Effect of three cooking methods on nutrient components and antioxidant capacities of bamboo shoot (Phyllostachys praecox C.D. Chu et C.S. Chao)

Effect of three cooking methods on nutrient components and antioxidant capacities of bamboo shoot ( Phyllostachys praecox C.D. Chu et C.S. Chao) first-author corresp ‡Corresponding Author ‡Corresponding Author †E-mail:yqhu@zju.edu.cn †E-mail:yqhu@zju.edu.cn yqhu@zju.edu.cn Department of Food Science and Nutrition, College of Biosystem Engineering and Food Science, Zhejiang University, Hangzhou 310029, China Department of Food Science and Nutrition, College of Biosystem Engineering and Food Science, Zhejiang University, Hangzhou 310029, China Three cooking methods, namely boiling, steaming, and stir-frying for 5 to 10 min, were used to evaluate the effect on nutrient components, free amino acids, L -ascorbic acid, total phenolic contents, and antioxidant capacities of bamboo shoots ( Phyllostachys praecox ). Results showed that boiling and stir-frying had a great effect on the nutrient components and they decreased the contents of protein, soluble sugar, and ash, and caused a great loss in the total free amino acids (decreased by 38.35% and 34.86%, respectively). Significant differences ( P < 0.05) in free amino acids were observed in the samples cooked by different methods. Stir-fried bamboo shoots had a high fat content which increased by 528.57% because of the addition of edible oil. After boiling, the L -ascorbic acid and total phenolic contents were significantly reduced, while steaming increased total phenolic content by 3.98% and stir-frying well-preserved L -ascorbic acid (78.87% of its previous content). Results of the antioxidative property study showed that stir-frying could increase antioxidant capacities of bamboo shoots. It is concluded that stir-frying is more suitable for bamboo shoots because it could obtain the maximum retention of antioxidant capacities. Cooking methods Bamboo shoot Nutrition Antioxidant capacities Introduction 1 Bamboo shoots are the young tender stalks emerging from nodes of the (pseudo-)rhizome of bamboo plants. The edible part consists of meristematic cell tissue with regions of rapid cell division and differentiation, which is enveloped in protective, non-edible leaf sheaths (Kleinhenz et al., 2000 2000 ). Bamboo shoots are rich in both amino acids and antioxidants and taste fresh, crisp with aromatic quality, and are delicious. Therefore, they are usually called “the top grade vegetable” (Xu et al., 2003 2003 ; Zhang et al., 2008 2008 ). The shoot of Phyllostachys praecox is a higher quality bamboo shoot among dozens of edible ones (Xu et al., 2003 2003 ) in China, and is widely distributed in various parts of the world. It is known to all that China has an extensive and profound food culture with many cooking methods, such as boiling, steaming, stir-frying, roasting, decocting, and deep-frying. These cooking treatments can not only increase the palatability but can also make the food more conducive to digestion and absorption (Kong et al., 2007 2007 ; Cho et al., 2008 2008 ). Many research reports and cooking practices have shown that different cooking methods have varying degrees of influence on food nutritional contents and functional ingredients (Dzudie et al., 2000 2000 ; Moynihan et al., 2000 2000 ). The bamboo shoots are available for consumption only after cooking. Because of heat treatments on the changes of protein, carbohydrates, free amino acids, L -ascorbic acid, flavonoids, phenolic acids, and other nutritional and functional components (Dzudie et al., 2000 2000 ; Kong et al., 2007 2007 ), the real nutritional value of cooked bamboo shoots is not equal to that of the fresh ones. However, the studies on the nutritive composition of bamboo shoots were mainly limited in their fresh or dried raw materials (Luo et al., 2008a 2008a ; 2008b 2008b ; Li et al., 2010 2010 ). Until now, no studies exist on the cooked bamboo shoots. The bamboo shoot ( Phyllostachys praecox ) was used as the material in this paper to elucidate the nutrients and antioxidant functional composition changes and the antioxidant capacity changes after three cooking methods, namely boiling, steaming, and stir-frying. This research could provide theoretical guidance for a modern healthy diet. Materials and methods 1 Bamboo shoot materials 2 The bamboo shoots ( Phyllostachys praecox C.D. Chu et C.S. Chao, cv. Ventricousinternode) were purchased from the market in the year of 2010 in Hangzhou, Zhejiang Province, China and stored at 4 °C. Sample treatments 2 The fresh bamboo shoots were cooked in the same way as when prepared for consumption. In brief, fresh bamboo shoots were firstly peeled and washed clean and then cut vertically into slices with about 2–3 mm thickness. The bamboo shoot slices were collected and weighed (~300 g). The first portion was used raw while the other three portions were subjected to steaming, boiling, or stir-frying as below. Each sample was cooked by the same chef to ensure its stable quality. Boiling treatment 3 Bamboo shoot slices were placed into a pot filled with 1 000 ml boiling water and then boiled for 10 min with a pot cover to prevent water loss. They were scooped up and cooled to room temperature. Steaming treatment 3 Bamboo shoot slices were placed in the centre of the top steamer with 1 000 ml boiling water at the bottom. After steaming for 10 min, they were scooped up and cooled to room temperature. Stir-frying treatment 3 A small amount (20 g) of peanut oil was placed in a pan, and then bamboo shoot slices were put into it and stir-fried for 5 min. They were scooped up and cooled to room temperature. Each 300 g of cooked bamboo shoot samples were divided into three groups: the first 100 g samples were used for the nutrient component detection; the second 100 g were used for free amino acid detection; the remaining samples were frozen quickly in liquid nitrogen, then freeze-dried in vacuum freeze-drying equipment (Taitec VD-400F, Japan), and crushed immediately after drying. The powder was stored at −80 °C for antioxidative property test. Nutrient component analysis 2 The moisture contents were measured by the difference of the sample weight after drying at 105 °C for 20 h. Ash, fat, soluble sugar, starch, and crude fibre contents were determined using the standard method of the Association of Official Analytical Chemists (AOAC, 1990 1990 ). The protein content was determined by the semi-micro Kjeldahl method (ISO 5983-1-2005 5983-1-2005 ). Free amino acid analysis 2 The free amino acids were extracted from bamboo shoot samples using the method mentioned by Antoine et al. ( 1999 1999 ). The amino acids’ compositions were analyzed according to Norziah and Ching ( 2000 2000 ), using the Waters Associates AccQ∙Tag method (pre-column derivatization of samples with AccQ∙Fluor reagent and analysis by reverse phase high-performance liquid chromatography (HPLC)). Identification of the amino acids in the sample was carried out by comparing their retention time with that of the standards. L -ascorbic acid analysis 2 L -ascorbic acid was immediately analyzed on the same day of sample preparation. A total of 10 g fresh or cooked bamboo shoot slices were homogenized with 30 ml of 1 mg/ml oxalic acid and centrifuged at 4 000 r/min for 5 min. The aquatic supernatant was collected, and the residue was again extracted with 10 ml of 1 mg/ml oxalic acid. The aquatic supernatants were combined and the volume was made up to 50 ml with 1 mg/ml oxalic acid, and then filtered through a 0.45-μm Advantec filter for HPLC analysis, according to Xu et al. ( 2008 2008 ). The L -ascorbic acid contents were calculated according to standard solution (80 mg/L). Antioxidative properties of bamboo shoots 2 Aqueous extracts of bamboo shoot samples (3.0 g powder samples) were prepared by homogenizing bamboo shoot powder in 30 ml of distilled water and filtered. The filtrates were made to 30 ml with distilled