optimisation of osmotic dehydration of carrot cubes in sucrose-salt solutions using response surface...

Eur Food Res Technol (2007) 225:157–165DOI 10.1007/s00217-006-0395-9

ORIGINAL PAPER

Optimisation of osmotic dehydration of carrot cubesin sucrose-salt solutions using response surface methodologyBahadur Singh · Parmjit S. Panesar · A. K. Gupta ·John F. Kennedy

Received: 23 January 2006 / Revised: 29 May 2006 / Accepted: 2 June 2006 / Published online: 15 August 2006C© Springer-Verlag 2006

Abstract Osmotic dehydration of carrot cubes in ternarysolution of water, sucrose and sodium chloride at differentsolution concentrations, temperatures and process durationswere analysed for water loss and solute gain during osmoticdehydration. The osmotically pre-treated carrot cubes werefurther dehydrated in a cabinet dryer at 65 ◦C and werethen rehydrated in water at ambient temperature of waterfor 10–12 h and were analysed for rehydration ratio, shrink-age and overall acceptability after rehydration. The processwas optimised for maximum water loss, rehydration ratioand overall acceptability of the rehydrated product, and forminimum solute gain and shrinkage of rehydrated productby response surface methodology. The optimum conditionsof various process parameters are 50◦B + 10% w/v aqueoussodium chloride concentration, 46.5 ◦C solution temperatureand 180 min process duration.

Keywords Osmotic dehydration . Carrots . Responsesurface methodology . Drying

B. Singh · P. S. PanesarDepartment of Food Technology, Sant Longowal Institute ofEngineering & Technology,Longowal (Sangrur) 148106, India

A. K. GuptaDepartment of Processing and Food Engineering, PunjabAgricultural University,Ludhiana 141004, India

J. F. Kennedy (�)Birmingham Carbohydrate and Protein Technology Group,School of Chemistry, University of Birmingham,Birmingham B15 2TT, UKe-mail: [email protected]

J. F. KennedyChembiotech Laboratories, University of Birmingham ResearchPark, Vincent Drive, Birmingham B15 2SQ, UK

Introduction

Vegetables are highly seasonal crops and only available inplenty at particular times of the year. In the peak season, theselling price decreases and this can lead to financial lossesby the grower. Also, due to the abundant supply during theseason, a glut in the market may result in the spoilage of largequantities. Preservation of these vegetables can prevent thehuge wastage and make them available in the off-season atremunerative prices [1].

Carrot (Daucus carota L.) is one of the important rootvegetable crops and is highly nutritious as it contains appre-ciable amount of vitamins B1, B2, B6 and B12 besides beingrich in β-carotene. It also contains many important miner-als. β-Carotene is a precursor of vitamin A and is reportedto prevent cancer [2]. Its maximum retention is of utmostimportance for the preservation of the attractive appearanceand dietary value of the product [2]. Carrots have a mois-ture content of 80–90% (wet basis) at the time of harvest[3]. They are seasonal in nature and highly susceptible tomoisture loss leading to wilting and loss of fresh appeal.

Out of various methods of extending the shelf life of per-ishable crops, osmotic dehydration is one of the simple andinexpensive alternate process, which is not only energy sav-ing and low-capital investment, but also offers a way to makeavailable this low cost, highly perishable and valuable cropavailable for the regions away from production zones andalso during off-season. Osmotic dehydration is the processin which water is partially removed from the cellular ma-terials when these are placed in a concentrated solution ofsoluble solute. Osmotic dehydration, which is effective evenat ambient temperature, preserves the colour, flavour andtexture of food from heat, and is used as a pre-treatment toimprove the nutritional, sensorial and functional propertiesof food. The amount of water remaining in the material after

Springer

158 Eur Food Res Technol (2007) 225:157–165

osmotic dehydration, however, does not ensure its stability,as water activity is generally higher than 0.9. When shelfstability is an ultimate process objective, other, complemen-tary methods of water removal, such as convective drying,freeze drying, freezing, etc. are suggested [4]. The influ-ence of the main process variables, such as concentration(conc.) and composition of osmotic solution, temperature(temp.), immersion time, pre-treatments, agitation, nature offood and its geometry, sample to solution ratio on the masstransfer mechanism and product quality have been studiedextensively [5, 6].

