OPTIMIZATION OF OSMOTIC DEHYDRATION PROCESS OF CARROT CUBES IN SUCROSE SOLUTION

OPTIMIZATION OF OSMOTIC DEHYDRATION PROCESS OFCARROT CUBES IN SUCROSE SOLUTION

BAHADUR SINGH1, PARMJIT S. PANESAR and VIKAS NANDA

Department of Food Engineering and TechnologySant Longowal Institute of Engineering and Technology

Longowal (Sangrur) 148106, India

Accepted for Publication April 6, 2007

ABSTRACT

For optimization of the osmotic dehydration process of carrot cubes insucrose solution by response surface methodology (RSM), the experimentswere conducted according to face-centered central composite design. Theindependent process variables for the osmotic dehydration process wereosmotic solution concentrations (45–55°Brix), temperature (35–55C) andprocess durations (120–240 min). Statistical analysis of results showed that allthe process variables had a significant effect on all the responses at 5% levelof significance (P < 0.05). The osmotic dehydration process was optimized byRSM for maximum water loss, rehydration ratio, retention of color, sensoryscore and minimum solute gain. The optimum process conditions were52.5°Brix sucrose syrup concentration, 49C osmotic solution temperature and150-min process duration.

PRACTICAL APPLICATIONS

The process of osmotic dehydration can be used for the preparation ofshelf-stable products for the purpose of use during off-season. The quality ofpreosmosed carrots is much superior to the product dehydrated with theconvectional method of convective dehydration. The osmotically dehydratedcarrots can be used for cooking as vegetables after rehydration or can be addeddirectly into soups, stews or casseroles before cooking. If the product isblanched before osmotic dehydration, the process can be used successfully forthe preparation of carrot candy.

1 Corresponding author. EMAIL: [email protected]

Journal of Food Process Engineering 31 (2008) 1–20. All Rights Reserved.© 2008, The Author(s)Journal compilation © 2008, Blackwell Publishing

1

mailto:[email protected]

INTRODUCTION

Carrots (Daucus carota L.) are one of the most important cool season rootvegetables grown extensively in various countries particularly during winterseason in tropical regions. Carrots are known for their nutrient contents, viz.,carotene and carotenoids, besides appreciable amounts of vitamins B1, B2, B6

and B12 and minerals. Carrot roots are used as salad, cooked vegetables, inpreparation of soups, stews, curries, sweetmeats, juices, flakes, fermentedpickles and soups. Hence, carrots occupy an important place in root vegetablesfor their multifaceted application, which has, in turn, resulted in the develop-ment of various processing operations for making different products and/or toextend shelf life. Fresh carrots cannot be stored for more than 3–4 days underordinary conditions, but shelf life can be extended to 7–8 months if stored incrates covered with perforated plastic film at 0C and 93–96% relative humidity(Chadda 2002). The other methods of extending shelf life are fermenting,pickling, canning or cold storage freeze-drying, etc. (Mudahar et al. 1989).Out of these methods, freeze-drying produces the highest quality food prod-ucts, but it is the expensive method of preservation. Therefore, there is a needfor a simple and inexpensive alternate process, which has low capital invest-ment and offers a way to save highly perishable products and make themavailable for the regions away from production zones. Osmotic dehydration isone of these new methods (Shi and Le Maguer 2002).

Osmotic dehydration is the process by which there is partial removal ofwater from the cellular materials when these are placed in a concentratedsolution of soluble solute. Osmotic dehydration, which is effective even atambient temperature and saves the color, flavor and texture of food from heat,is used as a pretreatment to improve the nutritional, sensorial and functionalproperties of food. The amount of water remaining in the material, however,does not ensure its stability, as water activity is generally higher than 0.9. Atwater activity of 0.9 and below, bacterial activity is inhibited. Water activityreflects a combination of water-solute and water-surface interactions and cap-illary forces. When shelf stability is an ultimate process objective, othercomplementary methods of water removal, such as convective drying, freeze-drying, vacuum drying, freezing, etc., are suggested (Nanjundaswamy et al.1978; Lenart and Cerkowniak 1996).

