Food Chemistry 201 (2016) 80–86

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Effect of cooking methods on nutritional quality and volatile compounds of Chinese chestnut (Castanea mollissima Blume) Qian Li a, Xianhe Shi a, Qiaojiao Zhao a, Yahui Cui a, Jie Ouyang a,⇑, Fang Xu b,⇑ a Department of Food Science and Engineering, College of Biological Sciences and Technology, Beijing Key Laboratory of Forest Food Processing and Safety, Beijing Forestry University, Beijing 100083, China b Analytical and Testing Center, Beijing Forestry University, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 4 October 2015 Received in revised form 17 December 2015 Accepted 18 January 2016 Available online 18 January 2016 Chemical compounds studied in this article: Sucrose (PubChem CID: 5988) Fructose (PubChem CID: 5984) Glucose (PubChem CID: 5793) L-Aspartic acid (PubChem CID: 5960) L-Glutamic acid (PubChem CID: 611) L-Arginine (PubChem CID: 6322) Oxalic acid (PubChem CID: 971) Malic acid (PubChem CID: 525) Ascorbic acid (PubChem CID: 54670067) Citric acid (PubChem CID: 311) Fumaric acid (PubChem CID: 444972)

a b s t r a c t This study aimed to evaluate the effects of different cooking methods on the content of important nutrients and volatiles in the fruit of Chinese chestnut. The nutritional compounds, including starch, watersoluble protein, free amino acids, reducing sugar, sucrose, organic acids and total flavonoids, of boiled, roasted and fried chestnuts were significantly (P < 0.05) lower than those of fresh chestnuts after cooking, while the amylose, fat, crude protein and total polyphenol content varied slightly (P > 0.05). L-Aspartic acid, L-glutamic acid and L-arginine were found to be the main reduced free amino acids in cooked chestnuts. The main aromatic compositions in fresh chestnuts were aldehydes and esters, while ketones, furfural and furan were formed in cooked chestnuts due to the Maillard reaction and degradation of saccharides, amino acids and lipids. Principle component analysis demonstrated that roasting and frying had a similar effect on the nutritional composition of chestnuts, which differed from that of the boiling process. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Chinese chestnut Cooking method Nutritional quality Volatile compound

1. Introduction Chestnuts belong to the family Fagaceae and are mainly distributed in Eastern and Southwestern Asia, Southern Europe and North America. Among the 12 world chestnut species, the annual fruit production of the Chinese chestnut (Castanea mollissima Blume) is 925,000 t, compared with 108,000 t for the European chestnut (C. sativa Miller) and 55,800 t for the North and South American chestnut (C. dentata Borkh) (De Vasconcelos, Bennett, Rosa, & Ferreira-Cardoso, 2010; Fernandes et al., 2011). The best development conditions are found at altitudes above 500 m and low winter temperatures, as in the Yan Mountain region of Northern China. Chestnuts are an important food resource with high nutritional value. Fresh Chinese chestnut fruits contain 52.0% ⇑ Corresponding authors. E-mail addresses: [email protected] (J. Ouyang), [email protected] (F. Xu). http://dx.doi.org/10.1016/j.foodchem.2016.01.068 0308-8146/Ó 2016 Elsevier Ltd. All rights reserved.

water, 42.2% carbohydrates, 4.2% proteins and 0.7% lipids (Yang, Pan, & Wang, 2009), while the starch, total sugar, crude protein and fat content in Spanish chestnuts are 42.2–59.8%, 9.5–22.2%, 4.8–6.9% and 1.7–4.0% d.m. (dry matter), respectively, varying by cultivar and region (Pereira-Lorenzo, Ramos-Cabrer, DíazHernández, Ciordia-Ara, & Ríos-Mesa, 2006). Chinese chestnut fruits can improve the function of kidneys according to the ancient encyclopedia of China Compendium of Materia Medica (Ben Cao Gang Mu) of the Ming Dynasty (A.D. 1590). From the various compositions and health studies, it is clear that chestnut fruits, and potentially other extracts from chestnut trees, have considerable potential as a functional food or as food ingredients (De Vasconcelos, Bennett, et al., 2010). Studies in the literature mainly focused on fresh materials (varieties) or assessed the impact of different heat treatments on European chestnut composition and antioxidant activity (Attanasio, Cinquanta, Albanese, & Di Matteo, 2004; Barreira,

