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Effect of superfine grinding on properties of ginger powder
Journal of Food Engineering 91 (2009) 217–222
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of superfine grinding on properties of ginger powder Xiaoyan Zhao a, Zaibin Yang b, Guosheng Gai a,*, Yufeng Yang a a b
Department of Material Science and Engineering, Tsinghua University, Room 2713, Yifu Building, Beijing 100084, China College Animal Science and Technology, Shandong Agriculture University, Shandong 271018, China
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 18 August 2008 Accepted 25 August 2008 Available online 5 September 2008 Keywords: Ginger Superfine grinding Particle size Physical and chemical properties
a b s t r a c t The superfine grinding could produce a narrow and uniform particle size distribution in dry ginger. The physical–chemical properties of five types of ginger powders with particles size of 300, 149, 74, 37 and 8.34 lm were investigated. The size was smaller for ginger powders, greater for the surface area (from 0.331 to 1.320 m2/g) and bulk density (from 0.3069 to 0.3426 g/ml) and smaller for the angle of repose (from 51.50° to 46.33°) and slide (from 45.80° to 39.50°). The values of water absorption index (WAI), water solubility index (WSI) and protein content significantly increased with decreasing the size of ginger particles (p < 0.05). Interestingly, the values of WAI, WSI and protein content of ginger powder with a particle size of 8.34 lm during soaking reached 0.52 g/g, 33.70% and 84.93% for 60 min, respectively. SEM observations revealed the shape and surface morphology of five types of ginger powders. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ginger (Zingiber officinale), a member of the tropical and subtropical Zingiberaceae, is widely used around the world in foods as spice, and extensively used in Traditional Chinese Medicine to treat headaches, nausea and colds. In Chinese, Ayurvedic and Western, the ginger also was applied in the treatment of arthritis, rheumatic disorders and muscular discomfort (Dedov et al., 2002; Wang and Wang, 2005; Tapsell et al., 2006). Due to these properties, ginger has gained considerable attention of as a botanical dietary supplement in the USA and Europe in recent years, and especially for its use in the treatment of chronic inflammatory conditions. The ginger contains biologically active constituents including the main pungent principles, the gingerols and shogaols. Jolad et al. (2004, 2005) examined the components of ginger, and identified a total of 115 compounds, including [6]-, [4]-, [7]-, [8]-, [10]-gingerol and [12]-shogaol etc. Recent studies have showed that the ginger protease could be applied in the tenderization of meat, and improve the quality of meat (Hashimoto et al., 1991; Naveena and Mendiratta, 2001). Wang and Ng (2005) found an antifungal protein in ginger rhizomes, and this protein had the stronger antifungal activity toward various fungi. However, the nutritive components of ginger have not been studied thoroughly. Superfine grinding technology is a new technology, which is a useful tool for making superfine powder with good surface properties like dispersibility and solubility (Tkacova and Stevulova, 1998). The surface of superfine powders could undergo some changes, which had brought out the following outstanding characteristics * Corresponding author. Tel.: +86 010 62792796; fax: +86 010 62791258. E-mail address: [email protected] (G. Gai). 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.08.024
that bulk materials do not possess. With these characteristics, namely, surface effect, mini-size effect, quantum effect, macroquantum channel effect, optical property, magnetic property, mechanical property, chemical and catalytic properties compared to conventional particle materials. Because of these notable characteristics, ultrafine powders have found many applications in ceramics, electric materials, chemicals and papermaking fields as well as in the pharmaceutical field (Yoshizawa and Hiroshi, 1991; Song et al., 2002). Nowadays superfine grinding technology has also been applied in biotechnology and food material, but only rarely. Zhang et al. (2005) found that the superfine powder of mushroom (Agrocybe chaxingu Huang) had good fluidity, water holding capacity and solubility, and was well suited to manufacture instant and convenient foods. Win and Stevens (2001) confirmed that the chitin superfine powder appeared to be a good substrate for the fungal deacetylase. Jin and Chen (2006) found in their investigations that the fiber size of superfine grinding rice straw powder had positive relations with enzymatic hydrolysis due to the cellulose accessibility. Rajkhowa et al. (2008) reported that the length of silk fiber via ultrafine grinding was reduced, and they also found the axial spitting and fragmentation of micro structure of silk fiber due to superfine milling. The studies suggested that the superfine grinding was a good way to fractionate bio-materials into easily bio-converted and hydrolyzed part. While very limited information is available on the effect of the superfine grinding on physical–chemical properties of ginger. The aim of our work is to investigate the application of the superfine grinding technology in ginger, and specific surface area, water holding capacity, water solution index, bulk density and protein solubility of different sized ginger particles. We have compared to the physical–chemical properties of ginger superfine powders with coarse ginger particles.
