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Production of silver carp bone powder using superfine grinding technology: Suitable production parameters and its properties
Journal of Food Engineering 109 (2012) 730–735
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Production of silver carp bone powder using superfine grinding technology: Suitable production parameters and its properties Gang-cheng Wu a, Min Zhang a,⇑, Ying-qiang Wang a, Kebitsamang Joseph Mothibe a, Wei-xing Chen b a b
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China Jiangsu Shanshui Food Company, Baoying 225800, China
a r t i c l e
i n f o
Article history: Received 23 September 2011 Received in revised form 11 November 2011 Accepted 12 November 2011 Available online 28 November 2011 Keywords: Silver carp bone Superfine grinding Powder properties
a b s t r a c t Superfine grinding technology is a new type of food processing that is employed to produce powders with outstanding properties such as high solubility, dispersion, adsorption, chemical reactivity and fluidity. The objective of this work was to study the superfine grinding technology on the silver carp bone and to optimize the operational parameters. The optimum processing conditions of superfine grinding were: feeding pressure of 0.85 MPa, grinding pressure of 1.0 MPa, feeding rate of 0.0325 g/s, feeding particle size of 150–300 lm and one pass through the grinder. Silver carp bone powder with average particle size (D50) of 7.65 lm was produced at these conditions. It was found that the smaller particle size of the powder, the higher its fluidity, solubility, solubility of protein, electric conductivity and water holding capacity of the powder were. So the quality of the products produced by the superfine grinding technology was said to be improved. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The silver carp (Hypophthalmichthys molitrix) is a species of freshwater cyprinid fish which is a variety of Asian carp common in north and northeast Asia. The silver carp is rich in minerals, amino acids, eicosapentaenoic acid and docosahexaenoic acid (Jin and Li, 1998). It is processed into various kinds of food products such as; fish chips, stewed fish frozen surimi and many others. During the fish-processing process, silver carp for its preparation is required to remove bones. However, silver carp bones are said to be fried and eaten in some areas of China but this still leaves lot of them wasted. They may cause environmental pollution if discarded, apart from that it is waste of resource (Takeshi and Nobutaka, 2000; Hammoumi et al., 1998). Many studies have shown that silver carp bones are rich in minerals, protein and amino acids (Chen et al., 2006). Therefore, the utilization of silver carp bones such as grinding it into powder and using it in feed as well as in nutritive foods for humans is really necessary. For production high quality powders, high technology processing techniques are required. Superfine grinding technology is a new technology that is a useful tool for making superfine powder with good surface properties like dispersibility and solubility (Tkacova and Stevulova, 1998). Compared with coarse particles, the surface of superfine powder ⇑ Corresponding author. Address: School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu Province, China. Fax: +86 (0)510 85807976. E-mail address: [email protected] (M. Zhang). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.11.013
possessed a series of important characteristics, such as surface effect, quantum effect, macro-quantum channel effect, mini-size effect, optical property, magnetic property, mechanical property, chemical and catalytic property (Zhao et al., 2010). To date, the superfine grinding technology is widely used in metallic materials and medicines (Hoeffl, 1987; Hua, 2000; Schmidt, 1989; Song et al., 2002; Yoshizawa and Hiroshi, 1991). Zhang (1998) found that the superfine grinding had the functions to decrease particle size and improve reactive surface to the greatest extent possible, and it had been considered to be less energy consuming than traditional mechanical grinding with respect to the increase of surface area. It was also found that this technique may also reduce the leaching of copper from conventional brass matrices and reduce the dissolution of tin in the packaging industries. Han et al. (2000) studied the use of ultrafine silk powder in cosmetic production and found results showed that the ultrafine powder can be absorbed well by the surface cells of the body. Nowadays, superfine grinding technology has also been applied in biotechnology and foodstuffs and has shown a high potential for many other commercial applications. Zhao et al. (2010) studied on properties of superfine ground powder of Astragalus membranaceus and reported that the superfine ground Astragalus membranaceus powder had high water holding capacity, high fluidity, high water solubility index and high protein solubility. Jin and Chen (2006) reported that superfine grinding was a good way to fractionate steam-exploded rice straw into easily bio-converted part and difficultly hydrolyzed part. The study on mushroom (Agrocybe chaxingu Huang) grinding has also shown that superfine grinding had many significant improvement in characteristics of
G.-c. Wu et al. / Journal of Food Engineering 109 (2012) 730–735
final product; the area of the superfine powder was increased after superfine grinding and had high fluidity, water holding capacity and solubility (Zhang et al., 2005). Superfine grinding has also increased the specific surface area of the superfine ginger powder and the solubility of the nutritive components, leading to better absorption by the body (Zhao et al., 2009). Nevertheless, so far little information is available on the preparation technology and properties of superfine ground powder of silver carp bone. Therefore, the objective of this study is to investigate the superfine grinding technology on silver carp bone and to optimize various operational parameters: grinding pressure, feed rate, feed particle size and times of grinding. Powder size and yield were discussed in the article. Furthermore, physical–chemical properties of the superfine powder such as fluidity, water holding capacity, bulk density and protein solubility of different sized bone particles were also studied and compared to conventional grinding powders in the paper. 2. Materials and methods Silver carps were obtained from the local supermarket of Wuxi City, Jiangshu Province, China. They were first thoroughly cleaned with water, removing innards and scales on the surface. Silver carp were then boiled for 10 min to make bones be easily dissociated from the flesh. Excess water in the dissociated bone was drained off using a net. Subsequently, the bones were placed in a mechanical drier at 50 °C and dried until their water content was less than 9% for about 10 h. The water content was determined by the standard oven method (Wu et al., 2010). Dried bones were subjected to coarse milling using an A11-analysis grinding machine (IKA Works Guangzhou, Guangzhou, China), and the coarse bone powder screened through different sized sieves. Finally, the uniform coarse particles were obtained. The uniform coarse particles were ground in an AO table type micronizer (Qingxin Powder Machinery Company, Yixing, Jiangsu, China), Fig. 1. This type of micronizer is jet mill. The operating principle of jet mill is to make materials impacting, colliding and friction by the agitation of high-speed gas or steam. In process optimization three important parameters (feeding rate, grinding pressure and feeding particle size) were taken into consideration. Firstly, uniform coarse bone particles were put into vibrating groove, where the feeding rate was controlled by vibration of the machine. Secondly, bone particles were fed to feed port by control-
