PELLETISER AND METHOD FOR MAKING FROZEN PELLETS

Abstract: A pelletiser (12) for making pellets from a liquid or semiliquid food product has two moulding rolls (28, 29) between which the food product is formed into pel­ lets. The rolls (28, 29) have teeth of complementary shape, and the teeth of at least one roll have axially spaced-apart recesses (34). The rolls are cooled with a refrigerant during the moulding operation. By means of the pelletiser, frozen pellets are made from a liquid or semiliquid food product which, prior to moulding, is refrigerated into mouldable consistency before being supplied to the nip between the two cooperating moulding rolls which are cooled during the moulding operation so as to form surface-frozen pellets which are there­ after subjected to final freezing.

Description: The present invention relates to a pelletiser for making pellets from a liquid or semiliquid food product which has been refrigerated into mouldable consistency, said pelletiser having two moulding rolls between which the refrigerated food product is formed into pellets. The invention also relates to a method for making frozen pellets from a liquid or semiliquid food product. The freezing of liquid or semiliquid food products in particle form, i.e. in the form of powders, granules, pellets or the like, confers advantages in several respects. Thus, quick freezing, as well as quick thawing, is made possible, which is advantageous for high quality. Freezing in particle form enables the producer to simplify packaging in portions of different sizes, while the consumer need only thaw the amount of food required on a particular occasion. For freezing liquid and semiliquid food products in the form of pellets, it is previously known to first freeze the product in block form and then divide the frozen blocks into particles which are smaller than the desired pellets, and thereafter compress the particles to form pellets of the desired shape. The major drawback of this method is that it is not sufficiently gentle on many products containing components in particle form. These components may then be damaged, both during disintegration and compression. Another method for freezing food products as defined above in the form of pellets consists in freezing the food product in containers of desired shape and volume, and expelling the food product from the containers after freezing. This method is complicated from production engineering aspects and also suffers from the disadvantage that it is difficult to prevent the product from freezing fast in the containers. Yet another method for freezing food products as defined above in pellet form involves spreading the product on a conveyor belt which may be planar, corrugated or provided with depressions of a desired shape. The product spread on the belt is conducted thereon through a freezing zone and, after passing through this zone, is separated from the belt. This method suffers essentially from the same drawbacks as the method previously described. It is further known, by resorting to substantial compression, to make pellets of optional materials which have been finely divided prior to compression. This technique however conditions that the components of material in the pellets produced are not adversely affected by the required, relatively high degree of compression. SE-A-453 554 (EP-A-0 250 381) discloses a process and an apparatus for freezing liquid or semiliquid foods in the form of essentially uniform pellets. The product is agitated within a container, while a liquid cooling gas is sprayed over the product. The partially frozen food product is then removed from the container before it is completely frozen but after formability has been achieved, and then is formed by extruding and cutting into essentially uniform pellets which are completely frozen outside the container. The object of the present invention therefore is to provide a pelletiser of the type stated in the introduction to this specification, which is able to make pellets without any of the shortcomings encountered in prior art techniques. According to the invention, a pelletiser of the type stated by way of introduction is characterised by a refrigerator for cooling at least one moulding roll so as to bring about surface freezing of the food product during moulding thereof into pellets. The moulding rolls suitably have teeth of complementary shape, and the teeth of at least one roll have axially spaced-apart recesses. In a preferred embodiment, the teeth of both rolls have recesses, the centre distance of which on each roll is at least equal to the axial width of the recesses, the recesses of one roll being axially located midway between the recesses of the other roll. The profile of the two rolls then is suitably identical. The recesses may advantageously have an undulating profile in an axial section of the teeth, and the teeth may further have a rounded shape, such that the moulded pellets will have a double saddle shape. To facilitate the removal of the moulded pellets from the recesses, these suitably extend throughout the entire tooth width in the circumferential direction and may be provided by turning. According to the invention, the method for making frozen pellets from a liquid or semiliquid food product is characterised by