GB2261807A - Thawing frozen food - Google Patents

Abstract

A method of thawing frozen food characterized by directing ultrasonic energy into frozen food subject to the conditions that (a) the frequency is high enough that the energy is not attenuated excessively at the surface of the food, and (b) the frequency is low enough that the energy is not attenuated excessively in the body of the thawed food, whereby the energy effecting the thawing results primarily from the absorption of the ultrasonic energy in the zone between any thawed portion and the substantially frozen portion, the thawing zone. A further condition may be that the cavitation threshold is increased for example by increasing the ambient pressure.

Description

THAWING FROZEN FOOD The present invention relates to the thawing of frozen materials, primarily frozen foodstuffs.

General background: The use of deep-freezing to preserve foods is now widely established. The food to be preserved is frozen, to a temperature at which the fraction of the water content of the food remaining in liquid form is small. Temperatures in the region of -10 C to -30 C are typical - the fraction of the freezable water content of raw meat, for example, remaining liquid is typically in the region of 10% at -10 C, 5% at -20 C, and 3% at -30 C. At such temperatures, food can often be stored for long periods of time, of the order of months or even years, without major deterioration.

The food may be frozen as large blocks, which may include many portions, or in smaller, even individual, pieces. Some relatively pure ice may be present around the food and/or between different portions.

When such frozen food is to be used, it must of course be thawed. Some foods (such as some fruits) need only be brought to ambient temperatures; some (such as pre-cooked meals) need to be brought to a high temperature for eating; and some (such as meat and fish) need to be cooked, i.e. brought to a high temperature and held at that temperature for a substantial time.

(It will be realized that by thawing we mean substantially complete conversion of the ice content to liquid form. This is distinct from procedures such as bringing the food up to a temperature at which it loses rigidity but still retains a significant fraction of its water content in the form of ice.) Natural thawing: The simplest technique for thawing consists of merely leaving the frozen food at normal ambient temperature. This results in a thawing time which is both variable and long. The time is variable because it is dependent on the ambient temperature, which is dependent on a variety of factors (such as time of year); it is long because the rate of heat transfer into the interior of the frozen food is low, being dependent on the temperature gradient (which is low and decreasing as thawing proceeds) and the thermal conductivity of the food (which is also low).

It will of course be realized that a large proportion of the heat required for thawing is absorbed in effecting the phase change (melting) of the water content of the food from ice to liquid water (the latent heat requirement). This melting occurs gradually, over a temperature range, and is generally substantially complete by about -0.5 C. The temperature gradient from the surface of the food to the thawing zone between the fully thawed and the substantially fully frozen regions is therefore the major gradient affecting the thawing time.

The length and variability of the thawing time which results from this technique produce significant problems.

In many cases, particularly those in which the food is to be heated or cooked, it is important to ensure that the food is fully thawed before heating or cooking. If there are any regions with a significant proportion of ice remaining in the interior of its mass at this stage, they will have an effectively very large thermal inertia, because of the latent heat required to melt this ice content. They are therefore likely to remain cool and uncooked. This is likely to make the resulting food less palatable.

More seriously, this may also result in a health hazard.

Some foods may contain a variety of bacteria which can cause food poisoning, and the importance of thorough cooking of such foods is well recognized. The presence of uncooked pockets in such foods is highly undesirable, especially if those pockets have been warmed, so promoting the growth of any bacteria which may be present.

In domestic situations, the time required for thawing food can be inconvenient. In commercial situations, it may be essential to minimize the time required for thawing. The presence of large quantities of food being thawed slowly not only puts a strain on the physical organization of the plant, but the more food there is in the process of thawing at any one moment, and the longer the thawing takes, the greater the chance of some item being taken for use before it is properly thawed.

There is therefore a need for a faster and more reliable thawing technique.

Thermal thawing: One possibility is to immerse the food in water, rather than leaving it in air. If the temperature of the water is controlled, this improves the speed and reliability of the process, since the time taken for thawing can be determined more accurately, so the safety margin which has to be allowed is greatly reduced.

However, this technique produces only a modest improvement in the thawing rate. The temperature of the surface of the food is forced towards the water temperature. (With air, the temperature of the surface of the food is likely to be be significantly below the ambient temperature, because of the low thermal capacity of air, even if forced circulation of the air is used.) The temperature gradient in the food will, however, remain relatively low. Also, there is a danger of contamination of the food with bacteria and other micro-organisms suspended in the water.

Another possibility is to increase the ambient temperature (using either air or water). This can produce a significant improvement in the thawing rate, because the temperature gradient in the food is substantially increased. However, this technique results in the surface portions of the food being held at elevated temperature for considerable times. This can result in partial cooking of those portions, and increases the danger of the growth of harmful bacteria in those portions.