Sugar, glucose, fructose, corn syrup and sodium chloride are the commonosmotic agents. The influence of the main process variables, such as concen-tration and composition of osmotic solution, temperature, immersion time,pretreatments, agitation, nature of food and its geometry, and solution tosample ratio on the mass transfer mechanism has been studied extensively(Rastogi and Raghavarao 1997; Kaymak-Ertekin and Sultanoglu 2000;Matusek and Meresz 2002; Singh and Gupta 2007; Singh, Kumar et al. 2007).

2 B. SINGH, P.S. PANESAR and V. NANDA

Statistical designs are powerful tools that can be used to account for themain as well as the interactive influences of different process parameters on theosmotic dehydration process. Among them, response surface methodology(RSM) is a collection of certain statistical techniques for designing experi-ments, building models, evaluating the effects of the factors and searching foroptimal conditions for desirable responses (Myers and Montgomery 2002).Therefore, during the last decades, RSM has been extensively applied in theoptimization of different processes in food applications. However, only limitedefforts have so far been made to optimize the osmotic dehydration of carrots.Mazza (1983) reported the effect of dipping carrot dices in sucrose and sodiumchloride solution for 30 min prior to air drying on moisture transport duringdehydration and on product quality. Effect of varying sizes of carrot piecesosmosed in cane sugar solution and of their subsequent vacuum drying on theosmotic and vacuum drying characteristics and on the product quality wasreported by Chopra and Verma (1994). The process of osmotic dehydration forcarrots has been optimized for sodium chloride salt solution and for mixture ofsucrose and salt (Singh et al. 2006; Singh, Panesar et al., 2007). The purposeof this work was to optimize the osmotic dehydration process of carrots byusing sucrose solution by RSM.

MATERIALS AND METHODS

Experimental Design

In this study, for the optimization of osmotic dehydration process of carrotcubes in sucrose solution, the experimental design used was face-centeredcentral composite design (CCF) for process variables (osmotic solution con-centration, temperature and immersion time), each at three levels and sixreplicates at the center (to account for pure internal error). This design is one ofthe three types of Box–Wilson Central Composite designs. Central compositedesign is the most popular of the many classes of RSM designs (Khuri andCornell 1987). A central composite design contains an imbedded factorial orfractional factorial design with center points that is augmented with a group of“star points” that allows estimation of curvature. If the distance from the centerof the design space to a factorial point is �1 unit for each factor, the distancefrom the center of the design space to a star point is �a with |a| > 1. In CCF, thestar points are at the center of each face of the factorial space, so a = �1. Thisdesign requires three levels for each factor, thus making the total number ofexperiments equal to 20 instead of 27 with full factorial design.

Total number of experiments no of variablesNo of variables= + ×+

2 2. .ccentral points

OSMOTIC DEHYDRATION OF CARROTS 3

Three different levels for each experiment in coded form are -1, 0, +1:

Coded value = =−( )

xX X

Rii i

i

2(1)

where Xi is the actual value in the uncoded (original) units of the ith factor; X̄i

is the average of the low and high levels for the ith factor; and Ri is thedifference (spacing) between the low and high levels of Xi.

The independent process variables were osmotic solution concentration,process duration and osmotic solution temperature. The low and high levels inthe actual (uncoded) form were taken as 45–55°Brix, 35–55C and 120–240 min for osmotic solution concentration, temperature and process time,respectively (Lazarides et al. 1995; Rastogi and Raghavarao 1997; Serenoet al. 2001; Ade-Omowaye et al. 2002). The sample/solution ratio was kept as1:5 (Simal et al. 1997; Kar and Gupta 2001, 2003; Pokharkar and Prasad2002). The levels of the input variables in coded (xi) and uncoded (Xi) form aregiven in Table 1.

Preparation of Sample

Fresh, well-graded carrots (5 kg) were procured from the local market.The weight of individual carrots varied between 40 and 70 g, the diameterbetween 2 and 3 cm, and lengths between 15 and 20 cm. The initial moisturecontent of the fresh carrots was 90.2–92.0% (wet basis). The carrots werewashed and peeled manually. The green parts of the carrots were removed toretain the uniform quality of the final product. Carrot cubes of dimensions1 cm3 were prepared by a manual dicer. The carrot cubes were washed withfresh water to remove the carrot fines adhering to the surface of the carrot

TABLE 1.THE LEVELS OF DIFFERENT PROCESS VARIABLES IN

CODED AND UNCODED FORM FOR OSMOTICDEHYDRATION

Coded levels Uncoded values of process variables

Concentration(°Brix)(X1)

Time (min)(X2)

Temperature (C)(X3)

-1 45 120 350 50 180 45

+1 55 240 55


cubes. No blanching was carried out prior to osmosis, as it was detrimental tothe osmotic dehydration process because of loss of semipermeability of thecell membrane and reduction of b-carotene (Kalra 1990; Bao and Chang 1994;Negi and Roy 2000).