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ling the vibrating groove. Finally, the superfine grinding powders were obtained. The average particle size (D50) was tested using a laser particle size instrument (LMS-24, Japan). The electric conductivity of powder was also tested by conductivity meter (DDSC-11T, Shanghai Rex Xinjing Instrument Co. Ltd.). Firstly, silver carp bone particles were put into the beakers containing deionized water at 60 °C for 60 min. Then the beakers were put in water bath to keep temperature constant during the experiment. Finally, the electric conductivity of powder was measured. 2.1. Determination of the powder yield The powder yield (PY) is an important index to measure the effects of superfine grinding, and was calculated using the following formulae:
Powder yield ðPYÞ ¼
W1 100% W2
ð1Þ
where W1 was the mass of the particle after superfine grinding and W2 was the mass of the material. All measurements were performed in triplicates. 2.2. Determination of protein content Protein content of the silver carp bones powder was determined using the Micro-Kjeldahl method (Ballentine and Gregg, 1947). 2.3. Test procedure for water holding capacity This parameter was determined according to Zhang et al. (2005). Firstly, the weights of cleaned centrifuge tubes (M) and the different particle size samples (Mi) were measured. Then the samples (Ms) were dispersed in water (Ma) according to Ms:Ma = 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 and 50 min separately and then they were placed in cold water for 30 min, followed by centrifugation for 20 min at 4500 rpm. The supernatant was removed and the centrifuge tubes with the powders (Mii) were weighed again. Water holding capacity (WHC) was calculated using the following equation:
WHC ðg=gÞ ¼
M ii M i M Mi
ð2Þ
Fig. 1. Schematic diagram of the superfine grinding system. (1) Pressure regulating valve, (2) Pressure gauges, (3) Mounting base, (4) Vibrating feeder controller, (5) Vibrational feeding machine, (6) Air pipeline, (7) Feed port, (8) Grinder, (9) Discharge port, (10) Receiver bag, (11) vibrating groove.
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2.4. Bulk density
4. Results and discussion
Bulk density was determined according to the method reported by Bai and Li (2006) with little modifications. The bulk density (kg/ m3) was included pores and interparticle voids were the density. Firstly, the weights of cleaned measuring cylinder (G1) were gauged. Then the different particle size samples were added in 10 ml measuring cylinder until they reached the 10 ml mark and were measured (G2). The bulk density of silver carp bone powder was calculated using the following equation:
4.1. Operation parameters of superfine grinding
G2 G1 103 10
ð3Þ
where G1 was the weight of the 10 ml measuring cylinder and G2 was the total weight of silver carp bone powder and 10 ml measuring cylinder. 2.5. Test procedure for the angle of repose The angle of repose (b) is defined as the maximum angle subtended by the surface of a heap of powder against the plane which supports it (Sun et al., 2000; 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 (H) between the paper and the outlet of the filler was 2 cm, and the filler was vertical to the paper. Then 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 (b) was calculated according to the following equation:
b ¼ arctan ðH=RÞ
ð4Þ
2.6. Test procedure for solubility of protein The powder which was coarse milled screened through different sieves into fractions: >500 lm, 300–500 lm, 150–300 lm, 75–150 lm. Different particle sized samples (W1) were accurately weighed, and put into the previously weighed tubes then mixed with exactly 20 ml distilled water and shaken well. The tubes were weighed again. The tubes were then reweighed and placed in a water bath at 94–100 °C, for different time intervals of 10, 20, 40, 60 and 100 min. After that, the tubes were cooled and weighed again. The lost water in the tubes during heating in the water bath was compensated by adding water, and it was put aside for a while. Then, the top portion of liquid was centrifuged at 4500 rpm for 20 min. Four milliliters of supernatant liquid was mixed with 1 ml of biuret reagent and led to stand for 45 min. Similarly, blank was prepared by mixing 4 ml distilled water with the 1 ml biuret reagent and the mixture allowed to stand for 30 min. The absorbances (Ai) of the reaction mixtures were determined at 540 nm by using a UV–Vis spectrophotometer. For comparison, weight of another superfine powder (W) was subjected to heat treatment for 240 min, and the absorbance (A) was measured using the same method (Zhang et al., 2005). The percentage of solubility of protein (P) was calculated using the following equation:
Ai W P¼ 100 A Wi
ð5Þ
3. Statistical analyzes
4.1.2. Superfine grinding pressures There are two types of the superfine grinding pressures: the feed pressure and the grinding pressure, and they have close relationship with each other. So it was discussed at two conditions: (a) feeding pressure = grinding pressure, (b) (grinding pressure feeding pressure) = 0.15 MPa. When feed particle size is 150–300 lm, feed rate is 0.0325 g/s and there is one grinding pass, different feeding and grinding pressures had different effects on the superfine grinding parameters (Fig. 3(a) and (b)). From Fig. 3(a), it could be seen that as feeding and grinding pressure increased the particle size decreased correspondingly. The superfine powder yield increased at the beginning and soon started to decrease as feeding and grinding pressure started to increase to 1.0 MPa. By analyzing these data, both the appropriate feed pressure and the grinding pressure were set to be equally 0.9 MPa. As shown in Fig. 3(b), the superfine powder yield and final particle size of the powder were increased with increase in both feeding and grinding pressure. Therefore, a grinding pressure of 1 MPa and a feed pressure of 0.85 MPa were selected as the optimum pressures for obtaining superfine ground powder. Compared with Fig. 3(a) and (b), it was observed that the powder yield and the particle size of the feed at grinding pressure of 1.0 MPa and feed pressure of 0.85 MPa were more appropriate than that at grinding pressures of 0.9 MPa. Therefore, it can be concluded that the combination of a grinding pressure of 1.0 MPa and a feed pressure of 0.85 MPa was best.
