Effect of infrared heating on quality and microbial decontamination in paprika powder

Available online at www.sciencedirect.com
Journal of Food Engineering 86 (2008) 17–24
www.elsevier.com/locate/jfoodeng
E?ect of infrared heating on quality and microbial decontamination
in paprika powder
N. Staack a,b, L. Ahrne´ a,*, E. Borch a, D. Knorr b
a SIK – The Swedish Institute for Food and Biotechnology, P.O. Box 5401, SE-402 29 Go¨teborg, Sweden
b University of Technology of Berlin, Ko¨nigin-Luise-Strasse 22, D-14195 Berlin, Germany
Received 16 March 2007; received in revised form 29 August 2007; accepted 3 September 2007
Available online 8 September 2007
Abstract
Infrared radiation (IR) was explored as a technique for decontaminating paprika powder. The e?ect of water activity (aw) and IR heat
?ux on paprika temperature and water loss were measured during near- or medium-IR heating. Paprika was evaluated in terms of colour,
aw, natural ?ora, and inoculated Bacillus cereus spores. Surface temperatures were considerably higher than temperatures inside the pow-
der, especially at low aw; greater di?erences were observed with medium- than with near-IR. Surface darkening was observed, though the
overall colour was not considerably a?ected. IR e?ectively removed water from paprika, especially at aw 0.5 and 0.8, resulting in unsat-
isfactory spore reduction. However, at aw 0.8, the load of the natural ?ora was reduced (P < 0.05). In aw 0.96 powder, areas with high
remaining aw displayed a reduction >6 log10 CFU/g for B. cereus (P < 0.05). In addition, no microbial counts of the natural background
?ora were observed in the paprika.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Infrared radiation; Water activity; Paprika powder; Colour; Bacillus cereus spores
1. Introduction
radiation. The wavelengths of IR range between 0.76 lm
and 1 mm, and are distinguished as near- (0.76–2 lm),
Commercial and legal requirements regarding the safety,
medium- (2–4 lm), and far-IR (4–1000 lm) and interacts
quality, and storage of food products have focused atten-
with foods due to re?ection, absorption, transmission,
tion on the development and improvement of decontami-
and scattering. The dissipation of radiative energy as heat
nation methods. Most preservation techniques have been
results in particular surface temperatures and penetration
designed for high-water-content foods. For pasteurising
depths speci?c to the products treated, depending on the
dried food products, suggested decontamination processes,
IR wavelength, food composition, aw, and product thick-
besides gamma irradiation, are high temperature short time
ness. Transmission energy can even be dissipated within a
treatments (Bueltermann, 1997; Laroche & Gervais,
powder, due to scattering and absorption, which can be
2003a), UV light irradiation (Sharma & Demirci, 2003),
important for thicker samples. The transmissivity of water
dry heat (Baron et al., 2003), steam (Schneider, 1993),
peaks at 0.8–1 lm and decreases to zero at 1.3–1.4 lm,
and microwave (Gerhardt & Romer, 1985) or IR heating
where the absorption reaches a maximum (Ginzburg,
(Hamanaka et al., 2000).
1969). IR heating has advantages over conventional heat-
IR is that part of the electromagnetic spectrum with
ing, as it heats the product directly, without being in?u-
wavelengths between those of ultraviolet and of microwave
enced by the air around the powder, in a fast and
e?ective thermal process (Ranjan, Irudayaraj, & Jun,
*
2002). Research has examined applying IR to inactivate
Corresponding author. Tel.: +46 31 335 56 42; fax: +46 31 83 37 82.
E-mail addresses: ns@sik.se (N. Staack), lilia.ahrne@sik.se (L. Ahrne´).
micro-organisms on surfaces, such as cottage cheese
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2007.09.004

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N. Staack et al. / Journal of Food Engineering 86 (2008) 17–24
(Rosenthal, Rosen, & Bernstein, 1996), corn ?our (Jun &
with air and a kanthal thread for medium-IR; their max-
Irudayaraj, 2003), and grains (Hamanaka, Uchino, Inoue,
imum emissions (corresponding temperature) were at
Kawasaki, & Hori, 2006).