A number of researchers conducted studies on the os-motic dehydration of carrots [7–9]. The multicritera opti-misation of apples for osmotic dehydration and responsesurface methodology for the optimisation of osmotic de-hydration of red paprika has also been applied earlier [10,11]. Only limited efforts have so far been made to studyoptimisation of osmotic dehydration of carrots. Mazza [12]reported the effect of dipping carrot dices in sucrose andsodium chloride solution for 30 min prior to air-drying onmoisture transport during dehydration and product qualitywhile studying the effect of several pre-drying treatments.Various osmotic agents such as sucrose, glucose, fructose,corn syrup and sodium chloride have been used individuallyor in some combinations for osmotic dehydration. Generally,sucrose solutions are used for fruits and sodium chloride so-lution is used for vegetables. Addition of small quantities ofsodium chloride to osmotic solutions increased the drivingforce of the drying process and synergistic effects betweensucrose and sodium chloride have been reported [13]. Hence,the purpose of this work was to optimise the osmotic dehy-dration process of carrots by using combinations of sucroseand sodium chloride by response surface methodology.

Materials and methods

Experimental design

For the optimisation of osmotic dehydration process, the ex-periments were conducted according to Central CompositeRotatable Design (CCRD) with three variables at five levelseach [14]. The CCRD design predict uniformly at all con-stant distances from their centre points. For rotatable designs

the variances and co-variances of the estimated coefficientsin the fitted model remain unchanged when the design pointsare rotated about its centres. The design was generated bycommercial statistical package, Design-Expert version 6.01(Statease Inc., Minneapolis, USA, Trial version). The vari-ables were process temperature, solute concentration and du-ration of osmotic dehydration process. The low level and highlevel in the actual (un-coded) form were taken as 50◦Brix +5% (w/v) aqueous sodium chloride to 50◦Brix + 15% (w/v)aqueous sodium chloride, 35 ◦C to 55 ◦C, and 120 min to240 min for osmotic solution concentration, temperature andprocess time, respectively, as shown in Table 1 [8, 11, 15, 16].The carrot material to solution ratio was kept as 1:5 [17, 18].

The experiments plan in coded and un-coded form ofprocess variables is as given in Table 2. The experimentswere conducted randomly.

Experimental procedure

For each experiment, the carrots were washed, peeled anddiced into 1 cm × 1 cm × 1 cm cubes. The carrot cubes werewashed in water to remove the carrot fines adhering to thesurface of the cubes. No blanching was done prior to os-mosis as it has been reported to be detrimental to osmoticdehydration processes due to loss of semi-permeability ofthe cell membranes [19] and reduction of β-carotene [20,21] Salt solution was chosen for osmosis, as it is an ex-cellent osmotic agent for vegetables retarding oxidative andnon-enzymatic browning [22] and sucrose was used to pre-vent any bleaching effect of salt [22]. Sodium metabisulphite(0.3%) was added to help retain carotenoids content of driedcarrots [23] and to increase the product storage life underadverse temperature conditions. Sodium metabisulphite wasadded to increase the shelf life and retention of colour of theproduct, because no blanching was done to the samples priorto osmotic dehydration.

For each experiment, known weights of carrot cubes wereput in stainless steel containers (of capacity 1200 ml each)containing calculated volumes of osmotic solutions of dif-ferent concentrations pre set at the desired temperature in ahot water bath. The temperature was maintained during os-mosis and agitation was given for reducing the mass transferresistance at the surface of the carrots and for good mixingand close temperature control in osmotic medium [24, 25].

Table 1 Range of differentvariables for osmoticdehydration process in codedand un-coded form

Coded values Un-coded valuesTime (min) Temperature (◦C) Concentration (50◦Brix + % w/v NaCl salt)

− 1.682 79.09 28.18 50◦Brix + 1.59% w/v NaCl salt− 1.000 120.00 35.00 50◦Brix + 5% w/v NaCl salt

0.000 180.00 45.00 50◦Brix + 10%w/v NaCl salt+ 1.000 240.00 55.0 50◦Brix + 15% w/v NaCl salt+ 1.682 280.90 61.81 50◦Brix + 18.4% w/v NaCl salt

Springer

Eur Food Res Technol (2007) 225:157–165 159

Tabl

e2

Exp

erim

enta

ldes

ign

ofpr

oces

sva

riab

les

and

valu

esof

expe

rim

enta

ldat

afo

ros

mot

icde

hydr

atio

n

Cod

edpr

oces

sva

riab

les

Un-

code

dpr

oces

sva

riab

les

Res

pons

esT

ime

(x1)

Tem

p.(x

2)

Con

c.(x

3)

Tim

e(m

in)

(X1)

Tem

p.(◦ C

)(X

2)

Con

c.(s

odiu

mch

lori

de%

w/v

)(X

3)

Wat

erlo

ss(Y

1)

Solu

tega

in(Y

2)

Reh

ydra

tion

ratio

(Y3)

Shri

nkag

e(%

)(Y

4)

Acc

epta

nce

(%)(

Y5)

0.00

00.

