Effect of drying methods on rehydration kinetics of potato slices

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1 Journal of Scientific & Industrial Research Vol. 63, May 2004, pp Effect of drying methods on rehydration kinetics of potato slices U Ghosh and H Gangopadhyay* Department of Food Technology & Bio-chemical Engineering, Jadavpur University, Kolkata Received 08 july 2003; accepted 23 February2004 The effect of drying methods on rehydration kinetics of potato slices is investigated at (l0, 35, 60 C), respectively. The effective diffusion coefficients for water and solute are determined, assuming the rehydration process to be governed by fickian diffusion and the apparent moisture (Dew) and solute (Des) diffusion coefficients are calculated. The effect of temperatue on dew, can be interpreted according to Arrhenius law. Low temperature dehydration is found to improve the rehydration characteristics of potato slices. Keywords: Rehydration kinetics, Potato slices, Diffusion coefficient IPC : Int.CI 7 : A 23 L 1/216, C 02 F 9/14 Introduction Potato is one of the most potential crop used by mankind as item of food. Its two principal disadvantages as bulk and comparatively short storage life compared to grains became apparent. People used to dehydrate I sufficient potatoes to provide for periods when potatoes were not available between crops or during its shortage. The drying method and the physico-chemical changes, that occur during drying seem to affect the quality''" such as colour, texture, density, porosity, and the sorption characteristics of the dehydrated product. All these properties characterize product quality and thus the effect of drying. method on these properties is important, so that high quality and convenient products are produced efficiently at competitive cost. Rehydration can be considered as a measure of the injury to the material caused by drying and treatments preceding dehydration+". The amount of water absorbed as well as the rates of absorption are adversely affected if cell disintegration occurs. A study of rehydration kinetics can be used to ascertain the net extent of injuries sustained by food material during dehydration and necessary pretreatments. Although lot of information is available on the drying of fruits and vegetables using different methods, there is a lack of information relating to rehydration. Further, there is no consistency in studying rehydration characteristics with regard to food to *Author for correspondence hganguly2@yahoo.com, Fax: 91-(033) ) water mass ratio, temperature of rehydration, levelof agitation, and procedure for the determination of moisture content". Conventional air drying is one of the most frequently used operation for food dehydration. Significant colour changes can take place during air drying". Vacuum dried materials tend to have higher porosity, depending on the pressure', Freeze drying is one of the most advanced dehydration methods. It provides a dry product witha porous II structure, supenor to taste and aroma retention, compared to products of other drying processes such as, air, vacuum, and microwave drying. This suggests that by choosing a suitable drying method with appropriate conditions the final product quality can be controlled. Therefore, in this study the effect of different drying methods on the rehydration of dried potato has been investigated. Experimental Procedure Materials and Methods Potatoes were procured from local market and washed thoroughly under running water. The potatoes after peeling were cut into slices of 40 mm ±. Thickness of slices was measured with a micrometer and it was found to be 5mm ± 0.2.Five hundred grams of sample were taken for blanching and dehydration. Tray load during dehydration was maintained at 5 kg/m'. One portion of potato slices was blanched in boiling water for 5 min and divided into three parts. Each part was dried separately in tray drier (60 C,

2 GHOSH & GANGOPADHY AY: EFFECT' OF DRYING METHODS ON REHYDRATION KINETICS OF POTATO SLICES h), fluidized bed drier (60 C, 4-5 h) and freeze dryer(-40 C, h). Another portion of potato was blanchedin microwave for 2-3 min and dried in tray drier(60 C, 4-5 h). All these samples were used for rehydrationstudies. Rehydration The dehydrated samples were rehydrated in water at10,35, and 60 C.The slices were weighed and"then subjected to rehydration. One gram of sample was taken for rehydration. The ratio of the weight of potato slices to volume of the medium was maintained at 1:50. The dry matter was estimated afterrehydration at different time intervals, according tothe method described in AOAC I2. The experiments were done in triplicate and average values are reported. Water and Solute Diffusivities During Rehydration For the mathematical analysis, mass transfer can be described by unsteady state Fick's law of diffusion 13, [Eq.(1)]. S -s 64 [ n 2 n2] Sr = _I e =-exp D t es' So-Se.n.{..(2... (4) Des, dew can be calculated from the plot of In [(m r - me)/(mo-me)] against time (t) and in [(sr-se)/so-se)] against time (t) for different process condition. The mass transfer eguation for moisture and solid mass transfer during rehydration is described is [Egs (5 and 6)]: dm -=Km(m -m) dt 1 c' ds --=Ks(s -s ) dt 1 e'... (5)... (6) Integration of [Eg. (5 and 6)] with the appropriate initial condition, resulted in [Egs (7 and 8)]: In ml-me K mt,... (7)... (1) where m is the moisture content (dry basis) D is the diffusion coefficient of moisture (m%), t is the time (s) and x is the distance for diffusion (m). The solution of unsteady state Fick's law of diffusion13 for an infinite slab results in, as described in [Eg. (2)], ml-m e _ I [-(2n-1)2n2 Dat] - 2 L..J 2 exp 2 ' mo-m e rt n=i(2n-1) 4[;... (2) where m, is the average moisture content (dry basis) at time t, m., is the initial moisture content (dry basis) L is the half thickness of sample (m), me is the equilibrium moisture content (dry basis). For sufficiently long rehydration times, only the first term of n = I in Eg. (2) can be used with small \. error. The geometry of the samples in experiments can be considered as a 2-D finite slab. The solution for the finite slab is obtained by applying Newman's rule".... (3) S -s In _I e =Kst, So-se... (8) Km, Ks can be calculated from the plot of In [(m r - me)/(mo-me)] against time (t) and in [( SISe)/So-se)] against time (t) for different process condition. Water absorption index of dry samples was measured according to the method described by Anderson et al.", Water soluble index difines the amount of water soluble solids recovered by evaporating the supernatant from the water absorption determination and was expressed as weight of solids/initial weight of the sample. Results and Discussion The solid and moisture content of the samples during rehydration were determined with time at rehydration temperatures of 10, 35 and 60 C and is shown in Figure 1(a-c) and Figure 2(a-c) respectively. The rate of change of moisture (dm/dt) as well as solid (ds/dt) content obtained from Figure 2(a-c) and Figure l(a-c), respectively, were plotted against average moisture and solid content Figure 4(a-c) and Figure 3(a-c), respectively, and the equilibrium moisture and solid content (me, Se) were inferred from these plots. The moisture and solid mass transfer