powder yield (%)
100 96
16
92
12
88
8
84
4 0
80 0.0046
Data was analyzed using SPSS 10.0, and analysis of variance (ANOVA) was conducted. Mean values were considered significantly different when p 6 0.05.
20
particle size (µm)
powder yield(%)
d¼
4.1.1. Feeding rate Primary operational parameters were; 0.8 MPa grinding pressure, 0.8 MPa feed pressure, feeding particle size of 150–300 lm and one grinding pass. It was found that different feed rate have different effects on the superfine grinding (Fig. 2). As shown in Fig. 2, it is clear that both superfine powder yield and the final particle size increased when feeding rate increased. The reason for this may be that when the feeding rate was faster, the silver carp bone seemed to have shorter holding time in the machine, which also led to an increased powder yield. However, the increased feeding rate did not provide enough time to grind and larger particle size powder was produced. Feed velocity of 0.0325 g/s was found to be optimum in order to obtain high powder yield of 92.46% while final average particle size (D50) was still kept small 10.20 lm.
particle size(µm)
732
0.0092
0.0185
0.0325
0.0625
feeding rate(g/s) Fig. 2. Effect of the feeding rate on the yield and final particle size of the powder.
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powder yield (%) 60
particle size (µm)
98
powder yield(%)
90
40
86
30
82
20
78
particle size(µm)
50
94
10
74 70
0 0.6
0.7
0.8
0.9
1
grinding pressure, 0.85 MPa feed pressure and one grinding pass. In the same figure, it can be seen that as the particle size of the feed decreased, the final particle size of the product decreased too. The particle size of the product decreased, while the powder yield increased. These data also revealed that when the particle size of the feed was less than 300 lm, the yield and final particle size of the powder did not have significantly difference with each other. For example, the yield for 150–300 lm final particle size and 7.65 lm particle size the feed was 93.46% while the yield for 75– 150 lm final particle size and 7.168 lm particle size the feed was 94.01%. However, experiments shown that it is difficult to reduce particle size with the ordinary grinding, for instance, reducing it to lower than 150 lm would be difficult. Therefore, 150–300 lm particle size of the feed could be selected as the suitable size by comprehensive analysis.
grinding pressure(Mpa)
(a) feeding pressure=grinding pressure 40
30
90 86
20
82 78
10
74 0
70
grinding pressure (feeding pressure) MPa
(b) grinding pressure-feeding pressure=0.15 Mpa Fig. 3. Effect of the pressure on the yield and final particle size of the powder.
powder yield (%)
100
16
4.2. Silver carp bone powder properties
12
4.2.1. The water holding capacity Effects of particle size and soaking times on the water holding capacity of the powder are shown in Fig. 7. From the figure, we
powder yield(%)
96 92
8 88 4
84 80
particle size(µm)
particle size (µm)
powder yield (%) particle size (µm)
90
0 500
300–500 150–300 75–150
50-75
feeding particle size (µm) Fig. 4. Effect of the feeding particle size on the yield and final particle size of the powder.
8
98
powder yield(%)
powder yield(%)
94
particle size(µm)
particle size (µm)
6
82 4
74 66
2
particle size(µm)
powder yield (%) 98
4.1.4. Superfine grinding pass All other parameters were kept constant, feed velocity of 0.0325 g/s, grinding pressure of 1.0 MPa, feeding pressure of 0.85 MPa and particle size of the feed of 150–300 lm while times of grinding was varied to find its effects on the superfine grinding (Fig. 5). As shown in Fig. 5, the powder yield decreased correspondingly when more times were used to grind the powder. However the increase in the number of grinding times was done, which did not have significant influence on the final particle size of powder. Therefore, not only from the process point of view it is advantageous but also from an economic point of view single grinding pass would be the best choice. Finally, the optimum processing conditions of superfine grinding were: feeding pressure of 0.85 MPa, grinding pressure of 1.0 MPa, feeding rate of 0.0325 g/s, feeding particle size of 150– 300 lm and one pass through the grinder. The distribution of particle size was shown in Fig. 6. The average particle size (D50) of superfine ground silver carp bone powder was 7.65 lm. The particle size was mainly distributed between 6 and 25 lm, and the amount of particle size which was larger than 25 lm was seldom. From the Fig. 6, it is also clear that the distribution curve was smooth, linear, narrow width, and the span of particle size distribution was narrow, which indicated that the distribution of particle size was very uniform.
58 4.1.3. Feeding particle size Particle size of the feeding has an important impact on the yield and the final particle size of the superfine ground powder. Fig. 4 shows that different particle sizes of the feed had different effects on the superfine grinding properties for 0.0325 g/s rate, 1.0 MPa
50
1
2
3
0
grinding passes Fig. 5. Effect of the grinding pass on the yield and final particle size of the powder.
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Fig. 6. The distribution of particle size.
3.6
500µm
water holding capacity (g/g)
3.4 3.2
Table 1 Bulk density, angle of repose and electric conductivity of different particle size.