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Pereira, Oliveira, & Ferreira, 2010; Morini & Maga, 1995a). The effects at the four sequential major stages of industrial processing (fresh, after storing for 3 months at ±0 °C and keeping the relative humidity at 90%, after industrial peeling by flame or fire at 800– 1000 °C, after freezing in a tunnel with a CO2 flow at
65 °C) on the nutritional content of six cultivars of C. sativa were thoroughly evaluated, including starch, fat, energy, fiber (De Vasconcelos, Bennett, Rosa, & Ferreira-Cardoso, 2009a), crude protein, free amino acids, phenolic phytochemicals (De Vasconcelos, Bennett, Rosa, & Ferreira-Cardoso, 2009b), minerals, free sugars, carotenoids and antioxidant vitamins (De Vasconcelos et al., 2010). The vitamin C content of fresh chestnuts varied from 400 to 693 mg/kg dry weight between the different European cultivars, and a significant decrease, 25–54% for the boiling process and 2–77% for the roasting process, was observed (Barros, Nunes, Gonçalves, Bennett, & Silva, 2011). The traditional methods for cooking Chinese chestnuts include frying them unshelled with sugar (tang chao li zi) or shelled and cooking them with other food materials. Currently, a type of industrially processed and packaged chestnut kernel has become popular in East Asia and is generally cooked via the boiling method. The kernel of a fresh chestnut has a weak smell of fruit, which becomes a strong flavor during cooking after thermal processing. A total of 30–33 components, including hydrocarbons, alcohols, aldehydes, ketones, furans, pyranone and acids, were firstly identified from the flavor extract of boiled and roasted Chinese chestnuts, respectively (Morini & Maga, 1995b). Monoterpenes and derivatives of butane, pentane, hexane and heptane were identified as important aroma impact compounds from roasted Italian chestnuts (Krist, Unterweger, Bandion, & Buchbauer, 2004). The purpose of this study is to analyze the nutritional and aromatic components in fresh and boiled, roasted and fried chestnuts to investigate the effects of different cooking methods on the contents of proximate composition and other chemical constituents. It is important to assess these changes so that the industrialized processing of chestnut fruits can be optimized for nutritional and aromatic qualities.

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nut powder was sealed in a polyethylene bag and stored at 2 °C in the dark before analysis. Water content was measured by oven drying at 105 °C until constant weight was achieved (AOAC method 925.40) (AOAC, 2000). Therefore, the calculation of the content of the chemical composition of chestnuts was based on d.m. 2.3. Analysis of total starch and amylose Chestnut starch was isolated by using the alkaline method as previously reported (Correia & Beirão-da-Costa, 2012). A chestnut sample of 500 g was immersed in 1000 mL of NaOH solution (0.2%, w/v) for 2 h. Then, the mixture was homogenized and filtered through a 75 lm stainless sieve to remove large particles. The filtrate was centrifuged at 4000g for 5 min; then, the mucilaginous layer was scraped away and the precipitate was washed with water three times. The extracted starches were dried for two days at 40 °C in a ventilated drying oven. Total starch content was determined according to the AOAC method 996.11 (AOAC, 1997). Amylose content was analyzed using a K-AMYL 07/11 Amylose/Amylopectin assay kit (Megazyme International Ireland, Ireland). Chestnut starch samples were completely dispersed by heating in dimethyl sulfoxide (DMSO) (Maršálková et al., 2010; https://secure.megazyme.com/Amylose-AmylopectinAssay-Kit). 2.4. Analysis of crude fat, crude protein and water-soluble protein The crude fat and nitrogen content was determined according to AOAC official methods 920.39 and 954.01 (AOAC, 1997). Crude protein content was calculated by multiplying the nitrogen content by 5.30 (FAO, 1986). Chestnut powder was extracted by using distilled water three times, and the watersoluble protein content was measured by using Coomassie Brilliant Blue (Bradford, 1976). Bovine serum albumin (BSA) was used as the standard. 2.5. Analysis of reducing sugar, sucrose, glucose and fructose

2. Materials and methods 2.1. Reagents The standards (amylose, sucrose, glucose, fructose, oxalic acid, malic acid, ascorbic acid, citric acid and fumaric acid) were purchased from Sigma–Aldrich (USA). Acetonitrile was HPLC grade (Fisher, USA). All other chemicals, unless otherwise noted, were of analytical grade and purchased from Sinopharm Chemical Reagent Beijing Co., Ltd of PR China. The water was obtained from a Milli-Q water purification system (Millipore, Belford, MA, USA). 2.2. Raw materials and sample preparation The Chinese chestnut cultivar ‘Zaofeng’ was purchased from a commercial market in Qianxi, Hebei province of North China. The fruits were harvested in September of 2013 and stored at 2 °C in the dark for one month before use. Chestnuts were divided into four groups: group 1 had no treatment, i.e., fresh chestnuts; group 2 was boiled in 100 °C water (chestnut: water = 1:2, w/v) for 20 min, i.e., boiled chestnuts; group 3 was roasted at 200 °C for 25 min in an electric oven (T3-L383b, Guangdong Midea Kitchen Appliance Manufacturing Co. Ltd., China), i.e., roasted chestnuts; group 4 was fried at 240 °C for 15 min in an electric pan (WK2102T, Guangdong Midea Kitchen Appliance Manufacturing Co. Ltd., China), i.e., fried chestnuts. Each group of 500 g was hulled, lyophilized, and ground using a 40-mesh, and the obtained chest-

