2021 Volume 27 Issue 1 Pages 63-68
During frying of food in oil, the high oil temperature allows water evaporation to occur inside the food due to the rapid heat transfer, and the evaporated water is replaced by oil. Moisture loss and oil absorption play an important role in the deep-frying process, providing a unique palatability and sensory characteristics to the food products. Rapeseed oil is often used as for frying. Extra virgin olive oil (EVOO) is also commonly used in kitchens. This study aimed to compare the moisture loss and oil absorption in French fries deep-fried in various combinations of these oils. Foods deep-fried in EVOO exhibited higher moisture loss and oil absorption than those deep-fried in rapeseed oil. Blending of EVOO with rapeseed oil enhanced moisture loss and oil absorption and also reduced the amount of acrolein formed during deep-frying. These results suggest that EVOO gives greater palatability to deep-fried foods and shows less deterioration of the frying oil than rapeseed oil.
Frying food is a fast cooking method that results in the development of a dried crust, crisp texture with added color and flavor. Frying can also trap flavor, taste, and nutritive compounds inside the food products. During frying of food in oil, heating and mass transfer take place simultaneously. Due to the high temperature of the oil (around 180 °C), rapid heat transfer occurs during a short cooking time. The water inside the food turns into steam and starts leaving the food. Oil adhering to the food surface is drawn into some of the voids left by the evaporated water and leads to its absorption into the food. The frying process involves dehydration and moisture loss, and it could play a role in promoting oil absorption (Saguy and Pinthus, 1995; Saguy et al., 1998; Razali and Bardri, 2003). The moisture content of food has a significant effect on its crispness, which plays an important role in the textural quality of fried foods and the interest of consumers towards fried food products (Katz and Labuza, 1981; Kawas and Moreira, 2001).
Rapeseed oil and olive oil are two of the most commonly used oils in the kitchen. Rapeseed oil is often used for frying or cooking foods at high temperature. It has a higher smoke point and a lower price compared to olive oil. However, rapeseed oil contains high levels of linoleic acid, an omega-6 polyunsaturated fatty acid that is suggested to lead to cardiovascular disease and inflammation (Simopoulos, 2008). Rapeseed oil also contains a high amount of α-linolenic acid, which is the main source of acrolein, a harmful compound that forms during heating of oil at high temperatures (Endo et al., 2013). Compared to rapeseed oil, olive oil contains less linoleic acid and more oleic acid, which oxidizes less than polyunsaturated fatty acids and has modulatory effects in a wide variety of physiological functions, such as immunity (Vingering et al., 2010; Sales-Campos et al., 2013). Extra virgin olive oil (EVOO), which is the highest grade of olive oil, contains a variety of compounds (1–2% of the oil), including antioxidants phenolic compounds (30–800 mg/kg) and α-tocopherol (98–370 mg/kg), that contribute to its stability and prevent certain chronic human diseases (International Olive Council, 2009, 2019; Romani et al., 2019; Psomiadou et al., 2000; Visioli and Galli, 2002). Therefore, when frying French fries, the formation of acrolein in EVOO was much lower than that in an α-linolenic-rich oil, such as salad oil (Kishimoto and Kashiwagi, 2018). Thus, there are several advantages to using olive oil for cooking and frying. In fact, retail sales of rapeseed oil have been falling recently in the oil market in Japan, while the sales of olive oil have been increasing (Agriculture and Agri-Food Canada, 2017).
The present study used near infrared spectroscopy (NIRS) to monitor the moisture and oil content of French fries deep-fried in rapeseed oil and EVOO. The objectives were to compare moisture loss and oil absorption in French fries deep-fried in these oils and to develop a method of frying French fries in rapeseed oil blended with EVOO to enhance moisture loss and oil absorption in French fries, which contributes to the palatability of fried food.
Materials Rapeseed oil, a medium-chain triglyceride oil (Nisshin Oillio Group, Ltd., Tokyo, Japan), and fresh potatoes (May Queen) were purchased at a market. EVOO was prepared by Shodoshima Healthyland Co., Ltd. (Kagawa, Japan). The percentage composition of fatty acids by type of the rapeseed oil and EVOO, respectively, was as follows (Yamazaki and Nagao, 1993; International Olive Council, 2019): palmitic acid (C16:0; 4.4% and 13.0%), palmitoleic acid (C16:1; 0.2% and 0.7%), stearic acid (C18:0; 1.7% and 2.2%), oleic acid (C18:1n-9; 56.1% and 68.9%), cis-vaccenic acid (C18:1n-7; 3.5% and 2.1%), linoleic acid (C18:2n-6; 21.2% and 11.3%), α-linolenic acid (C18:3n-3; 10.5% and 0.7%), arachidic acid (C20:0; 0.7% and 0.4%), 11-eicosenoic acid (C20:1n-9; 1.2% and 0.3%), behenic acid (C22:0; 0.3% and 0.1%), lignoceric acid (C24:0; 0.1% and 0.0%), and nervonic acid (C24:1n-9; 0.2% and 0.0%). Acrolein (purity, >95%) was purchased from Tokyo Chemical Industry, Co.,Ltd. (Tokyo, Japan).
