Journal of Scientific & Industrial ResearchVol. 63, May 2004, pp 452-457

water mass ratio, temperature of rehydration, levelofagitation, and procedure for the determination ofmoisture content". Conventional air drying is oneofthe most frequently used operation for fooddehydration. Significant colour changes can takeplace during air drying". Vacuum dried materials tendto have higher porosity, depending on the pressure',Freeze drying is one of the most advanceddehydration methods. It provides a dry product witha

II· dporous structure , supenor to taste an aromaretention, compared to products of other dryingprocesses such as, air, vacuum, and microwavedrying.

This suggests that by choosing a suitable dryingmethod with appropriate conditions the final productquality can be controlled. Therefore, in this study theeffect of different drying methods on the rehydrationof dried potato has been investigated.

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 700 032

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. Theeffective diffusion coefficients for water and solute are determined, assuming the rehydration process to be governed byfickian diffusion and the apparent moisture (Dew) and solute (Des) diffusion coefficients are calculated. The effect oftemperatue on dew, can be interpreted according to Arrhenius law. Low temperature dehydration is found to improve therehydration characteristics of potato slices.

Keywords: Rehydration kinetics, Potato slices, Diffusion coefficient

IPC : Int.CI7 : A 23 L 1/216, C 02 F 9/14

IntroductionPotato is one of the most potential crop used by

mankind as item of food. Its two principaldisadvantages as bulk and comparatively short storagelife compared to grains became apparent. People usedto dehydrate I sufficient potatoes to provide forperiods when potatoes were not available betweencrops or during its shortage. The drying method andthe physico-chemical changes, that occur duringdrying seem to affect the quality''" such as colour,texture, density, porosity, and the sorptioncharacteristics of the dehydrated product. All theseproperties characterize product quality and thus theeffect of drying. method on these properties isimportant, so that high quality and convenientproducts are produced efficiently at competitive cost.

Rehydration can be considered as a measure of theinjury to the material caused by drying and treatmentspreceding dehydration+". The amount of waterabsorbed as well as the rates of absorption areadversely affected if cell disintegration occurs. Astudy of rehydration kinetics can be used to ascertainthe net extent of injuries sustained by food materialduring dehydration and necessary pretreatments.Although lot of information is available on the dryingof fruits and vegetables using different methods, thereis a lack of information relating to rehydration.Further, there is no consistency in studyingrehydration characteristics with regard to food to

*Author for correspondenceE-mail: hganguly2@yahoo.com, Fax: 91-(033) 2414-6822)

Experimental ProcedureMaterials and Methods

Potatoes were procured from local market andwashed thoroughly under running water. The potatoesafter peeling were cut into slices of 40 mm ± 0.5.Thickness of slices was measured with a micrometerand it was found to be 5mm ± 0.2.Five hundred gramsof sample were taken for blanching anddehydration. Tray load during dehydration wasmaintained at 5 kg/m'.

One portion of potato slices was blanched inboiling water for 5 min and divided into three parts.Each part was dried separately in tray drier (60°C,

s

S -s 64 [ n2 n2]Sr = _I__ e =-exp- -+-- D t2 402 40 2 es'So-Se .n .{..(2

GHOSH & GANGOPADHY AY: EFFECT' OF DRYING METHODS ON REHYDRATION KINETICS OF POTATO SLICES 453

... (4)

Des, dew can be calculated from the plot of In [(mr-

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 solidmass transfer during rehydration is described is [Egs(5 and 6)]:

dm-=Km(m -m)dt 1 c'

... (5)

4-5 h), fluidized bed drier (60°C, 4-5 h) and freezedryer(-40°C, 22-24 h). Another portion of potato wasblanchedin microwave for 2-3 min and dried in traydrier(60°C, 4-5 h). All these samples were used forrehydrationstudies.

RehydrationThe dehydrated samples were rehydrated in water

at10,35, and 60°C.The slices were weighed and"thensubjected to rehydration. One gram of sample wastaken for rehydration. The ratio of the weight ofpotato slices to volume of the medium wasmaintained at 1:50. The dry matter was estimatedafterrehydration at different time intervals, accordingtothe method described in AOACI2

. The experimentswere done in triplicate and average values arereported.

