www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 68 (2005) 289–296

Combined treatments of blanching and dehydration:study on potato cubes

Carla Severini, Antonietta Baiano *, Teresa De Pilli, Barbara F. Carbone, A. Derossi

Faculty of Agriculture, Department of Food Science, University of Foggia, Via Napoli 25, 71100 Foggia, Italy

Received 23 January 2004; accepted 31 May 2004

Abstract

Samples of Solanum tuberosum var. Primura were submitted to combined treatments of blanching and dehydration. Blanching

was alternatively performed in hot distilled water, hot sugary-saline solution, by microwaves in distilled water or by microwaves in

saline solution. Drying was alternatively carried out in an air cabinet, a microwave oven or a belt drier. In terms of process speed,

colour retention and water absorption capacity, the best results were obtained combining microwave blanching with dehydration on

the belt drier. In particular, dehydration on the belt drier levelled eventual negative effects determined by the blanching treatments.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Blanching; Colour; Dehydration; Potato; Rehydration

1. Introduction

In the production of canned, frozen and dehydrated

vegetables, the main purpose of a blanching treatment

is the inactivation of enzymes such as polyphenoloxid-ases, responsible for browning (Mapson, Swain, &

Tomalin, 1963; Collins & McCarty, 1969), peroxidase,

catalase and phenolase which leads to the development

of off-flavours (Pinsent, 1962; Bizzarri, Andreotti, &

Massini, 1981). In particular, thanks to their heat resis-

tance or regeneration capacity, peroxidase, catalase and

phenolase are considered as indices of the heat treatment

efficacy. Among them, peroxidase is the most heat resis-tant. Smith (1977) found that it is necessary to per-

formed a blanching treatment at 100 �C for 3 min, to

obtain its complete inactivation in cubed potatoes des-

tined for dehydration. Bizzarri et al. (1981) obtained

the complete and irreversible inactivation of peroxidase

by blanching potatoes for 4 min at 97 �C.

0260-8774/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2004.05.045

* Corresponding author. Tel.: +39 881 589242; fax: +39 881 740211.

E-mail address: a.baiano@unifg.it (A. Baiano).

Blanching can be performed by exposing vegetables

to hot water (the most common method), hot and boil-

ing solutions containing acids and/or salts, by steam

(Kidmose & Martens, 1999) or by microwaving product

dipped in water or solutions (Chen, Collins, McCarty, &Johnston, 1971; Ramaswamy & Van de Voort, 1990;

Ponne, Van Remmen, & Bartels, 1991; Severini, De Pilli,

Baiano, Mastrocola, & Massini, 2001) for several sec-

onds or minutes.

The effectiveness of a drying process depends on dif-

ferent factors: method of heat transfer, continuity or dis-

continuity of the process, direction of the heating fluids

with respect to the product, pressure (atmospheric, low,deep vacuum).

Dehydration can be performed by using different

kinds of equipment: air cabinet, belt drier, tunnel drier,

fluidized bed, spray drier, drum drier, foam drier,

freeze-drier, microwave oven. An interesting drying

method is represented by osmotic dehydration. Krokida,

Maroulis, and Saravacos (2001) investigated the effects of

different drying methods on the colour of the obtainedproducts. They found that colour characteristics are sig-

nificantly affected by the drying methods and that the

290 C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296

changes in redness (a*) and yellowness (b*) follow a first

order kinetic model. In particular, air-, vacuum and

microwave drying caused extensive browning in fruit

and vegetables with a decrease inL* value and an increase

in a* and b* values whereas freeze-drying improved col-

our characteristics. Osmotically pre-treated samples werepreserved from browning to a certain measure.

Drying methods also affect the behaviour of dehy-

drated product during rehydration. Krokida, Kiranou-

dis, and Maroulis (1999) and Krokida and Maroulis

(2000, 2001) studied the viscoelastic behaviour of

dehydrated products during rehydration. They found

that dehydrated foods do not keep their viscoelastic

behaviour after rehydration due to structural damagesthat occur during drying. Freeze-dried products

showed the highest hysteresis after rehydration, losing

their elasticity and becoming more viscous. Osmotic

pre-treatment helped freeze-dried materials to keep

their elastic nature probably thanks to the solid gain.

Air and vacuum dried foods showed the smallest hys-

teresis tendency, keeping their viscoelastic characteris-

tics during rehydration close to those of driedmaterials.

The purpose of this work was to study the effects of

different combined systems of blanching and dehydra-

tion on dehydration speed, colour characteristics and

the behaviour during rehydration of cubed potatoes.