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2. Materials and methods 2.1. Materials Ginger was obtained from the local farmer in Laiwu country, Shandong Province, China, then sorted, cleaned and cut into small pieces. Excess water was drained off using a net, and subsequently the pieces were place in a mechanical drier at 40 °C. The drying was continued till the water content reached less than 9% for 6 h. The water content was determined by using AACC method No. 44–19 (AACC, 1995). The dried ginger was milled coarse particles by a disc-mill, which were screened through different sized sieves to separate granulates (d < 1 mm) (300 and 140 lm); the superfine powders with the size of 74, 37 and 8.34 lm were obtained in an HMB-701 type micronizer (Huanyatianyuan Machinery Company, Beijing, China), The planetary rubbing mill (power 0.75 kW) was a vertical batch type mill. Three rubbing rings were vertically equipped with pot holders on the turntable. The inner volume and diameter of the cylindrical shape pot were 300 ml and 400 mm, respectively. The pot was made of wear-resistant zirconia, and the rubbing rings with diameter of 300 mm were also made of the same material. The sample was mainly ground with pressure, collision and abrasion between rubbing rings and inner bottom of pot. The revolution speed of turntable was unfixed at 2500– 2800 rpm/min. The particle size distribution of ginger particles was determined using laser diffraction according to AFNOR standard NF X11-666 (1984). It was performed with a Mastersizer IP granulometer (Instruments 2000, UK). Diffraction pattern analysis was carried out in air on a stream of dry powder. Determinations were repeated three times. The particle size distributions of the powder were: D90 = 300, 149, 74, 37 and 8.34 lm as delivered by supplier. Three measurements were carried out for each ginger powder. 2.2. Determination of surface properties The specific surface area (m2/g) of the ginger particles was determined by measuring the adsorption of nitrogen at 77 K according to the Brunauer–Emmett–Teller (BET) principle (Sousa et al., 2002b) and using ASAP 2010 instrument (Micromeritics instrument Co., USA). Moreover, the volume and size of pores of precipitated materials were examined. The measurements were repeated four times after degassing of each sample for 24 h at 40 °C. The reproducibility measured on a reference sample of alumina (alumina CRM 169) on different days was 0.106 ± 0.009 m2/g, i.e. a variation coefficient of Ca. 8.5%. 2.3. Bulk density The bulk density (g/ml) was the density including pores and interparticle voids. Five types of ginger powders were filled in a 10 ml volumetric flask (W1) up to the mark and were weighed (W2) separately. The bulk density of the ginger powders was calculated (Bai and Li, 2006) as follows:
d0 ¼
W2 W1 10
ð1Þ
where W2 was the total weight of the ginger powder and flask, and W1 was the weight of the flask only. The experiments were repeated five times and the measurement of each sample was repeated three times. 2.4. Test procedure for the angle of repose and slide The angle of repose (h) was defined as the maximum angle subtended by the surface of a heap of powder against the plane which
supported it (Taser et al., 2005). The angle of repose was measured using the sequence of steps stated here. Firstly, a filler was fixed above some graph paper so that the distance of the paper from the outlet of the filler (H) was 3 cm, and the filler was vertical to the paper. Then the ginger powder of different size was 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 powder. The angle of repose (h) was calculated as the following formula:
h ¼ arctgð2R=HÞ
ð2Þ
The slide angle was determined according to Ileleji and Zhou (2008) method with some slight modification. The 5.000 g ginger samples with the different size were separately weighed. Then ginger powder was poured on glass plane with a length (L) of 130 mm and width of 100 mm. The sliding angle of repose was estimated by gradually lifting the glass plane until the surface of the ginger powder began to be slide. The angle between the inclined glass and horizontal was called the angle of slide. The vertical distance (H) between the top of inclined glass plane and the horizontal was measured. The angle of slide (a) was calculated as the following formula:
a ¼ arcsinðH=LÞ
ð3Þ
2.5. Test procedure for water holding capacity This parameter was determined using the method of Anderson (1982). Firstly, the weights of cleaned centrifuge tubes (M) and different sized samples (M1) were measured. Then the samples (M1) were dispersed in water (M2) according to M1:M2 = 0.05:1 at 20 °C and poured into the centrifuge tubes placed in a water bath at 60 °C. The tubes were held for 10, 20, 30, 40, 50 and 60 min separately and then they were placed in cold water for 30 min, followed by centrifugation for 20 min at 5000 r/min. The supernatant liquid was removed and the centrifuge tubes with the powders (M3) were weighed again. The formula to calculate water holding capacity (WHC) is as follows:
WHCðg=gÞ ¼
M1 M3 M
ð4Þ
2.6. Test procedure for water solubility index (WSI) The different sized samples (S1) were measured. Then the samples (S1) were dispersed in water (S2) according to S1:S2 = 0.02:1 at ambient temperature and put into the centrifuge tubes placed in a water bath at 80 °C, and shaken for different time intervals of 10, 20, 30, 40, 50 and 60 min. The WSI was reported as percentage, and determined by using AACC method of No. 44-19 (AACC, 1995). The resulting mixture was centrifuged at 6000 rpm for 10 min. Excess water of the clear supernatant solution was drained off by evaporation. Then the samples were dried at 105 °C and weighed (S3).
WSIð%Þ ¼
S3 100% S1
ð5Þ
2.7. Test procedure for solubility of protein Total nitrogen of the ginger powder was estimated by Kjeldahl’s method (Wathelet, 1999). The crude protein content (C) was calculated multiplying the nitrogen content by the approximate factor 6.25 (Wathelet, 1999). The weights of different sized samples (W1) were accurately weighed, and put into the previously weighed tubes and mixed
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with exactly 10 ml distilled water and shaken well. The tubes containing water and samples were weighed again. The tubes were then placed in a reciprocating water bath shaking instrument, and shaken at 60 °C for different time intervals of 10, 20, 30, 60 and 90 min. Then the tubes were taken out, cooled and weighed. The lost water in the tubes during heating in the water bath was compensated by adding water, and it was put aside for 20 min at ambient temperature. The resulting mixture was centrifuged at 4000 rpm for 15 min. The protein content of supernatant liquid of different sized ginger powders was determined by the Bradford method (1976). The supernatant liquid (1 ml) and Coomassie Brilliant Blue G-250 reagent (5 ml) were mixed and the mixture was laid aside for 5 min at room temperature. Similarly, a blank was prepared by mixing the 1 ml distilled water with the 5 ml Coomassie Brilliant Blue G-250 reagent and holding the mixture for 5 min. The absorbances (A) of the reaction mixtures were determined at 595 nm by using a UV–visual spectrophotometer. In order to test the linearity of calibration curve, various concentrations of standards ranging from 0.2 to 1.0 lg/ml were analyzed. The calibration was based on the triplicate analysis of each working solution at five concentration levels. Calibration curve was constructed from the peak height counts. The correlation coefficient was above 0.999, and relative standard deviation was 0.002–0.03% (n = 4). The solution protein of different sized ginger particles (S) was calculated to construct calibration curves. The percentage of solubility of protein (PSP) was calculated using the following formula:
PSBð%Þ ¼
S 100% C
ð6Þ
2.8. Scanning electron microscope (SEM) Morphological characterization of ginger particles was performed on images acquired using a scanning electron microscope (SEM), Quanta 200FEG-SEM (FEI Co. Netherlands) at 150 KV accelerated voltage and 10–15 mm working distance. The samples were coated with platinum of 10 nm thicknesses to make the samples conductive. 3. Statistical analyses Analyses of variance using the general linear models were conducted and averaged and reported along with the standard deviation (±SD). The differences in mean were calculated using the Duncan’s multiple-range tests for means with 95% confidence limit (p 6 0.05). Statistical analysis of the data was done using the SAS software (SAS Institute Inc., Cary, NC, USA).