The fresh and fried chestnuts, 10 g of each, were shelled and homogenized in 50 mL of Et-OH/H2O (80%, v/v). The slurry was shaken at 70 °C for 30 min and then centrifuged at 8000g for 15 min; Et-OH was removed from supernatant via vacuum evaporation at 60 °C. The concentrated extract was added to 0.5 mL of zinc acetate (1 M) and 0.5 mL of potassium ferrocyanide (0.25 M) and supplemented, using deionized water, to 5 mL. The mixture was kept at room temperature for 30 min and then centrifuged at 8000g for 15 min. The supernatant was filtered using 0.2 lm Millipore Express Membrane Filters (Millipore, Belford, MA, USA) and stored at 4 °C until analysis. The reducing sugar content was analyzed by using Fehling’s reagent titration method (Ayoola et al., 2008). The sucrose, glucose and fructose content in chestnuts was determined via HPLC (high performance liquid chromatography). HPLC was performed on a LUMTECH (Lumiere, German) system with a refractive index detector (50D) and a Waters WAT084038 NH2 column (4.6  250 mm, 5 lm). The analyses of sucrose, glucose and fructose were carried out separately. The mobile phase was acetonitrile–water (75:25, v/v), the detection wavelength was 285 nm, the injection volume was 20 lL and the flow rate was 1.0 mL/min. 2.6. Energetic value Energetic value was calculated as described by Fernandes et al. (2011):

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Energy ðkcal=100 gÞ ¼ ðg of protein þ g of starch þ g of sucrose þ g of glucose þ g of fructoseÞ  4

flow rate was 0.4 mL/min, the injection volume was 20 lL and the oven temperature was set at 57 °C.

þ ðg of lipidÞ  9 2.10. Analysis of volatile components 2.7. Analysis of total polyphenols and total flavonoids Three grams of chestnut powder was reflux-extracted with 75 mL of Et-OH/H2O (70%, v/v) at 70 °C for 30 min. The extract was filtered using 0.22 lm Millipore Express Membrane Filters and stored at 4 °C until analysis. For the determination of total polyphenols content, the reaction system included 1.0 mL of sample solution, 5.0 mL of Folin–Ciocalteu reagent and 4 mL of Na2CO3 (7.5%, w/v). The mixture was kept at room temperature for 60 min, and then the absorbance at 765 nm was measured (Ramanauskiene˙, Inke˙niene˙, Petrikaite˙, & Briedis, 2013). The concentration of polyphenols was calculated according to the standard curve, with gallic acid as the standard. The results were expressed as mg of gallic acid equivalent (GAE)/g d.m. To measure total flavonoid content, the reaction system consisted of 2 mL of sample and 0.5 mL of NaNO2 (5%, w/v). After reaction for 6 min, 0.5 mL of Al(NO3)3 (10%, w/v) was added and left to stand for 6 min; then, 4.0 mL of NaOH (4%, w/v) was added and 70% Et-OH/H2O (v/v) was supplemented to increase to 10 mL. The absorbance was measured at 510 nm after reaction for 15 min (Gao et al., 2011). The concentration of flavonoid was calculated according to the standard curve, with rutin as the standard. The results were expressed as mg of rutin equivalent/g d.m.

2.8. HPLC analysis of organic acids HPLC analysis was carried out according to a previous study (Ribeiro et al., 2007), with some modifications. 5 g of chestnut powder was extracted with 50 mL of Et-OH/H2O (80%, v/v) at room temperature for 20 min and then centrifuged at 8000g for 15 min. Et-OH was removed from the supernatant by flushing with nitrogen. The concentrated extract was added to 0.2 mL of phosphoric acid (1 M) and supplemented with deionized water to reach 10 mL. The solution was filtered using a 0.2 lm microfiltration membrane and stored at 4 °C until HPLC analysis. The determination of organic acids, including oxalic acid, malic acid, ascorbic acid, citric acid and fumaric acid, was performed on a Shimadzu LC-2010 with a C18 column (4.6 mm  250 mm, 5 lm; Shimadzu). The mobile phase was 0.01 M (NH4)2HPO4 (pH 2.7) and the detection wavelength was 210 nm. The injection volume was 20 lL and the flow rate was 1.0 mL/min.

