a preferred apparatus for transferring and portioning meat chunks is illustrated in the cross - sectional side views of fig1 and 2 . the preferred apparatus includes a twin vane concentric meat pump 10 having a cylindrical housing 12 comprised of a cylindrical side wall 14 , a circular , flat bottom wall 16 and removable cover 18 . the side wall 14 , bottom wall 16 and cover 18 of the pump housing 12 together define a hollow interior space having a substantially cylindrical shape . the removable cover 18 is provided with an inlet opening 20 ( fig2 ) which is in communication with a hopper 22 through which meat chunks are fed into the hollow interior of the pump 10 . the inlet opening 20 is provided proximate an edge of the cover 20 . the side wall 12 of housing 14 is provided with an outlet opening 36 ( fig1 ) through which the meat product is expelled from the pump 10 . as illustrated in fig2 the inlet opening 20 is spaced circumferentially from the outlet opening 36 such that the inlet opening 20 is not located directly above the outlet opening 36 , for reasons which will become apparent below . located within the hollow interior space of pump housing 12 is a cylindrical hub 24 which extends along a pump axis a . the hub 24 is comprised of two portions , a cylindrical upper portion 26 and a cylindrical lower portion 28 . the outside diameters of the upper and lower portions are preferably the same . the upper portion 26 of hub 24 is mounted on a drive shaft 30 which passes through a hollow passage 32 in the lower portion 28 of hub 24 . the lower portion 28 of hub 24 is connected to a hollow drive shaft 34 through which the shaft 30 extends . drive shafts 30 and 34 are driveable by separate driving means 31 and 35 , only schematically illustrated in fig1 . preferably , the driving means 31 , 35 each comprise a servo drive and are contained in a gearbox . attached to the upper portion 26 of hub 24 is a first vane 38 ( see fig2 and 4 ) having a proximal edge 40 extending parallel to the pump axis and a distal edge 42 extending parallel to the pump axis and being proximate the side wall 14 of housing 12 so as to form a substantial seal therewith . the upper portion of the proximal edge 40 is connected to the upper portion 26 of hub 24 , while the lower portion of proximal edge 40 is closely spaced relative to the lower portion 28 of hub 24 so as to form a substantial seal therewith . the first vane 38 has a height which is substantially the same as the height of the hollow interior space of the housing 12 such that the upper edge 44 of vane 38 forms a substantial seal with the cover 18 and the lower edge 46 of vane 38 forms a substantial seal with the bottom wall 16 of the housing 12 . attached to the lower portion 28 of hub 24 is a second vane 48 ( see fig1 and 4 ) having a proximal edge 50 , a distal edge 52 , an upper edge 54 and a lower edge 56 . second vane 48 has dimensions substantially identical to those of the first vane 38 to thereby form a substantial seal with the bottom wall , cover and side wall of housing 12 . the proximal edge 48 of the second vane 48 has a lower portion which is attached to the lower portion 28 of hub 24 , and an upper portion which is closely spaced relative to upper portion 26 of hub 24 so as to form a substantial seal therewith . as shown in fig4 the vanes 38 and 48 divide the hollow interior of housing 12 into a pair of chambers which are substantially sealed from one another . with the second vane 48 positioned between the circumferentially spaced inlet opening 20 and outlet opening 36 as shown in fig4 the pair of chambers comprises an inlet chamber 58 which communicates only with the inlet opening 20 and an outlet chamber 60 which communicates only with the outlet opening 36 . as will become apparent from the discussion below , the relative volumes of the inlet and outlet chambers 58 and 60 are variable with relative rotation of the first and second vanes 38 and 48 . located externally of the housing 12 at the outlet opening 36 is an outlet valve 62 ( fig1 ) which may preferably have a valve body with a spherical or cylindrical valve chamber 70 . inside valve chamber 70 is provided a rotatable valve element 72 having a circular cross - section as shown in fig1 and having a cylindrical bore 74 through which the meat product is expelled . the valve 62 is shown in the open position in fig1 and can be closed by rotating valve element by 90 degrees . as illustrated , the outlet valve is preferably in direct communication with a tubular stuffing horn 78 , such that the portioned meat product ( not shown ) is directly stuffed into a casing ( not shown ) as it leaves outlet valve 62 . the apparatus 10 further comprises pressure sensing means 64 ( fig1 ) to sense the pressure within the outlet chamber 60 , and volume determination means 66 ( fig1 ) to monitor the volume of the outlet chamber 60 . preferably , these means continuously monitor the pressure and volume inside the outlet chamber during pump operation . the pressure and volume data gathered by the pressure sensing and volume determination means are communicated to a control means 68 ( fig1 ) which controls the opening and closing of outlet valve 62 based on the supplied pressure and volume data . the operation of pump 10 according to the invention will now be described below with reference to fig5 to 9 , which illustrate the positions of vanes 38 and 48 during a first cycle of pump operation . in the example described below , the maximum volume of the outlet chamber is greater than two times , and less than three times , the volume of the portioned meat product which is produced by apparatus 10 . however , it will be appreciated that the preferred apparatus 10 is capable of producing portioned meat products of a variety of predetermined sizes . preferably , housing 12 is sufficiently large that the maximum volume of the outlet chamber 60 will be greater than the volume of the portioned meat product which is to be produced . fig5 illustrates the relative positions of the vanes 38 and 48 at the beginning of a first pumping cycle during operation of the pump 10 , and is essentially the same configuration described above in connection with fig4 . that is , the second vane 48 is positioned between the inlet