The use of this technique is largely restricted to situations where the food is to be heated or cooked straight away.

These techniques of thawing by air or water, which we can conveniently term simple thermal thawing, thus have significant drawbacks. The use of other forms of heat has therefore been considered.

Other thawing techniques: Vacuum thawing involves inducing a reservoir of water to boil at low temperature, the resulting water vapour condensing on the food to be thawed and so supplying its latent heat of condensation to the food. This is surface heating, with the disadvantages discussed above.

Infra-red heating is also essentially surface heating, and has similar drawbacks.

Microwave heating appears promising at first sight, but the penetration of microwaves is small. Also, it is found that microwaves are preferentially absorbed by liquid water rather than ice. Microwave thawing therefore produces localized hot pockets in the frozen food, which rapidly heat up and cook while the main body of the food remains frozen. The only way to thaw food effectively using microwaves is to keep the energy input low enough for thawing of the bulk of the food to occur by the diffusion of heat from the hot spots into the still frozen surrounding portions. The result is that there is little or no advantage over simple thermal thawing.

RF (dielectric) heating achieves better penetration than microwave heating, but suffers from the same drawback of runaway heating of thawed portions of the food.

Sonically assisted thermal thawing: Another approach, on which various proposals and investigations have been made, uses sonic or ultrasonic vibration, the material to be thawed being immersed in water into which the sonic energy is introduced. Various frequencies have been proposed, ranging from lowish audio frequencies up to typical ultrasonic frequencies.

In this technique, the sonic energy assists the process of thermal thawing, and we can can conveniently term the technique sonically assisted thermal thawing. The primary source of heat for the thawing is the water in which the material is immersed, and the sonic energy promotes the transport of heat into the material being thawed; the intensity of the sonic energy itself is generally a small fraction of the total energy required for thawing.

The sonic energy effectively produces a large increase in the thermal conductivity of the thawed material. Different explanations have been proposed for the mechanism by which the sonic irradiation promotes the transport of heat into the material being thawed; different explanations may be valid to varying extents and/or in different circumstances.

Thus microagitation and/or streaming or convection currents may increase the rate of heat transfer. The ultrasonic vibrations may cause agitation of the thawed liquid portions of the layer of food intermediate between the surface and the frozen core, thus increasing its heat conductive properties and thereby reducing the time to thaw the entire frozen food package. The change in density in thawing may involve a liquid mass transfer, which may in turn involve a liquid flow towards the solid region of the food and carrying heat with it; the thawing rate will thus be affected by the transfer rate, and the thickness of the phase change region between thawed and frozen will be increased by the sonic wave energy and so will increase the thawing rate.

Different investigations have also reported different results. One investigation (A.L. Brody et al., Food Technology, vol 13, Jan.-Dec. 1959, pages 109-112) concluded that the technique does not work. More specifically, it found excessive heating of the surface of the material being thawed.

Others, however, have reported or implied good results, with the rate of thawing being substantially increased without undue heating of the surface regions. The effectiveness of the technique is probably dependent on a variety of factors, such as the temperature of the water surrounding the material being thawed, the nature of the material, and the precise nature of the sonic irradiation.

Although the sonically assisted thermal thawing technique decreases thawing time, it has various drawbacks. It requires the use of warm or hot water as the primary heat source, and the thawing rate is therefore limited by the need to avoid heating the surface regions of the food to the point where they undergo changes. Further, the thawed food is warm, at least over its outer portions, and if by chance some item of thawed food should be left for a substantial time before being cooked, there is a serious danger of the growth of harmful bacteria.

Rosenberg et al., US PS 3846565, used a heated medium, such as water or oil, to reheat frozen food, especially precooked food packs for use in preparing meals. The reheating is assisted by sonic or ultrasonic vibratory wave energy.

Frequencies of 20, 40, 50, 72, 250 and 570 kHz are specifically mentioned as reducing reheating times, with those in excess of 100kHz indicated as preferred. However the purpose in Rosenberg is to produce foodheated to 1800F (some 750C) or more, not to only thaw the food pack.

Kissam, US PS 4464401, discloses the use of frequencies between 177Hz and 10,000Hz but only to supply a very small part of the heat needed, the low frequency and low power acoustics being referred to as appearing to stimulate heat transfer rather than apply energy. Kissam et al., Journal of Food Science, vol 47 (1981) pages 71-75 is relevant to this technique.

Various Japanese documents refer to the use of cavitation, resulting generally from bursting bubbles, to generate ultrasound in water surrounding food to be thawed (JP PS 59-055175, JP PS 59-169481, JP PS 62-157623). Two other Japanese documents emphasise the use of cavitation and a wide frequency range, 20kHz to 1MHz, without any specific frequency in the range being identified (JP PS 62-245143 and JP PS 63-157123).