2. Materials and methods

2.1. Raw material

Trials were performed on potato tubers (Solanum

tuberosum var. Primura) purchased at a local market.

This Italian variety, characterized by an early ripening

time, is the most commonly used in food industry

thanks to the regular shape and the good resistance tocooking of its big tubers.

2.2. Sample preparation

After grading, tubers were washed under running

water, wiped with blotting paper, hand-peeled and cut

into cubes (1 cm side).

2.3. Blanching

Blanching treatments were performed in four differ-

ent ways found as the optimal blanching conditions (in

terms of texture retention and enzyme inactivation) in

a previous screening:

� By immersion in a sodium-chloride solution (3%, w/w) at 98 �C for 5 min.

� By immersion in a corn syrup solution diluted to 70�Brix with distilled water and containing sodium-chlo-

ride (3%, w/w) at 100 �C for 5 min. The undiluted

corn syrup (commercial name ‘‘Glicosa’’, supplied

by Roquette Italia, Cassano Spinola, Italy) had the

following characteristics and chemical composition:refractometer grade 80.6� Brix, pH value 5.1 at 20

�C, 30% dextrose, 46% maltose, 10% trysaccharides,

14% polysaccharides.

� By microwaves in a domestic oven (De Longhi,

Milan, Italy) upon immersion in distilled water at

850 W for 5 min.

� By microwaves in a domestic oven (De Longhi,

Milan, Italy) upon immersion in a sodium-chloridesolution (3%, w/w) at 850 W for 5 min.

The product–solution ratio was always 1:5 (w/w).

After blanching, the samples were cooled in tap

water, drained and wiped with blotting paper.

Unblanched potato cubes were kept as controls.

2.4. Dehydration

Blanched and unblanched potato samples were sub-

mitted to three different methods of dehydration:

� in an air cabinet (I.S.C.O., Milan, Italy) at 100 �C;� in a microwave oven (De Longhi, Milan, Italy) at 85

W;

� in a monolayer on a concurrent belt drier (pilot plant,Sandvik Process Systems, Milan, Italy) at 100 �C.

During dehydration, samples were withdrawn at reg-

ular intervals (15 min) until the samples showed con-

stant weights.

2.5. Rehydration

The liquids used for rehydration trials were distilled

water and a vegetable stock obtained by dissolving

22 g of stock cubes in distilled water. Exactly weighed

potato cubes were dipped in the hot (initial tempera-

ture 80 �C) rehydration liquids (1:5 w/v ratio) for

0.5, 1, 2.5, 5, 7 and 10 min, recovered, drained and

weighed. Rehydration tests were also performed at room

temperature in water for 120 min with regular with-drawing.

2.6. Analyses

� Colour analysis: a tristimulus colorimeter (Chrom-

ameter-2 Reflectance, Minolta, Osaka, Japan)

equipped with a CR 300 measuring head was used.

Colour was expressed as L*, a* and b* (luminosity,red value and yellow value, respectively, on the Hun-

0

1

2

3

4

5

6

7

0 15 30 45 60 75 90 105120135150165180195 210225240255270285300315330345360375390405420Time (min)

W (g

H2O

/g d

ry m

atte

r) Air cabinet Microwave oven Belt drier

Fig. 1. Decrease in absolute moisture (g H2O/g dry matter) in

unblanced samples under different drying conditions as a function of

the dehydration time.

0.14

0.16Air cabinetMicrowave ovenBelt drier.

C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296 291

ter scale). The colorimeter was calibrated on a stand-

ard white tile (L*=93.5, a*=�1.0, b*=0.8) before

each series of measurements. For each samples, at

least five determinations were performed (coefficient

of variation less than 3% for L*, less than 45% for

a* and less than 48% for b*). Measurements were reg-ularly performed on samples withdrawn during dehy-

dration. The observer angle during colour analysis

was 90�.� Sample weight were measured using a balance Giber-

tini (mod. Europe 500, Milan, Italy). Determinations

were carried out in triplicate.

Kinetic constants (k) and correlation coefficients (r)were calculated from the linear regression on linear por-

tions of the curves using Excel 2002 software (Microsoft

Corporation, USA).

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.5 1 1.5 2 2.5 3 3.5 4

dW/d

t (g

H2O

/g d

ry m

atte

r*m

in)

W (g H2O/g dry matter)

Fig. 2. Dehydration speed of unblanced samples under different drying

conditions as a function of the absolute moisture.