3 454 J SCI IND RES VOL. 63 MAY C -'t C -0 Q) Q) on.. -0 c. E c 0 0 e 0 u u C) CU -0 - E E 0 0 C) on V') c.. on c 1.0 b-.. WI c 0.9 cu '- C) E c u 01 \oj E 0.3 Q) -0 O.? E E '" V') C) s on -0 -c Q).- E CJ) 0 -- u E -0 o 0 V') O SL-...l-...L.. --"-...J o Rehydration time (h) on -0 cu o s C 0.4 E c C) 0 ;-... u Q) 0 2 E '- 0) VI 0.' c Rehydration time (hi Fig. I(a-c)--Solid content with time during rehydration of potato slices atl 0, 35 and 60 C, respectively Fig. 2(a-c)--Moisture content with time during rehydrationof potato slices at 10,35 and 60 C, respectively

4 GHOSH & GANGOPADHYAY: EFFECT:, OF DRYING METHODS ON REHYDRATION KINETICS OF POTATO SLICES 455 coefficients (Kill, I(.)were calculated from semi log plottingof 111(111/111,,-111e against time and SI- s/so-se againstime. Rehydration kinetics of dried potato were analyzed according to the unsteady state Ficks law of diffusion13. Using the appropriate solution of the unsteadystate, diffusion equation for a 2-D finite slab (Eqs2 and 3), the apparent diffusion coefficients of water (Dew) and solid (Des)were determined by logarithmicplotting of e/111" - 111eand S( s; / So - s, against time for each sample which are shown in Figure 5(a-c) and Figure 6(a-c) respectively. The calculated values of Dew' Des together with mass transfer coefficients (KIll> 1(,) and also equilibrium moisture and solid content (111e, se) for samples dehydrated in different ways and rehydrated in different temperatures are given in Tablel. The diffusivity values for water infusion and solute loss 0 20r------, a 0.1' r-----, f/ "ti '; 0.' "ti '-:-.5-:0:'-:.6-0:-"-.70.':-e -:-'0.9 o.o,'-:--::'-::--='::--='::-::-' O.} 0: Avg. solid Avg. solid b 0.0,=---:-'---,--,'--' ,9 Avg. solid Fig.3(a-c)--Rate of change of solid (ds/dt) with average solid (kg/kg) during rehydration of potato slices at 10, 35 and 60 C, respectively 0.16 a 0 16 b 0.16 c a 0 7 "- 0. Vl "" Vl Time Time lh) lh) 6 7 b ' V') o.o.:,-::--:,-:--:!-c---::,-:-, ,) 0. O.} O ) 0. 0; ) ().4 0,,5 Avg. moisture Avg. moisture. AVQ. moisture Fig.4(a- c)--rate of change of moisture (dm/dt) with average moisture(kg/kg) during rehydration of potato slices at 10, 35 and 60 C, respectively 2 3" 4 5 r:me lhl 6 7 Fig. 5(a-c)--Solute ratio with time during rehydration of potato slices at 10,35 and 60 C,respectively

