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The Effects of Different Drying Methods on Nutrients and Antioxidant Activities of Agaricus bisporus LI Weiqin, LI Qin, DUAN Jianglian, XU Jianguo* College of Food Science, Shanxi Normal University, Linfen 041004 E-mail: [email protected] Abstract: The effects of different drying methods including hot air drying, vacuum drying, microwave drying, and freeze drying on sensory quality, nutrients and antioxidant activities of Agaricus bisporus were investigated. The results showed that the sensory quality, nutrients, phenolics, and antioxidant activities of Agaricus bisporus were greatly affected by drying methods. Products from freeze drying possessed the best sensory quality, nutrients, and antioxidant activities, followed by vacuum drying and microwave drying, the lowest for hot air drying. By contrast, products from hot air drying showed the highest carbohydrates, followed by vacuum drying and microwave drying, the lowest for freeze drying. Keywords: Agaricus bisporus; drying; nutrients; antioxidant activity

1. Introduction Agaricus bisporus is more and more paid attention from consumers because of taste delicious, nutritious [1]. Besides, Agaricus bisporus contained abundant bioactive compounds such as terpenoids, steroids, polysaccharides, phenolic compounds and so on. These compounds showed various biological activities including antidiabetic effect, anti-inflammatory, and antitumor actions as well as antibacterial and antioxidant activities

[2-4]

. However, mushrooms are one of the most perishable food products and tend to lose quality

immediately after harvest. The shelf life is reduced due to post-harvest changes, namely browning, cap opening, stipe elongation, cap diameter increase, weight loss and texture damage, related to their high respiration rate and moisture, relatively high protein content, and lack of physical protection to avoid water loss or microbial attack

[5-7]

. Drying is the most common method for preserving mushrooms, but different

drying methods have different effects on the quality of the products because of differences in characteristics and mechanism of drying and materials [8-10]. The effects of different drying methods including hot air drying, vacuum drying, microwave drying, and freeze drying on sensory quality, nutrients and antioxidant activities of Agaricus bisporus were investigated in this paper, which would provide some foundational information for the developing and application of Agaricus bisporus.

2. Materials and methods 2.1. Plant materials and reagents The fresh Agaricus bisporus was purchased from the local market of Linfen and stored at 4 °C until analysis. Gallic

acid,

2,2’-azino-bis

(3-ethylbenothiazoline-6-

sulfonic

acid)

diammonium

2,2-Diphenyl-1-picrylhydrazyl (DPPH), and ferrozine were from Sigma (United States).

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salts

(ABTS),

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2.2. Drying methods and processes The fresh Agaricus bisporus were washed, and cut into pieces (7-10 mm side length and 5 mm thickness), then samples were divided into four batches containing three sets of 250 g each, and were dried by using one of the following methods: (a) drying in a hot-air oven at 60 °C; (b) drying in a vacuum drying oven at 60 °C; (c) drying in a microwave oven at 400 W; and (d) drying in an vacuum freezing dryer at 20-30 °C. The drying conditions employed in each of these methods were selected after conducting trials to achieve a percentage moisture content of about 6%. The moisture content of the dried samples was determined in triplicate using a laboratory oven at 105 °C. The times needed for reaching the final drying point in each of the assayed drying methods were as follows: 10 h for oven drying at 60 °C, 120 min for vacuum drying oven at 60 °C, 20 min for microwave drying at 400 W, 14 h for vacuum freezing drying at 20-30 °C.

2.3. Determination of the sensory quality and nutrition compositions According to GB/T 22699-2008 ‘Puffed Food’ standard (China), Assessment of the sensory quality is carried out from the color, appearance, structure, and flavor of product. Moisture, protein, fat, carbohydrates and ash were determined following the AOAC procedures. The crude protein content (N×4.38) of the samples was estimated by the macro-Kjeldahl method; the crude fat was determined by extracting a known weight of the sample with petroleum ether, using a Soxhlet apparatus; the ash content was determined by incineration at 600 ± 15°C; total carbohydrates were calculated by difference: total carbohydrates (g) =100- (g moisture +g protein +g fat +g ash). Total energetic value (100 g) was calculated according to the following equation: energetic value (kcal) = 4× (g protein+ g carbohydrate) +9× (g fat).

