FIELD OF THE INVENTION

The present invention relates to a rotatable knife, a rotatable knife and hole plate combination, a grinder with axial and radial separators for separating hard constituents from food or feed products being ground. The invention also relates to a method of separating hard constituents from a food or feed product being ground by a grinder comprising the rotatable knife and having an axial separator and a radial separator for separating hard constituents from food or feed products being ground.

BACKGROUND OF THE INVENTION

Grinders for grinding meat and other products, particularly food products, where the grinder has a separator for separating hard constituents from other constituents are known. Herein, we refer to the hard constituents as B-products and the remaining, typically higher value product for A-products. When grinding food objects containing both A-products and B- products, it is an object to obtain good separation to avoid B-products in the A-products. By means of an example, the food object could be meat containing bone, gristle, and sinew. When such meat is grinded, it is important to separate the bone and gristle (B-product) from the meat and fat (A-product) and thereby provide an A-product which can be used inter alia in sausage manufacturing.

Prior art grinders with a separator have certain disadvantages, inter alia relative to the capacity and to the ability to efficiently separate the B-product from the A-product. If they are not capable of separating efficiently the hard constituents, it reduces the quality of the A- product. Particularly, if a large amount of hard constituents is present in the food or feed material to be ground, the lacking capacity can lead to blocking and/or a reduced quality of the A-product when B-product is not separated therefrom, and/or reduced yield when separating too much from the A-product, i.e. when A-product are separated with the B- product and therefor sold at very low price or discharged.

Also, the prior art grinders can be blocked by hard constituents accumulating in the knife house, and they sometimes produce a low value product of Mechanically Separated Meat i.e. meat where muscle fibre structure is lost or modified by excessive grinding. This reduces the value of the A-product.

When a grinder is blocked, the screw may continue working, and the meat gets heated. This causes smearing, and the meat must be discharged due to the heating process.

OBJECT OF THE INVENTION

It is an object of embodiments of the invention to overcome at least some or all the above problems related to grinders and separation of B-product from A-product in a grinder. It is a further object to increase the yield and the quality of the A-product and reduce the downtime of grinders.

SUMMARY OF THE INVENTION

According to these and other objects, the invention, in a first aspect, provides a rotatable knife configured for rotation in a direction of rotation about a rotor axis. By definition herein, the rotor axis extends in an axial direction.

The rotatable knife comprises a centre knife part, a peripheral ring forming an outer circumference, and at least two first arms extending between the centre knife part and the peripheral ring.

By definition herein, the arms define a leading edge toward the direction of rotation, i.e. facing forward when the rotatable knife rotates in the intended direction. The arms define a trailing edge at the opposite side, i.e. the side facing rearward relative to the direction of rotation.

A knife edge is provided on the arms for cutting meat or hard constituents.

Each first arm defines an elevation which projects in the axial direction above the centre knife part and above the peripheral ring.

The knife edge terminates the elevation in the axial direction and the centre knife part defines a central knife cavity with a floor which is recessed with respect to a height of the first arms - meaning that there is a surface portion, herein referred to as "floor" on the centre knife part which is at a lower height than the elevation of the arms, i.e. it is recessed. Due to the elevation above the centre knife part and above the peripheral ring, and due to the recessed floor of the centre cavity, B-products can be separated both in a radial direction over the peripheral ring, and in an axial direction via the centre cavity. This facilitates a better flow of the B-product away from the knife edge and thereby improves the ability to efficiently separate B-product from A-product.

The rotatable knife is configured for rotation which means that it is balanced and can rotate as a rotatable knife in a grinder. The direction of rotation defines what is seen as the leading edge and trailing edge of the arms. In use, the knife is therefore typically to be rotated in a specifically intended rotation direction about the rotor axis.

Since the centre knife part may be considered as a hub of a wheel, the arms as spokes of the wheel, and the peripheral ring as the rim of the wheel, the structure not only facilitates separation, but it also provides a solid and potentially rigid structure. The arms could be made in one piece with the centre knife part and the peripheral rim, e.g. by moulding, and the knife edge could constitute a machined edge of the one-piece moulded component, or they may be separate elements attached to the moulded component.

The recessed floor could be a flat/plane surface, or it could have any desired shape providing a cavity into which the B-product can be discharged for axial separation. The knife cavity floor may be recessed to a level of at least Vs, ¹ /4, ! , %, ¹ /2, %, or % of the height of the arms, where height is the dimension in the axial direction.

The knife edge may extend along the leading edge such that the knife edge can slide against a hole plate during a grinding process.

The leading edge may project above the trailing edge in the axial direction. This may be provided e.g. when the elevation is formed exclusively at the leading edge, or at least not at the trailing edge.