75–150µm
300–500µm
50-75µm
150–300µm
7.65µm
3 2.8
Silver carp bone powder (lm)
Bulk density ( 103 kg/m3)
Angle of repose (°)
Electric conductivity (mS/cm)
>500 300–500 150–300 75–150 50–75 7.65
0.365 ± 0.005a 0.382 ± 0.002a 0.396 ± 0.003a 0.414 ± 0.002b 0.421 ± 0.005b 0.456 ± 0.004c
50.62 ± 0.23d 48.24 ± 0.31c 47.76 ± 0.19c 45.53 ± 0.25bc 44.55 ± 0.25b 41.76 ± 0.14a
0.087 ± 0.002a 0.126 ± 0.001b 0.137 ± 0.005b 0.208 ± 0.004c 0.221 ± 0.003c 0.316 ± 0.002d
Note: Samples in same column with different letter differ significantly at p 6 0.05.
2.6 2.4 2.2 2 10
20
30
40
50
Time min Fig. 7. Effect of particle size and soaking time on the water holding capacity of the powder.
could see that the water holding capacity was affected by the different particle sizes of powders. It is clear that the water holding capacity of different particle sizes of silver carp bone powders slowly increased before 20 min. This might be due to the dissolution of the soluble components in silver carp before 20 min. However, the values of water holding capacity drastically increased after 20 min, especially for the smaller sizes of silver carp bone powder particles. There is an interesting phenomenon that the superfine ground powder of 7.65 lm final particle size, had a similar water holding capacity with other powders during soaking for 10 min, but after 10 min the water holding capacity increased with a decrease in powder particle size. This might be due to the fact that the surface properties of powder had changed drastically after superfine grinding, such as an increase in surface area and energy. These results indicated that the superfine ground powder of silver carp bone could have high solubility and high water retaining capacity. 4.2.2. The bulk density The different particle size powders had different bulk density. The bulk densities of the silver carp bone powders varied with their particle sizes (Table 1). From the table, it can be observed that the bulk densities of the silver carp bone fractions ranged from 0.365 103 kg/m3 to 0.456 103 kg/m3, and the bulk density increased as the particle size decreased. The results confirmed the significant effect of the particle size on the bulk density
(p 6 0.05). The reason for this might be that, as particle size decreased, pore spaces between particles decrease leading to increase in the bulk density (Zhao et al., 2010; Wang et al., 2004). The silver carp bone particle with the higher bulk density is advantageous in filling capsule products or preparing tablets (Costa et al., 2004). 4.2.3. The angle of repose The angle of repose is very important as it can be used in order to estimate the change in the fluidity of the powder. If the angle of repose values of powder was low then it had good fluidity (Zhao et al., 2009, 2010). The results for angle of repose are shown in Table 1. From the table, we could find that the larger the powder particle, the bigger the angle of repose. The angle of repose ranged from 50.62° (>500 lm) to 41.76° (7.65 lm), respectively (Table 1). The angle of repose of the silver carp bone powders varied significantly (p 6 0.05). Compared with the large particle sizes of the powders which had large angle of repose, the superfine ground powder had lower angle of repose, hence better fluidity and higher surface attachment of the powder (Ileleji and Zhou, 2008). Therefore, the qualities of the superfine ground powder are better, and their mixtures with solvents would be more stable. 4.2.4. The solubility of protein The chemical composition separation of the granulometric fractions could be changed by the superfine milling (Maarou et al., 2000). The content of protein of the silver carp bone powder was 25.98%. Protein solubility change with time and different sizes of bone particles was shown in Fig. 8. When the final particle size of the powder was the smallest, the solubility of protein was the highest. It can also be seen from the figure that the solubility of protein increased with the soaking time though not quite obvious. And different particle sizes powders had different protein solubility. The superfine particle size of 7.65 lm had the highest solubility and solubility of protein reached 84.78% after 40 min. However, the particle size of 50–75 lm did not attain the same solubility even after 100 min. The results indicated that the superfine ground silver carp bone powder could increase protein solubility. Therefore,
G.-c. Wu et al. / Journal of Food Engineering 109 (2012) 730–735
Acknowledgments
100
Protein solubility proportion(%)
735
The authors express their appreciation to China High-Tech (863) Plan for supporting our research work under contract No. 2011AA100802. The authors also thank Jiangsu Sanshui Food Company, China for supplying the testing materials and related services.
80
60
References
40
500µm
150–300µm
50-75µm
300–500µm
75–150µm
7.65µm
20
0 10
20
40
60
100
Time min Fig. 8. The change of the solubility of protein of different sized particles with time.