water. These extracts were used for determining total phenolic content, total antioxidant activity, and other in vitro antioxidant activities. Total phenolic content 3 Total phenolics were determined according to the method mentioned by Prabhasankar et al. ( 2009 2009 ). In a test tube, 0.2 ml of the aqueous extract was mixed with 4.0 ml of 0.02 g/ml Na 2 CO 3 and then 0.2 ml of 0.5 g/ml Folin Ciocalteau’s phenol reagent was added. The reaction mixture was mixed thoroughly and allowed to stand for 30 min at room temperature in the dark. After incubation, the solution was measured at 720 nm against a blank. Results were expressed as mg gallic acid equivalents per 100 g dry weight (mg GAE/100 g DW). Total antioxidant activity 3 The total antioxidant capacities of the bamboo shoot extracts were measured with the method mentioned by Abdel-Hameed ( 2009 2009 ). Based on the reduction of Mo(VI) to Mo(V) by the extract and subsequent formation of a green phosphate/Mo(V) complex at acid pH, 0.2 ml of sample was mixed with 0.1 ml distilled water and 3.0 ml reagent solution (0.6 mol/L sulphuric acid, 28 mmol/L sodium phosphate, and 4.0 mmol/L ammonium molybdate). A typical blank solution contained 3 ml of reagent solution and the appropriate volume of the same solvent used for the sample. All tubes were incubated at 95 °C for 90 min under water bath. After incubation, absorbance values of all the sample mixtures were measured at 695 nm against the blank using an ultraviolet-visible (UV-Vis) spectrophotometer. Total antioxidant activity is expressed as mg ascorbic acid equivalents per gram dry weight (mg AAE/g DW). 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activity 3 The DPPH radical-scavenging activity was measured with the method mentioned by Prabhasankar et al. ( 2009 2009 ). Briefly, 0.2 ml of the aqueous extract was mixed with 1.8 ml distilled water and 2.0 ml of 0.16 mmol/L DPPH solution (in methanol). The mixture was vortexed for 1 min and kept at room temperature for 30 min in the dark. The absorbance of the solution was measured at 517 nm. The scavenging effect (%) was calculated by using the formulae mentioned by Xu et al. ( 2007 2007 ). Superoxide radical-scavenging activity 3 Antioxidant capacity of the aqueous extract of bamboo shoot samples against superoxide radicals was determined by the method of Prabhasankar et al. ( 2009 2009 ). Briefly, in a test tube, 0.2 ml of the aqueous extract was mixed with 0.1 ml distilled water, 2.6 ml of 50 mmol/L phosphate buffer (pH 8.2), and 0.09 ml freshly prepared pyrogallol (3 mmol/L dissolved in 10 mmol/L HCl). The mixture was vortexed for 0.2 min. The inhibition rate of pyrogallol auto-oxidation was measured at 325 nm at every 1 min interval for 10 min. The scavenging activity (%) was calculated using the formula mentioned by Xu et al. ( 2007 2007 ). Statistical analysis 2 Data were analyzed by Duncan’s multiple range tests using statistical package Statistica V 5.5 software. A significant level was defined as P < 0.05. Determinations were carried out in triplicate. Results and discussion 1 Effects of cooking methods on nutrient components 2 Table 1 1 clearly indicates that the basic nutrient components of bamboo shoots had been influenced in different degrees by boiling, steaming, and stir-frying. After boiling, the crude protein, crude fat, soluble sugar, and ash had all been significantly influenced ( P < 0.05) and their loss ratios were 7.38%, 23.81%, 14.39%, and 7.71%, respectively. The losses of starch, crude fiber, and water were not obvious. These changes occurred because of the diffluent and volatile substances in bamboo shoots lost to the soup. After steaming, the losses of crude protein and crude fat were 1.23% and 4.76%, respectively, with tiny changes of other nutrient components. After stir-frying, the content of soluble sugar was significantly decreased by 17.42% ( P < 0.05). The addition of peanut oil during the treatment caused a crude fat increase of 528.57% and