Temperature Profiles in Dough Products during Microwave Heating with Susceptors

Czech J. Food Sci.
Vol. 20, No. 4: 151–160
Temperature Profiles in Dough Products during Microwave Heating
with Susceptors
JI?INA HOUŠOVÁ and KAREL HOKE
Food Research Institute Prague, Prague, Czech Republic
Abstract
HOUŠOVÁ J., HOKE K. (2002): Temperature profiles in dough products during microwave heating with susceptors. Czech
J. Food Sci., 20: 151–160.
The effect of food products on temperatures reached in the microwave heating with and without susceptors was followed in
experiments with certain types of food samples. A household microwave oven (650 W), susceptors from commercial packages for
microwave popcorn, samples of two commercial pizza products and two types of dough were used in the experiments together
with Luxtron temperature measurement system. The temperatures reached at the end of heating on the bottom surface of samples
varied between 103 and 115°C at the heating without susceptor, and between 110 and 155°C at the heating with susceptor. Not
only the susceptor but also the parameters of the heated samples (the moisture content, height/weight, the initial temperature)
influenced the increase of the temperature on the bottom surface of the samples. The highest temperatures were found at the end
of the heating of samples from dough with a lower content of moisture. The linear correlation between the temperature at the
bottom of the sample and the logarithm of the time of heating (ZUCKERMAN & MILTZ 1995) was proved only with the heating of
samples from one type of dough. The application of susceptor in the microwave heating alters not only the product temperature
in the places of contact with susceptor but also – to a certain extent – in other places of the product. The change in the shape of
the vertical temperature profile in the heated sample was found in the experiments with susceptor heating. For the optimal results
of the heating with susceptor, the optimization of certain product parameters (namely the moisture content and the dimensions)
have to be made
Keywords: microwave heating; susceptor; temperature profile in food; browning effect; crisping effect
Microwave heating has become very popular in the food
In order to achieve a significant browning and crisping
preparation. The microwave ovens are used for defrost-
reaction, the temperature of the food surface must be raised
ing of food and reheating of pre-cooked food products
well above 100°C (TURPIN 1989). As follows from differ-
both in households and catering. The popularity of mi-
ent measurements, the surface temperature of food prod-
crowave ovens leads the food producers to develop a
uct with a sufficient content of moisture does not exceed
new generation of products for microwave heating. Many
110°C during the heating in a microwave oven without
of these products can be heated and served in their pack-
additional heating systems (IR or/and hot air heating).
ages. To aid in achieving the optimum quality, special
The “susceptor packaging” technology was developed
package systems for microwaveable food products have
as one solution of how to overcome this problem.
been developed.
Susceptor is generally a lightly metallized PET (polyeth-
One problem in the microwave heating of food is the
ylenterephtalate) film which may be sandwiched within the
inability to crisp and brown the food surface when re-
package structure or laminated onto a rigid dimensionally
quired. Mainly those dough products which are expected
stable substrate as paperboard. At the optimum metalliza-
to be crisp and brown on the surface after preparation
tion, the susceptor itself absorbs the microwave energy
become soggy with warm damp surface when heated in
during the heating and converts it into heat. The amount
the microwave oven.
of this heat is a function of the resistance properties of
Supported by the National Agency for Agricultural Research of the Czech Republic (Grant No. EP 9177).
151

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Czech J. Food Sci.
the susceptor which depend on the thickness of metalli-
mensions on the temperature reached in the sample/sus-
zation (TURPIN 1989; MILTZ & ZUCKERMAN 1992, 1994;
ceptor contact area was confirmed. The linear rise of the
HOUŠOVÁ & HOKE 1998).
susceptor/dough interface temperature with the logarithm
The heat is transferred from susceptor to the product
of time was derived from the heating experiments. The
by conduction in the susceptor/product contact area cre-
slope of the line in this relationship depends on the weight
ating localized areas of high temperature on the product
and the diameter of the round sample of dough – the slope
surface and causing there water evaporation and brown-
decreases with the increasing of weight. The parameters
ing and crisping.
of the surface resistivity of susceptors, too, influence the
Susceptors have been used with some of microwave-
rise of the susceptor/dough contact temperature as fol-
able food products for more then 20 years. In spite of the
lows from other authors’ experiments (ZUCKERMAN &
experience with their application in the food production,
MILTZ 1997) – the parabolic (inverted) change of the con-
there is still relatively little information on their perfor-
tact temperature with the surface resistivity of susceptor
mance and on their changes during the microwave heat-
was deduced from experimental results.
ing.