1.2 lm (2100 °C) and 2.7 lm (800 °C), respectively. The
Herbs and spices comprise a large group of powdered
IR heat treatment of the sample was performed by plac-
foods. The use of spice mixes, aroma components, and
ing 30 g of paprika powder (10 mm bed thickness and
functional ingredients has increased in the food industry,
bulk density of 471 kg/m3) in the centre of the IR radia-
especially in functional foods, ready-to-eat meals, and
tor (Fig. 1).
highly spiced cuisine (Giese, 1994; Sloan, 1993). Despite
their contribution to taste, colour, and aroma in foods,
2.1. Determining heat ?ux
these powders are known to be a signi?cant source of
micro-organisms, due to high microbial contamination
The heat ?ux emitted by the near- and medium-IR radi-
caused by poor sanitary conditions during growing, har-
ators was calculated by the time required for the tempera-
vest, processing and storage (McKee, 1995).
ture to increase from 50 to 100 °C (DT = 50 °C) using a
One of the most frequently encountered spore-forming
black-painted
reference
copper
plate
(5 cm  5 cm Â
bacteria in spices is Bacillus cereus, with contamination
0.6 cm) with a thermocouple in the centre. The distance
commonly reaching 6–8 log10 CFU/g (McKee, 1995). It is
between radiators and the black body was 50 cm. The radi-
a food-poisoning bacterium causing diarrhoeal and emetic
ators produced heat ?uxes for near-IR of 11–23 kW/m2
illness, most of the outbreaks reportedly connected with
and for medium-IR of 5–11 kW/m2, corresponding to an
rice, meat, and milk dishes (Goepfert, Spira, & Kim,
output power between 25% and 100% for both wave-
1972; Murrell, 1989). Psychrotrophic strains of B. cereus
lengths. To guarantee uniform heat ?ux, the black body
can grow at storage temperatures of 5–8 °C (Meer, Baker,
was always placed in the same position in the oven (see
Bodyfelt, & Gi?ths, 1991). Thus, if powders are to be
Fig. 1).
incorporated into high-water-content food products, their
microbial quality is important. However, decontaminating
2.2. Wetting of powder
dried powders is di?cult as the micro?ora has adapted to
low aw by forming spores. The availability of water has a
Paprika powder was wetted to three di?erent aw levels:
pronounced e?ect on the heat resistance of spores, and
0.50, 0.80, and 0.96. At aw 0.50, the original paprika sample
the resistance of dried micro-organisms is many times
was used without adding water. The powder was wetted to
higher than the resistance of the same micro-organisms in
aw 0.8 by adapting the methods described by Cosßkun,
an aqueous solution (Laroche, Fine, & Gervais, 2005;
Yalcß?n, and O
¨ zarslan (2005) Muramatsu, Tagawa, and
Laroche & Gervais, 2003b).
Kasai (2005) as follows: 0.14 g water/g powder was sprayed
There is a lack of knowledge regarding the heating of
using an atomiser on thin layers of powder, which were
powdered food by IR, and the e?ects of IR heat ?ux and
then stored for 3 days at 5 °C in a refrigerator to enable
wavelength and of aw of the product on microbial inactiva-
the moisture to become uniformly distributed throughout
tion and product quality. This work addresses this lack by
the sample. Before using the wetted, partly agglomerated
assessing the suitability of IR treatment for decontaminat-
powder, it was sieved again to restore/maintain the status
ing powders. We examine the heating of paprika powder
of powder. Samples of aw 0.96 were prepared by adding
using di?erent IR heat ?uxes at near and medium wave-
lengths, and the e?ects of such heating on the temperature
pro?le, colour, and aw of the paprika powder bed and on
the numbers of natural background ?ora and spores of
Near- or medium-IR radiators with reflectors
B. cereus.
(lamps 44 cm long, distance between two lamps 10 cm)
2. Materials and methods
Paprika powder (Capsicum annuum), imported from
d = 50 cm
Israel and supplied by Nordfalks Industri AB (Mo¨lndal,
Sweden), was stored at 19 °C and 51% relative humidity.
