32 VR Food Packaging 2006
Packaging materials & Packs
I
n the perception of most microwave
oven users, all metal items includ-
ing food packages with aluminium
foil have to be banned from the oven.
This perception is enhanced by manu-
als
and general guidelines, which
strictly advise not to introduce metal
into the microwave oven. Aluminium
foil packages are not even mentioned
in most manuals. Interestingly, the
same manuals promote the use of
household foil to cover exposed parts
like the legs of a chicken during mi-
crowave heating [1].
Contradiction
Neglecting packages with aluminium
foil for microwaveable foods is how-
ever not justifi ed by microwave phys-
ics and is in contradiction to numer-
ous experimental studies in the past
[2, 3, 4, 5]. The studies dating from
the 1970ies through the 1990ies
agree that use of aluminium foil trays
in the microwave oven is perfectly
viable and can lead to good heating
results. A recent experiment carried
out at the Fraunhofer Institute for
Process Engineering enforces the
earlier results [6].
Modern microwave ovens are equipp-
ed with a glass or ceramic turntable
onto which food containers or dishes
sess microwave heating of food in
different packaging solutions con-
taining aluminium foil in a number of
modern microwave ovens for the
home kitchen. The tested packs in-
cluded aluminium foil trays in differ-
ent sizes, plastic containers with foil
or laminated foil lid, and dishes cov-
ered with household foil. Figure 2
shows two of the used foil trays. Per-
formance comparison was made
with similar plastic trays. Four differ-
ent ovens with microwave-power
ratings from 700 W to 1000 W were
used. The heated foods were egg bat-
ter, frozen lasagne, and minced
meat. Microwave heated foods were
investigated for minimum and maxi-
mum temperatures, temperature
patterns, and visual appearance of
food.
Results of the microwave heating
experiments with packages contain-
ing aluminium foil can be resumed as
follows:
– Microwave heating of food pack-
ages containing aluminium foil is
safe.
– Good heating quality can be
achieved in the microwave oven.
– Rapid heating can be attained.
Safe microwaving with
aluminium foil
More than 200 packages with alu-
minium foil were heated in micro-
wave ovens without a single case of
hazardous condition or damage of
oven. Electric sparks with aluminium
foil trays were only observed in se-
vere abuse situations, where empty
trays touched each other or touched
the oven wall during microwave expo-
sition. The abuse situations had to be
provoked and it is unlikely to create
them by careless operation. Thus this
situations will hardly occur during
normal operation of the microwave
ovens. Even though sparks can look
spectacular, they are not dangerous
nor do they damage the oven. Mostly
they result in small marks or burnt
Modern microwaveable packages
Aluminium foil packages are safe in microwave ovens
Photos: FhG
are placed (Figure 1). The turntable
promotes more uniform heating by
continuously turning the food con-
tainer through the microwave inter-
ference pattern inside the oven
chamber. The microwave energy en-
ters into the chamber through a mi-
crowave window, mostly at the right
side wall of the oven, and is refl ected
and scattered by the oven walls until
it is absorbed in the food. A small
part of the energy may be refl ected
through the window back to the mag-
netron, the device producing the
electromagnetic microwave fi eld. If,
however, no absorbing material is
present in the oven chamber, large
amounts of power can be refl ected
back and can lead to thermal wear of
the magnetron. The presence of food
containers with aluminium foil does
not alter the situation signifi cantly.
The contribution of such containers
to the back refl ection is small and
does not affect the operating life of
the magnetron [4].
A further concern in connection with
food packages containing aluminium
foil in microwave ovens is the forma-
tion of sparks between container and
metal walls of the oven chamber.
These sparks are safely avoided if
only a few millimetres wide air-gap is
maintained between the food pack-
age metal surface and the oven wall.
The glass turntable supports a safe
distance of package material to both,
oven chamber walls and fl oor, and
makes occurrence of sparks ex-
tremely unlikely.
The Fraunhofer Institute for Process
Engineering and Packaging recently
carried out a study in order to re-as-
Figure 1:
Cooking chamber
of microwave oven
with microwave
window in the right
side wall and with
glass turntable.
Cover of microwave
window is removed.
Figure 2:
Two aluminium
foil containers used
in the microwave
heating experi-
ments. Left: single
compartment
container. Right:
dual compartment
container.
