Mechanical properties of potato starch modified by moisture content and addition of lubricant**

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1 Int. Agrophys., 2014, 28, doi: /intg Mechnicl properties of potto strch modified y moisture content nd ddition of luricnt** Mteusz Stsik 1*, Mrek Molend 1, Józef Horik 1, Peter Mueller 2, nd Ireneusz Opliński 3 1 Institute of Agrophysics, Polish Acdemy of Sciences, Doświdczln 4, Lulin, Polnd 2 Deprtment of Mechnicl Process Engineering, Otto-von-Guericke University, Universitätspltz 2, Mgdeurg, Germny 3 Deprtment of Chemicl nd Process Engineering, Rzeszów University of Technology, Al. Powstńców Wrszwy 12, Rzeszów, Polnd Received Decemer 15, 2013; ccepted June 18, 2014 A s t r c t. Lortory testing ws conducted to deliver set of chrcteristics of structure nd mechnicl properties of pure strch nd strch with n ddition of luricnt mgnesium sterte. Considerle influence of moisture content of potto strch ws found in the cse of density, prmeters of internl friction, coefficients of wll friction nd flowility. Elsticity ws found to e strongly influenced y wter content of the mteril. Addition of mgnesium sterte ffected density nd prmeters of flowility, internl friction nd elsticity. Bulk density incresed from 604 to 774 kg m -3 with decrese in moisture content of potto strch from 17 to for 6%. Addition of mgnesium sterte resulted in pproximtely 10% decrese in ulk density. Angle of internl friction otined for 10 kp of consolidtion stress decresed from 33 to 24º with increse in moisture content, nd to pproximtely 22º with ddition of the luricnt. With n increse of moisture content from 6 to 18% nd with ddition of the luricnt, the modulus of elsticity during loding decresed from pproximtely 1.0 to 0.1 MP. Modulus of elsticity during unloding ws found in the rnge from 19 to 42 MP nd incresed with increse of moisture content nd mount of luricnt. K e y w o r d s: potto strch, mechnicl properties, friction, elsticity INTRODUCTION *Corresponding uthor e-mil: mstsik@ipn.lulin.pl **This work ws finnced from the udget for science s reserch project No. N ( ). The volume nd rnge of mterils in powdered form re still incresing in industry, yet simultneously powder hndling remins one of the lest understood res ssocited with solid processing plnts. Scientific knowledge regrding powder technology is still frgmentry, design of equipment relies on empiricl concepts, nd process control is lrgely sed on experts know-how (Cuq et l., 2011; Moy et l., 2013; Stsik et l., 2010). Predictle processing, ssuring qulity nd reduction of losses of products, re still sustntil issues. Incresing scle of industril opertions requires the design of efficient equipment nd relile processes tht in turn clls for more precise informtion out the physicl chrcteristics of sustrtes (Mzurkiewicz, 2011; Rmirez et l. 2009; Stsik et l., 2007). One of the frequently used powdered mterils ll over the world is potto strch, s chep nd esily ville. With the mrket shre of 6% in the world, it is one of the mjor components in the formultion of food products. Strch is one of the most widely used excipients in mnufcture of solid dosge form s inder nd dissenter in tlets formultion (Ahmd et l., 2012; Odeku et l., 2005; Ruppel et l., 2009; Wng et l., 2010). Strch is useful rw mteril for iodegrdle thermoplstics ecuse of its nturl undnce nd low cost (Mitrus nd Mościcki, 2009; Mościcki et l., 2012; Zvreze et l., 2012). However, its low flowility nd hydrophilicity generte serious technologicl prolems. Tht is why in formultions for thermoplstics production flows, gents re used to improve flowility nd to chnge chrcteristics from hydrophilic to hydrophoic (Chujing et l., 2009). These uthors proposed new dry process for the production of hydrophoic corn strch without solvent. To find the origins of properties of strch powders, investigtions focused on single grnule properties hve een conducted. Schroeter et l. (1992) otined prmeters of potto strch mesured on single grnule. Elstic modulus of 2.7 GP, tensile strength of 62 MP, indenttion hrdness of 113 MP found y these uthors were in rnge of vlues of those otined for rigid thermoplstics. Slerno et 2014 Institute of Agrophysics, Polish Acdemy of Sciences

