Field of the Invention

The present invention relates to equipment and methods for cooling food products such as ground meat.

Background of the Invention

In the commercial-scale preparation of food products, there can be stages in which the temperature of the food product increases and should be decreased. Lower temperatures are desirable to maintain the food products in sanitary condition, to avoid degradation of the physical characteristics of the food product, and to improve its processability and final product yield in subsequent unit operations. For example, the cutting and grinding operations that are performed in the production of ground meat may be expected to raise the temperature of the resulting ground meat product, and reducing the temperature of the ground meat product is highly desirable as soon as possible during or following the formation of the ground meat product.

Prior techniques for cooling food products have encountered drawbacks such as nonuniformity of cooling, as well as freezing of portions of the food product. Also, prior techniques that use cryogenic coolant are costly in that they consume considerable amounts of the coolant as well as energy and time for a given amount of cooling.

The present invention provides a system for cooling food product that avoids nonuniformity and freezing of the food product, while realizing unexpected efficiencies in cryogenic coolant and in time and energy requirements. Brief Summary of the Invention

One aspect of the present invention is a method for providing a desired amount of cooling to a quantity of food product in a vessel within a given period of time, comprising: (A) feeding cryogenic liquid which is not carbon dioxide out of a plurality of nozzle openings into a vessel directly into a formable mass of food product in the vessel at a flow rate effective to provide from each nozzle not greater than 1 ,900 BTUs of refrigeration to the formable mass of food product per minute from the cryogenic liquid delivered from each nozzle, while an impeller is moving the food product past the nozzle openings, wherein each nozzle opening is in direct contact with the food product and is at the interior surface of the vessel or is between the interior surface and the path of the impeller,

(B) while maintaining the rate at which the formable mass of food product is moved past each nozzle opening to be sufficiently high so that the mass of food product remains formable as it is being cooled by contact with the cryogenic liquid, and

(C) feeding said cryogenic liquid into said food product in accordance with steps (A) and (B) from a sufficient number of said nozzle openings to provide the desired amount of cooling to the quantity of food product within the given period of time.

In another aspect of the invention, cleaning fluid is intermittently fed into feed lines upstream from each nozzle opening, is passed through said feed lines and through and out of said nozzle openings into said vessel, and is removed from said vessel. In another aspect of the invention, the flow of cryogenic liquid out of the nozzles is intermittently interrupted by flowing nontoxic gas out of the nozzles into the food product. Figure 1 is a perspective view of a mixer suitable for use with the present invention.

Figure 2 is a top plan view of the mixer of Figure 1.

Figure 3 is a cross-sectional view of a portion of the mixer depicted in Figure 1, seen along the line 3 '-3" that appears in Figure 1.

Detailed Description of the Invention

The present invention is useful for cooling any of a wide variety of food products, especially products that are formable. A product is considered to be formable if it is sufficiently viscous that it can maintain for at least one second any particular shape into which it has been formed (e.g. formed by hand or by mechanical equipment) and if it is also capable of being moved or reconfigured into a different shape and, having been moved or reconfigured, then maintains that different shape for at least one second. As used herein, formable products also have to be able to be penetrated by streams of liquid directed at them or into them. Examples of formable products include ground meat (which includes mixtures of ground meat with other ingredients), and compositions (such as batters) that can be solidified in subsequent processing steps (such as baking) to create products such as baked goods, cookies, pet food kibbles, and the like.

The present invention is advantageously employed using equipment that can receive and hold a quantity of the food product to be cooled, in which there is at least one impeller inside the equipment which can move the product within the equipment. One example of such equipment is illustrated in Figure 1, wherein mixer 1 1 comprises side walls 12 and 13, end walls 14 and 15, and bottom 16, all of which are sealed to each other to form an enclosure. Lid 17 is hingedly attached to one wall, such as wall 13 as shown, so that lid 17 can be closed over product within mixer 11 and can be raised to permit access to the interior of mixer 11. Lid 17 includes one or more vents which can be opened or sealed, to permit vapor to be drawn out of the interior of mixer 11. The mixer 1 1 is supported on legs 19. Discharge hopper 26 is a receptacle that can receive cooled product from the interior of mixer 11 , through doorway 27 which is closed during the cooling operation and which is opened when cooling is completed. Discharge hopper 26 holds cooled product that is fed there from mixer 1 1, until doorway 28 in the bottom of discharge hopper 26 is opened at which point cooled product passes out through doorway 28 into a receptacle such as carrier 29 in which the cooled product can be held and moved to another location for further processing.

