Air-Frying Is a Better Thermal Processing Choice for Improving Antioxidant Properties of Brassica Vegetables

Abstract

Brassica vegetables have demonstrated many health benefits over the years due to their composition of phenolic, flavonoid, and glucosinolate contents. However, these bioactive molecules can be easily depleted during gastronomic operations. Therefore, a sustainable method that improves their phenolic content and antioxidant activity is required for both the processors and consumers. Thermal processing has been demonstrated as a method to improve the phenolic content and antioxidant status of Brassica vegetables. In the current study, four different thermal processing methods, including freeze-drying, sautéing, steaming, and air-frying, were employed for five different Brassica vegetables, including kale, broccoli sprouts, Brussels sprouts, red cabbage, and green cabbage. The total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activities were assessed using radical scavenging activity (DPPH and ABTS^•+), reducing power (FRAP), and the chelating ability of metal ions. Among the methods tested, air-frying at 160 °C for 10 min showed the highest TPC, TFC, and antioxidant activity of the Brassica vegetables, while sautéing showed the lowest. The steam treatments were preferred over the freeze-drying treatments. Within the vegetables tested, both kale and broccoli sprouts contained higher antioxidant properties in most of the employed processing treatments. The results also indicated that there is a strong correlation between the TPC, TFC, and antioxidant activity (p < 0.05). This study indicates that air-frying could be used as a sustainable thermal processing method for improving biomolecules in Brassica vegetables.

Keywords: Brassica vegetables; air-frying; antioxidants; broccoli sprouts; kale; thermal processing.

Figure 1

Summarized experimental approach for the processing methods. FRAP, ferric reducing antioxidant power; DPPH, 2,2-diphenyl-1-picrylhydrazyl; ABTS^•+, 2,2′-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid.

Figure 2

The effects of thermal processing techniques on the total phenolic content (TPC) of the selected Brassica vegetables. The bars represent means ± standard deviations (n = 3). The different letters for each vegetable indicate statistical differences based on a two-way analysis of variance. GAE, gallic acid equivalents; DW, dry weight; mg, milligram; g, gram; AF, air-fry; FD, freeze-dry.

Figure 3

The effects of thermal processing techniques on the total flavonoid content (TFC) of the selected Brassica vegetables. The bars represent means ± standard deviations (n = 3). The different letters for each vegetable indicate statistical differences based on a two-way analysis of variance. QE, quercetin equivalents; DW, dry weight; mg, milligram; g, gram; AF, air-fry; FD, freeze-dry.

Figure 4

The effects of thermal processing techniques on the antioxidant activity of the selected Brassica vegetables measured by DPPH radical scavenging activity. The bars represent means ± standard deviations (n = 3). The different letters for each vegetable indicate statistical differences based on a two-way analysis of variance. AF, air-fry; FD, freeze-dry; DPPH, 2,2-diphenyl-1-picrylhydrazyl.

Figure 5

The effects of thermal processing techniques on the antioxidant activity of the selected Brassica vegetables measured by the FRAP antioxidant assay. The bars represent means ± standard deviations (n = 3). The different letters for each vegetable indicate statistical differences based on a two-way analysis of variance. mM, millimoles; g, gram; TE, Trolox equivalent; AF, air-fry; FD, freeze-dry; FRAP, ferric reducing antioxidant power.

Figure 6

The effects of thermal processing techniques on the antioxidant activity of the selected Brassica vegetables measured by the chelating ability of the metals. The bars represent means ± standard deviations (n = 3). The different letters for each vegetable indicate statistical differences based on a two-way analysis of variance. mM, millimoles; g, gram; AF, air-fry; FD, freeze-dry; EDTA, ethylenediaminetetraacetic acid.

Figure 7

The effects of thermal processing techniques on the total antioxidant capacity of the selected Brassica vegetables measured by the ABTS radical scavenging assay. The bars represent means ± standard deviations (n = 3). The different letters for each vegetable indicate statistical differences based on a two-way analysis of variance. mM, millimoles; g, gram; TE, Trolox equivalent; AF, air-fry; FD, freeze-dry; ABTS, 2,2′-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid.