GB2044906A

GB2044906A - Defrosting frozen articles

Info

Publication number: GB2044906A
Application number: GB8007475A
Authority: GB
Original assignee: United Biscuits Ltd
Current assignee: United Biscuits Ltd
Priority date: 1979-03-16
Filing date: 1980-03-05
Publication date: 1980-10-22
Also published as: GB2044906B

Abstract

In the defrosting of a frozen article (particularly a food product), the article is subjected to a flow of heat transfer medium, such as air, and the temperature of the medium is controlled to provide and maintain a low temperature differential between the medium and the article, as the temperature of the latter rises during defrosting. The arrangement can include a low temperature heat source (9) (e.g. electric heating elements 10) which is located between a circulation fan (8) and the articles to be defrosted (e.g. food products in trays 7 carried in trolleys 5). The heat source (9) may be controlled by a dead zone thermostat and the heat transfer medium is preferably circulated around the articles at a comparatively high flow rate to prevent substantially any condensation forming thereon during defrosting. The apparatus can be incorporated in a cold room or a cold-storage transport vehicle. <IMAGE>

Description

SPECIFICATION Defrosting frozen articles This invention relates to a method of defrosting frozen food products and to apparatus for performing such a method.

A large number of frozen food products, such as pizzas, cooked pies and cooked meats, require to be defrosted before they are displayed at their point of sale where they are maintained, on display, in a chilled atmosphere.

For such display purposes, it is highly desirable to avoid the formation of visible condensation on the food product or its packaging, which condensation is formed during the defrosting process when carried out too rapidly.

Also, in many retail food outlets, it is necessary to defrost frozen food products overnight, say for twelve hours, for subsequent display at their point of sale the following morning.

Conventional defrosting methods are unable to provide such a comparatively short period for defrosting the products, without the formation of condensation on the products and/or their packaging. This is because a large amount of heat has to be applied tithe bulk-packaged, frozen product which not only produces condensation but can also cause uneven defrosting of the product and hot spots which, if above, say, 7"C can cause bacterial growth and consequential bacterial damage.

Furthermore, once the products have been defrosted to their display temperature, say, 2-3"C, it is highly advantageous to maintain them at this temperature not only at their point of sale but also in the chill store in which they are stored after defrosting, but before display, whereby substantially any bacterial action upon the defrosted products is prevented.

Known defrosting methods, such as natural warming in a heated room, microwave heating and warm air heating, have the above-mentioned disadvantages associated with them. For instance, with presently known methods of warm air defrosting, there is a high temperature differential between the heat transfer medium, namely, warm air, and the product to be defrosted, whereby condensation is formed thereon. Additionally, hot spots can easily be formed and heat transfer through the product is irregular.

One object of the invention is to provide a method of defrosting frozen products which eliminates the disadvantages associated with known defrosting methods, whilst another object of the invention is to provide apparatus for performing such a method.

In accordance with one aspect of the invention, there is provided a method of defrosting a frozen article comprising the steps of subjecting the article to be defrosted to a flow of heat transfer medium and controlling the temperature of the medium to provide and maintain a low temperature differential between the medium and the article as the temperature of the latter rises during defrosting.

In accordance with another aspect of the invention, there is provided an apparatus for defrosting a frozen article comprising means for subjecting an article to be defrosted to a flow of heat transfer medium and means for controlling the temperature of the medium to provide and maintain, in use, a low temperature differential between the medium and the article as the temperature of the latter rises during defrosting.

Preferably, the heat transfer medium is a gas, such as air, but it can comprise a liquid, which is circulated around the frozen article and whose temperature is controlled by a "black heater", that is, a low temperature heat source. Advantageously, the medium is circulated around the frozen article at a comparatively high flow rate which, in the case of a gas, prevents substantially any condensation forming on the article during defrosting. The heat transfer medium, in the case of a gas, is preferably circulated around the frozen article, for instance, a frozen food product, by means of a fan, a low temperature heat source being located in the path of the circulating medium between the fan and the article to be defrosted.

