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Journal of Food Science and Technology logoLink to Journal of Food Science and Technology
. 2012 Sep 5;51(11):3362–3368. doi: 10.1007/s13197-012-0827-4

Effect of different cooking methods on total phenolic contents and antioxidant activities of four Boletus mushrooms

Liping Sun 1,✉, Xue Bai 1, Yongliang Zhuang 1
PMCID: PMC4571224  PMID: 26396332

Abstract

The influences of cooking methods (steaming, pressure-cooking, microwaving, frying and boiling) on total phenolic contents and antioxidant activities of fruit body of Boletus mushrooms (B. aereus, B. badius, B. pinophilus and B. edulis) have been evaluated. The results showed that microwaving was better in retention of total phenolics than other cooking methods, while boiling significantly decreased the contents of total phenolics in samples under study. Effects of different cooking methods on phenolic acids profiles of Boletus mushrooms showed varieties with both the species of mushroom and the cooking method. Effects of cooking treatments on antioxidant activities of Boletus mushrooms were evaluated by in vitro assays of hydroxyl radical (OH·) -scavenging activity, reducing power and 1, 1-diphenyl-2-picrylhydrazyl radicals (DPPH·) -scavenging activity. Results indicated the changes of antioxidant activities of four Boletus mushrooms were different in five cooking methods. This study could provide some information to encourage food industry to recommend particular cooking methods.

Keywords: Boletus mushrooms, Cooking methods, Phenolics, Antioxidant activity

Introduction

Many species of mushrooms are traditionally used as food and medicine (Mdachi et al. 2004). They are appreciated not only for their texture and high content of flavor components but also because they are rich in proteins and amino acids and poor in calories. Some mushrooms have been reported as therapeutic foods, useful in preventing diseases such as hypertension, hypercholesterolemia, atherosclerosis and/or cancer (Wasser and Weis 1999; Sun et al. 2011).

The genus Boletus mushrooms has a worldwide distribution comprising of 24 species (Hall et al. 1998). Heleno et al. (2011) studied the contents of proteins, carbohydrates, fatty acids,sugars, vitamins and phenolic acids of six wild Boletus species and determined their antioxidant properties. The extract of Boletus scaber demonstrated antiulcer and antitumor activities and low acute toxicity (Ryzhova et al. 1997). The methanol extracts from the fruiting body of the mushroom Boletus calopus showed free radical-scavenging activity (Kim et al. 2006a). Boletus aereus, Boletus badius, Boletus pinophilus and Boletus edulis are dominant wild mushrooms belonging in Boletus species, in China. Although the edible wild mushrooms command higher prices than cultivated mushrooms, people prefer to consume them due to their flavor and texture (Elmastas et al. 2007). Nevertheless, Boletus mushrooms are becoming more and more important in our diet for their nutritional and pharmacological characteristics.

Cooking, such as boiling, microwaving, pressure-cooking, griddling, baking, steaming and frying, induces significant changes in the texture and chemical composition (Osman et al. 2010; Wolosiak et al. 2010; Mandge et al. 2011; Medoua and Oldewage-Theron 2011). The most mushrooms are commonly cooked before being consumed. Therefore, antioxidant activities of mushrooms are closely linked not only to species but also to cooking methods. Lower antioxidant activities were reported in cooked mushrooms (Manzi et al. 2004). While, the polyphenol concentration and antioxidant properties were increased by cooking treatments in L. edodes (Choi et al. 2006), However, the effects were reported to depend on the duration of cooking (Soler-Rivas et al. 2009). This supported the need to thoroughly evaluate the effects of cooking on antioxidant activities of edible mushrooms in view to develop the best cooking methods possible.

Because modern day consumers seek to avoid aggressive cooking methods which may affect the functioning of the food, there is growing interest in the phytochemical profiles and antioxidant activities of cooked mushroom. However, there are few investigations on the effects of cooking treatments on Boletus mushrooms. Therefore, the present work aimed at evaluating the effects of cooking on the total phenolic contents, phenolic acids profiles and antioxidant properties.