Osmotic Dehydration

For each experiment, 120 g of carrot cubes was put in stainless steelcontainers containing calculated volumes of osmotic solutions of differentconcentrations preset at the desired temperature in a hot water bath. To main-tain the desired level of sample/solution ratio, the volume (Vs) of the knownweights (Wo) of carrot cubes was measured by water displacement method andwas related by the equation

V Ws = × −0 993 0 5211. .o (2)

The temperature was maintained during osmosis, and agitation was givenfor reducing the mass transfer resistance at the surface of the carrots and forgood mixing and close temperature control in the osmotic medium (Mavroudiset al. 1998). During experimentation, it was assumed that the amount of solidleaching out of carrots during osmosis was negligible (Biswal and Bozorgmehr1992; Lazarides et al. 1995).

After the specified immersion times (Table 2), the carrot cubes wereremoved from the osmotic solutions and rinsed with water to remove surplussolvent adhering to the surfaces. These osmotically dehydrated cubes werethen spread on the absorbent paper to remove the free water present on thesurface. A proportion of the product carrot (15–20 g) was used for determina-tion of dry matter by oven drying. The water content was determined by dryingin a vacuum oven (70C at 100 mmHg) until a constant weight was reached(AOAC 1995). The remaining part of each product sample was dried to finalmoisture of 5% (wet basis) using a hot air drier preset at 65C air temperatureand 1.6-m/s air velocity. The dried samples were cooled in a desiccator con-taining silica gel for 1 h, packed in high-density polyethylene bags and kept atambient temperature for quality analysis for 10 days. The quality analysis wascarried out after the completion of all sets of experiments.

The experimental design along with values of various responses is given inTable 2. The experiments were conducted randomly to minimize the effects ofunexplained variability in the observed responses because of external factors.

Water Loss and Solute Gain during Osmotic Dehydration

The water loss and solute gain during osmotic dehydration were calcu-lated by the equations given by Ozen et al. (2002):


TAB

LE

2.E

XPE

RIM

EN

TAL

DE

SIG

NIN

CO

DE

DFO

RM

OF

PRO

CE

SSV

AR

IAB

LE

SA

ND

VA

LU

ES

OF

EX

PER

IME

NTA

LD

ATA

FOR

OPT

IMIZ

AT

ION

OF

OSM

OT

ICD

EH

YD

RA

TIO

NPR

OC

ESS

Cod

edpr

oces

sva

riab

les

Unc

oded

proc

ess

vari

able

sR

espo

nses

Con

cn(x

1)T

ime

(x2)

Tem

p(x

3)C

oncn

(°B

rix)

(X1)

Tim

e(m

in)

(X2)

Tem

p(C

)(X

3)

Wat

erlo

ss(Y

1)So

lute

gain

(Y2)

Reh

ydra

tion

ratio

(Y3)

Lva

lue

(Y4)

ava

lue

(Y5)

bva

lue

(Y6)

Sens

ory

scor

e(Y

7)

+1-1

-155

120

3524

.545

06.

1870

3.32

5735

.69

28.7

617

.04

6.5

00

050

180

4533

.856

911

.475

43.

0887

39.8

924

.35

18.1

18.

10

00

5018

045

33.9

658

11.4

879

3.08

5939

.92

24.3

818

.01

8.4

00

050

180

4533

.987

111

.480

93.

0808

39.9

424

.36

18.0

18.

2-1

+1-1

4524

035

28.2

340

9.12

563.

0892

34.4

825

.93

18.5

86.

70

-10

5012

045

29.4

257

9.99

893.

3011

38.6

724

.817

.69

7.5

+10

055

180

4534

.812

511

.599

93.

0327

40.1

124

.02

18.8

38.