4. Results and discussion 4.1. Surface properties The specific surface area, surface-number mean and volume– surface mean values of the different sized ginger powders were shown in Table 1. The ranges of surface area, surface-number mean and volume–surface mean values were from 0.331 to 1.320 m2/g, 18.113 to 4.556 lm and 107.512 to 11.488 lm, respectively. These parameters varied significantly among the ginger particles with the different size (p < 0.05). The results indicated that the surface parameters of ginger powder were directly related to the projected size of the corresponding ginger granules. The specific surface area increased as particle size decreased, with the stronger effect for the three finer fractions 74, 37 and 8.34 lm. Owing to a finer grinding, the ginger superfine powders with particle size of 74, 37 and 8.34 lm presented obviously higher specific surface area values
Table 1 Micromeritic parameters of the ginger particles of different size Ginger particle (lm)
Surface-number mean (lm)
Volume–surface mean (lm)
Specific surface area (m2/g)
300 149 74 37 8.34
18.113 ± 0.14d 10.392 ± 0.11c 8.224 ± 0.06b 7.302 ± 0.06b 4.556 ± 0.09a
107.512 ± 2.11e 35.391 ± 1.92d 20.487 ± 2.04c 13.107 ± 1.07b 11.488 ± 1.01a
0.331 ± 0.001b 0.577 ± 0.001c 0.730 ± 0.003d 0.822 ± 0.002d 1.320 ± 0.008d
Mean ± standard deviation. Values in the same column with different letters were significantly different (p < 0.05).
(0.730, 0.822 and 1. 320 m2/g, respectively). Finer particles tended to have a greater number of particles per unit weight, an indication of a higher potential for achieving homogeneity when mixing with other powder additives (Riley et al., 2008). 4.2. Bulk density The bulk density (DENbu.) of the ginger fractions ranged from 0.3069 to 0.3426 g/ml (Table 2). The bulk density of ginger powder (0.3426 g/ml) with a particle size of 8.34 lm was higher than other ginger particles. The reason might be attributed to high homogeneous of superfine particle size and form, which would lead to a probable decrease of the interparticle voids, and offer a larger contact surface with the surroundings. There were significant differences (p < 0.05) in bulk density among ginger particles with the different size. The ginger particle of the higher bulk density was beneficial to filling in preparing tablets or capsule products (Costa et al., 2004). 4.3. The angle of repose and slide The angle of repose and slide could reflect the change in the fluidity of the powder. The angle of repose and slide values of ginger particles with the different size were shown in Table 2. As shown in Table 2, the angle of repose and slide values ranged from 51.50° (300 lm) to 46.33° (8.34 lm) and 45.80° (300 lm) to 39.50° (8.34 lm), respectively. Significant differences (p < 0.05) existed in angle of repose and slide among the ginger particles. The ginger powder with a particle size of 8.34 lm had a lower angle of repose and slide than the others followed by 37 lm (47.70° and 40.61°) and 74 lm (49.37° and 41.88°), the highest were 300 lm (51.50° and 45.80°) and 149 lm (50.67° and 44.05°). As the angle of repose and slide increased so did the granular bulk become less flowable (Ileleji and Zhou, 2008). According to the angle of repose and slide criteria described, the superfine powders with 37 and 8.34 lm had good flow behavior and the surface attachment of the powder would also be higher. The result was in agreement with the investigation of Santomaso et al. (2003). The angle of repose and slide of ginger powders usually decreased with decreasing of the powder size, the reason might appear to be the formation of aggregates. The aggregates tended to arrange in a
Table 2 A parent and bulk density, angle of repose and slide of the ginger particles of different size Ginger particle (lm) 300 149 74 37 8.34
Bulk density (g/ml) a
0.3069 ± 0.002 0.3183 ± 0.001a 0.3277 ± 0.002b 0.3347 ± 0.002b 0.3426 ± 0.002c
Angle of repose (°) d
51.50 ± 0.33 50.67 ± 0.29c 49.37 ± 0.25b 47.70 ± 0.26a 46.33 ± 0.37a
Angle of slide (°) 45.80 ± 0.25d 44.05 ± 0.31c 41.88 ± 0.29b 40.61 ± 0.40a 39.50 ± 0.20a
Mean ± standard deviation. Values in the same column with different letters were significantly different (p < 0.05).
