- Email: [email protected]
Effect of potato starch modification on mechanical parameters and granules morphology
Journal of Food Engineering 102 (2011) 154–162
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of potato starch modification on mechanical parameters and granules morphology Mateusz Stasiak a,⇑, Robert Rusinek a, Marek Molenda a, Józef Fornal b, Wioletta Błaszczak b a b
Institute of Agrophysics, Polish Academy of Sciences, Lublin, Poland Institute of Animal Reproduction and Food Research, Polish Academy of Sciences, Olsztyn, Poland
a r t i c l e
i n f o
Article history: Received 22 February 2010 Received in revised form 5 July 2010 Accepted 24 July 2010 Available online 20 August 2010 Keywords: Food powders Starch Dextrins Potato protein Microstructure Mechanical properties Flowability Shear test
a b s t r a c t Research was conducted to recognise the interrelations between morphology and mechanical properties of powders used in the food industry (dried potato protein, granulated starch, gelatinising starch, potato starch, pudding flour, starch thickener, and six kinds of dextrins: 40% white dextrin, 60% white dextrin, yellow dextrins: S, N, W, WB). Microscopic examination, determination of particle size distribution, particle shape index, as well as direct shear tests were performed. Single particles of dextrins were found to have similar shapes and dimensions. Classification of materials based on the results of mechanical testing was found to be in close agreement with classification based on morphology. The values of unconfined yield strength of dextrins and other materials investigated were found to be close, and characteristic of easy flowing and cohesive materials. Oscillations in the shear stress–strain curves were observed for two experimental materials: potato starch and white dextrin 60%, while those for other experimental materials run smooth. The highest values of unconfined yield strength rc were found in a range from 4.5 to 8 kPa for dried potato protein, at major consolidation stress values of 4 and 10 kPa, respectively. The lowest values of rc, characteristic for easy flowing and free flowing materials, were found for potato starch and granulated potato starch. The results of this study confirmed the relevance of morphology to the mechanical properties of powders. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Parameters of internal friction and flow properties of powders influence handling and processing operations such as discharge from silos and hoppers, transportation, mixing, compaction or packaging (Knowlton et al., 1994; Izli et al., 2009). With increasing scale of industrial operations, there is a growing demand for information about food powder parameters for the design of reliable processes and efficient equipment. The flow characteristics of food powders have recently gained special importance because of their relationship to ex-factory product quality as well as to subsequent handling and on-shelf storage (Molenda and Stasiak, 2002; Domian and Poszytek, 2005). Flows occurring in silos can be described as: mass, funnel, intermediate and mixed flows (Jenike, 1961; Eurocode 1, 2006). During mass flow, the entire powder is in downward motion towards the discharge opening, while in the case of funnel flow, the powder discharges through a flow channel formed within the bulk and no sliding takes place along the wall. A serious industrial problem that may occur during the processing of food powders is cessation of flow. This is usually a result of ratholing or an arch forming across ⇑ Corresponding author. Tel.: +48 081 744 50 61; fax: +48 081 744 50 67. E-mail address: [email protected] (M. Stasiak). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.07.034
the discharge opening, which has strength sufficient to be self-supporting. Jenike (1961) proposed the theory of flow of granular material and methods of determination of material parameters, including the direct shear cell technique, for the determination of powder flow properties. Based on results of two-dimensional stress analysis, Jenike introduced the method of estimation of the minimum hopper opening dimension for mass flow from conical and wedge shaped hoppers. The design requires the determination of the following material characteristics: the flow function FF, the angle of internal friction u, the effective angle of internal friction d and the angle of wall friction uw . The flow function is a plot of unconfined yield strength rc of the powder against major consolidating stress r1, and represents the strength of the consolidated powder that must be surpassed to initiate flow of the powder. Regarding the values of flow function, powders may be characterised as free flowing, easy flowing, cohesive and strong cohesive. Based on linearised flow function, the flow index i is defined as the slope of the flow function (Schwedes, 2002). A typical application of the flow function as a material characteristic in industry is quality assessment of powders (Bell et al., 1994). It has been reported by many researchers that powder morphology strongly influences flow properties of powders. Fitzpatrick et al. (2003) determined flow properties of 13 food powders of various particle sizes, moisture contents, bulk densities and particle
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
densities using an annular shear cell. Based on the results obtained, the authors categorised the materials into groups from easy flowing to very cohesive. These authors concluded that particle size distribution and moisture content markedly influenced flowability, but no strong enough relationship was found to relate the flowability of the food powders solely on the basis of these physical properties. It was also stated that surface forces between particles influenced flowability to a considerable degree. Teunou et al. (1999) reported results of flowability determination in annular shear tester for four food powders and discussed possible relationships between flowability and physical properties and relative humidity of surrounding atmosphere. The authors presented an evaluation of the effect of storage time and consolidation on flowability. All tested food powders demonstrated time-consolidation effect such that their flowability was reduced with an increase in consolidation time. Currently a number of methods and testers are available to determine the strength and flow properties of bulk solids. Choosing the right method for the specific application requires knowledge and some experience in handling bulk materials, as outlined by Schwedes (2002). Most frequently, the flow properties of bulk solids have been determined by performing a shear test following slightly modified procedure proposed by Jenike (1961). As pointed out by industrial practitioners (Carson and Wilms, 2006) many different shear testers have been developed for different applications, but the translational shear tester (Jenike Shear Cell) has proven to be the most versatile for design applications and has become an industrial standard. Carson and Wilms attribute the success of direct shear test to the fact that Jenike proposed consistent theoretical background, the tester and the design procedure. The direct shear testing consists of two stages: consolidation under normal reference pressure and measurement of shear force. According to Schwedes (2002) repeatability of the test results may be attained only if the consolidation is identical (Schwedes, 2002). Application of shear testers for food powders poses specific problems associated with high deformability of particles and strong influence of moisture content on their properties. Ramírez et al. (2009) distinguished two modes of influence of moisture content on mechanical properties of powdered agricultural products: surface moisture content and hygroscopic moisture content. Samples were dried at 105–110 °C to determine the hygroscopic moisture content, while a temperature of 55 °C was used to obtain surface moisture content. These authors also used two methods: direct shear and triaxial compression tests for determination of friction angles. These parameters were found to be higher in the triaxial compression tests and the authors attribute the difference to the different states of stress during consolidation of the samples when using the two methods. Also during deformation the relationship between shear resistance (or internal friction) and inter-particle friction is subtle and non-linear as shown by Tykhoniuk et al. (2007). The bulk of powder showed internal friction even in the case where there was no inter-particle friction due to geometrical interlocking of asperities of particles surfaces. Inter-particle friction was found to affect internal friction only to a certain level, because surpassing the maximum value of coefficient of inter-particle friction resulted in rolling instead of sliding becoming the basic mechanism of shear deformation. Juliano et al. (2006) proposed that sliding of individual particles against each other was a predominant in resistance to shear measured in direct shear test. Based on the examination of food powders these authors found that values of coefficient of friction were statistically independent of pre-consolidation and shear stresses when testing at stress levels from 5 to 16 kPa. The authors stated that the values of coefficient of friction have the potential to be used for bulk characterisation. Starch is one of the major components in our diet and plays an important role in the formulation of food products. Amongst the
155
starches used in the whole world, the share of starch from maize is about 83%, followed by wheat (7.0%), potato (6.0%) and tapioca (4.0%). In the food sector starch is used in about 53% of its total production (sweets – 18%, soft drinks – 11%, other food – 24) whereas in the non-food sector (total share of 46%) 28% is used in paper, cardboard and corrugated board, and 13% in fermentation (Berghtaler, 2004). In Europe, due to climatic conditions, potato and wheat are the main sources of starch. In EU-15 countries, where total starch production was approximately 8.0 106 tons per year, starch from both sources amounted to over 60% of total production (Berghtaler, 2004). Potato starch is widely used not only in its native state, but also after converting to different products of desired physicochemical properties by chemical, physical and enzymatic processes. The potato protein, which is a by-product of starch extraction, is also of interest to the producers due to its biological value. Up to date, a lot of information is available on the chemical nature and physicochemical properties of different starches, but data on their mechanical and flow properties are rather scarce. The objectives of this paper are: (a) to report on the mechanical characteristics and parameters of 12 starch based powders, (b) to characterise the morphology (form and structure) and particle size distribution of the examined materials, and (c) attempt to find possible relationships between morphology and mechanical behaviour of examined materials.
