Effects of high hydrostatic pressure or high intensity electrical field pulse pre-treatment on dehydration characteristics of red paprika

Innovative Food Science & Emerging Technologies 2 Ž2001. 1᎐7

Effects of high hydrostatic pressure or high intensity electrical field pulse pre-treatment on dehydration characteristics of red paprika B.I.O. Ade-Omowaye1, N.K. Rastogi 2 , A. Angersbach, D. Knorr U Department of Food Biotechnology and Food Process Engineering, Berlin Uni¨ ersity of Technology, Konigin-Luise-Strasse 22, ¨ D-14195 Berlin, Germany Received 4 January 2000; accepted 18 September 2000

Abstract The effects of various pre-treatments Žhot water blanching, skin treatments, high pressure and high intensity electric field pulse treatment . on the dehydration characteristics of red paprika Ž Capsicum annuum L.. were evaluated and compared with untreated samples. Hot water blanching Ž100⬚C, 3 min. prior to dehydration Žfluidised bed dryer at 60⬚C, 6 h and 1 mrs. resulted in the permeabilisation of 88% of the cell membranes in paprika, which in turn resulted in a higher mass and heat transfer. Skin treatments Žsuch as lye peeling and acid treatment ., as practised conventionally, increased dehydration rates but affected only the skin permeability. The application of high hydrostatic pressure ŽHHP, 400 MPa for 10 min at 25⬚C. or high intensity electric field pulses ŽHELP, 2.4 kVrcm, pulse width 300 ␮s, 10 pulses, pulse frequency 1 Hz. pre-treatments resulted in cell disintegration indexes of 0.58 and 0.61, respectively. Cell permeabilisation of these physical treatments resulted in higher drying rates, as well as higher mass and heat transfer coefficients, as compared to conventional pre-treatments. 䊚 2001 Elsevier Science Ltd. All rights reserved. Keywords: High hydrostatic pressure; High intensity electric field pulses; Dehydration; Cell disintegration Industrial rele¨ ance: This report compares thermal pre-treatment with physical processes with the aim to increase mass transfer in products conventionally requiring skin pre-treatment. The higher drying rates at lower degree of cell disintegration as achieved with high pressure and electric field pulses are of interest because of the attractive energy efficient means of permeabilization and the time and energy savings during drying. Further, the lower and controllable degrees of permeabilization with the above methods as compared to hot water blanching is of relevance with regards to structure engineering of plant food materials.

1. Introduction Red paprika Ž Capsicum anuum L.. is produced in large quantities worldwide and is an essential part of agricultural production in the northern areas of NigeU

Corresponding author. Tel: q49-30-31471250; fax: q49-308327663. E-m ail address: foodtech@ m ailszrz.zrz.tu-berlin.de Ž D. Knorr.. 1 On German Academic Exchange Service ŽDAAD. fellowship from: Department of Food Science and Engineering, Ladoke Akintola University of Technology, Ogbomoso ŽNigeria.. 2 On German Academic Exchange Service ŽDAAD. fellowship from: Department of Food Engineering, Central Food Technological Research Institute, Mysore 570 013 ŽIndia..

ria. Its production is seasonal and its demand throughout the year has geared interest in products having qualities close to the fresh ones. Consequently, a great potential exists for the development of effective and efficient dehydration processes for paprika. The traditional method of sun drying of paprika depends to a large extent on the climatic conditions. The quality and yield of such dried products are negatively affected due to uncontrolled conditions of dehydration. Hot water blanching may also be employed in some cases prior to sun drying to hasten the drying rates. It has been reported that dehydration kinetics are governed predominantly by the skin permeability ŽMasi & Riva, 1988; Saravacos, Marousis & Raouzeos, 1988.. The

