Accelerated drying of welsh onion by far infrared radiation under vacuum (S. Mongpraneet, T. Abe , T. Tsurusaki).pdf

Journal of Food Engineering 55 (2002) 147–156

www.elsevier.com/locate/jfoodeng

Accelerated drying of welsh onion by far infrared radiation

under vacuum conditions

S. Mongpraneet, T. Abe * , T. Tsurusaki

Laboratory of Agricultural Process Engineering, Department of Biomechanical Systems, The United Graduate School of Agricultural Sciences,

Ehime University, Tarumi 3-5-7, Matsuyama 790-8566, Japan

Received 12 January 2001; accepted 28 January 2002

Abstract

Far infrared radiation has excellent radiation characteristics and high energy conversion rates can be achieved using ceramic

coated radiators. Using such a radiator, the dehydrating synergy, generated by far infrared radiation heating under vacuum con-

dition, the drying of welsh onion was studied. The radiation intensity levels inﬂuenced dramatically the drying rate and the product

qualities: the a for rehydrated onions, and the L and DE ab for dehydrated onions. A rising-rate period, a constant-rate period, and

a falling-rate period were ascribed to the drying behaviour. The radiation also had signiﬁcant eﬀects on chlorophyll content. The

long time in drying and the high temperature may have contributed to a decrease in rehydration properties.

Keywords: Welsh onion; Far infrared drying; Vacuum drying

1. Introduction

and enhance product quality have achieved considerable

attention.

The hot air drying method has been widely adopted

in manufacturing of conventional dried food. Freeze–

drying was developed later, when a higher quality prod-

uct was required. Nevertheless, there are still many

losses of thermal energy in hot air drying. However,

there is smaller energy loss in methods such as infrared

drying, since, unlike hot air drying, the electromagnetic

wave energy is absorbed directly by the dried food.

Infrared radiation has signiﬁcant advantages over

conventional drying. Among these advantages are higher

drying rates giving signiﬁcant energy savings, and uni-

form temperature distribution giving a better quality

product. Therefore, it can be used as an energy saving

drying method. At present, it has been developed in

various driers using infrared radiators. Using the char-

acteristics of these radiators, technology development

on the utilization of far infrared radiation is an energetic

advance that can give an increase in drying eBciency,

space saving, clean working environment, etc. (Ratti &

Mujumdar, 1995; Yamazaki, Hashimoto, Honda, &

Shimizu, 1992).

Earlier attempts to apply infrared to drying of agri-

cultural materials have been reported in the literature

Ginzburg (1969) and Yagi and Kunii (1951). Combined

Welsh onion (Allium ﬁstulosum LINN.) is a perish-

able vegetable that easily deteriorates at room temper-

ature and even if stored in a cold room. This is an

important vegetable, and local varieties exist in each

part of Japan. The aggregate planted area of welsh on-

ions in Japan from 1980 to 1997 averaged 24,317 hect-

ares or 546,283 tones production and was 3.76% of the

total planted area of vegetables or 5.0% of total vege-

table production (Kishida, 1998). This vegetable sees

widespread use in both the fresh and the dried forms.

The leaf part is rich in chlorophyll and dried welsh on-

ions in cut form are used widely in ready-to-eat Japanese

foods such as noodles, curry sauce, canned foods, etc.

The technique of dehydration is probably the oldest

method of food preservation practiced by mankind. The

use of artiﬁcial drying to preserve agricultural products

has expanded widely, creating a need for more rapid

drying techniques and methods that reduce the large

amount of energy required in drying processes. New

and/or innovative techniques that increase drying rates

* Corresponding author. Tel./fax: +81-89-9469827.

E-mail address: tabe@agr.ehime-u.ac.jp (T. Abe).