Referring to Figure 2, a pair of impellers 21 are shown. The invention can be practiced with embodiments having one impeller, or more than two impellers. Each impeller 21 includes an axle 22 which is mounted in conventional manner in each end wall so that each impeller can be rotated around its axle. Drive 23 includes a motor and suitable connections from the motor to the axles 22 to enable the motor to rotate each axle. Drive 23 also includes suitable controls to enable an operator to stop and start the rotation of the impellers and to regulate the speed at which the impellers 21 rotate.

The impellers 21 also include blades 25 which, in operation, contact the food product and urge it in a direction generally parallel to the axles 22 while also mixing the product. In Figure 2, each blade 25 is attached to an axle by a radial arm 24, and the blades 25 are angled relative to the sides of the mixer 11. While one embodiment of blades is illustrated in Figure 2, it will be understood that other configurations are also known and useable, such as ribbon-type and screw- type impellers and drives, to move the food product along the length of the mixer 1 1. The nozzle openings can be provided on only one side of the mixer 11 , to appear as shown in Figure 1, or they can be provided on both sides of the mixer 1 1, as shown in Figure 2.

Figure 3 illustrates an end-on cross-sectional view of mixer 1 1 showing one set of blades 25 and radial arms 24. It is preferred that the edges of blades 25 that are farthest from axle 22 are near, but not touching, the inner edge of side wall 12. A nozzle opening 35, which is at the open end of a cryogen feed line 34, is situated flush with the inner surface of wall 12, or (as illustrated by phantom lines 35 A) can be situated within the interior of mixer 11 provided that nozzle opening 35 does not contact blades 25 or any other part of the impeller.

Figure 3 also illustrates the preferred position of each nozzle opening 35 relative to the mixer. That is, each nozzle opening 35 is preferably located so that the angle A between a vertical radius from the axle 22 of the impeller 21 that is closest to the nozzle opening 35, and the radius extending to the nozzle opening 35 from the axle 22 of the impeller 21 that is closest to the nozzle opening 35, is preferably from 50 to 60 degrees. In addition, each nozzle opening 35 should be oriented so that the angle B between the central axis of the nozzle opening and a horizontal line passing through the nozzle opening is preferably from 12 to 20 degrees. Referring again to Figure 1 , each cryogen feed line 34 is open at one end as nozzle opening 35 and is connected at its other end to plenum 33 which is connected via line 32 to a source 31 of liquid cryogen such as an insulated tank with suitable control valves and meters. The liquid cryogen in the tank or other source 31 is typically at a pressure of 30 to 35 psig but can be as low as 25 psig and as high as 60 psig. Preferred cryogen includes liquid nitrogen. The liquid nitrogen can comprise 100% liquid (that is, with no vapor fraction present), the benefits of the present invention can be realized when some of the cryogen is in the vapor phase together with the balance being liquid. However, at least 70 % of the cryogen by weight should be liquid, and preferably at least 90 % and more preferably at least 95%, by weight should be liquid, as the benefits of the present invention generally increase with higher liquid content of the cryogen that is fed into the mixer 1 1.

While Figure 1 illustrates one plenum and all of the feed lines extending from the one plenum to the mixer, it will be recognized (and described below) that larger numbers of feed lines, some of which extend from second or additional plenums, can be utilized for larger capacity mixers, for providing higher overall refrigeration to the food product, or for providing refrigeration in a shorter period of time, though still subject to the surprising operational limits to the amount of cryogen injected per nozzle that have been determined to be necessary within the present invention. Reduction of the liquid portion (fraction) of the cryogen that is fed out of the nozzle openings can be avoided by insulating the lines 32 and 34 and plenum 33, and also by minimizing the pressure drop encountered by the cryogen between its source 31 and the nozzle openings 35. Pressure drop can be minimized by minimizing the number of bends in the lines 32 and 34 and by employing piping and nozzle openings that are not excessively constricting and that preferably minimize restrictions to flow. Inner diameters for the lines 32 on the order of 1 to 4 inches, and on the order of half an inch for the lines 34, are satisfactory.

Preferred pressure drop between the cryogen source 31 and the nozzle openings 35 is no more than 10 psi (pounds per square inch) and preferably no more than 5 psi.

