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 xe2x88x925xc2x0 C. and then brought further to a throughflow freezer, in which it is attempted to cool down the mass to a temperature of xe2x88x9215 or lower, preparatory to extruding the mass for the forming of the final ice-cream bodies for packing and final storing.
This xe2x80x98typexe2x80x99 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 teat 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 xe2x88x925xc2x0 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 xe2x88x9240xc2x0 C. was used.
It was found rather soon that the achievable results were entirely unusable in practice. It was found that is 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 notxe2x80x94and still is notxe2x80x94possible 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 xe2x80x98typexe2x80x99 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 uninteresting level.
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 conveyors 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 xe2x80x98typexe2x80x99 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 through flow 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 rotational 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 1xc2xd 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 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 xe2x88x9230xc2x0 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 xe2x88x9240xc2x0 C. and colder, e.g. down to 100xc2x0 C. and preferably in the range of xe2x88x9250 to xe2x88x9260xc2x0 C. for achieving a good efficiency by the freezing down of the mass to about xe2x88x9215xc2x0 through xe2x88x9222xc2x0 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 tool.
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 xe2x80x98typexe2x80x99 of process realizable in commercial practice.