In a zone herein referred to as "tangential zone" located between the leading edge and the trailing edge, the arm my reduce the axial height stepwise, i.e. an abrupt from a heigh axial dimension to a low axial dimension. Alternatively, the arm my reduce the axial height smoothly, e.g. linearly or in a curved height reduction.

The elevation may have the same height above both the peripheral ring and the centre part. However, to increase the ability of the B-product to separate radially, the elevation may be higher above the peripheral ring than above the centre knife part. The elevation, or more particularly the knife edge, may comprise an outer arm section, an inner arm section, and an intermediate arm section between the outer arm section and the inner arm section.

The intermediate arm section may protrude in the direction of rotation relative to the outer and inner arm sections. I.e. the outer and inner arm sections extend from the intermediate arm section in an inclined and a partly rearward direction relative to the direction of rotation such that the elevation or particularly the knife edge forms a V-shape or a U-shape when viewed in the axial direction.

The intermediate arm section of the first arms may have an angle 'B' between the inner arm section and the outer arm section, the angle optionally being above 90 degrees, such as above 100 degrees, such as above 110 degrees. The inner arm section of the first arm has an angle 'A' to a centre knife part, this angle is preferably below 90 degrees, such as about 85 degrees.

The inner arm section of the first arm may have an angle to a tangent of an outer circumference of the central knife cavity, this angle is preferably 90 degrees or below 90 degrees, such as about 85 degrees.

The outer arm section, the inner arm section and the intermediate arm section in cooperation and during rotation splits the hard constituents present adjacent to the first arms of the rotatable knife into two parts. The inner arm section being arranged to direct a first part of the hard constituents towards the central knife cavity, and the outer arm section being arranged to direct a second part of the hard constituents towards the outer circumference of the rotatable knife.

The outer arm section, or particularly the knife edge of the outer arm section, may extend straight from the intermediate arm section to a point at the peripheral ring where the elevation terminates. Alternatively, the outer arm section may extend in a curved shape from the intermediate arm section to a point at the peripheral ring where the elevation terminates.

The inner arm section, or particularly the knife edge of the inner arm section, may extend straight from the intermediate arm section to a point at the centre knife part where the elevation terminates. Alternatively, the inner arm section may extend in a curved shape from the intermediate arm section to a point at the centre knife part where the elevation terminates. The intermediate arm section may be curved or sharp pointed thereby forming above mentioned U-shape or V-shape.

The rotatable knife may comprise at least one second arm extending from the peripheral ring and terminating at an intermediate location in a space between the peripheral ring and the centre knife part. The at least one second arm is thereby shorter in the radial direction than the first arms.

The rotatable knife may have four arms, five arms, six arms or more, and the number of arms may be even or uneven, if the rotational balance of the knife is maintained to avoid generating vibrations. All arms of a rotatable knife may be first arms only, or the rotatable knife may comprise both first arms and second arms.

Particularly, the rotatable knife may comprise a plurality of first arms and second arms arranged circumferentially and sequentially with the second arms arranged between the first arms.

The second arm(s) define(s) a leading edge toward the direction of rotation and a trailing edge rearward relative to the direction of rotation and comprising at least one intermediate knife edge for cutting meat or hard constituents.

The intermediate knife edge may be along the leading edge of the second arm.

Each second arm may define an elevation projecting in the axial direction above the centre knife part and the peripheral ring. The intermediate knife edge may terminate the elevation in the axial direction such that the knife edge can slide against a hole plate during a grinding process.

The second arms may further be arranged to direct hard constituents towards the elevation of the first arms or, alternatively, outwards towards the periphery of the knife. The second arms are preferably arranged between adjacent first arms. The peripheral ring may connect the arms e.g. connecting one second arm with two first arms.

The second arms may be arranged and shaped such that they can direct the B-product towards any openings in the peripheral wall of a knife housing in which the rotatable knife rotates. This, radially separated, B-product can then enter a radial separator. The elevation of the second arms may extend straight from the intermediate location to a point at the peripheral ring (38) where the elevation terminates, or they may have a curved shape.

The central knife cavity may extend annularly about a central drive hole which facilitates attachment of a drive shaft of a motor, e.g. via a screw feeder feeding food or feed product to the rotatable knife.

In a second aspect, the invention provides a rotatable knife and hole plate combination comprising a rotatable knife as described herein, where the rotatable knife is located upstream of the hole plate seen in a material transport direction through the rotatable knife and hole plate combination, wherein, the central knife cavity makes up openings between the rotatable knife and the hole plate.

The rotatable knife and hole plate combination may comprise a rotatable knife as described herein relative to the first aspect.