Materials and methods

Sampling

Four fresh mushrooms fruit body (B. aereus, B. badius, B. pinophilus and B. edulis) were provided from the MuShuiHua fresh market in Kunming, in China, and identified by Yunnan Academy of Agriculture Sciences. They were cleaned from soil and substrate with a stainless knife. Three experiments were carried out, using 180 g of mushroom in each. The amount was divided into six variants (30 g each), raw and prepared for cooking in five methods. Mushrooms were then cut along their vertical axes into six pieces and the cuts were divided into six groups of 30 g within each variant, cut to slices 1–2 mm thick.

Cooking treatment

Steaming: Mushroom pieces (30 g) were placed in a pirex cylindrical bowl without additional water, and the bowl was placed in a domestic steam cooker. Then the samples were cooked for 8 min under atmospheric pressure.

Pressure-cooking: Mushroom pieces (30 g) were placed in a pirex cylindrical bowl without additional water, and the bowl was placed in a domestic high pressure cooker. Then the samples were cooked at high pressure in for 5 min until tender.

Microwaving: Mushroom pieces (30 g) were placed in a glass dish without additional water, and cooked in a domestic microwave oven at 900 watts for 1.5 min until tender.

Frying: Mushroom pieces (30 g) were placed in a frying pan with 100 mL hot peanut oil (160°C), and stirred for 3 min until the sample became crisp-tender.

Boiling: Mushroom pieces (30 g) were poured on a pot containing 300 mL boiling water. Then, they were cooked for 10 min and placed on a filter paper to drain water excess.

All samples, including raw and thermal processed, were freeze-dried to use in subsequent experiments.

Methanolic extracts

1 g freeze-dried sample was extracted by stirring with 10 mL 50 % methanol at room temperature for 24 h, and filtered through Whatman no. 1 paper (Savioe et al. 2008). The filtrates were concentrated with a rotary vacuum evaporator at 40 °C. The resultant extracts were stored at −20 °C until were used.

Total phenolic content determination

Total phenolics were determined according to the method of Mau et al. (2001) with some modifications. In brief, phenolic content was estimated by mixing 200 μL of deionized water, 50 μL of the diluted extracts, and 50 μL of Folin-Ciocalteau’a reagent. After 6 min, 500 μL of 7.5 % sodium carbonate solution were added to the mixture, which was adjusted to 1.3 mL with distilled water and allowed to stand at room temperature for 60 min. Then, the absorbance was read at 765 nm. Gallic acid (ranging from 0–100 μg/mL) was used to construct the calibration curve (R2 = 0.9982). The results were expressed as μg of gallic acid equivalents (GAE)/g fresh weight (FW).

Phenolic acids profiles determination

Samples preparation for analysis of phenolics followed Kim et al. (2006b). The 20 μL of extract was loaded on an Agilent 1100 HPLC system with a photodiode array detector equipped with an autoinjector (Agilent Technologies, Palo Alto, CA, USA). The separations were performed on a reversed-phase Zorbax SB-C18 HPLC column (250 mm × 4.6 mm i.d., 5 μm, Agilent Technologies) with a gradient elution consisting of methanol (eluent A) and aqueous 0.3 % phosphoric acid (eluent B) at a flow rate of 1 mL/min. The gradient pattern was a linear gradient increasing from 0 % to 100 % A in 60 min and keeping 100 % A for 10 min.

Eleven standard phenolics, p-coumaric acids, catechin, gallic acid, p-hydroxybenzioc acid, vanillin, luteolin, chlorogenic acid, protocatechuic, cinnamic acid, quercetin and benzoic acid, were used for calibration. Standard stock solutions (50, 100, 250, and 500 μg/g) were made, and all standard calibration curves showed high degrees of linearity (R2 > 0.99). Identification of the phenolic compounds in samples was done by comparison of the retention times and UV spectra with those of standard materials, and the quantification were done by comparing their peak areas with those of standard curves.