50

10

5024

045

36.1

985

12.2

545

3.03

1438

.77

23.2

017

.81

8.5

00

+150

180

5540

.698

913

.504

12.

9975

40.6

523

.84

17.6

18.

0-1

-1-1

4512

035

22.5

875

7.94

723.

3199

35.8

427

.97

18.0

56.

30

0-1

5018

035

27.3

998

7.81

343.

1769

36.6

526

.37

17.2

57.

0+1

-1+1

5512

055

36.9

253

12.1

042

3.18

9938

.57

24.8

118

.23

7.8

-10

045

180

4532

.712

512

.068

73.

0517

39.7

724

.30

19.1

66.

80

00

5018

045

33.9

754

11.4

790

3.07

8639

.88

24.3

718

.02

8.3

00

050

180

4533

.887

511

.498

53.

0785

39.7

924

.36

18.0

98.

2+1

+1+1

5524

055

44.6

852

15.4

012

2.81

4040

.66

23.4

918

.52

9.0

+1+1

-155

240

3530

.458

77.

7894

43.

1859

36.0

120

.36

17.5

67.

5-1

+1+1

4524

055

42.6

067

14.6

309

2.90

4339

.89

24.9

718

.41

6.8

-1-1

+145

120

5534

.856

711

.645

83.

3212

39.7

819

.89

18.3

66.

00

00

5018

045

33.9

902

11.5

024

3.09

0139

.90

24.3

917

.99

8.3


Water loss -g fresh carrots o t o

o

100 100=−( ) + −( ) ×

W W S S

Wt (3)

Solute gain -g fresh carrots t o

o

100 100=−

×S S

W(4)

where Wo is the initial weight of the carrots (g); Wt is the weight of the carrotsafter osmotic dehydration for any time t (g); So is the initial wt of solids (drymatter) in the carrots (g); and St is the weight of solids (dry matter) of carrotsafter osmotic dehydration for time t (g).

Rehydration Ratio

The rehydration of dried carrot cubes was determined by soaking 15 g ofeach sample in a sufficient volume of water (approximately 30 times theweight of dried carrots) at room temperature (Mazza 1983; Frederick 2000). Atthe end of the rehydration period, i.e., 12 h, which was found to be adequatefor the cubes to reach a constant weight, the cubes were weighed after remov-ing excess water with the help of absorbent paper. The rehydration ratio wascomputed as

Rehydration ratioWeight rehydrated carots g

Weight of dehydrat= ( )

eed carrots g( ) (5)

Color Measurement

Color is the most important parameter for the acceptability of the product.The color of rehydrated carrot was measured in terms of L, a and b values aftergrinding the samples. Variation in the values of L, a and b was observedbecause of different osmotic pretreatments and different proportions of thexylem (central core of carrot) and phloem (red portion) present in the samples.To reduce this variation, observations were recorded in triplicate after propermixing of the grinded sample. The color properties of the rehydrated productwere measured using a HunterLab MiniScan XE Plus colorimeter (Reston,VA). In Hunter scale, L varies from 0 to 100 and measures darkness orwhiteness. The chromatic portion of the solid is defined by +a (red), -a(green), +b (yellow) and -b (blue).

Sensory Evaluation

Organoleptic quality of dried carrot cubes and gazrella was determinedwith the help of a 10-member consumer panel using a 9-point hedonic scale.


The evaluation of dried product was carried out in its two forms – as driedcubes and as rehydrated cubes. In case of dried cubes, the organoleptic aspectsevaluated were color, appearance, odor and acceptability to purchase. Theaspects considered for rehydrated carrots were color, appearance, taste, flavorand overall acceptability. The average scores of all the 10 panelists werecomputed for different characteristics.

Statistical Analysis and Optimization

The second-order polynomial equation was fitted to the experimental dataof each dependent variable as given next:

Y B B x B x B x x ek k kii

i kiii

i kij i jj ii

k= + + + += = = +=∑ ∑ ∑∑0

1

3

1

32

1

3

1

2

(6)

where Yk = response variable (Y1 = water loss g/100-g fresh carrots; Y2 = solutegain g/100-g fresh carrots; Y3 = rehydration ratio; Y4, Y5 and Y6 = L, a and bvalues, respectively) and Y7 = sensory score of rehydrated carrots; xi representthe coded independent variables (x1 = solution concentration, x2 = processduration, x3 = process temperature); Xi represents the uncoded (actual)independent variables (X1 = solution concentration, X2 = process duration,X3 = process temperature); where Bko was the value of the fitted response at thecenter point of the design, i.e., point (0,0,0), Bki, Bkii and Bkij were the linear,quadratic and cross-product regression coefficients, respectively.