Deep-frying fresh potatoes in oil Peeled fresh potato sticks (0.5 × 0.5 × 6 cm) weighing 100 g were deep-fried in 300 g of rapeseed oil, EVOO, and blends of the two oils at 180 °C for 0–10 min. The French fries were placed on paper towels to absorb the excess oil and cooled to room temperature (25 ± 2 °C).
NIRS analysis To determine the moisture and oil content of fresh potatoes and French fries, the food samples were analyzed with NIRS using a SpectraStar 2600 XT-R (Unity Scientific, Milford, MA, USA) analyzer equipped with a reflectance post-dispersive optical configuration, an InGaAs (680–2600 nm) detector, and a rotating top, which allows for rapid analysis of unground, partially ground, or ground samples (Kashiwagi et al., 2019). The analytical conditions were as follows: food samples (5 uncooked potato sticks and 5 fried potato sticks) were homogenized, and a subsample of each was placed in nylon bags (Hiryu N-3, Asahi Kasei PAX Co., Tokyo, Japan) and then placed on the sample stage of the SpectraStar 2600 XT-R analyzer. The system was controlled by UScan chemometric software (Unity Scientific), incorporating calibrations for common constituents such as moisture and oil in foods. The calibration models for moisture and oil content showed good correlation with R2 values of 0.992 and 0.994, respectively. This system was used to determine actual moisture and oil content of fresh potatoes as 79.5% and 0.0 g/100 g, respectively. The standard values were 79.8% and 0.1 g/100 g, respectively (McCay et al., 1987), indicating excellent detection accuracy.
Quantification of acrolein To determine the amount of acrolein in each oil sample, a standard curve was established (Kishimoto and Kashiwagi, 2018). Samples of medium-chain triglyceride oils containing different concentrations of acrolein were prepared and subjected to flash gas chromatography analysis using the HERACLES II electronic nose (Alpha MOS, Toulouse, France). Acrolein in oil samples after frying was determined using the standard curve.
Statistical analysis Data are presented as mean ± standard deviation (SD) from three replicates. The statistical significance of differences between two groups was analyzed using Student's t-test in Microsoft Excel. The Tukey-Kramer test in Microsoft Excel was used to identify significant differences among means of multiple groups. The data were analyzed by one-way analysis of variance (ANOVA) followed by the Tukey-Kramer test in Microsoft Excel. Statistical significance was indicated by p < 0.05.
Moisture loss and oil absorption in French fries during frying Fig. 1A shows the change in moisture content of French fries deep-fried in rapeseed oil and EVOO at 180 °C. The moisture content of French fries deep-fried in rapeseed oil decreased over time, falling to 38% after 10 min of frying. In contrast, French fries deep-fried in EVOO exhibited lower moisture content at 27% after 10 min of frying. These moisture values were within those of commercial French fry products, which ranged from 25.3% to 55.1% (Stastny et al., 2014). Frying involves thermal conductivity of heat and mass transfer and interactions between the food and the frying medium. Thermal conductivity of olive oil is higher than that of rapeseed oil (Kaye and Higgins, 1928; Woolf and Sibbitt, 1954; Turgut et al., 2009; Sampawindé Zongo et al., 2019). Moreover, surface-active compounds are also known to be related to the interfacial tension between the oil and the food (Pinthus and Saguy, 1994). It was reported that polyphenols show surface-active properties (Zillich et al., 2015). Therefore, the observed increase in moisture loss in food when frying using phenolic-rich EVOO may be due to rapid heat transfer and reduction of interfacial tension between the oil and the food. For refined olive oil, most of the phenolic compounds are removed during refining procedures and refined olive oil may have lower phenolic compound content than EVOO, although the fatty acid composition is almost the same as EVOO (Ramirez-Tortosa et al., 1999).
Change in moisture content (A), oil content (B), and total weight (C) of French fries deep-fried at 180 °C for 0–10 min using rapeseed oil and EVOO. Data are expressed as mean ± SD (n = 3). Asterisk symbol on the EVOO mean values indicates a significant difference from the rapeseed oil mean values at the same time point (*p < 0.05, **p < 0.01, ***p < 0.001, Student's t-test).