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B.I.O. Ade-Omowaye et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 2 (2001) 1᎐7

thick epicuticular waxy layers present on the surface of peppers have a high resistance to mass transfer. It is been known that dipping waxy fruits for a few minutes in an emulsion of fatty acid derivatives, used as wetting agents and emulsifiers, such as ethyl oleate, can reduce dehydration time. Some work has been done on skin treatments of whole fruits, especially those having a waxy skin such as plums, cherries, and grapes for airdrying ŽRadley, 1964; Ponting & McBean, 1970; Saravacos et al., 1988.. Chemical Žsodium hydroxide, hydrochloric acid, ethyl oleate. and physical Žskin puncturing. skin treatments have been reported to influence mass transfer rates during the osmotic dehydration of tomatoes ŽShi, Maguer, Wang & Liptay, 1997.. Lye peeling and steam assisted peeling, are conventional methods adopted as pre-processing steps in the vegetable processing industry ŽGould, 1983; Floros & Chinnan, 1989.. Steam assisted peeling results in the loss of heat sensitive nutrients and possibly affects product texture to some extent. Lye peeling has been reported to adversely affect flavour, texture and sensory qualities of the product ŽShi et al., 1997.. In recent years, the consumer demand for minimally processed foods having fresh-like qualities has led to the development of physical processes which are nonthermal in nature and do not involve chemical treatment. High hydrostatic pressure ŽHHP. and high intensity electric field pulse ŽHELP. treatments are such alternatives. High intensity electric field pulses have been widely used for the irreversible permeabilisation of cell membranes, which has consequently improved the drying rate ŽBrodelius, Funk & Shillito, 1988; Knorr, 1994; Geulen, Teichgraber ¨ & Knorr, 1994; Angersbach & Knorr, 1997; Angersbach, Heinz & Knorr, 1997.. The application of high hydrostatic pressure affects cell wall structures, leaving the cells more permeable, which leads to significant changes in the tissue architecture ŽRastogi & Niranjan, 1998.. Our objective was to increase mass transfer rates during the dehydration of paprika after pre-treatments of blanching, skin treatments Žlye peeling and acid treatment ., high hydrostatic pressure and high intensity electric field pulse treatment. Heat and mass transfer coefficients were experimentally determined during the constant rate period for all the treatments and compared with controls. An attempt has been made to compare the conventional practice with the physical pre-treatments applied.

2. Materials and methods 2.1. Materials Fresh red paprika Ž Capsicum annuum L.. was procured from a local supermarket and cut into slices

of 1 cm perpendicular to their length axis. The paprika had an average moisture content of 91.20" 0.22% Žon wet basis. as determined by vacuum drying at 70⬚C for 24 h ŽAOAC, 1990.. 2.2. Methods The range of operating conditions utilised in this study were obtained from literature and preliminary analysis. Experiments were repeated three times and average values are reported. 2.3. Blanching pre-treatment Whole paprika was blanched in a boiling water for 3 min. The blanched samples were immediately cooled in tap water in order to arrest further softening due to thermal treatment and then water was drained. 2.4. Skin pre-treatments 2.4.1. NaOH pre-treatment (Lye peeling) Whole paprika samples were dipped in 5% wrv NaOH solution at 25⬚C or 35⬚C for 20 min. After the treatment, the paprika samples were washed with tap water in order to remove adhering solution. The skin for the paprika samples treated at 35⬚C was completely removed manually whereas it was not possible to peel the samples treated at 25⬚C. 2.4.2. Acid pre-treatment Whole paprika samples were treated with 5% vrv HCl solution at 25⬚C or 35⬚C for 20 min, and after the treatment they were washed with tap water and adhering water was wiped away gently with tissue paper. 2.5. Physical pre-treatments 2.5.1. High hydrostatic pressure (HHP) pre-treatment Sliced paprika samples were vacuum packed in polyethylene pouches and placed in a pressure vessel ŽNova Swiss, Model 9182.. The samples were treated at 400 MPa for 10 min at 25⬚C. The pressure unit had a 0.9-l volume vessel Žinternal diameter 45 mm., maximum pressure of 400 MPa at an operating temperature range of y10᎐50⬚C. During the pressure treatment, a thermocouple was fitted through the closure to measure the inner temperature of the vessel. A mixture of water and anti-corrosion fluid was used as a pressure-transmitting medium. The temperature in the vessel was controlled externally by flexible tubes coiled around the vessel and connected to a cryostat ŽD1-GH, HAAKE, Karlsruhe, Germany.. A high pressure reciprocating pump ŽDXSHF-602, Haskel Inc., Burbank California, USA. was used to build up the pressure in the vessel. The pressure was reached within 2 min, and the de-