The regression analysis of the experimental data was carried out toobserve the significance of the effect of various process parameters on thevarious responses by the statistical software Statistica for Windows 5.0 (1995).The relative effect of each process parameter was compared from the b valuescorresponding to that parameter. The b coefficients were the regression coef-ficients obtained by first standardizing the process variables to a mean of 0 andan SD of 1. The advantage of using b coefficient (as compared to B coeffi-cients, which were not standardized) was that the magnitudes of these valueshelped to compare the relative contribution of each independent variable in theprediction of the dependent variable (Statistica for Windows 5.0 1995). Thehigher the positive value of b of a parameter, the higher would be the effect ofthat parameter and vice versa.

The response surface and contour plots were generated for differentinteraction of any two independent variables, while holding the value of thethird variable as constant (at the central value). Such three-dimensional sur-faces could give accurate geometrical representation and provide usefulinformation about the behavior of the system within the experimental design.The optimization of the osmotic dehydration process was aimed at finding


the levels of independent variables, viz., osmotic solution concentration,temperature and process duration, which would give maximum possiblewater loss, rehydration ratio, L and a values and sensory score, and lowestsolute gain and b value of color (Themelin et al. 1997; Ade-Omowaye et al.2002). RSM was applied to the experimental data using a commercial sta-tistical package, Design-Expert version 6.01 (trial version, Stat-Ease Inc.,Minneapolis, MN). The same software was used for the generation ofresponse surface plots, superimposition of contour plots and optimization ofprocess variables.

RESULTS AND DISCUSSION

Diagnostic Checking of Fitted Models and Response Surfaces

The results of second-order response surface model in the form of analy-sis of variance (ANOVA) are given in Tables 3 and 4. Regression analyses fordifferent models indicate that the fitted quadratic models accounted for morethan 90% of the variation in the experimental data, which were highly signifi-cant (R2 > 0.90). Multiple regression equations were generated relatingresponses to both coded and uncoded forms (levels) of process variables. Thevalues of regression coefficients and P levels for the coded form of processvariables are presented in Tables 3 and 4.

The P values (Table 3) indicate that all linear terms of process variableshave significant effect (P < 0.05), whereas the quadratic terms of concen-tration and interactions of “concentration and time” and “concentration andtemperature” have a nonsignificant effect at 5% level of significance(P > 0.05) on water loss during osmotic dehydration. The relative magni-tudes of b values (Table 3) indicate the maximum positive effect ofosmotic solution temperature (b = 0.8781) followed by process duration(b = 0.44657) and solution concentration (b = 0.13762). These results indi-cate an increase in water loss with increase of osmotic solution temperatureand process duration. The quadratic and interaction terms of all the processparameters have a negligible effect on water loss as compared to the linearterms of process variables. Figure 1 also indicates an increase in water losswith increase in osmotic solution temperature and very slight increase withincrease of osmotic solution concentration.

Table 3 indicates that all linear, quadratic and interaction terms of processvariables have a significant effect (P < 0.05) on solute gain during osmoticdehydration. The magnitude b values indicate the maximum positive effectof osmotic solution temperature (b = 0.88835) followed by process dura-tion (b = 0.35374) and negative effect of osmotic solution concentration


TAB

LE

3.R

EG

RE

SSIO

NSU

MM

AR

YA

ND

AN

ALY

SIS

OF

VA

RIA

NC

EFO

RW

AT

ER

LO

SS,S

OL

UT

EG

AIN

AN

DR

EH

YD

RA

TIO

NR

AT

IOIN

CO

DE

DFO

RM

OF

PRO

CE

SSV

AR

IAB

LE

S

Sour

cedf

Res

pons

es

Wat

erlo

ssSo

lute

gain

Reh

ydra

tion

ratio

bB

Ple

vel

bB

Ple

vel

bB

Ple

vel

Mod

el9

––

<0.0

001

––

<0.0

001

––

<0.0

001

Con

stan

t1

–33

.89

––

11.4

9–

–3.