In the following text, the results are given of our own
The objective of this work was to follow the influence
experiments pursuing the temperature distribution in cer-
of the food product nature and parameters on the temper-
tain dough products or semi-products heated in the mi-
atures reached in the product during its heating in the
crowave oven with and without susceptor. A commercial
microwave oven using susceptor.
household microwave oven (650 W rated power), food
The attention paid to the temperatures really reached in
products of different nature, weight and dimension and
the microwave susceptor heating dates back to 90-ies
susceptors from commercial susceptor packages were
years. The research had to give an answer to the question
used in these experiments.
whether or not the susceptor package – as a very complex
system – could affect the safety of the heated product.
MATERIALS AND METHODS
Different experimental studies focused on the potential
migration of certain volatile and non-volatile compounds
Microwave oven
of susceptor packaging system into the heated food be-
gan at that time and relevant results are available in the
Moulinex, type FM 1515E, rated power output 650 W,
literature (RISCH 1993). The collaborative studies focused
power output according to IEC705 (1 l cold water) 660 W,
on the real values of the susceptor temperature during
cavity dimensions 190 × 175 × 290 mm, cavity usable vol-
the heating preceded the migration studies (KASCHTOCK
ume 14.7 l, removable glass plate on the cavity bottom.
et al. 1990). Experiments were carried out in 10 laborato-
Food samples
ries in U.S. with heating of three types of commercial pop-
corn and frozen cheese pizza using different types of
Samples prepared from the following types of dough
household microwave ovens (rated power about 700 W).
products or dough were used in the experiments: frozen
The temperature at the susceptor/product interface was
pizza cake “Bon Giorno”, frozen cheese pizza “Bon Gior-
measured in all laboratories by Luxtron system. Accord-
no”, “linecké t?sto” – dough, type 1, dough for salty crack-
ing to these studies, the temperatures reached at the end
er – type 2 (own preparation – recipe see Table 1). The
of heating were about 200°C (popcorn heating) and about
basic chemical compositions of the individual food prod-
180°C (pizza heating). The results of these experiments
ucts are given in Table 2 (POTÍŠKOVÁ 2001).
also indicated that many factors such as the susceptor
Round samples (diameter of 90 mm) were cut out from
parameters, the oven type and load and the parameters of
both pizza products. The dough was first rolled out to the
the product can affect the temperature reached by sus-
predetermined thickness (5.5; 7, and 10.5 mm – dough
ceptor during the microwave heating.
type 1, and 9 mm – dough type 2) and subsequently round
The temperature on the susceptor/food product inter-
samples (90 mm diameter) were cut out.
face during the microwave heating was experimentally
followed also in the work of ZUCKERMAN and MILTZ
(1995). Specially prepared susceptors of certain parame-
Table 1. Recipe of dough for salty cracker
ters, samples of dough (without information on the chem-
ical composition) of different weight and dimensions and
Ingredient
Quantity (g)
household microwave oven (700 W) were used by the
authors in the experiments together with Luxtron system
Wheat flour
333
for the temperature measurements. Susceptor/dough in-
Vegetable fat “Hera”
185
terface temperatures between 130 and 200°C were found
at the end of the heating of the dough samples tested by
Water
70
the authors. The influence of the sample weight and di-
Salt – alternative
3, 6 and 12
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Czech J. Food Sci.