2 502 M. STASIAK et l. l. (2014), in their tomic force microscope exmintions of individul grnules, otined elstic modulus of potto strch of 9.8 MP, thus firly close considering usully high vriility of properties of mterils of iologicl origin. Numerous reserch projects hve een performed to study the mechnicl properties of iopolymers such s strch, nd the influence of vrious process prmeters on these properties (Mitrus nd Mościcki, 2009; Mościcki et l., 2012; Zvreze et l., 2012) s well s properties of tlets mde with strch components (Odeku et l., 2005). Most frequently ddressed issues re: internl friction, flowility, elsticity nd friction (including slip-stick effect). These prmeters re of high importnce in technologies including compression, like extrusion nd tlet production, where pressure in equipment determines the necessry strength of construction memers of instlltion. Mgnesium sterte is widely used in tlet formultion ecuse of its luriction ction nd ility to form hydrophoic thin film on the surfce of the crrier grnules. Use of mgnesium sterte llows to reduce power consumption nd ejection forces in tlet production (Wng et l., 2010). Another fctor determining the mechnicl ehviour of powders is moisture content tht ffects flowility, cohesion, cking, density, sher strength, compction nd compression chrcteristics (Nokhodchi, 2005). Di Qu et l. (1998) investigted the influence of norml pressure, degree of conversion of strch, effective sher rte, temperture nd moisture content on sher stress in the powder to otin n eqution comining ll ove effects. Wxy corn strch ws tested in modified vne sher test device nd firly good correltion etween clculted nd sher resistnce ws otined. Ruppel et l. (2009) exmined flowility of corn strch powder with different dditions of flow regultor used in the phrmceuticl industry. Housner rtio nd sher stress of mixtures were mesured, moreover flowility prmeters were determined using modified outflow funnel. The uthors determined the conditions of est flow ehviour with the ddition of flow gent nd compred the new method of flowility determintion with those well estlished. Fitzptrick nd Ahrne (2005), in n rticle tht summrized 1-yer project focusing on food powder hndling nd processing, point out tht food powders chrcteriztion, design of powder storge nd trnsport is the most importnt knowledge rrier in industry nd mjor reserch need. Numericl modelling is promising ut just emerging tool for the design of processes of powder technology tht require specific mteril prmeters. One of currently developing pproches is DEM discrete element method. Di Qu et l. (1998) used DEM for modelling of extrusion nd tleting processes; tht tsk lso required knowledge of mteril properties, trnsport phenomen nd kinetics of rection. The ojective of the reported project ws to deliver set of structurl nd mechnicl chrcteristics of potto strch of different moisture contents nd with the ddition of luricnt. The modulus of elsticity, the slip-stick effect, the ngle of internl friction, the cohesion, flowility nd friction ginst typicl construction mterils were determined. The dt presented my e useful in design of equipment nd control of processes in the industril hndling nd conveying of potto strch. MATERIAL AND METHODS Commercil potto strch with moisture content of 6, 12 nd 17%, produced y Melvit in Ostrołęk, Polnd, ws used s the experimentl mteril. Prticle size distriution ws nlyzed using the Infrred Prticle Sizer IPS UA (Kmik, 2009). The method consists in mesurement, in continuous nd grduted mnner, of chnges of lser rdition strem scttered y moving prticles in the ir. The prmeters re defined s follows: the men dimeter for given mount of prticles (D n ), the men geometric dimeter (D geo ), the size t which 50% of the smple is smller nd 50% is lrger (D med ), the form fctor which is the rtio of two dimensions (height/width) of single prticle (WK) (for the sphere WK=1) (Kmik, 2009). Moistening of the powder ws relized y humidifier in closed cuic spce for 48 h. Moisture content (MC) (wet sis) ws mesured grvimetriclly y weighing 10 g of smple efore nd fter drying t 105ºC for 24 h. Dried mterils were otined y 24 h drying in lortory drier with forced ir circultion in stle temperture of 30ºC. The chrcteristics of the strches were quntified t the levels of moisture content occurring in prcticl technologicl processes. Crr prmeters with indexes descriing flowility were determined using Powder Chrcteristics Tester PT-S, Hosokw (Crr, 1965). Aerted density (r er ) is ulk density of prticulte contining much ir uilt up in vessel y dropping ccording to the injection method. The pcked density (r pck ) is ulk density of prticulte of loose ulk density uilt up y tpping. The compressiility (Co) is determined y the mesurement of erted nd pcked ulk densities. As the compressiility is higher, the flowility is poorer. The ngle of repose ( rep ) mens n ngle of hep of prticulte uilt up y dropping ccording to the injection method. Prticulte poor in flowility shows wide ngle of repose. The ngle of fll ( fll ) mens n ngle of residul prticulte hep resulting from collpse due to ppliction of certin shock to the prticulte forming the ngle of repose. As the prticulte is more lile to flow spontneously, the ngle of fll is nrrower. The ngle of difference ( diff ) is the difference etween the ngle of repose nd the ngle of fll. As the ngle of difference is wider, the flushing phenomenom is more likely to occur. Electron scnning microscope exmintions were performed using the JSM 5200 microscope t 10 kev. Specimens were plced on doule-side dhesive tpe mounted on luminium specimen holders, coted with gold in vcuum evportor JEOL 400.