the main factor that affects the solubility of protein was found to be final particle size of the powder. 4.2.5. The electric conductivity Effect of particle size on the electric conductivity of the powder is shown in Table 1. From the table, we could see that the electric conductivity was affected by the different particle sizes of powders. When the final particle size of the powder was the smallest, the electric conductivity was the strongest. The electric conductivity ranged from 0.087 to 0.316 mS/cm, respectively (Table 1). The electric conductivity of the silver carp bone powders varied significantly (p 6 0.05). Compared with the large powder particles, the superfine ground powders were more easily dissolved in water. So the electric conductivity of superfine ground powders was stronger than the others. The reason is that the surface area of superfine ground powders increased, which made powders were more easily dissolved in water (Wang et al., 2004). 5. Conclusions Suitable processing conditions for superfine grinding were determined in this study. The optimum processing conditions of superfine grinding were: feeding pressure of 0.85 MPa, grinding pressure of 1.0 MPa, feeding rate of 0.0325 g/s, feeding particle size of 150–300 lm and one pass through the grinder. And superfine grinding of silver carp bone had many significance characteristics: the surface area of the superfine powder was increased after superfine grinding, and high fluidity, high water holding capacity, high solubility of protein and high electric conductivity. Therefore, it could be used in snack and convenient foods.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Production of silver carp bone powder using superfine grinding technology: Suitable production parameters and its properties Gang-cheng Wu a, Min Zhang a,⇑, Ying-qiang Wang a, Kebitsamang Joseph Mothibe a, Wei-xing Chen b a b
State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China Jiangsu Shanshui Food Company, Baoying 225800, China
a r t i c l e
i n f o
Article history: Received 23 September 2011 Received in revised form 11 November 2011 Accepted 12 November 2011 Available online 28 November 2011 Keywords: Silver carp bone Superfine grinding Powder properties
a b s t r a c t Superfine grinding technology is a new type of food processing that is employed to produce powders with outstanding properties such as high solubility, dispersion, adsorption, chemical reactivity and fluidity. The objective of this work was to study the superfine grinding technology on the silver carp bone and to optimize the operational parameters. The optimum processing conditions of superfine grinding were: feeding pressure of 0.85 MPa, grinding pressure of 1.0 MPa, feeding rate of 0.0325 g/s, feeding particle size of 150–300 lm and one pass through the grinder. Silver carp bone powder with average particle size (D50) of 7.65 lm was produced at these conditions. It was found that the smaller particle size of the powder, the higher its fluidity, solubility, solubility of protein, electric conductivity and water holding capacity of the powder were. So the quality of the products produced by the superfine grinding technology was said to be improved. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The silver carp (Hypophthalmichthys molitrix) is a species of freshwater cyprinid fish which is a variety of Asian carp common in north and northeast Asia. The silver carp is rich in minerals, amino acids, eicosapentaenoic acid and docosahexaenoic acid (Jin and Li, 1998). It is processed into various kinds of food products such as; fish chips, stewed fish frozen surimi and many others. During the fish-processing process, silver carp for its preparation is required to remove bones. However, silver carp bones are said to be fried and eaten in some areas of China but this still leaves lot of them wasted. They may cause environmental pollution if discarded, apart from that it is waste of resource (Takeshi and Nobutaka, 2000; Hammoumi et al., 1998). Many studies have shown that silver carp bones are rich in minerals, protein and amino acids (Chen et al., 2006). Therefore, the utilization of silver carp bones such as grinding it into powder and using it in feed as well as in nutritive foods for humans is really necessary. For production high quality powders, high technology processing techniques are required. Superfine grinding technology is a new technology that is a useful tool for making superfine powder with good surface properties like dispersibility and solubility (Tkacova and Stevulova, 1998). Compared with coarse particles, the surface of superfine powder ⇑ Corresponding author. Address: School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu Province, China. Fax: +86 (0)510 85807976. E-mail address: [email protected] (M. Zhang). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.11.013
possessed a series of important characteristics, such as surface effect, quantum effect, macro-quantum channel effect, mini-size effect, optical property, magnetic property, mechanical property, chemical and catalytic property (Zhao et al., 2010). To date, the superfine grinding technology is widely used in metallic materials and medicines (Hoeffl, 1987; Hua, 2000; Schmidt, 1989; Song et al., 2002; Yoshizawa and Hiroshi, 1991). Zhang (1998) found that the superfine grinding had the functions to decrease particle size and improve reactive surface to the greatest extent possible, and it had been considered to be less energy consuming than traditional mechanical grinding with respect to the increase of surface area. It was also found that this technique may also reduce the leaching of copper from conventional brass matrices and reduce the dissolution of tin in the packaging industries. Han et al. (2000) studied the use of ultrafine silk powder in cosmetic production and found results showed that the ultrafine powder can be absorbed well by the surface cells of the body. Nowadays, superfine grinding technology has also been applied in biotechnology and foodstuffs and has shown a high potential for many other commercial applications. Zhao et al. (2010) studied on properties of superfine ground powder of Astragalus membranaceus and reported that the superfine ground Astragalus membranaceus powder had high water holding capacity, high fluidity, high water solubility index and high protein solubility. Jin and Chen (2006) reported that superfine grinding was a good way to fractionate steam-exploded rice straw into easily bio-converted part and difficultly hydrolyzed part. The study on mushroom (Agrocybe chaxingu Huang) grinding has also shown that superfine grinding had many significant improvement in characteristics of