Petri dish (Ø 9 cm) with
Teflon device (h = 1 cm)
The moisture content of the powder was 8 ± 0.5 (g water/
paprika sample (30 g)
with fixed Thermocouples
100 g dry matter) and aw was 0.50 ± 0.03.
Heating was done in a custom designed IR drying
Position of
oven (Ircon Drying Systems AB, Va
black body
¨nersborg, Sweden)
composed of two independently operating sections (each
Tray (aluminium net, 60 cm x 60 cm)
70 cm  70 cm  50 cm), one operating at near and the
Fig. 1. Schematic drawing of IR chamber (separate units for both near-
other at medium wavelength. The radiators comprised
and medium-IR wavelengths) with on a Te?on device ?xed thermocouples
of a quartz tube ?lled with halogen gas and containing
on surface and in depths of 1, 3 and 8 mm; and marked position of the
a tungsten thread for near-IR, and a quartz tube ?lled
black body.

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N. Staack et al. / Journal of Food Engineering 86 (2008) 17–24
19
water to paprika in a 1.7:1 ratio, and stirring the mixture
Pullman, WA, USA). Total water loss was measured by
until a homogeneous slurry was achieved.
weighing the whole sample before and after IR treatment.
The mean particle size was analysed for paprika at aw
2.3. Measurement of temperature pro?le
0.5 and 0.8. First, 100 g of powder was sieved, after which
the particle fractions obtained were measured using a lab-
Wetted paprika powder, placed in the IR chamber in the
oratory plansifter ?tted with six square-mesh sieves of
same position as the black body, was exposed to near- (23
ranging in mesh size from 40 to 1000 lm (the method
and 11 kW/m2) and medium-IR (11 and 5 kW/m2). The
was adapted from Chapelle, Melcion, Robert, & Bertrand,
temperature was measured every second in four di?erent
1989).
locations in the powder bed: directly under the surface and
The total and spore plate counts of B. cereus SIK 340 and
at depths of 1, 3, and 8 mm. Type T thermocouples (Pen-
total plate counts of natural bacterial background ?ora were
tronics, Gunnebo, Sweden), a logger, and a computer were
determined in triplicate for the surface and overall sample of
used to record the temperatures. The thermocouples were
paprika powder at aw 0.5, 0.8, and 0.96, and for inside the
attached to a Te?on device (see Fig. 1), in order to place
sample at aw 0.96. B. cereus concentration was estimated
them precisely in the desired locations in the powder bed.
for the inoculated paprika powder (spiking described under
2.4). The concentration of natural background ?ora was
2.4. Spiking of powder
de?ned for the uninoculated paprika powder.
The total plate counts were determined by mixing 1 g
Spores of the psychrotrophic strain of B. cereus, SIK 340,
paprika powder into 9 ml 0.1% peptone water and agitat-
were prepared, harvested, cleaned and stored as described
ing the mixture for 20 min (250 rpm); subsequently, 10
by Andersson and Ro¨nner (1998). The strain used was iso-
serial dilutions (using 0.1% peptone water) were plated
lated from a local dairy and previous tests showed a D95-
on TGE agar plates (24 g tryptone glucose extract agar/l
value of 2.5 min in paprika powder at aw 0.96 (unpublished
distilled water; Difco, BD, Franklin Lake, NJ, USA) and
data). The inocula were prepared by thawing 1 ml spores
incubated for 24 h at 30 °C.
suspension and diluting it in 99 ml 0.1% peptone water
Spore plate counts of B. cereus SIK 340 were determined
(1.0 g bacteriological peptone, 8.5 g NaCl/l Milli-Q water;
by placing 10 ml of the 1:10 diluted paprika powder (?lled
Difco, BD, Franklin Lake, NJ, USA) to achieve a concen-
in 20 ml test tube) in a water bath for 10 min at 80 °C.
tration of about 8 log10 spores/ml, after which it was stored
Temperature was measured in an uninoculated test tube
for up to 7 days at 4 °C. Screening tests indicated that the
using a digital thermometer. When the test tube reached
storage time did not induce germination. Spore solution
water bath temperature (within 3 min) the test commenced.
of about 3 ml was added to 27 g pre-wetted paprika powder
After heating the solution was quickly cooled down, plated
by spraying using an atomizer under simultaneous mixing.
and incubated, as described above.
