The effect of ohmic heating on vacuum drying rate ofsweet potato tissue

Tuoxiu Zhong, Marybeth Lima *

Department of Biological and Agricultural Engineering, Rm. 149, E.B. Doran Building, LSU Agricultural Center,

Baton Rouge, LA 70803-4505, USA

Received 11 June 2002; received in revised form 4 October 2002; accepted 6 October 2002

Abstract

Ohmically heating fruit and vegetable tissue has been shown to increase hot-air drying rate, shift desorption isotherms, and

increase juice extraction yields with respect to untreated, conventionally heated, and microwaved samples. The objective of this

study was to determine if ohmically heating sweet potato tissue would enhance the vacuum drying rate of these samples with respect

to untreated samples. Sweet potato cubes were ohmically heated to three endpoint temperatures using three electrical field strengths

and were then placed in a freeze dryer. Moisture content vs. time data were collected and modeled. Results showed that the vacuum

drying rates of ohmically heated samples were faster than raw samples for most treatment combinations, and that the maximum

reduction of drying time was 24%. Minimal ohmic treatment can result in a significant decrease in vacuum drying time, which could

have important economic and product quality implications.

� 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Ohmic heating is defined as a process wherein (pri-

marily alternating) electric currents are passed throughfoods or other materials with the primary purpose of

heating them (Sastry and Barach, 2000). Ohmic heating

has great potential in a large number of food processing

applications (Parrott, 1992), and the FDA recently

communicated that ‘‘A large number of potential future

applications exist for ohmic heating, including its use in

blanching, evaporation, dehydration, fermentation, and

extraction’’ (FDA, 2001). These unit operations are ofcritical importance in areas such as agricultural, food

and biological processing.

Prior literature has addressed electric heating and its

influence on mass transfer properties. Wigerstrom

(1976) found that electric fields enhanced moisture loss

during the blanching of potato slices. Carlon and La-

tham (1992) studied the drying rates of wetted materials

in electric fields and found that the drying time of wettedpaper towel discs decreased by a factor of six when the

electric field strength was increased from 0 to 7000 V/

cm. Lima and Sastry (1999) found that the lower the

frequency of alternating current used in ohmic heating,

the faster the hot-air drying rate of sweet potato. Wangand Sastry (2000) showed that ohmically treating sweet

potato prior to drying accelerated the hot-air drying rate

significantly compared to raw, conventionally treated

and microwaved samples.

Katrokha et al. (1984) used electrical heating to in-

crease the extraction of sucrose from sugar beets, and

Kim and Pyun (1995) used ohmic heating to extract

soymilk from soybeans. Halden et al. (1990), Schreieret al. (1993) and Lima et al. (2001) showed that the

diffusion of beet dye from beetroot into solution was

enhanced using an electric field, and that the extent of

enhancement was a function of electric field strength,

temperature, surface area, and electrical conductivity.

Lima and Sastry (1999) and Wang and Sastry (2000)

found that ohmically heating apple tissue prior to me-

chanical juice extraction significantly increased applejuice yields with respect to nontreated apple tissue, and

that the lower the frequency of alternating current, the

greater the extraction yield.

Because ohmic heating accelerates hot-air drying rates

and improves extraction yields, it could have important

Bioresource Technology 87 (2003) 215–220

*Corresponding author. Tel.: +1-225-578-1061; fax: +1-225-578-

3492.

E-mail address: mlima@gumbo.bae.lsu.edu (M. Lima).

0960-8524/03/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0960-8524 (02 )00253-5

commercial uses. The use of freeze drying (for frozen

samples) and vacuum drying (freeze drying unfrozen

samples) is time and energy intensive, thus any method

that would significantly decrease this time is important.

The objective of this study was to determine if ohmic

heating accelerated the vacuum drying rate of sweet

potato tissue.

2. Methods

The experimental apparatus used for ohmic heating

was identical to that of Lima and Sastry (1999). Sweet

potato cubes (1 cm3) were cut in half; one half of the

cube was ohmically treated, and the other half was used

as the raw (untreated) sample. Ohmic treatment con-

sisted of sandwiching the sweet potato samples (1 cm�1 cm� 0:5 cm) between two specially coated titanium

electrodes (coating provided by APV Company, Devon,

England) and heating the sample electrically until the

geometric center of the sample reached a given tem-

perature. Care was taken to ensure good contact be-

tween the sample surface and the electrodes, thus

uniform heating was assumed (Palaniappan and Sastry,

1991). A 120 V power supply was used to generate al-ternating current (60 Hz sine wave) for heating the

sample. A Teflon-coated thermocouple was inserted into

the geometric center of the sample to monitor temper-

ature. Calibrated voltage and current transducers (Ohio

Semitronics, Hilliard, OH) were used to measure voltage

and current. Raw (untreated) samples were also drilled

with the thermocouple to ensure that all samples were

comparable. Samples were ohmically treated immedi-ately after being cut to the proper dimensions.