2. Materials and methods The examined materials were potato starch in its native state and modified by physical and/or chemical treatment. Powdered potato protein obtained as by-product of starch extraction was additionally tested. Control (native) starch was at 7% of moisture content, while other materials had moisture content of approximately 15%. Only materials at equilibrium moisture content were investigated because the main interest of the project was to characterise parameters of powders used under industrial conditions of operation. Granulated starch was obtained by heat/moisture treatment; gelling and pudding starches (E 1404) were obtained by oxidation starch thickener (E 1420) by acetylation. White dextrins were produced by heating of dry acidified starch (40 and 60 mean% of average solubility at 20 °C). Yellow dextrins were obtained by dry roasting with the addition of an acid. Their symbols: S, N, W, WB correspond to the Hopler viscosity ranging from 70–100 mPas (yellow dextrin N) to 200–300 mPas (dextrin WB). Dextrins are used not only in the food industry but also in pharmaceutical, paper and iron industries. All examined materials were produced in the starch factory WPPZ SA Lubon´, Poland. Particle size distribution was analysed using Infrared Particle Sizer IPS UA (Kamika, Poland). The method consists of measurement, in continuous and gradual manner, of changes in laser radiation stream scattered by moving particles. The parameters are defined as follows: Dn in lm is the mean diameter for a given amount of particles, Dgeo in lm is the mean geometric diameter; Dmed is the size in lm at which 50% of the sample is smaller and 50% is larger, WK is the shape coefficient which compares two dimensions of a single particle (for the sphere WK = 1) (Kamika – Operator’s Guide, 2009). Microscope investigations were performed using the JSM 5200 microscope at 10 keV. Specimens were placed on double-side adhesive tape mounted on aluminium specimen holders, coated with gold in vacuum evaporator JEOL 400. Direct shear tests were performed in a shear tester using an apparatus 60 mm in diameter. The tests followed Eurocode 1 (2006) standard procedure for consolidation reference stresses rr
156
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
of 4, 6 and 10 kPa and speed of shearing V of 2 mm min 1. For determination of yield locus, values of maximum shear stresses at two levels of consolidating stress rr and ½rr (see Fig. 1) were used, following the recommendations of Eurocode 1. With yield locus determined, Mohr circles were drawn that gave values of unconfined yield strength rc and major consolidating stress r1. The relationship between these two parameters, rc (rl), is termed the flow function, FF (Molenda et al., 2006), which characterises the ability of a powder to flow.
3. Results 3.1. Microstructure and particle size distribution Microscopic images of investigated starch granules and potato protein particles are presented in Figs. 1 and 2. The control (native) starch granules are characterised by bimodal shape, typical for potato starches, and a broad range of diameters. Potato starch taken as control (Fig. 1a) is characterised by granules with smooth surface and diameters ranging from 5 to 70 lm. It is worth mention-
ing that large granules are elongated, whereas the small ones have rather round/circular shapes. The presence of granules with diverse shape and size can strongly influence their physical properties and behaviour during processing. Physical and/or chemical treatment changed the microscope images of starch drastically only in the case of granulated starch (Fig. 1b). Heat-moisture treatment of the granules caused their agglomeration due to partial surface gelatinisation and gluing of granules together. It resulted in the creation of very large irregular particles with lengths of hundreds of micrometers. Chemical modification of native starch did not cause any visible changes in granule morphology. Gelatinising starch (acetylated) (Fig. 1c), as well as pudding (Fig. 1d) and starch thickener (Fig. 1e – both oxidised), preserve natural shape without any visible changes on the granule surface. The starch industry byproduct – potato protein, due to its different chemical nature and processing parameters (acid-thermal coagulation) is characterised by large particles with porous structure (Fig. 1f). In comparison to the very solid structure of granulated starch, potato protein powder seems to be rather fragile. Treatment of starch in order to obtain dextrins of different properties: white dextrins (dry starch/moist steam) and yellow
Fig. 1. Microscope images of native and modified starches and of potato protein.
157
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Fig. 2. Microscope images of white and yellow dextrins.
ones (dry starch/roasting at 180–200 °C), did not markedly change the microscopic appearance of the granules (Fig. 2). Only slight swelling can be observed on single granules (Fig. 2b and c) and limitation of this process seems to be reduced moisture in the processed starch. Instrumental analysis of geometric features of the investigated materials was performed to supplement microscopic examination (Table 1). It was confirmed that the bulk of starch is
characterised by very broad diameter of individual granules, ranging from 5 to over 500 lm, as well as by coefficient of shape varying from 1.0 to 9.38. Granules having shape coefficient of 1.0 were of 5–15 lm diameter, while the WK of larger granules ranged from 2.04 to 2.71 for pudding flour and yellow dextrin WB, respectively (Figs. 1d and 2). The highest values of WB were found in the case of granulated starch and potato protein (Fig. 1b and f), as a result of
Table 1 Particle size analysis of experimental materials. Material
Dried potato protein
Granulated starch
Gelatinising starch
Potato starch
Pudding flour
Starch thickener
Dn (lm) Dgeo (lm) Dmed (lm) WK
41.3 18.7 90.5 9.88
36.6 23.7 44.3 9.38
12.5 9.7 15.5 2.11
13.1 10.0 16.5 2.22
12.1 9.3 15.9 2.04
12.8 9.8 17.1 2.19
Dn (lm) Dgeo (lm) Dmed (lm) WK
40% white dextrin
60% white dextrin
Yellow dextrin S
Yellow dextrin N
Yellow dextrin W
Yellow dextrin WB
12.2 9.3 15.7 2.06
12.8 10.0 17.1 2.25
14.5 10.9 18.6 2.54
13.5 10.0 17.7 2.34
15.2 11.1 19.5 2.61
17.5 12.2 24.9 2.71
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
3.2. Shear stress–strain relationships The relationships between shear stress s and relative displacement Dl/d were obtained from direct shear test results. In the case of control potato starch fluctuations of shear force were observed for all three values of consolidation stress used (Fig. 3a). A similar trend was shown by white dextrins 60 (Fig. 3b). However, the strongest stick–slip effect was observed in the case of white dextrin 60. In this powder, variation in shear stress had amplitude of approximately 3 kPa at a consolidation stress of 10 kPa and the same behaviour was obtained for three replications of the experiment. These results showed that even for materials of almost the same particle size distribution and shape of single granule, the slip–stick effect can occur. In the case of white dextrin 60 oscillations were probably caused by a more irregular shape of larger granules and this could be the reason for cyclic interlocking during shearing. Under the lowest consolidation stress of 4 kPa the slip–stick effect occurred in the case of control starch at relative displacement of approximately 0.03, while for white dextrin 60, at Dl/d of approximately 0.01. Such behaviour was probably a result of higher deformability and lower stiffness of single granules of white dextrin 60. Different results were obtained for other powders investigated (yellow dextrin W is shown as a representative) where the experimental curves took smooth paths (Fig. 3c). Slip–stick effects cause difficulties in the interpretation of results of testing. In the presented paper, frictional parameters of investigated powders in that fluctuations occurred were estimated using the maximum values of shear stress to obtain the worst case values. In terms of the theory of vibration, the measurement system may be treated as a linear system with one degree of freedom, excited by displacement causing an increase in shear force. Its response depends on the ratio of elasticity and damping present in the system and on the velocity of deformation. As shown by Stasiak and Molenda (2004), it is possible to reduce slip–stick effects by changing the stiffness of the measurement equipment. 3.3. Angles of internal friction and flow function Based on the experimental curves, the angle of internal friction (u), effective angle of internal friction (d), and flow functions (FF) of materials were determined. The relationships between the angle of internal friction and consolidation stress r1 are presented in Fig. 4. Values of the angle of internal friction u for the first group of materials were stable in the range of consolidation stress applied. The highest value of angle of internal friction – equal to 43.4° – was obtained for granulated starch at 4 kPa consolidation stress, while the lowest (24.5°) was for pudding flour at the same value of consolidation stress. The relationship between angle of internal friction and consolidation stress for control starch and granulated starch were located approximately 30% above the experimental curves for gelatinising starch, pudding flour and
a) 9 8 Potato starch
Shear stress τ [kPa]
7 6
consolidation 10 kPa
5 4
consolidation 6 kPa
3 2 consolidation 4 kPa
1 0 0.00
0.02
0.04
0.06 Δ l/D
0.08
b) 9
0.10
0.12
consolidation 10 kPa
8
White dextrin 60%
7 Shear stress τ [kPa]
processing parameters and strong agglomeration of individual particles. Variations in geometric parameters Dn, Dgeo and Dmed were also observed, however their values were lower than those that might be expected from the micrographs and earlier publications (Molenda et al., 2006; Singh et al., 2007; Kaur et al., 2004). The probable reason for such differences was in the varied nature of equipment used by different authors. Determination of the diameter in the bulk, based on measurements of thousands or even millions of individual particles, may give more reliable results as compared to microscopic analysis where only hundreds of particles are measured. Another probable reason for the discrepancy may be the use of ultrasonic feeder in Infrared Particle Sizer where ultrasonic waves might break agglomerates of particles, thereby reducing the size.