Hydroxyl radical (OH·) -scavenging activity assay

OH·-scavenging activity was assessed using the previous method (Smirnoff and Cumbes 1989) with a slight modification. 1 mL of extracts, 0.3 mL of 8 mM freshly prepared ferrous sulfate solution, 0.25 mL of 20 mM hydrogen peroxide, and 1 mL of 3 mM salicylic acid were injected into the test tube and incubated for 30 min at 37 °C; 0.45 mL of distilled water was then added to the test tube. The mixture obtained was centrifuged for 10 min at 3,000 × g. Distilled water was used instead of extracts as a control. OH·-scavenging activity was calculated according to the Eq. (1) and IC50 was expressed as the concentration of 50 % of OH·-scavenging activity.

graphic file with name M1.gif 1

Reducing power assay

The reducing power of extracts was determined by the method of Benjakul et al. (2005). Different concentrations of extract in 3.5 mL of phosphate buffer (0.2 M, pH 6.6) were mixed with 2.5 mL of 1 % potassium ferricyanide in test tubes. The mixtures were incubated for 20 min at 50 °C. At the end of the incubation, 2.5 mL of 10 % trichloroacetic acid were added to the mixtures, followed by centrifugation for 10 min at 650 × g. 2.5 mL of supernatant fluid was mixed with 2.5 mL of distilled water and 0.5 mL of 0.1 % ferric chloride, and the absorbance of the mixture was measured at 700 nm. The increase in the absorbance of the reaction mixture indicated the reducing power of extracts. EC50 was defined as the concentration of 0.500 of absorbance.

1, 1-diphenyl-2-picrylhydrazyl radicals (DPPH·) -scavenging activity assay

DPPH·-scavenging activity of extracts was determined by a modified method of Jing and Kitts (2004). Reaction mixture containing 2 mL of 0.1 mM DPPH· methanol solution and 0.4 mL of extracts was kept at room temperature in dark for 30 min. The absorbance at 517 nm was recorded. The DPPH·-scavenging activity was calculated according to the Eq. (1), and IC50 was expressed as the concentration of 50 % of DPPH·-scavenging activity.

Statistical analysis

Determinations were performed in triplicate in two independent experimental assays. Experimental results were expressed as mean ± standard deviation. Data were analyzed by one-way analysis of variance (ANOVA) using SPSS (version 11.0, Chicago, U.S.A.). A p value of <0.05 was taken as the level of statistical significance.

Results and discussion

Effects of cooking on total phenolic contents

Phenolics are the major contributors to the total antioxidant capacity of fruits, vegetables, and mushrooms (Heo et al. 2007). Previous researches showed that the relations between total phenolic contents of plants and their antioxidant activities were linear (Isabelle et al. 2010; Annegowda et al. 2011). In this study, total phenolic contents of raw mushrooms of B. aereus, B. badius, B. edulis and B. pinophilus were 209.5, 150.5, 184.5 and 220.0 μg of GAE/g FW, respectively. The effects of different cooking methods on the total phenolic contents of samples were shown in Table 1. For B. aereus, all cooking methods significantly decreased the total phenolic contents, and the loss during the boiling treatment was significantly higher than that of other cooking methods (p < 0.05). Compared with raw mushrooms, microwaving and steaming treatments showed insignificant effects on total phenolic content of B. badius. For B. edulis, the effect of microwaving treatment on total phenolic content was significantly higher than that of other cooking methods (p < 0.05). All cooking methods could keep total phenolic contents of B. pinophilus, except boiling, which significantly decreased its content (p < 0.05). In short, four mushrooms showed the highest retention of phenolics during the microwaving treatment compared with the other cooking methods, while boiling significantly decreased the total phenolic contents. Several reports on the retention of total phenolics in cooked mushroom are available in recent years. Khalil and Mansour (1995) stated that cooking treatments significantly decreased the phenolic contents. Barros et al. (2007) indicated that the cooking procedure with heating could destroy the structures of phenolics and decrease their contents. Ju et al. (2010) showed that steaming with pressure could increase the amounts of soluble phenolic acids of the Chaga mushroom (Inonotus obliquus).

Table 1.

Effect of different cooking methods on total phenolic contentsof Boletus mushroom (μg of gallic acid equivalents/g FW)

Cooking methods Raw Steaming Pressure-cooking Microwaving Frying Boiling
B. aereus 209.5 ± 5.52a 126.5 ± 1.04b 122.5 ± 1.51b 149.0 ± 4.95b 123.0 ± 1.48b 70.5 ± 1.46c
B. badius 150.5 ± 11.04a 139.5 ± 14.53a 127.0 ± 6.99b 146.0 ± 8.02a 117.5 ± 6.00b 57.0 ± 2.03c
B. edulis 184.5 ± 8.02a 137.0 ± 2.53c 140.0 ± 1.45c 161.0 ± 3.54b 91.5 ± 1.35d 65.0 ± 1.54e
B. pinophilus 220.0 ± 6.04a 217.5 ± 9.46a 217.0 ± 5.45a 216.5 ± 6.00a 217.5 ± 6.06a 102.0 ± 1.45b
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Values not sharing a common letter are significantly different at p < 0.05