0800

–C

oncn

10.

1376

1.04

<0.0

001

-0.0

73-0

.23

<0.0

001

-0.0

737

-0.0

140

<0.0

001

Tim

e1

0.44

663.

38<0

.000

10.

3537

1.13

<0.0

001

-0.7

648

-0.1

40<0

.000

1Te

mp

10.

8781

6.65

<0.0

001

0.88

832.

84<0

.000

1-0

.464

7-0

.087

<0.0

001

(Con

cn)2

1-0

.004

2-0

.045

0.44

49*

0.07

340.

33<0

.000

1-0

.135

5-0

.036

<0.0

001

(Tim

e)2

1-0

.092

8-0

.99

<0.0

001

-0.0

83-0

.38

<0.0

001

0.33

270.

088

<0.0

001

(Tem

p)2

10.

0226

0.24

0.00

15-0

.186

4-0

.84

<0.0

001

0.03

449.

1¥

10-3

0.12

05*

(Con

cn¥

time)

10.

0041

0.03

50.

3164

*0.

0257

0.09

2<0

.000

10.

0787

0.01

60.

0004

(Con

cn¥

tem

p)1

-0.0

005

-4.3

¥10

-30.

8965

*0.

1511

0.54

<0.0

001

-0.1

934

-0.0

41<0

.000

1(T

ime

¥te

mp)

10.

0582

0.49

<0.0

001

0.12

230.

44<0

.000

1-0

.252

-0.0

53<0

.000

1L

ack

offit

5–

–0.

067

––

0.08

––

0.05

15R

20.

9998

0.99

990.

9977

*N

onsi

gnifi

cant

effe

ctat

5%le

vel

ofsi

gnifi

canc

e.df

,deg

ree

offr

eedo

m.


TAB

LE

4.R

EG

RE

SSIO

NSU

MM

AR

YA

ND

AN

ALY

SIS

OF

VA

RIA

NC

EFO

RL

,a,b

AN

DSE

NSO

RY

SCO

RE

INC

OD

ED

FOR

MO

FPR

OC

ESS

VA

RIA

BL

ES

Sour

cedf

Res

pons

esL

valu

ea

valu

eb

valu

eSe

nsor

ysc

ore

bB

Ple

vel

bB

Ple

vel

bB

Ple

vel

BB

Ple

vel

Mod

el9

––

<0.0

001

––

<0.0

001

––

<0.0

001

––

<0.0

001

Con

stan

t1

–39

.88

––

24.3

7–

–18

.04

––

8.19

–C

oncn

10.

0489

80.

130.

0005

-0.0

58-0

.16

<0.0

001

-0.3

404

-0.2

40.

0002

0.56

044

0.67

0.00

02T

ime

10.

0482

10.

130.

0006

-0.2

965

-0.8

3<0

.000

10.

2159

60.

15<0

.000

10.

3680

50.

44<0

.000

1Te

mp

10.

7989

82.

09<0

.000

1-0

.443

7-1

.24

<0.0

001

0.37

90.

27<0

.000

10.

3011

30.

36<0

.000

1(C

oncn

)21

0.02

214

0.08

20.

1250

*-0

.053

3-0

.21

<0.0

001

0.96

579

0.95

<0.0

001

-0.2

635

-0.4

50.

0013

(Tim

e)2

1-0

.308

-1.1

4<0

.000

1-0

.093

8-0

.37

<0.0

001

-0.2

933

-0.2

90.

0002

-0.0

565

-0.0

950.