3. Results and discussion

3.1. Drying speed

The kinetic constants reported in Table 1 show that

drying speeds were in the order: belt drier>microwave

oven>air cabinet, for all the considered samples.In particular, the time necessary to obtain complete

dehydration (absolute moisture <0.11 g H2O/g dry mat-

ter) of the unblanched cubed potatoes (Fig. 1) amounted

to 195 min for the belt drier, 210 min for the microwave

oven and 330 min for the air cabinet. For the un-

blanched samples, Fig. 2 reports the dehydration speed

as a function of the absolute moisture. The initial abso-

lute moisture amounted to about 3.95 g H2O/g drymatter corresponding to 78.5% of water content. The

graphical representation allows visualization of the

Table 1

Drying speed (k), expressed as g H2O/(g dry matter·min), correlation coeffic

systems (p) for the combined blanching–drying treatments

Drying methods Unblanched cubed

potatoes

Potatoes traditionally

blanched in the

NaCl solution

Po

bla

Gli

Air cabinet

k 0.019 0.021 0

r 0.998 0.998 0

p <0.001 <0.001 <0

Microwave oven

k 0.043 0.064 0

r 0.998 0.937 0

p <0.001 <0.01 <0

Belt drier

k 0.117 0.151 0

r 0.974 0.940 0

p <0.001 <0.01 <0

drying speed independently by the time of treatment.

During the first phase of drying (characterized by high

moisture level), remarkable differences in dehydration

ient (r) and significance as a function of the degrees of freedom of the

tatoes traditionally

nched in the

cosa–NaCl solution

Potatoes blanched

by microwaves in

distilled water

Potatoes blanched

by microwaves in

the NaCl solution

.012 0.021 0.016

.998 0.999 0.998

.001 <0.001 <0.001

.021 0.070 0.045

.993 0.952 0.988

.001 <0.001 <0.001

.078 0.228 0.185

.962 0.993 0.999

.05 <0.001 <0.001

292 C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296

speeds among the three drying methods were seen. At

the end of process, drying speeds, expressed as g H2O/

(g dry matter·min), were similar for the belt drier and

the microwave oven (about 0.02 g H2O/g of dry matter)

whereas the process performed in the air cabinet was

slower. From Fig. 2, it is well evident that drying speedin the air cabinet was constant over a large range of

absolute moisture.

For cubed potatoes blanched in sodium-chloride

solution, all the kinetic constants were higher than the

corresponding ones for to the unblanched samples

(Table 1). This behaviour was probably due to softening

by the blanching treatment which facilitated the water

removal. In the case of the belt drier, the time necessaryfor complete dehydration was equivalent to those for the

unblanched samples. However, dehydration in the air

cabinet and in the microwave oven was longer (405

and 210 min, respectively, Table 2). Considering drying

speed as a function of the absolute moisture (data not

shown), the trends were similar to those illustrated for

the unblanched potatoes.

For samples blanched in the Glicosa–NaCl solution,kinetic constants (Table 1) were lower than the corre-

sponding k for the unblanched samples and potatoes

blanched in NaCl solution. This behaviour was proba-

bly due to the blanching in the hypertonic solution

which caused enrichment in soluble solids and conse-

quent reduction in water activity.

Kinetic constants for drying of samples blanched by

microwaves were generally higher than the correspond-ing k values for the unblanched potatoes and the cubes

submitted to blanching by immersion in hot solutions.

In particular, kinetic constants for the dehydration of

samples blanched by microwaves in distilled water were

higher than the k values calculated for dehydration of

cubed potatoes blanched by microwaves in a hypertonic

(Glicosa–sodium-chloride) solution. Also in this case,

these differences were due to the enrichment in solublesolids that increased the bound water.

All the correlation coefficient values demonstrate the

goodness of fit.

3.2. Drying time

Table 2 reports the drying time required to reduce the

absolute moisture value below 0.25 g H2O/g dry matter

Table 2

Dehydration times for the combined blanching–drying treatments: dehydra

content equal to 0.25 g H2O/g dry matter

Drying time (min) Unblanched

cubed potatoes

Potatoes traditionally

blanched in the

NaCl solution

Pot

blan

Glic

Air cabinet 255 >405 255

Microwave oven 145 210 180

Belt drier 105 105 180

(i.e., an uncomplete dehydration). A good correlation is

evident among the kinetic constants (Table 1) and dry-

ing times is evident for drying performed on the belt

drier. Furthermore, dehydration performed by the belt

drier on samples previously blanched by microwaves

showed the greatest efficiency. However, dehydrationperformed by the air cabinet on the same types of sam-

ples was among the worst. This demonstrates that the

efficiency of the dehydration systems in terms of water

removal can compensate for the unfavourable effects

deriving from the presence of moistening substances in

the blanching solution.

