Patent Application
20050136161
1. Field of the Invention
This invention generally pertains to systems and methods for freezing and for thawing food. More particularly, the present invention is directed to systems and methods of freezing food products that minimize damage to the food, such as aging, that may occur during the freezing process. The present invention also relates to systems and methods for thawing frozen foods to maximize taste.
2. Description of the Related Art
In conventional prior art freezing methods, food is reduced in temperature from room temperature to the frozen state in a matter of hours, typically 1 to 3 hours. When such conventional methods are applied to high water content foods such as sushi (which is a well known combination of cooked rice, raw fish and other toppings), a substantial portion of the water in the food is irreversibly lost. The loss of water is caused by an accelerated aging process that takes place when the food is exposed to a certain temperature zone for a relatively long period of time during conventional freezing processes. Exposure to this accelerated aging temperature zone for prolonged periods of time also results in the generation of ice crystals at a high rate. As a result, ice crystals that form will expand in size with time and rupture the cell structure of the food being frozen. When the food is defrosted, water generated from the ice crystals will be irreversibly lost from the food. Thus, conventional prior art food freezing methods have substantial drawbacks resulting from the substantial loss of moisture content, cell structure damage, thereby reducing freshness and changing the texture and desirability of the thawed food product.
In connection with efforts to improve conventional prior art freezing methods, many professional and industrial “quick” freezer systems use low temperature nitrogen gas or carbon dioxide gas as a cooling medium for more rapid (flash) freezing purposes. While nitrogen gas has a low temperature capability (−196° C.), its specific heat is only about 47 Kcal/gram/° C., and therefore is not sufficient in terms of heat absorption capacity to extract heat from the bulk of the food at high rates. While conventional freezers create fractured food cells due to ice crystal growth, quick freezer systems utilizing low calorie cooling sources may damage food cells due to rapid freezing of the food. In both cases, food cells are destroyed during the freezing process. Carbon dioxide gas has a higher specific heat than nitrogen gas (about 137 Kcal/gram/° C.), but has a much higher minimum temperature (about −79° C.). Quick freezing systems using carbon dioxide gas encounter the same problems with high water content foods as described above.
In another attempt to address shortcomings with conventional freezing techniques, it has been proposed to apply a magnetic field to the food during the freezing process. In this approach, according to U.S. Pat. No. 6,250,087, magnetic energy is applied to the food to be frozen in a conventional freezer to attempt to prevent cell fracture caused by ice crystal growth during the freezing process. The food is shaken by the application of the magnetic field to suppress crystallization. However, this approach uses conventional freezing technology and the process still takes a long time for complete freezing to take place (2 to 3 hours). While it is asserted that this approach maintains moisture in the cell and prevents dripping, such systems are complex, expensive, and have limited capacity.
For the foregoing reasons, there is a need for new and improved systems and methods for freezing and thawing food. The present invention overcomes these and other problems that occur with convention freezing techniques, and particularly in connection with freezing of higher water content foods.
In accordance with the foregoing and other objects, the present invention provides a method of freezing food-for later thawing and use. The method includes the steps of packing a food product in a container for freezing, cooling the food product substantially throughout the bulk thereof to about 10° C., and then cooling the food product substantially throughout the bulk thereof from about 10° C. to about 0° C. in less than approximately ten minutes.
According to another embodiment of the present invention, a method of freezing a food product is provided which includes a step of packaging a food product to be frozen after the temperature of the food product reaches a first predetermined temperature. The food product is then cooled until the temperature of the food product reaches a second predetermined temperature. The food product is then cooled so that the temperature of the food product decreases from the second predetermined temperature to a third predetermined temperature within a first predetermined period of time.
According to another embodiment of the present invention, a system for freezing a food product is provided which comprises a freezer and a control unit. The freezer maintains an interior temperature set to a first temperature and includes a first cooling unit and an adjustable cooling unit providing additional cooling energy. The control unit is coupled with the adjustable cooling unit and configured to adjust the additional cooling energy. The adjustable cooling unit provides additional cooling energy on demand.
