Influence of fatty acid composition on chemical changes in blends of sunflower oils during thermoxidation and frying
Highlights
► Influence of FA composition on sunflower oil performance was clarified using selected samples. ► Polymer formation followed zero-order kinetics independently of oil composition. ► There were no significant differences between frying oils and oils extracted from fried potatoes. ► The use of thermal oxidation assays to determine frying performance is demonstrated.
Introduction
Frying fats and oils range from hydrogenated oils designed for specific applications to non-hydrogenated refined oils. However, due to the negative nutritional implications of partially hydrogenated oils and saturated fats, the situation has drastically changed in the last few decades. The development of genetically modified seeds containing oils with fatty acid composition different from that of conventional oils is a promising trend to obtain high stability oils with nutritional properties compatible with consumer demands (Hazebroek, 2000, Kristott, 2003).
A huge number of papers on comparative performance of different oils and fats in the frying process are published every year. However, the results obtained are highly variable due to several reasons. Firstly, one oil sample is not representative of every oil of a same type, because differences in the initial quality and in the contents of minor compounds are important enough to modify significantly the frying performance. Secondly, important variables in the frying process, i.e. additives or the surface-to-oil volume ratio, are often omitted. Furthermore, repeatability of frying experiments is very low (Rodrigues Machado, Marmesat, Abrantes, & Dobarganes, 2007).
As a consequence, although formation of new compounds is expected to be accelerated by an increase in the degree of unsaturation, the differences found between oils of different degree of unsaturation are much lower or even opposite to those expected (Barrera-Arellano et al., 2002, Normand et al., 2001, Normand et al., 2006, Smith et al., 2007, Warner and Fehr, 2008, Warner and Gupta, 2003). These problems are diminished by avoiding the presence of the food, which in turn increases the repeatability of experiments (Barrera-Arellano et al., 1997, Gertz et al., 2000), or by modifying the degree of unsaturation by blending oils to partially eliminate the influence of other oil variables (Farhoosh et al., 2009, Su and White, 2004).
The objective of this paper was to evaluate the influence of the fatty acid composition on the formation of new compounds during thermoxidation and frying. Three sunflower oils with very different fatty acid composition, namely, high linoleic sunflower oil (HLSO), high oleic sunflower oil (HOSO) and high oleic, high palmitic sunflower oil (HOHPSO), were selected because no substantial changes in the minor components of their unsaponifiable fractions were found except for α-tocopherol in HOSO (Marmesat, 2007). Experiments at 180 °C in the absence of food on seven samples (the three oils and four binary mixtures) were performed to combine a strict control of main variables of the frying process, i.e. a wide range of fatty acid compositions and low differences in minor oil compounds. In addition, we performed frying experiments with the three oils using potatoes as the most consumed fried food. Being a non-lipid food, this substrate allowed us to evaluate the formation of new compounds at high temperature without interference from food lipids.
Section snippets
Samples
Sunflower (Helianthus annuus L.) seeds from the mutant line CAS-12 (high-oleic high-palmitic) (Fernández-Martínez, Mancha, Osorio, & Garcés, 1997) grown in Southern Spain were supplied by Advanta Ibérica (Marchena, Spain). From these seeds, HOHPSO was extracted and refined. HLSO and HOSO were supplied by KOIPE S.A. (Andújar, Jaén). Four binary mixtures of these three oils, i.e. HOHPSO/HLSO, 95:5 (w/w); HOSO/HLSO, 80:20 (w/w); HOSO/HLSO, 50:50 (w/w) and HOSO/HLSO, 25:75 (w/w), were also prepared
Results and discussion
Table 1 shows the main chemical characteristics of the seven samples used in the thermal oxidation assays, presented in increasing order of degree of unsaturation. The three basic oils enormously differed in their fatty acid composition. The linoleic acid content was very low for HOHPSO and HOSO (2.4% and 5.1%, respectively), in contrast to the high content in HLSO (56.8%). The preparation of the four binary mixtures was performed to give increasing concentrations of linoleic acid, which is the
Acknowledgements
This work was funded by the Spanish Ministry of Science and Innovation (MICINN) through Project AGL2010-18307 and Junta de Andalucía through Project P09-AGR-4622.
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