Rheological Characterization of Fiber Suspensions Prepared from Vegetable Pulp and Dried Fibers. A Comparative Study.

Transcription

1 ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 3, 5 Rheological Characterizatio of Fiber Suspesios Prepared from Vegetable Pulp ad Dried Fibers. A Comparative Study. Elea Bayod, Ulf Bolmstedt -, Fredrik Iigs 3, Eva Torberg Departmet of Food Egieerig, Lud Uiversity, P.O. Box 4, S- Lud, Swede. Tetra Pak Processig Compoets, P.O. Box 3, S- Lud, Swede. 3 Tetra-Pak Processig Systems, S-86, Lud, Swede. Correspodig author elea.bayod@livstek.lth.se ABSTRACT The effect of the water isoluble solids (WIS) cotet, the particle size, ad the water holdig capacity o the rheological properties of fiber suspesios has bee studied. The flow behavior of the suspesios was described by a power law model i the shear rate rage betwee ad s -. INTRODUCTION The rheological properties of food products are importat to cotrol, both durig processig ad storage, as they affect the quality of the fial product. Moreover, reliable rheological data allows a better desig of the process ad the processig equipmet. Vegetable foods cosist to a large extet of cell wall material. Whe this type of foods are mashed ito purees ad pastes they form a dispersio of fibers, whole cells, cell fragmets ad proteis (i.e. the pulp) i a cotiuous phase, usually a aqueous solutio of sugars (i.e. the serum). Literature o the rheology of fluid foods is wide ad ofte cotradictory, ad has bee rather focused o describig the effect of both the cocetratio ad temperature o viscosity,, 3. The role of the pulp cotet o the rheological properties of fluid foods such as tomato puree 4 or tomato juice 5 has previously bee studied. The structure of the cell wall material ad how it is disitegrated ad swolle i the cotiuous phase is of key importace to cotrol the rheological behavior of fibrous suspesios uder flow coditios. We have therefore chose to geerally characterize ad study the rheological behavior of cell wall materials as dried fibers ad vegetable pulps (i.e. differet swellig capacities) ad from differet origis (i.e. differet compositio) i suspesios of similar fiber cotet. MATERIAL AND METHODS Dry fiber suspesios. Three differet types of commercial dried fibers with similar particle size (3-35 µm) were used: potato fiber (Lyckeby Culiar), apple fiber (Vitacel AF 4-3) ad cellulose fibers (Vitacel L 6-3). Fibers were suspeded at differet cocetratios (.5,, 3, 5 ad 7 % w/w) i a aqueous sucrose solutio (ºBrix) with differet amouts of pectis HM- (CPKelco). The pecti cotet was chose to be.4 % for both potato ad apple fibers ad.8 % for cellulose fibers, i order to avoid precipitatio of the fibers. Pulp dilutios. Three differet types of pulp were chose: mechaically pressed potato pulp obtaied from the process of extractio of potato starch, commercial mashed apples, obtaied from mechaically pressed apples ad separated through a sieve ad two tomato pastes obtaied by hot break with ~3 ad ~9ºBrix, respectively. Potato pulp was diluted with water to 5 ad % w/w. Mashed apples were diluted (8, 7, 6, 5, 4, 3,, % w/w) i a aqueous 49

