Food Science and Technology Research
Online ISSN : 1881-3984
Print ISSN : 1344-6606
ISSN-L : 1344-6606
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Reducing the Formation of Acrolein from Linolenate-Rich Oil by Blending with Extra Virgin Olive Oil during Repeated Frying of Food at High Temperatures
Norihito Kishimoto Ayako Kashiwagi
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2018 Volume 24 Issue 6 Pages 1017-1020

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Abstract

Vegetable oils have long been used in cooking. However, repeated heating at a high temperature results in the gradual deterioration of the vegetable oils through oxidative degradation of fatty acids, leading to the formation of undesirable and irritating odors (e.g., acrolein). Linolenic acid is the main source of the acrolein formed during the heating of vegetable oils. The present study has developed a method to reduce the formation of acrolein during repeated frying with vegetable oils. Electronic nose analysis showed that the formation of acrolein from low-linolenic oil such as extra virgin olive oil (EVOO) was much lower than from a linolenic-rich oil, salad oil, when frying French fries. Blending EVOO with salad oil effectively reduced the amount of acrolein formed during repeated frying. These results suggest that EVOO can be used to reduce the formation of acrolein when repeatedly heating foods in thermosensitive vegetable oils.

Introduction

Cooking oils are plant, animal, or synthetic fats that are used for frying, baking, and other types of cooking. Vegetable oils are often used for cooking with heat by deep- and pan-frying, because these oils tend to be healthy, containing fats essential in the human diet. In particular, salad oil has long been popular as a cooking oil for deep frying in Japan. It can be a mixture of several vegetable oils, typically, soybean oil and rapeseed oil. However, excess heating of vegetable oils at high temperature causes them to undergo hydrolysis, oxidation and thermal reactions. Consequently, many by-products are formed, such as free fatty acids and free radicals that, in turn, combine to make monoglycerides, diglycerides and polymeric triglycerides (Tabee et al., 2009). All of these derived products generate polar compounds whose content is the most widely-used indicator for monitoring oil quality (Fritch, 1981). The prolonged heating of cooking oil also leads to the formation of potentially harmful compounds such as acrolein (Umano and Shibamoto, 1987).

Acrolein (2-propenal) is known to be formed during the heating of fats and oils. Acrolein vapor may irritate the eyes, and the nasal and respiratory tracts at low levels of exposure (Faroon et al., 2008; Gasee et al., 1996; Li and Holian, 1998; Uchida, 1999). Endo et al. (2013) found that linolenic acid was the main source of the acrolein formed during the heating of vegetable oils. Extra virgin olive oil (EVOO) can also form acrolein at low levels after being heated (Fullana et al., 2004; Katragadda et al., 2010; Molina-Garcia et al., 2017), because linolenic acid is usually present at a level of less than 1% (IOC, 2015).

Recently, EVOO has come to be used in most Asian countries as a premium edible oil because of its beneficial effects on health (Capogna and Gomez, 2016). EVOO is known as “the heart-friendly oil” and is preferred as a conventional cooking oil. EVOO has acquired the status of a “Functional Food” or “Health Food”, as these oils contain various antioxidants (Cunha et al., 2006) that prevent its autoxidation and are responsible for its high stability. Therefore, it is possible that the antioxidants in the oil help to prevent lipid oxidation. In particular, there are major benefits in using EVOO to reduce the effects of oxidative stress because EVOO is a readily available food source containing appreciable amounts of natural antioxidants. The present study used an electronic nose to monitor the amounts of acrolein formed during repeated frying of French fries in salad oil containing soybean oil and rapeseed oil, as a linolenic-rich oil, and in EVOO as a low-linolenic oil. The overall objective was to develop a method of blocking the formation of acrolein during the repeated frying of French fries in salad oil by blending it with a low-linolenic oil such as EVOO.

Materials and Methods

Materials    The salad oil and Medium-chain triglyceride (MCT) oil (Nisshin OilliO Group, Ltd., Tokyo, Japan) were purchased in a market. EVOO was prepared by Shodoshima Healthyland Co., Ltd. (Kagawa, Japan). The amount of α-linolenic acid (18:3n-3) in the salad oil and EVOO was 7.0 and 0.9 g per 100 g of oil, respectively. Acrolein (purity > 95%) and α-tocopherol (TP) (> 97%) were purchased from Tokyo Kasei Industry, Tokyo, Japan. Frozen French fries were purchased in a market.

Cooking French fries with oils    One hundred grams of frozen French fries were fried in oils at 180°C.

Rapid measurement of oil quality    The Testo 270 Deep-Frying Oil Tester (Testo AG, Lenzkirch, Germany) was used to rapidly measure the content of total polar compounds in the oil to an accuracy of ± 2%.