Effects of cooking on phenolic acids profiles

The phenolic acids profiles of raw and cooked mushrooms were presented in Table 2. For B. aereus, seven phenolics were identified. The content of gallic acid was the highest, showing 101.4 μg/g FW, and the content of p-coumaric acid was lowest. Microwaving treatment significantly increased the content of gallic acid (p < 0.05), while pressure, frying and boiling treatments significantly decreased its content (p < 0.05). All cooking methods significantly decreased the contents of chlorogenic acid and p-coumaric acid (p < 0.05). Microwaving treatment also increased the content of protocatechuic acid, depicting a 3-fold variation of the raw sample. In addition, p-hydroxybenzioc acid and p-coumaric acid were not detected in the fried and boiled samples. For B. badius, eight phenolics were identified. The content of gallic acid was the highest, showing 144.3 μg/g FW. The frying treatment could keep the content of gallic acid, and other cooking methods significantly decreased its content (p < 0.05). The content of catechin in raw sample was 54.9 μg/g FW, however, cooking treatment significantly decreased its content (p < 0.05). Frying treatment could significantly increase the contents of p-hydroxybenzioc acid, vanillin acid and protocatechuic acid (p < 0.05). p-hydroxybenzioc acid was not detected in the boiled sample. For B. edulis, only five phenolics were identified, including quercetin, chlorogenic acid, gallic acid, vanillin acid and protocatechuic acid. The content of protocatechuic acid was highest, showing 74.4 μg/g FW, followed by gallic acid, being 42.1 μg/g FW. All cooking treatments could significantly decrease the content of protocatechuic acid and gallic acid in B. edulis (p < 0.05). The frying treatment could significantly increase the content of chlorogenic acid and vanillic acid (p < 0.05). In addition, quercetin was not detected in the fried and boiled samples. For B. pinophilus, seven phenolics were identified. The contents of p-hydroxybenzioc acid and gallic acid were 56.0 and 58.4 μg/g FW, respectively. All cooking methods decreased the content of p-hydroxybenzioc acid. Microwaving treatment could increase the content of gallic acid, which was similar to the B. aereus, but different from B. badius and B. edulis. This might due to their different gene species and structures. Steaming and microwaving treatments could increase the content of p-coumaric acid and keep the content of catechin. However, catechin and chlorogenic acid were not detected in pressur-cooked and boiled samples.

Table 2.

Effect of different cooking methods on phenolic acids profiles of Boletus mushroom. (μg/g FW)