PII: S0260-8774(02)00058-4

148

S. Mongpraneet et al. / Journal of Food Engineering 55 (2002) 147–156

Nomenclature

c speed of the light

E total emissive power

p Planck constant

T temperature

d distance

Subscripts

heater

monochromatic

sample

Greek

emissivity

Boltzmann constant

black body

wavelength

emitter

Stefan–Boltzmann constant

infrared radiation and convection or vacuum drying has

also been reported as promising (Abe & Afzal, 1997;

Dontigny, Angers, & Supino, 1992; Hasatani, Harai,

Itaya, & Onoda, 1983; Hasatani, Itaya, & Miura, 1986).

In intermittent infrared and continuous convection

heating of a thick porous material the drying time was

two to two and a half times less compared to convection

alone while keeping good surface quality and high energy

eBciency (Dostie, Seguin, Maure, Ton-That, & Chat-

ingy, 1989). Far infrared drying of potato achieved high

drying rates with infrared heaters of high emissive power

(Masamura et al., 1988). The drying rate was also re-

ported to be increased when the electric power supplied

to the far infrared heater was increased and consequently

the temperature of the sample was also observed to be

high. Far infrared and near infrared drying using three

types of granular bed and their quantitative comparison

to hot air drying from the viewpoint of the heat transfer

has been reported by Hashimoto, Hirota, Honda, Shi-

mizu, and Watanabe (1991).

Infrared has its place in drying technology, but it is

not a panacea for all drying processes. It appeals be-

cause it penetrates and produces heating inside the ma-

terial being dried, but its penetrating powers are limited.

However, research which quantitatively analyzed heat-

ing and drying by infrared radiation are limited in

number in the literature.

This paper describes laboratory-scale experimental

results on the drying of the leaf parts of welsh onion by

far infrared radiation under vacuum condition. Because

inﬁltration distance of the far infrared radiation to

agricultural products in a drying chamber is short,

welsh onion was selected as a sample of this study.

Using conventional natural seasoning and hot-air dry-

ing method discoloration and loss of hue can occur with

only partial recovery when reconstituted in boiling

water. Signiﬁcant cost increases occur if vacuum or

freeze–drying methods are use as alternatives. A similar

study has not been carried out on the vacuum operation

(Itoh & Chung, 1995).

The objective of this study is to examine the drying

behavior of the combination of far infrared drying with

the vacuum operating condition on the leaf parts of

welsh onion by comparing the physical and thermal

parameters and the drying qualities. The outcome would

provide an innovative approach to further research and

development.

2. Experimental apparatus and procedures

2.1. Experimental apparatus

Fig. 1 depicts the experimental apparatus used for the

far infrared vacuum drying of the samples. The drying

procedure involved: a wire netting tray which contained

the material to be dried, ﬁtted in the interior of a acrylic

resin vessel (infrared drying chamber), capable of op-

erating at the desired vacuum level by means of a cold

aspirator; a vacuum meter for ease of seeing the vacuum

pressure charge; and a pressure controller for ﬁne ad-

justment of the pressure level. The materials are dried

by placing them in the vacuum drying chamber, and

then simultaneously reducing the pressure by means of

the aspirator while starting the heater. Vaporization is

promoted even at low temperature by the application of

a vacuum. Simply by putting the sample in the vacuum

atmosphere, a part of water immediately evaporates, and

the water in the remainder may add increased stiﬀness,

since heat of vaporization is taken from the surrounding

material. However, vacuum alone is not enough when it

is intended to completely dry the food. The heat neces-

sary for continuation of the boiling condition must be

provided externally and this is a role of the infrared

heater.

