Elsevier

Food Chemistry

Volume 197, Part B, 15 April 2016, Pages 1264-1270
Food Chemistry

Domestic cooking methods affect the phytochemical composition and antioxidant activity of purple-fleshed potatoes

https://doi.org/10.1016/j.foodchem.2015.11.049Get rights and content

Highlights

  • Domestic cooking methods on phytochemicals and antioxidant activity were systemic evaluated.

  • Stir-frying and air frying on phytochemicals and antioxidant were first studied.

  • Antioxidant activity was well accordance with total phenolic while showed least accordance with vitamin C.

Abstract

The effects of domestic cooking methods (boiling, baking, steaming, microwaving, frying, and stir-frying) and a new cooking method (air-frying) on the composition of phytochemicals (phenolics, anthocyanins, and carotenoids) and the antioxidant activity in purple-fleshed potatoes were investigated. Compared with raw potatoes, reductions of 23.59–90.42%, 7.09–72.44%, 7.45–83.15%, and 20.15–76.16% in the vitamin C, total phenolic, anthocyanin and carotenoid contents, respectively, was observed after cooking. Decreases of 7.88%, 21.55%, 22.48, 6.31%, and 61.38% in DPPH radical-scavenging activity was also observed after boiling, steaming, baking, microwaving and stir-frying, respectively, whereas an increase of 30.52% was noted after air-frying. A correlation analysis revealed that the antioxidant activity was in accordance with the total phenolic content and that this activity showed the lowest correlation with the vitamin C content. Among all of the cooking methods investigated in this study, stir-frying retained only slight levels of the phytochemicals and antioxidant activity observed in raw potatoes, whereas steaming and microwaving were able to retain most of the health-promoting compounds found in raw potatoes and may thus be suitable methods for cooking potatoes.

Introduction

Phytochemicals are bioactive non-nutrient plant compounds found in fruits, vegetables, grains and other plant foods and are broadly classified as carotenoids, phenolics, alkaloids, nitrogen-containing compounds, or organosulfur compounds (Nebeling, 2003). Numerous studies have demonstrated a negative correlation between the intake of phytochemicals and various diseases (e.g., chronic inflammation, cardiovascular diseases, cancer and diabetes) (Williams et al., 2013). As the dominant tuber crops worldwide (King & Slavin, 2013), potatoes contain numerous phytochemicals that are considered important due to their beneficial effects on health and therefore highly desirable in the human diet (Wolfe et al., 2008). Phytochemical extracts from potatoes have been reported to protect against acute liver injury and oxidative damage to erythrocytes (Singh & Rajini, 2008), reduce breast cancer in rats (Thompson et al., 2009), exhibit anti-inflammatory effects and even benefit heart and eye health (Ezekiel, Singh, Sharma, & Kaur, 2013).

Potatoes are usually cooked in different ways prior to consumption according to the recipes and culinary traditions of the various countries. For example, steaming, boiling and frying conform to the dietary habits of Western society, whereas stir-frying is used to prepare most homemade dishes in China (Ruiz-Rodriguez, Marín, Ocaña, & Soler-Rivas, 2008). Those cooking conditions are distinct (e.g. high temperature and excess oil involved in frying, limited oil, long cooking time and high temperature in air-frying, short cooking time and small size of strips in stir-frying) and induce a series of changes in the physical characteristics, chemical composition and enzyme modifications of foods (Rothwell et al., 2015). Most recent studies focused on the phytochemical and antioxidant activity undergone by potatoes during domestic cooking, but the conclusions were inconsistent and sometimes contradictory. For example, Blessington and coworkers (2010) reported that boiling, baking, frying and microwaving, significantly increased the total phenolic content, chlorogenic acid content and antioxidant activity in potatoes, whereas Xu, Li, Lu, Beta, and Hydamaka (2009) concluded that all cooking methods (boiling, baking and microwaving) induced decreases in phytochemical concentrations and antioxidant activity. Faller and Fialho (2009) showed that despite a significant increase in the total phenolic content, the antioxidant activity of potatoes decreased significantly, whereas Burgos et al. (2013) reported that boiling increased the total phenolic content and antioxidant activity and significantly decreased the total anthocyanin content. This difference may be attributed to the different species of potato examined in the studies, the pretreatment and cooking conditions applied, and the different analytical methods used. Thus, a systematic study of the effects of different cooking methods on phytochemical changes is needed.

Potatoes are food crops grown worldwide that are considered a good source of phytochemicals with interesting and health-promoting properties. With the increasing demand for novel functionality and health awareness, one variety of potato with purple-colored flesh containing high levels of anthocyanins has attracted much attention from researchers and the public (Ezekiel et al., 2013). However, the available information of the effects of cooking methods on selected phytochemicals and the antioxidant activity of purple-fleshed potatoes is insufficient. Purple potatoes contain an abundance of phytochemicals and should thus be a suitable model for evaluating the effects of domestic cooking methods on the phytochemical composition in potato.

Thus, in the present study, we conducted a systemic evaluation of the effects of different domestic cooking methods, including boiling, steaming, baking, microwaving, frying, on the phytochemical composition (i.e., total phenolics, phenolic acids, anthocyanins, carotenoids) and antioxidant activity, and the correlations between changes in these phytochemicals and antioxidant activity were also established. In particular, the effects of two other cooking methods (stir-frying, which is used to prepare most homemade dishes in China, and air-frying, a new technique for producing healthy fried potato strips) were evaluated for the first time in the present study.

Section snippets

Plant materials and cooking methods

Purple-fleshed potatoes (Heimeiren) (length 6–8 cm, diameter 3–4 cm, weight 100–120 g) and soybean oil were obtained from a local supermarket (Hangzhou, China). For boiling, steaming, baking and microwaving, whole tubers (unpeeled) were cooked, and a stainless-steel probe was inserted in the tubers to evaluate the cooking time (Table 1). For normal frying and air-frying, potatoes (unpeeled) were washed and cut into strips (8 × 7 × 60 mm) manually, whereas for stir-frying, the strips measured 2 × 3 × 60 mm.

Effect of cooking treatments on proximate composition

Table 2 shows the proximate composition of the samples after being subjected to different cooking conditions. The results show that frying, air-frying and stir-frying, significantly increased both the protein and crude fat contents (41.00%, 21.39% and 27.99% increases in protein and increases of 4.07 ± 0.15, 7.60 ± 0.37, and 3.49 ± 0.42 g/100 g DW in fat, respectively) (DW: dry weight). However, the other cooking methods, i.e., boiling, steaming and microwaving, induced no significant differences in

Conclusion

The present study clearly demonstrates that the antioxidant activity and the phytochemical composition of potatoes were affected by different cooking methods. Among the different cooking methods tested, steaming and microwaving were observed to retain the greatest amounts of phytochemicals and antioxidant activity, making these methods preferable for cooking potatoes from a health-promotion point of view (Palermo et al., 2014). However, the frying methods caused severe losses of vitamin C and

Conflict of interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by National Science-Technology Support Plan Projects (2014BAD04B01) and the Center for Basic Funds for Universities (2-2050205-15-001).

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