The ?nal spore concentration in the paprika powder after
spiking was 7.48 log10 spores/g.
2.6. Statistical analyses
2.5. Evaluating product quality
The detection limit for total and spore plate counts were
of 1 log10 CFU/g. Samples containing below 1 log10 CFU/g
The e?ects of IR treatment on the product quality of
were considered having no detectable organisms. Mean
paprika powder were evaluated in terms of colour, aw, total
values and their standard deviation (SD) were calculated
water loss, and microbial reduction. Furthermore, the
for colour, aw and microbial numbers and tested on their
e?ect of wetting paprika on the powder particle size was
signi?cant di?erences (P < 0.05) using Microsoft Excel
determined. Paprika samples were taken from di?erent
2003 (Microsoft, Redmond, WA, USA).
locations in the powder bed, namely the surface (to a depth
of 2 mm), inside (from depths of 2–10 mm), and overall
3. Results and discussion
(mixture of entire sample after IR heating). The overall
sample was prepared by mixing the entire powder bed with
3.1. Temperature pro?le in paprika powder during IR
a spatula until homogeneous.
heating
Colour was the parameter selected with which to assess
product quality. Colour measurements were made using a
The e?ects of aw 0.5, 0.8, and 0.96 on the heating of
Minolta CR-10 camera (Tokyo, Japan); the L* and a* colour
paprika powder at heat ?uxes of 23 and 11 kW/m2 (near-
parameters were measured, L* being the lightness and a* the
IR) and 11 and 5 kW/m2 (medium-IR) are presented in
red–green chromaticity co-ordinate. The reference white
Fig. 2.
ceramic plate had the co-ordinates L = 82.4 and a = 2.2.
Colour measurements were made in triplicate, as were the
3.1.1. E?ect of heat ?ux and aw
measurements of aw and total water loss in the paprika pow-
As expected, the higher the heat ?ux, the faster the
der. Aw was determined for the surface and overall sample
temperature increase on the surface of the paprika pow-
using an aw-measurement chamber (Decagon Devices Inc.,
der bed. Lower aw resulted in faster temperature increases

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N. Staack et al. / Journal of Food Engineering 86 (2008) 17–24
Fig. 2. Temperature of paprika powder measured on the surface (+) and at depths of 1 (s), 3 (M), and 8 mm (h) at aw levels of 0.5, 0.8, and 0.96 during
heating at near-IR heat ?uxes of 23 and 11 kW/m2 and medium-IR heat ?uxes of 11 and 5 kW/m2.
as well as greater temperature di?erences between the sur-
3.1.2. E?ect of wavelength
face of and inside the powder bed. At lower aw, powders
Comparing the two wavelengths at the same heat ?ux
have lower transmissivity and lower surface re?ection lev-
(near-IR of 11 kW/m2 and medium-IR of 11 kW/m2), it
els, i.e., higher energy absorption on the surface (Ginz-
was observed (Fig. 2) that at aw 0.5 and 0.8 higher surface
burg, 1969). Radiation is the dominant heat transfer
temperatures were achieved with medium-IR, while the
mechanism in?uencing the temperature of powders on
interior temperatures (at depths of 3 and 8 mm) di?ered
the surface and to a depth of 1 mm, while at depths of
by only a few degrees. For example, after 3 min of heating,
3 and 8 mm, the poor penetration of IR meant that the
the surface temperature of paprika of aw 0.5 was 160 °C
heat was transferred by conduction from the surface.
with medium-IR and 140 °C with near-IR, while the tem-
The poor thermal conductivity of powders can explain
peratures at a depth of 3 mm were 100 and 98 °C, respec-
the great temperature di?erence between the surface and
tively. For near-IR at aw 0.5 and 0.8 there was a slightly
inside of the powder bed. At higher aw, the temperature
smaller temperature di?erence between depths of 3 mm
gradient between the surface and inside decreased, mainly
and 8 mm, which may be explained by the higher penetra-
due to more e?ective heat conduction. The penetration
tion depth of near-IR. Longer wavelengths penetrate the
depth of IR is expected to decrease with increased aw.