2.4. Extraction and determination of phenolic compounds Initially, milled sample (5.0 g dry weight (DW)) was defatted two times with 50 mL of hexane at 30 °C. The defatted samples were blended with 25 mL of methanol for 60 min, and then the homogenates were centrifuged for 15 min at 4 °C and 4 000 g in a centrifuge. After centrifugation, the methanol supernatant was separated from the residue. The extraction was carried out at room temperature and in absence of light. Then supernatants were pooled, and vacuum-evaporated to dryness at 40 °C. The phenolic extracts were frozen at -4 °C until used. Total phenolic content was determined using the Folin-Ciocalteu colorimetric method. An aliquot (0.1 mL) of diluted extracts, 2.8 mL of deionized water and 0.1 mL of 1.0M Folin-Ciocalteu reagent were mixed and stirred. After 8 min, 2 mL of 7.5% sodium carbonate solution was added and mixed thoroughly. The absorbance of the reaction mixtures was measured using a spectrophotometer at 765 nm wavelength after incubation for 2 h at room temperature. Gallic acid was used for calibration of the standard curve.

2.5. Antioxidant Activities

DPPH Assay. The DPPH radical scavenging activity was determined according to the method of Xu et al.

[11]

. Briefly, each of sample solutions was serially diluted to various concentrations in methanol

respectively, and then a 0.5 mL of samples was mixed with 2.5 mL of 60 μM DPPH dissolved in methanol. The mixture was shaken and left to stand for 30 min in the dark. The scavenging rate was calculated according to the formula, scavenging rate (%) = [(Ao-A1)/Ao]×100, where Ao is the absorbance of the control solution, A1 is the absorbance in the presence of samples in DPPH solution. The scavenging activity of the sample on DPPH radicals was expressed by IC50 value.

ABTS Assay. The ABTS cation radical scavenging activity was determined according to the method described by Xu et al. [11] Briefly, ABTS cation radicals were generated by a reaction of 7 mmol/L ABTS and

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2.45 mmol/L potassium persulfate. The reaction mixture was allowed to stand in the dark at room temperature for 16-24 h before use and used within 2 days. The ABTS+ solution was diluted with methanol to an absorbance of 0.700 ± 0.050 at 734 nm. One hundred microliters of the diluted samples was mixed with 2.0 mL of diluted ABTS+ solution. The mixture was allowed to stand for 6 min at room temperature and the absorbance was immediately recorded at 734 nm. The scavenging rate and IC50 value were calculated using the equation described for DPPH assay.

Metal ion chelating assay. The ability of the sample to chelate iron (II) was estimated according to the method described by Xu et al. [11] with minor modifications. An aliquot of each sample (100 µL) was mixed with 500 µL of FeCl2 (0.5 mmol/L). After 5 min incubation, the reaction was initiated by the addition of 200 µL of ferrozine (5.0 mmol/L) and the mixture was adjusted to a total volume of 3 mL with methanol. After 10 min incubation at room temperature, the absorbance at 562 nm was recorded against a blank. The chelating ability and IC50 value of the sample were calculated using the equation described for DPPH assay.

3. Results and discussion 3.1. The effects of drying methods on sensory quality The effects of different drying methods on sensory quality are shown in Table 1. In appearance, surface of dried products were uniform color, however, the freeze drying products were white, which keep the original color; the vacuum drying and microwave drying products were light yellow, while hot air drying products were yellow tan. Surface of products from hot air drying and vacuum drying were severe shrinkage and integrity, but bubbles were found on the surface of vacuum drying products. The microwave drying products had shrunk slightly, while freeze drying had no obvious change. In the respect of organization structure, vacuum drying, microwave drying and freeze drying products were loose and crispy. In flavor, vacuum drying samples possessed rich mushroom flavor, followed by hot air drying, the lowest for freeze drying, which may be because low temperature was unfavorable for the formation of the volatile compounds [12]. Table 1 The effects of different drying methods on sensory quality of Agaricus bisporus appearance

