Research shows deep frying in EVOO improves nutritional value

The new research assessed the effect of deep frying in various cooking oils on the nutritional profiles and palatability of foods with different fat contents.

Countering the myths about high heat cooking with olive oil, recently-published research by the Modern Olives Laboratory team provides science-based evidence about the benefits of making EVOO your go-to cooking medium for deep frying. It’s great information to share with consumers at your farmers market stalls or farm shops, existing and potential new local stockists, and chefs wanting that healthful point of difference, so we’ve prepared a precis of the report for easy grower reference.

Introduction

When deep frying food, the quality of the frying oil and the fried food are intimately related, as the oil is absorbed during the frying process. When oil or fat is re-used multiple times it takes in moisture and air, resulting in thermal and oxidative decomposition, and the formation of elements harmful to health. Volatile decomposition products affect the flavour of the food, while the non-volatile compounds affect how long the oil can be used for frying. Naturally present or added antioxidants in oils and foods influence oil quality during deep-frying.

The oxidative breakdown of lipids also causes significant changes to the sensory properties and consumer acceptance of foods, affecting odour, flavour, colour, and texture, and sensory quality decreases with the number of frying cycles.

The aim of this study was to assess the effect of various cooking oils on the nutritional profiles and palatability of foods with different fat contents. Frozen chips, chicken, and broccoli were deep-fried in three cooking oils - extra virgin olive oil, canola oil, and grapeseed oil - and then evaluated for taste and other chemical changes such as products of degradation and antioxidants.

This work is a continuation of the research project Evaluation of chemical and physical changes in different commercial oils during heating.

Methodology

The trials consisted of four cycles of deepfrying at 180°C for four minutes, each using three litres of extra virgin olive oil (EVOO), canola and grapeseed oils. Standardised sample sizes of chicken nuggets, pre-cooked chips and broccoli were cooked separately, and added fresh to each cycle of cooking.

Samples of foods and oils were taken untreated and samples of used oil were taken after each cycle. Samples of food were taken after each cycle for sensorial analysis and after the first and fourth cycles for chemical analysis.

The experiments were carried out in triplicate.

Analysis

Sensory evaluation was performed blind by a nine-member untrained consumer panel. Samples were randomly coded and three sensory parameters (colour, texture, and flavour) were individually evaluated based on a nine-point hedonic scale (1: dislike and 9: extremely like).

Using recognised scientific methodologies the fat content in food samples was determined, along with chemical and nutritional elements of both the oils and foods including fatty acid content (FAC), total phenols, Vitamin E, squalene, free fatty acids (FFA), specific absorbance coefficient (K232 and K270), polar compounds and smoke point.

Results and discussion

Nutritional and organoleptic impact

When comparing the taste and preference of the food cooked with different oils, a statistically significant difference was only found between EVOO and canola oil on cooked chips. EVOO was preferred, with a fish odour and flavour detected in the food cooked with canola oil. This is consistent with previous research that shows that the oxidation of the linolenic acid in canola oil during deep-frying increases fishy odour and decreases fruity and nutty flavour, even at low concentrations.

Fat transfer between food and oils

In general, there were significant changes in fatty acid composition - saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), and polyunsaturated fatty acids (PUFAs) - of both the foods and oils used. These changes were greater when the food initially had less fat.

Key results • EVOO and canola oil deep-fried food were preferred by their colour, but canola fried food was disliked because of its flavour. • Results showed a transference between food and oils regarding fatty acid profile and antioxidant content as well as trans fatty acids (TFAs) and polar compounds (PCs): • all food presented more antioxidants and monounsaturated fatty acids after having been cooked with EVOO than after cooking with canola and grapeseed oils; • highest PCs in food were found when using canola oil and grapeseed oils; • EVOO was shown to decrease the PCs in chips and chicken nuggets; • PCs were not detected in raw broccoli, and broccoli cooked in EVOO showed the lowest PCs content; canola and grapeseed oils increased the TFAs in food, whereas EVOO decreased the TFAs in the chips and maintained the initial TFAs levels in chicken nuggets and broccoli. • EVOO improves the nutritional profile of the food when compared with canola and grapeseed oils when deep-frying without any negative impact on palatability or appearance.