powder only a little so the energy is mostly converted to
Dry powdered products, such as ?our or salt, display a
heat on the surface, whereas shorter wavelengths penetrate
penetration depth of 2 mm compared with a penetration
somewhat further into the material (Sakai & Hanzawa,
depth of 1 mm in a slurry product such as tomato paste
1994). Increasing aw decreased the di?erences in surface
(Ginzburg, 1969).
temperature between the near- and medium-IR treatments,

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N. Staack et al. / Journal of Food Engineering 86 (2008) 17–24
21
resulting in quite similar temperature curves for both wave-
3.2. Changes of product quality during IR heating
lengths at aw 0.96. Notably, at aw 0.8, up to 80 °C the same
temperature curve was observed for near- and medium-IR;
3.2.1. E?ect on colour
but as soon as the water was removed, a higher surface
Changes of the surface and overall colour of wetted
temperature was observed for medium-IR (Fig. 3). This
paprika powder during IR heating are presented in
may be explained by the di?erent behaviours of wet and
Fig. 4. Colour was expressed as the product of the L (light-
dry particles when absorbing radiative energy.
ness) and a (redness) colour parameters (i.e., a  L), values
Fig. 3. E?ect of near-IR heat ?uxes of 23 (+) and 11 (s) kW/m2 and medium-IR heat ?uxes of 11 (M) and 5 (h) kW/m2 on the surface temperature of
paprika powder at aw 0.5, 0.8, and 0.96.
Fig. 4. Surface (M) and overall (s) colour measurements (a  L) of paprika powder (±SD) at aw 0.50, 0.80, and 0.96 for near-IR of 23 and 11 kW/m2 and
medium-IR of 11 and 5 kW/m2.

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N. Staack et al. / Journal of Food Engineering 86 (2008) 17–24
>500, 500–300, and <300 being rated as red, medium red,
was observed after 5 min, however, as the temperature then
and dark, respectively (Ramakrishnan & Francis, 1973).
exceeded 130 °C. Further heating caused a rapid decrease
Dark samples having values of 300–200 were rated as
of colour value, due to further surface temperature
brown and <200 as black or charred.
increase, in agreement with the results of Ramakrishnan
In paprika powder, colour is a?ected by carotenoid
and Francis (1973).
presence, aw, temperature, and particle size (Schmalko,
Water loss, aw, and microbiology studies were done only
Scipioni, & Ferreyra, 2005). The initial colour of paprika
for near-IR of 11 kW/m2 and medium-IR of 5 kW/m2,
(a  L) at aw 0.5, 0.8, and 0.96 was 930, 850, and 580,
since these conditions result in better overall product
respectively. The mean particle size increased from
colour.
140 lm (range, 40–500 lm) to 340 lm (range, 250–
1000 lm) at aw 0.5 and 0.8, respectively, due to particle
3.2.2. E?ect on aw and total water loss
agglomeration during wetting (Hogekamp & Schubert,
The total water loss and change of aw during IR heating
1993). During heating, the colour value decreased signi?-
were studied for medium-IR of 5 kW/m2 and near-IR of
cantly (P < 0.05) on the surface, while overall colour values
11 kW/m2 (Fig. 5). The total water loss and change in over-
remained higher, as shown in Fig. 4.
all aw were higher for a heat ?ux at near-IR of 11 kW/m2
For near-IR of 23 kW/m2 and medium-IR of 11 kW/m2,
and aw 0.5 and 0.8, due to the higher surface temperatures.
the colour value decreased at all aw to an unacceptably
IR e?ectively removed water, especially at aw 0.5 and 0.8,
dark charred level within a few minutes. This property of
where 40–95% of the total water could be evaporated
IR could be used in processes aiming to create particular
within 9 min; however, at aw 0.96 only 10–30% of the total
surface colourations; however, in our case such surface col-
water could be removed within the same heating period.
ouration is undesired, as it decreases product quality.