opening 20 and outlet opening 36 and the first vane 38 is positioned just ahead of the inlet opening 20 . thus , inlet chamber 58 and outlet chamber 60 are formed , and are substantially sealed from one another by the vanes 38 and 48 . at the beginning of the cycle , the outlet valve 62 is closed , thereby preventing the pump from expelling the meat product through outlet opening 36 until it has been sufficiently pressurized . during the first portion of this pump cycle , first vane 38 is rotated in the clockwise direction ( toward the outlet opening 36 ) about the pump axis a , while the second vane 48 remains stationary between the inlet and outlet openings 20 and 36 . thus , during rotation of the first vane 38 , the volume of inlet chamber 58 increases while the volume of outlet chamber 60 decreases . therefore , a vacuum is created in the inlet chamber , allowing meat chunks to enter the inlet chamber 58 from the hopper 22 . simultaneously , the contents of the outlet chamber 60 are pressurized . as the first vane 38 is rotated toward the outlet opening 36 , with the outlet valve 62 closed , the pressure and volume inside the outlet chamber 60 are monitored by pressure sensing means 64 and volume determination means 66 . rotation of first vane 38 continues until the meat product in the outlet chamber 60 has been pressurized to a predetermined pressure , which is preferably up to about 35 bar . fig6 illustrates the relative positions of the vanes 38 and 48 at the instant that the pressure in the outlet chamber 60 reaches the predetermined pressure . if the volume of the outlet chamber 60 at this point is greater than the predetermined amount of the meat product to be expelled from the pump , the valve 62 is opened to allow the meat product to be expelled . preferably , the meat product is directly stuffed into a casing as it is expelled from the outlet valve 62 . with the outlet valve 62 open , rotation of the first vane 38 is continued in order to maintain the pressure in the outlet chamber 60 substantially equal to the predetermined pressure . as the meat product is expelled from the outlet chamber 60 , the volume inside the outlet chamber 60 is monitored by volume determination means 66 . once the volume inside the outlet chamber 60 is reduced by an amount equal to the predetermined amount , the outlet valve 62 is again closed , thereby separating the dispensed portion of meat from that remaining inside the apparatus 10 . as mentioned above , the casing is preferably stapled shut and is cut to sever the encased portion of meat from the apparatus 10 . fig7 illustrates the relative positions of the vanes 38 and 48 at the instant that the valve 62 is closed to sever the first portion of meat product from that remaining inside the apparatus 10 . with the pressure inside outlet chamber 60 maintained at the predetermined pressure , the volume of the outlet chamber is again determined by the volume determination means . in this example , the remaining volume of the outlet chamber 60 as shown in fig7 greater than the predetermined volume of the portioned meat product , and therefore the valve 62 is again opened to expel a second portion of meat product . as the meat is expelled through the valve 62 , the first vane 38 is further advanced to maintain the predetermined pressure in the outlet chamber 60 . after the predetermined amount of the meat product is expelled from the pump , the outlet valve 62 is again closed , at which point the vanes 38 and 48 have the relative positions shown in fig8 . again , with the pressure within outlet chamber 60 maintained at the predetermined pressure , the volume of the outlet chamber 60 is determined . in the configuration of fig8 the remaining volume of outlet chamber 60 is less than the predetermined amount of the meat product , and therefore the amount of meat product remaining in outlet chamber 60 is insufficient to produce another portioned meat product . accordingly , at this point , both the first vane 38 and the second vane 48 are advanced in the clockwise direction until they adopt the configuration shown in fig9 in which the first vane is now positioned between the inlet opening 20 and the outlet opening 36 , and in which the second vane is positioned ahead of the inlet opening 20 . the vanes are thus positioned for a second cycle of operation in which the second vane 48 is rotated clockwise while the first vane 38 is held stationary . the second cycle of operation is otherwise identical to the first cycle of operation . the steps to be followed during two complete cycles of pump operation are shown in the flow chart of fig1 . in the above - described example , the operation of the pump 10 was described after initial pump start - up , in which the outlet chamber 60 is full of meat chunks at the start of each cycle . it will be appreciated that , upon initial start - up of the pump 10 , the outlet chamber 60 will be empty , and therefore the outlet valve 62 is not opened during the first cycle of operation . during pump operation , the first and second cycles are repeated in the manner described above to continuously expel predetermined volumes of the meat product from the pump 10 . the pressurization of the outlet chamber 60 with the valve closed permits effective pressurization of the meat product therein , thereby eliminating the need for further pressurization of the meat product after it is expelled from the pump . furthermore , rotation of both vanes 38 and 48 in the same direction , in combination with simultaneous rotation of the vanes 38 and 48 by independent driving means at the end of each cycle , improves pump efficiency by avoiding loss of momentum due to stopping and changing the direction of rotation at the end of each cycle . in addition , the use of pressure and volume sensing means , and the use of a control means to control operation of the outlet valve , permits the production of accurately controlled portions of the meat product which eliminates the need for a separate portioning apparatus . although the invention has been described in connection with certain preferred embodiments , it is not intended to be limited thereto . rather , the invention includes all embodiments which may fall within the scope of the following claims .