During operation, the rotatable knife is located upstream of the hole plate seen in a material transport direction through the rotatable knife and hole plate combination and with cutting edges of the arms adjacent the hole plate. The central knife cavity forms at least one opening between the rotatable knife and a central cavity of the hole plate, such that, when the rotatable knife and the hole plate is in operation, the central knife cavity is in fluid communication with the centre hole plate cavity of the hole plate. The openings are further defined by the inner arm sections of the first arms. Further, the hole plate may have a centre hole plate cavity having an area larger than an area of the driving hole of the rotatable knife, to maximize the material flow through the central knife cavity through the centre hole plate cavity of the hole plate.

The floor may be circular, and a diameter of the floor may be substantially equal to a diameter of the centre hole plate cavity of the hole plate, since having a larger diameter does not greatly improve the material flow. The centre hold plate cavity of the hole plate may extend from the centre of the plate and to just inside of the innermost hole plate holes. The first knifes may have a length making it capable when in function to cut along a length from the innermost hole plate holes to the outermost hole plate holes, this length correspond to the U- or V-shape of the first knife.

Further, the first arms and the second arms may be the only material of the rotating knife that is making a distance between the rotatable knife and the hole plate. The knife and hole plate combination may further have a drive shaft for driving the rotatable knife, and wherein the hole plate is stationary. The central knife cavity may be adjacent to and surrounding the drive shaft in a location downstream of the knife and through the centre of the hole plate.

In a third aspect, the invention provides a grinder for grinding food or feed products comprising hard constituents, such as meat with bones and/or sinews and/or cartilage, the grinder comprises:

• A compression housing with a feed screw driven by a first motor, the feed screw directing food or feed products from an infeed end of the grinder towards an outfeed end of the grinder, and

• A knife housing having at least one set of a rotatable knife as described herein, and a stationary perforated hole plate in combination, the knife housing being located downstream of the compression housing, and

• One or more radial separators connected to a wall of the knife housing, for separating hard constituents from food or feed products being ground and

• An axial separator connected to the knife housing at the outfeed end for separating hard constituents from food or feed products being ground.

By connecting both a radial separator, or more radial separators, and an axial separator to a grinder this increases the amounts of hard constituents which can be removed when compared to having only one radial separator or one axial separator connected to a grinder. Also including the rotatable knife described herein further increases the amount of removed hard constituents, and it is possible to run the grinder with both radial separator(s) and an axial separator without smearing and/or clogging. The produced ground meat has as indicated a lower amount of hard constituents which increases the meat quality compared to meat having a higher amount of hard constituents. The quality is also increased as the obtained ground meat is a ground product where the fibre structure is still present in the product, which at least in some countries e.g. European countries increases the value of the meat.

Different kinds of food or feed products may be ground in a grinder described herein, however, as an example focus will be on meat and production of ground meat, especially ground beef meat. The meat products to be ground in the grinder may contain up to 25% hard constituents, such as 10-25%, e.g. 10-20% hard constituents. The grinder with the rotatable knife as described herein produces more high value ground meat than prior art grinders resulting in increased income as more ground product, such as ground meat, is obtained and can be sold as a high quality product.

Also, some grinders of the kind with a feed screw in a compression housing is described to be sorting off hard constituents under high pressure. However, 'sorting under high pressure' produces MSM (Mechanically Separated Meat) with loss or modification of muscle fibre structure. MSM is a low value product. With the present invention ground meat can be produced without reaching a pressure where the muscle fibre structure is lost and the value may thereby increase.

By 'hard constituents' is meant hard material normally being present in the food or feed product to be ground, though this material is not to be present in the final ground product, or at least the amount is to be minimized. 'Hard constituents' may also include foreign material. For meat to be ground, the hard constituents can be animal material such as bone fragments, cartilage/gristle, sinews/tendons, membranes, and it may also include non-animal material/foreign objects such as plastic and/or metal pieces.

The grinder may have a stationary perforated hole plate located at the outfeed end, and this hole plate may have an opening in the centre part. This opening may be connected to an axial outlet tube. Hereby hard constituents can be led from the knife housing to the axial separator.

The axial separator may comprise a separator screw located inside the axial outlet tube and being capable of directing hard constituents away from the knife housing. Preferably a second motor drives the separator screw. The hard constituents in the axial outlet tube is directed to a hard constituent outlet from where the hard constituents can be collected e.g. in boxes and be discharged or used for other purpose. It has been observed that the material of the hard constituents, when grinding meat with the grinder described herein, surprisingly does not include much soft or wet components, hereby the hard constituents make up a rather dry product.

The at least one radial separator of the grinder may be two or more radial separators which may be similar in their construction. The radial separator may comprise a radially located outlet tube connected to an opening in the knife housing. This opening is in the knife housing wall surrounding and adjacent to the circular knifes as described herein, and is preferably next to i.e. tangential to the most downstream knife in the grinder, however radial separators may also be located tangential to a rotating knife not being the most downstream knife in the grinder. Preferably, a radial separator is located radial to a rotating knife as hereby the rotating knife can exert a pressure onto the hard constituents being directed into a radial outlet tube.