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cone the angle of which was much lower than expected for ginger powders. Therefore, the quality of ginger superfine powders would be better, and the mixture would also be uniform and nonseparable. 4.4. The water holding capacity (WHC) and water solubility index (WSI) The different particle sized powders had different water holding capacity (Fig. 1). As shown in Fig. 1, the water holding capacity increased with decreasing the size of ginger particles. The values of water holding capacity of ginger with particle sizes of 300– 8.32 lm ranged from 0.26 to 0.29 g/g, 0.28 to 0.33 g/g, 0.31 to 0.41 g/g, 0.32 to 0.46 g/g, 0.32 to 0.49 g/g and 0.33 to 0.52 g/g for 10, 20, 30, 40, 50 and 60 min, respectively. At the same time, the WSI was investigated. As shown in Fig. 2, the WSI also increased with decreasing the size of ginger particles. The WSI values of ginger with particle sizes of 300–8.34 lm ranged from 27.81% to 30.07%, 28.06% to 30.68%, 28.53% to 31.82%, 28.91% to 31.93%, 29.35% to 32.60% and 29.86 to 33.70% for 10, 20, 30, 40, 50 and 60 min, respectively. The WHC and WSI values of different sized ginger particles slowly increased before 20 min. The reason might be due to the dissolution of the soluble components in ginger before 20 min, while after 20 min the values of WHC and WSI drastically increased with decreasing the size of ginger particles. However, the WSI was not significantly different for the particle sizes >150 lm. It was interesting to note that the WHC and WSI values of superfine powder with a particle size of 8.34 lm were higher than other particles during soaking. It might be due to the fact that after superfine grinding, the surface properties of super-
Water holding capacity (g/g)
0.6 0.5 300µm
0.4
149µm 74µm
0.3
fine powders had been changed, such as the increase of surface area and surface energy. Moreover, the hydrophilic groups in the cellulose and hemicelluloses of the ginger might had been exposed, which resulted in an easy integration with water, finally the value of WCH increased. In addition, because the changes of superfine powder surface led to the increase of dispersibility and solubility of superfine powder during soaking, the WSI value increased with decreasing the size of ginger particles. The results indicated that the size was smaller for the ginger particle and greater for the solubilization and retaining water capacity of ginger powder. 4.5. The solubility of protein The superfine milling could lead to marked differences in chemical composition separation of the granulometric fractions (Maarou et al., 2000). The crude protein content of ginger was 12.57%. The solubility of protein with different sized ginger particles was studied as shown in Fig. 3. The smaller the size, the higher the content of protein (ginger with particle sizes of 300–8.32 lm from 31.10% to 76.58%, 31.25% to 79.12%, 31.58% to 83.08%, 31.91% to 84.93% and 31.96% to 85.24% for 10, 20, 30, 40, 50 and 60 min, respectively). This evolution was regular. In particular, a step in the content evolution appeared for the finer fractions under 74 lm, with a marked an increase of the protein content. The superfine particle size of 8.34 lm had the highest solubility, the solubility rate of the protein reached 83.08% after 20 min. The ginger particle with a particle size of 300 lm attained the same solubility rate after 30 min, but at this time the particle size of 8.34 lm had achieved 85.24%. The results indicated that the superfine ground ginger could increase protein solubility. Regarding fraction characterization, the variations of chemical composition were due to a more or less deep and fast separation of different botanical constituents of the ginger. These differences in composition had to be avoided in close-circuits grinding whose purpose was to spare energy through a proper association of grinding and screening machines. Therefore, the main factor to affect the solubility of protein was shown to be the particle size and the area of the powder. 4.6. Microphotographs of ginger powder
37µm 8.34µm
0.2 0.1 10
20
30
40
50
60
Time (min) Fig.1. Effect of particle size and soaking time on the water holding capacity of different sized ginger particles.
The microphotographs showed the morphology of fragmented ginger granules (Fig. 4). As shown in Fig. 4, the rate of ginger particle size reduction was slow or even negative at the start. It then gradually reached a steady state. It was possibly attributed to the particle aggregation as revealed from the SEM images (Rajkhowa et al., 2008). Mechanical damage was a transformation from an ordered to a disordered (amorphous) structure via the breakage of intermolecular bonds. Broken ginger granules, depicting the gran-
90
33 300µm 31
149µm 74µm
29
37µm 8.34µm
27 25
Protein solubility rate (%)
Water solubility index (%)
35
300µm
60
149µm 74µm 37µm 30
8.34µm
0 10
20
30
40
50
60
Time (min) Fig. 2. Effect of particle size and soaking time on the water solubility index of different sized ginger particles.
10
20
30
60
90
Time (min) Fig. 3. The change of the solubility rate of protein of different sized ginger particles with time.
X. Zhao et al. / Journal of Food Engineering 91 (2009) 217–222
221
Fig. 4. SEM images of different sized ginger particles.
ular mechanical damage were observed. Extensive milling broke the particles into smaller fractions, the combination of flattening, aggregation and fracture resulted in various shapes of ginger particles, as could be seen in Fig. 4. At the onset of crack propagation and fracture, the aggregation of ginger particles might lead to an increase in size at the beginning. Once the rate of fracture surpassed the rate of aggregation, particle size was reduced. The morphology after milling changed considerably, which could have a significant impact on physical–chemical properties. Broken pieces that were in smaller fragments contributed to the amorphous domains, some of which became solubilized during heat/moisture treatment. 5. Conclusion Superfine grinding of ginger had many significance characteristics: the specific surface area of the superfine powder was increased after superfine grinding, and had the good fluidity, water holding capacity, water solubility index and protein solubility, and was well suited to manufacture instant and convenient foods; the superfine powder was easier to enter into the structure of the foods, so the dispersibility and solubility of superfine powder was good in foods, and the solubility of the nutritive components was increased after superfine grinding, leading to better absorption by the body, which would be more suitable for the development of functional foods than native ginger. The superfine ground ginger showed some physical–chemical properties that might be of potential use in the food industry. Acknowledgement Finance support from National High Technology Research and Development Program (2007AA100403) of China for this work is gratefully acknowledged.