According to the present invention, the calorie exchange rate of the freezer is adjusted to obtain the optimal freezing process to maintain the original taste and texture of the food. High water content foods, such as rice, can be frozen in a short period of time and in a manner that captures water in a food cell before large ice crystal clusters form and grow.
According to one embodiment of the present invention, dry ice is used as a cooling source in a double freezer configuration. When dry ice changes from its solid state to gas phase directly, a much higher calorie exchange rate is produced than when liquid carbon dioxide changes phase to gas. The present invention is a simple, low cost system suitable to freeze a large capacity of food. Also, the simple design of the present invention includes a continuous frozen food chamber that enables almost unlimited production of frozen foods.
According to another embodiment of the present invention, a method of thawing frozen food is provided which comprises the steps of placing a container of coolant on a side of the frozen food, and steaming the frozen food from a side that is opposite to the side where the container of coolant is placed. The food is steamed until the food is thawed to a desired temperature.
According to the present invention, food is preferably frozen in a reasonably short period of time to avoid exposing the food to the maximum ice crystal generation zone for extended periods of time which will cause damaging food by ice crystal growth. This is accomplished by using a high calorie cooling source, such as, for example dry ice. The freezing process of the present invention avoids the dehydration phenomenon resulting from conventional, quick freezing methods.
According to the present invention, a method is provided for thawing frozen food which includes a step of arranging a plurality of containers of frozen food in a tray. A package of coolant is placed on a side of each of said frozen food. A source of warm water is supplied to the tray until the plurality of containers of frozen food is thawed to a desired temperature.
With these and other objects, advantages and features of the invention that may become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims, and the drawings attached hereto.
The invention will be described in detail with reference to the following drawings, in which like features are represented by common reference numbers and in which:
FIGS. 9 is an illustration of a system for thawing a large volume of containers of frozen foods according to another embodiment of the present invention.
Although the present invention is applicable to the freezing and thawing of foods, and particularly those foods having high moisture content, the present invention will be described in connection with a preferred embodiment directed to freezing and thawing the food product sushi.
In accordance with the present invention, sushi refers to any food product known as sushi such as, for example, a food product in the form of cooked rice with some form of topping (e.g., fish, avocado, etc.). Sushi can also be in the form of rolls. Sushi typically has a moisture content of about 60% by weight. There are several important factors to be considered when freezing high water content food which is intended to be defrosted later for consumption. One factor is the aging process by which foods like rice can irreversibly lose their water content. In the case of sushi, this is a process by which a molecular chain of starch loses its regular array and turns into paste. The aging process in sushi is accelerated when the food is reduced to approximately below 10° C. and is most severe through a temperature range of about 6° C. to about 0° C. This temperature zone is referred to as the “accelerated aging temperature zone.”
A second factor is referred to as “the maximum ice crystal generation zone,” during which the water within the food forms into ice crystals. This occurs, in the case of sushi, in the range of from approximately 0° C. down to approximately −4 to −10° C. In this temperature zone, approximately 75% or more of the water in the food is transformed into ice crystals. The ice crystals damage the food during formulation by destroying cell structure, drying, etc. The present invention controls the freezing process to ensure that food is passed through those two temperature zones in the desired time, but also ensures that the freezing occurs throughout the bulk of the food as well.
Second freezer 106 includes one or more cooling units 108 which comprise a high calorie cooling source such as, for example, dry ice blocks. Dry ice may be provided in racks, as shown in
The system 100 may also include one or more cooling unit adjustment mechanisms 110 that adjust the cooling units 108 to provide more or less heat transfer (cooling) energy to the food 114 as needed depending on the size of the dry ice cluster and the volume of the food in the freezer. In one embodiment, the cooling adjustment mechanism is a rod or bar which is connected to each of the cooling units 108 so that those units can be moved or rotated in unison. For example, if cooling units 108 include dry ice blocks, then the adjustment unit 110 is preferably used to change the angle of the blocks relative to the circulation units 116 to increase or decrease heat transfer from the dry ice blocks to the food 114 by providing more or less surface area of dry ice in contact with circulating air. The adjustment mechanism 110 can be used in connection with the manual adjustment of the cooling units 108. In another embodiment, adjustment mechanism 110 can be used in connection with an automated adjustment of the-cooling units 108. In this embodiment, electronic movement of the adjustment mechanism and cooling units is controlled by the control unit 104.