Radial separators may, for letting hard constituents out of the separators, be controlled by different systems, such as by a valve control, where a valve controls the radial outlet tube such that hard constituents accumulated in the radial outlet tube are led away from the knife housing and out of the tube when the valve is open and prevented from leaving the tube when the valve is closed. The valve may be controlled by a valve controller which may e.g. control an air pressure towards a valve.

In the grinder, at least one rotatable knife is located upstream of the stationary perforated hole plate. The rotatable knife has a circular outer circumference. By the angle between the inner arm section and outer arm section, the rotatable knife is configured to direct a first portion of hard constituents towards a central axis of the knife housing, and configured to direct a second portion of hard constituents towards the inner wall of a tube-formed knife housing where an opening leads to a radial outlet tube. The knife, during rotation, exerts a pressure on the hard constituents, such that the hard constituents are directed towards and hereby into the radial outlet tube, i.e. the radial separator.

The grinder may have e.g. one, two, three or four radial separators. All radial separators may be located to receive hard constituents from the same rotatable knife i.e. all radial separators are located at the same circumference line or section of the knife housing. Alternatively, the radial separators may be located to receive hard constituents from e.g. two rotatable knives, i.e. at two different circumference line or sections of the knife housing. As an example, the grinder may have an axial separator and two radial separators, where the two radial separators may be located on opposite sides of a tube-formed knife housing, and the two openings in the knife housing may e.g. be in each end of one diameter line separating the circular form of the knife housing. The radial separators may be connected to openings in the knife housing, where each opening is associated with a separate rotatable knife for each radial separator.

Adjustments of the axial and radial separators may be made to adjust the amount of material including hard constituents being directed out through the separators. Adjustment of the axial separator may be performed by adjusting the rotation speed of the screw in the axial outlet tube, hereby a motor connected to the screw in the axial outlet tube may be used for regulating the rotation speed of the screw in the axial outlet tube. A high rotation speed of the screw in the axial outlet tube removes more material than if the speed is low. The feed screw in the compression housing and the screw in the axial outlet tube may be controlled independently to remove a pre-determined amount of hard constituents from each screw. For a radial separator an adjustment device may also be a screw in the radial outlet tube as described in relation to the axial separator. A radial separator may also include an adjustment device that may be connected to the radial outlet tube to adjust e.g. the opening area of the opening directing the hard constituents from the knife housing to the radial outlet tube. The opening area can then be increased or decreased if more or less material including hard constituents is to be removed from the ground material. The opening may be closed completely, when it is desirable to not remove hard constituents via the radial outlet tube.

A stationary perforated hole plate may be located at the outfeed end, and this hole plate preferably has an opening in the centre part. A stationary hole plate located downstream of the rotatable knife may have holes for ground meat to be directed towards an outlet end of the grinder, such holes may have a dimension of e.g. between 3-5 mm.

A rotatable knife according to the invention may be used as a retrofit to an existing central hole separator, since only the knives need to be changed and a radial hole arranged in the grinder housing. No change to central discharge hole plates need to be necessary.

In a fourth aspect, the invention provides a method for grinding food or feed products by use of a rotatable knife grinder as described herein, the method comprising a) Obtaining food or feed products comprising hard constituents, b) Directing the food or feed products into the infeed of the grinder, c) Grinding the food or feed products, wherein d) a first portion of hard constituents in the ground food or feed product is led into a radial separator, and e) A second portion of hard constituents in the ground food or feed product is led into an axial separator, and f) Obtaining ground meat with reduced amounts of hard constituents at the outfeed end of the grinder. The hard constituents may comprise animal material such as bone fragments, cartilage/gristle, sinews/tendons, membranes, or non-animal material/foreign objects such as plastic or metal.

As described above, the amounts of hard constituents removed from the food or feed product is impressive and the result is a ground product e.g. ground meat with a high quality.

In the method the food or feed products to be ground are preferably animal products such as meat products from pig, cattle, poultry or fish. Preferred is meat products from pig, cattle, poultry or fish. However, also vegetables can be ground by the described grinder.

In the method the hard constituents which are removed may be animal material such as bone fragments, cartilage/gristle, sinews/tendons, membranes and/or non-animal material/foreign objects such as plastic or metal. If processing vegetable the hard constituents may comprise seeds, vegetable stones e.g. plum stone, stems, etc. and/or nonvegetable material/foreign objects such as plastic, stone or metal.