6 5
consolidation 6 kPa
4 3
consolidation 4 kPa
2 1 0 0.00
0.01
0.02
0.03
0.04 Δl/D
0.05
0.06
0.07
0.08
c) 8 7
Yellow dextrin W consolidation 10 kPa
6 Shear stress τ [kPa]
158
5 4 consolidation 6 kPa
3 consolidation 4 kPa
2 1 0 0.00
0.01
0.02
0.03
0.04 Δl/D
0.05
0.06
0.07
0.08
Fig. 3. Shear stress s – relative displacement Dl relationship of potato starch, white dextrin 60 and yellow dextrin W for normal pressure r of 4, 6 and 10 kPa.
starch thickener. Such behaviour of these two powders is probably a result of their particle size distribution, size of single particles and their shape and roughness. The angle of internal friction for dried potato protein varied from 30.6 for 6 kPa of consolidation stress, to 34.2 for 4 kPa. For this material, standard deviation of the angle of internal friction was found to be higher than for other materials from the first group, and this is the effect of non-replicable structure of the material. Values of angles of internal friction u for dextrins were found to increase in the range of consolidation stress applied. The highest
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Fig. 4. Angle of internal friction for three values of consolidation stress.
value of angle of internal friction – equal to 43.3° – was obtained for white dextrin 60 at consolidation stress of 6 kPa, while the lowest of 21.3° was obtained for white dextrin 60 at 4 kPa consolidation stress. The relationship between angle of internal friction and consolidation stress for white dextrin 60 was located approximately 30% above the experimental curves for other powders at 6 and 10 kPa consolidation stress. In the case of dextrins, standard deviations of the angle of internal friction were found to be nearly equal for all consolidation stresses applied. The relationships between effective angle of internal friction d and consolidation reference stress rr are shown in Fig. 5. The highest values of effective angle of internal friction were obtained for dried potato protein and for granulated starch. The effective angle of internal friction was higher than the angle of internal friction due to slight cohesion observed in the examined materials. The highest value of d of 47.7° was found for white dextrin 60 at 6 kPa consolidation stress, while the lowest effective angle of internal friction of 31.2° was that of yellow dextrin WB at 4 kPa consolidation stress. 3.4. Cohesion Cohesion, an important parameter characterising powders, is the value of shear strength for zero of normal effective stress. Fig. 6 presents the relationships between cohesion and consolidation stress obtained during the experiments. In the case of the first group of materials the highest values of cohesion, in the range from
159
Fig. 5. Effective angle of internal friction of examined materials for three values of consolidation stress.
1.3 to 2.1 kPa, were calculated for dried potato protein. In the case of this material the influence of consolidation stress on cohesion was the strongest. This is probably a result of the state of surface of particles of this material, as shown in Fig. 1f. For other materials the values of cohesion were equal at the two values of consolidation stress (4 and 6 kPa). For all the materials tested, higher values of cohesion were observed at higher values of consolidation stress. The lowest values of cohesion, in the range from 0.35 to 0.9 kPa, were observed in the case of native potato starch. For all the dextrins tested, increase in consolidation stress resulted in an increase in cohesion. The highest values of cohesion were found for white dextrin 40 and yellow dextrin S, in the range from 0.9 to 1.24 kPa and from 0.96 to 1.3 kPa, respectively. In the case of white dextrin 60, strong slip–stick effect was observed, while cohesion ranged from 0.79 to 1.15 kPa.
3.5. Flow function Flow functions of the experimental materials, presented in Fig. 7, were found to be characteristic for easy flowing and cohesive materials. No differences in the values of FF in the case of dextrins were observed, probably because of similar shapes and dimensions of granules. In the first group of materials the highest values of FF were found for dried potato protein (highest WK and Dmed), the lowest for control and granulated potato starch. The reason for the nearly
160
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Fig. 6. Cohesion of examined materials for three values of consolidation stress.
same values of FF for potato starch and granulated potato starch (WK = 9.38, Dmed = 44.3) is breaking of clusters during shearing. 4. Discussion In this study, the influence of microstructure on mechanical parameters of two groups of powders used in food and also other industries was investigated. Knowledge of the values of internal friction and flowability is critical in designing technologies of processing, as well as accompanying equipment. It is well known that mechanical parameters of seemingly the same powders may differ over a wide range; therefore growing interest in their determination has been observed. Data on the properties of powdered potato products – starch and protein – are scarce. That is why two groups of powders, widely used in the food and non-food (paper, pharmaceutical, printing, textile etc.) industries, were chosen as experimental materials. Many authors are of the opinion that chemical modification does not, or only to a small extent, alter starch granule size and shape (Lewandowicz et al., 1998; Fornal et al., 1998; Singh et al., 2007). However, they have reported considerable changes during heat-moisture treatment. According to Bowler the theory of lensshaped starch granules deforming when heated in excess of water was proposed by Bowler et al. (1980) in which several stages of deformations due to radial and tangential swelling were observed using scanning electron microscope. This was also confirmed for other starches, including legume starches (Fornal, 2000). Soral-
Fig. 7. Flow functions of experimental materials.
Smietana et al. (1998) described drastic changes in starch granule morphology that can appear during spray-drying. In the case of the first group of materials (control, granulated, gelatinising starch, pudding flour, and potato protein), two of them were found to be significantly different in terms of the size and shape of single particles. Dried potato protein and granulated starch were found to posses the highest values of Dn, Dgeo, Dmed and WK. Distinct differences were noted for these two materials, caused by their chemical nature and the method of processing, both influencing dramatically the microstructure of the final products and mechanical properties of these powders. It is known that potato protein within the cells exists as minute granular deposits with diameters less than 1 lm, whereas starch granules have much larger size and internal lamellar structure. Both components are very sensitive to acid, moisture and temperature. Potato protein is obtained as a by-product of starch extraction from potato tubers. It is coagulated by acid and temperature, precipitated and dried. During this process small protein structures are clustered, with many air spaces inside. Steam and heat action on native starch granules causes partial gelatinisation and strong (depending on parameters used) adhesion of individual particles into larger agglomerates. This agglomeration influences markedly the mechanical properties of the powders. Both powders discussed above are characterised by significantly higher values of angle of internal friction and effective angle of internal friction. In the case of dried potato protein, cohesion was significantly higher than for other materials. The microstructure and shape of single granules also influenced the values of FF. In this case the material with coarse surface and the highest value of D – dried potato protein – had the highest values of the
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
flow function. This result proved that the state of the surface had a critical influence on flowability; even the powder with the largest granules, but with a rough and porous surface, had poor flowability. The values of FF of granulated starch were close to the values of FF of other modified starches. This may be a result of decomposition of agglomerated particles into smaller subunits or even to single granules during the shear test. Strong influence of particle size distribution on flowability and cohesion at low moisture content of wheat powders was observed by Landillon et al. (2008). Mathlouthi and Roge (2003) examined flowability of sugar of particle diameters from 50 lm to 1 mm in direct shear test and found a decrease in flowability with reduction in particle size. The flow index of the fraction of the highest diameter was 0.032, whereas for the fine fraction it was of 0.265, which classified the material as cohesive. Different behaviour of dried potato protein is confirmed by results of Shinohara et al. (2000). Those authors determined the influence of the shape of single granules of rust free steel powder on the angle of internal friction determined in triaxial compression test. Steel powder was treated in a rotational machine to obtain spheres, and then the shape index was determined. The diameter of initial material granules was 68.9 lm. The shape index was found to be in the range from 0.663 for untreated powder to 0.917 for powder with the longest treatment time, whose granules had shapes closest to being spherical. The authors concluded that the angle of internal friction increased with an increase in shape irregularity of single particles, similar to our results. Mullarney et al. (2003) based on testing pharmaceutical sweeteners, arrived at a similar conclusion. Podczeck and Miah (1996) analysed the influence of the particle size, shape and addition of lubricant on the angle of internal friction and flowability of powders and found that surface lubrication changed the non-linear dependence between the angle of internal friction and the size and shape of particles. Liu et al. (2008) proved experimentally and theoretically that flowability of powders is determined by particle size and particle size distributions. For two experimental materials – the control potato starch and white dextrin 60 – the slip–stick effect was observed. In the case of white dextrin 60 the effect occurred even though the geometric parameters Dn, Dgeo, Dmed and WK, determined in infrared particle size analyser, were close to the parameters of other dextrins. For white dextrin 60 the highest value of angle of internal friction (nearly 25%) was found, yet cohesion and flowability were found to be almost the same as for other dextrins. The slip–stick phenomenon can appear during handling of materials and it can cause unforeseeable outage of technological equipment. Sequences of dilations and compactions in the shearing zone are probably the reasons for the slip–stick phenomenon. Densification causes an increase in strength and ability to handle higher shear stress. Upon breakage of strength of material dilatation takes place. Similar slip–stick effects were reported by Stasiak et al. (2008) based on testing corn flour and wheat starch. Forsyth et al. (2002) studied the transition from free-flow to stick–slip using assemblies of glass spheres in humidity controlled air and steel spheres within a magnetic field. It was found in both systems that the transition was governed by a critical ratio of inter-particle force to particle weight. The authors suggested that the observed transition was due to the system achieving a certain critical level of interparticle force and not to an abrupt rise in cohesive force such as this generated by sudden formation of liquid bridges. The analysis has shown important effects influencing shear deformation, but changes in tangent inter-particle force were not considered.