3.3. Colour

Quality of potato cubes was evaluated during drying

through colour measurements. The average results of

the colour measurements performed on the raw potatoes

were the following: L* 52.55, a* �1.87, b* 15.84. Table 3

is a summary of the speeds of colour changes and re-

ports the kinetic constants calculated from the trends

in L*, a* and b* values during dehydration for all theconsidered samples. For unblanched potatoes, those

dried by microwave oven browned faster than the oth-

ers. The slowest browned cubes were those dried on

the belt drier. In fact, as is evident from data reported

in Table 3, L* kinetic constants were negative, indicating

a decrease in luminosity whereas a* and b* increased to

indicate an increase in the red and yellow value, respec-

tively. The increase in a* and b* values are indicesof Maillard reaction (Morales & van Boekel, 1998;

Carabasa-Giribet & Ibarz-Ribas, 2000). The better re-

sults obtained by the belt drier can be explained in the

following way: the potato cubes were brought faster

than the others to temperatures unfavourable to the ac-

tivity of enzyme responsible for browning; furthermore,

these temperatures were not high enough to induce non-

enzymatic browning (Maillard reaction).The colour of cubes blanched in the sodium-chloride

solution did not undergo great changes during drying

process (Table 3) with the exception of the b* value

which increased (higher yellow value) probably due to

the loss of water and the consequent colour concentra-

tion. It could be taken as an index of the start of

the Maillard reaction (Morales & van Boekel, 1998;

Carabasa-Giribet & Ibarz-Ribas, 2000).

tation was prolonged until the samples reached an absolute moisture

atoes traditionally

ched in the

osa–NaCl solution

Potatoes blanched

by microwaves in

distilled water

Potatoes blanched

by microwaves in

the NaCl solution

>360 325

240 240

85 75

Table 3

Speed of colour changes (k), expressed as colour parameter per min and correlation coefficient (r) for the combined blanching–drying treatments

Drying methods Unblanched cubed

potatoes

Potatoes traditionally

blanched in the NaCl

solution

Potatoes traditionally

blanched in the

Glicosa–NaCl

solution

Potatoes blanched by

microwaves in distilled

water

Potatoes blanched by

microwaves in the

NaCl solution

L* a* b* L* a* b* L* a* b* L* a* b* L* a* b*

Air cabinet

k �0.032 0.015 1·10�5 �0.010 0.005 0.052 �0.021 0.008 0.046 �0.010 0.005 0.052 �0.021 0.004 0.046

r 0.489 0.844 2·10�7 0.286 0.619 0.887 0.334 0.711 0.585 0.286 0.619 0.876 0.311 0.400 0.698

Microwave oven

k �0.118 0.024 0.030 �0.032 0.023 0.043 �0.053 0.025 0.006 �0.032 0.023 0.043 �0.040 0.011 0.054

r 0.777 0.828 0.612 0.371 0.868 0.588 0.542 0.886 0.065 0.371 0.868 0.588 0.212 0.707 0.737

Belt drier

k �0.002 0.011 0.009 �0.031 0.023 0.069 �0.027 0.022 0.018 �0.031 0.023 0.069 �0.071 0.030 0.093

r 0.028 0.767 0.272 0.467 0.887 0.721 0.472 0.852 0.248 0.467 0.887 0.721 0.560 0.977 0.714

Fig. 3. Increase in weight (g/100 g dry matter) in unblanched samples

as a function of the rehydration time in hot liquids.

C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296 293

In the case of samples blanched in the Glicosa–NaCl

solution, those dried by microwave oven browned faster

than the others (greater loss of luminosity and higher

red value, Table 3) probably due to the effects of micro-

waves on a higher solute concentration.

In samples blanched by microwaves (independently

by the kind of blanching solution), kinetic constants

for the b* index showed the greatest values and indicatea beginning of the Maillard reaction.

For all the considered samples, low correlation coef-

ficients were low. These low values could be explained in

terms of a very little dependence of the colour parame-

ters from the dehydration time.