In a sixth aspect, the invention provides a method for retrofitting a grinder for grinding food or feed products comprising hard constituents, such as meat with bones and/or sinews and/or cartilage. The grinder which is to be retrofitted comprises:

• A compression housing with a feed screw driven by a first motor, the feed screw directing food or feed products from an infeed end of the grinder towards an outfeed end of the grinder, and

• A knife housing having at least one set of a rotatable knife of the traditional kind, and a stationary perforated hole plate in combination, the knife housing being located downstream of the compression housing, and

• One or more radial or axial separators.

According to the sixth aspect, this grinder is retrofitted with at least one set of a rotatable knife according to the first aspect, and with an additional separator such that the resulting grinder comprises both at least one radial separator and an axial separator connected to the knife housing at the outfeed end for separating hard constituents from food or feed products being ground. LEGENDS TO THE FIGURE

Fig. 1 illustrates a schematic side or top view of a grinder, with a radial separator and an axial separator;

Fig. 2 illustrates an elevated perspective side view of a circular knife;

Fig. 3 illustrates a side view of the circular knife shown in Fig. 2;

Fig. 4 illustrates a top plan view of the circular knife shown in Fig. 2; and

Fig. 5 illustrates a sectional view along A-A of Fig. 4.

Fig. 6 illustrates a perspective view from the knife side of a rotatable circular knife and stationary hole plate combination;

Fig 7 illustrates a view along a central cut of Fig. 6;

Fig. 8 illustrates a perspective view from the hole plate side of a rotatable circular knife and stationary hole plate combination;

Fig. 9 illustrates a plan view of a half of the rotatable circular knife and stationary hole plate combination of Fig. 8;

Fig. 10 illustrates a perspective view from the knife side of a partial cut of the rotatable circular knife and stationary hole plate combination of Fig. 8;

Fig. 11 illustrates a perspective view from the hole plate side of a partial cut of the rotatable circular knife and stationary hole plate combination of Fig. 8;

Fig. 12 discloses details of the inner, outer, and intermediate arm sections;

Fig. 13 discloses a cross section of an arm; and

Fig. 14 illustrates an alternative embodiment with a different shape of the tangential zone of the arms. DETAILED DISCLOSURE OF THE INVENTION

Fig. 1 illustrates a grinder 1 with a radial separator 14 and an axial separator 15. The grinder

I grinds food or feed products 6, such as meat pieces. The grinder 1 has a compression housing 2 with a feed screw 3 driven by a first motor 4. The feed screw 3 directs food or feed products 6 from an infeed end 5 of the grinder 1 towards an outfeed end 7 of the grinder 1. A knife housing 8 is located downstream of the compression housing 2 and secured to the compression housing 2. The knife housing 8 is illustrated with a number of rotatable knives 9,

II and a number of stationary perforated hole plates 10, 12. The rotatable knives are driven by the first motor 4 via a central driving hole 40 (see Fig. 2) arranged in each knife, the central driving holes having a shape corresponding to the shape of a drive axle (not shown) connected to the first motor. The direction of flow of product 6 e.g. meat to be ground is indicated by the arrow 13. A radial separator 14 is connected radially to a wall of the knife housing 8, for separating hard constituents from food or feed products being ground. A radial outlet tube 16 directs hard constituents radially out of the knife housing 8. A valve 17, controlled by a valve controller 18, controls when hard constituents collected in the radial outlet tube 16 are directed to a first hard constituent outlet 19 from where hard constituents 24 can be collected and e.g. discharged. An axial separator 15 is connected axially to the knife housing 8 at the outfeed end 7 for separating hard constituents from food or feed products such as meat 6 being ground. An axial outlet tube 20 is connected axially to the knife housing 8. A screw 21 in the axial outlet tube is driven by a second motor 22 to direct hard constituents away from the knife housing 8. The hard constituents are directed into a second hard constituent outlet 23 from where the hard constituents 24 can be collected and e.g. discharged. Illustrated is also meat 6 to be ground in the compression housing 2 and that such meat 6 to be ground, when processed by the knives 9, 11 and hole plates 10, 12 leave the grinder axially as ground meat 25 through a final stationary hole plate 42, optionally having finer holes than the holes of other hole plates used in the grinder. This hole plate may be the hole plate 12. In the figure the knives 9, 11 and hole plates 10, 12 are illustrated as if space is located between these, however in sets (pairs) a knife and a hole plates are usually located close to each other with the hole plate in a downstream direction. The grinder 1 as illustrated can be connected to a system including e.g. a hopper (not shown) located upstream for receiving food or feed products to be processed. A further screw (not shown), or other feeding mechanism, may also be positioned upstream the screw 3 for directing food or feed products from a possible hopper to the screw 3. More than one radial separator may be located on the grinder, e.g. two radial separators.

