2014 Volume 20 Issue 4 Pages 731-738
The effects of physical modification methods, superfine grinding (SG) and high-pressure processing (HPP), on the physical and chemical properties of mushroom (A. chaxingu) powders were investigated. Scanning electron microscope observations revealed the shape and surface morphology of mushroom powders. The SG powder had smaller particle size and greater bulk density. The HPP powder had lower viscosity and a smaller angle of repose. The SG and HPP powders exhibited a water holding capacity of 3.82 g/g and 3.62 g/g, while the water solubility index was 32.6% and 28.3%, respectively. The chemical analysis indicated greater soluble dietary fiber (SDF) contents (9.63 g/100 g and 7.00 g/100 g), protein solubility (2.51% and 2.25%) and polysaccharide solubility (4.92% and 3.56%) in the SG and HPP powders, respectively. Overall, the application of SG and HPP may provide two novel approaches to achieve greater protein, SDF and polysaccharide contents with desirable properties.
Agrocybe chaxingu, also called Agrocybe cylindracea, is a cultivated mushroom in China that has become increasingly popular recently due to its delicious taste and unique texture (Tsai et al., 2008). Several studies refer to A. chaxingu as an ideal resource for a natural health food due to its abundant nutritive components, including protein, vitamins and minerals (Lee et al., 2009; Li et al., 2007, 2008). Lee et al. (2011) reported the amino acid and fatty acid compositions of A. chaxingu. A. chaxingu is also highly valued as a functional food for its antitumor activity, free radical scavenging activity, Cox-1 (Cox-2) inhibitory activity, hypoglycemic activity, and more (Kiho et al., 1994; Kim et al., 1997; Koshino et al., 1996; Ngai et al., 2005).
To extend the application of this mushroom, it can be further processed by physical modification. Physical modification processing involves synergism between different physical processes to transform raw materials into consumer-ready products. High-pressure processing (HPP) is a physical processing method that has demonstrated great potential in the food industry; it can maintain the quality of fresh foods with few effects on the flavor and nutritional value (Norton and Sun, 2008). Currently, HPP is being successfully applied to a variety of products, including fruit juices, sauces, desserts, rice dishes, oysters and meat products (Barba et al., 2012; Chen et al., 2010; Erkan et al., 2011; Sikes et al., 2009). Superfine grinding (SG) is an emerging technology demonstrating great potential in the manufacture of nutraceuticals and functional foods for the improvement of human health (Chen et al., 2006; Wang et al., 2009). The reduction of particle sizes of various materials leads to changes in the structure and surface area, and produces novel characteristics that bulk materials do not possess before modification (Zhao et al., 2009; Zhu et al., 2010). Micronization has been demonstrated to be an effective approach for modifying the texture of fiber-rich plant food materials (Huang et al., 2007; Wu et al., 2007). Zhang et al. (2005) found that superfine mushroom (A. chaxingu) powder had good fluidity, water holding capacity and solubility and was well suited for the manufacture of instant and convenience foods. We recently reported the antioxidant activities of the SG and HPP powders of A. chaxingu (Lv et al., 2011); the results revealed that the modified samples led to greater free radical scavenging activity, chelating activity and total antioxidant activity. The objective of this study was to investigate the effects of SG and HPP on the physical and chemical properties of A. chaxingu powders. In this study, SG and HPP were used to modify A. chaxingu, and the objective was to investigate some physical and chemical properties of the mushroom powders. The parameters were more comprehensive (such as: morphological characterization, viscosity, and dietary fiber, et al.) than previous papers (Zhang et al., 2005; Lee et al., 2009).
Materials The dry fruiting bodies of A. chaxingu were purchased at a local market in Hangzhou City, China. The mushrooms were cut into small pieces and placed in a mechanical drier at 60°C until the water content reached less than 9%. The moisture contents were determined according to an AACC method (2000). The dried materials were ground into a powder (through a 60 mesh sieve) by a CXP-100 pulverizer (Shanghai Shengxi Machinery, China) to be the control. The pulverized powders were then ground by pressure, collision and abrasion in a WZJ-6J type micronizer (Jinan Beili Powder Technology and Engineering Company, China) to obtain superfine ground (SG) powder. The high-pressure processing (HPP) powder was obtained as follows: Firstly, the dry fruiting bodies of the mushroom were cut into small pieces. Secondly, the small pieces fully absorbed water until the water content reached more than 95% and then were packaged into vacuum-sealed plastic bags. Thirdly, the packaged samples were HPP-treated. The treatment was performed at a pressure of 400 MPa at 25°C for a holding time of 15 min (UHPF-750, Baotou Kefa High Pressure Technology Co., Ltd, China), not including the pressure build-up and release times. After the HPP treatment, the mushrooms were dried at 60°C until the water content reached less than 9%, and they were then ground into powder (through a 60 mesh sieve) by a CXP-100 pulverizer (Shanghai Shengxi Machinery, China). All the chemicals used were of analytical grade.