In operation, formable food product to be cooled is placed in the mixer and the lid should then be closed. The impellers are turned on. The flow of liquid cryogen is also initiated, immediately or after some period of food product mixing or blending, from the source through the plenum and the individual feed lines and out of each nozzle opening into the food product in the mixer. The food product is in direct contact with the nozzle openings, so that liquid cryogen emerges out of the nozzle openings directly into the formable food product, wherein the liquid cryogen cools the food product by virtue of its low temperature, the liquid cryogen vaporizes in contact with the food product to provide refrigeration via the heat of vaporization, and the resulting cryogen vapor cools by virtue of its still- low temperature. The liquid cryogen just before it emerges from the nozzle openings is typically at a temperature on the order of -320 F to -250 F.

It has surprisingly been determined that unexpectedly good efficiency of cooling is obtained by limiting the flow rate of cryogen from each nozzle opening into the food product in the mixer, and by not exceeding a maximum amount of refrigeration obtained via each nozzle opening from the cryogen that is fed from each nozzle opening. The maximum expressed as refrigeration is up to 1,900 BTUs (preferably up to 1,500) of refrigeration per minute per nozzle.

Alternatively, adhering to a maximum amount of cryogen from each nozzle, as described more below, is likewise surprising and also accomplishes the results described herein. These findings are opposite to the conventional expectation in this field that increased cooling must be obtained by increasing the cooling rate and cryogen flow rate at each nozzle. This finding also enables the operator to obtain efficient, substantially uniform, and rapid cooling while avoiding freezing of the food product. That is, the food product remains formable as defined herein, even as cooling of the food product continues and its temperature decreases.

In general, meat mixing is an application with uniquely high refrigeration rate requirements to remove significant heat (on the order of 50,000 to 100,000+ BTUs) often from several tons of meat in a short period of time (several minutes). Thus, the use of a cryogen such as liquid carbon dioxide or liquid nitrogen in direct contact with the meat is generally advantageous. At the same time, this invention has found that limiting the liquid nitrogen refrigeration rate per injection nozzle is highly advantageous from the point of view of cryogen efficiency and food product quality. Efficiency here refers to the amount of cooling obtained per amount of cryogen fed into the mixer for contact with the food product being cooled. The present invention provides an unexpectedly high efficiency compared to prior techniques using liquid cryogen, even where the prior techniques use liquid nitrogen as the cryogen.

This outcome is produced by a combination of operating conditions. The impeller is operated at a rate that moves the food product past each nozzle opening rapidly enough to avoid freezing of the food product by its contact with the liquid cryogen from the nozzle opening. Typically this means that the food product moves past each nozzle opening at a rate on the order of 0.2 to 3.0 meter/second, with higher rates preferred. The desired rate of movement of the food product can be achieved by controlling the rate at which the impeller rotates. Impellers such as those in typical commercial use would be rotated at rates on the order of 3 to 45 rpm (rotations per minute), again with higher rates preferred although attention must be given to avoiding rates that are so high that the physical integrity or the quality of the food product is damaged. Effective rates of movement of the food product can readily be determined by evaluating whether or not the food product undergoes any stiffening due to the onset of freezing, under a given set of conditions of cryogen flow rate out of the nozzle opening and rate of movement of the food product past the nozzle opening.

In addition, another unexpected operating condition to attain the surprising improvement in cooling efficiency by the present invention is to limit the refrigeration that is provided out of each nozzle opening. This is provided by not exceeding a maximum of 1,900 (preferably a maximum of 1,500) BTUs of refrigeration to the formable food product per minute from the cryogen liquid delivered from each nozzle opening. The heat to be removed during cooling of the food product in the mixer as the food product goes from thermodynamic state 1 to 2 is Δ H _f = ¾ (T ₂ , 2) - H _f i (Ti, i) with units of BTU/lb or kJ/kg, where H _f is the enthalpy of the food product, T is the temperature of the food product, and x is the phase or frozen fraction of the food product. The total amount of refrigeration required to be delivered to the food product per batch (or during the cooling residence time of the food product in the vessel during continuous operation) is Δ¾ m _f / At with units of BTU/min or kJ/min, where m _f is the mass of the food product in the vessel, and At is the cooling batch time for batch operation (or cooling residence time in the vessel for continuous operation). Refrigeration actually delivered into the food product by the cryogenic fluid directly injected inside the vessel to contact the food product is m _{c c} with units of BTU/min or kJ/min, where m _c is the mass flow rate of the cryogenic fluid (kg/min or lb/min), and _c is the cryogen refrigeration utilization efficiency (BTU/lb or kJ/kg).