5 456 J SCI IND RES VOL. 63 MAY : , during rehydration were more in freeze dried sample than tray dried or fluidized bed samples. The diffusivity increased with rehydration temperature. Characteristics of potato powders are shown in Table 2. 7 Conclusion Samples dehydrated at high temperature (tray and fluidized bed) could not absorb more water as compared with low temperature dehydrated, (Freeze dried) sample. This may be due to cellular and structural disruption 16 and resulting in the inabilityof shrunken capillaries to imbibe sufficient water to '.'rehydrate fully. 0.7 b Acknowledgement The authors gratefully acknowledges the financial assistance provided by Department of Food Processing Industries and Horticulture (GOWB) in carrying out this work Time (h) 0.7 o s L-L_..l-_L- L:_---JL-_::' o Time (h) Fig. 6(a c}--moisture ratio with time during rehydration of potato slices at 10, 35 and 60 0 C, respecti vely References I Woolfe J A, Processing, in The potato in human diet (Cambridge University Press, London, UK) Krokida M K & Maroulis Z B, Effect of drying methodon shrinkage and porosity, Drying Technol, 10 (1997) Krokida M K & Maroulis Z B, The effect of drying method on viscoelastic behaviour of dehydrated fruits and vegetables, J Food Sci Technol, 35 (2000) Krokida M K, Zogzas N P & Maroulis Z B, Modelling shrinkage and. porosity during vacuum dehydration, 1111 J Food Sci Technol, 32 (1997) Krokida M K, Tsarni E& Maroulis Z B, Kinetics of colour changes during drying of some fruits and vegetables, Drying Technol, 16 (1998) Krokida M K, Karathanos V T & Maroulis Z B, Viscoelastic behaviour of dehydrated carrot and potato, Drying Technol, 16 (1998) Saravocas G D, Technological developments in fruits& vegetable dehydration, in Food flavour,ingredients and composition, edited by G Charalambous (Elsevier, London) 1981, Okes M R, Narishman G, Singh R K & Weitnauer A C, Food dehydration, in Handbook of food engineering (Marcel Dekker, New York) McMinn W A M & Magee T R A, Physical characteristics of dehydrated potatoes, J Food Eng, 33 (1997) Lewicki P P, Some remarks on rehydration of dried foods,j Food Eng, 36 (1998) 81. II Karathanos V T, Anglea S A & Karel M, Structure collapse of plant materials during freeze drying, J Therrn Analys, 46 (1996) AOAC methods of analysis, lothed (AOAC,Washington DC) Crank I Mathematics of diffusion, 2 nd ed (Oxford Clarendon Press, Oxford) 1975,24.

6 GHOSH & GANGOPADHYA Y: EFFECT..OF DRYING METHODS ON REHYDRATION KINETICS OF POTATO SLICES 457 s - e Mr- Sr - dmldt - dsldt - Km- Kstable I-Effective diffusion coefficients of water (Dew) and solute (Des) during rehydration of potato slices at 10,35 and 60 C Sample Rehydration, me s, K M x10 5 Ksx 10 5 Dewxl0 10 Des xlolo (0C) (kg/kg) (kg/kg) (i/s) (i/s) (m 2 /s) (m2/s) A B C A B C ) A B C A-Tray drying, water, blanching, B-Tray drying, microwave blanching, C-Fluidized bed drying, D-Freeze drying Table 2--Characteristics of potato powder dried in different dryers 51. Sample Bulk WAI WSI Activation Relation of diffusivity with No. density (gig) (gig) energy temperature (g/cc) (kj/mol) Tray drying water blanching "2.18 Dew= 5.31 X e( rr).78 Des= 2.14 x e(-932rr) 2 Tray drying microwave blanching "1.99 Dew= 5.07 X 10-10e( ff) blo.75 Des= 0.76 X e(-1287rr) 3 Fluidized bed drying "2.72 Dew= 6.61 X 10-10e( ff) b Des= 0_02 X e(-1486ff) 4 Freeze drying 0_86 6_ "1.26 Dew= 3.94 X 10-10e( ff) b4.79 Des= 0.11 X e( rr) a - activation energy for moisture diffusion, b - activation energy for solute diffusion 14 Treybal R E, Mass transfer operations, 2 nd ed (McGraw Hill, New York) 1968, Anderson R A, Conway H F, Ppcfer V F & Griffin E P, Gelatinization of corn grits by roll and extrusion cooking, Cereal Sci Today,14 (1969) Jayaraman K S, Dasgupta D K & Babu Rao N, Effect of pre treatment with salt and stability of dehydrated cauliflower, lilt J Food Sci Teclmol, 2S (1990) 47. Nomenclature 0- diffusion coefficient of moisture, (m2/s) t- time,(s) x - distance for diffusion, m m,- average moisture content at time t, dry basis mom e - L- Dew - Des - initial moisture content, dry basis equilibrium moisture content, dry basis Half thickness of sample, III effective diffusivity of water nlls effective diffusivity of solute m 2 1s average solid content at time t, dry basis Initial solid content, dry basis equilibrium solid content, dry basis moisture ratio Solute ratio rate of change of moisture rate of change of solid mass transfer coefficient for moisture, lis mass transfer coefficient for solid, lis