3 MECHANICAL PROPERTIES OF POTATO STARCH 503 Mgnesium sterte ws used s the luricnt in mounts of 1, 2 nd 6% of weight of the potto strch smple nd mixed for 1 h in stndrd lortory mixer. This chemicl is widely used luricnt for tlet compression ecuse of its expedient mechnicl properties: low coefficient of friction, very smll prticles nd hydrophoic properties (Wng et l., 2010). Direct sher tests were performed in sher ox 60 mm in dimeter. The tests followed Eurocode 1 (2006) stndrd procedure for consolidtion reference stresses ( r ) of 4, 6 nd 10 kp nd speed of shering (V) of mm s -1. Bsed on the experimentl curves, the ngle of internl friction ( ), the effective ngle of internl friction ( ), flow functions (FF), cohesion (C), wll friction coefficient ( ) on lck, glvnized nd stinless steel smples were determined. Yield locus ws determined sed on the vlues of the mximum sher stresses t two levels of consolidtion stress s recommended y Eurocode 1 (2006). Next, Mohr circles were drwn, which provided sis for clculting the vlues of unconfined yield strength nd mjor consolidting stress. The reltionship etween these two prmeters regrded s FF, ws lso clculted (Molend et l., 2006). Prototype unixil compression tester with horizontl stress control ws used to determine poured ulk density ( ), ulk density of consolidted mteril ( cons ), modulus of elsticity (E) during loding nd unloding nd Poisson rtio ν. Mximum consolidtion stress of 10 kp ws pplied s recommended y Eurocode 1 nd incrementl vlue of verticl stress ws 2 kp. The unixil compression tester is presented in Fig. 1. Supports of two sic components of the tester: force trnsducer nd of electronic micrometer re mounted on the durlumin se, on which lso the cylindricl chmer of the pprtus is plced. Cylindricl chmer, 25 mmm high nd 60 mm in dimeter, consists of two semicirculr hlves connected with hinge. One of the hlves is connected rigidly to the ottom plte, while the second one is connected with the structure through force trnsducer to restore the shpe of the cylinder. Top nd ottom ses of the chmer re rigid t the lod level pplied nd their inner surfces re smooth. Consolidtion stress ws enforced grvittionlly through yoke which is plced on the top plte of the chmer. The height of smple nd its chnges were mesured y electronic micrometer with the ccurcy of mm. A verticl stress is pplied to the smple constrined ginst horizontl deformtion. As the verticl stress increses, the chnge in induced horizontl stress nd the chnge in verticl displcement is mesured. The E for loding is clculted from these mesurements. The verticl stress should then e decresed nd the chnge in induced horizontl stress nd the chnge in verticl displcement is mesured nd from these dt E for unloding is clculted. Detils of the procedure of determintion of modulus of elsticity during loding nd unloding were s descried in Eurocode 1 (2006). RESULTS AND DISCUSSION Microscopic imges nd results of size nlysis re presented in Fig. 2. The grnules re chrcterized y imodl shpe, typicl for potto strch, nd rther rod rnge of dimeters. Surfces of the grnules re smooth nd dimeters rnge from 5 to 70 µm. Lrge grnules re elongted in shpe, wheres the smll ones re closer to circulr. These results re in good greement with the wide reserch of Biegnowski et l. (2011) focusing on the geometry of potto strch grnules. These results corroorte results of erlier investigtion tht potto strch grnules re ovl nd irregulr or cuoidshped while corn nd whet strch grnules re ngulr, sphericl nd lenticulr-shped (Singh et l., 2003). Fig. 1. Prototype unixil tester with mesurement of horizontl stress. D n D geo D med WK Fig. 2. Microscopic imges nd UPS Kmik prticle size nlysis of potto strch (Stsik et l., 2011).