B.I.O. Ade-Omowaye et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 2 (2001) 1᎐7

compression time was approximately 10 s. The maximum temperature attained during pressurisation was 30⬚C. The pressure in the vessel was measured using a pressure transducer ŽHHP28, Intersonde Ltd., Watford, England. connected at the inlet of the vessel. The samples were dried immediately after the pressure treatment. 2.5.2. High intensity electric field pulse (HELP) pretreatment Cut paprika samples were subjected to HELP treatment Žpeak field strength in the sample, Es 2.4 kVrcm, pulse duration, t s 300 ␮s, pulsing rate 1 Hz, number of pulses, n s 10.. A high voltage generator ŽPure Pulse Technologies Inc., San Diego CA. produced a high voltage charge, which supercharged the capacitor. The capacitor was then discharged at 1 Hz through the food material in tap water Žconductivity 0.8 mSrcm. placed between parallel electrodes. The electrodes were 140 cm2 each and spaced 2.14 cm apart. The exponential decay pulse was used for the treatment. The pulses were monitored on line with an oscilloscope ŽPhilips, Model PM 3335. during treatment, and voltage as well as pulse duration was recorded. The temperature increase due to HELP treatment at 25⬚C was less than 1⬚C for the treatment. The specific energy input over the pulse duration was calculated from the pulse shape as an integration of voltage time dependent V 2 Ž t . R over the pulse duration. The total specific energy input in the present case was 3.0 kJrkg product. 2.6. Fluidised bed drying The differently pre-treated cut samples were dried in a pilot plant fluidised bed dryer ŽATP, Berlin, Germany, Model MS 1016. at 60⬚C for 6 h and the velocity of air was 1 mrs. During the dehydration of paprika subjected to different pre-treatments Žsuch as blanching, skin pre-treatments, and physical pre-treatments, e.g. HHP and HELP., the moisture contents Ž X . at different drying times were experimentally determined. The weight loss of samples was recorded at 30-min intervals. The wet bulb and dry bulb temperatures were 30 and 63⬚C, respectively. The drying equipment was of a closed loop type with hot air re-circulation, having a power driven fan blowing air through a heating section from where the heated air branched into an air velocity controlling valve and air bypass valve, and then through the drying chamber consisting of a metal container with a mesh base which allowed a through-flow of the heated air.

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bach, Heinz & Knorr, 1999; Knorr & Angersbach, 1998; Angersbach & Knorr, 1997.. The cell integration index Ž Z p . was defined as: Zp s 1 y b

Ž K ⬘h y K ⬘l . ; 0 F Zp F 1 Ž Kh yKl .

Ž1.

Where bs K hrK ⬘h ; K l and K ⬘l are the electrical conductivity of control and treated samples at low frequency field Ž1᎐5 kHz. and K h and K ⬘h are the electrical conductivities of control and treated samples in a high frequency field Ž3᎐50 MHz.. The cell disintegration index characterises the proportion of cells with highly permeable cell walls. Z p is between 0 and 1, corresponding to 100% intact cells and total cell disintegration, respectively. The conductivity for control and treated samples was determined with impedance measurement equipment ŽElectronic Manufacture Company, Mahlsdorf, Germany. between parallel disc electrodes Ž9.7 mm diameter. spaced 10 mm apart. The phase voltages were each of equal amplitude Žtypically between 1 and 5 V peak-to-peak. and the frequency changed in the range from 3 kHz to 50 MHz. 2.8. Heat and mass transfer calculations The classical approach of psychometry was used to determine heat and mass transfer coefficients. The drying rate per unit area during the constant rate period Ž R dc . is expressed as ŽGeankoplis, 1983.: R dc s ymrAŽ dXrdt .

Ž2.

It is assumed that the rate of heat transfer is taking place only through convection. The rate of removal of water vapour Ždrying. is controlled by the rate of heat transfer to the evaporating surface, which furnishes the latent heat of evaporation for the water. During the constant rate drying, the moisture movement within the solid is sufficient to keep the surface saturated. At steady state, the rate of moisture transfer balances the rate of heat transfer and at that condition the product is considered at wet bulb temperature and rate of drying can be expressed in terms of following equations ŽGeankoplis, 1983; Okos, Narsimhan, Singh & Weitnaver, 1992.. R dc s kMa Ž Ha y Hs .