refrigerating the food product into mouldable consistency, feeding the mouldable food product into the nip between two cooperating moulding rolls, cooling the moulding rolls during the moulding operation so as to form surface-frozen pellets, and thereafter subjecting the pellets to final freezing. Suitably, the cooling of the moulding rolls is so adjusted that the moulded pellets are completely enclosed by a frozen crust. The refrigerator of the pelletiser preferably is arranged to cool both moulding rolls, suitably by means of a refrigerant which is supplied to a cavity in each roll. The cavity may advantageously be cylindrical, and the refrigerant, e.g. liquid nitrogen, can be injected through a nozzle provided in the cavity. The invention will now be described in more detail hereinbelow with reference to the accompanying drawings. Fig. 1 schematically shows an embodiment of a refrigerating device in which the inventive pelletiser is included. Fig. 2 is a side view in cross-section of an embodiment of the pelletiser according to the invention. Fig. 3 is a top plan view of a first embodiment of the rolls included in the pelletiser. Figs. 4 and 5 are a side view and a top plan view, respectively, of a second embodiment of the rolls included in the pelletiser. Fig. 6 is a longitudinal section of a preferred embodiment of one of the moulding rolls of the pelletiser. Fig. 7 is a cross-section taken along the line VII-VII in Fig. 6. In Fig. 1, there is shown a container 1 for a liquid or semiliquid product to be frozen in pellet form. The product in the container 1 can be discharged therefrom through a conduit 2 by means of a pump 3. The conduit 2 is connected to a nozzle 4 from which the product is caused to drop or flow down into one end of a second insulated container 5 having an agitator and a conveyor 6. The container 5 is elongate and accomodates a screw conveyor 7 extending in the longitudinal direction of the container 5 from an inlet end thereof at the nozzle 4 to an opposite outlet end. The screw conveyor 7 has at least one screw flight 8 mounted on a central drive shaft 9 in spaced-apart relation thereto, such that it also serves as agitator. The screw conveyor may advantageously include two parallel screws, each comprising one screw flight. At the outlet end of the container 5, an opening is provided in the bottom of the container 5. In a discharge conduit 10 connected to this opening, a screw conveyor 11 is mounted for feeding the product concerned through the discharge conduit 10 to a pelletiser 12 mounted at the outer end of the discharge conduit. In the upper part of the container 5, there are mounted a plurality of spray nozzles 13 which via conduits 14 can be supplied with a refrigerating gas in the liquid state, preferably liquid nitrogen. The nozzles 13 are such that the liquid refrigerating gas is sprayed in a very finely divided state over the product fed into the container 5. Further, electric motors 15, 16, 17 and 18 are provided for driving the pump 3, the agitator and the conveyor 6, the screw conveyor 11 and the pelletiser 12, respectively. The excess of refrigerating gas in the container 5 is conducted through a conduit 19 and a throttle valve 20, either into a gap 21, which covers at least the underside of the container 1, and from there into the container 1, or to an outlet 22. The arrangement described above operates as follows. The liquid or semiliquid precooled product is pumped by means of the pump 3 from the container 1 to the nozzle 4 and into the container 5. In the container 5, the product supplied is agitated while the liquid refrigerating gas is sprayed over it in a freezing zone between the inlet end and the outlet end of the container 5. Very small droplets of the liquid refrigerating gas will thus be distributed over the surface of the product in the agitator and the conveyor 6, providing quick and gentle freezing of the product. The simultaneous agitation of the product prevents the formation of too large ice crystals in the product, i.e. the fluidity of the product decreases as the product is conveyed slowly through the agitator and the conveyor 8. The refrigerating gas, passing into gaseous form when contacting the product, flows through the conduit 19 and the throttle valve 20 down into the gap 21 and from there into the container 1 for precooling the product contained therein. By suitably controlling the feeding speed of the conveyor screw 7 and the amount of refrigerating gas supplied in liquid form, it is possible to impart to the food product concerned the required mouldability at the outlet end of the conveyor 6. From the outlet end, the mouldable food product is then fed by the screw conveyor 11 through the discharge conduit 10 to the pelletiser 12. This drops the moulded pellets onto a conveyor 23 which conveys the food product in pellet form into a freezer 24 for final freezing. The required mouldability is such that the moulded pellets will essentially retain their shape during the transport on the conveyor belt 23 into the freezer 24. The pelletiser 12, shown in more detail in Fig. 2, is mounted on the outer end of the discharge conduit 10 and consists of a substantially parallelepepidal housing 25 provided in its top side with an inlet opening 26 to which the discharge conduit 10 is