Figs. 2 to 5 illustrate a rotatable circular knife 9, 11 of a grinder 1, the circular knife can be a second knife 11 i.e. the last knife in the grinder. This second knife 11 may be used in a grinder 1 as illustrated in Fig. 1. The second knife 11 in Fig. 2 is illustrated with a view from downstream the second knife 11 when located in the knife housing 8. The knife 11 is illustrated with a number of first arms 26, and second arms 27. The first arms 26 and the second arms 27 are in this example located alternately. The arms 26, 27 each comprises an inner cutting edge 31, for cutting meat passing through holes of an adjacent hole plate (not illustrated). Between each of the arms 26, 27 space is present. At the circumference 38 of the knife, knife material 32 is present between the arms 26, 27 at least if the second arms are not connected to the centre knife part 42. The height of the knife material 32 between the arms 26, 27 at the circumference 38 is lower than the height of the arms 26, 27. When located in the knife housing 8 of the grinder 1 the rotatable circular knife 9, 11 is located such that the part of the arms 26, 27 which is illustrated above the knife material 32 is positioned to run inside of an opening in the knife housing 8 to which opening a radial outlet tube 16 is positioned. Knife material 32 bridging the arms 26, 27 at the circumference 38 need not be present if the knife only has first arms 26 connected to the centre knife part 42.

An arrow 34 indicates the rotating direction of the knife 11. The first arms 26 have an inner arm section 29 and an outer arm section 30 connected to each other in an intermediate arm section 33 which has an angle 'B' 36 (Fig. 4) which preferably is greater than 90 degrees. The inner arm section 29 of the first arm 26 has an angle 'A' 35 (Fig. 4) to an inner arm section of the knife 11, this angle 35 is preferably smaller than 90 degrees.

When grinding food or feed products, the knife 11 is driven in the direction of the arrow 34, causing the intermediate arm sections 33 of the first arms 26 to separate at least the hard constituents present and adjacent to the first arms 26 into two parts.

One part of hard constituents is directed along the inner arm section 29 towards the centre of the knife, and to the central knife cavity 43 in the centre knife part 42 (Fig. 4) and further through an opening 49 and a centre hole plate cavity 45 (Fig. 6-11) of the stationary hole plate 12, where the one part of hard constituents can enter the axial separator 15 illustrated in Fig. 1. The inner arm section 29 is arranged such that an extended line along the inner arm section 29 towards the central driving hole 40 intersects the central driving hole 40 to the left in the figure, that is, the angle formed between the inner arm section 29 and a line through the rotational centre of the central driving hole 40 and the intermediate arm section 33 of the first arm 26 is less than 45 degrees, to facilitate the movement of hard constituents towards the at least one axial opening 41 of the stationary hole plate 12. The dotted line 41 extends in the axial direction.

The other part of the hard constituents is directed towards the outer part of the knife 11 along the outer arm section 30 of the first knife. An amount of the hard constituents enter into the radial outlet tube 16 each time an arm 26, 27 does not cover the entrance to the radial outlet tube 16. The outer arm section 30 is arranged such that an extended line along the outer arm section 30 towards the radial outlet tube 16 intersects a line through the tangent of the outline 38 of the knife 11, at a beginning of the radial outlet tube, at an angle that is less than 90 degrees, to facilitate the movement of hard constituents towards the radial outlet tube. When an arm 26, 27 passes by the entrance to the radial outlet tube 16, the outer cutting edge 39 will cut into the hard constituents and let the part in the radial outlet tube 16 be further directed into the radial outlet tube 16.

Thus, a rotatable knife 9, 11 according to the invention has preferably at least two arms 26, 27, each arm having at least one knife edge 31 for cutting meat or hard constituents. A central knife cavity 43 is arranged at a centre knife part 42 of the rotatable knife 11 at a downstream side of the rotatable knife. The central knife 43 cavity is defined by a floor 46 that is recessed with respect to a thickness 47 of the arms 26, 27 of the rotatable knife. The central knife cavity 43 may be annularly arranged about a central drive hole 40 of the knife

9, 11.

In Fig. 2 openings 49 are illustrated by dotted lines. The upper part of these openings 49 is where a hole plate 12 would be located when in the grinder 1. The openings 49 are where hard constituents are directed by the first arms 26 towards the central knife cavity 43 which is in open connection with the centre hole plate cavity 45. The large volume of the central knife cavity 43 and centre hole plate cavity 45 are illustrated in Fig. 11. These cavities surround a drive axle (not shown) going through the driving hole 40.

Figs. 6 to 11 show a rotatable knife 9, 11 and hole plate 10, 12 combination. Reference numbers correspond to those used for Figs. 1 to 5, unless indicated, and where holes 37 are holes of hole plate 10, 12.