Notably, when cooking with canola and grapeseed oils, MUFAs decreased and PUFAs levels increased in the cooked food compared with the raw food. After deepfrying with EVOO, there was an increase in MUFAs and a decrease in PUFAs in the cooked chicken nuggets and chips in comparison with the raw food.

The FAP obtained from the cooked broccoli was the same as that of each oil used.

The changes in these fatty acid contents were the opposite in the oils used. These results correspond well to previous studies that suggest the type of food being fried alters the composition of the frying oil because fatty acids are released from fatcontaining foods, and their concentration in the frying oil increases with continued use.

When analysing the influence of cooking cycles in the FAP of the oils being reused, only grapeseed oil showed significant FAP changes when cooking chips and chicken nuggets. MUFAs increased and PUFAs decreased in grapeseed oil after cycles three and four of cooking chips and chicken nuggets.

Antioxidant transfer of oil to food

Antioxidant content

After deep-frying with the different oils, the highest antioxidant content was seen in broccoli, followed by chips and chicken nuggets (Figures 1–3). Oil absorption is essentially a quantitative water replacement process, therefore the more water the food has, the more oil will be absorbed, and thus the more antioxidants are transferred or present in the food.

All cooked food presented more antioxidants after cooking with EVOO (~6653 ppm) than after cooking with canola (~407 ppm) and grapeseed oils (~584 ppm). This correlates with the oil’s initial antioxidant content and supports that the quality of oils during frying process and the quality of the final product are related. Phenol content

While phenol content was low in raw chips and in the raw chicken nuggets, after cooking with EVOO the phenol content increased. Canola and grapeseed oils initially showed only slight traces of these antioxidant compounds, reflected in only minor changes in phenol content in the chips and chicken nuggets after deep-frying with these oils.

The highest phenol content in raw food was seen in broccoli and, while broccoli showed an increase in phenols after deepfrying with all three oils, the highest phenol value was found after cooking with EVOO (177.8 ± 70.8 ppm vs 97 ± 0.6 ppm). These results add to previous studies that have shown that cooking vegetables in EVOO increased the phenols and their antioxidant content. Note: the apparent increase of phenol content in broccoli after cooking with canola and grapeseed oils may be explained by the concentration of phenolic compounds due to water loss rather than transfer from the oil as is the case with EVOO.

The level of phenols in EVOO decreased over time but remained significantly higher than in canola and grapeseed oils after deepfrying the food. After cycle four of deepfrying with EVOO, phenols also decreased in food: when moisture-containing foods are fried, phenolic antioxidants are lost by steam distillation and also consumed by oxidative reactions.

Vitamin E content

Vitamin E content was initially highest in chips (134 ± 0 ppm) then chicken nuggets (61.60 ± 0 ppm) and was not detected in broccoli, however after cooking with EVOO, broccoli showed the highest increment in vitamin E. The vitamin E content in chips remained the same after the first cycle of cooking with EVOO but decreased after the first cycle of cooking with canola and grapeseed oils. The vitamin E content in chicken nuggets remained the same after cooking with the three oils, decreasing after cycle four and almost to zero after cooking with grapeseed oil.

Vitamin E in all oils decreased over time when deep-frying. After four cycles of reusing oils, the highest vitamin E content was seen in EVOO used to cook broccoli (174.3 ± 11.2 ppm), followed by EVOO after cooking chips (119 ± 4.4 ppm) and chicken nuggets (110.99 ± 6.9 ppm). Canola oil showed the lowest vitamin E content after cooking chicken nuggets (87.58 ± 23.4 ppm). This may be attributed to the initial vitamin

300

Total ) s( ppm phenol

200

100

0 Total phenols in food

ChipsChicken nuggetsBroccoli Type of food Initial in EVOO C1 in EVOO C4 in Canola C1 in Canola C4 in Grapeseed C1 in Grapeseed C4

E content in the oils used (being higher in EVOO in all cases and EVOO’s very good resistance to oxidation.

Squalene content

Squalene was not detected in raw broccoli, with uncooked chips presenting the highest squalene values (978 ppm). Chips, chicken nuggets, and broccoli all showed a significant increment in squalene after deep-frying with EVOO and squalene content was significantly higher using EVOO (~10000 ppm) than canola and grapeseed oils (~200 ppm). This result is consistent with previous studies' findings that one of the most important differences between olive oil and other vegetable oils is the amount of squalene present in the oil. Olive oil even when it is refined contains 25 to 30 times more squalene than seed oils [46].