2.1.1. Far infrared heater

A stainless steel heating coil comprises the heating

element and is eﬀectively made into a ﬂuororesin plate

heater by covering the surface in ﬂuororesin which it has

an excellent radiation eBciency for far infrared radia-

tion with a thickness as little as 1 mm, it is outstanding

in water resistance, and corrosion resistance, in addition

to having a very rapid response rate. The coated ﬂuorine

S. Mongpraneet et al. / Journal of Food Engineering 55 (2002) 147–156

149

Fig. 1. Schematic view of experimental far infrared dryer with vacuum extractor.

resin plastic board infrared heater was 23 18 cm in

area and operated at 100 V, with a maximum power of

150 W.

Every body emits radiation due to its temperature

level. It is called thermal radiation because it generates

heat in the wavelength range of 0.1–100 lm within the

spectrum. The total amount of radiation released by a

body per unit area and time is identiﬁed as its total

emissive power, E, and depends on the temperature and

the surface characteristics of the body. This energy is

emitted from a surface in all directions and at all

wavelengths. A black body is also deﬁned as one that

emits the maximum radiation per unit area and its

emissive power, E b , depends only on its temperature.

The emissivity of a body, e, is then deﬁned as the ratio of

its total emissive power to that of a black body at the

same temperature, e ¼ E = E b (Nicholas, 1943; Ratti &

Mujumdar, 1995).

The distribution of the spectral emissivity of the

heater used in this experiment is uniform for each

wavelength, and, when averaged over the surface, the

radiation characteristics are over about 0.7 (Fig. 2). An

infrared radiation body of this type may be deemed

similar to a black body or a high-eBciency radiator. The

features of such a far infrared radiator are that the

heater can radiate the far infrared radiation maximally

even at low temperature, because spectral emissivity is

high in the full wavelength range.

For a black body, this power is expressed by

(Planck’s law of radiation):

Fig. 2. Emissivity characteristics of FIR heater.

dynamic equilibrium (which requires all surfaces to be at

the same temperature), the monochromatic absorptivity

and emissivity of a body are equal. Eq. (1) has a maxi-

mum that is related to the temperature by the following

expression (Wein’s displacement law):

k max T ¼ 2897 : 6 lmK

ð 2 Þ

Eq. (1) may be integrated over all wavelengths to obtain

the total emissive power for a black body (Stefan–

Boltzmann law):

E b ¼ Z 1

ð 3 Þ

2pc 2 pk 5

exp cp = jkT

E b ; k ¼

Þ 1

ð 1 Þ

where r is the Stefan–Boltzmann constant.

As pointed out above, the total emissive power

includes the energy from all the wavelengths in the

spectrum of the radiation. On the other hand, the

So the monochromatic emissivity of a body is deﬁned as

e k ¼ E k = E b ; k . Kirchhoﬀ’s law states that under thermo-

E b ; k dk ¼ rT 4

150

S. Mongpraneet et al. / Journal of Food Engineering 55 (2002) 147–156

Fig. 3. Monochromatic emissive power of heater at various absolute

temperature.

room temperature. For each run, the sample was placed

on a tray (20 25 3 cm) made from the wire gauze

and connected to an electronic balance (Shimadzu Co.,

model BL-1200H, Tokyo, 0.01-g accuracy). The tray

was spaced to permit air circulation between the mate-

rials. The initial mass of sample was set to be about 50 g

by spreading one or two layers on the tray. The initial

moisture content of the leaf part of the welsh onion for

each experiment was determined by drying at least three

replications in a forced convection oven at 65 C for 5 h

(Resource Investigative Committee, Science & Tech-

nology Agency edition, 1982). The values varied be-

tween 91.3% and 93.5% wet basis. The drying was ended

when the moisture content of the sample reached 5% wet

basis. The drying rate was calculated as the amount of

moisture evaporating in 1 min per original kg of the

sample.

monochromatic emissive power E k is the radiant energy

contained between wavelengths k and k þ dk.