000

0.00

018

0.00

45.0

050

◦ B+

10.0

045

.198

13.4

752.

932

099

1.00

01.

000

−1.0

0024

0.00

55.0

050

◦ B+

5.00

55.3

4616

.22

2.74

00

880.

000

0.00

00.

000

180.

0045

.00

50◦ B

+10

.00

45.2

0113

.481

2.95

10

990.

000

0.00

00.

000

180.

0045

.00

50◦ B

+10

.00

45.1

7913

.453

2.96

50

991.

682

0.00

00.

000

280.

9045

.00

50◦ B

+10

.00

46.3

6316

.115

2.67

80

950.

000

0.00

0−1

.682

180.

0045

.00

50◦ B

+1.

5940

.519

12.3

453.

077

089

−1.0

001.

000

−1.0

0012

0.00

55.0

050

◦ B+

5.00

47.0

8113

.582

3.03

62

900.

000

0.00

01.

682

180.

0045

.00

50◦ B

+18

.40

45.0

6714

.847

2.78

10

920.

000

0.00

00.

000

180.

0045

.00

50◦ B

+10

.00

45.1

5813

.449

2.94

50

97−1

.682

0.00

00.

000

79.0

945

.00

50◦ B

+10

.00

35.2

6611

.654

3.21

53

850.

000

0.00

00.

000

180.

0045

.00

50◦ B

+10

.00

45.2

0313

.490

2.94

20

98−1

.000

−1.0

001.

000

120.

0035

.00

50◦ B

+15

.00

44.3

0012

.010

3.15

91

84−1

.000

−1.0

00−1

.000

120.

0035

.00

50◦ B

+5.

0034

.903

10.0

383.

273

481

0.00

01.

682

0.00

018

0.00

61.8

150

◦ B+

10.0

060

.377

16.4

842.

751

194

0.00

0−1

.682

0.00

018

0.00

28.1

850

◦ B+

10.0

048

.388

11.4

533.

117

290

1.00

0−1

.000

−1.0

0024

0.00

35.0

050

◦ B+

5.00

41.3

0112

.626

3.00

80

951.

000

−1.0

001.

000

240.

0035

.00

50◦ B

+15

.00

49.2

5614

.619

2.83

20

951.

000

1.00

01.

000

240.

0055

.00

50◦ B

+15

.00

51.3

2417

.102

2.61

52

94−1

.000

1.00

01.

000

120.

0055

.00

50◦ B

+15

.00

44.5

3914

.533

2.88

00

960.

000

0.00

00.

000

180.

0045

.00

50◦ B

+10

.00

45.1

6913

.448

2.97

60

98

Springer


At the specified times the carrot cubes were removedfrom the osmotic solutions and rinsed with water to removesurplus solvent adhering to the surfaces. These osmoticallydehydrated cubes were then spread on the absorbent paperto remove the free water present on the surface. Out of theosmosed sample, about 15–20 g sample was used for thedetermination of dry matter by oven drying. The remainingpart of each product sample was dried to final moisture of5% (wet basis) using a hot air drier pre set at 65 ◦C airtemperature and 1.6 m/s air velocity [26]. The dried sampleswere cooled in a desiccators containing silica gel for 1 h,packed in HDPE (high density polyethylene) bags and keptat ambient temperature for quality analysis.

The dehydrated samples were rehydrated in water at ambi-ent temperature for 8–10 h and were analysed for rehydrationratio, shrinkage (%) and overall acceptability.

Statistical analysis and optimisation

The second order polynomial equation was fitted to the ex-perimental data of each dependent variable as given below

Yk = Bk0 +n∑

i=1

Bki xi +n∑

i=1

Bkii x2i

+n−1∑

i=1

n∑

j=i+1

Bki j xi x j + ek (1)

where Yk = response variable (Y1 = water loss g/100 gfresh carrot, Y2 = solute gain g/100 g fresh carrot, Y3 = re-hydration ratio, Y4 = shrinkage (%) of rehydrated product,Y5 = overall acceptability (%) of rehydrated product), xisrepresent the independent variables (x1 = process duration,x2 = process temperature, x3 = solution concentration).Where Bk0 is the value of fitted response at the centrepoint of design, i.e. point (0, 0, 0), Bki, Bkii and Bkij are thelinear, quadratic and cross-product regression coefficients,respectively.

