Agricultural and Food Science in Finland, Vol. 11 (2002): 209–218

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Vol. 11 (2002): 209–218.

A combined infrared/heat pump drying technology
applied to a rotary dryer

Kirsti Pääkkönen
Department of Food Technology, PO Box 27, FIN-00014 University of Helsinki, Finland,

e-mail: kirsti.paakkonen@helsinki.fi

The short drying time and low product temperature makes it suitable for drying such heat-sensitive
materials as herbs and vegetables. The purpose of this work was to develop a small-scale dryer for
herbs and vegetables. A prototype rotary dryer combining infrared radiation with a so-called heat
pump drying method was applied in drying experiments for several herbs and vegetables. The drying
experiments were performed under actual crop production conditions. The drying curves for leaves
of birch (Betula spp.), rosebay willowherb (Epilobium angustifolium) and dandelion (Taraxacum
spp.) as well as slices of red beet (Beta vulgaris) and carrot (Daucus carota) are presented. During
the drying operation, temperature and humidity of the drying air were recorded, as well as the energy
consumed in drying. The quality parameters measured were water content, colour and rehydration
ratio. In the present rotary dryer design, intermittent irradiation and mixing of the product enable to
avoid overheating, which is particularly important for maintaining product quality. In this dryer de-
sign the drying drum slowly rotates and simultaneously mixes the product. The infrared heaters are
attached to a panel, allowing the product to receive infrared radiation periodically.

Key words: Betula, Beta vulgaris, Daucus carota, Epilobium angustifolium, Taraxacum, infrared dryers,
colour, drying curves, microbial flora, rehydration

role in optimizing the utilization of a given dry-
er because they can be used to define the opti-
mal conditions for drying time and energy con-
sumption (Tsamparlis 1992). Low-temperature
drying with infrared radiation has been shown
to be a potentially useful method for preserving
heat-sensitive natural products since it is gentle
and shortens the processing time significantly
(Pääkkönen et al. 1999). Studies focusing on
drying kinetics of products show that the drying

Introduction

In recent years, attention has focused on the de-
sign and operation of a small-scale dryer for
natural and biologically cultivated products. The
underlying objective is to improve the quality
of the dried products and to reduce the manpower
required to conduct the drying operation. In the
present study, drying curves play an important

mailto:kirsti.paakkonen@helsinki.fi

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Pääkkönen, K. A combined infrared/heat pump drying technology

temperature and water content of the drying air
are the main factors controlling the drying rate.
The initial stage of the convection drying of
herbs and vegetables may be predicted by heat
and mass transfer equations (Techasena et al.
1992, Belghit et al. 2000). If the shrinkage ef-
fect dependent on water diffusion and on tem-
perature are taken into account, it is a difficult
task to define the physical constants needed to
solve the equations (Pabis 1999, Yaldýz and
Ertekýn 2001). In convection drying the temper-
ature of the drying air is the main factor in-
fluencing the drying rate, if no secondary heat
sources are considered. In near-infrared drying,
radiation energy is transferred from the heating
element to the product surface without heating
the surrounding air. Drying temperature is de-
pendent on the distance of the material from the
emitter (Chu et al. 1992, Parrouffe et al. 1992,
Ratti and Crapiste 1992, Zbicinski et al. 1992,
Abe and Afzal 1997). Umesh Hebbar and Ras-
togi (2001) have studied mass transfer and Ran-
jan et al. (2002) heat and mass transfer in in-
frared drying systems. In the current design the
drying drum rotates slowly and simultaneously
mixes the product. The infrared heaters are at-
tached to a panel, allowing the product to receive
infrared radiation periodically. Low product tem-
perature makes the device suitable for drying
heat-sensitive plant material. In drying at tem-
peratures below 50°C, the drying capacity of the
air at the dryer entrance can be increased by forc-
ing it first through the heat pump dryer, where
its moisture content is reduced due to condensa-
tion of water vapour. Rossi et al. (1992) demon-
strated that drying food with a heat pump result-
ed in energy savings and better product quality
due to shorter processing times.