The dual-freezer configuration of the present invention provides a very stable reference cooling temperature in the interior freezer 106. One skilled in the art will understand that single freezer arrangements can also be used. In single freezer arrangements, various loading systems may be used to prevent loss in cooling energy during loading and unloading of food to be frozen, in order to maintain a steady interior temperature of the freezer. For example, suitable loading systems could include a loading chamber unit attached to a freezer with a door on the loading side and another door on the freezer side with an air tight seal. During the loading process, a door on the loading side is open, but the door on the freezer side remains closed. Once the food rack is loaded into the loading chamber, a door on the loading side is closed first and then the door on the freezer side is open to allow the food rack to enter inside of the freezer. When the food is completely frozen as described in the detailed description of the invention, the food rack is preferably taken out in the reverse order as described in connection with the loading process.
The thermal exchange with the food to be frozen can be performed smoothly using a high calorie cooling unit, such as dry ice, which has a very high calorie heat transfer coefficient. Food placed inside the second freezer 106 can have its temperature passed through the accelerated aging temperature zone and maximum ice crystal generation zone within a short period time by using a high calorie cooling source.
The control unit 104 is coupled to the adjustment unit 110, variable cooling source 112b and circulation means 116, as well as to one or more temperature sensors 118 which measure the temperature of the interior of freezer 106 and/or of the food 114. The control unit 104 may include a computer processor or the like, a memory unit and appropriate input/output devices (not shown) for communicating with and controlling adjustment unit 110, variable cooling source 112b and circulation means 116, and for receiving temperature data from the one or more temperature sensors 118. The control unit is preferably programmed with computer software for facilitating the processes of the present invention, which are described in more detail below.
The size of freezer in accordance with the present invention can be of any suitable size depending on the quantity of food to be frozen. In one embodiment, freezer 206 is approximately 8′×8′×8′ and can be used to freeze approximately two to three 200 pound batches of sushi according to the present invention. In this embodiment, approximately 400 pounds of dry ice is placed in racks 108. Also, the freezers are preferably capable of maintaining a positive air pressure inside of approximately 5 psi to maintain the dry ice and to allow the dry ice to sublimate properly for the desired cooling. To maintain the pressure, a pressure relief valve (not shown) may be provided to vent the freezer when necessary if the pressure is increasing.
The temperature sensors 118 may also be placed in the vicinity of the food 114 or any other location within freezers 106 and 206 to allow proper monitoring thereof. For example, as shown in
In another embodiment of the present invention, a temperature sensor is positioned to measure the surface temperature food product. The surface temperature of the food produce, which largely corresponds to the temperature of the interior of the freezer, may be used to provide additional information for freezing food products in accordance with the present invention.
The control unit 104 is configured to control the speed of the circulation of air over the dry ice. Also, control unit 104 may control the interior temperature of the freezer 106, including the variable cooling source 112a and 112b as needed to ensure that the food 114 is cooled at the proper rate. For example, if the temperature of food to be frozen is not decreasing at the desired rate, the variable cooling may be initiated to further reduce the temperature inside second freezer 106 or freezer 206 at the desired rate. The control unit 104 also may reduce or terminate the variable cooling to prevent the outside region of the food from cooling too quickly so that the food is frozen throughout its bulk properly. For example, carbon dioxide gas may be discharged into second freezer 106 or freezer 206 via nozzle 112a for a predetermined amount of time (e.g., a few seconds), or until the environment or food (surface and/or core) reaches a selected temperature.
