the saturated fatty acids in their TAG molecules and hence, different α_sat coefficients, which were calculated in function of their TAG composition. These oils were of the high-oleic high-stearic type, which are of interest given their stability and possible application as alternatives to the undesirable palm, lauric and hydrogenated trans fats in confectionery (Fernández-Moya et al., 2005; Pleite et al., 2006; Flickinger and Huth, 2004; Wassell and Young, 2007; List, 2004; Legrand and Rioux, 2015). The melting and crystallization behavior are the most important properties of confectionery and plastic fats (Narine and Marangoni, 1999; Marangoni, 2002; Smith et al., 2013) and thus, the phase transitions of these oils were studied by differential scanning calorimetry (DSC). Thus, the possibility of using the α_sat factor to predict the physical properties of oils and fats is discussed in function of the results.

In the present work, oils with different saturated fatty acid distributions at the sn-1 and sn-3 positions of TAGs were prepared by mixing sunflower oils and sunflower oil fractions enriched in disaturated TAGs. Within each series, the oils investigated contained approximately the same composition of saturated fatty acids but a different distribution between the 1 and 3 positions of the TAG. These differences in saturated fatty acids can be quantified through the so called α_sat (or simply α) coefficient. Such variation is usually found in oils from different species and it is caused by the different substrate specificity of the enzymes participating in TAG biosynthesis. Thus, the TAGs in rice oil have a symmetric saturated fatty acid distribution with an α value of 0.49, whereas the high asymmetry in walnut oil TAGs is reflected by an α value of 0.04 (Martínez-Force et al., 2004). The distribution of fatty acids was also demonstrated to be a function of the fatty acid composition of the oil (Martínez-Force et al., 2009). However, while different distributions have been reported for oils from different species, no systematic studies of the impact of this parameter on the physical properties of oils have been carried out to date. The oils studied here were grouped into three series containing about 27, 30 and 34% of total saturated fatty acids, and they correspond to different oils and fats that could be obtained from high-oleic high-stearic sunflower mutant seeds. These oils are of great interest because they are an alternative to cocoa butter and other tropical fats as a source of high-stearic disaturated TAGs (Salas et al., 2009). Although these oils have a similar fatty acid composition, they contain different TAG species, as seen in Table 2. To construct this table we considered that the disaturated TAG species were mostly of the saturated-unsaturated-saturated type. Previous studies on the saturated fatty acid content at the sn-2 position of sunflower oils indicated that this is a valid approximation (Martínez-Force et al., 2009) and thus, they were ordered according to their α values. Higher values of this parameter reflect more symmetry in terms of the saturated fatty acids and hence, a higher disaturated TAG content at the expense of other TAG species, mainly monosaturated like 1-stearoyl-2,3-oleoyl glycerol (StOO) or triunsaturated like trioleoyl glycerol (OOO). The most abundant disaturated TAG in these oils was 1,3-distearoyl-2-oleoyl-glycerol (StOSt), with lower amounts of 1-palmitoyl-3-stearoyl-2-oleoyl glycerol (POSt) or similar derivatives containing arachidic or behenic fatty acids that gave rise to 1-arachidoyl-2-oleoyl-3-stearoyl glycerol (AOSt) or 1-behenoyl-2-oleoyl-3-stearoyl glycerol (BOSt).

The effect of the fatty acid distribution in TAGs can be observed in thermograms showing the melting and crystallization curves of the oils (Figure 1 and 3). The melting profiles of these oils displayed two peaks at approximately 5 and 18 °C, whose relative dimension changed in function of the α values. Moreover, an additional signal appeared at lower temperatures (−5 °C) in the series 3 oils. There was a clear increase in the higher temperature signal in series 1 and 2 oils, evident at the expense of the lower temperature peak and as reflected by the values of α. This was due to the increase in disaturated TAGs that display higher melting points than monosaturated and triunsaturated ones. This effect has a direct impact on the solid fat content curves obtained by the integration of the melting thermograms (Figure 2), where the oils with the higher α coefficients displayed the highest solid contents during the melting interval of the oil from −10 °C to 25 °C. This situation differed in series 3 oils, which represent combinations of TAGs that are not usually found in natural oils. These oils have weak TAG symmetry and their high levels of saturated fatty acids reflect a larger proportion of monosaturated TAGs (mainly StOO) at the expense of triunsaturated (like OOO) and disaturated TAGs. As the value of α increases, so does the level of triunsaturated and disaturated TAGs. Nevertheless, the higher disaturated fatty acid content of these oils assures they have the maximum solid content.

The distribution of fatty acids within TAGs also determines their crystallization behavior, an aspect that is important in many food products such as margarine, chocolate, butter and spreads. Thus, the complex behavior of some fats in terms of crystallization, polymorphic transformation, crystal morphology and aggregation depends directly on the properties of their TAGs (Foubert et al., 2006; Foubert et al., 2007, De Graef et al., 2012). The effects of different tristearin (StStSt) and StOSt ratios in TAGs on the crystallization behavior of different fat blends with the same fatty acid composition has been studied (Vereecken et al., 2009). Accordingly, StStSt does not appear to exert a seeding effect on the other TAGs present in the TAG mixture, unlike fats containing tripalmitin and dipalmitoyl glycerol. The present study did not use fat blends containing trisaturated TAGs since they were not present in highly saturated sunflower oils. To study crystallization behavior we applied a temperature ramp from 30 to −70 °C and examined the resulting heat flow (Figure 3). In the three oil series studied, three exothermic signals were evident at approximately 5, −20 and −50 °C, corresponding to fractionated crystallization of different TAG species. The temperatures at which crystallization started were in function of the value of the α coefficient, meaning that oils with a higher symmetry coefficient, and therefore a higher disaturated content, started to solidify at higher temperatures. This indicated that the higher content of disaturated TAGs forced the crystallization of the oil at higher temperatures and as such, oils with a higher symmetry will tend to form crystals at higher temperatures than oils with lower α coefficients.

The thermal properties of a high-oleic high-stearic oil series containing very similar fatty acid compositions changed in function of the distribution of saturated fatty acid in the TAGs. At saturated fatty acid concentrations of 30% or less, more symmetry in the distribution of saturated fatty acids involves a greater content of solids at temperatures from 0 to 20 °C and crystallization at higher temperatures (see Fig. 4, which shows the content of solids in the oils of the different series at 3 different temperatures). The content of solids tends to increase when the saturated fatty acids are more symmetrically distributed between the external TAG positions. These changes tended to be less noticeable at lower temperatures in series 1 and 2, whereas there was still a high degree of variation in the solid content in series 3 oils, probably since the changes in the α coefficient in this saturated fatty acid content involved more abrupt modifications in the TAG compositions of the oils. The physical properties of the oils studied clearly depend largely on the distribution of fatty acids in the TAGs, as well as on the fatty acid composition itself. Moreover, the α coefficient may be valid to quantify this distribution in terms of the symmetrical distribution of saturated fatty acids between the external positions of TAGs.