Ž3.

or R dc s h Ž T y Ts . r␭ . w

Ž4.

2.7. Determination of cell disintegration index

For water vapour᎐air mixtures, the psychometric ratio Ž hrMa k . is approximately equal to its enthalpy:

The conductivity᎐frequency spectra of treated and control samples of paprika were determined ŽAngers-

h U s Ž 1.005q 1.88 H . 10 3 Ma k

Ž5.

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Table 1 Effect of various pre-treatments of paprika on drying rates, heat and mass transfer coefficients and cell disintegration indexes during its dehydration in a fluidised bed dryer at 60⬚C Pre-treatments

Critical moisture content Žkgrkg ds.

Constant drying rate Rd c = 104 Žkgrm2 s.

Heat transfer coefficient, h ŽWrm2 K.

Mass transfer coefficient, k Žkgrm2 s.

Cell disintegration index, Zp

Control Blanched 5% NaOH Ž25⬚C. 5% NaOH Ž35⬚C. 5% HCl Ž25⬚C. 5% HCl Ž35⬚C. High pressure HELP

6.10" 0.20 5.75" 0.10b 5.66" 0.11b 4.70" 0.08b 6.09" 0.10a 5.21" 0.16b 5.18" 0.13b 5.16" 0.05b

9.68" 0.15 13.20" 0.21b 11.03" 0.31b 12.25" 0.13b 10.04" 0.31b 10.50" 0.45b 11.07" 0.54b 13.02" 0.35b

73.13" 0.10 99.72" 0.96b 83.29" 0.61b 92.54" 0.72b 75.81" 0.55b 79.32" 0.85b 83.61" 0.78b 98.36" 0.93b

0.043" 0.005 0.059" 0.007b 0.049" 0.008b 0.054" 0.003b 0.044" 0.005a 0.047" 0.002b 0.049" 0.003b 0.058" 0.001b

0.00 0.88" 0.04 0.00 0.00 0.00 0.00 0.58" 0.02 0.61" 0.03

a b

Insignificant difference to control at PF 0.05. Significant difference to control at PF 0.05.

Fig. 1. Variation of moisture content with drying time for Ža. physical and thermal; Žb. chemical pre-treatments. Paprika slices Ž1.0 cm. were cut perpendicular to the length axis.

B.I.O. Ade-Omowaye et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 2 (2001) 1᎐7

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Fig. 2. Characteristic drying rate curves for Ža. physical and thermal; Žb. chemical pre-treatments.

where H is the saturation humidity of drying air at T s Ts ŽGeankoplis, 1983.. From the experimentally determined moisture content, the rate of change of moisture content Žd Xrdt . were evaluated. The drying rate Ž R dc . was calculated using Eq. Ž2.. The drying curves were plotted ŽFig. 2. using these R dc values against average moisture content values. The R dc values at the constant drying period were intuitively obtained from the drying curves at the critical moisture content where the slope of the drying curve changes from falling rate period to constant rate period. From Eq. Ž3., the mass transfer coefficient Ž k . was determined with Ha and Hs values being 0.014 kg H 2 Orkg dry air and 0.028 kg H 2 Orkg dry air, respectively, obtained from the psychometric charts at corresponding temperatures. The heat transfer coefficient Ž h. was determined from the values of k using Eq. Ž5..

2.9. Drying time calculations The total drying time is the summation of time required for a constant rate period and falling rate period drying up to a certain moisture content. t s t dc q t f

Ž6.

where t dc s

tf s

m Ž X 1 y X dc . AR dc

m A

X dc

HX

2

dX R

Ž7.

Ž8.

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B.I.O. Ade-Omowaye et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 2 (2001) 1᎐7

Fig. 3. Drying time during constant and falling rate period drying as well as total drying time for different pre-treatments. Drying time during falling rate period was calculated up to a moisture content of 0.112 kgrkg ds for all the samples.

2.10. Statistical analysis Statistical analyses were carried out using PlotIT software ŽScientific Program Enterprises, 1993.. Means were compared using the t-test.