In another embodiment of the present invention, the freezer system may be configured for continuous high volume operation by providing conveyor mechanism or the like for loading and unloading units of food to be frozen. One example of a continuously operating freezer 300 is shown in
Referring to
In a preferred embodiment, the food products to be frozen, such as sushi, should first be packaged into a container, such as a bag, and hermetically sealed after de-aeration. Such packaging locks flavor into the product and helps prevent the food from drying. Shrink wrapping or vacuum bagging the food allows good results and is preferred.
Operational aspects of the present invention are discussed in connection with a discussion of the temperature characteristics of the environment of the interior of the second freezer 106 and of the food during freezing. For example, in an experiment, an arbitrary volume of cooked rice (2 lbs) was cooled to room temperature (about 22° C.) and stored in a bag after it was determined to be in a balanced condition. The package was de-aerated and sealed. The food was then stored in the interior of freezer 106 maintained at a temperature of −60 to −70° C. Temperature sensors were used to measure (1) the environment or reference temperature of the interior space of second freezer 106, and (2) the core temperature of the food 114.
The results of the experiment are shown in
Curve A reflects the measured interior environment temperature of freezer 106, which also reflects the cooling capacity of the freezer. The interior environment temperature A of the freezer changes as a function of time because of thermal energy exchange using the air in the freezer as a catalyst. In other words, the environment temperature A shows the transition in the freezer caused by thermal transmission from the outside surface of food product, such as a rice cluster, which is warmer than the environment temperature, as air passes over the food product. This temperature inclination changes the degree of the angle by the freezer capability per unit of a chiller source, wind velocity and size of transfer surface area, etc., however it can be read that the change of inclination has a general tendency which is affected by the thermal capacity of the rice cluster.
Cooling control of the freezer can be determined from the curves, such as the curves represented in
The freezing activity is achieved by seeking the phase inversion, by passing the temperature of the food through its freezing point artificially. A complex group of solid-state properties has many different freezing points, especially food which is a complex of hydrous substances, like sushi, the ingredients of which may have significantly different water characteristics to be carefully treated. Since curve A is the curve of the controllable buffer zone in a cooling process, it shall be considered as a control region such that the cooling heat energy, the transmission speed for the heat exchange, etc. and cooling transmission temperature control should be applied within this zone.
Curve B is considered as the cooling heat conduction area of the rice cluster by which the cooling heat transmission is undertaken, and it should be understood as an analytical area for a proper control of the hydrous properties of the food. That is, from curve B, it can be determined how to adjust the cooling within the freezer 106, as more or less energy is required to achieve the desired cooling of the food.
It can be observed that curve B has a shallow angle as the temperature goes below 0° C. and continues until a point where curve B reaches approximately −10° C. From this observation, it can be understood that the heat conduction ratio of the food reduces following the progression of ice precipitation in the food between the surface and the core of the food due to ice precipitation of menstruum (free water) at the surface of a rice cluster. Also, each rice grain is individually affected by the changes in the thermal conductivity from the outside to the core of the rice cluster, and it is therefore understood that curve B reflects the heat exchange rate of the area between the surface and the core of the food as the aggregate of average complicated heat flow speed.
Curve B also shows the similar tendency as curve A. However, while curve A corresponds to a transmission rate with comparatively high efficiency by the direct heat dissipation transfer to the environment temperature, curve B shows a widening temperature difference from curve A by relaying to the layer where the conduction efficiency is low in the progression of heat flux process from the curve A, and in spite of the rapid declining angle of curve A, continues as being indicated an aspect of passing through a temperature zone of the specific food. Meanwhile, each layer from the exterior side to the core side of the food advances mainly the phase changes of free water and relay descent in the direction where the constituent is frozen, and the temperature thereof passes through the maximum ice crystal generation zone.
At this stage, curve B shifts to the steep angle. The difference between the temperatures of core side and the exterior side becomes narrow and finally, overlap each other, and the thermal conductivity of the each layer of rice cluster become almost equivalent, and the freezing is deepened in proportion to the heat transmission capability from this point. This indicates that all the food throughout its bulk has been cooled passed the maximum ice crystal generation zone.