color

structure

flavor

hot air drying

severe shrinkage, shape integrity

uniform, yellow tan

densification

good

vacuum drying

severe shrinkage, air bubbles

uniform, light yellow

loose

good

microwave drying

slight shrinkage, shape integrity

uniform, light yellow

loose

weaken

freeze drying

no shrinkage, shape integrity

uniform, white

loose

weaken

3.2. The effects of drying methods on nutrients The effects of drying methods on nutrients of Agaricus bisporus are shown in Table 2. The results showed that four drying methods had no effects on the content of ash and energy value of Agaricus bisporus. The content of crude protein and fat was the highest in freeze drying product, followed by microwave drying and vacuum drying, the lowest for hot air drying, which might be related to the heating temperature and time. Conversely, the carbohydrates content was the highest in hot air drying product, the lowest for freeze drying. Table 2 The nutrients (g/100g) and energy value (kcal/100g) of Agaricus bisporus after drying moisture

protein

fat

carbohydrates

ash

energy value

hot air drying

5.4±0.1a

3.5±0.1 b

27.2±0.8 c

56.1±0.8a

7.8±0.2 a

365.6±2.5a

vacuum drying

5.3±0.1 a

3.6±0.2 b

29.5±0.5 b

53.8±0.5b

7.8±0.1 a

364.7±1.8a

microwave drying

5.3±0.2 a

3.7±0.1 b

29.2±0.8 b

53.9±0.8b

7.9±0.1 a

365.7±3.2a

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freeze drying

5.2±0.1 a

4.2±0.2 a

32.4±0.5 a

50.1±0.4c

8.1±0.1 a

367.8±2.0a

3.3. The effects of drying methods on phenolics The effects of drying methods on phenolic content of Agaricus bisporus are shown in Figure 1. The results showed that the drying methods had a satisfactory effect on the phenolics of Agaricus bisporus. During freeze drying, the loss of phenolics was the lowest, followed by vacuum drying, microwave drying, and hot air drying. Investigates its reason, the loss may be come from enzymatic and non enzymatic reaction of phenolic compounds during the drying process [6,13]. The internal temperature of materials was very high although the heating time was short during microwave drying, while the heating time was longer during hot air drying.

phenolic content (mg GAE/g extracts)

Therefore, the higher drying temperature and the longer drying time lead to oxidation of phenolic compounds. 30 a

25

b

20 15

c d

10 5 hot air

vacuum

microwave

freeze

drying methods

Figure 1 The effects of different drying methods on phenolic content

3.4. The effects of drying methods on antioxidant activities The effects of drying methods on the antioxidant activities of Agaricus bisporus are shown in Table 3. The results showed that the drying methods had obvious effects on the antioxidant activities. The scavenging activity of freeze drying products was the highest with the IC50 value of 3.5 mg/mL, followed by vacuum drying, microwave drying, and the lowest for hot air drying with the IC50 value of 7.6 mg/mL. The scavenging activity of freeze drying and vacuum drying products on ABTS radicals was the highest, followed by microwave drying, and the lowest for hot air drying. In metal ion chelating assay, as observed in the DPPH radical-scavenging activity measurements, the chelating ability of freeze drying products was the highest with the IC50 value of 9.4 mg/mL, followed by microwave drying, vacuum drying, and the lowest for hot air drying with the IC50 value of 17.8 mg/mL. In addition, we found that the content of phenolics was highly associated with antioxidant activity, indicating that the phenolic compounds contributed significantly to the antioxidant activity of Agaricus bisporus. which was in agreement with the previous studies [13,14]. Table 3 The effects of different drying methods on antioxidant activities of Agaricus bisporus IC50 value/mg·mL-1 DPPH

ABTS

chelating ability

hot air drying

7.6±0.2 a

4.5±0.2 a

17.8±1.5 a

vacuum drying

4.7±0.1 c

1.8±0.1 c

13.2±1.2 b

microwave drying

5.2±0.1 b

2.3±0.2 b

14.5±1.1 b

freeze drying

3.5±0.1 d

1.6±0.1 c

9.4±0.8 c

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4. Conclusion The different drying methods significantly affected the sensory quality, nutrients and phenolics as well as antioxidant activities of Agaricus bisporus. Products from freeze drying possessed the best sensory quality, nutrients, and antioxidant activities, followed by vacuum drying and microwave drying, the lowest for hot air drying. By contrast, Products from hot air drying showed the highest carbohydrates, followed by vacuum drying and microwave drying, the lowest for freeze drying. According to advantages and disadvantages of different drying methods, these results will provide certain theoretical and practical basis for the development of products from Agaricus bisporus.

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