Particle size and scanning electron microscope (SEM) analysis The particle size of the three mushroom powders was measured by a BT9300-H laser particle size analyzer (Dandong Baite Instrument Co., Ltd., China). The mushroom powders were dispersed in ethanol solution (concentration was 0.2%), and then were dispersed by vortex vibration. The treatment samples were used for particle size analysis. The morphological characterization of the mushroom powders was performed on images acquired using a SEM (Philips XL-30-ESEM, Holland) at 20 KV accelerated voltage.
Bulk density and viscosity measurement The bulk density was measured by gently pouring 2 g of the mushroom powder into a 10 mL measuring cylinder, and then holding the cylinder on a vortex vibrator for 1 min to obtain a constant volume of the sample. The volume of the sample was recorded against the scale on the cylinder. The bulk density value was calculated as the ratio of the mass of the powder and the volume occupied in the cylinder (Bai and Li, 2006).
The viscosity was measured by taking a 20 g sample of each powder and homogenizing it with 1000 mL of water. Each solution was stirred for 30 min, and the viscosity was measured at 25°C using a Brookfield LVDV-II + Pro viscometer (Brookfield Engineering Lab, Stoughton, MA, USA). A LV 1 spindle was used.
Angle of repose measurement The angle of repose (θ) is defined as the maximum angle subtended by the surface of a heap of powder against the plane that supports it (Taser et al., 2005). The angle of repose was measured using the following sequence of steps. First, a filler was fixed above graph paper so that the distance of the paper from the outlet of the filler (H) was 2.0 cm, and the filler was vertical to the paper. Next, different powders were separately poured into the filler until the tip of the powder cone touched the outlet of the filler. The diameter (2R) of the cone was measured for each type of powder. The angle of repose (θ) was calculated using the following formula: θ = arctg H/R.
Determination of hydration properties The water holding capacity (WHC) was determined according to the method of Anderson (1982) with some modifications. First, a cleaned centrifuge tube (M, g) was weighed, and approximately 1.0 g of powder (M1, g) was poured into it. Water (M2, g) was added to disperse the powder with a powder/water ratio of 1/20 (w/w) at ambient temperature. The dispersion was stirred at 25°C for 4 h. Then, the tube was centrifuged at 5000 rpm for 20 min. The resulting supernatant was removed, and the centrifuge tube with the sediment (M3, g) was weighed again. The WHC was calculated using the following formula: WHC (g/g) = (M3-M)/M1.
The water solubility index (WSI) was determined using the AACC method of No. 44 - 19 (2000). The different powders (S1) were dispersed in water (S2) according to S1 / S2 = 1/50 (w/w) at ambient temperature, placed into the centrifuge tubes in a water bath at 80°C, and shaken for 30 min. The resulting mixture was centrifuged at 6000 rpm for 10 min. Excess water from the clear supernatant solution was drained off by evaporation. The samples were then dried at 105°C and weighed (S3). The WSI was calculated using the following formula: WSI (%) = S3 / S1 × 100%.
The swelling capacity (SC) was determined according to a previously reported method (Lecumberri et al., 2007). The initial 1.0 g of powder (M) was poured into a graduated cylinder, and its occupied bed volume (V1) was recorded. Next, 10 mL of water was added into the tube, and the tube was shaken until a homogeneous dispersion was achieved. The dispersion was incubated in a water bath at 25°C for 24 h to allow the complete swelling of the powder. The new volume (V2) of the wetted powder was then recorded. The SC was calculated using the following formula: SC (mL/g) = (V2-V1) / M.
Chemical analysis The chemical compositions of the powders, including the ash, protein, fat, insoluble dietary fiber (IDF) and total dietary fiber (TDF), were measured using AOAC methods (1997). Ash content was analyzed at 550°C for 2 h (g ash/100 g sample). Protein content (g protein/100 g sample) was analyzed according to the Kjeldahl method. Fat content (g fat/100 g sample) was calculated by the weight lost after a 6-cycle extraction with petroleum ether in a Soxhlet apparatus. TDF and IDF were determined following AOAC methods. SDF was calculated by subtracting the IDF proportion from the TDF.
