Olive oil stability under deep-frying conditions

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Abstract

The suitability of different commercial olive oil categories for domestic frying was investigated. Oil samples were taken every 3 h of frying and evaluated for free acidity, peroxide and p-anisidine values, specific extinction coefficients, oxidative stability, fatty acids, vitamin E, β-carotene and total phenols, until the total polar compounds achieved the maximum legal value (25%). All olive oils were fried during more time than the commercial vegetable oil blend taken for comparison (from 24 to 27 h, against 15 h). The extra-virgin Protected Designation of Origin (PDO) olive oil was characterized by reduced levels of oxidation and hydrolysis, and superior amounts of minor antioxidant compounds. The “olive oil” commercial category behaves similarly, but “Cobrançosa” olive oils performance was slightly worse, and clearly different between years, highlighting the importance of blending different cultivars. The vegetable oil, despite containing significantly higher amounts of vitamin E, was highly susceptible to oxidation under frying conditions when compared to all olive oils.

The results also show that the chemical composition of olive oils, particularly the amount of natural antioxidants, are important parameters in their predictive behavior along the frying process, but mostly that olive oil is clearly resistant to frying conditions, independently to the commercial category chosen.

Introduction

Frying is one of the most popular culinary processes worldwide, both for industrial and domestic food preparation procedures. Fried products have unique organoleptic and sensorial properties, including flavor, texture, and appearance, which turn them largely enjoyed by consumers. Also, this procedure considerably reduces cooking time (Sánchez-Muniz and Bastida, 2006) and is regarded as inducing equal or even smaller nutrient losses when compared with other common culinary processes (Fillion and Henry, 1998). Frying can enhance the food nutritive value due to the simultaneous incorporation of important lipid components, namely vitamin E and essential fatty acids, providing that it is consumed under a balanced diet, because the lipid uptake might increase significantly the total daily energy intake.

The high temperatures used during frying, in the presence of oxygen and water, induce important chemical changes of the oils, namely by oxidation, polymerization, cyclization, and hydrolysis (Paul and Mittal, 1997, Saguy and Dana, 2003), inevitably reducing their shelf life and affecting directly the quality of the final fried food (Kochhar, 2001). These chemical reactions are influenced by the type and quality of the oil, the food properties, and the food/oil ratio, among other parameters (Saguy and Dana, 2003), altogether determining the frying oil performance (Andrikopoulos et al., 2002). Each vegetable oil is characterized by typical stabilities against oxidation, dependent on the fatty acids composition, particularly the insaturation degree, and the content and composition of minor compounds such as tocopherols (particularly γ-tocopherol), certain sterols, hydrocarbons (squalene), carotenoids, polyphenols, and trace metals (Hoffman, 1989, Kochhar, 2001).

Several studies support a relationship between the Mediterranean diet and a lower incidence of some important diseases of our century, including cancer (Assmann et al., 1997) and cardiovascular diseases (Covas, 2007). Olive oil is typically the main lipid source in the Mediterranean diet, being used as salad dressing and for frying purposes. Its beneficial properties are associated with the richness in monounsaturated fatty acids, but other minor compounds also take an important part (Varela and Ruiz-Roso, 2000). The differences to other common vegetable oils are enhanced by the fact that olive oil is mostly obtained by cold pressure procedures, without being subjected to refining – virgin olive oil – retaining therefore higher amounts of important bioactive components of the olive fruit. More recently, however, mostly supported by economical reasons and reduced consumer information, the consumption of “olive oil”, a blend of refined with virgin olive oils in undeclared proportions, is increasing, together with other refined vegetable oils blends, including sunflower, soybean, and high oleic acid seed oils, particularly for frying purposes.

The objective of the present work is to compare the stability under real domestic frying conditions of several olive oils categories. The vegetable oil blend most frequently consumed in Portugal, sunflower based, was also included in the study. Aware that predictions based on the oxidative stability alone generally do not correlate with the real oxidation kinetics under frying conditions, and in the presence of food (Velasco and Dobarganes, 2002), this study was performed under real domestic frying conditions. The frying media degradation was evaluated taking the actual law as rejection point, particularly the total polar compounds, while complemented by several chemical evaluations in order to support and understand the degradation patterns and potential nutritional losses.

Section snippets

Materials and methods

All reagents were of analytical, chromatographic or spectroscopic grade and were supplied from Merck (Darmstadt, Germany) or Sigma–Aldrich (St. Louis, USA). All experiments and analytical determinations were performed at least in duplicate.

Changes in the total polar compounds (TPC)

In accordance with the Portuguese law and as mandatory in several countries, the legal rejection point for frying oils is 25% of TPC, including under this label the hydrolysis products (diglycerides, monoglycerides and free fatty acids), the oxidation and polymeric derivatives, all formed at temperatures below 180 °C (Portaria No. 1135/95). Under real frying facilities, mostly restaurants and industries, this parameter is usually evaluated by commercial rapid tests, mostly based on colorimetric

Conclusions

In the present work is proved that olive oil, independently of its category label, is clearly resistant to degradation under domestic frying conditions (170 °C). Among the olive oil categories, the extra-virgin PDO olive oil was more resistant under the conditions of this study. Being from the “Trás-os-Montes” region, this olive oil is particularly rich in “Cobrançosa” cultivar. Nevertheless, when this cultivar was evaluated individually, the monovarietal olive oils were clearly less resistant,

Conflict of Interest

The authors declare that there are no conflicts of interest.

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