3. Results and discussion The plots of variation of moisture contents with dehydration time are shown in Fig. 1a,b and compared with untreated samples. The rate of change of moisture content was obtained from the graph and plotted against average moisture contents as per Eq. Ž2. ŽFig. 2a,b.. Fig. 2a,b shows that a large portion of the moisture from paprika is removed when the rate of drying is constant Žconstant rate period. and further drying rate was reduced with respect to moisture contents Žfalling rate period.. The relevant values of critical moisture contents are reported in Table 1 for all treatments. The cell disintegration indices Ž Z p . used as an approximation of the extent of cell permeabilisation were compared for the various pre-treatments as shown in Table 1. Hot water blanching treatment resulted in the highest cell disintegration index of 0.88 due to thermal effect and 0.61 and 0.58 cell disintegration indexes were recorded for HELP and HHP, respectively. For the skin treatments ŽNaOH and acid pre-treatments ., the cell permeabilisation was 0%, indicating that the treatments only affected the skin permeability but did not permeabilise the tissue. The calculated values of mass and heat transfer coefficients during the constant rate period are reported in Table 1. It can be inferred from Table 1 that all the pre-treatments increased the drying rate during

the constant rate period as compared to the control samples. The heat and mass transfer coefficients were also found to increase due to these treatments, except for 5% HCl at 25⬚C, which did not significantly increase the mass transfer coefficient ŽTable 1.. The increase as a result of acid and 5% NaOH at 25⬚C pre-treatments might be due to partial removal of the waxy layer from the skin surface, thus reducing the resistance to mass transfer. The results were similar to the tomato treatment ŽShi et al., 1997.. The 5% NaOH treatment at 35⬚C resulted in the separation of skin from the paprika and provided an increased surface area and less resistance to heat and mass transfer, which resulted in high heat and mass transfer coefficients Ževen though the cell disintegration index was zero.. Physical pre-treatments such as HHP and HELP treatments were equally effective regarding drying rates as compared to blanching, without the disadvantages of blanching and other processes used as a reference. This showed that non-thermal permeabilisation of paprika cells was beneficial as a pre-treatment to increase the drying rates as well as heat and mass transfer coefficients. Comparison of drying times Žas calculated as per Eqs. Ž6. ᎐ Ž8.. during the constant rate and falling rate periods as well as total drying time ŽFig. 3. for the different pre-treatments showed that the drying time during the constant rate period was not significantly different for the different pre-treatments, except for HELP and hot water blanching pre-treatments with slightly lower times, whereas the drying time during the falling rate period was significantly different. The drying time during the falling rate period was shortest for HELP pre-treated paprika samples. For HHP pre-

B.I.O. Ade-Omowaye et al. r Inno¨ ati¨ e Food Science & Emerging Technologies 2 (2001) 1᎐7

treated paprika samples, drying times were comparable with blanching treatment. The NaOH treatment at 35⬚C could further reduce the drying time as compared to the blanching pre-treatment. However, the shortcoming of this treatment was the reduction in the colour intensity of the peeled sample, which eventually impaired the quality of dehydrated paprika products.

4. Conclusions The application of high hydrostatic pressure or high intensity electrical field pulse pre-treatments could induce cell permeabilisation and resulted in increased drying rates as well as increased heat and mass transfer during constant rate drying. These pre-treatments could be alternatives to chemical pre-treatments and contribute to minimising environmental pollution from chemicals. The total drying time following these pretreatments was also reduced. Blanching of paprika was also found to be effective as compared to these physical pre-treatments, but leaching and destruction of nutrients and subsequent nutritional as well as environmental effects are considered to be major drawbacks of this conventional pre-treatment.

5. Nomenclature A h Ha H k m Ma Rdc t tdc tf T Ts X Xc Xt X1 X2 ␭w

surface area of sample Žm2 . heat transfer coefficient ŽWrm2 K. humidity of drying air Žkgrkg air. saturation humidity of drying air at T s Ts mass transfer coefficient Žkgrm2 s. mass of dry solid Žkg. molar mass of air, 29 kgrkmol. drying rate during constant rate period Žkgrm2 s. time Žs. drying time for constant drying period drying time for falling rate period temperature of drying air ŽK. product’s surface temperature ŽK. average moisture content Žkgrkg ds. critical moisture content Žkgrkg ds. average moisture content at any time Žkgrkg ds. initial moisture content final moisture content latent heat of evaporation at T s Ts ŽJrkg.