the energy increase of 34.4%. Data presented as arithmetic mean±SD ( n =3). Mean values with different letters in the same row are significantly different ( P < 0.05). FW: fresh weight 2 Macronutrient Raw 2 Boiling 2 Steaming 2 Stir-frying Content (g/100 g FW) Content (g/100 g FW) Loss (%) Content (g/100 g FW) Loss (%) Content (g/100 g FW) Loss (%) Water 91.25±0.01a 91.76±0.12a −0.56±0.13 91.65±0.09a −0.44±0.09 90.31±0.25b 1.03±0.25 Crude protein 3.25±0.02a 3.01±0.04b 7.38±1.12 3.21±0.02a 1.23±0.61 3.20±0.05a 1.54±0.97 Crude fat 0.21±0.01b 0.16±0.01c 23.81±4.77 0.20±0.01b 4.76±3.75 1.32±0.05a −528.57±23.81 Soluble sugar 1.32±0.02a 1.13±0.02b 14.39±1.52 1.31±0.02a 0.76±1.26 1.09±0.03b 17.42±2.27 Starch 1.75±0.02a 1.72±0.03a 1.71±0.07 1.75±0.01a 0.00±0.07 1.74±0.02a 0.57±0.36 Crude fiber 1.37±0.01a 1.38±0.02a −0.73±1.46 1.37±0.01a 0.00±0.09 1.37±0.01a 0.00±0.00 Ash 0.85±0.01a 0.81±0.01b 4.71±1.18 0.85±0.01a 0.00±0.00 0.84±0.01a 1.18±0.95 After three conventional cooking treatments, the crude protein content all had decreased, as well as the crude fat content in boiling and steaming samples. These phenomena were related to the protein denaturation and fat oxidation at high temperature (Wei et al., 2009 2009 ). In addition, during the high temperature process of stir-frying, both protein and its decomposition products and soluble sugar participated in Maillard reaction, then resulted in their content decline. Effects of cooking methods on free amino acids 2 In fresh bamboo shoots, we detected out 12 free amino acids altogether, included 6 unessential amino acids (UAA, namely, Asp, Glu, Gly, Ala, Tyr, and His) and 6 essential amino acids (EAA, namely, Val, Met, Ile, Leu, Phe, and Lys). The total free amino acid (TFAA) content was 550.92 mg/100 g fresh weight (FW) and total free essential amino acid (TFEAA) content was 163.77 mg/100 g FW, which demonstrated that bamboo shoots belonged to the vegetables with high free amino acid content. As important flavor substances, the free amino acids in cooked samples showed a variety of different types of free amino acids in the boiled, steamed, and stir-fried samples when compared to those in fresh bamboo shoots (Table 2 2 ). Moreover, their contents also had various degrees of difference after cooking treatments. The boiled bamboo shoots’ free amino acids decreased substantially ( P < 0.05) with TFEAA by 38.85% and TFAA by 38.35%. This is possible due to the loss of free amino acids to the fluid which the samples were boiled in. TFEAA and TFAA contents in steamed bamboo shoots had no significant changes, but the contents of Glu, Gly, Tyr, and Ile increased substantially ( P < 0.05) by 23.18%, 128.40%, 7.07%, and 3.92%, respectively. In stir-fried bamboo shoots, most free amino acid contents reduced dramatically ( P < 0.05) with TFEAA by 39.20% and TFAA by 34.86%; however, Gly content increased by 5.64%. Data presented as arithmetic mean±SD ( n =3). Mean values with different letters in the same row were significantly different ( P < 0.05) Essential free amino acid TFEAA: total free essential amino acids; TFAA: total free amino acids; ND: not detected 2 Amino acid Raw 2 Boiling 2 Steaming 2 Stir-frying Content (mg/100 g FW) Content (mg/100 g FW) Loss (%) Content (mg/100 g FW) Loss (%) Content (mg/100 g FW) Loss (%)  Asp 32.17±2.11a 19.82±2.07c 38.39±2.51 27.61±1.27b 14.17±0.74 20.87±2.58c 35.13±4.21  Ser ND 1.75±0.07b − 2.31±0.21a − 2.06±0.43a −  Glu 15.62±0.61b 11.06±0.92c 29.19±3.11 19.24±1.04a −23.18±1.11 12.53±0.41c 19.78±1.12  Pro ND ND − ND − ND −  Gly 5.14±0.21b 3.92±0.13c 23.74±2.01 11.74±0.51a −128.40±9.08 5.43±0.21b −5.64±0.31  Ala 55.03±3.17a 21.94±1.05c 60.11±4.07 45.86±3.25b 16.62±1.05 22.01±1.07c 59.98±5.07  Cys ND ND − 0.12±0.07a − 0.17±0.03a −  Tyr 163.87±11.97a 102.78±8.32bc 37.28±2.95 175.45±10.42a −7.07±0.93 121.2±6.72b 26.04±1.92  His 115.35±10.61a 78.21±5.62c 32.19±2.76 107.58±10.65ab 6.74±0.61 75.16±5.63c 34.84±4.66  Arg ND ND − ND − 0.11±0.02 −  Val * * 39.62±2.51a 21.37±1.61bc 