Influence of different cooking methods on color, bioactive compounds, and antioxidant activity of kale

Figures & data

Figure 1. A typical HPLC chromatogram of ascorbic acid compound of cooked kale sample.

Table 1. Some chemical properties of kale leaves.

	Kale
Dry matter (%)	13.9 ± 1.2
°Brix	9.4 ± 0.9
pH	6.0 ± 0.2
Titratable acidity (% citric acid)	0.3 ± 0.0
Ash (%)	1.7 ± 0.1
Protein (%; n = 6.25)	3.2 ± 0.6

Data are expressed on a fresh weight basis.

Table 2. Changes in the surface color of kale cooked by different methods.

	L	a	b
Fresh	32.0 ± 1.1^a	–6.0 ± 0.1^a	8.1 ± 1.7^a
Steaming	27.5 ± 1.2^bc	–6.7 ± 0.8^a	9.1 ± 1.8^a
Boiling	25.2 ± 2.2^c	–6.0 ± 0.5^a	7.9 ± 1.9^a
Microwaving	30.9 ± 3.4^ab	–8.5 ± 1.5^b	11.2 ± 1.9^a
Stir-frying	27.2 ± 0.8^bc	–7.3 ± 0.8^ab	8.2 ± 1.6^a

Data are expressed as mean ± standard deviation of three replicates.

Numbers followed by different superscript lowercase letters within the same column are significantly different (p < 0.05) according to Duncan’s multiple range test.

Table 3. Effects of cooking methods on chlorophyll, carotenoid, and vitamin-C content of kale.

	Total chlorophylls (mg/kg dw)	Total carotenoids (mg β-car eq/kg dw)	Ascorbic acid (mg/kg dw)
Fresh	8890.1 ± 534.8^a	560.1 ± 24.9^a	4917.8 ± 559.7^a
Steaming	7227.6 ± 760.7^a	555.6 ± 12.1^a	4777.8 ± 0.3^a
Boiling	6977.3 ± 242.5^a	567.6 ± 22.2^a	2306.2 ± 71.3^b
Microwaving	3975.9 ± 137.4^b	559.0 ± 42.0^a	4395.5 ± 6.6^a
Stir-frying	2580.2 ± 277.6^b	402.0 ± 45.5^b	2218.2 ± 443.5^b

Data are expressed as mean ± standard deviation of three replicates.

Numbers followed by different superscript lowercase letters within the same column are significantly different (p < 0.05) according to Duncan’s multiple range test.

Figure 2. The amount of flavonols in fresh and cooked kale samples.

Table 4. Effects of cooking methods on phenolics and antioxidant activity of kale.

	Fresh	Steaming	Boiling	Microwaving	Stir-frying
Sinapic acid (mg/kg dw)
Free	2.8 ± 0.7^a	2.2 ± 0.0^a	n.d.	2.8 ± 1.9^a	n.d.
Bound	47.6 ± 18.9^a	38.0 ± 3.1^ab	38.9 ± 1.7^ab	22.0 ± 0.5^b	19.3 ± 4.1^b
Total	50.5 ± 18.3^a	40.1 ± 3.1^ab	38.9 ± 1.7^ab	24.9 ± 1.4^b	19.3 ± 4.1^b
Phenolic content (mg ga eq/kg dw)
Free	12922.1 ± 1878.6^a	11871.0 ± 2163.4^ab	7541.39 ± 7.7^c	12008.7 ± 138.1^ab	8153.7 ± 2464.9^c
Bound	7946.1 ± 1439.3^a	7286.7 ± 392.6^ab	7237.5 ± 977.6^ab	5635.8 ± 305.6^bc	4523.4 ± 338.3^c
Total	20868.2 ± 3139.5^a	19157.7 ± 1770.8^ab	14778.9 ± 985.3^bc	17644.5 ± 443.7^abc	12677.2 ± 2803.2^c
DPPH (mmol Trolox eq/g dw)
Free	18.6 ± 1.7^a	17.6 ± 2.1^a	12.6 ± 0.2^b	18.6 ± 1.6^a	13.8 ± 0.6^b
Bound	11.8 ± 3.5^a	8.8 ± 1.4^ab	8.8 ± 1.2^ab	6.1 ± 1.5^b	4.8 ± 0.2^b
Total	30.4 ± 4.6^a	26.4 ± 3.5^ab	21.4 ± 1.0^bc	24.6 ± 2.2^abc	18.6 ± 0.3^c
ABTS (mmol Trolox eq/g dw).
Free	39.1 ± 1.7^a	36.8 ± 5.6^a	29.2 ± 8.2^a	37.7 ± 2.3^a	33.4 ± 0.8^a
Bound	52.8 ± 9.3^a	41.6 ± 13.4^ab	41.4 ± 2.9^ab	35.3 ± 0.9^ab	27.7 ± 1.4^b
Total	91.9 ± 7.3^a	78.4 ± 16.1^ab	70.6 ± 8.6^b	73.1 ± 2.1^b	61.1 ± 1.3^b

Data are expressed as mean ± standard deviation of three replicates.

Numbers followed by different superscript lowercase letters within the same row are significantly different (p < 0.05) according to Duncan’s multiple range test.