Fig. 3 describes the quantity of radiant energy emit-

ted at each individual wavelength from a heater source

at selected temperatures. As can be seen, varying the

source’s temperature cause changes in the amount of

energy radiated by the heater. Increasing the tempera-

ture of the heater brought about a shift in the peaks to

shorter wavelengths. This heater can be deemed to have

radiation characteristics suitable for this experiment,

because the experiment is carried out under vacuum and

the temperature cannot be high in such an enclosed

chamber. Although the spectral emissivity in the small

wavelength region may be small for one type of far in-

frared radiation heater, in the long wavelength region it

can be high and can be regarded as a large heater. A low

spectral emissivity in the short wavelength area can

be advantageous to the heating eBciency, because the

conversion of radiation energy distribution to the small

wavelength area suppresses the radiation of near infra-

red radiation and visible rays which have little or no

heating eﬀects. At high temperatures with heaters of this

type, there is a signiﬁcant amount of both the near in-

frared radiation and visible rays. Nevertheless, while

there is no clear guideline for the selected application of

the heater of either types, about 300 C is regarded as

the heater surface temperature forming the boundary

between the two areas, and when below this tempera-

ture, the similarity to a black body may be regarded as

an advantage.

2.1.3. Drying chamber

The drying chamber used for this experiment was

a vacuum desiccator (Iuchi Co., model VW) that was

made from a transparent acrylic resin with 40 30 40

cm internal dimensions, and 133 Pa vacuum tolerance.

All inner walls of the drying chamber were covered with

aluminum foil from which the infrared rays were re-

ﬂected (Ginzburg, 1969). An electronic balance, not di-

rectly exposed to the infrared rays was placed under the

sample in the drying chamber.

For vacuum drying, the pressure in the drying cham-

ber was lowered to approximate 1/76 of atmospheric

pressure so that the water boiled and evaporated at a

temperature as low as 10. As is normal in vacuum

drying, the temperature of dried material was rapidly

lowered as evaporation commenced. The drying then

stops, as long as there is no supply of heat of vapor-

ization, when it approaches the boiling temperature

appropriate to the degree of vacuum.

2.1.4. Aspirator

Since the drying was to be accomplished in the vac-

uum state, an aspirator was used. An aspirator is a

pump forcing liquid through a pipe that has a con-

striction in its diameter and using the ensuing pressure

drop to pull a vacuum in the chamber (through a tubing

connected to the point of constriction). It is necessary to

lower the water temperature in order to increase the

level of vacuum, because there is the relationship be-

tween a water vapor pressure and the water temperature

(e.g. water vapor pressure is 0.93 kPa at 5 C). It is also

possible to bring the water temperature to 10 C, by

mixing ethylene glycol into the water. For this experi-

ment, the mass ﬂow rate of cold water through the as-

pirator (Iwaki Glass Co., Ltd., model CLU33-ASP) was

19 l/min, and the ultimate vacuum was 0.28, 0.61, 1.22,

and 2.32 kPa when using water at 10, 0, 10, and 20 C,

respectively.

2.1.2. Sample

Welsh onions of uniform initial moisture content were

bought directly from the countryside. Only the leaf part

was used for this experiment. The samples were cut into

5 and 10 mm lengths and immediately wrapped in a

plastic ﬁlm, and stored at 5 C in a refrigerator. Before

each run, the total amount of samples required was

taken from the refrigerator and left to equilibrate to the

S. Mongpraneet et al. / Journal of Food Engineering 55 (2002) 147–156

151

In the vacuum state, the heat supplied is of course

radiant heat, because there is no air for convection.

EBcient drying can be expected, since 3 lm far infrared

radiation is radiated from the far infrared radiation

plate heater used in this experiment, and since it radiates

uniformly from the full face of the heater. Also, there is

some water in the pores of the food since diﬀusion of

water molecule from the interior of the food is rapid.

coordinates show the degree of brightness, the degree of

redness ( þ a) or greenness ( a), and the degree of yel-

lowness ( þ b) or blueness ( b), respectively. Hue angle

can be computed from the a and b parameters (Chen,

Koh, & Park, 1999). In the Lab color solid, as the dis-

) from the origin increases, the color

becomes more vivid. For each sample, this was mea-

sured directly on the product seven times in diﬀerent

locations to determine the average values. Color diﬀer-

ence values, L , a , and b , were thus quantiﬁed (Nsonzi

& Ramaswamy, 1998; Venkatachalapathy, 1997):