2 suspedig medium ( % sugar,.8 % pectis HM-). Fially, tomato pastes were diluted to 8, 6, 5, 4, ad % w/w i a suspedig medium (% sugar ad.8% pectis HM-). Geeral characterizatio. Particle size ad shape was studied by light microscopy (Olympus BX5) ad a laser diffractio method (Coulter LS 3) applyig the Frauhofer optical model. Water isoluble solids (WIS) were calculated from the differece betwee the dry matter ad the water soluble solids cotet, Eq. (), ad it is assumed to be the isoluble fiber cotet. TS SS WIS = () SS Dry matter (TS) was measured i a vacuum ove for 6 h at 7ºC. Water soluble solids (SS) was measured with a maual refractometer as ºBrix. Rheological characterizatio. A rotatioal cotrolled-stress rheometer was used to study the suspesios uder steady-state coditios at ºC. The viscosity of the fiber suspesios ad of the mashed apple dilutios was measured i a cocetric cylider rheometer (mm gap, smooth surface). The viscosity of both tomato pastes was measured i a serrated-cocetric cylider rheometer ( mm gap) i order to avoid slippage at the wall. The viscosity of potato pulp was measured i a tube viscometer (L=3 m, d i =4 mm), i order to avoid disturbaces due to the large particles of this sample. Power law model, Eq. () was applied to the data, ad both, K ad parameters were determied i the rage of shear rates betwee ad s -. τ & = K γ () Flow properties were determied at ºC because this is a relevat temperature durig pumpig. The depedece o the temperature was studied i a rage of to 3ºC. Tomato ad potato pulp showed o depedece withi the studied rage. The mashed apple suspesios were sesitive to temperature. The effect of temperature o the dried fibers was studied i a separate work ad the viscosity slightly decreased with icreasig temperature. However, the effect of the pulp cotet ad structure seems to be much greater tha the possible effect of temperature. Thus, the temperature effect o the dried fiber ad pulp suspesios will ot be cosidered further. RESULTS Particle size. The dried fibers were a mixture of heterogeeous particles with a aspect ratio ragig betwee.5 ad 7 ad a equivalet volume based diameter of 3-35µm (Fig. a, b). Pulps were a mixture of whole cells, cell fragmets, skis ad free fibers (Fig. c, d), with a equivalet volume diameter of ~8, ~4 ad ~47 µm for tomato, potato ad apple, respectively (Table ). It has to be emphasized that the volume-based particle size distributio was uimodal but wide. A C Figure. Typical photomicrographs of both dried fibers ad pulp suspesios: (a) potato fibers, bar µm, (b) cellulose fibers, bar µm, (c) mashed apples, bar µm, (d) detail of the fiber aggregatio i tomato paste ~9 ºBrix, bar 5µm. B D 5

3 Table. Compositioal characteristics of both the dried fibers ad the pulp suspesios. Cotet Particle size ph SS WIS moisture separatio of water % µm - ºBrix % % by Potato fibers 7. 33,6 ± 66, , gravity Apple fibers 7. < ,6 gravity Cellulose fibers 7. 33, ± 63, ,9 gravity Potato pulp. 49, ± 46 - ~ o-separatio Mashed apples. 463, ± ~ gravity Tomato paste~3. 8, ± , cetrifugatio Tomato paste~9. 7,9 ± , cetrifugatio Accordig to the supplier Soluble solids measured o tomato serum Water holdig capacity. The cotet of water i 3 ad 9ºBrix tomato paste is 76.7 ad 69.6 %, respectively; i the apple pulp it is 87.7 % ad i potato pulp it is 78.5 %. For the 7% fiber suspesios the water cotet was aroud 7 %, idepedetly of the type of fiber (Table ). Water is held iside the capillaries betwee the fibers. For the o diluted potato pulps it was impossible to release water by cetrifugatio at 3 g for mi, eve if the water cotet of those pulps was high. For tomato paste, oly about 9.9 % of the serum was separated by cetrifugatio uder the same coditios. However, the water cotaied i the apple pulp was easily separated by gravity. Water was easily separated by cetrifugatio i the potato pulp dilutios, but it was more difficult to separate by gravity. Rheological characterizatio. The suspesios of the fibers ad the pulps preseted a very differet viscosity behavior. Figure shows the depedecy of the power-law parameters, the cosistecy coefficiet (K) ad the flow behavior idex,9 A 5,9 B 5,8 4,8,7,7,6,5 3 K,6,5 5 K,4,4,3,3,, 5,, %fibers (WIS) % fibers (WIS) Figure. K ad parameters as a fuctio of the fiber cocetratio, determied i a shear rage of ad s - applyig a power law model (Eq. ), to the viscosity curve. A) Fibers: potato ( ), apple ( ) ad cellulose ( ) ad B) Pulps: tomato paste 3 ad 9ºBrix ( ad, respectively), mashed apples ( ) ad potato pulp ( ). Filled symbols are K-values ad empty symbols are -values. 5

