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No literature data were available on the crystal polymorph structure associated with the crystallization/melting of TAG fractions of extra virgin olive oil until the publication of the article by Barba, Arrighetti and Calligaris in this issue of the European Journal of Lipid Science and Technology [p. 322–329]. The authors determined crystal structure of extra virgin olive oil TAGs using DSC coupled with XRD technique. These findings are of great importance as an increasing number of formulated foods (e.g. salad dressings, sauces, chilled and frozen ready-to-eat products) and new products are nowadays produced with this high quality vegetable oil as lipid phase. The significance of these results is underlined in this commentary.
Differential scanning calorimetry (DSC) is a widely known calorimetric technique mainly employed in the field of oil and fat technology. Its application does not require time-consuming manipulation and treatment of the sample, avoiding the use of toxic chemicals that could be hazardous for the environment, giving several advantages to the analyst.
A strict relation was demonstrated among DSC profiles obtained by cooling and heating transition and extracted thermal properties, and the chemical composition of oils and fats 1. On this basis, several studies were reported in literature about its potential application for the determination of iodine value 2, the prediction of fatty acid compositions 3, the estimation of adulteration 4, the evaluation of both induction time 5, and expected oxidation stage after conventional and innovative heating treatments 6 for several oils and fats.
Recently, some of these applications were also reported for extra virgin olive oil (EVOO), starting to fill the literature gap, with the aim to expand knowledge about the employment of this thermal technique in the field of this high quality vegetable oil 7–9. Despite these applications and their potentiality in the field of food technology, DSC does not allow the direct identification of the TAG crystal species formed in a fat or oil system. However, they are conveniently determined using DSC coupled with X-ray diffraction (XRD) technique that can be considered the best method for the characterization of the dynamic phase behaviour and polymorphism of lipids 10.
It is well known that EVOO phase transitions show two main consequential thermal events (A and B of Figure 1) associated with the crystallization/melting of fractions containing different ratios among saturated and unsaturated TAG 1, 7 but it is very surprising that no literature data are available on the crystal polymorph structure emerging as a consequence of these transitions up to the publication in this issue of the European Journal of Lipid Science and Technology of the article by Barba et al. 11.
Figure 1. Representative DSC cooling and heating transitions of EVOO obtained at a scanning rate of 2°C/min. Main exothermic and endothermic events are indicated with letters. A, B foor cooling and A0, A1 and B1 for heating.
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It must be underlined that the increasing knowledge of both the physical and thermal properties of such oils as those derived from olives is of remarkable importance with respect to functional, sensorial and nutritional properties of a number of formulated foods, such as salad dressings, sauces, chilled and frozen ready-to-eat products, where olive oils represent the lipid phase. In addition, data obtained on EVOO crystal structure could be used for the development of new products formulated with this oil, for example ice cream or Italian gelati
Data obtained on EVOO crystal structure could be used for the development of new products formulated with this oil, for example ice cream or Italian gelati
where it is nowadays added as a functional ingredient.
The authors of the paper realized a first attempt to systematically investigate the EVOO crystal polymorphism during cooling and subsequent heating of an oil sample at 2°C/min from 60 to −60°C and viceversa by means of DSC coupled with XRD at wide (WAXD) and small (SAXD) angle, respectively, giving a complete picture of the EVOO crystal morphology.
Their results allowed to relate the two endothermic events upon cooling (see A and B of Figure 1) that the authors found at about −10°C and at −33.5°C, to the formation of a triple-chain length (3L) having a c parameter of about 58.38 Å and a quadruple chain length structure (4L) with a c parameter of about 89.99 Å, respectively, evidencing a cell packing arrangement ascribable to a β′ form. In particular, two crystalline β′ phases were found; the so called β′a, which emerged at higher temperatures (peak B of Figure 1) and largely constituted by TAG rich in palmitic and/or stearic acid and a β′b phase, linked with DSC peak A of Figure 1, probably related to crystals formed by TAG mainly composed by oleic and/or linoleic acids, as previously hypothesized by other authors 1, 7.
The authors also stated that “Considering the solid fraction calculated from the partial integration of calorimetric melting curve, it can be assumed that about 17% w/w of the oil is crystallized under refrigerated storage conditions (4°C). Only when temperature is reduced below 0°C, the crystallization of the oil fraction rich in unsaturated fatty acid could take place”. This consideration also related to the type of the different polymorph crystals determined could be of great importance for all food products where olive oils are ingredient or constituent that are generally stored under refrigerated or frozen condition.
Finally, Barba et al. found that a part of the metastable β′ crystals rearranged into a more thermodynamically stable β form upon heating (see A0 of Figure 1), giving effort to the suppositions from other authors 7, 9. This event was followed by the melting of the two crystal structures β′a and β′b (see A1 and B1 of Figure 1) upon further heating, which was completed at about 11°C.
Obviously this is an initial study which will require follow-up work, as also stated by the authors, as only one sample was investigated. EVOO presents a unique chemical composition, with high but variable levels of fatty acids (e.g. palmitic acid 7.5–20%; oleic acid 55–83%), almost all largely influenced by different drupe cultivars, geographical origins, agronomical practices, harvesting periods and processing technologies. Thus, these very interesting results must be confirmed highlighting the influence of these factors on the phase transition behaviour and polymorphism of this unique vegetable oil.
The author has declared no conflict of interest.