Effect of various drying methods on texture and color of tomato halves (Gholam Reza Askari, Zahra Emam-Djomeh).pdf

EFFECT OF VARIOUS DRYING METHODS ON TEXTURE AND

COLOR OF TOMATO HALVES

GHOLAM REZA ASKARI, ZAHRA EMAM-DJOMEH 1 and

MARYAM TAHMASBI

Transfer Phenomena Laboratory, Department of Food Science, Technology and

Engineering, Faculty of Biosystems Engineering, Agricultural Campus

University of Tehran

Karaj, 31587-11167, Iran

Accepted for Publication May 6, 2009

ABSTRACT

Tomatoes were pretreated with osmotic solutions (NaCl and sucrose) at

different concentrations and then dried using hot air (75C, 1.5 m/s), a vacuum

(55C, 75 kPa) or hot-air drying followed by microwave treatment (400 W,

10 s). The effects of pretreatment and drying method on the drying kinetics

were examined. A puncture test and scanning electron microscopy were used

to analyze the effects of these processes on texture and microstructure. Hunter

values ( L, a, b ) were used to measure color. Measurements showed that two

osmotic solutions, S 3 (40% sucrose, 5% NaCl) and S 4 (40% sucrose, 10%

NaCl), performed better, reducing drying times and having a positive effect on

microstructure, but an adverse effect on hardness. Apart from the type of

process, dehydration reduced ﬁrmness and collapsed the structure of tomato

halves. The subsequent microwave treatment then caused further damage,

especially on the surface of the dried samples, but enhanced their color when

combined with appropriate osmotic treatment.

PRACTICAL APPLICATIONS

This study shows that the color and structural changes of tomato during

drying can be reduced using appropriate procedure. This may ﬁnd application

in the production of dried tomato with better appearance and lower drying cost.

KEYWORDS

Color, drying kinetics, hot-air drying, microstructure, microwave drying,

osmotic pretreatment, texture, vacuum drying

1 Corresponding author. TEL: +98-21-8879-6165; FAX: +98-21-8879-6165; EMAIL: emamj@ut.ac.ir

Journal of Texture Studies 40 (2009) 371–389.

371

372

G.R. ASKARI, Z. EMAM-DJOMEH and M. TAHMASBI

INTRODUCTION

Dehydration is an important process in the chemical and food processing

industries. The basic objective of drying food products is the removal of water

from a solid to the point where microbial spoilage and deteriorating chemical

reactions are greatly minimized. Tomato, being a popular fruit, ﬁnds numerous

uses in both fresh and processed forms. Processed products include ketchup,

sauces, pastas and juice. However, drying is not a popular way to process

tomatoes because of its adverse effect on the quality of the ﬁnal product. The

fruit tissue darkens upon drying (Gupta and Nath 1984) and a strong distinc-

tive ﬂavor develops. Nevertheless, interest in the production of dried tomatoes

is increasing as a result of their potential use in pizza toppings, snacks and

other savory dishes.

Anumber of methods are used to dry fruits and vegetables. Hot-air drying

is the most common method. However, this method can cause an unpleasant

taste and color and reduce the nutritional content of the product (Silveira et al.

1996; Goula et al. 2006; Toor and Savage 2006). It can also bring about a

decline in porosity and water absorbance capacity and a shifting of the solutes

from the internal part of the drying material to the surface over the long drying

period at high temperatures (Feng and Tang 1998; Drouzas et al. 1999;

Maskan 2001). Also, low thermal conductivity of food materials in the falling

rate period limits heat transfer to the inner part of food during conventional

heating (Feng and Tang 1998).

The elimination of these problems, preventing signiﬁcant quality loss and

achieving fast and effective thermal processing, has resulted in the increasing

use of microwaves for drying food. Microwave drying is rapid, more uniform

and energy efﬁcient compared with conventional hot-air drying (Drouzas and

Schubert 1996). In the microwave process, energy is converted into the kinetic

energy of the water molecules and then into heat when the water molecules

realign in the changing electrical ﬁeld and interact with the surrounding

molecules (Mudgett 1989; Khraisheh et al. 1997).

Predrying treatment and drying substantially affect the quality of the

products. Osmotic pretreatment preceding air drying was found to be advan-

tageous to the quality of the products (Collins et al. 1997; Shi et al. 1999;

Lewicki et al. 2002). The combination of osmotic dehydration and microwave-

convective drying has been proposed by a number of researchers for fruits and

vegetables to reduce drying time and introduce into the products solutes such

as sucrose, salt and calcium (Torreggiani 1993; Ertekin and Cakaloz 1996).

In addition, osmotic dehydration is effective at relatively low temperatures

with minimal damage to color and texture (Silveira et al. 1996; Moreno et al.

2000; Valencia Rodriguez et al. 2003; Stojanovic and Silva 2007). However,

there is little information about the effect of combined methods such as

TEXTURE AND COLOR CHANGES IN DRYING TOMATOES

373

osmotic hot-air and osmotic-convective microwave drying on texture, color

and, especially, on the microstructure of dried tomatoes.

The objective of this investigation was to compare the drying kinetics,

texture, color and microstructure of tomato halves using a combination of

drying techniques. The effect of the type of osmotic solution used (brine or

sugar; single or binary) was also evaluated.