The purpose of this work was to develop a
small-scale dryer for herbs and vegetables com-
bining infrared radiation with heat pump drying.
The specific objectives for the drying experi-
ments were to determine the temperatures inside
the dryer and the dynamic moisture changes oc-
curring during drying as well as the effect of
drum rotation on the thickness and width of
product slices.

Material and methods

Birch (Betula spp.), dandelion (Taraxacum spp.)
and rosebay willowherb (Epilobium angustifo-
lium) were grown at Savonlinna, Finland. Car-
rot (Daucus carota) and red beet (Beta vulgaris)
were grown at Mikkeli, Finland. Birch, dandeli-
on and rosebay willowherb leaves were harvest-
ed before blooming and dried immediately after
harvesting. Slices of 2.5–5 cm of carrot and red
beet were dried within 24 h. The dryer was load-
ed with 27–43 kg of fresh product. When drying
was carried out using a drum even mixing and
temperature distribution within the cuttings, as
well as drying curves for herbs and vegetables
could be obtained.

Rotary dryer
A prototype rotary dryer (Fin patent no. 3729)
using infrared radiation combined with a heat
pump was built by Kesvent Ltd., Kesälahti, Fin-
land. The walls of the dryer are stainless-steel
sheets with a layer of rock wool insulation. The
d r y i n g c h a m b e r m e a s u r e s 1 2 6 0 × 1 2 6 0 ×
1550 mm equipped with a drum of overall length
870 mm and diameter 430 mm. The drum is di-
vided radially into eight compartments covered
with galvanized wire of ISS 200 mesh size (cor-
responding to a 2.5-mm width of the openings).
The near-infrared panel heater was composed of
two lamps, each 300 mm in length. The nominal
power of each lamp was 3 kW. The infrared heat-
ers are attached to a panel at a distance of 300
mm from the drying drum. The rotation of the
drum timed the infrared radiation period and
duration: the speed of rotation was 0.3 rpm, and
the direction of rotation changed at intervals of
180 s so that the product in each compartment is
heated during about 22.5 s every 3 min. In this
design the drying drum slowly rotates and si-
multaneously mixes the product. The periods of
irradiation alternated with periods of exposure
to ambient air temperature. During the cooling
periods the temperature gradient within the ma-

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terial reversed, and the displacement of mois-
ture towards the evaporation zone intensified. A
heat pump with an open-type, reciprocal com-
pressor linked to a 5-HP electric motor served
as a dehumidifier-heater for the ambient air used
in drying of products. The refrigerant (R22), used
with an air condenser employing dry expansion
evaporation (20 m2), was suitable for condens-
ing and draining moisture from the inlet air. The
air flowed into the drying chamber at a rate of
13 ms–1, versus a calculated average rate of
1.2 ms–1 in the drying drum. The outlet air tem-
perature was measured and controlled using a
manual variable transformer. The construction
of the dryer is shown in Fig.1.

Drying experiments
The drying curve was established by taking sam-
ples at 1 h intervals during the drying process.
The moisture contents of the samples were de-
termined by heating in an oven at 105°C for at
least 12 h and by weighing the samples to con-
stant weight with an electric balance (± 0.001 g)
(Precisa 180A).

Rehydration
The product (2.5 g) was immersed in (50 ml)
distilled water at 23°C for 15 min.; excess water
was drained off using filter paper. The ratio of
water absorbed by unit weight of the dried sam-
ples was determined. The rehydration measure-
ments were repeated 3 times and the mean ra-
tios reported. The rehydration capacity of the
dried slices was also determined by measuring
their width and thickness (24 samples) before
and after rehydration with an electronic digital
calliper (Wurth AG, Germany).