5. Conclusions Chemical modification of potato starch did not influence markedly the morphology of starch granules. Only in the case of steam-
161
heat treatment agglomeration of individual granules was found. Heat treatment of dry starch during dextrinisation had only a limited influence on granule morphology, however differences in granule size were observed. Dried potato protein, due to differences in the single particle diameter and processing parameters, differed significantly from starch products. Irregular shape of particles led to an increase in the angle of internal friction up to 43.4° of granulated starch. Rough and porous surface of potato protein powder and granulated starch were responsible for the highest values about 45° of effective angle of internal friction. Influence of particle morphology on cohesion was observed. The highest were values of cohesion of dried potato protein which surface is rough. The highest values of cohesion were observed at the highest value of consolidation stress for all examined materials. In the case of the first group of materials the highest values of cohesion, in the range from 1.3 to 2.1 kPa, were calculated for dried potato protein. In the case of this material the influence of consolidation stress on cohesion was the strongest. For other materials the values of cohesion were equal at two values of consolidation stress of 4 and 6 kPa. For some investigated materials (control starch and white dextrin 60) the slip–stick effect was observed. It was connected with the highest angle of internal friction and effective angle of internal friction. No relationship between the slip–stick effect and the size of single particles of powders was found. The investigated materials were characterised by different flowability. The highest values of FF were found for dried potato protein (highest WK and Dmed), the lowest for control starch and granulated potato starch. No differences in FF values between of dextrins were found.
References Bell, A., Ennis, B.J., Grygo, R.J., Scholten, W.J.F., Schenkel, M.M., 1994. Practical evaluation of the Johanson hang-up indicizer. Bulk Solids Handling 14, 117– 125. Berghtaler, W., 2004. The status of wheat starch and gluten production and uses in Europe. In: Tomasik, P., Yuryev, V.P., Bertoft, E. (Eds.), Starch: Progress in Structural Studies, Modifications and Applications. PSFT Malopolska Branch, pp. 417–436. Bowler, P., Williams, M.R., Angold, R.E., 1980. A hypothesis for the morphological changes which occur on heating lenticular wheat starch granules. Starch 32, 186–189. Carson, J.W., Wilms, H., 2006. Development of an international standard for shear testing. Powder Technology 167 (1), 1–9. Domian, E., Poszytek, K., 2005. Wheat flour flowability as affected by water activity, storage time and consolidation. International Agrophysics 19 (2), 119–124. Eurocode 1, Part 4, 2006. Basis of Design and Actions on Structures. Actions in Silos and Tanks. EN 1991-4. Fitzpatrick, J.J., Barringer, S.A., Iqbal, T., 2003. Flow property measurement of food powders and sensitivity of Jenike’s hopper design methodology to the measured values. Journal of Food Engineering 61, 399–405. _ ´ c´ 2 (23), 59– Fornal, J., 2000. Structural properties of starch in food systems. Zywnos 71. Fornal, J., Błaszczak, W., Lewandowicz, G., 1998. Microstructure of starch acetates from different starch sources. Polish Journal of Food and Nutrition Sciences 7/48 (3), 86–95. Forsyth, A.J., Hutton, S., Rhodes, M.J., 2002. Effect of cohesive interparticle force on the flow characteristics of granular material. Powder Technology 126 (2), 150– 154. Izli, N., Unal, H., Sincik, M., 2009. Physical and mechanical properties of rapeseed at different moisture content. International Agrophysics 23, 137–145. Jenike, A.W., 1961. Gravity flow of bulk solids. Bulletin Engineering Experiment Station (52), 29. Utah State University. Juliano, P., Muhunthan, B., Barbosa-Cánovas, G.V., 2006. Flow and shear descriptors of preconsolidated food powders. Journal of Food Engineering 72 (2), 157–166. Kamika Operation Guide, 2009. Kamika Instruments, Warsaw. Kaur, L., Singh, N., Singh, J., 2004. Factors influencing the properties of hydroxypropylated potato starches. Carbohydrate Polymers 55, 211–223. Knowlton, T.M., Carson, J.W., Klinzing, G.E., Yang, Wen-Ching, 1994. The importance of storage, transfer and collection. Chemical Engineering Progress 90 (4), 44–54. Landillon, V., Cassan, D., Morel, M.-H., Cuq, B., 2008. Flowability, cohesive and granulation properties of wheat powders. Journal of Food Engineering 86, 178– 193.
162
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Lewandowicz, G., Błaszczak, W., Fornal, J., 1998. Effect of acetylation on microstructure of potato starch. Polish Journal of Food and Nutrition Sciences 7/78 (2), 78–84. Liu, L.X., Marziano, I., Bentham, A.C., Litster, J.D., White, E.T., Howes, T., 2008. Effect of particle properties on the compaction of ibuprofen powders. International Journal of Pharmaceutics 362, 109–117. Mathlouthi, M., Roge, B., 2003. Water vapour sorption isotherms and the caking of food powders. Food Chemistry 82, 61–71. Molenda, M., Stasiak, M., 2002. Determination of elastic constants of cereal grain in uniaxial compression. International Agrophysics 16 (1), 61–65. Molenda, M., Stasiak, M., Horabik, J., Fornal, J., Błaszczak, W., Ormowski, A., 2006. Microstructure and mechanical parameters of five types of starch. Polish Journal of Food and Nutrition Sciences 15/16 (20), 161–168. Mullarney, M.P., Hancock, B.C., Carlson, G.T., Ladipo, D.D., Langdon, B.A., 2003. The powder flow and compact mechanical properties of sucrose and three highintensity sweeteners used in chewable tablets. International Journal of Pharmaceutics 257, 227–236. Podczeck, F., Miah, Y., 1996. The influence of particle size and shape on the angle of internal friction and flow factor of unlubricated and lubricated powders. International Journal of Pharmaceutics 144, 187–194. Ramírez, A., Moya, M., Ayuga, F., 2009. Determination of the mechanical properties of powdered agricultural products and sugar. Particle and Particle Systems Characterization 26 (4), 220–230.
Schwedes, J., 2002. Consolidation and flow of cohesive bulk solids. Chemical Engineering Science 57 (2), 287–294. Shinohara, K., Oida, M., Golman, B., 2000. Effect of particle shape on angle of internal friction by triaxial compression test. Powder Technology 107, 131–136. Singh, J., Kaur, L., McCarthy, O.J., 2007. Factors influencing the physico-chemical, morphological, thermal and rheological properties of some chemically modified starches for food applications – a review. Food Hydrocolloids 21, 1–22. Soral-Smietana, M., Fornal, J., Wronkowska, M., 1998. Microstructure and functional properties of wheat and potato resistant starch preparations. Polish Journal of Food and Nutrition Sciences 7/48 (3), 79–85. Stasiak, M., Molenda, M., 2004. Direct shear testing of food powders flowability (influence of deformation speed and apparatus stiffnes). Acta Agrophysica 111, 4(2), 557–564. Stasiak, M., Opalinski, I., Molenda, M., 2008. Mass and microscopic properties of food and industry granular materials. I. Comparison of mechanical properties. Przemysł Chemiczny 87 (2), 199–202 (in Polish). Teunou, E., Fitzpatrick, J.J., Synnott, E.C., 1999. Characterization of food powder flowability. Journal of Food Engineering 39, 31–37. Tykhoniuk, R., Tomas, J., Luding, S., Kappl, M., Heim, L., Butt, H.J., 2007. Ultrafine cohesive powders: from interparticle contacts to continuum behaviour. Chemical Engineering Science 62 (11), 2843–2864.