46.06±3.55 38.17±2.31a 3.66±0.50 25.91±1.47b 34.60±3.53  Met * * 10.56±1.51a 7.29±0.57bc 30.97±2.52 9.76±0.65a 7.58±0.31 6.51±0.52c 38.35±4.51  lle * * 25.28±0.91a 20.12±0.93b 20.41±1.98 26.27±2.95a −3.92±0.33 17.73±0.91c 29.86±1.93  Leu * * 32.71±2.93a 19.25±0.83b 41.14±3.91 31.81±2.95a 2.75±0.91 17.16±1.75b 47.54±3.92  Phe * * 30.30±1.25a 13.19±0.51d 56.46±4.22 28.75±1.23ab 5.12±0.21 16.51±1.32c 45.51±3.27  Lys * * 25.30±1.21a 16.21±0.82c 35.92±2.25 22.87±1.17b 9.60±0.61 14.61±1.28d 42.25±3.21  Thr * * ND 2.71±0.21a − 2.25±0.21b − 1.13±0.21c − 8 TFEAA 163.77±11.27a 100.14±9.21b 38.85±3.55 159.88±12.22a 2.37±0.21 99.56±7.61b 39.20±3.28 TFAA 550.92±0.61a 339.62±27.25b 38.35±2.62 549.67±22.21a 0.23±0.01 358.82±37.21b 34.86±3.73 Under the heat treatments, some proteins and short peptides may have degraded into free amino acids (Cho et al., 2008 2008 ), then reacted with reducing sugar at high temperature and produced not only specific food flavors but also colored substances. On the other hand, after cooking, some free amino acids were run off along with bamboo shoots’ juice. Effects of cooking methods on L -ascorbic acid 2 Processing methods utilizing heat can irreversibly inactivate ascorbate oxidase, thus counteracting enzymatic decomposition; however, reduction of L -ascorbic acid may also occur as a function of thermal decomposition and leach into the cooking medium (Davey et al., 2000 2000 ). In fresh bamboo shoots, the content of L -ascorbic acid was 12 mg/100 g FW. After three conventional cooking treatments, L -ascorbic acid content in bamboo shoots all dropped apparently (Fig. 1 1 ). The retention was 47.37% in boiled bamboo shoots and 57.83% in steamed ones, which could be attributed to the fact that L -ascorbic acid is very soluble in water and not stable at high temperature. However, the short time stir-frying retained the maximum content of L -ascorbic acid at 78.87%, much more than the other two. This result was consistent with the observation of Somsub et al. ( 2008 2008 ) who also found that conventional cooking methods (blanching, boiling, or stir-frying) led to excessive loss of L -ascorbic acid in most Thai vegetables. Effects of cooking methods on total phenolic content and total antioxidant activity 2 Phenolic compounds are commonly found in plants and have been reported to possess biological activity similar to antioxidants (Udayakumar et al., 2010 2010 ). Aqueous extracts of uncooked and cooked bamboo shoots exhibited high phenolic content (Fig. 2a 2a ). Total phenolic contents in both boiled and stir-fried samples decreased slightly while steaming made phenolic content increase. On the one hand, heat treatment had a decomposition effect on phenolic compounds (Ranilla et al., 2010 2010 ), which also may have been lost to the solution, in which the bamboo shoots were boiled; on the other hand, heat treatment could make the existed polyphenol oxidases inactive so that prevented polyphenols’ decomposition (Yamaguchi et al., 2003 2003 ), and heat treatment could promote dietary fiber-binding type polyphenols decomposition into free phenolic compounds and then made the detected value much higher (Stewart et al., 2000 2000 ). In contrast with fresh bamboo shoots, the antioxidant capacity of boiled samples reduced apparently, steamed ones remained unchanged, and stir-fried ones increased slightly (Fig. 2b 2b ). The antioxidant capacity of bamboo shoot was closely related to L -ascorbic acid and total phenolic compounds. After boiling, a lot of the antioxidant substances were running off into the boiling medium, which then led to the decrease of their antioxidant capacities. This result was similar to the research of Chuah et al. ( 2008 2008 ) who indicated that almost all of the antioxidant substances in colored peppers run off and dissolve in the water during boiling process. Kenny and O′Beirne ( 2009 2009 ) indicated that the loss of antioxidant activity was relative to the contact area between vegetables and water as