Colour measurements
Sample colour was measured using a Minolta
Crome CR-200 colour meter (Minolta Camera

Co. Ltd, Japan). Three measurements were tak-
en at random locations of sliced fresh and dried
samples, and the mean was reported. The colour
readings were expressed by the ICI chromatici-
ty coordinates system, in which subscript 0 re-
fers to the colour reading of the fresh sample
and L*, a* and b* indicate brightness, redness
and yellowness, respectively. The colour differ-
ence from the fresh samples ∆E*ab was used to
describe the colour change during drying. The
larger the value for ∆E*ab, the greater the col-
our change from the reference colour of the fresh
sample. Samples were analysed in triplicate for
colour content.

Microbiological analysis
The microbiological quality of the fresh and
dried material was determined for total bacterial
count according to ISO 4833/91, coliforms were
determined according to ISO 4832/91 and
moulds and yeasts according to NMKL (1995).

Fig. 1. Schematic of dryer; control thermocouples (1–8)
and measuring thermocouple (9).

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Pääkkönen, K. A combined infrared/heat pump drying technology

Monitoring
Dryer operation was monitored continuously for
the period of the experiment with a Honeywell
DPR 3000 and PC-based measuring system.
Eight thermoelements measured temperature
changes inside the chamber throughout the
drying process. The temperatures and humidi-
ties of the intake and exhaust air were measured
with sensors. Exhaust airflow was recorded with
an Alnor Thermo-Anemometer GGaA-65. Elec-
trical power was measured with an Enermet
K320NXEp meter.

Results and discussion

The drying experiments were performed as a
function of the operating parameters, namely,
temperature and humidity, drying time and en-
ergy consumption. The only continuously con-
trolled parameter was temperature of the circuit
airflow inside the drying chamber. Thermal in-
sulation from the ambient air is important in the
drying process. The temperature gradients inside
the dryer during the drying operation are shown
in Fig. 2. Temperature changes monitored inside
the drying chamber during drying indicated the
temperature variation with time as well as loca-
tion. The average temperatures obtained at the
different locations in the dryer for three levels
of dry air temperature are shown at Table 1. The
standard deviation characterizes threir fluctua-
tion with time. The first hundred minutes to ac-
quire the steadiness of the temperature are not
included when calculating the temperatures (See
in Fig. 2.). The drying temperature was depend-
ent on the infrared radiation and the tempera-
ture of the drying air. When plant materials are
dried, the maximum temperature reached by the
product is a very critical variable. The tempera-
ture of the wet air remained relatively low; how-
ever, towards the end of the drying process the
air temperature generally rose to the average lev-
el existing inside the drying chamber. With the
temperature control system (See in Fig. 1.) ad-
justed for a given temperature, the cooling ef-

Table 1. Temperature deviations inside drying chamber during drying at different temperatures.

Temperatures inside drying chamber (°C)

Mean STD Mean STD Mean STD

Wet air 27.3 2.4 35.5 1.6 45.3 1.2
Dry air 32.0 3.9 36.4 2.2 48.2 4.9

Botton right 30.2 3.6 34.0 3.4 46.5 4.8
Botton left 30.4 3.7 38.4 1.8 48.1 2.8

Top right 32.0 3.5 37.4 2.3 50.0 3.4
Top left 30.0 3.8 38.8 1.6 46.8 5.0

STD = Standard deviation

Fig. 2. Temperature gradients inside the dryer during dry-
ing at 50°C; Diagrams (upper) near infrared lamps, (mid-
dle) in drying chamber, (lower) after condensing.

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fect of convection allows higher radiative con-
ditions and shorter drying time. In experiments
the temperature inside the dryer was maintained
at 40–50°C. The drying experiments were car-
ried out at a constant air flowrate. The relative
humidity of the drying air was 16.7– 14.1% rH
in the first drying period and 11.4–10.2% rH at
the end of drying. The relative humidity of the
inlet air and the evaporation temperature were
considered to be the major parameters limiting
the applicability of this system. Table 2 shows
the results of the drying experiments: drying tem-
perature and time, moisture content of the prod-
uct and energy consumption.