The powder sample (1.0 g) was dispersed in 30 mL of distilled water and incubated in a water bath (60°C for protein solubility and 100°C for polysaccharide solubility) for the required time, varying from 20 min to 100 min. After incubation, the mixture was cooled and centrifuged at 5000 rpm for 20 min and the supernatant was collected for further measurements (the water lost during incubation was replaced to obtain the volume prior to incubation). The amount of protein in the above-obtained supernatant was determined by the Coomassie Brilliant Blue method, as developed by Bradford (1976). The polysaccharide in the supernatant was quantified using a phenol-sulfuric acid method (Dubois et al., 1951). Protein solubility (%) was expressed as the percentage of the mass of protein of the supernatant to that of the powder and polysaccharide solubility (%) was expressed as the percentage of the mass of polysaccharide in the supernatant to that of the powder.
Statistical analysis All experiments were performed in triplicate and the results were expressed as the mean ± standard deviation (SD). The difference between the means was determined by Duncan's multiple range tests using the SPSS v. 11.5 statistical software (SAS Inc., NC, USA). The results were considered statistically significant at p < 0.05.
Particle size and SEM The particle size distributions of the powders obtained by a laser particle size analyzer are displayed in Table 1. The particle size distributions were characterized by D0.1, D0.5 and D0.98 values (Giry et al., 2006). The agglomeration ratio, D0.5, was considered to be the average median diameter that was representative of the degree of powder cohesiveness. SG significantly reduced the average particle size of the powder. This result is in agreement with the investigation of Zhang et al. (2011). The width of the particle size distributions was measured by span according to Zhang et al. (2012). A smaller span value indicated a narrower particle size distribution and more uniform size. The span values of the SG (2.56) and HPP (2.92) powders were lower than that of the control (3.91) (p < 0.05).
| Volume diameters (µm) | Powders | ||
|---|---|---|---|
| Control | SG | HPP | |
| D0.10 | 7.05 ± 0.13a | 5.56 ± 0.13b | 10.80 ± 0.29c |
| D0.50 | 56.26 ± 1.11a | 18.48 ± 0.64b | 82.44 ± 1.12c |
| D0.98 | 226.67 ± 4.65a | 52.94 ± 0.87b | 251.46 ± 4.37c |
| Span | 3.91 ± 0.01a | 2.56 ± 0.45b | 2.92 ± 0.01c |
Mean ± standard deviation. Values in the same row with different letters were significantly different (p < 0.05). D0.10, D0.50 and D0.98 are the equivalent volume diameters at 10, 50, and 98% cumulative volumes, respectively; Span is determined by the equation: Span = (D0.98 − D0.10) / D0.50. Control, without physical modification; SG, superfine grinding; HPP, high-pressure processing.
Morphological changes in the mushroom after physical modification were observed by imaging the powder surface with SEM (Fig. 1). Grinding led not only to particle size reduction but also to a deep structural modification. After SG, the cottony surface was observe and the larger bulk density in later was confirm the loose microstructure. Extensive milling broke the particles into smaller fractions; the combination of flattening, aggregation and fracture resulted in various shapes of particles. Mechanical damage was a transformation from an ordered to a disordered (amorphous) structure via the breakage of intermolecular bonds. After HPP, the structure of stratiform clouds was observed in the surface that revealed slight ruptures compared to the control. However, these results demonstrate that the surface of the sample was greatly destroyed after high-pressure treatment. Similar changes in the morphology, as assessed by SEM, were also observed on the surface of R. coreanus after high-pressure treatment (Seo et al., 2011). In the progress of HPP, the pressure increased firstly, and then the pressure reduced, which resulted to the sample inner structure change. Some bonds may be broken.
SEM images of different physically modified mushroom (A. chaxingu) powders. Control, without physical modification; SG, superfine grinding; HPP, high-pressure processing.