The experimental design for this study was a com-

pletely randomized design with a factorial (3� 3� 2)

treatment structure. There were two types of heating

(Heat), ohmic or treated, and nonheated or untreated.

Samples were ohmically treated with combinations of

three electrical field strengths (EFS ¼ 50, 70 and 90 V/

cm) and three endpoint temperatures (EPT ¼ 45, 60 and80 �C). Currents during ohmic heating ranged from 0.1

A (30 �C and 50 V/cm) to 2.3 A (80 �C and 90 V/cm).

Immediately after ohmic heating treatment, the samples

(both ohmic and nonheated) were placed into a pre-

weighed glass vial, weighed together, and loaded into a

freeze dryer (Labconco Kansas City, MO) for drying.

The freeze drying vacuum pressure was maintained at

50� 10�3 bar, and the condenser temperature was )50�C. Moisture content data were obtained by weighing the

samples after 0, 5, 10, 15, 20, 30, 45, 60, 90, and 120 min

of drying. Subsequently, the samples were transferred toa 120 �C oven for final moisture content determination

(for details see Lima and Sastry, 1999). There were four

replications for each treatment combination.

The mean dimensionless moisture ratio (MR) was

determined using the equation

MR ¼ ðMðtÞ �MeÞðM0 �MeÞ

;

where MðtÞ is moisture content at time t ¼ f0; 5; 10; 15;20; 30; 45; 60; 90; or 120g, M0 is the initial moisture

content, and Me is the equilibrium (final) moisture con-

tent. The moisture ratio concept was developed by

Hukill (1947, 1954) to determine moisture content dur-

ing the drying of grain products.

A separate nonlinear exponential decay model was

fitted to each of the 18 treatment combinations. The

nonlinear model is

MR ¼ aþ be�ct;

where a is the horizontal asymptote, b is the distance

from the asymptote to the intercept of the vertical axis,

and c is the slope or rate. For this investigation, the rateparameter, c, is the effect of the drying time for a specific

treatment combination. The larger the value of c, thefaster a particular treatment combination reduced theMR. The model used is similar to published drying

models (ASAE, 2000) and was used to generate a best fit

for the data.

A nonlinear analysis of covariance was used to eval-

uate treatment combination differences (Hinds and

Milliken, 1987). Based on the results of the analysis of

covariance, Fisher�s least significant difference tests

(Milliken and Johnson, 1984) were conducted to evalu-ate differences between the rate parameters (indicating

significantly different vacuum drying rates) and asymp-

totes (indicating the degree to which moisture was re-

Nomenclature

EFS electrical field strength (V/cm)

EPT endpoint temperature (�C)M moisture content (% dry basis)

MðtÞ moisture content at time t (% dry basis)

Me equilibrium moisture content (% dry basis)

M0 initial moisture content (% dry basis)

MR mean dimensionless moisture ratio

O ohmically treated sample

p probability

U untreated (raw) sample

V volts

216 T. Zhong, M. Lima / Bioresource Technology 87 (2003) 215–220

moved). All statistical analyses were conducted using theS-Plus (2001).

3. Results and discussion

Results of the nonlinear analysis of covariance are

shown in Table 1. The results indicated that ohmic

heating significantly altered (increased) the vacuum

drying rate (p-value < 0:05), but that endpoint temper-ature and electrical field strength were not significant

factors in affecting vacuum drying rate (p-values >0:05). These results are different from prior studies in-

volving hot-air drying rate (Lima and Sastry, 1999;

Wang and Sastry, 2000), which showed that increasing

the electrical field strength increased the drying rate. The

statistically significant three-way interaction (EFS�EPT�Heat, p-value < 0:05) suggests that all 18 non-linear models, one for each treatment combination, are

needed to adequately describe the relationship between

MR and time. The effect of ohmic heating is different

depending on the endpoint temperature and electrical

field strength. Initial moisture contents of raw and oh-

mically heated samples were determined to verify that

no significant dehydration had taken place during the

heating. Results were not significantly different, indi-cated that the difference in drying rate was not due to

initial dehydration effects.