The rotatable knife 9, 11 and hole plate 10, 12 combination comprises a rotatable knife 9, 11, where the rotatable knife is located upstream of the hole plate seen in a material transport direction 13 through the rotatable knife and hole plate combination. The central knife cavity 43 makes up openings 49 between the rotatable knife 9, 11 and the hole plate

10, 12. When the rotatable knife and the hole plate is in operation, the central knife cavity 43 is in fluid communication with a centre hole plate cavity 45 of the hole plate 10, 12. The openings 49 are further defined by the inner arm sections 29 of the first arms 26. The hole plate 10, 12 has a centre hole plate cavity 45 having an area being larger than an area of the driving hole 40 of the rotatable knife 9, 11. A diameter of the floor 46 is substantially equal to a diameter of the centre hole plate cavity 45 of the hole plate 10, 12. The first arms 26 and the second arms 27 are the only material making a distance between the rotatable knife 9, 11 and the hole plate 10, 12.

The knife and hole plate combination further comprises a drive shaft (not shown) for driving the rotatable knife 9, 11 via the driving hole 40. The hole plate 10, 12 is stationary, i.e. it does not rotate with the drive shaft.

Figs. 12 and 13 illustrate further details of the angle B, indicated by numeral 36 in Fig. 4. The angle can be varied depending on the need for increased or reduced transport of B-product to the radial and axial separator. The angle B is illustrated relative to the dotted lines following the knife edges of the inner and outer arm sections 29, 30.

Fig. 12 also illustrates that the knife edge of the inner and the outer arm sections 29, 30 are straight whereas the knife edge at the intermediate arm section 33 is circular and protrudes forward in the direction 34 of rotation relative to the inner and outer arm sections 29, 30.

Fig. 12 also illustrates a zone referred to as "tangential zone". This zone is indicated by the arrow 50 and shown in a cross section in fig. 13 located between the leading edge 51 and the trailing edge 52.

Fig. 13 illustrates how the arm is reduced in the axial height in the tangential zone. In the embodiments of Figs. 12, 13, the reduction is linear, i.e. the arm has a tapered shape indicated by the line 53.

In the embodiment of Fig. 14, the reduction is stepwise, i.e. in the direction from the leading edge to the trailing edge, there is an abrupt change from a large height at the leading edge and thereby at the knife edge, to a small height at the trailing edge, away from the knife edge. The abrupt change defines a sharp corner 54.

LIST OF NUMBERED EMBODIMENTS

1. A rotatable knife (9, 11) comprising : at least two arms (26, 27), each arm having at least one knife edge (31, 39) on one side of the rotatable knife for cutting meat or hard constituents, wherein a central knife cavity (43) is arranged at a centre knife part (42) of the rotatable knife (11) at a downstream side (44) of the rotatable knife or at the same side of the knife edges (31, 39) of the at least two arms (26, 27), the central knife cavity being defined by a floor (46) that is recessed with respect to a height (47) of the arms (26) of the rotatable knife.

2. The rotatable knife (9, 11) according to embodiment 1, wherein the central knife cavity (43) is annularly arranged about a central drive hole (40) of the knife (9, 11).

3. The rotatable knife (9, 11) according to any of the preceding embodiments, wherein the knife cavity floor (46) is recessed to a level of at least Vs, ¹ /4, ! , %, ¹ /2, %, or % of the height of the arms.

4. The rotatable knife (9, 11) according to any of embodiments 1 to 3, wherein at least some of the arms are extending from the centre knife part (42) to the outer circumference (38) forming first arms (26), and wherein at least some of the first arms (26) have an outer arm section (30) and an inner arm section (29) connected to each other at a intermediate arm section (33) protruding in a rotating direction (34) of the rotatable knife (11) during operation, the inner arm section (29) being arranged to direct hard constituents towards the central knife cavity (43), and the outer arm section (30) being capable of directing hard constituents towards the outer circumference (38) of the rotatable knife (11).

5. The rotatable knife (9, 11) according to any of embodiments 1 to 4, wherein at least some of the arms are located towards the outer circumference (38) of the rotatable knife (9, 11) forming second arms (27); wherein the second arms (27) have a sharp edge forming an outer cutting edge (39) arranged at the outer ends of the second arms (27) and the second arms are arranged to direct hard constituents towards the intermediate arm sections (33) of the first arms, or alternatively, towards the periphery of the rotatable knife.

6. The rotatable knife (9, 11) according to any of embodiments 1 to 5, wherein the second arms (27) are arranged between adjacent first arms (26).

7. The rotatable knife (9, 11) according to any of the preceding embodiments, wherein the rotatable knife has four arms, five arms, six arms or more, and the number of arms may be even or uneven.

8. The rotatable knife according to any of the preceding embodiments, wherein the intermediate arm section (33) has an angle 'B' between the inner arm section (29) and the outer arm section (30), the angle optionally being above 90 degrees, such as above 100 degrees, such as above 110 degrees. 9. The rotatable knife according to any of the preceding embodiments, where the inner arm section (29) of the first arm (26) has an angle (35) to a tangent of an outer circumference

(48) of the central knife cavity (43), this angle is preferably below 90 degrees, such as about 85 degrees.