4 504 M. STASIAK et l. The presence of grnules with diverse shpe nd size cn strongly influence the physicl properties nd ehviour of the powder during processing. Bldwin et l. (1998), sed on AFM imges, hve shown tht the surfces of whet nd potto strch grnules present sustntilly different topogrphies. Potto strch grnules hve rougher surfce thn tht of whet strch grnules, consisting of lrge rised nus (50 to 300 nm in dimeter) ove fltter surfce contining structures of dimeters from pproximtely 10 to 50 nm. The vlue of D n ws 13 µm, D geo 10.8 µm nd the form fctor of potto strch ws pproximtely 3. Moisture content ws found to influence strongly the density of potto strch. Poured nd consolidted density determined in unixil chmer decresed with n increse of moisture content from 774 to 604 nd from 895 to 788 kg m -3, respectively. Density decresed lso with the ddition of mgnesium sterte to potto strch, t 6% MC nd resulted in decrese of poured density to out 730 kg m -3 with the ddition of 2 nd 6% of the luricnt. No influence of the ddition of the luricnt on consolidted density ws oserved (Tle 1). This result is contrry to wht should e expected from the ddition of luricnt nd wht is oserved in tleting. The possile explntion of such ehviour is the lrge difference in pressure level of 10 kp in compction testing compred to out 10 MP in tlet formtion. Proly, t high level of pressure, deformtion or even rekge of grnules tke plce tht re fcilitted y the ction of luricnt. At low pressure level, the ddition of mgnesium sterte would oppose or did not influence compction. These results re lso in opposite to results otined y Bos et l. (1987). In their investigtion the density of luricted potto strch were 900 kg m -3 during unluricted 708 kg m -3. T l e 1. Density of potto strch determined in unixil chmer Prmeter Moisture content (%) Poured density r (kg m -3 ) Consolidted density r cons Addition of mgnesium sterte (% ) Fig. 3. Modulus of elsticity for: loding nd unloding s function of powder moisture content nd mount of luricnt ddition. Points denote men vlues nd verticl rs denote the 0.95 confidence intervls; F, p nlysis of vrince prmeters. The higher vlue of test function F mens thn given effect hs stronger influence on determined prmeter. The p-vlue is significnce.

5 MECHANICAL PROPERTIES OF POTATO STARCH 505 Moduli of elsticity (E lo ) nd (E un ) determined following Eurocode1 recommendtion in prototype unixil tester re shown in Fig. 3. There is still need for developing new device for the mesurements of prmeters of powdered mterils. One of such device is tht proposed y Brlett nd Poletto (2013) who invented two hlf-cells with fixed one nd moile one using single stndrd lod cell without the need to pply strin guges on custom uilt prts, s in pprtus presented in this rticle. Vlues of the prmeter otined under the loding regime were pproximtely 30 times lower thn those for unloding. Modulus of elsticity during loding, E lo, decresed from pproximtely 1 to 0.1 MP with n increse of moisture content from 6 to 17%. No influence of MC ws oserved for unloding modulus of elsticity. The high sctter of results oserved in these exmintions llows for concluding out tendencies rther thn firm reltionships. Loding modulus of elsticity decresed strongly from out 1 MP to out 0.1 MP with ddition of the luricnt (Fig. 3). Further increse in the ddition of mgnesium sterte did not influence the ehviour of this prmeter. Opposite tendency ws oserved for unloding where incresing mount of the luricnt resulted in n increse in modulus of elsticity. The verge vlues of unloding modulus incresed from pproximtely 20 MP without the luricnt up to pproximtely 45 MP with the highest ddition of the luricnt. Results of this prmeter for single grnules otined y Slerno et l. (2014) is ten time higher from loding modulus. Crr indexes of potto strch re presented in Tle 2. The ngle of repose strongly incresed with growing moisture content of powder, from 38 for 6% moisture content of powder to 58. The softest cone formed with powder ws otined for potto strch with 12% of moisture content (ngle of difference 28 ). The ddition of mgnesium sterte lso resulted in n increse in the ngle of repose, ut only wek tendency ws oserved. The most unstle powder cone ws otined for the highest ddition of the luricnt. In the cse of mteril with the luricnt higher vlues of erted density were found, ut still lower thn in unixil compression test. Aerted ulk density ws found equl to this otined in unixil compression nd decresing with n increse in moisture content from 6 to 17% dequtely from 763 to 593 kg m -3. Pcked ulk density ws higher thn consolidted density (Tle 2). This is the effect of stronger compction