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Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of superfine grinding on properties of ginger powder Xiaoyan Zhao a, Zaibin Yang b, Guosheng Gai a,*, Yufeng Yang a a b
Department of Material Science and Engineering, Tsinghua University, Room 2713, Yifu Building, Beijing 100084, China College Animal Science and Technology, Shandong Agriculture University, Shandong 271018, China
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 18 August 2008 Accepted 25 August 2008 Available online 5 September 2008 Keywords: Ginger Superfine grinding Particle size Physical and chemical properties
a b s t r a c t The superfine grinding could produce a narrow and uniform particle size distribution in dry ginger. The physical–chemical properties of five types of ginger powders with particles size of 300, 149, 74, 37 and 8.34 lm were investigated. The size was smaller for ginger powders, greater for the surface area (from 0.331 to 1.320 m2/g) and bulk density (from 0.3069 to 0.3426 g/ml) and smaller for the angle of repose (from 51.50° to 46.33°) and slide (from 45.80° to 39.50°). The values of water absorption index (WAI), water solubility index (WSI) and protein content significantly increased with decreasing the size of ginger particles (p < 0.05). Interestingly, the values of WAI, WSI and protein content of ginger powder with a particle size of 8.34 lm during soaking reached 0.52 g/g, 33.70% and 84.93% for 60 min, respectively. SEM observations revealed the shape and surface morphology of five types of ginger powders. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ginger (Zingiber officinale), a member of the tropical and subtropical Zingiberaceae, is widely used around the world in foods as spice, and extensively used in Traditional Chinese Medicine to treat headaches, nausea and colds. In Chinese, Ayurvedic and Western, the ginger also was applied in the treatment of arthritis, rheumatic disorders and muscular discomfort (Dedov et al., 2002; Wang and Wang, 2005; Tapsell et al., 2006). Due to these properties, ginger has gained considerable attention of as a botanical dietary supplement in the USA and Europe in recent years, and especially for its use in the treatment of chronic inflammatory conditions. The ginger contains biologically active constituents including the main pungent principles, the gingerols and shogaols. Jolad et al. (2004, 2005) examined the components of ginger, and identified a total of 115 compounds, including [6]-, [4]-, [7]-, [8]-, [10]-gingerol and [12]-shogaol etc. Recent studies have showed that the ginger protease could be applied in the tenderization of meat, and improve the quality of meat (Hashimoto et al., 1991; Naveena and Mendiratta, 2001). Wang and Ng (2005) found an antifungal protein in ginger rhizomes, and this protein had the stronger antifungal activity toward various fungi. However, the nutritive components of ginger have not been studied thoroughly. Superfine grinding technology is a new technology, which is a useful tool for making superfine powder with good surface properties like dispersibility and solubility (Tkacova and Stevulova, 1998). The surface of superfine powders could undergo some changes, which had brought out the following outstanding characteristics * Corresponding author. Tel.: +86 010 62792796; fax: +86 010 62791258. E-mail address: [email protected] (G. Gai). 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.08.024
that bulk materials do not possess. With these characteristics, namely, surface effect, mini-size effect, quantum effect, macroquantum channel effect, optical property, magnetic property, mechanical property, chemical and catalytic properties compared to conventional particle materials. Because of these notable characteristics, ultrafine powders have found many applications in ceramics, electric materials, chemicals and papermaking fields as well as in the pharmaceutical field (Yoshizawa and Hiroshi, 1991; Song et al., 2002). Nowadays superfine grinding technology has also been applied in biotechnology and food material, but only rarely. Zhang et al. (2005) found that the superfine powder of mushroom (Agrocybe chaxingu Huang) had good fluidity, water holding capacity and solubility, and was well suited to manufacture instant and convenient foods. Win and Stevens (2001) confirmed that the chitin superfine powder appeared to be a good substrate for the fungal deacetylase. Jin and Chen (2006) found in their investigations that the fiber size of superfine grinding rice straw powder had positive relations with enzymatic hydrolysis due to the cellulose accessibility. Rajkhowa et al. (2008) reported that the length of silk fiber via ultrafine grinding was reduced, and they also found the axial spitting and fragmentation of micro structure of silk fiber due to superfine milling. The studies suggested that the superfine grinding was a good way to fractionate bio-materials into easily bio-converted and hydrolyzed part. While very limited information is available on the effect of the superfine grinding on physical–chemical properties of ginger. The aim of our work is to investigate the application of the superfine grinding technology in ginger, and specific surface area, water holding capacity, water solution index, bulk density and protein solubility of different sized ginger particles. We have compared to the physical–chemical properties of ginger superfine powders with coarse ginger particles.