Fig. 1B shows the change in the oil content of French fries deep-fried in rapeseed oil and EVOO. French fries fried in EVOO showed higher level of oil absorption compared to fries deep-fried in rapeseed oil. Increase in oil content results from continuous oil absorption, which is part of an exchange between the frying oil and water content evaporated from the food (Dana and Saguy, 2001). The oil content of fried foods was perceived negatively due to its high caloric value and increased consumer awareness of the relationship between food and health. However, consumption of EVOO is related to low incidence of heart disease (Roche et al., 2000). It has been reported that deep-frying using EVOO can efficiently transfer phenolics to the food through absorption of oil and that the antioxidant capacity of foods deep-fried in EVOO is increased (Ramírez-Anaya et al., 2015). Dietary intake of antioxidants can improve endogenous antioxidant defenses and counter the excess of free radicals generated by pathological conditions (Finley et al., 2011). French fries deep-fried in EVOO had 20 g/100 g of oil content, falling within the range of commercial French fry products (11.1–22.3 g/100 g), which is lower than that of typical fried food products such as potato, corn, and tortilla chips (exceeding 30 g/100 g) (Saguy et al., 1998; Kawas and Moreira, 2001; Stastny et al., 2014).
Fig. 1C shows the change in the total weight of French fries deep-fried in rapeseed oil and EVOO. French fries deep-fried in EVOO showed lower total weight compared to fries deep-fried in rapeseed oil. The reason for this was that the moisture loss during deep-frying in EVOO was higher than that in rapeseed oil, as moisture accounted for a larger percentage of the total weight of the fries.
Reduction of the moisture content of French fries by blending rapeseed oil with EVOO for frying Based on these results, we investigated changes in the moisture content of French fries after frying in rapeseed oil blended with 5–15% EVOO. At 5% EVOO, the moisture content is significantly higher. Blends with 10% and 15% EVOO resulted in a moisture content of the fries at approximately the same level as using EVOO alone (Fig. 2A), suggesting that reduction of moisture content is dependent on blending EVOO with the rapeseed oil. Further, a lower quantity of EVOO in the rapeseed oil and EVOO blend increased the level of the moisture content of French fries compared to the fries deep-fried in rapeseed oil with 15% EVOO (Fig. 2A). These results suggest that enhancement of moisture loss depended on the amount of EVOO blended with rapeseed oil.
Moisture (A) and oil (B) content of French fries deep-fried at 180 °C for 10 min in rapeseed oil, EVOO, and their blends. Data are expressed as mean ± SD (n = 3). a–bMean values with different letters are significantly different (p < 0.05, Tukey-Kramer multiple comparison test).
The increase in the oil absorption in the French fries depended on the level of the moisture loss (Fig. 2B). This inference is in agreement with the findings of previous studies (Saguy and Pinthus, 1995; Saguy et al., 1998; Razali and Bardri, 2003).
Formation of acrolein in oils after frying The presence of acrylamide, which is probably carcinogenic to humans, in foods prepared by heating at high temperatures was reported by Tareke et al. (2002). It has been proposed that acrylamide is formed from acrolein, which is produced by lipid degradation (Ibrahim et al., 2020; Lingnert et al., 2002). When vegetable oils rich in α-linolenic acid, including rapeseed oil, are heated to frying temperature, acrolein, which is an irritating and off-flavor compound, is formed in the oil (Endo et al., 2013). Napolitano et al. (2008) reported that high phenolic EVOO can efficiently inhibit acrylamide formation in deep-frying. In this study, we examined the formation of acrolein after deep-frying French fries for the use of rapeseed oil, EVOO, and three blends (Fig. 3). The highest formation of acrolein was observed for rapeseed oil, and the formation of acrolein decreased depending on the concentration of EVOO in the blends of rapeseed oil and EVOO. This observation is consistent with previous results which showed that the formation of acrolein in salad oil (rich in α-linolenic acid) is reduced by blending it with EVOO for repeated use in frying food (Kishimoto and Kashiwagi, 2018). Antioxidants such as α-tocopherol play an important role in reducing the formation of acrolein. Therefore, the addition of refined olive oil for the reduction of the formation of acrolein may have lower activity than that of EVOO because the oil has a lower level of α-tocopherol content than that of EVOO (Ergönül and Köseoğlu, 2014).
The formation of acrolein in rapeseed oil, EVOO, and their blends after deep-frying of French fries at 180 °C for 10 min. Data are expressed as mean ± SD (n = 3). a–dMean values with different letters are significantly different (p < 0.05, Tukey-Kramer multiple comparison test).
In this study, our results demonstrated that French fries fried in EVOO exhibit a higher level of moisture loss and oil absorption than those fried in rapeseed oil. Moisture loss and oil absorption of food during frying can be enhanced by blending rapeseed oil with 10% EVOO. In addition, this blending can reduce the formation of acrolein during frying of the foods in rapeseed oil, which is rich in α-linolenic acid. This blending blocks the thermo-oxidative deterioration of the frying oil during heating. The blend shows inhibited formation of potentially harmful compounds like acrolein in vegetable oil during heating and also contributes to the palatability of deep-fried food, which is pleasing for consumers.
Acknowledgements We would like to thank Hiroyuki Hara and Shun Nakayama (BL TEC K.K., Osaka, Japan) for their technical advice about NIRS and Editage (www.editage.com) for English language editing.