From
A dotted line curve in
At step 5-3, the temperature inside the freezer 106 is measured via temperature sensors 118. As described above, the temperature of the food (surface and/or core) may be estimated using a temperature inclination of the atmospheric temperature from the chart of
When the temperature of the food 114 reaches the upper limit of the accelerated aging temperature zone (e.g., for sushi, approximately 10° C.), a cooling pattern is generated to cool the food through the accelerated aging temperature zone. For example, the control unit 104 controls the adjustment unit 110 and the fans 116 to create an operative cooling pattern (i.e., the fans blow air over the dry ice). Control unit 104 may also initiate variable cooling via variable cooling units 112, if cooling is too slow. Variable cooling injection then can be combined with circulation control by the control unit 104, and the temperature of the food is decreased through the accelerated aging zone at the appropriate rate. Preferably, the temperature of the food is reduced quickly to properly freeze the food throughout its bulk without damage to the food cells. Preferably, the accelerated aging temperature zone (approximately 6° C. to about 0° C.) is traversed in 1-10 minutes, and preferably 3-5 minutes.
At step 5-4, when temperature of the surface of the food reaches the upper limit of the ice crystal generation zone (e.g., for sushi ˜0° C.), variable cooling is adjusted again, if necessary, in response to heat transmission of the food. Variable cooling may be terminated if the temperature of interior freezer 106 is sufficient to continue cooling of the food through the ice crystal generation zone at an adequate rate and to prevent the food from cooling too quickly. Variable cooling may not be necessary to freeze food at the proper rate. If the temperature of the food does not reach approximately −5° C. to approximately −7° C. within approximately 10-15 minutes after the food is introduced into the freezer, variable cooling may be initiated to force the temperature to go down momentarily as shown with the dotted line of curve A in
The food is cooled from 0° C. to −10° C. in approximately 10 to approximately 40 minutes. The food is preferably cooled from 0° C. to −10° C. in approximately 15 to approximately 30 minutes. In another preferred embodiment, the food is cooled from 0° C. to −7° C. in approximately 10 to approximately 40 minutes.
Next, the food is preferably cooled from about −10° C. to about −30° C. within approximately 30 minutes to approximately 90 minutes. The food is more preferably cooled from about −10° C. to about −30° C. within approximately 40 to 60 minutes. By the time the food reaches −30° C., the fans will most likely become unnecessary and may be shut off. At this temperature, the water inside the food is frozen completely.
Next, the food is cooled to about −60° C., in order to freeze composite water that may exist, such as water mixed with oil. Preferably, the food is cooled to −60° C. in approximately 5 to approximately 50 additional minutes. More preferably, the food is cooled to −60° C. in approximately 10 to approximately 30 additional minutes. At this point, the food is completely frozen throughout.
The velocity of coolant circulated in the freezer, such as by a fan, is preferably set to be proportional to the heat transmission efficiency. It is considered that the stronger the velocity of the coolant, the better the heat exchange rate is. However, the velocity of the coolant in the freezer shall be controlled in consideration of the whirlpool motion of air circulating therein and the proper heat exchange in the relation between the flow and the obstruction.
As for the variable cooling, liquid nitrogen and a liquid carbon dioxide can be considered as a coolant. From the aspect of the evaporation temperature and the evaporation latent heat, the nitrogen has −196° C./47 Kcal and carbon dioxide has −78.9° C./137 Kcal. A coolant which has more evaporation latent heat within the range of −60° C. is most suitable. Carbon dioxide gas is preferred.
Temperatures and times described herein are described in connection with preferred embodiments. One skilled in the art will understand that the temperatures and times may differ based on the composition of the food, the size and type of the freezer, etc.
In accordance with another aspect of the present invention, a system and method for thawing frozen food is described with reference to
As illustrated in
Another embodiment of the present invention is shown in
Similar to the method described with reference to
With the system 700, a large volume of frozen food may be thawed at the same time.
As illustrated in
Thus, the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. It is to be understood that the invention is not to be limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