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2. Materials and methods 2.1. Materials Ginger was obtained from the local farmer in Laiwu country, Shandong Province, China, then sorted, cleaned and cut into small pieces. Excess water was drained off using a net, and subsequently the pieces were place in a mechanical drier at 40 °C. The drying was continued till the water content reached less than 9% for 6 h. The water content was determined by using AACC method No. 44–19 (AACC, 1995). The dried ginger was milled coarse particles by a disc-mill, which were screened through different sized sieves to separate granulates (d < 1 mm) (300 and 140 lm); the superfine powders with the size of 74, 37 and 8.34 lm were obtained in an HMB-701 type micronizer (Huanyatianyuan Machinery Company, Beijing, China), The planetary rubbing mill (power 0.75 kW) was a vertical batch type mill. Three rubbing rings were vertically equipped with pot holders on the turntable. The inner volume and diameter of the cylindrical shape pot were 300 ml and 400 mm, respectively. The pot was made of wear-resistant zirconia, and the rubbing rings with diameter of 300 mm were also made of the same material. The sample was mainly ground with pressure, collision and abrasion between rubbing rings and inner bottom of pot. The revolution speed of turntable was unfixed at 2500– 2800 rpm/min. The particle size distribution of ginger particles was determined using laser diffraction according to AFNOR standard NF X11-666 (1984). It was performed with a Mastersizer IP granulometer (Instruments 2000, UK). Diffraction pattern analysis was carried out in air on a stream of dry powder. Determinations were repeated three times. The particle size distributions of the powder were: D90 = 300, 149, 74, 37 and 8.34 lm as delivered by supplier. Three measurements were carried out for each ginger powder. 2.2. Determination of surface properties The specific surface area (m2/g) of the ginger particles was determined by measuring the adsorption of nitrogen at 77 K according to the Brunauer–Emmett–Teller (BET) principle (Sousa et al., 2002b) and using ASAP 2010 instrument (Micromeritics instrument Co., USA). Moreover, the volume and size of pores of precipitated materials were examined. The measurements were repeated four times after degassing of each sample for 24 h at 40 °C. The reproducibility measured on a reference sample of alumina (alumina CRM 169) on different days was 0.106 ± 0.009 m2/g, i.e. a variation coefficient of Ca. 8.5%. 2.3. Bulk density The bulk density (g/ml) was the density including pores and interparticle voids. Five types of ginger powders were filled in a 10 ml volumetric flask (W1) up to the mark and were weighed (W2) separately. The bulk density of the ginger powders was calculated (Bai and Li, 2006) as follows:
d0 ¼
W2 W1 10
ð1Þ
where W2 was the total weight of the ginger powder and flask, and W1 was the weight of the flask only. The experiments were repeated five times and the measurement of each sample was repeated three times. 2.4. Test procedure for the angle of repose and slide The angle of repose (h) was defined as the maximum angle subtended by the surface of a heap of powder against the plane which
supported it (Taser et al., 2005). The angle of repose was measured using the sequence of steps stated here. Firstly, a filler was fixed above some graph paper so that the distance of the paper from the outlet of the filler (H) was 3 cm, and the filler was vertical to the paper. Then the ginger powder of different size was 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 powder. The angle of repose (h) was calculated as the following formula:
h ¼ arctgð2R=HÞ
ð2Þ
The slide angle was determined according to Ileleji and Zhou (2008) method with some slight modification. The 5.000 g ginger samples with the different size were separately weighed. Then ginger powder was poured on glass plane with a length (L) of 130 mm and width of 100 mm. The sliding angle of repose was estimated by gradually lifting the glass plane until the surface of the ginger powder began to be slide. The angle between the inclined glass and horizontal was called the angle of slide. The vertical distance (H) between the top of inclined glass plane and the horizontal was measured. The angle of slide (a) was calculated as the following formula:
a ¼ arcsinðH=LÞ
ð3Þ
2.5. Test procedure for water holding capacity This parameter was determined using the method of Anderson (1982). Firstly, the weights of cleaned centrifuge tubes (M) and different sized samples (M1) were measured. Then the samples (M1) were dispersed in water (M2) according to M1:M2 = 0.05:1 at 20 °C and poured into the centrifuge tubes placed in a water bath at 60 °C. The tubes were held for 10, 20, 30, 40, 50 and 60 min separately and then they were placed in cold water for 30 min, followed by centrifugation for 20 min at 5000 r/min. The supernatant liquid was removed and the centrifuge tubes with the powders (M3) were weighed again. The formula to calculate water holding capacity (WHC) is as follows:
WHCðg=gÞ ¼
M1 M3 M
ð4Þ
2.6. Test procedure for water solubility index (WSI) The different sized samples (S1) were measured. Then the samples (S1) were dispersed in water (S2) according to S1:S2 = 0.02:1 at ambient temperature and put into the centrifuge tubes placed in a water bath at 80 °C, and shaken for different time intervals of 10, 20, 30, 40, 50 and 60 min. The WSI was reported as percentage, and determined by using AACC method of No. 44-19 (AACC, 1995). The resulting mixture was centrifuged at 6000 rpm for 10 min. Excess water of the clear supernatant solution was drained off by evaporation. Then the samples were dried at 105 °C and weighed (S3).
WSIð%Þ ¼
S3 100% S1
ð5Þ
2.7. Test procedure for solubility of protein Total nitrogen of the ginger powder was estimated by Kjeldahl’s method (Wathelet, 1999). The crude protein content (C) was calculated multiplying the nitrogen content by the approximate factor 6.25 (Wathelet, 1999). The weights of different sized samples (W1) were accurately weighed, and put into the previously weighed tubes and mixed
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with exactly 10 ml distilled water and shaken well. The tubes containing water and samples were weighed again. The tubes were then placed in a reciprocating water bath shaking instrument, and shaken at 60 °C for different time intervals of 10, 20, 30, 60 and 90 min. Then the tubes were taken out, cooled and weighed. The lost water in the tubes during heating in the water bath was compensated by adding water, and it was put aside for 20 min at ambient temperature. The resulting mixture was centrifuged at 4000 rpm for 15 min. The protein content of supernatant liquid of different sized ginger powders was determined by the Bradford method (1976). The supernatant liquid (1 ml) and Coomassie Brilliant Blue G-250 reagent (5 ml) were mixed and the mixture was laid aside for 5 min at room temperature. Similarly, a blank was prepared by mixing the 1 ml distilled water with the 5 ml Coomassie Brilliant Blue G-250 reagent and holding the mixture for 5 min. The absorbances (A) of the reaction mixtures were determined at 595 nm by using a UV–visual spectrophotometer. In order to test the linearity of calibration curve, various concentrations of standards ranging from 0.2 to 1.0 lg/ml were analyzed. The calibration was based on the triplicate analysis of each working solution at five concentration levels. Calibration curve was constructed from the peak height counts. The correlation coefficient was above 0.999, and relative standard deviation was 0.002–0.03% (n = 4). The solution protein of different sized ginger particles (S) was calculated to construct calibration curves. The percentage of solubility of protein (PSP) was calculated using the following formula:
PSBð%Þ ¼
S 100% C
ð6Þ
2.8. Scanning electron microscope (SEM) Morphological characterization of ginger particles was performed on images acquired using a scanning electron microscope (SEM), Quanta 200FEG-SEM (FEI Co. Netherlands) at 150 KV accelerated voltage and 10–15 mm working distance. The samples were coated with platinum of 10 nm thicknesses to make the samples conductive. 3. Statistical analyses Analyses of variance using the general linear models were conducted and averaged and reported along with the standard deviation (±SD). The differences in mean were calculated using the Duncan’s multiple-range tests for means with 95% confidence limit (p 6 0.05). Statistical analysis of the data was done using the SAS software (SAS Institute Inc., Cary, NC, USA).