Water and Solute Diffusivities During RehydrationFor the mathematical analysis, mass transfer can be

described by unsteady state Fick's law of diffusion 13,

[Eq.(1)].

... (1)

ds--=Ks(s -s )dt 1 e'

... (6)

where m is the moisture content (dry basis) D is thediffusion 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 ofdiffusion13 for an infinite slab results in, as describedin [Eg. (2)],

ml-me _~ ~ I [-(2n-1)2n2 Dat]- 2 L..J 2 exp 2 '

mo-me rt n=I(2n-1) 4[;

Integration of [Eg. (5 and 6)] with the appropriateinitial condition, resulted in [Egs (7 and 8)]:

In ml-me Kmt ,

... (2)

where m, is the average moisture content (dry basis) attime t, m., is the initial moisture content (dry basis) Lis the half thickness of sample (m), me is theequilibrium moisture content (dry basis).

For sufficiently long rehydration times, only thefirst term of n = I in Eg. (2) can be used with small

\ .error. The geometry of the samples in experimentscan be considered as a 2-D finite slab. The solutionfor the finite slab is obtained by applying Newman'srule".

... (3)

... (7)

S -sIn _I__ e =Kst ,So-se

... (8)

Km, Ks can be calculated from the plot of In [(mr-

me)/(mo-me)] against time (t) and in [(SI~Se)/So-se)]against time (t) for different process condition.

Water absorption index of dry samples wasmeasured according to the method described byAnderson et al.", Water soluble index difines theamount of water soluble solids recovered byevaporating the supernatant from the water absorptiondetermination and was expressed as weight ofsolids/initial weight of the sample.

Results and DiscussionThe solid and moisture content of the samples

during rehydration were determined with time atrehydration temperatures of 10, 35 and 60°C and isshown in Figure 1(a-c) and Figure 2(a-c) respectively.

The rate of change of moisture (dm/dt) as well assolid (ds/dt) content obtained from Figure 2(a-c) andFigure l(a-c), respectively, were plotted againstaverage moisture and solid content Figure 4(a-c) andFigure 3(a-c), respectively, and the equilibriummoisture and solid content (me, Se) were inferred fromthese plots. The moisture and solid mass transfer

coeploaga..I.•..

C-'t C-0 Q) on Q) acc-0

...•.. c. diftE c e 00 0 u uns--... u C) (Eq~

__ CU-0 ~ ~ watE ~- E .•.. log;0 0 C) onV')

0 - s,0·5 ~ Fig!

0 2 4 6 2 4 - 6 calctranmoi

1.0 b- deh•.. 0.5 diff•.. cWI c 0.9 cu diff-0 41 .•.. 0·4•.. on cE c -0 0

0 u01 \oJ E 0.3'- C) Q)~ -0 O.? --... ~

~E - ~ •.. 0.201 0 E '"V') 0.6

C) 0 0.10.5 s .;

0 2 6 --4 ..•-0

454 J SCI IND RES VOL. 63 MAY 2004

o·s c.•..

on C 0.4-0 cu.•..E cC) 0;-..... u~ Q) 0·2E '-~0) 0.'VI

00.0~ 0 2 4 6

Rehydration time (hI

-cQ)

E ~CJ) 0-- u~E -0

o 0V')

on-0.-

O·SL- __ ....l- __ .....L..__ --"-...Jo 2 4 6

Rehydration time (h)

Fig. I(a-c)--Solid content with time during rehydration ofpotato slices atl 0, 35 and 60° C, respectively

Fig. 2(a-c)--Moisture content with time during rehydrationofpotato slices at 10,35 and 60° C, respectively

Fig .(kg/krespe

o

ooo

~(

~ (

o

o

Fig.moistl60°C,

GHOSH & GANGOPADHYAY: EFFECT:, OF DRYING METHODS ON REHYDRATION KINETICS OF POTATO SLICES 455

coefficients (Kill, I(.)were calculated from semi logplottingof 111(111/111,,-111e against time and SI- s/so-se

againsttime.Rehydration kinetics of dried potato were analyzed

according to the unsteady state Ficks law ofdiffusion13. Using the appropriate solution of theunsteadystate, diffusion equation for a 2-D finite slab(Eqs2 and 3), the apparent diffusion coefficients ofwater (Dew) and solid (Des)were determined bylogarithmicplotting of 1111 - 111e/111" - 111eand S( s; / So