well as processing time. It was clear that the contact areas in steaming and stir-frying processes were much smaller than that in boiling, so their antioxidant substances lost relatively very little. The higher content of total phenolic compounds, the stronger antioxidant activity (Baardseth et al., 2010 2010 ). During steaming, the increase of total phenolic compounds made up for the loss of L -ascorbic acid, so that their antioxidant capacities corresponded to the fresh ones. However, in short time and high temperature stir-frying, both L -ascorbic acid and total phenolic contents in bamboo shoots decreased but the antioxidant capacity increased. This might be due to the Maillard reaction. Since the surface temperature of stir-fried bamboo shoots could reach 150 °C, the soluble sugar decreased by 17.42%, the TFEAA decreased by 39.2% and strong aromatic flavors were generated, it is reasonable to assume that the Maillard reaction occurred, which could promote the generation of antioxidant substances and increase their antioxidant capacities (Nicoli et al., 1997 1997 ). Effects of cooking methods on in vitro antioxidative properties 2 DPPH has been used extensively as a free radical to evaluate reducing substances and is a useful reagent for investigating in vitro free radical-scavenging activities of compounds. As shown in Fig. 3a 3a , all the four bamboo shoot samples exhibited appreciable scavenging properties against DPPH radicals. It was evident that the stir-fried bamboo shoots were proven to be the most powerful antioxidant, as their radical-scavenging activities were significantly different from those of any other kinds of cooked bamboo shoots. However, the boiled samples exhibited the lowest radical-scavenging activity and this could be attributed to the heat-degraded compounds from carbohydrates having leached into the cooking medium. (a) DPPH radical-scavenging activity; (b) Superoxide radical-scavenging activity Sawa et al. ( 1999 1999 ) suggested that studies on vegetables concerning superoxide anion-scavenging activity were of increasing interest due to the harmful effect of the superoxide. The superoxide-scavenging activity of fresh bamboo shoots was 35.31%, which decreased dramatically to 13.17% after boiling for 10 min. Steamed and stir-fried bamboo shoots exhibited higher superoxide-scavenging activity as compared to fresh ones (Fig. 3b 3b ). In general, the difference of these three cooked bamboo shoots’ antioxidant activities could be attributed to reaction products with radical-scavenging activities generated by different cooking methods (Sawa et al., 1999 1999 ). Conclusions 1 After three conventional cooking treatments, the boiled bamboo shoots had serious losses of crude protein, soluble sugars, ash, free amino acids, and L -ascorbic acid, meanwhile their antioxidant capacities also decreased significantly ( P < 0.05); in steamed bamboo shoots, the basic nutrient components, TFAA, and antioxidant capacity changed very little, and however, each free amino acid had changed to various extents and the L -ascorbic acid content was considerably reduced with a loss percentage of 42.17%; during stir-frying, the addition of peanut oil increased the crude fat content, while both the soluble sugar and TFAA contents decreased dramatically ( P < 0.05). 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J Anhui Agric Sci , 2008 . 36 ( 3 ): 841 - 842 . (in Chinese) . Analysis and determination of the nutritive composition in the phyllostachy spraecox shoot 10.1631/jzus.B1000251 Project supported by the Fundamental Research Funds for the Central Universities (No. KYJD09033), the National Key Technology R & D Program of China (No. 2008BAD91B04), and the Science and Technology Department of Zhejiang Province (Nos. 2010C12012 and 2008C02015), China 2010-07-01 2011-07-27 2010-11-28 2022-01-05 Copyright © Zhejiang University and Springer-Verlag Berlin Heidelberg 2011 2011 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 09 12