Vol. 20, No. 4: 151–160
Table 2. Basic chemical composition (in %) of individual food products (POTÍŠKOVÁ 2001)
Food product
Water
Drymatter
Fat
Proteins
Saccharides
Ash
NaCl
Pizza cake
33.01
66.99
7.28
8.48
47.79
3.44
0.83
Dough type 1
14.53
85.47
25.09
6.25
53.60
0.53
0.03
cake
38.48
61.52
5.58
7.79
45.72
2.43
0.75
Cheese pizza topping
58.57
41.43
9.28
13.93
14.57
3.65
1.18
Dough type 2
12
88
30
–
–
–
–
Before heating, the samples were kept in a refrigerator
Temperature measurement
(dough samples and defrosted pizza cake) or a freezer (sam-
Luxtron model 755 with four fluoroptic probes MIW-2
ples from pizza and pizza cake).
was used for the measurement of the food sample temper-
Susceptors
ature during the heating. The temperature data were col-
lected at intervals of 2 s by the four-channel acquisition
Round samples of susceptors (diameter of 90 mm) were
system and transferred to the computer via RS 232 serial
cut out from the active part of commercial packages for
port.
microwave popcorn. The average value of susceptors
The temperature of samples was measured in four pre-
optical density (O.D.) was 0.220 ± 0.002 (separate mea-
determined positions (Fig. 1) in several horizontal levels –
surements with 10 samples of susceptors – POTÍŠKOVÁ
in the distance of 0.5 mm from the susceptor or glass plate
2001), the average value of the microwave power absorbed
(the temperature of samples near their bottom surface)
in susceptor was 42.55 ± 1.23% of the incidental power
and other two or three levels (according to the height of
(separate measurement with 10 samples, ?ESNEK 2001 –
samples). To fix the locations of probes, a special holder
personal commun.).
from PTFE was used.
Insulating pads
Experimental procedure
In several measurements, the following types of ther-
mal insulation were tested to prevent the loss of heat from
The sample of susceptor together with a food sample of
susceptor by transition to the glass plate in the micro-
certain dimensions, weight and initial temperature was
wave oven: plate from PTFE, 100 × 100 mm, thickness of
placed in the middle of the oven glass plate. Luxtron
5 mm, and corrugated paperboard, 120 × 120 mm, thick-
probes were installed into the sample. An additional load
ness of 4 mm.
(two glasses each with 100 ml of cold water) was installed
4
2
1
3
Temperature probes
Susceptor
Sample of food
Corrugated paperboard
Microwave oven glass plate
Fig. 1. The position of temperature measuring probes in food sample during the heating in the microwave oven with susceptor and
an insulation pad from corrugated paperboard
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Czech J. Food Sci.
on the glass plate in order to lower the increase of the
As follows from the graphs, during the microwave heat-
temperature of small food samples during the heating. The
ing without susceptor the food surface temperature did
food samples were heated for predetermined time (2 to
not exceed very much the water boiling temperature, namely
5 min) at 100% power output setting on the microwave
if the food had enough moisture. During the heating of
oven. After the heating, the weight and the dimensions of
samples from pizza cake and cheese pizza, the bottom sur-
samples were controlled and the sample appearance
face temperature of samples remained at 100°C for most of
(namely of the bottom surface) was visually judged.
the heating time; only at the end of the heating it increased
For comparison, the heating of samples of each type of
to 105 (cheese pizza) or 110°C (pizza cake). A little higher
food without susceptors was also performed.
temperature was reached in the heating of samples pre-
The heating procedure was repeated three times for each
pared from both types of dough with lower moisture con-
process, food parameter, and vertical placement of tem-
tents, where the maintenance of temperature at 100°C was
perature probes in food samples. The oven was allowed
shorter.
to cool between the runs (30 min breaks).
The use of susceptor in the heating influenced posi-
The temperatures reported in the following text and
tively – but not to the same extent – the increase of the
graphs are average values of the temperatures measured
bottom surface temperature in all types of samples. It is
in two places in the central parts of samples (probes 1 and
evident from the graphs that the effect of the additional
2) in three repeated runs.
heat from susceptor began to show after some time of
heating. In the initial period of heating, the bottom sur-
face temperature of the samples was lower in the heating
RESULTS AND DISCUSSION
with susceptor compared to the heating without suscep-
tor. During the heating of pizza type samples, a signified
Temperature of food sample near its bottom surface
during the microwave heating with and without susceptor
shortening of the temperature delay at 100°C was found if
susceptor was used. The temperatures reached at the end
The increase of the sample temperature near its bottom
of heating with susceptor were higher compared to the
surface (0.5 mm from the susceptor or glass plate) during
heating without susceptor in all food samples tested. An
the microwave heating with and without susceptor is
average temperature of about 130°C was found at 180 s of
shown in the two following figures. Fig. 2 presents the
susceptor heating of frozen and defrosted pizza cake sam-
results obtained in the heating of defrosted pizza cake
ples, and of about 155°C at the end of the heating of sam-
samples and Fig. 3 refers to the heating of the samples
ples from dough, type 1.
from dough type 1.