The analysis was carried out according to the method described by Krist et al. (2004), with some modifications. The fresh, boiled, roasted and fried chestnuts were each hulled and cut into particles of 2 mm  2 mm  2 mm. The volatiles of the samples were analyzed by using a dynamic headspace sample (DHS) and automatic thermal desorption-gas chromatography–mass spectrometric (ATD-GC/MS) system. The adsorption tubes were pretreated in a desorption disposer (TP-2040, Beijing BeiFenTianPu Instrument Co. Ltd, China) to remove impurities by flushing with helium at 270 °C for 120 min. The chestnuts were put into a sampling bag (Reynolds, 406 mm  444 mm, Richmond, VA, USA); the air in the bag was pumped out for 40 min and then filtrated air was pumped into the bag. The sampling bag was then sealed and left alone for 90 min to accumulate volatiles. The volatiles were pumped out by using an Atmosphere Sampling Instrument (QC1S, Beijing Municipal Institute of Labor Protection, China) for 60 min and adsorbed in an adsorption tube. The filled-in adsorption tube was put in the ATD (Perkin Elmer Turbo Matrix 650, USA); GC–MS analyses were performed using a gas chromatograph and mass spectrometer (Perkin Elmer Clarus 600 Gas Chromatograph & 600 Mass Spectrometer, USA) equipped with a DB-5MS capillary column (30 m  0.25 mm, 0.25 lm film thickness). The first stage of desorption was carried out at 260 °C, with helium as the carrier gas and the flow rate being 1.5 mL/min. The desorbed volatiles were adsorbed in cold hydrazine (
25 °C), followed by the second stage of desorption. The cold hydrazine temperature increased from
25 °C to 300 °C at a rate of 40 °C/s. The re-desorbed volatiles flowed into GC–MS through a pipe (250 °C). Column temperature was initially kept at 40 °C for 2 min, then increased to 200 °C at a rate of 6 °C/min, held for 5 min and finally raised to 270 °C at 20 °C/min, holding for 5 min. Helium was used as the carrier gas at a constant flow of 1.5 mL/min. Mass spectra were recorded in EI mode, with a 29–500 amu scan range and 0.2 s scan time. Interface temperature was 250 °C; ion source temperature was 220 °C; ionization voltage was 70 eV. Statistical analyses of the data were conducted via a computer search using digital libraries of mass spectral data (NIST2008). Retention indices (RI) of constituents were determined using standard C8–C25 straight chain hydrocarbons (Shanghai Anpel Co. Ltd., China). The RI references were obtained from the NIST Chemistry WebBook http://webbook.nist.gov/chemistry/. 2.11. Statistical analysis

2.9. HPLC analysis of free amino acids (FAA) The fresh and cooked chestnuts, 10 g of each, were shelled and homogenized in 150 mL of Me-OH/H2O (80%, v/v). The slurry was centrifuged at 8000g for 15 min, and the sediment was homogenized in 150 mL of Me-OH/H2O (80%, v/v) for a second time and centrifuged at 8000g for 15 min again. The two supernatants were combined, vacuum concentrated and lyophilized. The obtained powder was dissolved in 5% (w/v) trichloroacetic acid to reach a constant volume of 25 mL, filtered using a 0.2 lm microfiltration membrane, and stored at 4 °C until HPLC analysis. The samples were hydrolyzed using 7.5 M HCl at 110 °C for 24 h. The concentrations of amino acids were analyzed via HPLC on a Hitachi Automatic Amino Acid Analyzer L-8900 (Hitachi, Japan) system with aUV–VIS spectrometer and a Hitachi #2622pH column (4.6  60 mm). The mobile phase was 0.075 M of sodium citrate buffer (pH 3.3) and the detection wavelength was 570 nm. The

All experiments were carried out in triplicate, and the results were expressed as mean values based on dry matter. For HPLC analyses of the organic acids and FAA, the extractions were performed in triplicate, and each sample was quantified in duplicate. Means were compared using Tukey’s honestly significant difference (HSD) multiple comparison test by SPSS17.0 software (IBM Corporation, Armonk, NY, USA). A principal component analysis (PCA) of the data was performed using UnscramblerÒ X10.2 (CAMO software, Oslo, Norway). 3. Results and discussion 3.1. Effect of cooking on the variation of proximate composition Results obtained for the proximate composition of fresh, boiled, roasted and fried chestnuts are shown in Table 1, which includes

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Q. Li et al. / Food Chemistry 201 (2016) 80–86 Table 1 Proximate composition of fresh and cooked chestnuts. Chestnuts

Total starch (%, d.m.)

Amylose in starch (%, d.m.)

Crude fat (%, d.m.)

Crude protein (%, d.m.)

Water-soluble protein (%, d.m.)

Reducing sugar (%, d.m.)

Sucrose (%, d.m.)

Glucose (%, d.m.)

Fructose (%, d.m.)