- s, against time for each sample which are shown inFigure 5(a-c) and Figure 6(a-c) respectively. Thecalculated values of Dew' Des together with masstransfer coefficients (KIll> 1(,) and also equilibriummoisture and solid content (111e, se) for samplesdehydrated in different ways and rehydrated indifferent temperatures are given in Tablel. Thediffusivity values for water infusion and solute loss

0·20r------,

0.1'

0·20r-----,a b

0.16

0.14

0.12

• 0.10"tI

f/'; 0.'"tI

0·6

0·4

0.2 0·2

o .O,'-:--::'-::--='::--='::-::-'O.} 0:6 0.7 0·80.9

Avg. solid

0·~'-:-.5-:0:'-:.6-0:-"-.7~0.':-e-:-'0.9

Avg. solid

0.0,=---:-'---,--,'--'0.5 0.6 0.7 0·8 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

0.1'

0.12

0·0 0.0 o.o.~:,-::--:,-:--:!-c---::,-:-,0·1 0·2 0,) 0.• O.} O~ 0·2 0·) 0.• 0.5 0;1 0·2 0.) ().4 0,,5

Avg. moisture Avg. moisture. AVQ. moisture

Fig.4(a- c)--Rate of change of moisture (dm/dt) with averagemoisture(kg/kg) during rehydration of potato slices at 10, 35 and60°C, respectively

0.90·8 a0·70.60.5

"- 0 .•Vl

0·3

2 3 4 5

Time lh)

0.70.60.5

""0·4

Vl

0·3

2 345Time lh)

•..V')

0.50·4

0·3

0·2

2 3" 4 5r:me lhl

Fig. 5(a-c)--Solute ratio with time during rehydration ofpotato slices at 10,35 and 60° C ,respectively

6 7

b

6 7

6 7

456 J SCI IND RES VOL. 63 MAY 2004

0·7.------:--------,0.6

0·1L-L_..l-_L-~ _ _L:_---JL-_::'o 2 345 6 7Time (h)

during rehydration were more in freeze dried samplethan tray dried or fluidized bed samples. Thediffusivity increased with rehydration temperature.Characteristics of potato powders are shown inTable 2.

ConclusionSamples dehydrated at high temperature (tray and

fluidized bed) could not absorb more water ascompared with low temperature dehydrated, (Freezedried) sample. This may be due to cellular andstructural disruption 16 and resulting in the inabilityofshrunken capillaries to imbibe sufficient water to

'.'rehydrate fully.

AcknowledgementThe authors gratefully acknowledges the financial

assistance provided by Department of FoodProcessing Industries and Horticulture (GOWB) incarrying out this work.

ReferencesI Woolfe J A, Processing, in The potato in human diet

(Cambridge University Press, London, UK) 1987.2 Krokida M K & Maroulis Z B, Effect of drying methodon

shrinkage and porosity, Drying Technol, 10 (1997) 1145.3 Krokida M K & Maroulis Z B, The effect of drying method

on viscoelastic behaviour of dehydrated fruits andvegetables, J Food Sci Technol, 35 (2000) 291.

4 Krokida M K, Zogzas N P & Maroulis Z B, Modellingshrinkage and. porosity during vacuum dehydration, 1111 JFood Sci Technol, 32 (1997) 445.

5 Krokida M K, Tsarni E& Maroulis Z B, Kinetics of colourchanges during drying of some fruits and vegetables, DryingTechnol, 16 (1998) 667.

6 Krokida M K, Karathanos V T & Maroulis Z B,Viscoelastic behaviour of dehydrated carrot and potato,Drying Technol, 16 (1998) 687.

7 Saravocas G D, Technological developments in fruits&vegetable dehydration, in Food flavour,ingredients andcomposition, edited by G Charalambous (Elsevier,London) 1981,389.

8 Okes M R, Narishman G, Singh R K & Weitnauer A C,Food dehydration, in Handbook of food engineering(Marcel Dekker, New York) 437.