connected, and an outlet opening 27 in its bottom side. Within the housing 25, two rolls 28 and 29 are mounted for simultaneous rotation while engaging each other. More specifically, the rolls 28 and 29 have teeth of a complementary shape and are mounted on shafts 30, 31 in such a spaced-apart relationship that the teeth 32 and 33 of the rolls 28 and 29, respectively, mesh with each other when the rolls 28, 29 are rotated by means of the motor 18 (not shown in Fig. 2). In the embodiment of the pelletiser according to the invention shown in Fig. 2, the teeth 32 of the roll 28 are considerably wider than the teeth 33 of the roll 29. The teeth 32 are further formed with axially spaced-apart recesses 34. The shape of these recesses appears more clearly from Fig. 3. In a longitudinal section of the roll 28, the recesses 34 thus are arcuate, e.g. circular-arc shaped. Their depth is smaller than the height of the teeth 32, and their centre distance in the axial direction is at least equal to the axial width of the recesses 34. The teeth 33 of the roll 29 may be formed by milling. The teeth 32 of the roll 28 may also be provided by milling, whereupon the recesses 34 can be formed by turning. These two working operations may also be performed in the reverse order. For guiding the product supplied through the opening 26 in between the rolls 28, 29, two guide blocks 35, 36 are mounted in the housing 25 above the respective roll. The guide block 35 has a profile which is complementary to that of the roll 28 in Fig. 3, while the guide block 36 has a circular-arc shaped, concave surface 37, the radius of curvature of which substantially agrees with the maximum radius of the roll 29. Alternatively, the guide block may have the shape shown by dashed lines in Fig. 2. The product fed through the opening 26 can thus pass through the pelletiser 12 only between the rolls 28, 29. Below the roll 28, a doctor blade 38 is fixedly mounted. It has the same profile as the guide block 35. Below the roll 29, a doctor blade 39 is mounted on a shaft 40 and biased by a spring clockwise. The shaft 40 in turn is mounted on a shaft 40′ and also biased by a spring clockwise. When the roll 29 rotates clockwise, the doctor blade 39 will thus follow the outer circumferential surface of the roll 29 as a cam. The pelletiser 12 shown in Figs. 1-3 operates as follows. The product, plastically mouldable by partial freezing and supplied through the opening 26, will fill the space above the rolls 28, 29 and between the guide blocks 35, 36. When the rolls 28, 29 rotate anticlockwise and clockwise, respectively, pellets will be moulded from the product in the spaces which are formed by the almost complete closure of the recesses 34 by the roll 29, due to the cooperation between the teeth 32 of the roll 28 and the tooth gaps of the roll 29. The feed pressure should then be relatively high, such that the exposed recesses and tooth gaps are completely filled. The recesses 34 in the roll 28 are again exposed as the engagement between the tooth 32 on the roll 28 and the corresponding tooth gap on the roll 29 ceases. Thus, the resulting pellets will drop through the opening 27 onto the conveyor 23 for conveyance into the freezer 24. The doctor blades 38 and 39 ensure that the moulded pellets drop through the opening 27, preventing them from accompanying either of the rolls 28, 29 back to the inlet opening 26. Figs. 4 and 5 show an alternative design of rolls 41, 42 having teeth of identical profile. In this embodiment, the teeth of both rolls 41, 42 have recesses 43 and 44, respectively, the centre distance of which on each roll is at least equal to the axial width of the recesses 43, 44. Further, the recesses 43 of one roll 41 are axially located midway between the recesses 44 of the roll 42. The area of the recesses 43 (like that of the recesses 34) suitably is larger than the area of the surface closing the recesses, thereby facilitating the release of the pellets from that surface. To the same end, the side walls of the recesses may closely join radial planes through the respective roll and may also have axial grooves which, suitably, are also formed by turning. The rolls 41 and 42 can also be manufactured by milling and turning, the recesses (as in the roll 28) extending throughout the entire tooth width. It is evident that several different profiles are possible for the rolls included in the pelletiser. The teeth suitably have such a profile that they will slide relative to each other during meshing. The tooth height may advantageously also be reduced relative to the roll diameter, for instance be 1/10-1/20 thereof. It is also essential that the shape of the rolls is such that no substantial compression takes place of the plastically mouldable material to be formed into pellets. Several modifications of the pelletiser described above are possible within the scope of the invention as claimed. Thus, the doctor blade 39 may be replaced by an axially extended wire which is caused to follow the outer circumferential surface of the roll 29. The roll length may also be varied from the diameter of the screw conveyor 11 up to the width of the conveyor 23. In another embodiment of the rolls 41, 42, the recesses 43, 44 are deeper than the height of the teeth, and the teeth have such a shape