Subscripts a ds s w

air dry solid surface water

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Acknowledgements The authors B.I.O. Ade-Omowaye and N.K. Rastogi gratefully acknowledge the support of German Academic Exchange Service ŽDAAD. for providing research fellowships. References Angersbach, A., Heinz, V., & Knorr, D. Ž1997.. Elektrische Leitfahigkeit als Maß des Zellaufschlußgrades von zellularen Ma¨ ¨ terialien durch Verarbeitungsprozesse. Lebensmittel und Verpackungstechnik ŽLVT. 42, 195᎐200. Angersbach, A., & Knorr, D. Ž1997.. High intensity electric field pulses as pre-treatment for affecting dehydration characteristics and rehydration properties of potato cubes. Nahrung 41, 194᎐200. Angersbach, A., Heinz, V., & Knorr, D. Ž1999.. Electrophysical model of intact and processed plant tissues: cell disintegration criteria. Biotechnology Progress 15, 753᎐762. AOAC Ž1990.. Official method of analysis Ž15th ed.. Arlington, VA: Association of Official Analytical Chemists. Brodelius, P. E., Funk, C., & Shillito, R. D. Ž1988.. Permeabilisation of cultured plant cells by electroporation for release of intracellularly stored secondary products. Plant Cell Report 7, 186᎐188. Floros, J. D., & Chinnan, M. S. Ž1989.. Determining the diffusivity of sodium hydroxide through tomato and capsicum skin. Journal of Food Engineering 9, 12᎐132. Geankoplis, C. J. Ž1983.. Transport Processes and Unit Operations Ž2nd ed... Boston: Allyn and Bacon, pp. 524᎐540. Geulen, M., Teichgraber, P., & Knorr, D. Ž1994.. High electric field ¨ pulses for cell permeabilisation ŽZFL.. Z. Lebensmittelwirtschaft. 45, 24᎐27. Gould, W. A. Ž1983.. Tomato Production, Processing and Quality E¨ aluation Ž2nd ed.. Wesport, Connecticut: AVI Publishing Company, INC. Knorr, D. Ž1994.. Plant cell and tissue cultures as model systems monitoring the impact of unit operations on plant foods. Trends in Food Science and Technology 5, 328᎐331. Knorr, D., & Angersbach, A. Ž1998.. Impact of high electric field pulses on plant membrane permeabilisation. Trends in Food Science and Technology 9, 185᎐191. Masi, P., & Riva, M. Ž1988.. Modelling grape drying kinetics. In: S. Bruin, Reconcentration and Drying of Food Material. Amsterdam: Elsevier Pub. Okos, M. R., Narsimhan, G., Singh, R. K., & Weitnaver, A. C. Ž1992.. Food dehydration. In: D. R. Heldman, D. B. Lund, Handbook of Food Engineering. New York: Marcel Dekker. Ponting, J. D., & McBean, D. M. Ž1970.. Temperature and dipping treatment effects on drying rates and drying times of grapes, prunes and other waxy fruits. Food Technology 24, 84᎐88. Radley, F. Ž1964.. The prevention of browning during drying by the cold dipping treatment of sultana grapes. Journal of Food Agriculture 15, 864᎐867. Rastogi, N. K., & Niranjan, K. Ž1998.. Enhanced mass transfer during osmotic dehydration of high pressure treated pineapple. J of Food Science 63Ž3., 508᎐511. Saravacos, G. D., Marousis, S. N., & Raouzeos, G. S. Ž1988.. Effect of ethyl oleate on the rate of air drying of foods. Journal of Food Engineering 7, 263᎐270. Shi, J. X., Maguer, M. L., Wang, S. L., & Liptay, A. Ž1997.. Application of osmotic treatment in tomato processing ᎏ effect of skin treatments on mass transfer in osmotic dehydration of tomatoes. Food Research International 30 Ž9., 669᎐674. Scientific Program Enterprises Ž1993.. Computer software PlotIT Graphics and statistics version 2.0 ŽICS GmbH, Frankfurt..