In drying at 40°C about 12 h are required to
reduce the moisture content to a level of 12%.
To compare drying methods, the energy require-
ments per unit weight of water removed during
drying is a relevant criteria. It was the lowest
(1.5 kWh kg–1 H2O) in drying of vegetable slic-
es having an initial water contents of about 90%.
For the herbs (2.9–4.5 kWh kg–1 H2O) it also
depended on the loading of the drying drum.
Energy consumption was maximum with the
minimum loading. In the earlier static bed infra-
red drying experiments with air convection
(Pääkkönen et al. 1999), the mean energy con-
sumption (4.6 kWh kg –1 H 2O), was higher
than in the present drum drying experiments
(3.3 kWh kg–1 H2O) confirming that drum dry-
ing with a heat pump results in energy savings.
The drying time can be shortened and energy
consumption reduced by drying at higher tem-

peratures (Fig. 3, Table 2). However, the low
temperature contributes to retain useful proper-
ties. Although the drying study of Keinänen and
Julkunen-Tiitto (1996) at 40°C and 80°C showed
that flavonoids and glycosides of birch leaf are
relatively thermostable, it must also be consid-
ered that thermal denaturation of enzymes usu-
ally begins at temperatures of about 40–50°C.
The drying temperature will be selected accord-
ing to the expected properties of the end prod-
ucts.

Water evaporation rate in the capillary con-
densation phase was very high (Figs. 4 and 5).
Water evaporation is dependent on the structure
and surface properties of the material and water
sorption, eg. the bound water phase, is depend-
ent on the molecular structure and chemical com-
position of the material (Pääkkönen 1987). It is

Table 2. Sample weight, drying temperature and time, moisture of fresh and dried plants and energy consumption in infra-
red drying experiments.

Drum initial Drying Drying Moisture of Moisture of Energy
Plant load, temperature, time, fresh plant, dried plant, consumption,

kg °C h % w/w % w/w kWh kg–1 H2O

Birch leaf 40 40 12 69 12 3.6
43 50 9 69 10 3.1

Dandelion 27 40 12 80 12 4.5
Rosebay willowherb 38 40 10 81 12 2.9
Red beet 40 40 12 87 12 1.5
Carrot 30 40 12 91 18 1.6

Fig. 3. Drying curve of birch leaves at 40 and 50°C; I and II
are two different experiments.

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Pääkkönen, K. A combined infrared/heat pump drying technology

shown in irregular shape of the drying curves.
Fig. 4 shows the drying rate curves of herb leaves
at 40°C. The average drying rate for rosebay wil-
lowherb was 0.474 kg H2O kg

–1 DM (dry mat-
ter) during the first 7 hours and is similar to that
for dandelion (0.411 kg H2O kg

–1 DM). Figure 5
shows the drying rate curves for the slices of red
beet and carrot. The average drying rate during
the first 7 hours for the carrot slices was 1.206
and for beet root 0.783 kg H2O kg

–1 DM, which
were remarkably higher than for the herb leaves
(Fig. 4). This results from the differing bound
water content in the cell structure of fresh plants
and the water evaporation in the unshielded sur-
face of the cuttings.

The fresh plant is an essential factor in de-
termining the quality of the dried herbs. Although
closer identification of the aerobic plate count
(APC) group in herbs was not attempted, aero-
bic sporeformers and moulds were detected in
almost all herb samples (Malmsten et al. 1991).
The drying experiment of Pääkkönen et al.
(1999) indicated that infrared radiation in low-
temperature drying did not affect the microbial
quality of herbs, but that drying temperature
clearly affected microbial quality. Microbial
analysis of the herbs during the drying opera-
tion (Table 3) shows that at the first half of dry-

Fig. 4. Drying kinetics of rosebay willowherb and dandeli-
on at 40°C.

Fig. 5. Drying kinetics of carrot and red beet at 40°C.

Table 3. Number of microflora per gram in fresh herbs and during infrared drying. Two samples were taken from each plant.

