The present invention relates to the production of ice-cream material
and more specifically to a production method of the type, by which the material
in the form of the so-called mix with a substantial content of air is first cooled
down to a conventional forming temperature of typically -5°C and then brought further
to a throughflow freezer, in which it is attempted to cool down the mass to a
temperature of -15° or lower, preparatory to extruding the mass for the forming
of the final ice-cream bodies for packing and final storing.
This 'type' of process is known from the literature, cf. DE-C-39
18 268, but not really from practice as far as usual ice-cream is concerned, since
the process has been found to involve quite marked problems. At the principal
level the process type is highly attractive, because ideally it would make it possible
to form and pack the ice bodies directly to the final storing, without the conventional
use of an intermediate and expensive low freezing system-between the packing station
and the final storage. Moreover, an intensive cooling of the mass will enable an
improved product quality, in particular when producing larger block products.
The direct starting point of the invention was a test system including
a conventional throughflow freezer having a driven, scraping conveyor worm,. dimensioned
for a further conveying of the flow from the preceding, ordinary continuous freezer,
which cools the mass down to some -5°C. As the flow remains unchanged it was natural
to select increased or unchanged pipe dimensions. At the outset, a standard mix
of ice-cream with a so-called overrun (degree of swelling) of 100% was used, and
in the throughflow freezer an evaporator temperature of approximately -40°C was
It was found rather soon that the achievable results were entirely
unusable in practice. It was found that it was difficult to reach the desired low
temperature of the ice-cream, and moreover the overrun was decreased quite unacceptably,
down to 30-50 %. Changed process parameters made no difference in this picture,
but demonstrated that the drastical drop of the overrun was noticeably influenced
by such changes.
A solution of the said problem was made difficult by the fact that
it was not - and still is not - possible to precisely indicate the reason why the
overrun turns out to be decreased.
However, according to the invention a surprising solution to the
problem has been found, viz. by introduction of a controlled resistance in the
flow from the throughflow freezer. From a processing point of view this will not
be any particularly attractive solution, but it will be attractive anyway as long
as it seems to be the only possibility of making the discussed 'type' of process
practically usable at all. Also, the said resistance will not in any way need to
be so high that it will indirectly reduce the production. capacity to some commercially
Thus, some additional energy should undeniably be used for the forcing
out of the mass from the throughflow freezer, but this amounts to almost nothing
in view of the . fact that in return the discussed type of process can then be
used in practice for achieving a really usable result, i.e. providing a final
product having the desired overrun, structure and low temperature.
It could be desirable that it would be possible to introduce as a
simple measure the said delivery flow restriction as a permanent pipe narrowing,
but the further efforts in connection with the invention have shown that this
will not normally be sufficient, as the optimum constriction is depending not only
of the mechanical process parameters, but also of the formulation of the mix and
the relevant process parameters. In practice, therefore, it seems to be a necessity
to use a controllable, variable flow resistance. This may be realized by the use
of an adjustable throttling valve or pressure regulating valve or by the use of
a controlled, partial heating of a narrowed discharge pipe.
In that the flowthrough freezer should operate with a heavy cold
transfer at extra low temperature, there is currently formed, on the inside of
the freezer, an ice layer which should be scraped off. As it is also desired to
effect a positive conveying of the ice-cream mass inside the cylinder, there will
be no technical problem in combining such a scraping and conveying, viz. in using
a scraping worm conveyor, which is a known machine element. However, with a test
system using such a known worm conveyor freezer the result is rather discouraging,
as it is observed that in order to effect the scraping and the conveying of the
ice-cream mass it is required to supply so much energy that the freezer becomes
ineffective because of the applied scraping, kneading and pumping energy, which
will reveal itself as a heat development, directly opposing the the freezing. This
can be counteracted by using a furtherly lowered temperature on the cooling side,
but only with the result that the building up of the said ice layer is promoted
such that still more energy will be required for the scraping function, and it
has been found that also this basic condition must be responsible for the discussed
process 'type' not so far having been realized commercially.
On this background and in connection with the invention it has been
considered whether it could be.possible to provide an entirely different and more
effective throughflow freezer. Surprisingly enough, however, it has been found
possible to maintain the relatively effective and simple concept of a worm conveyor,
when only the traditional design thereof is drastically changed with respect to
the rotation speed of the rotor and the pitch of the worm winding or windings.
For worm conveyors in connection with flowthrough freezers it is
customary to use a rotor rotation at some 100-1000 r.p.m., least for larger cylinders
and highest for cylinders with small diameter. For a representative worm conveyor
with an inner worm diameter of 105 mm the rotor speed will typically be 200-600
r.p.m. which, by a typical worm pitch of between a whole and a half time the outer
diameter of the worm will result in an axial scraping speed of 1-3.5 m/sec.
With the invention it has been found possible and optimal to operate
with a revolution figure of only some 5-20 r.p.m. as well as with a worm pitch
that is unusually large, viz. between one to two times the outer diameter of the
worm, preferably between 1 and 1 1/2 times this diameter. The said scraping speed
will thus occur at a reduced value of only some 1-10 % of the conventional standard,
but it has been found that in return it is then possible to realize the. process
in practice. What is actual is a practically usable compromise between the effect
of the applied energy being sufficient for conveying and scraping without causing
undesired heating. It is a surprising result that the low scraping speed and the
associated low scraping frequency is sufficient for keeping the heat exchanger
surface clean to such an extent that it is possible to operate with a practically
acceptable efficiency of the heat exchange.