A moisture gradient developed between the sample sur-
For treatments at near-IR of 11 kW/m2 and medium-IR
face (dry zone formation) and centre, similar to the temper-
of 5 kW/m2, surface browning occurred, but overall colour
ature and colour gradients previously described. After
stayed within acceptable colour levels (a  L $ 500). The
4 min of heating, paprika wetted to aw 0.5 and 0.8
overall colour values for the cited heat ?uxes after 9 min
decreased signi?cantly in surface wetness to aw 0.3 and
and 12 min were similar to the surface colour values after
0.4, respectively, due to the surface heating. The overall
4 min and 5 min, respectively. During the ?rst 3 min of
aw was not signi?cantly (P > 0.05) a?ected in that time,
heating, no signi?cant surface colour changes (P > 0.05)
though further heating did decrease the overall aw as well.
were observed, as the temperature was under 100 °C. A sig-
At aw 0.96, no changes were observed in either the surface
ni?cant reduction in colour value on the surface (P < 0.05)
or overall aw (P > 0.05) after 5 min heating.
Fig. 5. E?ect of IR heating for near-IR heat ?ux of 11 kW/m2 and medium-IR heat ?ux of 5 kW/m2 on surface (h) and overall (D) aw (±SD) and on total
water loss (s) in paprika powder at aw 0.5, 0.8, and 0.96.

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N. Staack et al. / Journal of Food Engineering 86 (2008) 17–24
23
Table 1
Concentration of B. cereus SIK 340 spores and natural background ?ora (±SD) in di?erent locations in paprika powder with aw 0.5, 0.8, and 0.96 after
exposure to IR heat ?ux from medium-IR of 5 kW/m2 and near-IR of 11 kW/m2
Heat ?ux (kW/m2)
Location in powder
Microbial counts after IR heating (log10 CFU/g)
Bacillus cereus
Natural ?ora
Total plate count
Spore plate count
Total plate count
aw 0.50
5 med-IR
Surface
7.12
–
4.67
Overall
7.15 (±0.04)
7.16 (±0.08)
4.45 (±0.11)
11 near-IR
Surface
6.78
–
4.59
Overall
7.05 (±0.06)
6.95 (±0.06)
4.39 (±0.07)
aw 0.80
5 med-IR
Surface
6.87
–
4.02
Overall
7.17 (±0.09)
7.01 (±0.45)
4.14 (±0.14)
11 near-IR
Surface
6.77
–
3.65
Overall
6.34 (±0.13)
6.23 (±0.30)
3.24 (±0.15)
aw 0.96
5 med-IR
Surface
7.02 (±0.25)
6.98 (±0.19)
4.72 (±0.08)
Inside
2.21 (±0.59)
2.11 (±0.38)
2 (±0.24)
Overall
6.19 (±0.49)
6.05 (±0.27)
3.49 (±0.19)
11 near-IR
Surface
6.58 (±0.21)
6.50 (±0.42)
<1
Inside
<1
<1
<1
Overall
5.77 (±0.24)
5.71 (±0.17)
<1
The initial population was 7.48 log10 B. cereus spores/g in inoculated samples and 4.90 log10 CFU/g of the background ?ora in uninoculated samples.
3.2.3. E?ect on microbial numbers
No reduction was observed in the level of natural back-
The e?ect of IR heat treatment on microbial numbers
ground ?ora at aw 0.5 (P > 0.05). However, at aw 0.8, an
was studied for medium-IR of 5 kW/m2 and near-IR of
overall reduction of 1.6 and 0.7 log10 CFU/g was obtained
11 kW/m2. The IR treatments tested and their e?ect on
for treatments of 11 and 5 kW/m2, respectively, and at aw
microbial counts of B. cereus SIK 340 spores and natural
0.96, no microbial counts were observed in the paprika.
background ?ora are presented in Table 1. The tempera-
ture history of the product during IR heating is shown in
4. Conclusions
Fig. 2. The total IR treatment time was chosen based on
acceptable overall colour (a  L $ 500) and on process
IR could be used to reduce microbial numbers in
time in the product with a temperature over 95 °C (when
paprika powder. If so, process parameters such as IR
reached at a depth of 1 mm in the powder bed). For
power, wavelength, and aw should be carefully selected to
11 kW/m2, after 8, 9 and 15 min (corresponding aw 0.50,
signi?cantly reduce microbial numbers and avoid changes
0.80 and 0.96, respectively) the overall colour reached the
in product quality.