140
120
100
(
°
C)
80
60
Temperature
with susceptor
40
without susceptor
20
0
0
30
60
90
120
150
180
210
Time (s)
Fig. 2. Microwave heating of defrosted pizza cake samples with or without susceptor. The increase of temperature on the bottom
surface of the samples during the heating
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Vol. 20, No. 4: 151–160
180
160
140
120
(
°
C)
100
80
Temperature
60
with susceptor
40
without susceptor
20
0
0
30
60
90
120
150
180
Time (s)
Fig. 3. Microwave heating of samples from dough type 1 (height 7 mm) with or without susceptor. The increase of temperature
on the bottom surface of samples during the heating
The appearance of the bottom surface of the samples
ceptor and a brown and crispy one after the heating with
after the heating correlated well with the final tempera-
susceptor was the result of the visual judgement of these
tures in these parts of the samples: a pale, soggy, and
samples after the heat treatment.
damp surface of the pizza after the heating without sus-
160
140
120
100
(
°
C)
80
defrosted pizza cake (13 mm)
defrosted pizza cake (13 mm)
60
Temperature
dough, type 1 ( 5.5 mm)
dough, type l (5.5 mm)
40
dough, type 2–0.5% NaCl (9 mm)
dought, type 2 – 0.5% NaCl (9 mm)
20
dough, type 2–2% NaCl (9 mm)
dough, type 2 – 2% NaCl (9 mm)
0
0
20
40
60
80
100
120
140
160
180
200
220
240
260
Time (s)
Fig. 4. Microwave heating of samples from defrosted pizza cake (height 13 mm), dough type 1 (height 5.5 mm), dough type 2
(height 9 mm) with the addition of 0.5 and 2% NaCl. The increase of temperature on the bottom surface of the samples during the
heating with susceptor
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Czech J. Food Sci.
180
160
140
120
(
°
C)
100
80
Temperature
60
height 5.5 mm, weight 40.4 g
height 7 mm, weight 50.4 g
40
height 10.5 mm, weight 76.9 g
20
0
0
30
60
90
120
150
180
210
Time (s)
Fig. 5. Microwave heating of samples from dough type 1 of different heights. The influence of the height (weight) of the samples
on the increase of temperature on the bottom surface of the samples during the heating with susceptor
As the results of the experiments confirmed, it was not
of temperature at its bottom surface during the heating.
only susceptor that influenced the product temperature
The time–temperature relations for the heating of defrost-
reached in the contact area at the end of the microwave
ed pizza cake, dough type 1 (5.5 mm height), and dough
heating. Fig. 4 illustrates the influence of the nature and
type 2 with the addition of 0.5 and 2% NaCl (height 9 mm)
physical parameters of the heated product on the increase
are compared in this figure.
180
experimental data
160
140
calculation
120
(
°
C)
100
80
T = –194.3 + 148.6 log t
Temperature
60
40
20
0
0
20
40
60
80
100
120
140
160
180
200
220
Time (s)
Fig. 6. The increase of the temperature on the bottom of samples (dough type 1, 0.5 mm thickness) during the heating with
susceptor. The comparison of the experimental values and the values predicted by the use of the relationships by ZUCKERMAN
and MILTZ (1995)
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Czech J. Food Sci.