The regression analysis of the experimental data wascarried out to observe the significance of the effect ofvarious process parameters on the various responses by thestatistical software Statistica 5.0 [27]. The relative effect ofeach process parameter was compared from the β valuescorresponding to that parameter, the β coefficients being theregression coefficients obtained by first standardising theprocess variables to a mean of zero and standard deviationto one. The advantage of using β coefficient (as comparedto B coefficients which are not standardised) is that themagnitudes of these values allow us to compare the relativecontribution of each independent variable in the predictionof the dependent variable [27]. The higher the positive valueof β of a parameter; the higher would be the effect of thatparameter and vice versa.

The response surface and contour plots were generatedfor different interactions of any two independent variables,while holding the value of the third variable as constant. Suchthree-dimensional surfaces could give accurate geometricalrepresentation and provide useful information about the be-haviour of the system within the experimental design. Theoptimisation of the osmotic dehydration process was aimedat finding the levels of independent variables, viz. osmoticsolution concentration, temperature and process duration,which would give maximum water loss, rehydration ratio,overall acceptability and minimum solute gain and shrink-age (%) of the rehydrated carrot cubes [15]. Response sur-face methodology was applied to the experimental data us-ing a commercial statistical package, Design-Expert version6.01 (Statease Inc., Minneapolis, USA, Trial version) for thegeneration of response surface and contour plots. The samesoftware was used for superimposition of contour plots andoptimisation of process variables.

Mathematical calculations

Water loss and solute gain during osmotic dehydration

During osmotic dehydration, the phenomena of water lossand solution gain take place simultaneously. During osmoticdehydration process, the weight of the carrots is reduced dueto water loss, but at the same time there will be an increasein weight due to solute gain. Therefore, water loss is the sumof weight reduction and solute gain.

Weight Reduction (g) = WR = (W0 − Wt ) (2)

Solute Gain after osmotic dehydration for time t (g) = SG

= (St − S0) (3)

Water Loss = Weight Reduction + Solute gain

The water loss and solute gain during osmotic dehydrationwere calculated by the following equations [28].

Water loss/100 g Fresh Carrots

= (W0 − Wt ) + (St − S0)

W0× 100 (4)

Solute gain/100 g Fresh Carrots =(

St − S0

W0

)× 100 (5)

where W0 is the initial weight of carrot cubes (g), Wt theweight of carrot cubes after osmotic dehydration for anytime t (g), S0 is the initial weight of solids (dry matter) inthe carrots (g), and St is the weight of solids (dry matter) ofcarrots after osmotic dehydration for time t (g).

Springer


Tabl

e3

Reg

ress

ion

sum

mar

yan

dA

NO

VA

for

wat

erlo

ss,s

olut

ega

inan

dre

hydr

atio

nra

tio

Wat

erlo

ssSo

lute

gain

Reh

ydra

tion

ratio

Sour

cedf

βB

Sum

ofsq

uare

sp-

leve

lβ

BSu

mof

squa

res

p-le

vel

βB

Sum

ofsq

uare

sp-

leve

l

Mod

el9

––

639.

169

1.37

E−

23–

–63

.177

2.41

E−

18–

–0.

585

4.31

E−

09C

onst

ant

1–

45.1

86–

1.51

E−

32–

13.4

66–

4.54

E−

27–

2.95

2–

2.33

E−

21T

ime

10.

482

3.30

014

8.72

15.

77E

−23

0.60

91.

311

23.4

789.

77E

−19

−0.7

24−

0.15

10.

309

2.98

E−

10Te

mp

10.

521

3.56

517

3.60

72.

66E

−23

0.70

11.

508

31.0

872.

4E−

19−0

.569

−0.

118

0.19

13.

16E

−09

Con

c1

0.19

71.

349

24.8

874.

39E

−19

0.34

00.

732

7.32

93.

28E

−16

−0.3

75−

0.07

80.

083

1.72

E−

07T

ime×

Tim

e1

−0.2

35−1

.554

34.7

808.

24E

−20

0.07

10.

149

0.31

81.

89E

−09

−0.0

01−

4E−

042.

1E−

060.

950a

Tem

p.×

Tem

p1

0.49

13.

243

151.

603

5.24

E−

230.

086

0.17

80.

458

3.18

E−

10−0

.024

−0.

005

0.00

00.

436a

Con

c.×

Con

c1

−0.1

29−0

.854

10.5

123.

26E

−17

0.02

20.

046

0.03

18.

23E

−05

−0.0

33−

0.00

70.