Cooking methods Raw Steaming Pressure-cooking Microwaving Frying Boiling
B. aereus
 p-hydroxybenzioc acid 6.9 ± 0.03c 9.0 ± 0.07b 12.8 ± 0.92a 4.1 ± 0.01d nd nd
 p-coumaric acid 2.7 ± 0.52a 1.5 ± 0.31b 0.1 ± 0.00c 1.1 ± 0.26b nd nd
chlorogenic acid 54.3 ± 1.61a 25.6 ± 1.94c 22.6 ± 1.54c 44.3 ± 2.94b 26.2 ± 2.87c 39.2 ± 4.26b
 gallic acid 101.4 ± 7.63b 97.8 ± 3.64b 52.8 ± 2.13d 120.5 ± 7.42a 64.6 ± 1.69c 55.1 ± 2.02d
 vanillin acid 21.1 ± 2.89a 16.6 ± 1.04b 16.6 ± 0.69b 21.8 ± 2.27a 25.6 ± 2.43a 13.7 ± 0.92c
 protocatechuic acid 20.9 ± 1.23c 38.8 ± 5.06b 22.6 ± 1.65c 68.6 ± 3.65a 32.1 ± 3.49b 21.4 ± 1.98c
B. badius
 p-hydroxybenzioc 10.1 ± 1.81b 12.5 ± 1.03b 13.7 ± 1.47b 4.8 ± 0.01c 19.3 ± 0.45a nd
 Cinnamic acid 0.3 ± 0.01c 0.3 ± 0.02b 0.3 ± 0.01c 0.2 ± 0.01c 0.4 ± 0.01a 0.3 ± 0.04c
 p-coumaric acid 0.1 ± 0.01d 3.9 ± 0.03c 5.3 ± 0.82b 3.9 ± 0.01c 10.8 ± 0.54a 6.3 ± 0.75b
 catechin 54.9 ± 2.22a 7.5 ± 0.51c 7.9 ± 0.07c 8.4 ± 0.94bc 4.2 ± 0.06d 9.5 ± 2.03b
 chlorogenic acid 4.4 ± 0.63c 11.8 ± 1.85ab 8.7 ± 0.81b 5.5 ± 0.74c 14.9 ± 0.59a 13.3 ± 0.81a
 gallic acid 144.3 ± 7.56a 50.7 ± 4.94b 52.7 ± 1.42b 56.7 ± 2.15b 131.3 ± 8.29a 32.9 ± 0.96c
 vanillin acid 13.7 ± 2.11b 11.1 ± 0.59b 10.5 ± 1.91b 12.3 ± 0.86b 23.5 ± 1.62a 9.3 ± 2.08b
 protocatechuic acid 3.2 ± 0.61b 1.3 ± 0.01d 3.7 ± 0.47b 4.6 ± 0.51b 19.4 ± 0.68a 2.7 ± 0.13c
B. edulis
 quercetin 1.1 ± 0.21a 1.5 ± 0.11a 1.0 ± 0.01b 1.0 ± 0.01b nd nd
 chlorogenic acid 15.5 ± 0.49b 6.2 ± 0.55d 8.2 ± 0.18c 2.7 ± 0.19e 29.3 ± 1.41a 7.8 ± 0.77cd
 gallic acid 42.1 ± 1.28a 7.1 ± 0.06d 11.1 ± 0.83c 9.7 ± 0.37c 29.0 ± 1.08b 9.6 ± 0.48c
 Vanillin acid 14.9 ± 0.08b 13.7 ± 0.15b 13.8 ± 0.09b 13.5 ± 0.17b 26.4 ± 0.19a 11.6 ± 0.11c
 protocatechuic acid 74.4 ± 4.81a 50.0 ± 2.89b 41.1 ± 3.22bc 37.6 ± 3.01cd 44.3 ± 2.83b 32.2 ± 2.21d
B. pinophilus
 p-hydroxybenzioc 56.0 ± 2.02a 32.7 ± 1.49b 11.7 ± 0.91c 35.3 ± 1.72b 35.4 ± 1.46b 7.4 ± 0.11d
 p-coumaric acid 38.1 ± 2.11b 44.6 ± 3.21a 10.2 ± 0.92d 45.8 ± 3.42a 28.9 ± 0.96c 6.1 ± 0.03e
 catechin 5.0 ± 0.50a 5.3 ± 0.47a nd 4.0 ± 0.54a nd nd
 chlorogenic acid 10.9 ± 0.69a 3.1 ± 0.31c nd 3.4 ± 0.22c 6.6 ± 0.12b nd
 gallic acid 58.4 ± 3.85b 49.5 ± 2.53c 73.4 ± 3.21a 48.6 ± 0.96c 61.3 ± 1.93b nd
 vanillin acid 21.0 ± 1.03b 15.3 ± 0.67c 13.2 ± 1.26cd 18.6 ± 0.86b 31.7 ± 1.02a 11.1 ± 1.34d
 protocatechuic acid 21.0 ± 0.25a 15.0 ± 1.18b 6.5 ± 0.05c 16.5 ± 0.47b 14.4 ± 1.91b 3.0 ± 0.02d
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Values not sharing a common letter are significantly different at p < 0.05