4. Results and discussion 4.1. Surface properties The specific surface area, surface-number mean and volume– surface mean values of the different sized ginger powders were shown in Table 1. The ranges of surface area, surface-number mean and volume–surface mean values were from 0.331 to 1.320 m2/g, 18.113 to 4.556 lm and 107.512 to 11.488 lm, respectively. These parameters varied significantly among the ginger particles with the different size (p < 0.05). The results indicated that the surface parameters of ginger powder were directly related to the projected size of the corresponding ginger granules. The specific surface area increased as particle size decreased, with the stronger effect for the three finer fractions 74, 37 and 8.34 lm. Owing to a finer grinding, the ginger superfine powders with particle size of 74, 37 and 8.34 lm presented obviously higher specific surface area values
Table 1 Micromeritic parameters of the ginger particles of different size Ginger particle (lm)
Surface-number mean (lm)
Volume–surface mean (lm)
Specific surface area (m2/g)
300 149 74 37 8.34
18.113 ± 0.14d 10.392 ± 0.11c 8.224 ± 0.06b 7.302 ± 0.06b 4.556 ± 0.09a
107.512 ± 2.11e 35.391 ± 1.92d 20.487 ± 2.04c 13.107 ± 1.07b 11.488 ± 1.01a
0.331 ± 0.001b 0.577 ± 0.001c 0.730 ± 0.003d 0.822 ± 0.002d 1.320 ± 0.008d
Mean ± standard deviation. Values in the same column with different letters were significantly different (p < 0.05).
(0.730, 0.822 and 1. 320 m2/g, respectively). Finer particles tended to have a greater number of particles per unit weight, an indication of a higher potential for achieving homogeneity when mixing with other powder additives (Riley et al., 2008). 4.2. Bulk density The bulk density (DENbu.) of the ginger fractions ranged from 0.3069 to 0.3426 g/ml (Table 2). The bulk density of ginger powder (0.3426 g/ml) with a particle size of 8.34 lm was higher than other ginger particles. The reason might be attributed to high homogeneous of superfine particle size and form, which would lead to a probable decrease of the interparticle voids, and offer a larger contact surface with the surroundings. There were significant differences (p < 0.05) in bulk density among ginger particles with the different size. The ginger particle of the higher bulk density was beneficial to filling in preparing tablets or capsule products (Costa et al., 2004). 4.3. The angle of repose and slide The angle of repose and slide could reflect the change in the fluidity of the powder. The angle of repose and slide values of ginger particles with the different size were shown in Table 2. As shown in Table 2, the angle of repose and slide values ranged from 51.50° (300 lm) to 46.33° (8.34 lm) and 45.80° (300 lm) to 39.50° (8.34 lm), respectively. Significant differences (p < 0.05) existed in angle of repose and slide among the ginger particles. The ginger powder with a particle size of 8.34 lm had a lower angle of repose and slide than the others followed by 37 lm (47.70° and 40.61°) and 74 lm (49.37° and 41.88°), the highest were 300 lm (51.50° and 45.80°) and 149 lm (50.67° and 44.05°). As the angle of repose and slide increased so did the granular bulk become less flowable (Ileleji and Zhou, 2008). According to the angle of repose and slide criteria described, the superfine powders with 37 and 8.34 lm had good flow behavior and the surface attachment of the powder would also be higher. The result was in agreement with the investigation of Santomaso et al. (2003). The angle of repose and slide of ginger powders usually decreased with decreasing of the powder size, the reason might appear to be the formation of aggregates. The aggregates tended to arrange in a
Table 2 A parent and bulk density, angle of repose and slide of the ginger particles of different size Ginger particle (lm) 300 149 74 37 8.34
Bulk density (g/ml) a
0.3069 ± 0.002 0.3183 ± 0.001a 0.3277 ± 0.002b 0.3347 ± 0.002b 0.3426 ± 0.002c
Angle of repose (°) d
51.50 ± 0.33 50.67 ± 0.29c 49.37 ± 0.25b 47.70 ± 0.26a 46.33 ± 0.37a
Angle of slide (°) 45.80 ± 0.25d 44.05 ± 0.31c 41.88 ± 0.29b 40.61 ± 0.40a 39.50 ± 0.20a
Mean ± standard deviation. Values in the same column with different letters were significantly different (p < 0.05).