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final product; the area of the superfine powder was increased after superfine grinding and had high fluidity, water holding capacity and solubility (Zhang et al., 2005). Superfine grinding has also increased the specific surface area of the superfine ginger powder and the solubility of the nutritive components, leading to better absorption by the body (Zhao et al., 2009). Nevertheless, so far little information is available on the preparation technology and properties of superfine ground powder of silver carp bone. Therefore, the objective of this study is to investigate the superfine grinding technology on silver carp bone and to optimize various operational parameters: grinding pressure, feed rate, feed particle size and times of grinding. Powder size and yield were discussed in the article. Furthermore, physical–chemical properties of the superfine powder such as fluidity, water holding capacity, bulk density and protein solubility of different sized bone particles were also studied and compared to conventional grinding powders in the paper. 2. Materials and methods Silver carps were obtained from the local supermarket of Wuxi City, Jiangshu Province, China. They were first thoroughly cleaned with water, removing innards and scales on the surface. Silver carp were then boiled for 10 min to make bones be easily dissociated from the flesh. Excess water in the dissociated bone was drained off using a net. Subsequently, the bones were placed in a mechanical drier at 50 °C and dried until their water content was less than 9% for about 10 h. The water content was determined by the standard oven method (Wu et al., 2010). Dried bones were subjected to coarse milling using an A11-analysis grinding machine (IKA Works Guangzhou, Guangzhou, China), and the coarse bone powder screened through different sized sieves. Finally, the uniform coarse particles were obtained. The uniform coarse particles were ground in an AO table type micronizer (Qingxin Powder Machinery Company, Yixing, Jiangsu, China), Fig. 1. This type of micronizer is jet mill. The operating principle of jet mill is to make materials impacting, colliding and friction by the agitation of high-speed gas or steam. In process optimization three important parameters (feeding rate, grinding pressure and feeding particle size) were taken into consideration. Firstly, uniform coarse bone particles were put into vibrating groove, where the feeding rate was controlled by vibration of the machine. Secondly, bone particles were fed to feed port by control-
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ling the vibrating groove. Finally, the superfine grinding powders were obtained. The average particle size (D50) was tested using a laser particle size instrument (LMS-24, Japan). The electric conductivity of powder was also tested by conductivity meter (DDSC-11T, Shanghai Rex Xinjing Instrument Co. Ltd.). Firstly, silver carp bone particles were put into the beakers containing deionized water at 60 °C for 60 min. Then the beakers were put in water bath to keep temperature constant during the experiment. Finally, the electric conductivity of powder was measured. 2.1. Determination of the powder yield The powder yield (PY) is an important index to measure the effects of superfine grinding, and was calculated using the following formulae:
Powder yield ðPYÞ ¼
W1 100% W2
ð1Þ
where W1 was the mass of the particle after superfine grinding and W2 was the mass of the material. All measurements were performed in triplicates. 2.2. Determination of protein content Protein content of the silver carp bones powder was determined using the Micro-Kjeldahl method (Ballentine and Gregg, 1947). 2.3. Test procedure for water holding capacity This parameter was determined according to Zhang et al. (2005). Firstly, the weights of cleaned centrifuge tubes (M) and the different particle size samples (Mi) were measured. Then the samples (Ms) were dispersed in water (Ma) according to Ms:Ma = 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 and 50 min separately and then they were placed in cold water for 30 min, followed by centrifugation for 20 min at 4500 rpm. The supernatant was removed and the centrifuge tubes with the powders (Mii) were weighed again. Water holding capacity (WHC) was calculated using the following equation:
WHC ðg=gÞ ¼
M ii M i M Mi
ð2Þ
Fig. 1. Schematic diagram of the superfine grinding system. (1) Pressure regulating valve, (2) Pressure gauges, (3) Mounting base, (4) Vibrating feeder controller, (5) Vibrational feeding machine, (6) Air pipeline, (7) Feed port, (8) Grinder, (9) Discharge port, (10) Receiver bag, (11) vibrating groove.
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2.4. Bulk density
4. Results and discussion
Bulk density was determined according to the method reported by Bai and Li (2006) with little modifications. The bulk density (kg/ m3) was included pores and interparticle voids were the density. Firstly, the weights of cleaned measuring cylinder (G1) were gauged. Then the different particle size samples were added in 10 ml measuring cylinder until they reached the 10 ml mark and were measured (G2). The bulk density of silver carp bone powder was calculated using the following equation:
4.1. Operation parameters of superfine grinding
G2 G1 103 10
ð3Þ
where G1 was the weight of the 10 ml measuring cylinder and G2 was the total weight of silver carp bone powder and 10 ml measuring cylinder. 2.5. Test procedure for the angle of repose The angle of repose (b) is defined as the maximum angle subtended by the surface of a heap of powder against the plane which supports it (Sun et al., 2000; 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 (H) between the paper and the outlet of the filler was 2 cm, and the filler was vertical to the paper. Then 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 (b) was calculated according to the following equation:
b ¼ arctan ðH=RÞ
ð4Þ
2.6. Test procedure for solubility of protein The powder which was coarse milled screened through different sieves into fractions: >500 lm, 300–500 lm, 150–300 lm, 75–150 lm. Different particle sized samples (W1) were accurately weighed, and put into the previously weighed tubes then mixed with exactly 20 ml distilled water and shaken well. The tubes were weighed again. The tubes were then reweighed and placed in a water bath at 94–100 °C, for different time intervals of 10, 20, 40, 60 and 100 min. After that, the tubes were cooled and weighed again. The lost water in the tubes during heating in the water bath was compensated by adding water, and it was put aside for a while. Then, the top portion of liquid was centrifuged at 4500 rpm for 20 min. Four milliliters of supernatant liquid was mixed with 1 ml of biuret reagent and led to stand for 45 min. Similarly, blank was prepared by mixing 4 ml distilled water with the 1 ml biuret reagent and the mixture allowed to stand for 30 min. The absorbances (Ai) of the reaction mixtures were determined at 540 nm by using a UV–Vis spectrophotometer. For comparison, weight of another superfine powder (W) was subjected to heat treatment for 240 min, and the absorbance (A) was measured using the same method (Zhang et al., 2005). The percentage of solubility of protein (P) was calculated using the following equation:
Ai W P¼ 100 A Wi
ð5Þ
3. Statistical analyzes
4.1.2. Superfine grinding pressures There are two types of the superfine grinding pressures: the feed pressure and the grinding pressure, and they have close relationship with each other. So it was discussed at two conditions: (a) feeding pressure = grinding pressure, (b) (grinding pressure feeding pressure) = 0.15 MPa. When feed particle size is 150–300 lm, feed rate is 0.0325 g/s and there is one grinding pass, different feeding and grinding pressures had different effects on the superfine grinding parameters (Fig. 3(a) and (b)). From Fig. 3(a), it could be seen that as feeding and grinding pressure increased the particle size decreased correspondingly. The superfine powder yield increased at the beginning and soon started to decrease as feeding and grinding pressure started to increase to 1.0 MPa. By analyzing these data, both the appropriate feed pressure and the grinding pressure were set to be equally 0.9 MPa. As shown in Fig. 3(b), the superfine powder yield and final particle size of the powder were increased with increase in both feeding and grinding pressure. Therefore, a grinding pressure of 1 MPa and a feed pressure of 0.85 MPa were selected as the optimum pressures for obtaining superfine ground powder. Compared with Fig. 3(a) and (b), it was observed that the powder yield and the particle size of the feed at grinding pressure of 1.0 MPa and feed pressure of 0.85 MPa were more appropriate than that at grinding pressures of 0.9 MPa. Therefore, it can be concluded that the combination of a grinding pressure of 1.0 MPa and a feed pressure of 0.85 MPa was best.