Vol. 20, No. 4: 151–160
200
180
160
140
120
(
°
C)
100
80
Temperature
with susceptor
60
without susceptor
40
paperboard and susceptor
20
0
0
30
60
90
120
150
180
Time (s)
Fig. 7. The influence of the insulation pad (corrugated paperboard) placed under susceptor on the bottom surface temperature of
samples during the heating (defrosted pizza cake)
Fig. 5 illustrates the influence of the height (or weight)
our experiments. However, great differences between the
of a food sample (dough type 1) on the rate of the in-
experimental and the predicted values of temperature were
crease of the bottom surface temperature. The decrease
found. It seems that the correlation proposed by Zucker-
of the sample surface temperature in the place of contact
man and Miltz will be available only for food products of
with susceptor with the increase of the sample weight
certain compositions.
corresponds well with the experiments of ZUCKERMAN
The bottom surface temperature of the samples heated
and MILTZ (1995).
in the microwave oven with susceptors can be positively
The data obtained in the heating experiments with sus-
influenced using certain insulation under the susceptor.
ceptor were correlated using the linear dependence of the
Good results were obtained mainly in the tests with corru-
bottom surface temperature of the sample (T) on the log-
gated paperboard – see Fig. 7 (the heating of samples
arithm of the heating time (t) as proposed by ZUCKER-
from defrosted pizza cakes).
MAN and MILTZ (1995): T = A + B × log t, where A and B
are coefficients affected by the product.
Vertical temperature profiles in food samples in the
microwave heating with and without susceptors
Fig. 6 illustrates the results of this correlation for the
heating of the samples from dough type 1 with the height
Susceptor used at the microwave heating of food sam-
of samples of 10.5 mm. As seen from the figure, the pro-
ples influences mainly the temperature in the susceptor/
posed relationships is relatively satisfactory for this type
sample contact area but a certain influence on the temper-
of food product. With the decrease of the sample height
atures in other regions of samples was also found in our
(weight), the slope of the lines increases. The respective
experiments.
values of both coefficients were: A coefficient –194.3
In the following two figures, the development is com-
(10.5 mm), –147.4 (7 mm) and –145.0 (5.5 mm) and B coeffi-
pared of temperatures monitored gradually in different
cient 148.6 (10.5 mm), 134.6 (7 mm) and 132.4 (5.5 mm). For
heights in food samples during their heating with and
the salt dough with 2% of NaCl, the value of A coefficient
without susceptor.
was –221.1 and that of B coefficient 153.2, respectively,
In Fig. 8, the results are shown of the heating of cheese
but the agreement between the experimental and calculat-
pizza samples and in Fig. 9, those of the tests with sam-
ed values of temperature was not so satisfactory in this
ples from dough the type 1 (7 mm).
case.
In both figures it can be seen that the susceptor appli-
The application of the proposed relationships was test-
cation in the heating affects not only the bottom surface
ed also for the other two types of food samples used in
temperature of the sample but – to a certain extent – it
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Czech J. Food Sci.
140
start
120
60 s
120 s
100
180 s
(
°
C)
240 s
80
300 s
60
start (x)
60 s (x)
Temperature
40
120 s (x)
180 s (x)
20
240 s (x)
300 s (x)
0
-20
0
2
4
6
8
10
12
14
16
18
Distance of temperature probes from susceptor/glass plate (mm)
Fig. 8. Vertical temperature profiles in samples of cheese pizza (24 mm) during the microwave heating without and with
susceptor (×)
changes also the shape of the vertical temperature pro-
ature in the whole of the sample (in all points in which the
file. The influence of the food product type is also evi-
temperature was measured). After the rise of temperature
dent from the comparison of the figures.
to 100°C approx., a marked increase of it was observed on
In the initial period of heating, the application of sus-
the bottom surface in the case of the heating with suscep-
ceptor resulted in a slow-down of the increase of temper-
tor but only a slow increase of temperature in other layers
160
start
30 s
140
60 s
90 s
120
120 s
150 s
100
170 s
(
°
C)
start (x)
80
30 s (x)
60 s (x)
60
Temperature
90 s (x)
120 s (x)
40
150 s (x)
170 s (x)
20
0
0
1
2
3
4
5
6
Distance of temperature probes from susceptor/glass plate (mm)
Fig. 9. Vertical temperature profiles in samples from dough type 1 (7 mm) during the microwave heating without and with
susceptor (×)
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Czech J. Food Sci.
Vol. 20, No. 4: 151–160
of the samples. At the end of the heating with susceptor,
6. The application of susceptor in the microwave heat-
the highest temperatures were found on the bottom sur-
ing which exerts an influence mainly on the temperature
face of the samples, contrarily to the heating without sus-
of the heated product in the contact area, alters to a cer-
ceptor in which the highest temperatures were found on
tain extent also the local temperatures in other parts of the
the upper surface of the samples.
product. A certain change in the shape of the vertical
temperature profile was derived in all types of samples
Conclusions
from the comparison of the results of the heating with and
without susceptor.