In order that the invention may be more fully understood, an apparatus, in accordance therewith, will now be described by way of example and with reference to the accompanying drawings in which: Figure lisa schematic, elevational side view of a defrosting apparatus; and Figure 2 is a diagrammatic view of a thermostat incorporated in the apparatus of Figure 1.

Referring to Figure 1, a chill room hasthermallly- insulated side walls 2 and roof 3. On the floor 4 of the room 1 stands two wheeled trolleys 5 provided with shelves upon which are supported trays 7 containing frozen food products, such as pizzas, to be defrosted.

Afan 8 is supported above the room floor 4 and between the output of the ean and the trolleys 5 is located a "black" or low temperature heater 9 comprising a frame 11 carrying a plurality of spaced electric heating elements 10 and a wire mesh grill which acts as a collimator 12 for the air passing over the heating elements from the output of the fan at a flow rate of 4,200 cubic feet per minute. Thus, air passing from the output of the fan 8 passes over the heating elements 10, is heated thereby, and then, via the collimator 12, circulates around the food products contained in the trays 7, where it loses heat to the products, and back to the input of the fan, as indicated by the arrows, for re-heating and recirculation.The heat provided by the heating elements 10 and the high throughput of the fan 8 are such that a low temperature differential between the heated air passing around the frozen food products is maintained throughout the defrosting process.

This low temperature differential prevents substantially any undesirable condensation or hot spots forming on the surface of the products or their packaging.

Once the products have been defrosted, at a thawing temperature of, say, from -5 C to 0"C, it may be necessary to maintain the defrosted products at a chill temperature of, say, 2"-6"C, thereby preventing them from rising to a high temperature at which bacterial growth can start. In this regard, reference is now made to Figure 2 which shows a particular type of thermostat 21 known as a dead zone thermostat.

The dead zone thermostat 21 comprises a temperature sensor 22 which is filled with a temperatureexpansive fluid, such as alcohol, in communication with a bellows 3. An actuating element 24 is secured to the bellows 23 and is arranged to operate a switch A. Similarly, another actuating element 25 is also secured to the bellows 23 and is arranged to operate a switch B. In its "ON" position (ann), the switch A causes the heating elements 10 to be operated and this "ON" position can be effected manually to start the defrosting process.When the sensor 22 senses the temperature of the food products at a predetermined value, say, a few degrees C above the thawing temperature of the products, the consequential expansion of the bellows 23 causes the element 24 to engage and operate the switch A into the "OFF" (toff) position, whilst the switch B is still maintained in the "OFF" position (Boff). Thus, at this predetermined temperature the heating elements 10 are switched off and the lem peratu re of the food products will rise slowly due to heat leakage into the room 1.If the temperature of the products rises to another, higher predetermined value, say, a few degrees C above the first predetermined temperature, then consequential expansion of the bellows 23 causes the element 25 to engage and operate the switch B into the "ON" position (boy). Because, in the "ON" position, switch B causes a refrigeration unit (not shown), associated with the fan 8, to start operating, the cold airflow produced by the refrigeration unit and fan 8 causes the products to begin to cool. When the products have cooled to the first predetermined temperature, that is to say, the temperature which is a few degrees C above the thawing temperature of the products, the switch B is operated, under bias, to its "OFF" position, as a consequence of the contraction of the bellows 23 and the consequential disengagement of the element 25 from the switch.Thus, the refrigeration unit is stopped and, once again, the temperature of the chilled product begins to rise. Therefore, it can be seen that, once the products have reached the first predetermined temperature after the defrosting process, they can be maintained within a preset temperature range by means of the dead zone thermostat and the associated refrigeration unit.

The switches A and B, and their respective elements 24, 25, can be adjusted to provide any required temperature range in which the products are to be maintained once they have been defrosted.

Tests on the defrosting of pizzas were carried out as follows: 1360 kgs of pizzas were both loosely and closely stacked in trays upon trolleys, as drscr;had above, located within a cold room having an interior volume of 85 cubic metres. The pizzas were at a normal cold storage temperature of -26 to -208). Twenty-four stainlessstell, íKW heating elements 10 were then actuated at half power, to provide an output of 12KW, together with the fan 8. The circulating air flow, provided by the fan 8, as described above, provided a temperature differential between the air and the frozen pizzas of 2-3 degrees C. Overnight, that is to say, in about 12 hours, the pizzas had passed their thawing temperature of -3" to -2 .