001

0.29

4a

Tim

e×

Tem

p1

0.05

20.

462

1.70

82.

83E

−13

0.00

00.

001

1.2

E−

050.

904a

0.01

40.

004

0.00

00.

628a

Tim

e×

Con

c1

−0.0

41−0

.365

1.06

72.

94E

−12

−0.0

02−0

.006

0.00

00.

560a

−0.0

14−

0.00

40.

000

0.63

5a

Tem

p.×

Con

c1

−0.3

34−2

.990

71.5

012.

25E

−21

−0.0

95−0

.267

0.56

81.

11E

−10

0.00

40.

001

9.5E

−06

0.89

4a

Res

idua

l10

––

0.00

7–

––

0.00

7–

––

0.00

5–

Lac

kof

fit5

––

0.00

50.

129

––

0.00

60.

090

––

0.00

40.

123

Pure

erro

r5

––

0.00

2–

––

0.00

1–

––

0.00

1–

Cor

rect

edto

tal

19–

–63

9.17

6–

––

63.1

85–

––

0.59

1–

R2

0.99

90.

999

0.99

1

a Non

-sig

nific

anta

t5%

leve

l.

Rehydration ratio

The rehydration of dried carrot cubes was determined bysoaking a known weight (10–12 g) of each sample in a suf-ficient volume of water (approximately 30 times the weightof dried carrots) at room temperature [12]. At the end ofthe rehydration period, i.e. 12 h, which was found to be ad-equate for the cubes to reach a constant weight, the cubeswere weighed after removing excess water with the help ofabsorbent paper. Before taking weights, the volume of therehydrated carrot cubes was also measured by a water dis-placement method for the purpose of measurement of bulkshrinkage of rehydrated carrot cubes [29]. The rehydrationratio was computed using the equation [12]:

Rehydration ratio

= Weight of rehydrated carrots (g)

Weight of dehydrated carrotsbefore rehydration (g)

(6)

Measurement of shrinkage

The bulk shrinkage of the carrot cubes during rehydrationof dehydrated carrot cubes, as measured by the water dis-placement method, was calculated as the percentage changefrom the initial apparent volume. The shrinkage (%) wascalculated as

% Shrinkage =[

1 −(

Vt

V0

)]× 100 (7)

where Vt is the volume displaced by 100 g rehydrated carrotcubes at time t, V0 the volume displace by 100 g fresh carrotcubes.

Results and discussion

The p and β values in Tables 3 and 4 indicate that for osmoticdehydration process, the process duration has significant andpositive effect on responses water loss, solute gain and over-all acceptability and has negative effect on rehydration ratioand shrinkage of the rehydrated product. Therefore, therewill be an increase in all the response values with increase ofprocess duration except rehydration ratio and shrinkage ofrehydrated product. The negative effect of process durationindicates that, with increase of process duration there willbe a decrease in rehydration ratio and shrinkage of the rehy-drated product. Therefore, only three-dimensional responsesurfaces have been generated for the osmotic solution con-centration and temperature only. The detailed analysis of var-

Springer


Table 4 Regression summary and ANOVA for shrinkage (%) and overall acceptability

Source df Shrinkage (%) Overall acceptability (%)β B Sum of squares p-level β B Sum of squares p-level

Model 9 – – 26.811 3.42E − 06 – 537.108 8.35E − 08Constant 1 – −0.002 – 0.987 – 98.333 – 1.84E − 20Time 1 −0.516 −0.736 7.388 4.7E − 06 0.438 2.769 104.724 6.72E − 07Temp 1 −0.137 −0.196 0.526 0.039 0.228 1.444 28.495 0.000Conc 1 −0.154 −0.22 0.659 0.024 0.232 1.467 29.422 0.000Time × Time 1 0.395 0.544 4.263 5.11E − 05 −0.483 −2.947 125.159 2.92E − 07Temp. × Temp 1 0.395 0.544 4.263 5.11E − 05 −0.367 −2.240 72.302 3.67E − 06Conc. × Conc 1 0.009 0.013 0.002 0.869a −0.454 −2.770 110.594 5.22E − 07Time × Temp 1 0.335 0.625 3.125 0.000 −0.439 −3.625 105.125 6.61E − 07Time × Conc 1 0.469 0.875 6.125 1.08E − 05 −0.044 −0.375 1.125 0.281a

Temp. × Conc 1 0.201 0.375 1.125 0.006 0.136 1.125 10.125 0.006Residual 10 – – 0.938 – – – 8.691 –Lack of fit 5 – – 0.938 – – – 5.358 0.307Pure error 5 – – 0 – – – 3.333 –Corrected total 19 – – 27.75 – – – 545.800 –R2 0.966 0.984

aNon-significant at 5% level.