powder yield (%)
100 96
16
92
12
88
8
84
4 0
80 0.0046
Data was analyzed using SPSS 10.0, and analysis of variance (ANOVA) was conducted. Mean values were considered significantly different when p 6 0.05.
20
particle size (µm)
powder yield(%)
d¼
4.1.1. Feeding rate Primary operational parameters were; 0.8 MPa grinding pressure, 0.8 MPa feed pressure, feeding particle size of 150–300 lm and one grinding pass. It was found that different feed rate have different effects on the superfine grinding (Fig. 2). As shown in Fig. 2, it is clear that both superfine powder yield and the final particle size increased when feeding rate increased. The reason for this may be that when the feeding rate was faster, the silver carp bone seemed to have shorter holding time in the machine, which also led to an increased powder yield. However, the increased feeding rate did not provide enough time to grind and larger particle size powder was produced. Feed velocity of 0.0325 g/s was found to be optimum in order to obtain high powder yield of 92.46% while final average particle size (D50) was still kept small 10.20 lm.
particle size(µm)
732
0.0092
0.0185
0.0325
0.0625
feeding rate(g/s) Fig. 2. Effect of the feeding rate on the yield and final particle size of the powder.
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powder yield (%) 60
particle size (µm)
98
powder yield(%)
90
40
86
30
82
20
78
particle size(µm)
50
94
10
74 70
0 0.6
0.7
0.8
0.9
1
grinding pressure, 0.85 MPa feed pressure and one grinding pass. In the same figure, it can be seen that as the particle size of the feed decreased, the final particle size of the product decreased too. The particle size of the product decreased, while the powder yield increased. These data also revealed that when the particle size of the feed was less than 300 lm, the yield and final particle size of the powder did not have significantly difference with each other. For example, the yield for 150–300 lm final particle size and 7.65 lm particle size the feed was 93.46% while the yield for 75– 150 lm final particle size and 7.168 lm particle size the feed was 94.01%. However, experiments shown that it is difficult to reduce particle size with the ordinary grinding, for instance, reducing it to lower than 150 lm would be difficult. Therefore, 150–300 lm particle size of the feed could be selected as the suitable size by comprehensive analysis.
grinding pressure(Mpa)
(a) feeding pressure=grinding pressure 40
30
90 86
20
82 78
10
74 0
70
grinding pressure (feeding pressure) MPa
(b) grinding pressure-feeding pressure=0.15 Mpa Fig. 3. Effect of the pressure on the yield and final particle size of the powder.
powder yield (%)
100
16
4.2. Silver carp bone powder properties
12
4.2.1. The water holding capacity Effects of particle size and soaking times on the water holding capacity of the powder are shown in Fig. 7. From the figure, we
powder yield(%)
96 92
8 88 4
84 80
particle size(µm)
particle size (µm)
powder yield (%) particle size (µm)
90
0 500
300–500 150–300 75–150
50-75
feeding particle size (µm) Fig. 4. Effect of the feeding particle size on the yield and final particle size of the powder.
8
98
powder yield(%)
powder yield(%)
94
particle size(µm)
particle size (µm)
6
82 4
74 66
2
particle size(µm)
powder yield (%) 98
4.1.4. Superfine grinding pass All other parameters were kept constant, feed velocity of 0.0325 g/s, grinding pressure of 1.0 MPa, feeding pressure of 0.85 MPa and particle size of the feed of 150–300 lm while times of grinding was varied to find its effects on the superfine grinding (Fig. 5). As shown in Fig. 5, the powder yield decreased correspondingly when more times were used to grind the powder. However the increase in the number of grinding times was done, which did not have significant influence on the final particle size of powder. Therefore, not only from the process point of view it is advantageous but also from an economic point of view single grinding pass would be the best choice. Finally, the optimum processing conditions of superfine grinding were: feeding pressure of 0.85 MPa, grinding pressure of 1.0 MPa, feeding rate of 0.0325 g/s, feeding particle size of 150– 300 lm and one pass through the grinder. The distribution of particle size was shown in Fig. 6. The average particle size (D50) of superfine ground silver carp bone powder was 7.65 lm. The particle size was mainly distributed between 6 and 25 lm, and the amount of particle size which was larger than 25 lm was seldom. From the Fig. 6, it is also clear that the distribution curve was smooth, linear, narrow width, and the span of particle size distribution was narrow, which indicated that the distribution of particle size was very uniform.
58 4.1.3. Feeding particle size Particle size of the feeding has an important impact on the yield and the final particle size of the superfine ground powder. Fig. 4 shows that different particle sizes of the feed had different effects on the superfine grinding properties for 0.0325 g/s rate, 1.0 MPa
50
1
2
3
0
grinding passes Fig. 5. Effect of the grinding pass on the yield and final particle size of the powder.
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Fig. 6. The distribution of particle size.
3.6
500µm
water holding capacity (g/g)
3.4 3.2
Table 1 Bulk density, angle of repose and electric conductivity of different particle size.