3647

*(T

emp)

21

-0.3

269

-1.2

1<0

.000

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(b = -0.07302). These results indicate an increase in solute gain with increaseof osmotic solution temperature and process duration, and a decrease in solutegain with increase of osmotic solution concentration. The decrease in solutegain with increase in osmotic solution concentration might be due to highviscosity of more concentrated sucrose solution, which imparted resistance tothe solute penetration at solution and carrot cube interface. The results wereconsistent with the findings of Hawkes and Flink (1978) and Lenart and Flink(1984). The quadratic terms of time and temp have a negative effect on solutegain, and the interaction terms of all the process parameters have positive andnegligible effects on solute gain as compared to the linear terms of the processvariables. Figure 2 also indicates an increase in solute gain with increase inosmotic solution temperature and process duration, and decrease with increaseof osmotic solution concentration. Similarly, Table 3 indicates that all linear,quadratic and interaction terms of process variables have significant effect(P < 0.05) on the rehydration ratio of osmotically pretreated carrot cubes, andthe sign of b values indicates that with an increase of all the process variables,there will be a decrease in rehydration ratio; however, this decrease in rehy-dration ratio was negligible with change in concentration (b = -0.0737) as

28.46

30.67

32.88

35.10

37.31

W

ate

r lo

ss g

/ 1

00

g f

resh

ca

rro

ts

45.00

47.50

50.00

52.50

55.00

35.00

40.00

45.00

50.00

55.00

Conc.(Degree Brix)Temp (Deg C)

FIG. 1. EFFECT OF TEMPERATURE AND CONCENTRATION ON WATER LOSS DURINGOSMOTIC DEHYDRATION AT 180 MIN OF PROCESS DURATION


compared to time (b = -0.76482) and temperature (b = -0.4647). Figure 3also indicates the similar change in rehydration ratio with concentrationand time.

For measurement of color of rehydrated carrot cubes, the values of L, aand b were measured. The effect of all linear, quadratic and interaction termsof process variables were significant (P < 0.05) on L, a and b values afterrehydration of product, only with the exception of the quadratic term ofconcentration on the L value, and the interaction of “concentration and time”on the b value (Table 4). Therefore, all the process parameters had bearingeffect on color, i.e., quality of the osmotically dehydrated product. The bvalues and Fig. 4 indicate an increase in L value with an increase of tem-perature (b = 0.79898), as compared to very low increase with concentration(b value of 0.04898) and process duration (b = 0.04821). Similarly, the bvalues (Table 4) also indicate decreased a value and increased b valuewith increase of osmotic solution temperature and process duration. Theosmotic solution concentration had a negligible effect on the a value of theproduct.

Table 4 indicates that all the linear terms, quadratic terms of concentra-tion and temperature, and interaction term of “concn and temp” had significant

7.37

9.06

10.75

12.44

14.13

S

olu

te g

ain

g/1

00 g

fre

sh c

arr

ots

45.00

47.50

50.00

52.50

55.00

35.00

40.00

45.00

50.00

55.00

Conc. (Deg. Brix) Temperature (Deg. C)

FIG. 2. EFFECT OF TEMPERATURE AND OSMOTIC SOLUTION CONCENTRATION ONSOLUTE GAIN DURING OSMOTIC DEHYDRATION AT 180 MIN OF

PROCESS DURATION


effect on the sensory score of the dehydrated and rehydrated product(P < 0.05) except interaction of “concentration and time” at 5% level ofsignificance. The magnitudes of b values indicate that osmotic solution con-centration had more effect (b = 0.56044) on the sensory score, and the processduration and osmotic solution temperature had almost the same effects.Similar effects are also indicated in Fig. 5.

Optimization of Osmo-convective Dehydration Process

A graphical multiresponse optimization technique was adopted to deter-mine the workable optimum conditions for the osmotic dehydration of carrotcubes. The contour plots for all responses were superimposed, and regions thatbest satisfy all the constraints were selected as optimum conditions. The maincriterion for constraint optimization was maximum possible water loss, rehy-dration ratio, L and a values and sensory score, and lowest solute gain and b

2.91

2.98

3.05

3.12

3.18

R

ehydra

tion R

atio

45.00

47.50

50.00

52.50

55.00

120.00

150.00

180.00

210.00

240.00

Conc. (Deg. Brix) Time (minutes)

FIG. 3. EFFECT OF TIME AND CONCENTRATION ON REHYDRATION RATIO OFOSMO-CONVECTIVELY DEHYDRATED CARROT CUBES AT 45C OSMOTIC

SOLUTION TEMPERATURE


value of color. (Themelin et al. 1997; Ade-Omowaye et al. 2002). Theseconstraints resulted in “feasible zone” of the optimum solutions (shaded areain the superimposed contour plots). Superimposed contour plots havingcommon superimposed area for all responses for osmo-convective dehydrationin sucrose solution are as shown in Figs. 6 and 7. The points in the range ofosmotic solution concentration, 49–53°Brix, 42–48C osmotic solution tem-perature and 150- to 185-min process duration, were found to be optimum forosmo-convective dehydration.