1. The use of susceptors in the microwave heating of
7. For the optimal result of the microwave heating with
the food samples tested affected positively their final ap-
susceptor, a certain optimization of the food product pa-
pearance. The expected “baking effect” of susceptors was
rameters (moisture, dimensions) seems to be needed.
observed to a certain extent after the heating with all food
samples tested. The appearance of the samples after heat-
R e f e r e n c e s
ing correlated well with the temperatures measured at the
end of the heating on the bottom surface of the samples.
HOUŠOVÁ J., HOKE K. (1998): Závislost tepelného efektu meta-
Higher values of temperature were found in these loca-
lizovaných fólií pro mikrovlnný oh?ev na stupni jejich
tions at the end of the heating with susceptor in all types
pokovení. Czech J. Food Sci., 10: 655–661.
of food samples as compared to the heating without sus-
KASHTOCK M.E., WURTS C.B., HAMLIN R.N. (1990): A multi-
ceptor.
lab study of food/susceptor interface temperatures measured
2. The temperatures reached on the bottom surface of
during microwave preparation of commercial food products.
the samples at the end of the heating were influenced not
J. Pack. Technol., 3: 14–19.
only by the application of susceptor but also by the prod-
POTÍŠKOVÁ J. (2001): Studium vlastností susceptor?. [Diplo-
uct itself confirmed in the experiments. Higher surface
mová práce.] VŠCHT, Praha.
temperatures were found at the end of the heating with
RISCH S. (1993): Safety assessment of microwave susceptors
susceptor on the bottom of samples from dough type 1
and other high temperature packaging materials. Food Addit.
and type 2 with a lower moisture content in comparison to
Contam., 10: 655–661.
the samples from pizza (cake, cheese) with a higher mois-
TURPIN CH. (1989): Browning and crisping. Microwave World,
ture content.
10 (6): 8–13.
3. Except the moisture content, the dimensions and the
ZUCKERMAN H., MILTZ J. (1992): Characterisation of thin
weight of samples also affected the temperature reached
layer susceptors for the microwave oven. J. Food Proces.
on their surface at the end of the heating. An increase of
Preserv., 16: 193–204.
the height (and the weight) of samples resulted in a slow-
ZUCKERMAN H., MILTZ J. (1994): Changes in thin-layer
down of the increase of the sample bottom temperature.
susceptors during microwave heating. Pack. Technol. Sci., 7:
4. The initial temperature of the food product is also
21–26.
important for the result of the microwave heating. The
ZUCKERMAN H., MILTZ J. (1995): Temperature profiles at
susceptor application influenced very positively the re-
susceptor/product interface during heating in microwave oven.
sult of the microwave heating of the frozen samples.
J. Food Proces. Preserv., 19: 385–398.
5. The relationships proposed by Zuckerman and Miltz
ZUCKERMAN H., MILTZ J. (1997): Prediction of dough brown-
for the description of the time-temperature dependence
ing in the microwave oven from temperatures at the susceptor/
for the food product/susceptor interface seems not to be
product interface. Lebensm.-Wiss. u-Technol., 30: 519–524.
available commonly but only for food products of certain
chemical compositions.
Received for publication March 12, 2002
Accepted May 20, 2002
Souhrn
HOUŠOVÁ J., HOKE K. (2002): Teplotní profily ve výrobcích z t?sta b?hem mikrovlnného oh?evu s použitím susceptor?.
Czech J. Food Sci., 20: 151–160.