75–150µm
300–500µm
50-75µm
150–300µm
7.65µm
3 2.8
Silver carp bone powder (lm)
Bulk density ( 103 kg/m3)
Angle of repose (°)
Electric conductivity (mS/cm)
>500 300–500 150–300 75–150 50–75 7.65
0.365 ± 0.005a 0.382 ± 0.002a 0.396 ± 0.003a 0.414 ± 0.002b 0.421 ± 0.005b 0.456 ± 0.004c
50.62 ± 0.23d 48.24 ± 0.31c 47.76 ± 0.19c 45.53 ± 0.25bc 44.55 ± 0.25b 41.76 ± 0.14a
0.087 ± 0.002a 0.126 ± 0.001b 0.137 ± 0.005b 0.208 ± 0.004c 0.221 ± 0.003c 0.316 ± 0.002d
Note: Samples in same column with different letter differ significantly at p 6 0.05.
2.6 2.4 2.2 2 10
20
30
40
50
Time min Fig. 7. Effect of particle size and soaking time on the water holding capacity of the powder.
could see that the water holding capacity was affected by the different particle sizes of powders. It is clear that the water holding capacity of different particle sizes of silver carp bone powders slowly increased before 20 min. This might be due to the dissolution of the soluble components in silver carp before 20 min. However, the values of water holding capacity drastically increased after 20 min, especially for the smaller sizes of silver carp bone powder particles. There is an interesting phenomenon that the superfine ground powder of 7.65 lm final particle size, had a similar water holding capacity with other powders during soaking for 10 min, but after 10 min the water holding capacity increased with a decrease in powder particle size. This might be due to the fact that the surface properties of powder had changed drastically after superfine grinding, such as an increase in surface area and energy. These results indicated that the superfine ground powder of silver carp bone could have high solubility and high water retaining capacity. 4.2.2. The bulk density The different particle size powders had different bulk density. The bulk densities of the silver carp bone powders varied with their particle sizes (Table 1). From the table, it can be observed that the bulk densities of the silver carp bone fractions ranged from 0.365 103 kg/m3 to 0.456 103 kg/m3, and the bulk density increased as the particle size decreased. The results confirmed the significant effect of the particle size on the bulk density
(p 6 0.05). The reason for this might be that, as particle size decreased, pore spaces between particles decrease leading to increase in the bulk density (Zhao et al., 2010; Wang et al., 2004). The silver carp bone particle with the higher bulk density is advantageous in filling capsule products or preparing tablets (Costa et al., 2004). 4.2.3. The angle of repose The angle of repose is very important as it can be used in order to estimate the change in the fluidity of the powder. If the angle of repose values of powder was low then it had good fluidity (Zhao et al., 2009, 2010). The results for angle of repose are shown in Table 1. From the table, we could find that the larger the powder particle, the bigger the angle of repose. The angle of repose ranged from 50.62° (>500 lm) to 41.76° (7.65 lm), respectively (Table 1). The angle of repose of the silver carp bone powders varied significantly (p 6 0.05). Compared with the large particle sizes of the powders which had large angle of repose, the superfine ground powder had lower angle of repose, hence better fluidity and higher surface attachment of the powder (Ileleji and Zhou, 2008). Therefore, the qualities of the superfine ground powder are better, and their mixtures with solvents would be more stable. 4.2.4. The solubility of protein The chemical composition separation of the granulometric fractions could be changed by the superfine milling (Maarou et al., 2000). The content of protein of the silver carp bone powder was 25.98%. Protein solubility change with time and different sizes of bone particles was shown in Fig. 8. When the final particle size of the powder was the smallest, the solubility of protein was the highest. It can also be seen from the figure that the solubility of protein increased with the soaking time though not quite obvious. And different particle sizes powders had different protein solubility. The superfine particle size of 7.65 lm had the highest solubility and solubility of protein reached 84.78% after 40 min. However, the particle size of 50–75 lm did not attain the same solubility even after 100 min. The results indicated that the superfine ground silver carp bone powder could increase protein solubility. Therefore,
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Acknowledgments
100
Protein solubility proportion(%)
735
The authors express their appreciation to China High-Tech (863) Plan for supporting our research work under contract No. 2011AA100802. The authors also thank Jiangsu Sanshui Food Company, China for supplying the testing materials and related services.
80
60
References
40
500µm
150–300µm
50-75µm
300–500µm
75–150µm
7.65µm
20
0 10
20
40
60
100
Time min Fig. 8. The change of the solubility of protein of different sized particles with time.
the main factor that affects the solubility of protein was found to be final particle size of the powder. 4.2.5. The electric conductivity Effect of particle size on the electric conductivity of the powder is shown in Table 1. From the table, we could see that the electric conductivity was affected by the different particle sizes of powders. When the final particle size of the powder was the smallest, the electric conductivity was the strongest. The electric conductivity ranged from 0.087 to 0.316 mS/cm, respectively (Table 1). The electric conductivity of the silver carp bone powders varied significantly (p 6 0.05). Compared with the large powder particles, the superfine ground powders were more easily dissolved in water. So the electric conductivity of superfine ground powders was stronger than the others. The reason is that the surface area of superfine ground powders increased, which made powders were more easily dissolved in water (Wang et al., 2004). 5. Conclusions Suitable processing conditions for superfine grinding were determined in this study. The optimum processing conditions of superfine grinding were: feeding pressure of 0.85 MPa, grinding pressure of 1.0 MPa, feeding rate of 0.0325 g/s, feeding particle size of 150–300 lm and one pass through the grinder. And superfine grinding of silver carp bone had many significance characteristics: the surface area of the superfine powder was increased after superfine grinding, and high fluidity, high water holding capacity, high solubility of protein and high electric conductivity. Therefore, it could be used in snack and convenient foods.
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