10. The rotatable knife according to any of the preceding embodiments, where at least some of the first arms comprise a sharp edge at their outer ends located towards and/or parallel with the periphery of the rotatable knife.

11. A rotatable knife (9, 11) and hole plate (10, 12) combination comprising a rotatable knife according to any preceding embodiment, where the rotatable knife is located upstream of the hole plate seen in a material transport direction (13) through the rotatable knife and hole plate combination, wherein, the central knife cavity (43) makes up openings (49) between the rotatable knife (9, 11) and the hole plate (10, 12).

12. The knife and hole plate combination according to embodiment 11, wherein the openings

(49) are further defined by the inner arm sections (29) of the first arms (26).

13. The knife and hole plate combination according to embodiment 11 or 12, wherein the hole plate (10, 12) comprises a centre hole plate cavity having an area being larger than an area of the driving hole (40) of the rotatable knife (9, 11).

14. The knife and hole plate combination according to any of embodiments 11 to 13, wherein a diameter of the floor (46) is substantially equal to a diameter of the centre hole plate cavity (45) of the hole plate (10, 12).

15. The knife and hole plate combination according to any of embodiment 11 to 14, wherein the first arms (26) and the second arms (27) are the only material making a distance between the rotatable knife (9, 11) and the hole plate (10, 12).

16. The knife and hole plate combination according to any of the embodiments 11 to 15, further comprising a drive shaft for driving the rotatable knife (9, 11), and wherein the hole plate (10, 12) is stationary, and wherein the central knife cavity (43) is adjacent to and surrounding the drive shaft in a location downstream of the knife (9, 11) and through the centre of the hole plate (10, 12). 17. A grinder (1) for grinding food or feed products comprising hard constituents, such as meat (6) with bones and/or sinews and/or cartilage, the grinder (1) comprising

• A compression housing (2) with a feed screw (3) driven by a first motor (4), the feed screw (3) directing food or feed products from an infeed end (5) of the grinder (1) towards an outfeed end (7) of the grinder (1), and

• A knife housing (8) having at least one set of a rotatable knife (9, 11) and a stationary perforated hole plate (10, 12) in combination according to any of embodiments 11 to 16, the knife housing (8) being located downstream of the compression housing (2), and

• A radial separator (14) connected to a wall of the knife housing (8), for separating hard constituents from food or feed products being ground and

• An axial separator (15) connected to the knife housing (8) at the outfeed end

(7) for separating hard constituents from food or feed products being ground.

18. The grinder according to embodiment 17, wherein a stationary perforated hole plate (12) is located at the outfeed end (7), and the hole plate has an opening in the centre part.

19. The grinder according to embodiment 17 or 18, wherein the axial separator (15) comprises a separator screw (21) for directing hard constituents away from the knife housing

(8).

20. The grinder according to embodiment 19, wherein a second motor (22) drives the separator screw (21).

21. The grinder according to any of embodiments 17 to 20, wherein the radial separator (14) comprises a radially located radial outlet tube (16) connected to an opening in the knife housing (8).

22. The grinder according to embodiment 21, wherein the opening in the knife housing (8) and the radial outlet tube (16) is located next to the rotatable knife (11).

23. The grinder according to embodiment 21 or 22, wherein a valve (17) controls the radial outlet tube (16) such that hard constituents accumulated in the radial outlet tube (16) are led away from the knife housing (8) and out of the radial outlet tube (16) when the valve (17) is open. 24. The grinder according to any of embodiments 17 to 23, wherein the rotatable knife, such as a second knife (11), is located upstream of the stationary perforated hole plate, such as upstream of a second stationary perforated hole plate (12).

25. A method for grinding food or feed products such as meat (6) in a grinder (1) according to any of embodiments 17 to 24, the method comprising a) Obtaining food or feed products comprising hard constituents, b) Directing the food or feed products into the infeed of the grinder (1), c) Grinding the food or feed products, wherein d) a first portion of hard constituents in the ground food or feed product is led into a radial separator (14) and e) A second portion of hard constituents in the ground food or feed product is led into an axial separator (15), and f) Ground meat (25) with reduced amount of hard constituents is obtained at the outfeed end (7) of the grinder (1).

26. The method according to embodiment 25, wherein the food or feed products are meat (6) products from pig, cattle, poultry or fish.

27. The method according to any of embodiment 25 or 26, wherein the hard constituents comprises animal material such as bone fragments, cartilage/gristle, sinews/tendons, membranes or non-animal material/foreign objects such as plastic or metal.

It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.