It is even to notice that for good reasons it is required to use
a refrigerant with an evaporation temperature lower than the approximately -30°C,
which to the skilled persons has been considered as a minimum evaporation temperature
in connection with continuous ice-cream freezers; it has previously been found
that with still lower temperatures there will occur a too heavy solid freezing
of the ice-cream on the heat exchanger surface. Apparently it is a paradox that
with the invention and the associated reduced scraping it is possible to operate
effectively with freezing temperatures of -40°C and colder, e.g. down to -100°C
and preferably in the range of -50 to -60°C for achieving a good efficiency by
the freezing down of the mass to about -15° through -22°C. It can only be confirmed,
however, that the good results have been achieved by the use of the said modified
continuous freezer, in which it is the worm itself that acts as the effective scraper
There has been found no reason to assume that the aforementioned
and in fact similarly important effect with respect to the preserving of the overrun
should be particularly depending of the use of the discussed modified freezer,
but.on the other hand it can be confirmed that the relevant good result can be
achieved also by the use of this freezer, such that the combined result renders
the said 'type' of process realizable in commercial practice.
The invention is illustrated in the drawing, in which:
- Fig. 1 is a schematic diagram for illustrating the process, while
- Fig. 2 is a schematic representation of a throughflow freezer according to
The processing system for producing extruded ice-cream products as
schematically shown in Fig. 1 comprises a continuous freezer 2 which, from a supply
4, is supplied with 'mix' passing a pump 6 and a mixing chamber 8, in which the
mix is mixed with air from a compressed air source 10 for achieving a overrun
of traditionally some 100%. This ready made ice-cream substance is cooled in the
continuous freezer 2 down to a temperature of approximately -5°C as fully conventional
for a subsequent portioning out and shaping of the substance.
In connection with the invention, however, it is desirable to convey
the cooled substance further through a continuous freezer 12 for a subsequent extrusion
at a temperature of -12 to -25°C, such that the cut ice bodies can be packed for
transfer directly to the freezing store. The freezer 12 should be positively conveying,
i.e. it should comprise a conveyor worm 14 driven by a motor, which is here designated
W in order to indicate that this driving will incur a certain supply of heat energy,
partly for the conveying function itself and partly for the scraping work to be
effected by the worm for scraping off the solid frozen ice-cream mass.
Owing to the associated increased viscosity of the mass it would
be natural to use a somewhat enlarged dimension of the discharge pipe 18 compared
with the supply pipe 16, but as mentioned it has been found that the final result
of this is in fact unusable with respect to the overrun of the extruded mass.
With the invention it has been found that this major problem can be solved by providing
a discharge resistance R in the pipe 18. This resistance is relatively critical,
inasfar as it should be noticeable for achieving the desired result with respect
to the overrun, but not so noticeable that it gives rise to the conveying resistance
in the freezer 12 increasing to a level at which the required conveying energy
reveals itself as an unacceptable heat generation in the freezer.
This in itself is a noticeable problem, because it . may imply that
it is very difficult to achieve the desired cooling of the ice-cream, practically
no matter how much the freezer is cooled from the outside. This will be considered
in more detail below.
First, it is important to note that normally the required flow resistance
R should be statically or dynamically adjustable, as extensive tests have shown
that the optimum resistance depends of various process parameters, including the
formulation of the mix and the discharge temperature and capacity of the ice-cream.
It is customary that in a given system there will be produced products with different
formulations and process conditions, and the resistance R should be adjustable
accordingly, based on gained experiences. Normally, as a standard, the pipe dimension
at the discharge side of the freezer 12 should be slightly reduced, but the resistance
should still be adjustable. This will be achievable by a differentiated partial
heating of the narrower pipe, but preferably a controlled throttling valve or a
pressure regulating valve should be used, for example a controllable constriction
of a hose portion inserted in the discharge pipe
Next, it is important that the continuous freezer 12 operates with
a relatively very low temperature at the primary side, e.g. in the range of -40°
to -100°C, and that it is made with a special geometry as far as the conveying/scraping
worm is concerned, in connection with an equally unusually low rotational speed
of the worm, preferably as low as 5-20 r.p.m.
The freezer unit 12,14 is indicated in more detail in Fig. 2 with
the following designations of dimensions.
- D1 =
- diameter of rotor core;
- D2 =
- outer diameter of worm on this core;
- D3 =
- inner diameter of surrounding freezing cylinder;
- L =
- length of freezing cylinder and worm; and
- P =
- pitch of worm.
With the invention the following relations are preferred:
L / (D3) = 5-10;
P / (D2) = 150 / (105) = 1-2;
D2 / (D1) = 105 / (75-90) = 1,1-1,4 (height of worm winding).
The pitch P should not necessarily be constant along the length L,
as it may vary as desired for an optimized design and for reducing the ice-cream
pressure during the conveying thereof through the freezer 12.