Plant Drying time, h APC Coliform Moulds Yeasts

Birch leaf
fresh 0 < 1 000 < 10 400 29 000
dried (40°C) 10 4 000 0.1 1 100 900

11 2 000 0.1 500 1 400
dried (50°C) 2 < 1 000 < 10 200 1 400

4 4 000 < 10 < 100 53 000
9 < 1 000 < 10 200 < 100

Rosebay willowherb
fresh 0 104 000 < 0 < 100 18 000
dried (40°C) 2 480 000 < 0 < 100 28 800

5 168 000 < 0 < 100 35 600
10 310 000 < 0 < 100 23 000

APC = Aerobic plate count

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ing, the temperature and humidity conditions in
the drying chamber might be favourable for mi-
crobes. The APC and yeast counts were slightly
lower for fresh herbs than for samples dried at
40°C; only yeasts were destroyed in drying at
50°C.

In drying experiments colour and rehydration
capacity of the different dried samples were com-
pared as individual quality attributes for the
fresh/dried products. Colour plays an important
role in consumer acceptance of a product because
the primary impression of food is a visual one.
Beet pigments are natural food colourants; loss-
es of the main red beet pigments (betanine and
vulgaxanthin) during dehydration occur simul-
taneously with respect to both temperature and
moisture content (Saguy et al. 1980, Krokida et
al. 2001). The results of colour analysis of dried
birch and rosebay willowherb leaves and dried
beet root and carrot slices are presented in Ta-
ble 4. The colour difference values ∆E* for the
birch leaves dried at different temperatures (40
and 50°C) were similar, suggesting that the tem-
perature difference did not affect their colour.
The colour of the rehydrated sample is also a
critical quality factor for the dried product. In
the case of red beet and carrot slices the ∆E*
value for the rehydrated sample was lower than

that for dried. The colour difference value be-
tween fresh and rehydrated carrot slices was very
much lower (7.90) than that between fresh and
dried slices (16.37) suggesting that the colour
of the processed food is mainly dependent on
the water content of the products. However, in
processing, permanent degradation of colour is
caused by oxidation of pigments and interstitial
melting.

Ideally, rehydration of a food product uptakes
the amount of moisture lost during dehydration.
The capacity of dried foods to absorb water var-
ies with different drying treatments (Schalde et
al. 1983). For comparison a sample of red beet
slices was dried in a static bed infrared dryer
(Pääkkönen et al. 1999) for use in rehydration
analysis. The rehydration capacity for the red
beet slices dried in the drum dryer and in static
dryer and for carrot slices in the drum dryer was
96%, 93% and 99%, respectively. In static bed
drying the shape of the slices remained un-
changed, while the drum-dried slices were crook-
ed, which was caused by the rotation effect of
the drying drum. The results of the rehydration
experiment of the differently dried red beet slic-
es, which was performed to examine the quality
of the dried product, are shown in Fig. 6 togeth-
er with the results of a t-test comparison of the

Table 4. Colour analysis of fresh, dried and rehydrated samples.

Lightness Redness Yellowness Colour difference
L* a* b* ∆E*

Birch
fresh 33.84 –10.20 13.51
dried (40°C) 40.50 –8.83 12.43 6.93
dried (50°C) 39.48 –8.20 17.12 6.96

Rosebay willowherb
fresh 33.88 –13.03 14.18
dried (40°C) 33.22 –7.94 13.12 5.37

Beet root
fresh 33.63 11.17 –0.31
dried (40°C) 38.31 1.77 –1.16 8.83
rehydrated 34.03 3.07 –0.44 6.32

Carrot
fresh 60.78 32.50 34.80
dried 54.59 27.70 20.42 16.37
rehydrated 54.29 32.33 30.29 7.90

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Pääkkönen, K. A combined infrared/heat pump drying technology

mean scores of the drum-dried carrot and sam-
ples of the dried beet root. Comparison of the
data of drum- and static-dried samples in the case
of red beet revealed significant differences (P <
0.1) between thickness of the dried samples and
between width of the rehydrated samples. In
drying the maximum temperature relating to
the plants is a very critical variable. In static dry-
ing infrared radial heating caused melting by
rapid reduction of surface moisture that could
be seen as a perseptible reduction in rehydra-
tion capacity of the static-dried compared with
the drum-dried sample. In the present drum dryer
design, intermittent irradiation and mixing of the
product enable to avoid overheating, which is
particularly important for maintaining product
quality.