nd not detected

Some literature reports indicated that increasing molecular weight of phenolic compounds lead to enhanced antioxidant activity of the tested compounds. These powerful antioxidant compounds may be explained by the fact that they possess a great number of hydroxyl groups. In addition, antioxidant activities of phenolic compounds also depend on their phenolic acids composition. Jacobo-Velzquez and Cisneros-Zevallos (2009) reported that the specific antioxidant activity of pure quercetin (Q) and chlorogenic acid (C) were 2,638 and 1,470 μg Trolox/mg phenolic, respectively, while mixtures of Q/C (75/25) and Q/C (25/75) resulted in specific antioxidant activity values of 2,172 and 1,907 μg Trolox/mg phenolic, respectively. However, there are great variations in the antioxidant activities of the different phenolic acids reported by different authors, due to the methods of analysis. Yokozawa et al. (1998) reported the IC50 values of gallic acid and ellagic acid were 8.14 and 4.60 μmol/L. Thitilertdecha et al. (2010) indicated that the IC50 values of gallic acid and ellagic acid of 2.49 and 1.64 μmol/L. Furthermore, some phenolic compounds were unstable and easily became non antioxidative under heating. Heat used in the cooking procedure could destroy the hydroxyl groups of phenolics (Barros et al. 2007) or transform phenolics with high antioxidant activity into different phenolics with low antioxidant activity (Jacobo-Velzquez and Cisneros-Zevallos 2009), and then cause decrease in their antioxidant activity. Therefore, cooking not only decreased the total phenolic contents, but also changed the type and relative amounts of phenolics (Sun et al. 2011).

Effects of cooking on antioxidant activities

OH·-scavenging activities

The OH·-scavenging activities of four mushrooms submitted to steaming, pressure-cooking, microwaving, frying and boiling with respect to raw samples were shown in Table 3. For B. aereus, all cooking methods significantly decresed the OH·-scavenging activity (p < 0.05), and the IC50 values of boiled and pressured samples reached 599.5 and 586.4 μg/mL, which were 2.99 and 2.92 folds of the raw samples, respectively. For B. badius, all cooking methods kept OH·-scavenging activities, except boiling, which significant decreased OH·-scavenging activities of B. badius (p < 0.05). For B. edulis, IC50 values of microwaved, steamed and pressured sapmles showed no significantly difference with that of raw mushroom, but boiling and frying treatments significantly decreased its OH·-scavenging activity (p < 0.05). For B. pinophilus, microwaving and steaming treatments kept OH·-scavenging activities. However, boiling significantly increased the IC50 value (p < 0.05), while pressure-cooking and frying increased its OH·-scavenging activity (p < 0.05).

Table 3.

Effect of different cooking methods on antioxidant activities of Boletus mushroom

Cooking methods Raw Steaming Pressure-cooking Microwaving Frying Boiling
IC50 of scavenging OH· activity (μg/mL)
 B. aereus 200.8 ± 6.53d 450.6 ± 4.76c 586.4 ± 5.69a 387.3 ± 5.22b 410.2 ± 4.63b 599.5 ± 7.31a
 B. badius 302.3 ± 5.75b 318.9 ± 5.11b 311.4 ± 6.18b 310.5 ± 5.25b 307.9 ± 6.33b 421.3 ± 5.81a
 B. edulis 102.3 ± 5.20c 105.8 ± 2.75c 102.0 ± 3.26c 98.6 ± 3.08c 410.3 ± 3.44a 305.9 ± 3.45b
 B. pinophilus 520.4 ± 5.56b 512.6 ± 6.02b 421.3 ± 4.60c 509.5 ± 5.68b 386.1 ± 5.01c 645.8 ± 5.84a
EC50 of reducing power (μg/mL)
 B. aereus 49.3 ± 1.38b 98.8 ± 2.76a 100. 8 ± 2.03a 89.7 ± 2.47a 99.7 ± 2.03a 122.3 ± 2.33a
 B. badius 114.8 ± 3.14c 123.2 ± 3.06b 129.7 ± 3.00b 98.4 ± 2.92d 128.4 ± 2.2c 187.1 ± 3.8a
 B. edulis 41.9 ± 1.43c 78.0 ± 2.46a 74.9 ± 3.07a 76.9 ± 2.21a 75.9 ± 2.43b 75.6 ± 2.11a
 B. pinophilus 66.5 ± 2.10a 58.7 ± 2.78b 49.7 ± 1.89c 63.4 ± 3.07ab 60.9 ± 1.47b 70.7 ± 2.27a
IC50 of scavenging DPPH· activity (μg/mL)
 B. aereus 135.4 ± 4.67b 139. 8 ± 3.02a 144.3 ± 4.00a 137.9 ± 5.17a 145.3 ± 4.13b 147.1 ± 5.22a
 B. badius 120.7 ± 5.01b 125.0 ± 5.02b 137.4 ± 3.38c 106.8 ± 4.11a 134.2 ± 3.02c 222.8 ± 5.84a
 B. edulis 91.0 ± 2.68b 97.4 ± 2.78b 106.2 ± 3.89a 97.3 ± 3.12b 101.7 ± 4.67a 93.2 ± 3.02b
 B. pinophilus 102.8 ± 3.08a 98.9 ± 4.14a 66.0 ± 2.11c 80.5 ± 3.02b 64.9 ± 2.32c 108.3 ± 2.37a
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Values not sharing a common letter are significantly different at p < 0.05