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of potato starch modification on mechanical parameters and granules morphology Mateusz Stasiak a,⇑, Robert Rusinek a, Marek Molenda a, Józef Fornal b, Wioletta Błaszczak b a b
Institute of Agrophysics, Polish Academy of Sciences, Lublin, Poland Institute of Animal Reproduction and Food Research, Polish Academy of Sciences, Olsztyn, Poland
a r t i c l e
i n f o
Article history: Received 22 February 2010 Received in revised form 5 July 2010 Accepted 24 July 2010 Available online 20 August 2010 Keywords: Food powders Starch Dextrins Potato protein Microstructure Mechanical properties Flowability Shear test
a b s t r a c t Research was conducted to recognise the interrelations between morphology and mechanical properties of powders used in the food industry (dried potato protein, granulated starch, gelatinising starch, potato starch, pudding flour, starch thickener, and six kinds of dextrins: 40% white dextrin, 60% white dextrin, yellow dextrins: S, N, W, WB). Microscopic examination, determination of particle size distribution, particle shape index, as well as direct shear tests were performed. Single particles of dextrins were found to have similar shapes and dimensions. Classification of materials based on the results of mechanical testing was found to be in close agreement with classification based on morphology. The values of unconfined yield strength of dextrins and other materials investigated were found to be close, and characteristic of easy flowing and cohesive materials. Oscillations in the shear stress–strain curves were observed for two experimental materials: potato starch and white dextrin 60%, while those for other experimental materials run smooth. The highest values of unconfined yield strength rc were found in a range from 4.5 to 8 kPa for dried potato protein, at major consolidation stress values of 4 and 10 kPa, respectively. The lowest values of rc, characteristic for easy flowing and free flowing materials, were found for potato starch and granulated potato starch. The results of this study confirmed the relevance of morphology to the mechanical properties of powders. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Parameters of internal friction and flow properties of powders influence handling and processing operations such as discharge from silos and hoppers, transportation, mixing, compaction or packaging (Knowlton et al., 1994; Izli et al., 2009). With increasing scale of industrial operations, there is a growing demand for information about food powder parameters for the design of reliable processes and efficient equipment. The flow characteristics of food powders have recently gained special importance because of their relationship to ex-factory product quality as well as to subsequent handling and on-shelf storage (Molenda and Stasiak, 2002; Domian and Poszytek, 2005). Flows occurring in silos can be described as: mass, funnel, intermediate and mixed flows (Jenike, 1961; Eurocode 1, 2006). During mass flow, the entire powder is in downward motion towards the discharge opening, while in the case of funnel flow, the powder discharges through a flow channel formed within the bulk and no sliding takes place along the wall. A serious industrial problem that may occur during the processing of food powders is cessation of flow. This is usually a result of ratholing or an arch forming across ⇑ Corresponding author. Tel.: +48 081 744 50 61; fax: +48 081 744 50 67. E-mail address: [email protected] (M. Stasiak). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.07.034
the discharge opening, which has strength sufficient to be self-supporting. Jenike (1961) proposed the theory of flow of granular material and methods of determination of material parameters, including the direct shear cell technique, for the determination of powder flow properties. Based on results of two-dimensional stress analysis, Jenike introduced the method of estimation of the minimum hopper opening dimension for mass flow from conical and wedge shaped hoppers. The design requires the determination of the following material characteristics: the flow function FF, the angle of internal friction u, the effective angle of internal friction d and the angle of wall friction uw . The flow function is a plot of unconfined yield strength rc of the powder against major consolidating stress r1, and represents the strength of the consolidated powder that must be surpassed to initiate flow of the powder. Regarding the values of flow function, powders may be characterised as free flowing, easy flowing, cohesive and strong cohesive. Based on linearised flow function, the flow index i is defined as the slope of the flow function (Schwedes, 2002). A typical application of the flow function as a material characteristic in industry is quality assessment of powders (Bell et al., 1994). It has been reported by many researchers that powder morphology strongly influences flow properties of powders. Fitzpatrick et al. (2003) determined flow properties of 13 food powders of various particle sizes, moisture contents, bulk densities and particle
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
densities using an annular shear cell. Based on the results obtained, the authors categorised the materials into groups from easy flowing to very cohesive. These authors concluded that particle size distribution and moisture content markedly influenced flowability, but no strong enough relationship was found to relate the flowability of the food powders solely on the basis of these physical properties. It was also stated that surface forces between particles influenced flowability to a considerable degree. Teunou et al. (1999) reported results of flowability determination in annular shear tester for four food powders and discussed possible relationships between flowability and physical properties and relative humidity of surrounding atmosphere. The authors presented an evaluation of the effect of storage time and consolidation on flowability. All tested food powders demonstrated time-consolidation effect such that their flowability was reduced with an increase in consolidation time. Currently a number of methods and testers are available to determine the strength and flow properties of bulk solids. Choosing the right method for the specific application requires knowledge and some experience in handling bulk materials, as outlined by Schwedes (2002). Most frequently, the flow properties of bulk solids have been determined by performing a shear test following slightly modified procedure proposed by Jenike (1961). As pointed out by industrial practitioners (Carson and Wilms, 2006) many different shear testers have been developed for different applications, but the translational shear tester (Jenike Shear Cell) has proven to be the most versatile for design applications and has become an industrial standard. Carson and Wilms attribute the success of direct shear test to the fact that Jenike proposed consistent theoretical background, the tester and the design procedure. The direct shear testing consists of two stages: consolidation under normal reference pressure and measurement of shear force. According to Schwedes (2002) repeatability of the test results may be attained only if the consolidation is identical (Schwedes, 2002). Application of shear testers for food powders poses specific problems associated with high deformability of particles and strong influence of moisture content on their properties. Ramírez et al. (2009) distinguished two modes of influence of moisture content on mechanical properties of powdered agricultural products: surface moisture content and hygroscopic moisture content. Samples were dried at 105–110 °C to determine the hygroscopic moisture content, while a temperature of 55 °C was used to obtain surface moisture content. These authors also used two methods: direct shear and triaxial compression tests for determination of friction angles. These parameters were found to be higher in the triaxial compression tests and the authors attribute the difference to the different states of stress during consolidation of the samples when using the two methods. Also during deformation the relationship between shear resistance (or internal friction) and inter-particle friction is subtle and non-linear as shown by Tykhoniuk et al. (2007). The bulk of powder showed internal friction even in the case where there was no inter-particle friction due to geometrical interlocking of asperities of particles surfaces. Inter-particle friction was found to affect internal friction only to a certain level, because surpassing the maximum value of coefficient of inter-particle friction resulted in rolling instead of sliding becoming the basic mechanism of shear deformation. Juliano et al. (2006) proposed that sliding of individual particles against each other was a predominant in resistance to shear measured in direct shear test. Based on the examination of food powders these authors found that values of coefficient of friction were statistically independent of pre-consolidation and shear stresses when testing at stress levels from 5 to 16 kPa. The authors stated that the values of coefficient of friction have the potential to be used for bulk characterisation. Starch is one of the major components in our diet and plays an important role in the formulation of food products. Amongst the
155
starches used in the whole world, the share of starch from maize is about 83%, followed by wheat (7.0%), potato (6.0%) and tapioca (4.0%). In the food sector starch is used in about 53% of its total production (sweets – 18%, soft drinks – 11%, other food – 24) whereas in the non-food sector (total share of 46%) 28% is used in paper, cardboard and corrugated board, and 13% in fermentation (Berghtaler, 2004). In Europe, due to climatic conditions, potato and wheat are the main sources of starch. In EU-15 countries, where total starch production was approximately 8.0 106 tons per year, starch from both sources amounted to over 60% of total production (Berghtaler, 2004). Potato starch is widely used not only in its native state, but also after converting to different products of desired physicochemical properties by chemical, physical and enzymatic processes. The potato protein, which is a by-product of starch extraction, is also of interest to the producers due to its biological value. Up to date, a lot of information is available on the chemical nature and physicochemical properties of different starches, but data on their mechanical and flow properties are rather scarce. The objectives of this paper are: (a) to report on the mechanical characteristics and parameters of 12 starch based powders, (b) to characterise the morphology (form and structure) and particle size distribution of the examined materials, and (c) attempt to find possible relationships between morphology and mechanical behaviour of examined materials.