Squalene content of food after cooking with EVOO was higher in broccoli, followed by chips and then chicken nuggets, and these increments remained stable after four cycles of reusing EVOO. This can be attributed to squalene’s protective affect against oxidative breakdown and, in this study, the fact that deep-fat frying has two main advantages over other cooking methods: the temperature inside the food remains below 100°C as long as some liquid water remains, and that frying times are usually very short.

Oil deterioration and impact on food

FFA content

FFA content expressed as oleic acid is an important measure for assessing the suitability of vegetable oils for human consumption. FFA amounts are also directly correlated with upper temperature limits due to their lower boiling points. While FFA increased slightly and proportionally with frying time in this study, no significant changes occurred during the cooking process.

UV coefficients

The lowest formation of UV coefficients, secondary products of oxidation, was found in EVOO. UV coefficients were initially high in raw chips and chicken nuggets and not found in broccoli. A decrease in these parameters was observed after the fourth cycle of cooking with EVOO and canola oil, with a slight increase when cooking with grapeseed oil. Once again, this can be attributed to an increased resistance to oxidation due to the EVOO’s low polyunsaturated acid content.

Polar compounds

The highest polar compound (PC) levels were found in chips deep fried with canola and grapeseed oils, followed by chicken nuggets then broccoli. EVOO was shown to decrease the PCs in the chips and chicken nuggets by 20%, whereas grapeseed oil decreased PCs in chips by 8% and increased in chicken nuggets by 28%. The PCs in all oils increased over deep-frying cycles, with the highest levels in grapeseed oil. These results are consistent with previous research findings of linolenic acid content as a critical factor affecting the quality of oil during frying: PCs are derived from oxidation and thermal reaction of oils during frying, and oils with a greater amount of linoleic and linolenic acids are more susceptible to oxidation.

Trans fatty acids

Figure 5 shows the trans fatty acids (TFAs) results. The TFAs content decreased by approx. 70% or remained stable in the food cooked with EVOO and increased when cooking with canola and grapeseed oils (in some cases over 100%), showing the highest production with grapeseed oil. The same behaviour was observed with the oils: the lowest TFAs production was in EVOO and the highest in grapeseed oil. TFAs are formed during partial hydrogenation of oils and diets high in hydrogenated fat and/or trans fatty acids has been shown to have an adverse effect with respect to cardiovascular disease risk.

Conclusion

The results confirmed that there is a consistent transference between food and oils regarding fatty acid profile and antioxidant content, as well as TFAs and PCs. The changes observed in the cooked food show that the absorption of oil changes the composition ~of the food.

This study indicates that frying with EVOO delivers a better nutritional profile of the food when compared with canola and grapeseed oils, producing higher levels of MUFAs and antioxidants. Furthermore, food fried with EVOO had lower levels of undesirable products of degradation such as TFAs and PCs when compared with canola and grapeseed oils, when deep-frying under normal cooking conditions. This significantly better nutritional profile of EVOO fried food was obtained without compromising palatability.

15000

(ppm ) Squalene

10000

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0 Squalene in food

ChipsChicken nuggetsBroccoli Cooking cycles Initial in EVOO C1 in EVOO C4 in Canola C1 in Canola C4 in Grapeseed C1 in Grapeseed C4

4

% TFA

3

2

1

0 TFA in food

ChipsChicken nuggetsBroccoli Type of food Initial in EVOO C1 in EVOO C4 in Canola C1 in Canola C4 in Grapeseed C1 in Grapeseed C4

This is a precis of the report ‘Evaluation of Chemical and Nutritional Changes in Chips, Chicken Nuggets, and Broccoli after DeepFrying with Extra Virgin Olive Oil, Canola, and Grapeseed Oils’: de Alzaa, F., Guillaume, C. & Ravetti, L. Journal of Food Quality. 2021; https:// doi.org/10.1155/2021/7319013.

The full report is accessible from the Olive Science section of the Olive Wellness Institute website – www.olivewellnessinstitute.org.