OH: Hydroxyl radical; DPPH·: 1, 1-diphenyl-2-picrylhydrazyl radicals

Reducing powers

Reducing powers of four samples submitted to steaming, pressure-cooking, microwaving, frying and boiling with respect to raw samples are shown in Table 3. For B. aereus, all cooking method showed the significant decreases in reducing power (p < 0.05). For B. badius, microwaving treatment significantly increased reducing power (p < 0.05), and boiling significantly decreased its activities (p < 0.05). For B. edulis, effects of cooking methods on the reducing power was similar to that of B. aereus and all cooking method significantly decreased the reducing power of B. edulis (p < 0.05). All cooking methods kept reducing powers of B. pinophilus, except pressure-cooking, which increased its activity with significant difference (p < 0.05).

DPPH·-scavenging activities

Table 3 showed the DPPH·-scavenging activities of mushrooms submission to different cooking methods. The IC50 values of DPPH·-scavenging activity of B. aereus cooked by five methods showed no significant difference. For B. badius, microwaving treatment significantly increased DPPH·-scavenging activities (p < 0.05), and boiling significantly decreased its activity (p < 0.05). It was similar to the effects of cooking treatments on its reducing power. For B. edulis, boiling, microwaving and steaming kept DPPH·-scavenging activities, however, pressure-cooking and frying decreased its activity with significant difference (p < 0.05). Microwaving, pressure-cooking and frying significantly increased DPPH·-scavenging activities of B. pinophilus (p < 0.05), and the IC50 values of pressured and fried samples were 66.0 and 64.9 μg/mL, which were lower than that of raw mushroom significantly (p < 0.05).

Some possibilities are suggested for the increase in antioxidant activity of some fruits, vegetables, and mushrooms after cooking. On the one hand, the thermal destruction of cell wall and sub cellular compartment could effectively cause liberation of high amounts of antioxidant components, such as microwaving treatment could accelerate release of phenolic compounds (Hayat et al. 2009). On the other hand, thermal chemical reaction could product stronger radical-scavenging antioxidants (Jimnez-Monreal et al. 2009). Ju et al. (2010) studied steaming of high pressure could increase the antioxidant activity of the Chaga mushroom. Third, new non-nutrition antioxidants or the formation of novel compounds were produced in the thermal process, such as Maillard reaction products with antioxidant activities (Morales and Babel 2002). It has been shown that the thermal processing of sweet corn, tomato and other vegetables increasing antioxidant activity, perhaps as a result of Maillard reaction production (Nindo et al. 2003). However, it is not clear to what extent each possible factor contributes to the increase antioxidant activities of fruits, vegetables, and mushrooms.

Conclusion

This study evaluated the effects of five cooking methods (steaming, pressure-cooking, microwaving, frying and boiling) on the total phenolic centents, phenolic acids profiles and antioxidant activities (OH·-scavenging activity, reducing power and DPPH·-scavenging activity) of four Boletus mushrooms (B. aereus, B. badius, B. pinophilus and B. edulis). Total phenolic contents of four mushrooms were all decreased during the five cooking treatments. However, microwaving treatment was better in retention of total phenolics than others. The effects of five treatments on the phenolic acids profiles showed varieties with both the species of mushroom and the cooking method. The changes of antioxidant activities during the five cooking treatments were also varieties with both the species of mushroom and the cooking method. The results of the study mainly provided some information on the effects of different cooking methods on the phenolics and antioxidant activities of Boletus mushrooms, which might encourage the food industry to recommended particular cooking methods. However, further study on effect of cooking on bioaccessibility in vivo should be carried out in the future.

Acknowledgments

This work was financially supported by Yunnan Natural Science Foundation (2010ZC032)

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