Energetic value (kcal/100 g)

Fresh Boiled Roasted Fried

71.08 ± 0.12a 57.69 ± 0.66c 61.25 ± 0.14b 62.91 ± 2.27b

19.87 ± 0.52a 18.88 ± 0.95a 21.21 ± 0.28a 19.65 ± 0.22a

2.27 ± 0.06a 2.27 ± 0.10a 1.96 ± 0.08b 1.42 ± 0.12c

8.27 ± 0.75a 8.40 ± 0.81a 8.49 ± 0.86a 8.44 ± 0.89a

2.65 ± 0.31a 0.89 ± 0.08b 0.74 ± 0.07c 0.77 ± 0.09c

2.06 ± 0.04a 1.15 ± 0.03b 0.67 ± 0.01d 0.80 ± 0.01c

9.85 ± 0.87a 5.90 ± 0.65c 9.21 ± 0.91b 9.26 ± 0.82b

0.22 ± 0.03a 0.19 ± 0.02b 0.23 ± 0.02a 0.18 ± 0.03a

0.18 ± 0.02a 0.11 ± 0.01c 0.17 ± 0.02ab 0.16 ± 0.02b

405 ± 43a 354 ± 41b 318 ± 28c 324 ± 39c

Values are expressed as means ± SD (n = 3). Different letters within one column represent significant difference at P < 0.05.

Table 2 Free amino acid content of raw, boiled, roasted and fried chestnuts. Free amino acid (mg/g DW)

Fresh chestnut

Boiled chestnut

Roasted chestnut

Fried chestnut

Asp Thr Ser Glu Pro Gly Ala Val Ile Leu Tyr Phe Lys His Arg Total

4.23 ± 0.32a 0.10 ± 0.02a 0.15 ± 0.02a 1.29 ± 0.11a 0.14 ± 0.02a 0.09 ± 0.01a 0.15 ± 0.02a 0.08 ± 0.01a 0.03 ± 0.01a 0.04 ± 0.01a ND 0.11 ± 0.02a 0.04 ± 0.01a 0.11 ± 0.01a 1.11 ± 0.09a 7.67 ± 0.67a

2.12 ± 0.17b 0.02 ± 0.00b 0.05 ± 0.01b 1.09 ± 0.09b 0.04 ± 0.01b 0.05 ± 0.01c 0.11 ± 0.02b 0.03 ± 0.01b 0.01 ± 0.00a 0.03 ± 0.00a ND 0.02 ± 0.00b 0.02 ± 0.00ab 0.03 ± 0.01b 0.11 ± 0.02b 3.73 ± 0.36b

1.82 ± 0.21c 0.03 ± 0.00b 0.05 ± 0.01b 0.97 ± 0.10c 0.05 ± 0.01b 0.05 ± 0.01c 0.12 ± 0.03b 0.03 ± 0.00b 0.02 ± 0.00a 0.03 ± 0.01a ND 0.02 ± 0.00b 0.02 ± 0.00ab 0.03 ± 0.00b 0.11 ± 0.02b 3.35 ± 0.40c

2.07 ± 0.20b 0.04 ± 0.01b 0.06 ± 0.01b 0.96 ± 0.08c 0.05 ± 0.01b 0.07 ± 0.01b 0.10 ± 0.02b 0.03 ± 0.01b 0.01 ± 0.00a 0.02 ± 0.00a ND 0.02 ± 0.00b 0.01 ± 0.00b 0.04 ± 0.01b 0.11 ± 0.01b 3.78 ± 0.37b

Values are expressed as means ± SD (n = 3). Different letters within one row represent significant difference at P < 0.05. ND: not detected.

total starch, amylose, crude fat, crude protein, water-soluble protein, reducing sugar, sucrose, glucose, fructose and calculated energetic value. The total starch content in fresh chestnuts was 71.08% d.m., higher than that (53.8%) of Chinese chestnuts (Liu, Wang, Chang, & Wang, 2014) and (48.74–53.89%) of Portuguese chestnuts (De Vasconcelos, Bennett, et al., 2010). After cooking, the total starch content significantly (P < 0.01) decreased in boiled (57.69%), roasted (61.25%) and fried (62.91%) chestnuts, which may be attributed to the starch degradation at high temperature (Bryce & Greenwood, 1963). Meanwhile, the starch content in boiled chestnuts was lower than that of other cooked chestnuts because part of the water-soluble starch was dissolved in water during boiling. The ratios of amylose/total starch in fresh, boiled, roasted and fried chestnuts were 19.87%, 18.88%, 20.21% and 19.65%, respectively, exhibiting no difference (P > 0.05). In previous studies, high temperature treatment had the effect of lowering starch content; the starch content decreased from 53.83% to 51.19% d.m. in chestnut fruits (C. sativa) after industrial peeling by flame or fire at 800–1000 °C (De Vasconcelos et al., 2009a ) and reduced from 58.3% to 56.2% and 53.9% d.m. after drying at 40 and 60 °C, respectively (Attanasio et al., 2004). Meanwhile, the ratio of amylose also increased from 32.9% to 43.3% and 57.8% after drying at 40 and 60 °C, respectively. The fresh chestnut granules appeared to be round or oval and were changed to being shapeless, with their surfaces being quite rough after drying (Attanasio et al., 2004). The crude fat content was 2.27% d.m. in fresh chestnuts, which was in accordance with the previously reported 2.1–2.4% (Liu et al., 2014) and 1.91–4.39% (De Vasconcelos, Bennett, et al., 2010). After cooking, the crude fat content became 2.27%, 1.96% and 1.42% in boiled, roasted and fried chestnuts, respectively. It seems that boiling had no effect on chestnut fat, but roasting and frying could lower crude fat content by decomposing fat at a high temperature