MATERIALS AND METHODS

Sample Preparation

Fresh tomatoes ( Lycopersicon esculentum var. Roma) obtained from a

local market in Tehran, Iran and were sorted visually for color (bright red),

ﬁrmness, size (diameter 4–5 cm) and lack of blemishes. In comparison with

other varieties, Roma has a ﬁrm and pulpy tissue with lower moisture content

and is therefore suitable for drying. The fresh tomatoes were placed at an

ambient temperature (20C) for 24 h before the experiments. Prior to drying,

the tomatoes were cut into halves and placed in small hermetic containers.

Three replications were run for each experiment.

Osmotic Pretreatment

The halves were osmotically dehydrated in NaCl and NaCl–sucrose solu-

tions (Table 1) at a regulated temperature (30 2C) and agitation of 150 rpm.

The halves (30 g) were placed in 600-mL beakers containing the osmotic

solution and maintained inside a temperature-agitation controlled bath. The

weight ratio of the fruit medium to osmotic medium was less than 1:10 to

avoid signiﬁcant dilution of the medium and a subsequent decrease of the

driving force during the process. The samples were removed from the solution

at 15, 30, 60, 120, 180 and 240 min of immersion, drained and the excess

TABLE 1.

TYPE OF OSMOTIC SOLUTION

Solution

% NaCl

% Sucrose

374

G.R. ASKARI, Z. EMAM-DJOMEH and M. TAHMASBI

solution on their surfaces was removed with absorbent paper. Pretreated

samples were completely dried using one of the following drying methods.

Hot-Air Drying

Tomato halves were dried in a pilot plant hot-air drier (tray dryer, Arm-

ﬁeld, Hampshire, England). The drying was operated at an air velocity of

1.5 m/s parallel to the drying surface of the slices at 75C dry bulb temperature.

The operation mode was controlled using a computer connected to the dryer.

To obtain the drying curves, moisture loss was recorded with a digital balance

(Cobos, Homburg, Germany) at 5-min intervals beginning 30 min after the

start of drying until 30 min before end of drying, after which point it was

measured every 10 min. Hot-air drying was conducted until a moisture content

of 0.2 kg/kg dry matter was reached.

Vacuum Drying

Vacuum conditions were maintained using a vacuum pump and moni-

tored with a manometer. Two steel plates heated by electric resistance provided

the thermal energy. An automatic regulator controlled the temperature of the

plates. The experimental procedure consisted of putting food samples on the

hot plate, closing the door of the chamber and putting the chamber under a

vacuum. Tomato samples were withdrawn from the dryer at set intervals and

their weights determined using an analytical balance with accuracy to 0.001 g.

The temperature of the plate was set at 55C and the pressure of the chamber at

75 kPa.

Microwave-Assisted Hot-Air Drying

Hot-air drying was conducted as previously described until a moisture

content of 0.3 kg/kg dry matter was reached. Initial observation revealed that

using a higher moisture content produced a lower quality product; thus,

samples with a low moisture content of 0.3 kg/kg were used. After this point,

to obtain uniform moisture distribution in the samples, they were placed in a

hermetically sealed container for 30 min. Next, the samples were transferred to

a programmable domestic microwave oven (Butane MR-1, Butane, Tehran,

Iran, maximum output of 1,000 W at 2,450 MHz.) for the microwave treat-

ment. It was observed that charring and sample boiling occurred at 800 and

600 W, respectively. Thus, only the 400 W power level was chosen for 5-, 10-

and 15-s treatment times. Samples were placed at the centre of the turntable in

the microwave (400 W, 10 s). The use of the turntable was necessary to achieve

uniform heating of the samples and to reduce the level of microwave power on

TEXTURE AND COLOR CHANGES IN DRYING TOMATOES

375

the magnetron (Khraisheh et al. 1997). After the microwave treatment, none of

the samples had the same moisture content. The maximum recorded moisture

content was 0.18 kg/kg.

The microwave power available to the load was measured using an IEC

Standard Method 60705 (IEC, 2004) with some modiﬁcations. Cold ethanol

was used instead of water to verify the microwave heating efﬁciency. This

liquid was chosen because of its low dielectric constant (e

″

) and high loss

) that represent good absorption and low reﬂection of microwave

energy. The measured efﬁciency of the cavity was approximately 70% (Pereira

et al. 2007).

Microstructure

Scanning electron microscopy (SEM) was used to analyze microstruc-

tural changes during drying. To obtain the SEM images, small pieces were

taken from both the inner parts and surface of the tomato slices. The samples

were coated with a very thin layer of gold under high vacuum and analyzed

using a scanning electron microscope (XL-30, Philips, Amsterdam, the

Netherlands).

Mechanical Properties

Firmness was evaluated by measuring the stress at maximum force using

a texture analyzer (H5KS-Hounsﬁeld, Redhill, England). Samples were kept at

20C until analysis to minimize the inﬂuence of temperature on the textural

results. Stress at maximum force is related to the hardness and ﬁrmness of the

samples. Measurements were performed at a constant speed of 1 mm/s using

a cylindrical puncture ﬂat-head probe (d = 1.6 mm).The samples were cut into

halves using a sharp knife and their texture ﬁrmness was analyzed by punching

the newly cut surface of each half. Stress (s), in MPa, was then calculated

using Eq. (1):

σ =×

10 6

(1)

where F is the maximum force in Newtons read by the texture analyzer and A

is the area of the puncture probe in mm 2 . Changes in textural hardness were

reported as the ratio between the maximum force obtained for treated samples

to that observed for fresh ones (Heredia et al. 2007).

Color

Color evaluation of the tomato samples was performed using a Hunter-

Lab ColorFlex, A60-1010-615 model colorimeter (Hunter-Lab, Reston, VA)

factor (e

′

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