3.4. Rehydration

After dehydration, samples were submitted to rehy-

dration trials to check their capability to rehydrate in

liquids of different nature. Rehydration in hot liquids

was carried out for only 10 min to avoid cooking the po-

tato. For the same reason, the temperature of the liquids

was not kept at 80 �C for all the time but decreased as a

result of both the addition of potato cubes and the lack

of heating. From Figs. 3–7, which report the rehydra-tion curves of all the samples, it is evident that potato

cubes absorbed more liquid if rehydrated in distilled

water rather than in vegetable stock. This behaviour

was probably due to the slower diffusion in the inner po-

tato cubes of liquid containing soluble solids and a fat

fraction. Furthermore, the behaviour of samples during

reconstitution was different depending on the drying

method to which they where submitted.Fig. 3 shows the increase in weight of unblanched

samples during rehydration. Potato cubes dehydrated

in the air cabinet and by microwaves had a slower rehy-

dration than samples dehydrated on the belt drier. The

latter absorbed a quantity of liquid amounting to about

100% of their weight during the first minute of immer-

sion and, by the end of the rehydration, they had ab-

sorbed the highest quantity of liquid compared to the

other samples.

These differences were amplified in the case of sam-

ples blanched in the sodium-chloride solution (Fig. 4).

Potato cubes dehydrated on the belt drier acquired after10 min a quantity of liquid (water or vegetable stock)

amounting to about 200% of their dry matter whereas

samples dehydrated in the air cabinet and by micro-

waves absorbed liquid amounting 50% and 100% of

their dry matter, respectively.

Fig. 6. Increase in weight (g/100 g dry matter) in samples blanched by

microwaves in distilled water as a function of the rehydration time in

hot liquids.

Fig. 5. Increase in weight (g/100 g dry matter) in samples traditionally

blanched in the glicosa–Nacl solution as a function of the rehydration

time in hot liquids.Fig. 7. Increase in weight (g/100 g dry matter) in samples blanched by

microwaves in the NaCl solution as a function of the rehydration time

in hot liquids.

Fig. 4. Increase in weight (g/100 g dry matter) in samples traditionally

blanched in the NaCl solution as a function of the rehydration time in

hot liquids.

294 C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296

Samples blanched in the Glicosa–sodium-chloride

solution showed slower rehydration kinetics and ac-

quired the smallest quantity of liquid (Fig. 5). This

behaviour was probably due to the acquisition of

soluble solids during the blanching, which resulted in a

decrease in porosity. The effect of the blanching treat-

ment minimized the differences due to the drying

methods.

Table 4

Rehydration time in distilled water at room temperature for the combined blanching–drying treatments

Rehydration time (min) Unblanched

cubed potatoes

Potatoes traditionally

blanched in the

NaCl solution

Potatoes traditionally

blanched in the

Glicosa–NaCl solution

Potatoes blanched by

microwaves in distilled

water

Potatoes blanched by

microwaves in the

NaCl solution

Air cabinet 70 >120 95 >120 >120

Microwave oven >120 >120 >120 >120 >120

Belt drier >120 >120 85 90 38

C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296 295

Fig. 6 shows the rehydration curves of samples

blanched by microwaves in distilled water. In this

case, microwaves induced a modification of the starch

granules whose capacity to absorb liquid during rehy-

dration was increased, according to the results repor-

ted by Sikora, Tomasik, and Pielichowski (1997). Thiseffect was particularly evident in samples dehydrated

on the belt drier which absorbed liquids amounting to

about 300% of their dry matter. Among samples

blanched by microwaves in distilled water, those show-

ing the worse behaviour during rehydration were those

dehydrated by microwaves. This can be explained

considering that these samples were treated (blanched

and de-hydrated) exclusively by microwaves whichdetermined the complete starch gelatinization and, con-

sequently, the minor water holding capacity during re-

hydration.

Potato cubes blanched by microwaves in the sodium-

chloride solution showed a behaviour similar to those

blanched by microwaves in distilled water during rehy-

dration (Fig. 7) even if the effects of microwaves were re-

duced by the presence of salt.Table 4 reports the time of rehydration in distilled

water at room temperature necessary to bring samples

to their natural water content. From this point of view,

samples showing the best results were those blanched by

microwaves in the sodium-chloride solution and succes-

sively dehydrated on the belt drier. The other samples

needed double or triple rehydration times.

4. Conclusions

Dehydration performed by the air cabinet was the

slowest and did not yield, within the first phase of rehy-

dration at least, a satisfying water holding capacity.

Dehydration by microwaves gave a good drying

speed coupled to a good water holding capacity and col-our retention. Nevertheless, a blanching-drying process

totally carried out by microwaves would be too expen-

sive.

Dehydration performed on the belt drier was the

most effective both for speed and water absorption dur-

ing rehydration, independently of the kind of blanching.

In terms of process speed and quality (colour and re-

hydration capability) of the obtained samples, the best

combinations were by the blanching by microwaves cou-

pled to dehydration on the belt drier.

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