2. Materials and methods The examined materials were potato starch in its native state and modified by physical and/or chemical treatment. Powdered potato protein obtained as by-product of starch extraction was additionally tested. Control (native) starch was at 7% of moisture content, while other materials had moisture content of approximately 15%. Only materials at equilibrium moisture content were investigated because the main interest of the project was to characterise parameters of powders used under industrial conditions of operation. Granulated starch was obtained by heat/moisture treatment; gelling and pudding starches (E 1404) were obtained by oxidation starch thickener (E 1420) by acetylation. White dextrins were produced by heating of dry acidified starch (40 and 60 mean% of average solubility at 20 °C). Yellow dextrins were obtained by dry roasting with the addition of an acid. Their symbols: S, N, W, WB correspond to the Hopler viscosity ranging from 70–100 mPas (yellow dextrin N) to 200–300 mPas (dextrin WB). Dextrins are used not only in the food industry but also in pharmaceutical, paper and iron industries. All examined materials were produced in the starch factory WPPZ SA Lubon´, Poland. Particle size distribution was analysed using Infrared Particle Sizer IPS UA (Kamika, Poland). The method consists of measurement, in continuous and gradual manner, of changes in laser radiation stream scattered by moving particles. The parameters are defined as follows: Dn in lm is the mean diameter for a given amount of particles, Dgeo in lm is the mean geometric diameter; Dmed is the size in lm at which 50% of the sample is smaller and 50% is larger, WK is the shape coefficient which compares two dimensions of a single particle (for the sphere WK = 1) (Kamika – Operator’s Guide, 2009). Microscope investigations were performed using the JSM 5200 microscope at 10 keV. Specimens were placed on double-side adhesive tape mounted on aluminium specimen holders, coated with gold in vacuum evaporator JEOL 400. Direct shear tests were performed in a shear tester using an apparatus 60 mm in diameter. The tests followed Eurocode 1 (2006) standard procedure for consolidation reference stresses rr
156
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
of 4, 6 and 10 kPa and speed of shearing V of 2 mm min 1. For determination of yield locus, values of maximum shear stresses at two levels of consolidating stress rr and ½rr (see Fig. 1) were used, following the recommendations of Eurocode 1. With yield locus determined, Mohr circles were drawn that gave values of unconfined yield strength rc and major consolidating stress r1. The relationship between these two parameters, rc (rl), is termed the flow function, FF (Molenda et al., 2006), which characterises the ability of a powder to flow.
3. Results 3.1. Microstructure and particle size distribution Microscopic images of investigated starch granules and potato protein particles are presented in Figs. 1 and 2. The control (native) starch granules are characterised by bimodal shape, typical for potato starches, and a broad range of diameters. Potato starch taken as control (Fig. 1a) is characterised by granules with smooth surface and diameters ranging from 5 to 70 lm. It is worth mention-
ing that large granules are elongated, whereas the small ones have rather round/circular shapes. The presence of granules with diverse shape and size can strongly influence their physical properties and behaviour during processing. Physical and/or chemical treatment changed the microscope images of starch drastically only in the case of granulated starch (Fig. 1b). Heat-moisture treatment of the granules caused their agglomeration due to partial surface gelatinisation and gluing of granules together. It resulted in the creation of very large irregular particles with lengths of hundreds of micrometers. Chemical modification of native starch did not cause any visible changes in granule morphology. Gelatinising starch (acetylated) (Fig. 1c), as well as pudding (Fig. 1d) and starch thickener (Fig. 1e – both oxidised), preserve natural shape without any visible changes on the granule surface. The starch industry byproduct – potato protein, due to its different chemical nature and processing parameters (acid-thermal coagulation) is characterised by large particles with porous structure (Fig. 1f). In comparison to the very solid structure of granulated starch, potato protein powder seems to be rather fragile. Treatment of starch in order to obtain dextrins of different properties: white dextrins (dry starch/moist steam) and yellow
Fig. 1. Microscope images of native and modified starches and of potato protein.
157
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Fig. 2. Microscope images of white and yellow dextrins.
ones (dry starch/roasting at 180–200 °C), did not markedly change the microscopic appearance of the granules (Fig. 2). Only slight swelling can be observed on single granules (Fig. 2b and c) and limitation of this process seems to be reduced moisture in the processed starch. Instrumental analysis of geometric features of the investigated materials was performed to supplement microscopic examination (Table 1). It was confirmed that the bulk of starch is
characterised by very broad diameter of individual granules, ranging from 5 to over 500 lm, as well as by coefficient of shape varying from 1.0 to 9.38. Granules having shape coefficient of 1.0 were of 5–15 lm diameter, while the WK of larger granules ranged from 2.04 to 2.71 for pudding flour and yellow dextrin WB, respectively (Figs. 1d and 2). The highest values of WB were found in the case of granulated starch and potato protein (Fig. 1b and f), as a result of
Table 1 Particle size analysis of experimental materials. Material
Dried potato protein
Granulated starch
Gelatinising starch
Potato starch
Pudding flour
Starch thickener
Dn (lm) Dgeo (lm) Dmed (lm) WK
41.3 18.7 90.5 9.88
36.6 23.7 44.3 9.38
12.5 9.7 15.5 2.11
13.1 10.0 16.5 2.22
12.1 9.3 15.9 2.04
12.8 9.8 17.1 2.19
Dn (lm) Dgeo (lm) Dmed (lm) WK
40% white dextrin
60% white dextrin
Yellow dextrin S
Yellow dextrin N
Yellow dextrin W
Yellow dextrin WB
12.2 9.3 15.7 2.06
12.8 10.0 17.1 2.25
14.5 10.9 18.6 2.54
13.5 10.0 17.7 2.34
15.2 11.1 19.5 2.61
17.5 12.2 24.9 2.71
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
3.2. Shear stress–strain relationships The relationships between shear stress s and relative displacement Dl/d were obtained from direct shear test results. In the case of control potato starch fluctuations of shear force were observed for all three values of consolidation stress used (Fig. 3a). A similar trend was shown by white dextrins 60 (Fig. 3b). However, the strongest stick–slip effect was observed in the case of white dextrin 60. In this powder, variation in shear stress had amplitude of approximately 3 kPa at a consolidation stress of 10 kPa and the same behaviour was obtained for three replications of the experiment. These results showed that even for materials of almost the same particle size distribution and shape of single granule, the slip–stick effect can occur. In the case of white dextrin 60 oscillations were probably caused by a more irregular shape of larger granules and this could be the reason for cyclic interlocking during shearing. Under the lowest consolidation stress of 4 kPa the slip–stick effect occurred in the case of control starch at relative displacement of approximately 0.03, while for white dextrin 60, at Dl/d of approximately 0.01. Such behaviour was probably a result of higher deformability and lower stiffness of single granules of white dextrin 60. Different results were obtained for other powders investigated (yellow dextrin W is shown as a representative) where the experimental curves took smooth paths (Fig. 3c). Slip–stick effects cause difficulties in the interpretation of results of testing. In the presented paper, frictional parameters of investigated powders in that fluctuations occurred were estimated using the maximum values of shear stress to obtain the worst case values. In terms of the theory of vibration, the measurement system may be treated as a linear system with one degree of freedom, excited by displacement causing an increase in shear force. Its response depends on the ratio of elasticity and damping present in the system and on the velocity of deformation. As shown by Stasiak and Molenda (2004), it is possible to reduce slip–stick effects by changing the stiffness of the measurement equipment. 3.3. Angles of internal friction and flow function Based on the experimental curves, the angle of internal friction (u), effective angle of internal friction (d), and flow functions (FF) of materials were determined. The relationships between the angle of internal friction and consolidation stress r1 are presented in Fig. 4. Values of the angle of internal friction u for the first group of materials were stable in the range of consolidation stress applied. The highest value of angle of internal friction – equal to 43.4° – was obtained for granulated starch at 4 kPa consolidation stress, while the lowest (24.5°) was for pudding flour at the same value of consolidation stress. The relationship between angle of internal friction and consolidation stress for control starch and granulated starch were located approximately 30% above the experimental curves for gelatinising starch, pudding flour and
a) 9 8 Potato starch
Shear stress τ [kPa]
7 6
consolidation 10 kPa
5 4
consolidation 6 kPa
3 2 consolidation 4 kPa
1 0 0.00
0.02
0.04
0.06 Δ l/D
0.08
b) 9
0.10
0.12
consolidation 10 kPa
8
White dextrin 60%
7 Shear stress τ [kPa]
processing parameters and strong agglomeration of individual particles. Variations in geometric parameters Dn, Dgeo and Dmed were also observed, however their values were lower than those that might be expected from the micrographs and earlier publications (Molenda et al., 2006; Singh et al., 2007; Kaur et al., 2004). The probable reason for such differences was in the varied nature of equipment used by different authors. Determination of the diameter in the bulk, based on measurements of thousands or even millions of individual particles, may give more reliable results as compared to microscopic analysis where only hundreds of particles are measured. Another probable reason for the discrepancy may be the use of ultrasonic feeder in Infrared Particle Sizer where ultrasonic waves might break agglomerates of particles, thereby reducing the size.