4 (), o the fiber cocetratio, for both dried fibers (Fig. A) ad pulp suspesios (Fig. B). From Fig. A, it is observed that -values icrease slightly with decreasig cocetratio ad are sigificatly higher tha those i pulps (Fig. B). I additio, K- values were much lower (<5 Pa s ) i the suspeded fibers, whereas i the pulps they raged betwee 5 ad.5 Pa s. For the pulps, there is a drastic icrease of the - value below % fiber cocetratio ad a more or less expoetial decrease of K with decreasig fiber cocetratio (Fig. B). Note that the calculated K ad parameters are oly valid i the studied rage of shear rates (- s - ). DISCUSSION The reaso for this large differece i rheological behavior betwee dried fibers ad pulps of similar fiber cotet might be due to the fact that the origial physical structure of dried fibers have bee destroyed durig extractio ad dryig. There was o swellig of these fibers, as o chage of the fiber diameter was observed after days i a aqueous solutio. Earlier, Harper ad El Sahrigi 6 showed that the viscosity of recostituted tomato cocetrate was oly about oe third of that of the correspodig origial sample. I our study, moreover, the smaller particle size of the dried fibers alog with their arrower particle size distributio gave rise to a better packig of the fiber particles ad hece, the volume that those fibers occupied i the suspesios was probably very small compared to that of the pulps, at the same fiber cotet. I pulps, fibers ad cells are i a swolle state, which icreases their volume fractio ad hece, the umber of cotacts betwee the particles, resultig i a higher viscosity. Figure 3 clearly shows the eormous differece i viscosity betwee two tomato pastes ad some of the dried fiber suspesios, at similar fiber (or WIS) cotet. The origi of the material gave rise to large differeces i the rheological behavior of pulps (Fig. B), whereas smaller or o differeces were oted for the dried fibers (Fig. A). viscosity (Pa s),,, shear rate (/s) Figure 3. Viscosity as a fuctio of shear rate: ( ) tomato paste ~3 ºBrix, ( ) tomato paste ~9 ºBrix, (x) potato fibers at 7 %, ( ) potato fibers at 3 %, ( ) apple fibers at 7 %, (*) apple fibers at 3 %, (-) cellulose fibers at 7%, (+) cellulose fibers at 3%. Ideed, tomato pastes of similar particle size showed differet flow properties, ad potato pulp (~4 µm) behaved closer to the tomato paste of 3 Brix (~8 µm) tha to the mashed apple suspesio (~47 µm). This suggests that ot oly the particle size is importat, but also how these particles iteract ad form a etwork plays a importat role o the flow behavior of the suspesios. It is speculated that the mashed apple icludes more water i its etwork tha the potato pulp does. Iterestigly, the apple pulp gave rise to the largest viscosity per gram of fiber (or WIS). I fact, this sample was the most mildly processed pulp compared to the others, where the latter were cocetrated to high solid level by more severe mechaical 5

5 treatmet. It is also oted that the 9 Brix tomato paste was processed to a lesser extet tha the 3 Brix paste. The mechaical history of a tomato juice has earlier bee foud to greatly affect its viscosity 5. Below a specific level of fiber cotet (about %), the pulp suspesios behave i a differet fashio, deoted by a drastic chage i the slope of the -value (Fig. B). This could be a idicatio of a chage i the flow regime from plug to poiseuille, i.e. from a yield stress fluid to a o-yieldig oe. CONCLUSIONS The flow behavior of both dried fibers ad pulp suspesios is pseudoplastic ad the rheological data is adequately described by the power law model, withi the covered rage of shear rates (- s-). Large differeces i the rheological behavior betwee dried fibers ad pulps are foud. For pulp suspesios, the flow behavior idex () is early idepedet of the cocetratio of fiber up to a specific fiber cotet (about %), whereas the cosistecy coefficiet (K) decreases expoetially with decreasig WIS i the cocetratio rage studied (-7 % WIS). The structure of the fibers ad the extet to which they were previously processed seem to be the key parameters determiig the rheological properties of these suspesios. ACKNOWLEDGMENTS The authors wish to thak Orkla Foods, Kiviks Musteri ad Lyckeby Culiar for providig the pulp samples. REFERENCES. B. Abu-Jdayil, F. Baat, R. Jumah, S. Al-Asheh ad S. Hammad, A comparative study of rheological characteristics of tomato paste ad tomato powder solutios. It. J. Food Prop., vol. 7, pp , 4.. N. Grigelmo-Miguel, A. Ibarz-Ribas ad O. Martí-Belloso, Rheology of peach dietary fiber suspesios. J. Food Egieerig, vol. 39, pp. 9-99, M.K. Krokida, Z.B. Maroulis ad G.D. Saravacos, Rheological properties of fluid fruit ad vegetable puree products: compilatio of literature data. It. J. Food Prop., vol. 4, pp. 79-,. 4. B.Yoo ad M.A.Rao, Effect of uimodal particle size ad pulp cotet o rheological properties of tomato puree. J. Texture Studies, vol. 5, pp , N. Beresovsky, I.J. Kopelma ad S. Mizrahi, The role of pulp iterparticle iteractio i determiig tomato juice viscosity. J. Food Processig Preservatio, vol. 9, pp.33-46, J.C. Harper ad A.F. El Sahrigi, Viscometric behavior of tomato cocetrates. J. Food Sci., vol. 3, pp ,