Based on the statistically significant three-way inter-

action, further analyses were conducted to evaluate

differences between rate (c) parameter estimates and

between asymptote (a) parameter estimates. The results

of separate analysis of variances indicate that there is no

statistically significant difference between asymptote es-timates (p-value > 0:05) but there are statistically sig-

nificant differences between rate parameter estimates

(p-value < 0:05) (Table 2).

To isolate the rate parameter differences, Fisher�sleast significant difference was used to construct 95%

confidence intervals around the rate parameter estimates

(Table 3). The rate parameter estimates show that ohmic

heating increases the vacuum drying rate for six of the

nine treatment combinations, though statistical signifi-

cance was observed in only three cases as indicated in

Table 3.

It is important to note that even when rate parame-

ters were not significantly different, ohmically treated

samples dried faster than untreated samples. Times forthe MR to reach 0.2 and 0.5 with treated and untreated

samples are illustrated in Table 4. For the three cases in

which the treated rate parameter was higher than the

untreated rate parameter but the differences were not

statistically significant, vacuum drying times were 6.9–

17.4% faster. In the remaining three cases, ohmic

treatment combinations (EPT ¼ 45 �C, EFS ¼ 90 V/cm;

EPT ¼ 80 �C, EFS ¼ 70 V/cm; EPT ¼ 80 �C, EFS ¼ 90V/cm) increased the drying rate parameter, though these

differences were not statistically significant, and drying

times were almost identical (see Table 4). Plots of the

95% confidence bands about the difference between

predicted MR for treated and untreated data are shown

in Fig. 1.

Table 1

Nonlinear analysis of covariance table for a completely randomized

design with a factorial treatment structure (3� 3� 2) for the nonlinear

model: MR ¼ aþ be�ct

Source of variation Degrees of

freedom

Sum of

squares

F -value Prob

(P F -value)

EPT 6 0.00305 1.46 0.20

EFS 6 0.00112 0.54 0.78

Heat 3 0.04335 41.5 0

EPT� EFS 12 0.07849 18.8 0

EPT�Heat 6 0.03626 17.4 0

EFS�Heat 6 0.03819 18.3 0

EPT� EFS�Heat 12 0.16168 38.7 0

Error 126 0.04388

Table 2

Analysis of variance results for rate (c) and asymptote (a) parameter

differences

Asymptote (a) Rate (c)

F -value 0.98 2.38

MSEPooled 3:4� 10�4 3:4� 10�4

prob(F -value) 0.49 0.003

dfðMSEPooledÞ 126 126

Table 3

Parameter estimates and 95% confidence intervals for the rate para-

meter

Treatment

combination

a b c 95% LCL

for c95% UCL

for c

O/45/50a 0.0277 0.962 0.0339 0.0306 0.0372

U/45/50a 0.0478 0.9449 0.0273 0.0236 0.0307

O/45/70 0.0456 0.9385 0.0306 0.0261 0.0351

U/45/70 0.0506 0.9444 0.0272 0.0233 0.0313

O/45/90 0.0307 0.9437 0.0267 0.0220 0.0314

U/45/90 0.0376 0.9575 0.0281 0.0243 0.0319

O/60/50 0.0556 0.9285 0.0323 0.0287 0.0358

U/60/50 0.0296 0.9692 0.0281 0.0248 0.0314

O/60/70a 0.0452 0.9497 0.0351 0.0323 0.0379

U/60/70a 0.0346 0.9523 0.0259 0.0216 0.0302

O/60/90 0.0264 0.9571 0.0308 0.0270 0.0346

U/60/90 0.0384 0.9631 0.0287 0.0249 0.0325

O/80/50a 0.0981 0.876 0.0320 0.0266 0.0374

U/80/50a 0.0361 0.9578 0.0261 0.0223 0.0299

O/80/70 0.0241 0.9557 0.0300 0.0260 0.0340

U/80/70 0.0407 0.9651 0.0302 0.0264 0.0340

O/80/90 0.0336 0.9459 0.0295 0.0255 0.0335

U/80/90 0.0623 0.9409 0.0305 0.0260 0.0350

a Indicates that ohmically treated (O) and untreated (U) rate pa-

rameters (c) are significantly different (P 6 0:05). Values in the left

column represent endpoint temperature/electrical field strength.