limitation of acceptable overall colour of 503, 501 and
Higher heat ?ux signi?cantly degraded product quality
456 (see Fig. 4). The respective process time over 95 °C
on the surface of the powder bed. This occurred due to
was 6, 6 and 8 min. For 5 kW/m2, the colour changes are
overheating, lower aw resulting in a higher temperature
not pronounced as for 11 kW/m2, and therefore the process
gradient in the powder due to poor thermal conductivity.
time was then chosen in order to achieve a similar process
At the same heat ?ux and at low aw, higher surface tem-
time over 95 °C for both treatments. Thus, for 5 kW/m2 the
peratures were observed for medium-IR, while near-IR
process time over 95 °C were of 6, 7 and 9 min, which
produced a greater penetration depth. However, at higher
required a total IR treatment of 9, 10 and 28 min (corre-
aw, there were no observed temperature di?erences between
sponding aw 0.50, 0.80 and 0.96, respectively).
the two wavelengths.
A reduction by a maximum of 1 log10 spores of B. cer-
For near-IR of 11 kW/m2 and medium-IR of 5 kW/m2,
eus/g was obtained in the surface sample and also in the
changes in colour and aw were observed on the surface of
sample representing the overall powder bed (Table 1) at
the powder bed, though overall colour and aw were not
aw 0.5 and 0.8. This low reduction is explained by the
greatly a?ected. However, aw was the most important fac-
low aw in the paprika sample, which increased the heat
tor in terms of reducing microbial numbers. No signi?cant
resistance of the spores. A similar small reduction was also
reduction in B. cereus spores were obtained at aw 0.5 and
achieved on the surface of the sample with an initial aw of
0.8. At aw 0.96, the surface was the most critical part of
0.96. During heat treatment, aw decreased during the long
the powder bed, as aw had already decreased during warm-
warming-up period. In the centre parts of aw 0.96 sample
ing-up, resulting in poor reduction of microbial numbers.
(with a high remaining aw), a signi?cant reduction of 5
Nevertheless, areas with high remaining aw displayed com-
log10 spores/g was obtained at 5 kW/m2 and of over 6
plete inactivation, at a detection limit of 1 log10 CFU/g.
log10 spores/g at 11 kW/m2.
The natural background ?ora showed to be more sensible

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N. Staack et al. / Journal of Food Engineering 86 (2008) 17–24
against IR heating: at a
Goepfert, J. M., Spira, W. M., & Kim, H. U. (1972). Bacillus cereus: Food
w 0.8, a reduction of 1–2 log10 CFU/
g was obtained for treatments of 11 and 5 kW/m2, respec-
poisoning organism. a review. Journal of Milk and Food Technology,
35(4), 213–226.
tively, whereas at aw 0.96, no microbial survivors were
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Con?ict of interest statement
Laroche, C., Fine, F., & Gervais, P. (2005). Water activity a?ects heat
resistance of microorganisms in food powders. International Journal
This publication re?ects the authors’ views and not nec-
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ination of powdery products (PATENT WO 02071853). http://ww.u-
is provided as is and no guarantee or warranty is given that
bourgogne.fr/GPAB/presentation_GB/ppal.htm.
(accessed
2.
3.
the information is ?t for any particular purpose. The user
2006).
thereof uses the information at its sole risk and liability.
Laroche, C., & Gervais, P. (2003b). Unexpected thermal destruction of
dried, glass bead-immobilized microorganisms as a function of water
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Document Outline

• Effect of infrared heating on quality and microbial decontamination in paprika powder
  • Introduction
  • Materials and methods
    • Determining heat flux
    • Wetting of powder
    • Measurement of temperature profile
    • Spiking of powder
    • Evaluating product quality
    • Statistical analyses
  • Results and discussion
    • Temperature profile in paprika powder during IR heating
      • Effect of heat flux and aw
      • Effect of wavelength
    • Changes of product quality during IR heating
      • Effect on colour
      • Effect on aw and total water loss
      • Effect on microbial numbers
  • Conclusions
  • Conflict of interest statement
  • Acknowledgements
  • References

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