ious responses for the osmotic dehydration of carrot cubes isdescribed as under:

Diagnostic checking of fitting mode and surface plotsfor various responses

Water loss

The magnitude of p and β values in Table 3 indicates themaximum positive contribution of osmotic solution temper-ature followed by process duration and solution concentra-tion on the water loss during osmotic dehydration. It impliesincreased water loss with increase of solution temperatureand process time. The quadratic terms of time and concen-tration have negative, and that of temperature have positiveeffect on water loss. Further, the interactions of ‘time andtemp.’ have positive and significant effect, whereas the in-teractions of ‘time and conc.’ and ‘temp. and conc.’ havenegative and significant effect on water loss. Figure 1 alsoindicates an increase in water loss with increase of osmoticsolution temperature and osmotic solution concentration at180 min of process duration. It also indicates the morepronounced effect of osmotic solution temperature than os-motic solution concentration on water loss during osmoticdehydration.

Solute gain

The magnitude of p and β values in Table 3 indicates the max-imum contribution of osmotic solution temperature followedby process duration, whereas, the solution concentration hasleast but significant effect on the solute gain during osmotic

39.59

43.25

46.90

50.55

54.20 W

ater

loss

g/1

0 0f

fres

h ca

rrot

s

5.007.50

10.0012.50

15.00

35.00 40.00

45.00 50.00

55.00

Conc (50ºBrix +% Salt) Temp (ºC)

Fig. 1 Effect of osmotic solution temperature and concentration onwater loss during osmotic dehydration of carrot cubes at 180 min pro-cess duration

dehydration. It implies increased solute gain with increase ofosmotic solution temperature, process duration and solutionconcentration.

The quadratic terms of time, temperature and concen-tration have positive and significant effect on solute gain.The magnitude of β values indicates that the effects of thequadratic terms of all the process parameters have negligi-ble effect on solute gain in comparison to effect of linearterms. Further, only the interaction of ‘temp. and conc.’ hassignificant and negative effect on solute gain at 5% level ofsignificance, however, the effect was negligible in compari-son to the effect of linear terms of process parameters. Fig-ure 2 also indicates an increase in solute gain with increaseosmotic solution temperature and osmotic solution concen-tration at 180 min of process duration. It also indicates the

Springer


11.18

12.30

13.42

14.55

15.67

Sol

ute

gain

g/ 1

00 g

fres

h ca

rrot

5.00

7.50

10.00

12.50

15.00

35.0040.00

45.0050.00

55.00

Conc (50ºBrix +% Salt)Temp (ºC)

Fig. 2 Effect of osmotic solution temperature and concentration onsolute gain during osmotic dehydration of carrot cubes at 180 minprocess duration

more pronounced effect of osmotic solution temperature incomparison to solution concentration.

Rehydration ratio

The magnitude of p and β values in Table 3 indicates themaximum negative and significant contribution of processduration followed by solution temperature, solution concen-tration on the rehydration ratio. It implies the decrease inrehydration ratio with increase of process duration and os-motic solution temperature and concentration. This is be-cause the rehydration ratio is inversely related to the solutegain during osmotic dehydration process, which has to beleached out during rehydration process. As all the processvariables favours the solute gain during osmotic dehydra-tion, therefore their effect on rehydration ratio is negative.The effects of all the quadratic and interaction terms werenon-significant on rehydration ratio. Figure 3 indicates a lin-ear decrease in rehydration ratio with increase of osmoticsolution concentration and osmotic solution temperature at180 min of process duration.

Shrinkage of rehydrated carrot cubes

The magnitude of p and β values in Table 4 indicates the max-imum negative contribution of process duration, and leastnegative contribution of solution concentration and temper-ature on the shrinkage (%) of rehydrated product. It impliesdecrease in shrinkage of rehydrated product with the ad-vancement of process duration. The quadratic terms of timeand temperature have positive and significant effect, whereasthat of concentration has non-significant effect on shrinkage(%) of rehydrated product. Further, all the interaction termsof all process parameters has positive and significant effect onshrinkage (%) of rehydrated product. The maximum positivecontribution was of interaction of ‘time and conc.’ followed

2.74

2.84

2.94

3.04

3.14

Reh

ydra

tion

Rat

io

5.007.50

10.00 12.50

15.00

35.00 40.00

45.00 50.00

55.00


Fig. 3 Effect of osmotic solution temperature and concentration onrehydration ratio during osmotic dehydration of carrot cubes at 180 minprocess duration

by ‘time and temp.’ and ‘temp. and conc.’ Figure 4 also in-dicates a non-linear decrease in shrinkage with increase ofosmotic solution temperature and concentration at 180 minof process duration. The figure indicates that the shrinkageis less at the highest osmotic solution concentration and ismore at low and high levels of osmotic solution temperatures.The minimum shrinkage was observed in osmotic solutiontemperature range of 45 to 50 ◦C.