The present invention has determined that this is generally achieved by not exceeding a flow rate of cryogen liquid into the food product in the range of 5 to 30 pounds of cryogen per minute per nozzle, preferably 5 to 25 pounds of cryogen/minute/nozzle, typically on the order of only about 10 to 20 pounds of cry ogen/ minute/nozzl e.

The high efficiency that is achieved by the present invention is also enabled by providing a high liquid content (fraction) of the liquid cryogen that is fed out of the nozzle openings, in accordance with the guidance that is described hereinabove. Liquid fractions of at least 90 wt.% are to be preferred.

The number of nozzle openings (as well as the number of feed lines feeding liquid cryogen to the nozzle openings, and the number of plenums associated with the feed lines) will be determined by the overall desired amount of cooling to be provided to the quantity of food product being cooled, and by the length of time within which the cooling is to be achieved.

As one typical example, in a commercial 4000-pound mixer, satisfactory high-efficiency cooling is provided in a range up to 10 minutes to provide heat removal of 25 BTU/pound of food product, by adjusting and balancing the cryogen flow rates out of each nozzle opening, the number of nozzle openings, and the liquid content of the cryogen that is fed into the food product, and providing cryogen liquid as described herein through the nozzle openings into the food product.

A preferred overall sequence of steps is the following: Mixing only (optional)

Some food products require a mixing only period to mix the various ingredients that have been added to the mixer. Typical times are 1 to 3 minutes. During this period, inert nontoxic gas such as gaseous nitrogen may optionally be injected through the nozzle openings periodically to keep them un-blocked.

Typically, this gas would be injected through one plenum for 5 seconds, followed by 5 seconds through the second plenum, and so on. All the manifolds will be injected through at least once and the cycle may be continued multiple times. Gaseous nitrogen pre-purging (+ mixing)

Prior to the liquid cryogen injection, gas such as nitrogen is injected through all of the nozzle openings to clear out any food product that may be present in the nozzle openings. Each manifold should be injected with this gas at least once and preferably twice. Each injection should be at least 5 seconds and preferably 10 seconds. Up to 2 manifolds at one time can be injected with gas. Typically, the total time for this pre-purge is between 20 to 40 seconds. Liquid nitrogen injection (+ mixing)

The liquid nitrogen injection period preferably includes alternating injections of liquid nitrogen and of gaseous nitrogen injection through each manifold. Typically, this period can be between 3 to 15 minutes. The total period consists of multiple cycles. Each cycle consists of a period of liquid nitrogen injection followed by a period of gaseous nitrogen injection. Typically, each cycle is 30 to 60 seconds in duration. Typically, the liquid nitrogen injection duration is 50% to 90% of the total cycle time. As an example, an injection cycle of 60 seconds can consist of 45 seconds of liquid nitrogen injection followed by 15 seconds of gas nitrogen injection.

Gaseous nitrogen post-purging (+mixing) 30 sees to 1.5 mins

After the liquid nitrogen injection, gaseous nitrogen is injected through the manifolds to clear out the nozzles and warm up the nozzle openings and the surrounding mixer wall (warm, that is, relative to the temperature of cryogenic liquid) to prevent food product from freezing and sticking to them. This period also gives time for the food product to be well mixed so as to ensure more uniform final cold product temperatures. Typically, the duration of this period is between 30 to 90 seconds. Each manifold should be injected in this step with gaseous nitrogen at least once and preferably twice. Each injection should be at least 5 seconds and preferably 10 seconds. Up to 2 manifolds at one time can be injected with gas. Sanitation considerations

Periodically, such as at the end of a production shift or workday, the manifolds can be cleaned and sanitized from the inside. This can be carried out by closing all valves feeding liquid cryogen and inert nontoxic gas (as described above) into the mixer. Then, preferably, compressed air is blown for at least 2 minutes through each manifold to ensure that all food product is removed from the nozzles. Next, hot water is flowed through each manifold and feed line to each nozzle opening, followed by detergent solution and finally sanitizer solution. As shown in Figure 1, the manifolds are self-draining, i.e. all liquid in the manifold flows into the feed lines and out of the nozzle openings and no liquid remains collected at any location in the manifold. This is important so that no liquid remains in the manifold which could freeze when liquid nitrogen or other liquid cryogen is flowed through the manifold.