DL ¼ L L 0 ; Da ¼ a a 0 ; Db ¼ b b 0

a 2 þ b 2

2.1.5. Data acquisition system

All experimental measuring equipments were con-

nected to a personal computer to continuously record

the weight loss and the other data as a function of

drying time without removing samples from the drying

chamber. A computer program written in BASIC was

used for this purpose. Type K thermocouples (chromel

vs. alumel) with a thickness of 0.3 mm were ﬁxed at two

locations to record the infrared heater surface temper-

ature and one for the drying chamber temperature.

Three samples of the food material temperature were

recorded by 0.1 mm diameter of type-T thermocouples

(copper vs. constantan).

ð 4 Þ

where L , a , and b are the measured values of ground

dried onion and after soaking in distilled water for 2h,

L 0 , a 0 , and b 0 are the values of the initial onion. The

total color diﬀerence (DE ab ) was deﬁned using the Mi-

nolta equation as follows:

h þ D ð 2 þ D ð 2 i 0 : 5

ð 5 Þ

2.3.3. Rehydration ratio

The measurement of the water rehydration rate was

based on the following procedure. 200 g of distilled

water was brought to a temperature of 90 C in a con-

stant temperature water bath. Then a precisely weighed

1 g sample of the dried material was placed in a wire

gauze basket and soaked for 20 min. Afterwards, the

samples were centrifuged to remove free water and

the weight of each was taken (Itoh & Chung, 1995). The

ratio of the mass after the water rehydration to the pre-

drying mass of the sample was calculated as a recovery

ratio (Fasina, Tyler, & Pickard, 1997; Nsonzi & Ra-

maswamy, 1998; Venkatachalapathy, 1997).

2.1.6. Heat power regulator

The radiation intensity was varied by regulating the

voltage and hence output of the heater. As a result, the

power of heater was regulated at 40, 50, 60, 70, 80, 90,

and 100 2W. Each level was tested for at least ﬁve

drying runs.

2.2. Experimental procedure

The sample set was placed at the middle of the drying

chamber directly facing the far infrared heater. The

vacuum operation was achieved through the sidewall of

the chamber. Seven levels of distance between the sam-

ple and the far infrared radiation heater were used and

tested for the selected radiation intensity levels. These

were 7.5, 10, 12, 15.2, 17, 20, and 22.5 cm.

3. Results and discussion

3.1. Eﬀect on the preliminary study

2.3. Measurement and evaluation of drying quality

2.3.1. Chlorophyll content

Chlorophyll was extracted from the sample in abso-

lute (100%) acetone, and the absorbance at 644 and 662

lm of the ﬁltered acetone extract was obtained using a

spectrophotometer (Shimadzu Co., model UV-120-01,

Tokyo, 325–1000 nm sensitivity). The chlorophyll con-

tent was calculated using the equations quoted in the

literature Cupina (1969) and Holm (1954).

It has been mentioned previously that a range of

distances between the samples and the heater were used.

The electric power was 80 W and the samples were of 5

mm length. Fig. 4 shows that all spacings gave a gradual

moisture content decrease at the beginning of drying

and a dramatic decrease after approximately 50 min.

Decreasing the spacing gave a more rapid moisture

content decrease. This is because the radiation pene-

trated into the product directly. However, the dehy-

drated products at the lower spacings were scorched due

to intense radiation. On the other hand, at the 17 cm

spacing and greater, the eﬀect seems to be marginal

because of the dissipation of thermal radiation. As a

result, the 10 cm spacing was used entirely for this study.

This conclusion is consistent with Dontigny et al. (1992),

2.3.2. Color

The color and hue of both the dried onion in each

drying run and fresh onion sample were measured using

a colorimeter (Minolta Camera Co., Ltd., model CR-

100). In tristimulus color measurements, L , a , and b

tance (

DE ab ¼ D ð 2

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