Overall acceptability of rehydrated carrot cubes

The magnitude of p and β values in Table 4 indicates themaximum positive, significant contribution of process du-ration, followed by solution concentration and temperatureon the overall acceptability (%) of rehydrated product. Thequadratic terms of the entire process variable have negativeand significant effect on overall acceptability of rehydrated

-0.22

0.17

0.56

0.95

1.35

Shr

inka

ge (

%)

5.00

7.50

10.00

12.50

15.00

35.00 40.00

45.0050.00

55.00 Conc (ºBrix +% Salt)

Temp (ºC)

Fig. 4 Effect of osmotic solution temperature and concentration on“shrinakage (%) of rehydrated product” during osmotic dehydration ofcarrot cubes at 180 min process duration

Springer


91.54

93.37

95.21

97.04

98.88

Ove

rall

acce

ptan

ce (

%)

5.00

7.50

10.00

12.50

15.00

35.00

40.00

45.00

50.00

55.00


Fig. 5 Effect of osmotic solution temperature and concentration onoverall acceptability (%) of rehydrated product during osmotic dehy-dration of carrot cubes at 180 min process duration

product. The quadratic term of time has maximum negativeeffect followed by solution concentration and temperature onoverall acceptability (%) of rehydrated product. The interac-tion of ‘time and concentration’ has non-significant effect onoverall acceptability of the rehydrated product. The interac-tion term of ‘time and temperature’, has maximum negativeeffect followed by positive effect of ‘temperature and con-centration’ on overall acceptability of rehydrated product.Figure 5 also indicates a non-linear increase in overall ac-ceptability with increase of osmotic solution temperatureand concentration at 180 min of process duration. However,a slight decrease in overall acceptability has been observedwith increase of osmotic solution concentration between 12and 15% salt in 50◦Brix sucrose solution and osmotic solu-tion temperature from 50 to 55 ◦C.

Optimisation of osmo-convective dehydration process

A graphical multi-response optimisation technique wasadopted to determine the workable optimum conditions forthe osmotic dehydration of carrot cubes. The contour plotsfor all responses were superimposed and regions that best sat-isfy all the constraints were selected as optimum conditions.The overlaid contours were generated between two processvariables by fixing the third variable at its central value (45 ◦Ctemperature and 50◦B + 10% w/v sodium chloride for so-lution concentration). The main criterion for constraints op-timisation was maximum possible water loss, rehydrationratio, overall acceptability, and lower solute gain, shrink-age as low as possible [10, 11]. These constraints resultedin ‘feasible zone’ of the optimum solutions (shaded areain the superimposed contour plots). Superimposed contourplots having common superimposed area for all responsesfor osmo-convective dehydration in solution sucrose–sodiumchloride mixture are as shown in Figs. 6 and 7. The pointsin the range of osmotic solution concentration 50◦B + 8.5%

Overlay Plot

X:Time (minutes)Y:Temperature (ºC)

120.00 150.00 180.00 210.00 240.00

35.00

40.00

45.00

50.00

55.00

WL: 41.59

WL: 45.02

SG: 13.23

SG: 14.00RR: 2.96

RR: 3.04

Shrinkage: 0.02

Shrinkage: 1.56

Acceptance: 97.64

Acceptance: 98.86

Fig. 6 Overlaid contours for time and osmotic solution temperatureat 50◦Brix + 10% sodium chloride salt concentration

w/v aqueous sodium chloride to 50◦B + 11% w/v aqueoussodium chloride, 43–48 ◦C osmotic solution temperature and160–190 min process duration were found to be optimum forosmo-convective dehydration.