6 5
consolidation 6 kPa
4 3
consolidation 4 kPa
2 1 0 0.00
0.01
0.02
0.03
0.04 Δl/D
0.05
0.06
0.07
0.08
c) 8 7
Yellow dextrin W consolidation 10 kPa
6 Shear stress τ [kPa]
158
5 4 consolidation 6 kPa
3 consolidation 4 kPa
2 1 0 0.00
0.01
0.02
0.03
0.04 Δl/D
0.05
0.06
0.07
0.08
Fig. 3. Shear stress s – relative displacement Dl relationship of potato starch, white dextrin 60 and yellow dextrin W for normal pressure r of 4, 6 and 10 kPa.
starch thickener. Such behaviour of these two powders is probably a result of their particle size distribution, size of single particles and their shape and roughness. The angle of internal friction for dried potato protein varied from 30.6 for 6 kPa of consolidation stress, to 34.2 for 4 kPa. For this material, standard deviation of the angle of internal friction was found to be higher than for other materials from the first group, and this is the effect of non-replicable structure of the material. Values of angles of internal friction u for dextrins were found to increase in the range of consolidation stress applied. The highest
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Fig. 4. Angle of internal friction for three values of consolidation stress.
value of angle of internal friction – equal to 43.3° – was obtained for white dextrin 60 at consolidation stress of 6 kPa, while the lowest of 21.3° was obtained for white dextrin 60 at 4 kPa consolidation stress. The relationship between angle of internal friction and consolidation stress for white dextrin 60 was located approximately 30% above the experimental curves for other powders at 6 and 10 kPa consolidation stress. In the case of dextrins, standard deviations of the angle of internal friction were found to be nearly equal for all consolidation stresses applied. The relationships between effective angle of internal friction d and consolidation reference stress rr are shown in Fig. 5. The highest values of effective angle of internal friction were obtained for dried potato protein and for granulated starch. The effective angle of internal friction was higher than the angle of internal friction due to slight cohesion observed in the examined materials. The highest value of d of 47.7° was found for white dextrin 60 at 6 kPa consolidation stress, while the lowest effective angle of internal friction of 31.2° was that of yellow dextrin WB at 4 kPa consolidation stress. 3.4. Cohesion Cohesion, an important parameter characterising powders, is the value of shear strength for zero of normal effective stress. Fig. 6 presents the relationships between cohesion and consolidation stress obtained during the experiments. In the case of the first group of materials the highest values of cohesion, in the range from
159
Fig. 5. Effective angle of internal friction of examined materials for three values of consolidation stress.
1.3 to 2.1 kPa, were calculated for dried potato protein. In the case of this material the influence of consolidation stress on cohesion was the strongest. This is probably a result of the state of surface of particles of this material, as shown in Fig. 1f. For other materials the values of cohesion were equal at the two values of consolidation stress (4 and 6 kPa). For all the materials tested, higher values of cohesion were observed at higher values of consolidation stress. The lowest values of cohesion, in the range from 0.35 to 0.9 kPa, were observed in the case of native potato starch. For all the dextrins tested, increase in consolidation stress resulted in an increase in cohesion. The highest values of cohesion were found for white dextrin 40 and yellow dextrin S, in the range from 0.9 to 1.24 kPa and from 0.96 to 1.3 kPa, respectively. In the case of white dextrin 60, strong slip–stick effect was observed, while cohesion ranged from 0.79 to 1.15 kPa.
3.5. Flow function Flow functions of the experimental materials, presented in Fig. 7, were found to be characteristic for easy flowing and cohesive materials. No differences in the values of FF in the case of dextrins were observed, probably because of similar shapes and dimensions of granules. In the first group of materials the highest values of FF were found for dried potato protein (highest WK and Dmed), the lowest for control and granulated potato starch. The reason for the nearly
160
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Fig. 6. Cohesion of examined materials for three values of consolidation stress.
same values of FF for potato starch and granulated potato starch (WK = 9.38, Dmed = 44.3) is breaking of clusters during shearing. 4. Discussion In this study, the influence of microstructure on mechanical parameters of two groups of powders used in food and also other industries was investigated. Knowledge of the values of internal friction and flowability is critical in designing technologies of processing, as well as accompanying equipment. It is well known that mechanical parameters of seemingly the same powders may differ over a wide range; therefore growing interest in their determination has been observed. Data on the properties of powdered potato products – starch and protein – are scarce. That is why two groups of powders, widely used in the food and non-food (paper, pharmaceutical, printing, textile etc.) industries, were chosen as experimental materials. Many authors are of the opinion that chemical modification does not, or only to a small extent, alter starch granule size and shape (Lewandowicz et al., 1998; Fornal et al., 1998; Singh et al., 2007). However, they have reported considerable changes during heat-moisture treatment. According to Bowler the theory of lensshaped starch granules deforming when heated in excess of water was proposed by Bowler et al. (1980) in which several stages of deformations due to radial and tangential swelling were observed using scanning electron microscope. This was also confirmed for other starches, including legume starches (Fornal, 2000). Soral-
Fig. 7. Flow functions of experimental materials.
Smietana et al. (1998) described drastic changes in starch granule morphology that can appear during spray-drying. In the case of the first group of materials (control, granulated, gelatinising starch, pudding flour, and potato protein), two of them were found to be significantly different in terms of the size and shape of single particles. Dried potato protein and granulated starch were found to posses the highest values of Dn, Dgeo, Dmed and WK. Distinct differences were noted for these two materials, caused by their chemical nature and the method of processing, both influencing dramatically the microstructure of the final products and mechanical properties of these powders. It is known that potato protein within the cells exists as minute granular deposits with diameters less than 1 lm, whereas starch granules have much larger size and internal lamellar structure. Both components are very sensitive to acid, moisture and temperature. Potato protein is obtained as a by-product of starch extraction from potato tubers. It is coagulated by acid and temperature, precipitated and dried. During this process small protein structures are clustered, with many air spaces inside. Steam and heat action on native starch granules causes partial gelatinisation and strong (depending on parameters used) adhesion of individual particles into larger agglomerates. This agglomeration influences markedly the mechanical properties of the powders. Both powders discussed above are characterised by significantly higher values of angle of internal friction and effective angle of internal friction. In the case of dried potato protein, cohesion was significantly higher than for other materials. The microstructure and shape of single granules also influenced the values of FF. In this case the material with coarse surface and the highest value of D – dried potato protein – had the highest values of the
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
flow function. This result proved that the state of the surface had a critical influence on flowability; even the powder with the largest granules, but with a rough and porous surface, had poor flowability. The values of FF of granulated starch were close to the values of FF of other modified starches. This may be a result of decomposition of agglomerated particles into smaller subunits or even to single granules during the shear test. Strong influence of particle size distribution on flowability and cohesion at low moisture content of wheat powders was observed by Landillon et al. (2008). Mathlouthi and Roge (2003) examined flowability of sugar of particle diameters from 50 lm to 1 mm in direct shear test and found a decrease in flowability with reduction in particle size. The flow index of the fraction of the highest diameter was 0.032, whereas for the fine fraction it was of 0.265, which classified the material as cohesive. Different behaviour of dried potato protein is confirmed by results of Shinohara et al. (2000). Those authors determined the influence of the shape of single granules of rust free steel powder on the angle of internal friction determined in triaxial compression test. Steel powder was treated in a rotational machine to obtain spheres, and then the shape index was determined. The diameter of initial material granules was 68.9 lm. The shape index was found to be in the range from 0.663 for untreated powder to 0.917 for powder with the longest treatment time, whose granules had shapes closest to being spherical. The authors concluded that the angle of internal friction increased with an increase in shape irregularity of single particles, similar to our results. Mullarney et al. (2003) based on testing pharmaceutical sweeteners, arrived at a similar conclusion. Podczeck and Miah (1996) analysed the influence of the particle size, shape and addition of lubricant on the angle of internal friction and flowability of powders and found that surface lubrication changed the non-linear dependence between the angle of internal friction and the size and shape of particles. Liu et al. (2008) proved experimentally and theoretically that flowability of powders is determined by particle size and particle size distributions. For two experimental materials – the control potato starch and white dextrin 60 – the slip–stick effect was observed. In the case of white dextrin 60 the effect occurred even though the geometric parameters Dn, Dgeo, Dmed and WK, determined in infrared particle size analyser, were close to the parameters of other dextrins. For white dextrin 60 the highest value of angle of internal friction (nearly 25%) was found, yet cohesion and flowability were found to be almost the same as for other dextrins. The slip–stick phenomenon can appear during handling of materials and it can cause unforeseeable outage of technological equipment. Sequences of dilations and compactions in the shearing zone are probably the reasons for the slip–stick phenomenon. Densification causes an increase in strength and ability to handle higher shear stress. Upon breakage of strength of material dilatation takes place. Similar slip–stick effects were reported by Stasiak et al. (2008) based on testing corn flour and wheat starch. Forsyth et al. (2002) studied the transition from free-flow to stick–slip using assemblies of glass spheres in humidity controlled air and steel spheres within a magnetic field. It was found in both systems that the transition was governed by a critical ratio of inter-particle force to particle weight. The authors suggested that the observed transition was due to the system achieving a certain critical level of interparticle force and not to an abrupt rise in cohesive force such as this generated by sudden formation of liquid bridges. The analysis has shown important effects influencing shear deformation, but changes in tangent inter-particle force were not considered.