When they had reached 2" to 3"C, the switch A of the dead zone thermostat was switched from the "ON" to the "OFF" position, whereby the heating elements were deactivated, the switch B at this time being in its "OFF" position. The temperature of the defrosted pizzas then slowly rose to 50C at which point the switch B was operated from the "OFF" to the "ON" position, thus activating the refrigeration unit. By this means, the temperature of the chilled pizzas was returned to 2 -3 C and was prevented from rising to 7"C at which temperature bacterial growth would be promoted on the pizzas. At 2"-3"C, the switch B was operated to its "OFF" position whereby the refrigeration unit was deactivated.Thus, it was able to maintain the defrosted but chilled pizzas with a temperature range of 2"-5"C. The fan was maintained operative during the whole process to provide constant circulation, and hence agitation, of the air, whereby an even temperature was maintained throughout the atmosphere in the room.

During the defrosting process, the temperature differential between the airflowing around the pizzas and the pizzas themselves was maintained at 2-3 degrees C and, as a consequence, no visible condensation was formed on the surface of the pizzas or their packaging. Also, the formation of hot spots was prevented, due not only to the low temperature differential but also to the comparatively high airflow provided by the fan.

Additionally, a similar trial was performed in which only one trolley of pizzas was located comparatively close to the collimator 12 of the heating elements 10. Again, the formation of hot spots and visible condensation was avoided throughout the whole defrosting process.

Although the above-described trials were carried out with the pizzas both loosely and closely packed upon the trolleys, the defrosting process can be carried out upon tightly and densely packed frozen food products. However, it will be appreciated that longer defrosting times will be required, due to the increased amount of heat required to pass into the interior of the products, in view of the high weighty surface area ratio.

It should be noted that, although the above description in relation to the defrosting of frozen pizzas specifies a temperature differential between the circulating air and pizzas of 2-3 degrees C, the temperature differential applicable to the defrosting of other frozen food products can be different but, preferably, such that no condensation is formed on the defrosting product, or its associated packaging, if any, and no hot spots are formed on its surface. The temperature differential employed is dependent upon such factors as the specific heat, the density, the heat absorbtion characteristics of the surface, the emissivity and the contour of the surface of the frozen food product to be defrosted. For instance, if the surface of the product is generally irregular and has numerous upstanding filamentary elements, such as the grated cheese filaments on the pizza, then hot spots will tend to occur at these elements at temperature differentials of greater than, say, 2-3 degrees C. However, for generally smooth-surfaced products, such as pork pies, the temperature differential can be slightly greater than 2-3 degrees C at, say, 4-6 degrees C. However, it has been found that a temperature differential of greater than 10 degrees C tends to be unsuitable, in that it causes condensation to be formed on the defrosting food product and/or its associated packaging, as well as undesirably high hot spots, whatever the product.

Of course, it will be appreciated that formation of condensation and hot spots can also be dependent upon the rate of flow of the circulating heat transfer medium, higher rates tending to reduce the chance of condensation and hot spots.

Once the food product has been defrosted, it can, as discussed above with reference to pizzas, be maintained within a pre-set temperature range, in order to reduce substantially undesirable bacterial action. In the case of pizzas, this temperature range is preferably 2 -5 C. However, for other defrosted food products this range can be or can extend below or above this range, depending upon the nature of the defrosted product and the liklihood of bacterial growth occurring in or on the particular product at given temperatures.

The fan, heating elements and associated collimator, and dead zone thermostat of the apparatus described above can be incorporated into a bulk, cold-storage road transport vehicle for defrosting the transported frozen products between a large wholesale cold store and a retail food outlet, such as, a supermarket, whereby the defrosted and chilled products are transferred from the vehicle to the point of sale or into a chill room for subsequent display.