that each recess 43, 44, in a certain position of rotation of the rolls 41, 42, is completely closed, with the exception only of the surface portion between adjacent recesses in the same radial plane, which is the result of the recess depth being larger than the height of the teeth. When the pelletiser is used for making partially frozen products, it may of course be combined with other types of freezers than that shown in Fig. 1. Further, it may advantageously be connected directly to the discharge end of the screw conveyor 11, without using any discharge conduit 10. Figs. 6 and 7 show the most preferred embodiment of the moulding rolls in the pelletiser according to the present invention. These Figures show only one roll 45, but the roll cooperating therewith is of identical design. The roll 45 has a cylindrical cavity 46 and is rotatably mounted on a fixed shaft 49 by means of bearings 47, 48. On the roll 45 there is fixedly mounted a sprocket 50, by means of which the roll 45 is rotated. From one end of the shaft 49 extends an axial bore 51 through the shaft and over the entire length of the cavity 46. Radial through holes 52 are provided in the shaft 49 within the extent of cavity 46. The holes 52 thus connect the cavity 46 to the bore 51. An outlet pipe 53 is connected to the open end of the bore 51. Further, a separate conduit 54 extends through the bore 51 and out through one of the holes 52 and is terminated in the cavity 46 by a nozzle 55. During the operation of the pelletiser according to Figs. 6 and 7, a refrigerating medium is supplied through the conduit 54 and sprayed through the nozzle 55 into the cavity 46 where it will thus cool the outer wall of the cavity 46 and, consequently, the teeth of the roll 45. The refrigerating medium, which suitably is liquid nitrogen, is then gasified and led through the openings 52 and the bore 51 out through the outlet pipe 53. By means of the above-described embodiment of the moulding roll 45 and the roll cooperating therewith, the food product, when being moulded into pellets between the moulding rolls, will be refrigerated to such an extent that the moulded pellets will have a frozen crust of a thickness of one or a few millimetres, this facilitating the removal of the moulded pellets from the moulding rolls and ensuring that the moulded pellets retain their shape. Final freezing of the moulded pellets then suitably takes place, as previously described, in a separate freezer of optional type. To obtain a suitable crust thickness of the moulded pellets, the temperature of the moulding roll 45 is suitably sensed by means of a suitable sensor, and the flow of refrigerant is fed through the conduit 54 into the cavity 46 depending on the temperature sensed, such that it is maintained at a value required for the desired crust thickness.

Claims: 1. A pelletiser for making pellets from a liquid or semiliquid food product which has been refrigerated to mouldable consistency, said pelletiser having two moulding rolls (28, 29; 41, 42; 45) between which the food product is formed into pellets, characterised by a refrigerator (54, 55) for cooling at least one moulding roll (45) so as to bring about surface freezing of the food product during moulding thereof into pellets. 2. Pelletiser as claimed in claim 1, characterised in that said one moulding roll (45) has a cavity (46) into which said refrigerator (54, 55) feeds a refrigerant. 3. Pelletiser as claimed in claim 2, characterised in that said one moulding roll (45) is rotatably mounted on a fixed shaft (49), through a bore in which said refrigerant is fed into the cavity (46) of said roll (45). 4. Pelletiser as claimed in any one of claims 1-3, characterised in that the rolls (28, 29; 41, 42; 45) have teeth of complementary shape and identical profile, that the teeth of both rolls have axially spaced-apart recesses (43, 44), the centre distance of which is at least equal to the axial width of said recesses, and that the recesses of one roll are axially located midway between the recesses of the other roll. 5. Pelletiser as claimed in claim 4, characterised in that the radial depth of the recesses (43, 44) is larger than the height of the teeth. 6. Pelletiser as claimed in claim 4 or 5, characterised in that the recesses (43, 44) have an arcuate profile in an axial section of the teeth. 7. Pelletiser as claimed in any one of claims 4-6, characterised in that the teeth (32, 33) have a rounded shape. 8. Method for making frozen pellets from a liquid or semiliquid food product, characterised by refrigerating the food product into mouldable consistency, feeding the mouldable food product into the nip between two cooperating moulding rolls, cooling the moulding rolls during the moulding operation so as to form surface-frozen pellets, and thereafter subjecting said pellets to final freezing. 9. Method as claimed in claim 8, characterised in that the cooling of the moulding rolls is so adjusted that the moulded pellets are completely enclosed by a frozen crust. 10. Method as claimed in claim 8 or 9, characterised in that said cooling is carried out with liquid nitrogen.

Inventors: ANDERSSON, ALVAR
Applicants: FRIGOSCANDIA FOOD PROCESS SYSTEMS AKTIEBOLAG
Application year: 1989
Main classification: A23L3
IPC codes: B01J2, A23L3, A23P1, B30B11, B30B15