(Das, Babylatha, Pavithra, & Khatoon, 2013). Similar results were observed by Gonçalves et al. (2010), who found crude fat content in fresh and roasted chestnuts of 3.20% and 3.08% d.m., respectively. Künsch et al. (2001) also found that total fatty acids decreased after roasting in Switzerland native chestnuts (C. sativa Mill). Crude protein in fresh chestnuts was 8.27% d.m., which was similar to the 8.5% of Liu et al. (2014), and increased slightly after boiling (8.40%), roasting (8.49%) and frying (8.44%). The crude protein content increased from 48.9–49.1 to 50.2–53.9 mg/g d.m. after industrial peeling by fire (De Vasconcelos et al., 2009b ). Gonçalves et al. (2010) noted that the cooking processes significantly (P < 0.0001) affected the primary and secondary metabolites of chestnuts, with the protein content in roasted and boiled chestnuts being 67.1 and 62.8 mg/g d.m., respectively, varying from that of fresh chestnuts (65.1 mg/g). Water-soluble protein, mainly peptides and hydrophilic proteins, decreased significantly (P < 0.01) when subject to the Maillard reaction during cooking. It’s well known that the Maillard reaction is the main formation mechanism of aromatic components in thermal processed nuts; thus, the analysis of the variation of FAA in fresh and cooked chestnuts is very important. The content of fresh chestnuts with regard to 14 types of FAA was analyzed (Table 2), with a total amount of 7.67 mg/g d.m., which mainly included L-aspartic acid (4.23 mg/g), L-glutamic acid (1.29 mg/g) and L-arginine (1.11 mg/g). Similarly, the free amino acid profiles in Portugal chestnuts were dominated by L-aspartic acid (0.70–1.41% d.m.), followed by L-glutamic acid (0.61–1.03%), leucine (0.40–0.74%), L-alanine (0.45–0.74%) and L-arginine (0.22–1.16%) (Borges, Gonçalves, de Carvalho, Correia, & Silva, 2008). The total FAA decreased by 50.7% to 3.78 mg/g after frying, while in boiled and roasted chestnuts, it decreased by 51.4% and 56.3%, respectively. The sum of decreasing amounts of L-aspartic acid, L-glutamic acid and L-arginine accounted for

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Table 3 Total polyphenols, total flavonoid and organic acids of fresh and cooked chestnuts (mg/g d.m.). Chestnuts Fresh Boiled Roasted Fried

Total polyphenols a

2.24 ± 0.05 2.03 ± 0.02b 2.26 ± 0.01a 2.08 ± 0.05b

Total flavonoids a

2.62 ± 0.13 2.12 ± 0.09b 2.25 ± 0.11a 2.13 ± 0.08b

Malic acid

Citric acid a

1.2593 ± 0.1024 1.1355 ± 0.0988a 0.7979 ± 0.0803b 0.7979 ± 0.0752b

Oxalic acid a

0.8569 ± 0.0832 0.6941 ± 0.0456b 0.6669 ± 0.0733b 0.2806 ± 0.0410c

Ascorbic acid a

0.1226 ± 0.0111 0.1011 ± 0.0095c 0.1077 ± 0.0080c 0.1124 ± 0.0092b

Fumaric acid a

0.0475 ± 0.0051 0.0281 ± 0.0032b 0.0136 ± 0.0014d 0.0157 ± 0.0011c

0.0207 ± 0.0031a 0.0032 ± 0.0006c 0.0069 ± 0.0007b 0.0066 ± 0.0007b

Values are expressed as means ± SD (n = 3). Different letters within one column represent significant difference at P < 0.05.

