Effect of different cooking methods on phenolic compounds and antioxidant properties of Cirsium vulgare stem

Introduction: In recent years, increasing attention has been paid by humans to the role of diet in human health. Epidemiological researches have demonstrated that a high intake of plant-originated foods is strongly associated with a reduced risk of a number of chronic diseases, such as atherosclerosis and cancer, neurodegenerative diseases, including Parkinson's and Alzheimer's diseases (Şengül et al., 2014). These beneficial effects have been partly attributed to the compounds present in plants that possess antioxidant activity. Antioxidant compounds, which are found in plants (fruits, vegetables, medicinal herbs, etc.), are free radical scavenging molecules, such as phenolic compounds (e.g., phenolic acids, flavonoids, stilbenes, quinones, coumarins, lignans, tannins), nitrogen compounds (amines, alkaloids, betalains), vitamins (C, E), endogenous metabolites, and terpenoids (including carotenoids) (Cai et al., 2004).
Artichoke is one of the plants with high health properties and usually heat treated before consumption. Stems of artichoke present significant antioxidant activity (Pereira et al., 2013), and for this purpose they are the main ingredients in many dietary supplements and drugs. In particular, stems and leaves are the most common plant part used for therapeutic purposes, since they have numerous medicinal properties including antitumor, antioxidant, antibacterial, antifungal, and hepatoprotective effects and so forth, mostly attributed to their high content of phenolic compounds (Zhu et al., 2004). Vegetables are commonly cooked by different cooking processes, including steaming, boiling in water or microwaving, before being consumed. These processes change the physical and chemical properties of vegetables. Cooking processes may have an effect on the antioxidant content of food due to antioxidant release, destruction, or creation of redox-active metabolites (Wachtel-calor et al., 2008) Antioxidant compounds, such as ascorbic acid and some carotenoids, are very sensitive to heat and storage. On the other hand, polyphenols have shown a certain stability when exposed to high temperatures. Turkmen et al., (2005) reported that after cooking, total antioxidant activity increased or did not change depending on the type of vegetable but not type of cooking. Zhang and Hamauzu (2004) showed that cooking affected the antioxidant compound contents of broccoli. Ismail and et al., (2004) demonstrated that heating decreased the total phenolic content in some vegetables such as swamp cabbage, kale, spinach, cabbage, and shallots. Important parameters in the cooking quality of vegetables, which may strongly influence consumer preferences, are texture and color. Thus, the aim of the current study was to investigate the effects of different cooking methods (boiling, steaming, and microwaving) on artichoke stem. The antioxidant capacity, and physical characteristic changes after cooking were also investigated.
Material and methods: In this research, the effects of different cooking methods (boiling, steaming and microwaving) in three levels of time on the antioxidant properties of the artichoke stem were used. Fresh artichoke was used in this research and obtained from a market in Sari, Iran and immediately cleaned by removed manually non edible parts with a sharp knife. Artichoke carefully washed with water (in consumer conditions), dried air and stems were cut into almost equal small pieces, mixed well. The samples were taken and divided into four portions. One portion was kept raw as control and stored at 4 ◦C in the refrigerator in home consumer conditions, others were subjected for four thermally treatments in triplicate. The best cooking times were determined as previously described and according to common cooking techniques [9] which conducted by trained researchers (Boiling 5, 10 and 15 min, steaming 5, 10 and 15 min, Microwaving 2, 4 and 6 min). Cooking conditions were examined, with a preliminary experiment in our laboratory. Then, total phenol content, total flavonoid content, DPPH radical scavenging, total antioxidant capacity and ferric reducing power and also chemical composition and sensory properties of samples were evaluated.
Result and discussion: The results showed that boiling at all times caused a significant increase in moisture content in the samples (P<0.05). Steaming also increased the moisture content of the samples, but this increase was not significant (P<0.05). There was no significant difference between the control and the cooked samples by microwave in 5 minutes (P<0.05). The fat content of the samples varied from 0.11 to 0.16%. Methods and cooking times did not have a significant effect on fat and protein (P<0.05). Protein levels also varied from 1.2 to 7.2%. Also, there was no significant difference between the amount of ash of the control sample and in boiled, steamed and microwaved samples at different times (P<0.05) except the microwaved sample in 6 minutes, which ash content was 1.9%. The cooking method has a significant effect on total phenol content, total flavonoid and antioxidant activity (P<0.05). The total phenol content and total flavonoid content of the artichoke stem under microwave processing at 2 and 4 min and the steaming process at 5 minutes increased compared to fresh artichoke stem samples and the boiling process decreased significantly (p <0.05). In boiling method, stems are immersed in water and in the loss of phenolic compounds because of leaching (leakage of compounds into water from artichoke). The antioxidant activity of the stem in the microwave cooking process at 2 min and steaming at 5 min showed the highest activity. In the reducing power assays, the antioxidants present in the artichoke stems convert the oxidized form of iron (Fe⁺³) in ferric chloride to the ferrous (Fe⁺²) form. The reducing power of the microwave method was significantly higher than other methods (P<0.05). In the sensory evaluation, the microwave method for 2 min and steaming for 5 min obtained the highest overall acceptance. Therefore, the microwaving and steaming methods are recommended in short time for cooking of artichoke and the optimal use of the valuable and useful compounds of this plant.
Conclusion: Therefore, microwave and steaming methods in short time for cooking artichoke stem and optimal use of the beneficial compounds and health of this plant are recommended. In addition, artichoke stems of the studied genotypes showed different bioactive compound profiles and significant antioxidant properties, and could be further used in the food and nutraceuticals industries as a cheap source of phenolic compounds and antioxidants.