5. Conclusions Chemical modification of potato starch did not influence markedly the morphology of starch granules. Only in the case of steam-
161
heat treatment agglomeration of individual granules was found. Heat treatment of dry starch during dextrinisation had only a limited influence on granule morphology, however differences in granule size were observed. Dried potato protein, due to differences in the single particle diameter and processing parameters, differed significantly from starch products. Irregular shape of particles led to an increase in the angle of internal friction up to 43.4° of granulated starch. Rough and porous surface of potato protein powder and granulated starch were responsible for the highest values about 45° of effective angle of internal friction. Influence of particle morphology on cohesion was observed. The highest were values of cohesion of dried potato protein which surface is rough. The highest values of cohesion were observed at the highest value of consolidation stress for all examined materials. In the case of the first group of materials the highest values of cohesion, in the range from 1.3 to 2.1 kPa, were calculated for dried potato protein. In the case of this material the influence of consolidation stress on cohesion was the strongest. For other materials the values of cohesion were equal at two values of consolidation stress of 4 and 6 kPa. For some investigated materials (control starch and white dextrin 60) the slip–stick effect was observed. It was connected with the highest angle of internal friction and effective angle of internal friction. No relationship between the slip–stick effect and the size of single particles of powders was found. The investigated materials were characterised by different flowability. The highest values of FF were found for dried potato protein (highest WK and Dmed), the lowest for control starch and granulated potato starch. No differences in FF values between of dextrins were found.
References Bell, A., Ennis, B.J., Grygo, R.J., Scholten, W.J.F., Schenkel, M.M., 1994. Practical evaluation of the Johanson hang-up indicizer. Bulk Solids Handling 14, 117– 125. Berghtaler, W., 2004. The status of wheat starch and gluten production and uses in Europe. In: Tomasik, P., Yuryev, V.P., Bertoft, E. (Eds.), Starch: Progress in Structural Studies, Modifications and Applications. PSFT Malopolska Branch, pp. 417–436. Bowler, P., Williams, M.R., Angold, R.E., 1980. A hypothesis for the morphological changes which occur on heating lenticular wheat starch granules. Starch 32, 186–189. Carson, J.W., Wilms, H., 2006. Development of an international standard for shear testing. Powder Technology 167 (1), 1–9. Domian, E., Poszytek, K., 2005. Wheat flour flowability as affected by water activity, storage time and consolidation. International Agrophysics 19 (2), 119–124. Eurocode 1, Part 4, 2006. Basis of Design and Actions on Structures. Actions in Silos and Tanks. EN 1991-4. Fitzpatrick, J.J., Barringer, S.A., Iqbal, T., 2003. Flow property measurement of food powders and sensitivity of Jenike’s hopper design methodology to the measured values. Journal of Food Engineering 61, 399–405. _ ´ c´ 2 (23), 59– Fornal, J., 2000. Structural properties of starch in food systems. Zywnos 71. Fornal, J., Błaszczak, W., Lewandowicz, G., 1998. Microstructure of starch acetates from different starch sources. Polish Journal of Food and Nutrition Sciences 7/48 (3), 86–95. Forsyth, A.J., Hutton, S., Rhodes, M.J., 2002. Effect of cohesive interparticle force on the flow characteristics of granular material. Powder Technology 126 (2), 150– 154. Izli, N., Unal, H., Sincik, M., 2009. Physical and mechanical properties of rapeseed at different moisture content. International Agrophysics 23, 137–145. Jenike, A.W., 1961. Gravity flow of bulk solids. Bulletin Engineering Experiment Station (52), 29. Utah State University. Juliano, P., Muhunthan, B., Barbosa-Cánovas, G.V., 2006. Flow and shear descriptors of preconsolidated food powders. Journal of Food Engineering 72 (2), 157–166. Kamika Operation Guide, 2009. Kamika Instruments, Warsaw. Kaur, L., Singh, N., Singh, J., 2004. Factors influencing the properties of hydroxypropylated potato starches. Carbohydrate Polymers 55, 211–223. Knowlton, T.M., Carson, J.W., Klinzing, G.E., Yang, Wen-Ching, 1994. The importance of storage, transfer and collection. Chemical Engineering Progress 90 (4), 44–54. Landillon, V., Cassan, D., Morel, M.-H., Cuq, B., 2008. Flowability, cohesive and granulation properties of wheat powders. Journal of Food Engineering 86, 178– 193.
162
M. Stasiak et al. / Journal of Food Engineering 102 (2011) 154–162
Lewandowicz, G., Błaszczak, W., Fornal, J., 1998. Effect of acetylation on microstructure of potato starch. Polish Journal of Food and Nutrition Sciences 7/78 (2), 78–84. Liu, L.X., Marziano, I., Bentham, A.C., Litster, J.D., White, E.T., Howes, T., 2008. Effect of particle properties on the compaction of ibuprofen powders. International Journal of Pharmaceutics 362, 109–117. Mathlouthi, M., Roge, B., 2003. Water vapour sorption isotherms and the caking of food powders. Food Chemistry 82, 61–71. Molenda, M., Stasiak, M., 2002. Determination of elastic constants of cereal grain in uniaxial compression. International Agrophysics 16 (1), 61–65. Molenda, M., Stasiak, M., Horabik, J., Fornal, J., Błaszczak, W., Ormowski, A., 2006. Microstructure and mechanical parameters of five types of starch. Polish Journal of Food and Nutrition Sciences 15/16 (20), 161–168. Mullarney, M.P., Hancock, B.C., Carlson, G.T., Ladipo, D.D., Langdon, B.A., 2003. The powder flow and compact mechanical properties of sucrose and three highintensity sweeteners used in chewable tablets. International Journal of Pharmaceutics 257, 227–236. Podczeck, F., Miah, Y., 1996. The influence of particle size and shape on the angle of internal friction and flow factor of unlubricated and lubricated powders. International Journal of Pharmaceutics 144, 187–194. Ramírez, A., Moya, M., Ayuga, F., 2009. Determination of the mechanical properties of powdered agricultural products and sugar. Particle and Particle Systems Characterization 26 (4), 220–230.
Schwedes, J., 2002. Consolidation and flow of cohesive bulk solids. Chemical Engineering Science 57 (2), 287–294. Shinohara, K., Oida, M., Golman, B., 2000. Effect of particle shape on angle of internal friction by triaxial compression test. Powder Technology 107, 131–136. Singh, J., Kaur, L., McCarthy, O.J., 2007. Factors influencing the physico-chemical, morphological, thermal and rheological properties of some chemically modified starches for food applications – a review. Food Hydrocolloids 21, 1–22. Soral-Smietana, M., Fornal, J., Wronkowska, M., 1998. Microstructure and functional properties of wheat and potato resistant starch preparations. Polish Journal of Food and Nutrition Sciences 7/48 (3), 79–85. Stasiak, M., Molenda, M., 2004. Direct shear testing of food powders flowability (influence of deformation speed and apparatus stiffnes). Acta Agrophysica 111, 4(2), 557–564. Stasiak, M., Opalinski, I., Molenda, M., 2008. Mass and microscopic properties of food and industry granular materials. I. Comparison of mechanical properties. Przemysł Chemiczny 87 (2), 199–202 (in Polish). Teunou, E., Fitzpatrick, J.J., Synnott, E.C., 1999. Characterization of food powder flowability. Journal of Food Engineering 39, 31–37. Tykhoniuk, R., Tomas, J., Luding, S., Kappl, M., Heim, L., Butt, H.J., 2007. Ultrafine cohesive powders: from interparticle contacts to continuum behaviour. Chemical Engineering Science 62 (11), 2843–2864.