Pr?b?h teplot p?i mikrovlnném oh?evu n?kolika typ? potravin za použití susceptor? byl monitorován za použití mikrovlnné
trouby pro domácnosti (650 W – štítkový výkon). Byly zjišt?ny konkrétní hodnoty teplot dosažené p?i mikrovlnném oh?evu
vzork? n?kolika r?zných potravin bez susceptoru a s ním. Ú?inek susceptoru byl sledován p?i mikrovlnném oh?evu vzork?
plochého tvaru, p?ipravených ze dvou druh? t?st (linecké a k?ehké s p?ídavkem NaCl) a ze dvou komer?ních výrobk? (sýrová
159

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Czech J. Food Sci.
pizza a korpus pro pizzu). Všechny vzorky m?ly kruhový tvar s pr?m?rem 90 mm. Výška vzork? závisela na typu potraviny
a pohybovala se od 5,5 mm (nejnižší vzorky z lineckého t?sta) do 24 mm (zmrazená sýrová pizza). Vzorky susceptor? byly
vyst?iženy z komer?ních obal? pro popcorn ur?ený k p?íprav? v mikrovlnné troub?. Teploty vzork? potravin byly postupn?
monitorovány pomocí termometrického systému Luxtron. Vedle teploty spodního povrchu vzork? (0,5 mm od susceptoru) byly
zjiš?ovány teploty v dalších dvou nebo t?ech místech jejich výšky. Na konci oh?evu se susceptorem (2 až 5 min podle druhu
vzorku) byly u spodního povrchu vzork? nam??eny pr?m?rné teploty v rozmezí 110–155 °C (podle druhu vzorku), p?i oh?evu
bez susceptoru jen 105–115 °C. Soub?žn? provád?né vizuální hodnocení vzhledu vzork? po oh?evu dob?e korelovalo s nam??enými
kone?nými teplotami. Výsledky zkoušek potvrdily výrazný vliv samotného vzorku na pr?b?h m??ených teplot, zejména pak
vliv obsahu vlhkosti a výšky, resp. hmotnosti vzorku. U vzork? s vyšším obsahem vlhkosti (pizza) byly nam??eny výrazn? nižší
kone?né povrchové teploty než u vzork? z obou t?st s nižší vlhkostí. Pomalejší vzr?st teploty spodního povrchu vzork? byl
zaznamenán p?i zvyšování výšky (resp. hmotnosti) vzork? (zkoušky se vzorky lineckého t?sta). Vzr?st teploty spodního
povrchu jednotlivých typ? vzork? p?i mikrovlnném oh?evu se susceptorem byl porovnáván s empirickým vztahem, který
doporu?ili ZUCKERMAN a MILTZ (1995), tj. s lineární závislostí teploty potraviny v míst? kontaktu se susceptorem na logaritmu
?asu. Podle výsledk? výpo?t? není použitelnost tohoto vztahu obecná, je omezena pouze na n?které typy výrobk?. V daném
p?ípad? byla zjišt?na dobrá shoda nam??ených teplot s údaji, vypo?tenými podle empirického vztahu, pouze pro oh?ev vzork?
z lineckého t?sta, p?íp. k?ehkého t?sta. V ?lánku jsou uvedeny hodnoty p?íslušných koeficient? empirické závislosti, závisející na
parametrech vzork? (složení, výška, resp. hmotnost). Pomocí nam??ených hodnot je dokumentován i p?íznivý vliv použití
izola?ní podložky pod susceptor, který snižuje ztráty tepla odvodem do sklen?né desky mikrovlnné trouby. Aplikace susceptoru
p?i mikrovlnném oh?evu, ovliv?ující vzr?st a kone?nou výši teploty potraviny v míst? jejího kontaktu se susceptorem, m?ní do
ur?ité míry i teplotu v ostatních vrstvách potraviny. Míra vlivu susceptoru na vertikální rozložení teplot ve vzorcích b?hem
oh?evu souvisí s typem potraviny. Výsledky experimentální studie potvrdily, že pro dosažení optimálního ú?inku aplikace
susceptoru p?i mikrovlnném oh?evu je t?eba ur?ité optimalizace parametr? samotného výrobku (rozm?ry, obsah vlhkosti).
Klí?ová slova: mikrovlnný oh?ev; susceptor; teplotní profil potravin; hn?dnutí potravin; k?ehkost potravin
Corresponding author:
Ing. JI?INA HOUŠOVÁ, CSc., Výzkumný ústav potraviná?ský Praha, Radiová 7, 102 31 Praha 10-Hostiva?, ?eská republika
tel.: + 420 2 96 79 23 35, fax: + 420 2 72 70 19 83, e-mail: k.hoke@vupp.cz
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