using tpping in the whole mss of smple compred to compression through the top plte of unixil compression tester. Proly in the unixil compression pprtus lyers of powder close to the upper lid were consolidted stronger thn those close to the ottom of the chmer s n effect of friction cting on the wll. The highest compressiility equl to 30 ws otined for powder with the highest moisture content. These vlue ws nerly equl to compressiility of the mteril with n ddition of the luricnt, where the mount of mgnesium sterte dded did not influence the result. Similr investigtions were performed y Crouter nd Briens (2014). Authors stted tht flowility determined y Crr indexes decresed with incresing moisture content, once criticl point f monolyer wter coverge ws reched, due to n increse in cohesion from stronger interprticle liquid ridges. Experimentl reltionships otined in direct sher tester re presented in Fig. 4. Strong disturnces of friction force were oserved in the cse of 6% MC. potto strch for consolidtion stresses of 4, 6 nd 10 kp. Such fluctutions of experimentl reltionships oserved in mechnicl systems with frictionl dmping re termed the slip-stick effect nd re ttriuted to the difference etween coefficients of sttic nd dynmic friction (Ado et l., 2010). Fluctutions could e the reson of surfce condition of single prticles T l e 2. Crr indexes of potto strch Angle Bulk density Crr prmeter repose rep fll fll difference diff erted r er pcked r pck Compressiility Co ( ) (kg m -3 ) Moisture content (%) Addition of mgnesium sterte to potto strch (%)

6 506 M. STASIAK et l. Fig. 4. Sher stress reltive displcement Dl/D reltionship of potto strch for: different moisture content nd ddition of mgnesium sterte. of powder mterils which for therml modified strches fter drum or hot ir drying is more rough thn for ntive strch (Ydv et l., 2006). The highest mplitude of oscilltions ws oserved for the highest consolidtion pressure of 10 kp. This result is typicl for food powders nd ws otined y Bgg et l. (2012) for crem powder. In their investigtion for crem powder increse in norml lod from 2.52 to 9.59 kp resulted in 6 fold higher mplitude of oscilltions. Slip-stick effects cuse difficulties in interprettion of testing results. Frictionl prmeters for mterils showing fluctutions were estimted using the mximum vlues of sher stress to otin the lrgest (or the worst cse) vlues. As shown y Stsik nd Molend (2004), it is lso possile to reduce the slip-stick effects y chnge in the stiffness of mesurement equipment. Increse in moisture content resulted in cesstion of the slip-stick effect. Only smll slip stick ws oserved for the highest norml stress of 10 kp for moisture content of 12%. Proly wter which is on the surfce of grnules cts s luricnt, llowing smooth sliding. Moreover. increse in moisture content resulted in erlier stiliztion of experimentl curves. Addition of mgnesium sterte resulted in lower vlues of mximum stle vlues of sher stresses. Moreover, even the lowest ddition of the luricnt of 1% resulted in disppernce of the slip-stick effect. Angle of internl friction ws found to e influenced y moisture content nd ddition of mgnesium sterte s shown in Fig. 5. Averge vlues of decresed shrply with n increse in moisture content, from pproximtely 25 for 6% moisture content to pproximtely 16 for 12% moisture content. For 17% moisture content slight decrese ws oserved, to 14. Prole reson of such ehviour ws tht the powder reches the stte of sturtion nd wter cts s luricnt. For the highest moisture content, free wter my pper on the surfce of grnules nd decrese friction in contcts. It is opposite ehviour to tht determined in Hosokw Powder Tester, where the ngle of repose incresed with n increse in moisture content. These results re in greement to results otined y Gun nd Zhng (2009) which determined higher ngle of internl friction for drier mteril. In the cse of 1 nd 2% ddition of the luricnt, ngle of internl friction decrese from pproximtely 28 to 16 ws oserved. The lowest vlue of the ngle of internl friction ws otined for the highest ddition of the luricnt. Also these results re opposite to the ehviour of ngle of repose oserved in HPT. The reson for this difference in frictionl ehviour is proly the difference of level of pressure, nerly zero (own weight) in testing in free fll nd mximum 10 kp in direct sher test. Fig. 5. Reltionships etween ngle of internl friction t: different moisture content nd ddition of mgnesium sterte. Points denote men vlues nd verticl rs denote the 0.95 confidence intervl; F, p - nlysis of vrince prmeters. The higher vlue of test function F mens thn given effect hs stronger influence on determined prmeter. The p-vlue is significnce.