Alternatively, a cold store in a retail outlet, such as, a supermarket, could be modified to include the fan, the heating elements and associated collimator, and thermostat of the apparatus, whereby frozen food products could be defrosted overnight and maintained at a chilled temperature for immediate display at the point of sale the following morning.

Therefore, it can be seen that the present invention provides an inexpensive method and apparatus for defrosting frozen articles, without forming conde nsation or hot spots on the surface thereof, during the defrosting process, and that, as a subsidiary feature, the defrosted articles can be maintained within a preset temperature range, if necessary.

Particular components of the apparatus need not be limited to those described above. For instance, the dead zone thermostat described with reference to Figure 2 can be replaced by any other suitable dead zone thermostat, such as one which is electro nically operated.

Claims

1. A method of defrosting a frozen article com prising the steps of subjecting the article to be defrosted to a flow of heat transfer medium and controlling the temperature of the medium to pro vide and maintain a low temperature differential between the medium and the article as the tempera ture of the latter rises during defrosting.

2. A method as claimed in claim 1, in which the heat transfer medium is circulated around the frozen article during defrosting.

3. A method as claimed in claim 1 or 2, in which the flow of heat transfer medium is at a comparatively high rate, to prevent substantially any condensation forming on the article during defrosting.

4. Method as claimed in claim 1,2 or 3, in which the temperature of the heat transfer medium is controlled by a "black heater" constituted by a low temperature heat source.

5. A method as claimed in any preceding claim, in which the flow of heat transfer medium is caused byafan.

6. A method as claimed in claim 5, when dependent upon claim 4, in which the low temperature heater is located between the fan and the article to be defrosted.

7. A method as claimed in any preceding claim, in which the heat transfer medium is a gas.

8. A method according to any preceding claim, in which the temperature of the medium is controlled thermostatically.

9. A method as claimed in any preceding claim, in which the temperature differential is not greater than 10 degrees C.

10. A method as claimed in claim 9, in which the temperature differential is 2-3 degrees C.

11. A method as claimed in any preceding claim, in which the article, when defrosted, is maintained at a temperature sufficient to reduce substantially any bacterial growth in or on the defrosted article.

12. A method according to claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

13. Apparatus for defrosting a frozen article comprising means for subjecting an article to be defrosted to a flow of heat transfer medium and means for controlling the temperature of the medium to provide and maintain, in use, a low temperature differential between the medium and the article as the temperature or the latter rises during defrosting.

14. Apparatus as claimed in claim 13 including means for circulating the heat transfer medium around the frozen article during defrosting.

15. Apparatus as claimed in claim 13 or 14, in which said temperature control means comprises a "black heater" constituted by a low temperature heat source.

16. Apparatus as claimed in claim 13,14, or 15, in which said means for subjecting the article to a flow of heat transfer medium comprises a fan.

17. Apparatus as claimed in claim 16, when dependent upon claim 15, in which the low temperature heat source is located between the fan and article to be defrosted.

18. Apparatus as claimed in any of claims 13to 17 including a gas as the heat transfer medium.

19. Apparatus as claimed in any of claims 13 to 18 including a thermostat for controlling said tem perature controlling means.

20. Apparatus as claimed in claim 19, in which the thermostat is a dead zone thermostat.

21. Apparatus as claimed in any of claims 13 to 20, in which said temperature control means is arranged to maintain the temperature differential at not more than 10 degrees C.

22. Apparatus as claimed in claim 21, in which said temperature control means is arranged to maintain the temperature differential at 2-3 degrees C.

23. Apparatus as claimed in any of claims, 13 to 22, in which said temperature control means is arranged to maintain the article, when defrosted, at a temperature sufficient to reduce substantially any bacterial growth in or on the defrosted article.

24. Defrosting apparatus substantially as hereinbefore described with reference to the accompanying drawings.

25. A cold room incorporation defrosting apparatus as claimed in any of claim 13 to 24.

26. A cold-storage transport vehicle incorporating defrosting apparatus as claimed in any of claims 13 to 24.

27. An article defrosted by a method in accord ante with any of claims 1 to 12.