Conclusions

The drum drying technique developed here ef-
fectively combined infrared drying and heat
pump drying operations in one device. In this
dryer design the drum slowly rotates and simul-
taneously mixes the product, allowing the prod-
uct to receive infrared radiation periodically,
leading to a very short drying time. Its capabili-
ty for drying small particulate foods was inves-
tigated. Consequently the short drying time and
low drying temperature are the factors which
make this procedure suitable for drying such
heat-sensitive materials as herbs and vegetables.

Sponsoreship. National Techology Agency

Fig. 6. Comparison of (1) thick-
ness and (2) width of static-dried
and drum-dried red beets and
drum-dried carrot and (3) thick-
ness and (4) width of rehydrated
ones. Bars indicate the standard
deviation of the mean; n = 24.
Means with columns followed by
the different letter are significant-
ly different at the 99% level using
Tukey’s test.

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Pääkkönen, K. A combined infrared/heat pump drying technology

SELOSTUS
Yrttien ja vihannesten infrapunakuivaus rumpukuivurissa

Kirsti Pääkkönen
Helsingin yliopisto

Infrapunakuivaus on yrttien ja vihannesten kuivauk-
seen soveltuva menetelmä, jonka käyttö vaatii lait-
teiston, jossa kuivattava materiaali liikkuu niin, että
kuivuminen tapahtuu tasaisesti. Kokeissa kuivattiin
koivun, horsman ja voikukan lehtiä sekä porkkana-
ja punajuurilastuja. Tutkimukseen rakennettiin ruos-
tumattomasta materiaalista putkirunkoinen ja ulko-
pinnoiltaan lämpöeristetty kuivauslaitteisto.

Kuivausrumpu on jaettu kahdeksaan segmenttiin,
joissa on luukut täyttöä ja tyhjennystä varten. Rum-
mun ulkopinta on teräsverkkoa. Laitteistossa on il-
mankiertojärjestelmä, jossa kuivaa ja valitun lämpöis-
tä ilmaa kierrätetään kuivattavan materiaalin läpi.
Ilma kuivataan ja lämmitetään lämpöpumpun avulla
ja puhalletaan uudelleen kuivattavan materiaalin läpi.
Kuivumista nopeutetaan infrapunalamppujen avulla.

Kuivausrummun pyörivä liike jaksottaa infrapu-
nalamppujen säteilyn materiaaliin. Kerrallaan kuivat-
tiin 30–40 kg. Kuivauslämpötilat olivat 40–50°C.
Kuivurin sisälämpötila pysyi kuivauslämpötilan ase-
tuksen mukaisena koko kuivauksen ajan. Kuivaus-
rummun jälkeinen kuivausilman vesipitoisuus oli
noin 12 % kuivauksen lopussa. Kuivauslämpötilan
nostaminen nopeutti kuivumista. Yrttien lehdet kui-
vuivat 12 %:n vesipitoisuuteen 12 tunnissa. Hiivojen
pitoisuus pieneni kuivattaessa 50°C:ssa. Tutkimuksen
tulokset osoittivat, että rumpukuivauslaitteisto, jos-
sa kuivaus tapahtuu yhdistetyllä infrapuna- ja lauh-
dekuivaustekniikalla sopii hyvin yrttien lehtien ja sil-
puttujen juuresten kuivaukseen. Materiaali kuivui
nopeasti ja lopputuote oli hyvälaatuista.

Title
Introduction
Material and methods
Results and discussion
Conclusions
References
SELOSTUS