Flash gas chromatography electronic nose analysis    The headspace of the oil samples was analyzed using the HERACLES II electronic nose (Alpha MOS, Toulouse, France) (Kishimoto et al., 2017). The HERACLES II was equipped with two identical gas chromatography columns working in parallel mode: a non-polar column (MXT-5: 10-m length and 180-µm diameter) and a polar column (MXT-WAX: 10-m length and 180-µm diameter) to produce two chromatograms simultaneously. It was also equipped with an HS100 auto-sampler (CTC Analytics AG, Zwingen, Switzerland) to automate the sample incubation and injection. For calibration, an alkane mix (from n-heptane to n-hexadecane) was used to convert retention times into Kovats indices. The analytical procedure was as follows: an aliquot of oil (2.0 g) was placed in a 20-mL vial then sealed with a magnetic cap. The vial was placed in the auto-sampler, which was placed in the HERACLES's shaker oven where it was incubated for 15 min at 60°C, while being shaken at 500 rpm. A syringe sampled 5 mL of the headspace then injected it into the gas chromatograph. The thermal program started at 40°C (held for 10 s) then increased to 250°C at 1.5°C/s. The final temperature was held for 60 s. The total separation time was 120 s. Data were acquired and processed using AlphaSoft software v.14 (Alpha MOS, Toulouse, France) then an AroChemBase module (Alpha MOS, Toulouse, France) was used to identify the volatile compounds.

Quantification of acrolein    To determine the amounts of acrolein in the oil samples, we first established a standard curve. Different concentrations of acrolein were prepared in MCT oil, and then subjected to flash gas chromatography electronic nose analysis. The amounts of acrolein in the sample oils after heating were determined from the standard curve. Experiments were performed in triplicate, and the results were presented as mean values.

Statistical analysis    Data are presented as means ± standard deviation from three replicates. The significance of differences between the mean values of acrolein formation in salad oil and EVOO was tested using Student's t-test in Microsoft Excel. Data were analyzed by one-way analysis of variance (ANOVA) followed by Tukey-Kramer test in Microsoft Excel. Values of p < 0.05 were considered to be statistically significant.

Results and Discussion

Flash gas chromatography electronic nose analysis    Figure 1 shows a representative chromatogram of the headspace gases obtained from acrolein in MCT oil and salad oil after repeated frying of French fries at 180°C, when the MXT-WAX column was used. The peak of acrolein was eluted at a retention time of 25 s. At the same retention time, the peaks of the major volatile compound of salad oil were also detected.

Fig. 1.

A representative chromatogram of head space gases obtained from MCT containing acrolein and salad oil after frying 5 batches of French fries.

Quantification of acrolein    The standard curve for acrolein which was used to determine its concentration in the oil samples. The concentration of acrolein was correlated with the peak area over the range between 25 and 2 500 ppb. The linear correlation coefficient (R = 0.996) indicated that this standard curve allowed quantification of acrolein in oils to a high accuracy.

The formation of acrolein in oils after repeated frying use    Figure 2 shows the changes in the levels of acrolein formed during the repeated frying of French fries at 180°C. The amounts of acrolein in the salad oil increased as frying repetitions increased, reaching 639 ppb after the 5th frying repetition. In contrast, EVOO exhibited a much lower level of acrolein, 92 ppb, during frying when compared with salad oil.

Fig. 2.

The relationship between the formation of acrolein from vegetable oils and the number of batches of French fries prepared using Salad Oil and EVOO. Asterisks on the EVOO mean values indicate a significant difference from the Salad oil mean values at the same time point (p < 0.05, Student's t-test).

Significance of differences of mean values compared with that of salad oil - ** p < 0.01, *** p < 0.001.

Reduction in the formation of acrolein from salad oil by blending with EVOO during frying    Based on these results, we investigated changes in the levels of acrolein after frying 5 batches of French fries in salad oil blended with 10% EVOO. This blend effectively kept the acrolein content at almost the same low level as using EVOO alone (Fig. 3), suggesting that this blocking effect depended on blending the salad oil with EVOO. Blending with 10% EVOO had an inhibitory effect on the level of total polar compounds formed in the salad oil (Fig. 4).

Fig. 3.

The formation of acrolein in vegetable oils after frying 5 batches of French fries. Mean values without a common letter differ significantly (p < 0.05, Tukey-Kramer multiple comparison test).

Fig. 4.

The formation of total polar compounds from vegetable oils after frying up to 5 batches of French fries. a–c Mean values with different letters for frying 5 batches of French fries are significantly different (p < 0.05, Tukey-Kramer multiple comparison test).

We next examined the formation of acrolein after frying using salad oil blended with less than 10% EVOO. A reduction in the quantity of EVOO blended with salad oil increased the levels of acrolein after the 5th frying repetition when compared with salad oil blended with 10% EVOO (Fig. 3). This suggested that the reduction in the formation of acrolein during frying depended on how much EVOO was blended with the salad oil.

EVOO contains substantial amounts of antioxidants, such as α-TP (Cunha et al., 2006). The content of α-TP in EVOO in the present study was 16.2 mg/100 g. The level of acrolein from salad oil including 8 mg α-TP was the same as that with 10% added EVOO, so the reduction was the same level as that for salad oil blended with 5% EVOO (Fig. 3). This suggests that EVOO contains an inhibitory agent other than α-TP for reducing the formation of acrolein during frying with salad oil.

In conclusion, our results demonstrated that the formation of acrolein in salad oil, a linolenate-rich oil, is reduced by blending with EVOO during repeated frying of food at high temperature, so this could be a method to block the degradation of oil during heating. EVOO has a crucial role in promoting health, but its benefits are due not only to its properties such as preventing diseases but also to its ability to inhibit the formation of harmful compounds from vegetable oil during heating when deep-frying foods.

Acknowledgments    We would like to thank Philip Creed, PhD, from Edanz Group for editing a draft of this manuscript.

References
 
© 2018 by Japanese Society for Food Science and Technology
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