Bulk density and viscosity Due to the great variability of shape of food powders particles, interactions between assemblies of particles and fluid may become more complex in food powders than in inert powders. Bulk property determinations may be considered, therefore, it is a very critical issue for food powder characterizations, as well as for evaluating their effects on processing (Ortega-Rivas, 2009). The bulk density of the mushroom powders produced by different methods is displayed in Table 2. The bulk density of the SG powder (0.475 mL/g) was clearly greater than the other powders (p < 0.05), which may be attributed to that the smaller particle size having a greater contact surface with the surroundings and a greater homogeneous form, which would lead to a decrease the pore spaces between the particles and increase the bulk density value (Zhao et al., 2010). The mushroom powders with high bulk densities are potential ingredients that could be used in instant beverages.
| Powders | Bulk density (g/mL) | Viscosity (mPa·s) | Repose angle (°) |
|---|---|---|---|
| Control | 0.455 ± 0.009a | 15.4 ± 0.7a | 43.4 ± 1.3a |
| SG | 0.475 ± 0.007b | 21.8 ± 1.1b | 46.6 ± 1.3b |
| HPP | 0.437 ± 0.005c | 12.2 ± 0.6c | 40.3 ± 1.4c |
Mean ± standard deviation. Values in the same column with different letters were significantly different (p < 0.05). Control, without physical modification; SG, superfine grinding; HPP, high-pressure processing.
The powder was added to water to create suspensions. Viscosity is one of the most important functional characteristics of suspensions. The viscosity caused by materials prevents complete digestion of foods and reduces the uptake of digested products. The apparent viscosity of different powders ranged from 12.2 – 21.8 mPa·s (Table 2). The HPP sample demonstrated a lower apparent viscosity than the others (p < 0.05). The high-pressure treatment led to the formation of a smooth and compact surface, which reduced the resistance between the particles. In the case of suspension, littler particle size increased the friction of particles and the viscosity increased. More soluble polysaccharide and protein contents also increased the viscosity. Above all, viscosity is the result of multiple factors.
Angle of repose The angle of repose is very important, as it can be used to estimate the change in the fluidity of the powder. A low angle of repose value indicates that the powder has good fluidity (Wu et al., 2012). As displayed in Table 2, the micronized powder had significantly greater angle of repose value than the pulverized powder derived from the same material. The angle of repose tended to increase with a decrease in the powder size; the reason for this trend may be due to the adhesive force increasing among particles with a decreasing of the powder size. According to the angle of repose criteria described above, the HPP powder (θ = 40.3°, p < 0.05) had good flow behavior, and the surface attachment of the powder was also lower.
Hydration properties The hydration capacity is the ability of a moist material to retain water when subjected to an external centrifugal gravity force or compression; it consists of the sum of bound water, hydrodynamic water and, mainly, physically trapped water (Vázquez-Ovando et al., 2009). Hydration properties, including the WHC, WSI and SC of the powders, are displayed in Table 3. The WSI value increased in the order of Control < HPP < SG. The WSI is related to the soluble protein, soluble polysaccharide and SDF contents. The control, SG and HPP powders exhibited WHC value 3.32, 3.82 and 3.62 times their own weight (p < 0.05), respectively, which is in agreement with results reported by Zhang et al. (2005). The hydrophilic groups in the cellulose and hemicellulose of the A. chaxingu might have been exposed, resulting in an easy integration with water that led to an increased WHC. High-WHC materials swell by soaking up water in stomach that tricks people into feeling they have eaten enough before their stomach is full. It is good food for the obese. So, the strong ability to hold water allows them to be used as functional food additives. The SC value of the HPP powder was lower than that of the control powder, while the SC value of the SG powder was significantly greater. This may be because after the SG and HPP modification, the surface properties of the powders had changed drastically, as with the increase in the surface energy.
| Powders | Hydration properties | ||
|---|---|---|---|
| WHC (g/g) | WSI (%) | SC (mL/g) | |
| Control | 3.32 ± 0.09a | 25.7 ± 1.0a | 3.93 ± 0.13a |
| SG | 3.82 ± 0.07b | 32.6 ± 0.8b | 4.45 ± 0.08b |
| HPP | 3.62 ± 0.04c | 28.3 ± 0.8c | 2.63 ± 0.07c |
Mean ± standard deviation. Values in the same column with different letters were significantly different (p < 0.05). Control, without physical modification; SG, superfine grinding; HPP, high-pressure processing; WHC, water holding capacity; WSI, water solubility index; SC, swelling capacity.