84.0–89.7% of the total FAA loss, which indicated that they had a close relationship with aroma formation in chestnuts. The reducing sugar content in fresh chestnuts was 2.06% d.m. and decreased to 1.15%, 0.67% and 0.80% in boiled, roasted and fried chestnuts, respectively. The concentrations of sucrose, glucose and fructose in varieties of chestnut fruits from Tenerife (Spain) were between 31.10–99.40, 0.25–1.90 and 0.25–1.53 g/kg d.m., respectively (Hernández Suárez, Rodriguez Galdón, Rios Mesa, Diaz Romero, & Rodriguez Rodriguez, 2012). In the present study, the sucrose content in Chinese chestnuts was 9.85% d.m. and decreased significantly (P < 0.05) after thermal processing, while the glucose and fructose content changed slightly. The variation of reducing sugar, sucrose, glucose and fructose in cooked chestnuts is mainly due to four processes: the hydrolysis of starch to oligosaccharide and monosaccharide, decomposition of sucrose to glucose and fructose (Bernárdez, De la Montaña Miguélez, & Queijeiro, 2004), the caramelization and degradation of sugars, and the Maillard reaction. The sucrose content in fresh Italian chestnuts (29.7%) decreased significantly (P < 0.05) to 22.3% after drying at 60 °C, while fructose (1.9%) and glucose (1.4%) content remained unchanged under the same drying condition (Attanasio et al., 2004). The energetic value of fresh chestnuts was 405 kcal/100 g d.m., which decreased to 318–354 kcal/100 g d.m. after cooking owing to the degradation of the proximate composition, especially starch, which is the main energy source (approximately 70%) in chestnuts.

The energetic value of Portuguese chestnuts (C. sativa) was 402 kcal/100 g d.m., which changed to 396 kcal/100 g d.m. after 30-day storage (Fernandes et al., 2011). 3.2. Effect of cooking on the variation of total polyphenols, total flavonoids and organic acids The total polyphenol content in fresh chestnuts was 2.24 mg/g d.m., which remained unchanged after roasting (2.26 mg/g) and decreased to 2.03 and 2.08 mg/g after boiling and frying, respectively (Table 3). In the previous studies, total polyphenols remained unchanged (P > 0.05) after boiling but increased significantly (P < 0.05) after roasting (Gonçalves et al., 2010) and increased significantly (P < 0.05) after industrial peeling by flame (De Vasconcelos et al., 2009b ). Polyphenols may transfer from the chestnut shell to the kernel, by which the content of polyphenols in a chestnut kernel is increased (Gonçalves et al., 2010). On the other hand, polyphenols decompose during cooking, which leads to a decrease. The content of total flavonoids decreased from 2.62 to 2.12, 2.25 and 2.13 mg/g d.m. after boiling, roasting and frying, respectively. All five tested organic acids, including malic acid, citric acid, oxalic acid, ascorbic acid and fumaric acid, decreased significantly (P < 0.01) after cooking. The total content of the above organic acids in fresh chestnut was 2.307 mg/g d.m., which decreased by 50.6% after frying, while only being reduced by 15% after boiling. Gonçalves et al. (2010) found that citric acid

Fig. 1. Principal component analysis score plot for the classification of fresh, boiled, roasted and fried chestnuts. The vertical and horizontal lines show the 95% confidence interval.

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Q. Li et al. / Food Chemistry 201 (2016) 80–86 Table 4 Comparison of volatile components of raw, boiled, fried and roasted chestnuts. Compounds

RI (cal)

RI (lit)

Ethyl acetate Spiro[2,4]hepta-4,6-diene Hexanal Dihydro-2-methyl-3(2H)-furanone Butyl acetate Furfural Ethyl benzene 3-Methyl-1-butanol acetate 3-Heptanone 2-Hydroxy-2-cyclopenten-1-one 4-Hydroxy-2-butanone 3-Carene 1R-a-pinene 1-(Methylencyclopropyl)-ethanol Benzaldehyde (2-Hexenoic acid, methyl ester) 6-Methyl-5-hepten-2-one 2-Pentyl-furan Octanal Acetophenone Nonanal Dodecane Decanal Total

– – 811 820 825 842 873 892 901 908 936 942 944 965 971 972 1002 1005 1021 1083 1122 1217 1223

610 – 802 810 810 830 849 876 894 926 – 1007 937 – 964 – – 991 1000 1062 1102 1200 1206

Percentages Fresh chestnut

Boiled chestnut

Roasted chestnut

Fried chestnut

92.46 – 1.54 – 1.20 – 0.67 0.34 – – – – 0.33 – 0.55 – 0.18 – 0.36 0.11 1.81 0.17 0.28 100

96.40 0.43 0.77 – 0.85 – 0.17 0.10 0.04 – – – 0.14 – 0.16 – 0.11 – 0.14 0.06 0.50 0.05 0.08 100

94.82 – 1.26 0.04 0.97 0.37 0.59 0.10 – 0.85 – – 0.24 – 0.22 – 0.05 – 0.09 0.02 0.26 0.06 0.06 100

– – 14.18 – – 36.58 0.28 1.35 8.87 1.42 6.76 1.15 – 0.56 3.32 0.30 4.67 0.73 3.42 1.41 10.73 0.94 3.33 100