In order to optimize the process conditions for the osmotic dehydrationprocess by numerical optimization technique, equal importance of “3” wasgiven to all the three process parameters (viz., osmotic solution concentration,process duration and solution temperature). However, based on their relativecontribution to the quality of the final product, the importance given to differ-ent responses was 4, 2, 3, 3, 4, 2 and 5 for water loss; solute gain; rehydrationratio; L, a, b values and sensory score, respectively. Maximum importance wasgiven to the sensory score because it includes a number of parameters likecolor, flavor, firmness and appearance, etc.

The optimum operating conditions for concentration, temperature andprocess duration were 52.5°Brix, 49C and 150 min, respectively.

38.36

38.81

39.26

39.71

40.16

"L

- valu

e"

45.00

47.50

50.00

52.50

55.00

35.00

40.00

45.00

50.00

55.00

Conc (Deg. Brix)

Temp (Deg. C)

FIG. 4. EFFECT OF TEMPERATURE AND CONCENTRATION ON L VALUE AFTERREHYDRATION OF OSMO-CONVECTIVELY DEHYDRATED CARROT CUBES FOR

180-MIN PROCESS DURATION


CONCLUSIONS

RSM was effective in optimizing process parameters for the osmoticdehydration of carrot cubes in osmotic aqueous solutions of sucrose havingconcentrations in the range of 45–55°Brix, temperature of 35–55C and processduration of 120–240 min. The regression equations obtained in this study canbe used for optimum conditions for desired responses within the range ofconditions applied in this study. Graphical techniques, in connection withRSM, aided in locating optimum operating conditions, which were experimen-tally verified and proven to be adequately reproducible. The optimum solutionby numerical optimization obtained was 52.5°Brix, 49C solution temperatureand 150-min process dehydration to get maximum possible water loss, rehy-dration ratio, L and a values and sensory score, and lower solute gain and bvalue of color.

6.66

7.22

7.77

8.33

8.89

S

en

so

ry S

co

re

45.00

47.50

50.00

52.50

55.00

120.00

150.00

180.00

210.00

240.00

Conc (Deg. Brix)

Time (minutes)

FIG. 5. EFFECT OF TIME AND CONCENTRATION ON “SENSORY SCORE” OFREHYDRATED CARROT CUBES AFTER OSMO-CONVECTIVE DEHYDRATION

PROCESSED AT 45C OSMOTIC SOLUTION TEMPERATURE


Overlay Plot

Conc (Deg. Brix)

Tem

p (

Deg. C

)

45.00 47.50 50.00 52.50 55.00

35.00

40.00

45.00

50.00

55.00

WL: 30.00

WL: 35.58

SG: 10.00

SG: 12.06

RR: 3.12

L: 37.00

L: 40.34a: 24.08

a: 24.08

a: 24.55

b: 18.07b: 18.07

Sensory: 8.20

FIG. 7. OVERLAID CONTOURS FOR CONCENTRATION AND TEMPERATURE TOOPTIMIZE THE OSMOTIC DEHYDRATION PROCESS FOR 180-MIN PROCESS DURATION

WL, water loss; SG, solute gain; RR, rehydradion ratio.

Overlay Plot

Conc (Deg. Brix)

Tim

e (

min

ute

s)

45.00 47.50 50.00 52.50 55.00

120.00

150.00

180.00

210.00

240.00

WL: 30.00

WL: 34.70

SG: 10.00

SG: 11.72

RR: 3.12

L: 39.94

a: 24.14

a: 24.14

a: 25.00

b: 18.09b: 18.09Sensory: 8.06

FIG. 6. OVERLAID CONTOURS FOR TIME AND CONCENTRATION TO OPTIMIZE THEOSMOTIC DEHYDRATION PROCESS AT 45C SOLUTION TEMPERATURE

WL, water loss; SG, solute gain; RR, rehydradion ratio.


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OPTIMIZATION OF OSMOTIC DEHYDRATION PROCESS OF CARROT CUBES IN SUCROSE SOLUTION

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