In order to optimise the process conditions for osmoticdehydration process by numerical optimisation technique,

Overlay Plot

X:Time (minutes)Y:Conc (50˚B +% NaCl salt)

120.00 150.00 180.00 210.00 240.00

5.00

7.50

10.00

12.50

15.00

WL: 41.59

WL: 45.02

SG: 13.23 SG: 14.00RR: 2.96

RR: 3.04

Shrinkage: 0.02

Shrinkage: 0.02

Shrinkage: 1.56

Acceptance: 97.64

Acceptance: 98.86

Fig. 7 Overlaid contours for time and concentration at 45 ◦C osmoticsolution temperature

Springer


equal importance of ‘3’ was given to all the three processparameters (viz. process duration, osmotic solution temper-ature and concentration). However, based on their relativecontribution to quality of final product, the importance givento different responses was 4, 2, 3, 4 and 5 for water loss,solute gain, rehydration ratio, shrinkage (%) and overall ac-ceptability, respectively. Maximum importance was given tothe overall acceptability (%), because it includes a numberof parameters like colour, flavour, firmness and appearance,etc.

The optimum operating condition for osmotic solu-tion concentration, temperature and process duration were50◦Brix + 10% w/v aqueous sodium chloride, 46.5 ◦C and180 min, respectively.

Conclusions

Response surface methodology is effective in optimising pro-cess parameters for the osmotic dehydration of carrot cubesin osmotic aqueous solutions of sucrose–sodium chloridemixture. The recommended process variables are 50◦Brix+ 10% w/v aqueous sodium chloride, 46.5 ◦C solution tem-perature and 180 min process dehydration to get maximumwater loss, subsequent rehydration ratio, and overall accept-ability, and minimum solute gain and shrinkage of rehydratedproduct.

References

1. Prakash S, Jha SK, Datta N (2004) J Food Eng 62:305–3132. Ong DE, Chytil F (1983) In: Aurbach GD (ed) Vitamins and hor-

mones, Academic Press, New York, pp 105–1123. Kalra CL, Kulkarni SG, Berry SK (1987) Indian Food Packer Nov–

Dec, 46–73

4. Lenart A, Cerkowniak M (1996) Polish J Food Nutr Sci 5:73–825. Rastogi NK, Raghavarao KSMS (1994) Lebensmittel Wissenschaft

Technol 27:564–5676. Kaymak-Ertekin F, Sultanoglu M (2000) J Food Eng 46:243–2507. Singh S, Shivhare US, Ahmed J, Raghavan GSV (1999) Food Res

Int 32:509–5148. Rastogi NK, Raghavarao KSMS (1997) J Food Eng 34:429–4409. Matusek A, Meresz P (2002) Periodica Polytech SER Chem Eng

46:83–9210. Themelin A, Raoult-Wack AL, Lebert A, Danzart M (1997) Drying

Technol 15:2263–227911. Ade-Omowaye BIO, Rastogi NK, Angersbach A, Knorr D (2002)

J Food Sci 67:1790–179612. Mazza G (1983) J Food Technol 18:113–12313. Lerici CL, Pinnavaia G, Dalla Rosa M, Bartolucci L (1985) J Food

Sci 50:1217–121914. Khuri AI, Cornell JA (1987) Response surfaces design and ana-

lysis. Marcel Dekker Inc., New York15. Lazarides HN, Katsanidis E, Nickolaidis A (1995) J Food Eng

25:151–16616. Sereno AM, Moreira R, Martinez E (2001) J Food Eng 47:43–4917. Pokharkar SM, Prasad S (2002) J Food Sci Technol 39:384–38718. Azoubel PM, Murr FEX (2004) J Food Eng 61:291–29519. Ponting JD (1973) Process Biochem 8:18–2220. Sharma GK, Semwal AD, Arya SS (2000) J Food Sci Technol

37:196–20021. Reyes A, Alvarez PI, Marquardt FH (2002) Drying Technol

20:1463–148322. Jackson TH, Mohammed BB (1971) Sudan J Food Sci Technol

3:18–2223. Mohammed S, Hussein R (1994) J Food Process Preserv 18:343–

34824. Mavroudis NE, Gekas V, Sjoholm I (1998) Osmotic dehydration

of apples – effects of agitation and raw material characteristics. JFood Eng 35:191–209

25. Chopra CS (2001) Beverage Food World September 15–1726. Andrzej L (1991) In: Mujumdar AS, Filkova I (eds) Drying 91,

Elsevier Science, pp 489–49827. Statistica for Windows 5.0. (1995) Computer program manual.

StatSoft, Inc., Tulsa28. Ozen BF, Dock LL, Ozdemir M, Floros JD (2002) Int J Food Sci

Technol 37:497–50229. Johnson PNT, Brennan JG, Addo-Yobo FY (1998) J Food Eng

37:233–242

Springer

optimisation of osmotic dehydration of carrot cubes in sucrose-salt solutions using response surface...

Documents