RI (lit) from the NIST Chemistry WebBook (http://webbook.nist.gov/chemistry/).

increased after boiling and roasting, while malic acid decreased. The content of ascorbic acid decreased by 33% and 37% after boiling and roasting, respectively (Barros et al., 2011). To establish the relationship between the different variables in fresh and cooked chestnuts, PCA was applied to total starch, amylose, crude fat, crude protein, water-soluble protein, reducing sugar, sucrose, glucose, fructose, total polyphenols, total flavonoids and organic acids (Fig. 1). An eigenvalue of 100% was achieved using two PCs (PC1 = 99%, PC2 = 1%). Fresh chestnuts were clearly distinguished from boiled, roasted and fried chestnuts, while roasted chestnuts were similar to fried chestnuts because of similar cooking conditions. 3.3. Volatile components in fresh and cooked chestnuts The main volatiles in fresh chestnuts were esters and aldehydes, which mainly included ethyl acetate (92.46%), nonanal (1.81%), hexanal (1.54%), butyl acetate (1.20%), and benzaldehyde (0.55%), 13 compounds in total (Table 4). The primary volatiles in fried chestnuts were furfural (36.58%), hexanal (14.18%), nonanal (10.73%), 3-heptanone (8.87%), and 4-hydroxy-2-butanone (6.76%). Ethyl acetate (96.40%), butyl acetate (0.85%), hexanal (0.77%), nonanal (0.50%) and spiro[2,4]hepta-4,6-diene (0.43%) were found in boiled chestnuts; ethyl acetate (94.82%), hexanal (1.26%), butyl acetate (0.97%) and 2-hydroxy-2-cyclopenten1-one (0.85%) were found in roasted chestnuts. There was still a large amount of esters and aldehydes in the thermal processed chestnuts; moreover, ketones, furfural and furan were found. The aromatic components of cooked chestnuts mainly come from the degradation of saccharides, protein and lipids, caramelization of saccharides, and Maillard reaction between reducing sugar and amino acids (Morini & Maga, 1995b). Furfural, 3-heptanone, 2-hydroxy-2-cyclopenten-1-one, 4-hydroxy-2-butanone, 3-carene, 1-(methylencyclopropyl)-ethanol, 2-hexenoic acid methyl ester and 2-pentyl-furan were observed in fried chestnuts but not in fresh chestnuts, which means that they were formed during thermal processing. Comparing the obtained results with previous studies, hexanal, 4-hydroxy-2-butanone, and decanal were also identified in thermal processed Chinese chestnuts (Morini &

Maga, 1995b); furfural (6.3%) and benzaldehyde (7.2%) were also found to be main components in roasted Italian chestnuts (Krist et al., 2004). 4. Conclusions After thermal processing, the proximate composition, including starch, fat, water-soluble protein, reducing sugar, L-aspartic acid, Lglutamic acid, L-arginine, sucrose and other nutritional compounds, decreased significantly, which led to a decrease in nutritional value. However, the decrease in reducing sugar and free amino acids made a great contribution to the flavor formation. The main volatile components in cooked chestnuts were ketones, furfural and furan, in addition to the esters and aldehydes that originated in fresh chestnuts. Acknowledgements The authors are thankful for the support of the Forestry Industry Research Special Funds for Public Welfare Projects (No. 201204401) from the Ministry of Forestry of the People’s Republic of China and the Fundamental Research Funds for the Central Universities (2015ZCQ-SW-04). References AOAC (1997). Official methods of analysis. Arlington VA, USA: Association of Official Analytical Chemists. AOAC (2000). Official methods of analysis. Arlington VA, USA: Association of Official Analytical Chemists. Attanasio, G., Cinquanta, L., Albanese, D., & Di Matteo, M. (2004). Effects of drying temperatures on physico-chemical properties of dried and rehydrated chestnuts (Castanea sativa). Food Chemistry, 88(4), 583–590. Ayoola, G., Coker, H., Adesegun, S., Adepoju-Bello, A., Obaweya, K., Ezennia, E., et al. (2008). Phytochemical screening and antioxidant activities of some selected medicinal plants used for malaria therapy in southwestern Nigeria. Tropical Journal of Pharmaceutical Research, 7, 1019–1024. Barreira, J. C., Pereira, J. A., Oliveira, M. B., & Ferreira, I. C. (2010). Sugar profiles of different chestnut (Castanea sativa Mill.) and almond (Prunus dulcis) cultivars by HPLC-RI. Plant Foods for Human Nutrition, 65(1), 38–43. Barros, A. I. R. N. A., Nunes, F. M., Gonçalves, B., Bennett, R. N., & Silva, A. P. (2011). Effect of cooking on total vitamin C contents and antioxidant activity of sweet chestnuts (Castanea sativa Mill.). Food Chemistry, 128(1), 165–172.

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