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cone the angle of which was much lower than expected for ginger powders. Therefore, the quality of ginger superfine powders would be better, and the mixture would also be uniform and nonseparable. 4.4. The water holding capacity (WHC) and water solubility index (WSI) The different particle sized powders had different water holding capacity (Fig. 1). As shown in Fig. 1, the water holding capacity increased with decreasing the size of ginger particles. The values of water holding capacity of ginger with particle sizes of 300– 8.32 lm ranged from 0.26 to 0.29 g/g, 0.28 to 0.33 g/g, 0.31 to 0.41 g/g, 0.32 to 0.46 g/g, 0.32 to 0.49 g/g and 0.33 to 0.52 g/g for 10, 20, 30, 40, 50 and 60 min, respectively. At the same time, the WSI was investigated. As shown in Fig. 2, the WSI also increased with decreasing the size of ginger particles. The WSI values of ginger with particle sizes of 300–8.34 lm ranged from 27.81% to 30.07%, 28.06% to 30.68%, 28.53% to 31.82%, 28.91% to 31.93%, 29.35% to 32.60% and 29.86 to 33.70% for 10, 20, 30, 40, 50 and 60 min, respectively. The WHC and WSI values of different sized ginger particles slowly increased before 20 min. The reason might be due to the dissolution of the soluble components in ginger before 20 min, while after 20 min the values of WHC and WSI drastically increased with decreasing the size of ginger particles. However, the WSI was not significantly different for the particle sizes >150 lm. It was interesting to note that the WHC and WSI values of superfine powder with a particle size of 8.34 lm were higher than other particles during soaking. It might be due to the fact that after superfine grinding, the surface properties of super-
Water holding capacity (g/g)
0.6 0.5 300µm
0.4
149µm 74µm
0.3
fine powders had been changed, such as the increase of surface area and surface energy. Moreover, the hydrophilic groups in the cellulose and hemicelluloses of the ginger might had been exposed, which resulted in an easy integration with water, finally the value of WCH increased. In addition, because the changes of superfine powder surface led to the increase of dispersibility and solubility of superfine powder during soaking, the WSI value increased with decreasing the size of ginger particles. The results indicated that the size was smaller for the ginger particle and greater for the solubilization and retaining water capacity of ginger powder. 4.5. The solubility of protein The superfine milling could lead to marked differences in chemical composition separation of the granulometric fractions (Maarou et al., 2000). The crude protein content of ginger was 12.57%. The solubility of protein with different sized ginger particles was studied as shown in Fig. 3. The smaller the size, the higher the content of protein (ginger with particle sizes of 300–8.32 lm from 31.10% to 76.58%, 31.25% to 79.12%, 31.58% to 83.08%, 31.91% to 84.93% and 31.96% to 85.24% for 10, 20, 30, 40, 50 and 60 min, respectively). This evolution was regular. In particular, a step in the content evolution appeared for the finer fractions under 74 lm, with a marked an increase of the protein content. The superfine particle size of 8.34 lm had the highest solubility, the solubility rate of the protein reached 83.08% after 20 min. The ginger particle with a particle size of 300 lm attained the same solubility rate after 30 min, but at this time the particle size of 8.34 lm had achieved 85.24%. The results indicated that the superfine ground ginger could increase protein solubility. Regarding fraction characterization, the variations of chemical composition were due to a more or less deep and fast separation of different botanical constituents of the ginger. These differences in composition had to be avoided in close-circuits grinding whose purpose was to spare energy through a proper association of grinding and screening machines. Therefore, the main factor to affect the solubility of protein was shown to be the particle size and the area of the powder. 4.6. Microphotographs of ginger powder
37µm 8.34µm
0.2 0.1 10
20
30
40
50
60
Time (min) Fig.1. Effect of particle size and soaking time on the water holding capacity of different sized ginger particles.
The microphotographs showed the morphology of fragmented ginger granules (Fig. 4). As shown in Fig. 4, the rate of ginger particle size reduction was slow or even negative at the start. It then gradually reached a steady state. It was possibly attributed to the particle aggregation as revealed from the SEM images (Rajkhowa et al., 2008). Mechanical damage was a transformation from an ordered to a disordered (amorphous) structure via the breakage of intermolecular bonds. Broken ginger granules, depicting the gran-
90
33 300µm 31
149µm 74µm
29
37µm 8.34µm
27 25
Protein solubility rate (%)
Water solubility index (%)
35
300µm
60
149µm 74µm 37µm 30
8.34µm
0 10
20
30
40
50
60
Time (min) Fig. 2. Effect of particle size and soaking time on the water solubility index of different sized ginger particles.
10
20
30
60
90
Time (min) Fig. 3. The change of the solubility rate of protein of different sized ginger particles with time.
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Fig. 4. SEM images of different sized ginger particles.
ular mechanical damage were observed. Extensive milling broke the particles into smaller fractions, the combination of flattening, aggregation and fracture resulted in various shapes of ginger particles, as could be seen in Fig. 4. At the onset of crack propagation and fracture, the aggregation of ginger particles might lead to an increase in size at the beginning. Once the rate of fracture surpassed the rate of aggregation, particle size was reduced. The morphology after milling changed considerably, which could have a significant impact on physical–chemical properties. Broken pieces that were in smaller fragments contributed to the amorphous domains, some of which became solubilized during heat/moisture treatment. 5. Conclusion Superfine grinding of ginger had many significance characteristics: the specific surface area of the superfine powder was increased after superfine grinding, and had the good fluidity, water holding capacity, water solubility index and protein solubility, and was well suited to manufacture instant and convenient foods; the superfine powder was easier to enter into the structure of the foods, so the dispersibility and solubility of superfine powder was good in foods, and the solubility of the nutritive components was increased after superfine grinding, leading to better absorption by the body, which would be more suitable for the development of functional foods than native ginger. The superfine ground ginger showed some physical–chemical properties that might be of potential use in the food industry. Acknowledgement Finance support from National High Technology Research and Development Program (2007AA100403) of China for this work is gratefully acknowledged.
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