7 MECHANICAL PROPERTIES OF POTATO STARCH 507 c Fig. 6. Reltionships etween: cohesion nd consolidtion stress, cohesion nd moisture content, nd c cohesion nd ddition of the luricnt. Explntions s in Fig. 5. Cohesion incresed with consolidtion stress from pproximtely 0.25 kp to pproximtely 0.5 kp with n increse in consolidtion stress. No influence of moisture content on cohesion ws oserved (Fig. 6). Addition of the luricnt in n mount of 1% resulted in decrese in cohesion nd ngle of internl friction. Reching 6% concentrtion, cohesion incresed lmost to the vlue otined when no luricnt ws present. Significnt differences in FF of exmined smples were oserved s shown in Fig. 7. Vlues of FF were chrcteristic for free nd esy flowing mterils. Vlues of FF incresed y 50% with n increse in moisture content from pproximtely 0.9 to 1.4 kp for the lowest consolidtion stress, nd from pproximtely 1.6 to 2.2 kp for consolidtion stress of 10 kp. In the cse of Fig. 7. Flow functions of exmined powders of potto strch. ddition of mgnesium sterte significnt decrese in FF vlue ws oserved. For consolidtion stress of 4 kp the c d Fig. 8. Coefficient of friction of potto strch s dependent on: consolidtion stress, moisture content, c mount of luricnt dded nd d type of steel. Explntions s in Fig. 5.

8 508 M. STASIAK et l. vlues of unconfined yield strength decresed from 0.9 kp to pproximtely 0.4 kp for ech concentrtion of mgnesium sterte. In the cse of the highest vlue of consolidtion stress the vlues of FF decresed roughly y 25% for the ddition of 1% of mgnesium sterte. For 2 nd 6% of luricnt ddition the FF decresed y out 50%. Mechnicl strength of the ed nd worsening powder flowility with n increse of moisture content were oserved y Opliński et l. (2012). Coefficient of friction ws determined ginst construction mterils widely used in industril technologies, three kind of steel: lck, glvnized nd stinless. Averge vlues of friction coefficient re presented in Fig. 8. The highest friction coefficient of the tested mterils ws oserved for lck steel, while vlues otined for glvnized nd stinless steel were nerly equl. No significnt differences in friction coefficient were oserved t vrious consolidtion stresses. Coefficient of friction incresed y pproximtely 40% with incresing moisture content. CONCLUSIONS 1. Vlues of mechnicl prmeters of potto strch were found to e strongly influenced y moisture content of the mteril nd mount of luricnt dded. 2. During loding of the strch with ddition of 1% of luricnt, the modulus of elsticity decresed with increse in moisture content. Further increse in the content of luricnt did not influence the elsticity. During unloding the modulus of elsticity did not chnge with increse in moisture content nd incresed with increse in ddition of mgnesium sterete. 3. Slip-stick effect oserved during experiments decresed with increse of moisture content nd ddition of mgnesium sterte. 4. Angle of internl friction decresed with increse in wter content in whole rnge of moisture content nd even minimum ddition of luricnt, while prcticlly no considerle influence ws oserved in the cse of cohesion. 5. 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