The effect of the hydrophilic and lipophilic herbal extracts on the oxidative stability of cooking plant oils during low- and high-temperature heating

Background

Recently, the most commonly used cooking oils in the world are obtained from corn germ, soybean, sunflower and rape seeds. Also, refined flaxseed oil is preferred for low-heat cooking in some countries. Use these oils instead of solid fats (including butter, shortening, lard and margarine) and tropical oils (including palm and coconut oil) can be healthy choice, because of lower saturated fat content. Unfortunately, repeated or high-temperature heating may result in oil decompose to form unwanted hazardous substances which can pose harmful effects to the health. The well-known natural method to limit oil oxidation is the used of herbs and spices, which also can improve the taste and aroma properties of food. The aim of the study was to evaluate the effect of added herbal extracts on the oxidative stability of two cooking plant oils (refined rapeseed oil, refined flaxseed oil) during low- and high-temperature heating.

Methods

Hydro- and lipophilic extracts were prepared from popular commercial dried herbs (thyme, parsley, caraway, oregano, basil, dill) using 70% methanol and n-hexane, respectively. Cooking plant oils were characterized by quality indices, fatty acid composition and content of bioactive compounds, while in the herbal extracts' composition and content of pigments, total phenolic compounds, flavonoids, tocols and antioxidant capacity were examined. The oxidative stability of cooking plant oils, without and with the addition of herbal extracts, was determined in the Rancimat test at 110 and 150 °C for refined rapeseed oil and 80 and 110 °C for refined flaxseed oil.

Results

It was found that the addition of herbal extracts had varied effect on oxidative stability of flaxseed and rapeseed oils at low- and high-temperature heating. Flaxseed oil was generally well-protected by thyme hydro- and lipophilic extracts regardless of temperature heating (induction period increase by 18–24%). Also, hydrophilic parsley and lipophilic oregano extracts improved this oil oxidative stability at low temperature (induction period increase by 10–17%), while lipophilic extracts from caraway and dill were more effective at high temperature (induction period increase by 20–26%). In turn, all extracts improved the rapeseed oil oxidative stability under low-temperature heating (induction period increase by 11–24%), but they were ineffective during high-temperature heating. The better additives for this oil were both type extracts obtained from thyme and oregano, which were characterized by the highest content of antioxidants, total phenolic compounds and tocopherols, mainly α-tocopherol. The results suggest a greater difference in content of phenolic compounds for extracts obtained by using of different solvents then from different herbs. In turn, the herb type had a strong impact on the content of pigments and tocopherols.

Conclusions

Results confirmed the possibility of using herbal extracts to increase the oxidative stability of cooking plant oils during thermal heating. However, the type of solvent and herb should be adjusted individually to cooking oil type and heating temperature.