Chemical composition The chemical compositions of the different powders are presented in Table 4. The powders revealed protein contents of 22.2% – 23.1% and fat contents of 3.29% – 3.35%, which are similar to previously reported finding that in the fruiting body of A. chaxingu, the proportions of crude protein and crude fat are 22.7% and 3.64% by weight, respectively (Lee et al., 2009). The HPP powder of A. chaxingu may be a good source due to its low crude fat content and higher protein content.
| Powders | Protein (g/100 g) | Fat (g/100 g) | Ash (g/100 g) | TDF (g/100 g) | SDF (g/100 g) | IDF (g/100 g) |
|---|---|---|---|---|---|---|
| Control | 22.2 ± 0.9a | 3.35 ± 0.11a | 2.45 ± 0.09a | 58.1 ± 0.9ab | 6.60 ± 0.20a | 51.5 ± 0.7a |
| SG | 24.3 ± 0.8b | 3.42 ± 0.10a | 2.24 ± 0.09b | 59.7 ± 1.1a | 9.63 ± 0.25b | 50.0 ± 0.9a |
| HPP | 23.1 ± 1.1ab | 3.29 ± 0.08a | 2.37 ± 0.10ab | 57.8 ± 1.1b | 7.00 ± 0.30a | 50.4 ± 0.8a |
Mean ± standard deviation. Values in the same column with different letters were significantly different (p < 0.05). Control, without physical modification; SG, superfine grinding; HPP, high-pressure processing; TDF, total dietary fiber; SDF, soluble dietary fiber; IDF, insoluble dietary fiber.
The major chemical component found in the control powder was the TDF, with a total of 58.1%, of which 51.5% corresponded to IDF and 6.60% to SDF. The content of TDF in the A. chaxingu was greater than that reported for shiitake mushrooms (49.09%) by Martínez-Flores et al. (2009). The SDF fraction in the SG powder was significantly greater than that in the control powder (p < 0.05), up to 9.63 g/100 g. It is valuable to emphasize that the micronization method greatly increased the SDF fraction in the powder. A previous report indicated an increased SDF fraction in a carrot insoluble fiber-rich fraction and water caltrop pericarp after ball milling micronization (Chau et al., 2007). This fact was explained by the redistribution of fiber components from the insoluble to the soluble fractions. IDF is beneficial to intestinal function because it helps increase fecal bulk and enhances intestinal peristalsis. SDF has beneficial properties associated with its significant role in human physiology function, including reductions in cholesterol levels and blood pressure, the prevention of gastrointestinal problems and protection against the onset of several cancers.
The solubilities of the protein and polysaccharides in the powders as a function of soaking time were displayed in Fig.2 and Fig.3. Both the protein and polysaccharide solubilities were increased with the prolonging of soaking time from 20 min to 100 min, but the increment was small and different treatment powders had different protein and polysaccharide solubilities. We can see from the figure that the SG powder had the greatest solubility and the solubility rate of the polysaccharides and protein reached 3.06% and 2.01%, respectively, after 20 min. The control powder attained the same solubility rate after 100 min; however, at this time, the SG powder had achieved 4.92% for polysaccharides and 2.51% for protein. The solubility of the polysaccharides and protein of the HPP powder was less than that of the SG powder but greater than that of the control powder and reached 3.56% and 2.25%, respectively, after 100 min. This study demonstrated that the A. chagxingu powder, which was treated by physical modifications, had increased polysaccharides and protein solubility. Previously, we reported that physically modified powders of A. chaxingu had better antioxidant activities in vitro, and the solubility of the total phenols was 2.26 mg/g for the control, 2.60 mg/g for the SG and 4.20 mg/g for the HPP powder (Lv et al., 2011).
Effects of physical modification methods and soaking time on the solubility of protein from different mushroom (A. chaxingu) powders.
Control, without physical modification; SG, superfine grinding; HPP, high-pressure processing.
Effects of physical modification methods and soaking time on the solubility of polysaccharides from different mushroom (A. chaxingu) powders.
Control, without physical modification; SG, superfine grinding; HPP, high-pressure processing.
In this study, the physical and chemical properties of mushroom (A. chaxingu) powders prepared using physical modification methods, namely SG and HPP, were investigated and compared. The SG and HPP powders were shown to be nutritionally desirable. The physical modification played a dominant role in the physical and chemical properties of the mushroom powders. In contrast to the powder without treatment (Control), the SG powder revealed a smaller particle size and greater WSI, WHC and SC; the HPP powder also had a lower viscosity and greater fluidity. The SG and HPP powders both had greater SDF contents and protein and polysaccharide solubility than the control. These improved properties facilitate the application of mushroom powders in food additives and convenience food products.
Acknowledgements This research was supported by grants from great project of Science Technology Department of Zhejiang Province (No.2011C12046) and Project of Innovation and Promotion of Zhejiang Academy of Agriculture Science. We acknowledge all staff for their valuable assistance in conducting this study.