T. Zhong, M. Lima / Bioresource Technology 87 (2003) 215–220 217

The data in this study exhibit some similarities to

prior work done with enhancing the hot-air drying rate

of sweet potato using ohmic heating. Nonlinear model

data show that the biggest difference in drying rate be-tween treated and untreated samples occurred during

the initial and intermediate stages of drying (see Fig. 1).

This trend is also evident in Fig. 2; the treatment com-

bination of a 70 V/cm electrical field and 60 �C endpoint

temperature shown in this figure resulted in the largest

difference of MR compared to the untreated sample.These curves are characteristic of those obtained by

Lima and Sastry (1999) and Wang and Sastry (2000),

with the biggest differences in drying rates occurring

between MRs of 15–50%. According to parameter esti-

mates for this case, freeze drying sweet potatoes to a

MR of 20% would take 51.7 min with ohmic treatment

and 67.6 min if untreated. This represents a 24% de-

crease in freeze drying time, which is significant in termsof time and energy savings.

Asymptotes for the model data were also tested to

determine if final moisture contents were significantly

different as a result of heating (Table 2). The final MR

values were not significantly different, indicating that at

the treatment conditions, ohmic heating did not alter the

extent to which moisture could be liberated from the

sweet potato sample. This suggests that ohmic heatingdoes not alter the internal resistance to moisture transfer

at low moisture contents.

While confounding factors render it difficult to make

broad recommendations regarding optimal conditions

of ohmic heating for accelerating freeze drying, it ap-

pears that minimal ohmic heating (EFS ¼ 50 V/cm,

EPT ¼ 45 �C) can result in a significant reduction (22–

24%) in freeze drying time. Lima and Sastry (1999)showed that minimal ohmic treatment had a significant

effect on extraction yield; a treatment of EFS ¼ 40 V/cm

and EPT ¼ 40 �C increased the extraction yield of apple

juice from apple from 486.4 to 586.9 ml/kg. It has been

theorized that mild electroporation occurs during ohmic

Table 4

Drying times to specific MR

Treatment combinations

(Heat, EPT, EFS)

Time (min) to

MR ¼ 0:5

Time (min) to

MR ¼ 0:2

O/45/50 21.0 50.7

U/45/50 27.0 66.9

O/45/70 23.7 59.0

U/45/70 27.3 67.8

O/45/90 26.2 64.3

U/45/90 25.9 63.1

O/60/50 22.8 57.6

U/60/50 25.7 61.9

O/60/70 21.0 51.7

U/60/70 27.6 67.6

O/60/90 22.8 55.4

U/60/90 25.6 62.2

O/80/50 24.3 67.2

U/80/50 27.8 67.6

O/80/70 23.2 56.4

U/80/70 24.6 59.7

O/80/90 24.0 58.9

U/80/90 25.1 63.0

Fig. 1. Plots of the 95% confidence band aroud the difference of the predicted values for ohmically treated and untreated samples for the nine

combinations of EPT and EFS.

218 T. Zhong, M. Lima / Bioresource Technology 87 (2003) 215–220

heating (Imai et al., 1995; Sastry and Barach, 2000)

because the low frequency of ohmic heating (6 60 Hz)

allows cell walls to build up charges and form pores.

4. Conclusions

The vacuum drying rate of ohmically heated sweet

potato samples was faster than raw samples for most

electrical field strength and endpoint temperaturetreatment combinations. The maximum rate parameter

difference was obtained with an EPT of 60 �C and an

EFS of 70 V/cm. Minimal ohmic heating (EFS ¼ 50 V/

cm, EPT ¼ 45 �C) resulted in a significant reduction

(22–24%) in vacuum drying time. Benefits of ohmic

heating such as enhancing vacuum drying rate and en-

hancing extraction yields could be ideal for processes

such as supercritical fluid extraction, and could be im-portant in the recovery of high value commodities from

biological materials.

Acknowledgements

This study was supported by the Louisiana Agricul-

tural Experiment Station. The authors acknowl-

edge Tom Bride, James Finney and the late Malcolm

Gaspard for technical assistance. Dr. Vicki Lancaster

(Department of Experimental Statistics, LSU) helped

immensely with data analysis and model preparation.Dr. Sam Godber reviewed the manuscript, Ms. Jane

Honeycutt proofread the manuscript, and Ms. Julianne

Forman assisted with final manuscript preparation.

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