9 McMinn W A M & Magee T R A, Physical characteristicsof dehydrated potatoes, J Food Eng, 33 (1997) 49.

10 Lewicki P P, Some remarks on rehydration of dried foods,JFood Eng, 36 (1998) 81.

II Karathanos V T, Anglea S A & Karel M, Structure collapseof plant materials during freeze drying, J Therrn Analys, 46(1996) 1541.

12 AOAC methods of analysis, lOthed (AOAC,WashingtonDC) 1965.

13 Crank I Mathematics of diffusion, 2nd ed (Oxford ClarendonPress, Oxford) 1975,24.

7

0.7b0.6

0.5

0·4

.. 0·3~

1·2

0-10 2 3 4- 5

Time (h)

0.70.6o·s0••

•..~ 0·3

0·2

Fig. 6(a·c}--Moisture ratio with time during rehydration ofpotato slices at 10, 35 and 600 C, respecti vely

GHOSH & GANGOPADHYA Y: EFFECT ..OF DRYING METHODS ON REHYDRATION KINETICS OF POTATO SLICES 457

14 Treybal R E, Mass transfer operations, 2nd ed (McGrawHill, New York) 1968,717.

15 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) 4.

16 Jayaraman K S, Dasgupta D K & Babu Rao N, Effect of pretreatment with salt and stability of dehydrated cauliflower,lilt J Food Sci Teclmol, 2S (1990) 47.

mO-me-

L-Dew -Des -

s -e

Mr-Sr -dmldt -dsldt -Km-Ks-

table I-Effective diffusion coefficients of water (Dew) and solute (Des) during rehydration of potato slices at 10,35 and 60 C

SampleRehydration, me s, KMx105 Ksx 105 Dewxl010 DesxlOlO

(0C) (kg/kg) (kg/kg) (i/s) (i/s) (m2/s) (m2/s)

A 10 0.32 0.60 6.35 1.02 2.26 0.36B 10 0.34 0.50 6.41 2.16 2.28 0.77C 10 0.45 0.49 6.42 2.71 2.29 0.980 10 0.65 0.39 6.43 2.83 2.29 1.01A 35 0.41 0.64 6.41 1.31 2.28 0.47B 35 0.43 0.56 6.55 3.26 2.33 1.16C 35 0.50 0.54 6.43 3.43 2)9 1.230 35 0.68 0.43 6.79 4.65 2.42 1.66A 60 0.42 0.76 6.81 3.47 2.43 1.23B 60 0.46 0.58 6.93 4.49 2.47 1.59C 60 0.52 0.56 6.96 5.01 2.48 1.770 60 0.70 0.52 7.05 5.34 2.51 1.91

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 withNo. density (gIg) (gIg) energy temperature

(g/cc) (kj/mol)

Tray drying water blanching 0.84 6.52 0.09 "2.18 Dew= 5.31 X 10.10e(-260.75rr)

~.78 Des= 2.14 x 10-10e(-932rr)

2 Tray drying microwave blanching 0.81 6.51 0.09 "1.99 Dew= 5.07 X 10-10e(-238.94ff)blO.75 Des= 0.76 X 10-10e(-1287rr)

3 Fluidized bed drying 0.88 6.49 0.09 "2.72 Dew= 6.61 X 10-10e(-325.48ff)b12.4 Des= 0_02 X 10-10e(-1486ff)

4 Freeze drying 0_86 6_65 0.08 "1.26 Dew= 3.94 X 10-10e(-150.48ff)b4.79 Des= 0.11 X 10-10e(-573.73rr)

a - activation energy for moisture diffusion, b - activation energy for solute diffusion

Nomenclature0- diffusion coefficient of moisture, (m2/s)t- time,(s)x - distance for diffusion, mm,- average moisture content at time t, dry basis

initial moisture content, dry basisequilibrium moisture content, dry basisHalf thickness of sample, IIIeffective diffusivity of water nllseffective diffusivity of solute m21saverage solid content at time t, dry basisInitial solid content, dry basisequilibrium solid content, dry basismoisture ratioSolute ratiorate of change of moisturerate of change of solidmass transfer coefficient for moisture, lismass transfer coefficient for solid, lis