None of the above provides a solution to the problem of speedily and efficiently thawing a large frozen block of food without raising the temperature excessively.

The present invention - general principles: We have discovered that under appropriate conditions, ultrasonic vibration directed into frozen food which is partially thawed is preferentially absorbed in the zone between the thawed and substantially frozen portions (the thawing zone). The present invention is based on this discovery, and in one aspect consists in essence of a method of thawing frozen food by directing ultrasonic energy into frozen food under appropriate conditions to effect thawing, the energy effecting the thawing being supplied in the form of ultrasonic energy and the thawing resulting primarily from the absorption of that energy at the thawing zone. We have found that substantially all of the ultrasonic energy reaching the thawing zone is absorbed, with only a small fraction passing on through into the remaining frozen parts of the food.

As noted above, the phase change of the water content of the food from ice to liquid water occurs gradually, over a temperature range. During thawing, there is therefore a thawing zone rather than an ideal boundary surface. The effectiveness of the thawing is dependent on the degree of absorption in this zone. The front (fully thawed) edge of this zone, which we can term the thawed/frozen interface, is fairly well defined as the boundary between fully thawed (ice-free) food and food which still contains a significant proportion of ice; the back edge is less precisely defined, but can generally be taken as where the proportion of freezable water which is still liquid becomes small. We have found that the absorption of the ultrasonic energy is in fact at its greatest in the region of the thawed/frozen interface.

Any energy which does pass on through the thawing zone will of course undergo attenuation in the frozen parts, warming them and so reducing the amount of heat eventually required to effect thawing in the thawing zone. This effect will however be small, for two reasons. One is that we have found that almost all the ultrasonic energy reaching the thawing zone is absorbed there, so the amount which passes beyond it into the still frozen food is small; the other is that the absorption of the ultrasonic energy in the frozen food beyond the thawing zone is small (though larger than that in the thawed region).

We have found that there are two major factors affecting the conditions under which this technique is effective.

The first condition is that the ultrasonic energy must be adequately coupled to the food to be thawed. For this, a liquid couplant such as water will generally be needed between the ultrasonic transducer and the food being thawed. It may be desirable to put the food into a bag of thin plastics material for thawing, to prevent loss of liquid food into the water or penetration of the water into the food. If this is done, then care should be taken to avoid air bubbles or voids in the bag.

This can be done by evacuating the bag at least before starting thawing or, if convenient, during thawing as well. In this way the bag will initially conform with the food and, if evacuation continues, conformity will be retained even if the food changes shape or voids collapse, also gas which may be released by thawing can be removed. To reduce voids in the food the food maybe compacted before thawing and even during thawing, in addition to any elevated ambient pressure, although this latter alone may also be used.

Further, coupling losses must be kept modest compared to the energy being transmitted into the food. We have found that at certain combinations of frequency and power density, there is a very high energy absorption in the region of the surface of the food; these regions of the frequency/power density plane must therefore be avoided. We believe that this effect is due to cavitation, and we will use the term "cavitation" to refer to it.

More specifically, the frequency below which this effect is significant is (under simple conditions) in the region of 430 kHz, dependent on factors such as the power level. We have found that this frequency is quite sharply defined.

This condition represents a major distinction from the sonically assisted thawing technique discussed above. The proposals using that technique generally use frequencies which are either in the audio range (below 20 kHz) or in the lower parts of the ultrasonic range (say up to around 100 kHz).

These frequencies are all significantly below the lower frequency limit just mentioned for our technique.

The second condition is that the attenuation of the ultrasonic energy by the thawed food sets a limit to the depth to which thawing can be effectively carried. The absorption of ultrasonic energy by the thawed food will be greatest at the surface of the food, and this should be modest compared to the amount of energy reaching and being absorbed at the thawing zone within the food. We have found that this attenuation is dependent on frequency, rising with rising frequency.

More specifically, if the frequency is raised to around 740 kHz, the depth to which thawing can effectively be carried out is likely to be too small to be of much practical use, though we have found that the limiting frequency is not sharply defined.

There has been only one specific investigation in the prior art involving a frequency which is above the limit set by the first condition. The frequency involved in that investigation was 1 MHz, which is well above the limit set by this second condition, and the result reported was that effective thawing was not achieved.

Improvements over the basic principles: As noted above, we have found that there are two major factors, cavitation and attenuation, affecting the conditions under which the present technique is effective. With both these factors, what limits the effectiveness of the present process.is the absorption of a substantial amount of energy in the surface regions of the food.

Considering first the absorption coefficient factor, the maximum rate of absorption is in the surface regions of the food, where the energy density is greatest, though absorption will of course occur throughout the thawed region, with the energy reaching the thawing zone decreasing exponentially as its depth increases. If the absorption in the surface regions of the food is -too high compared with the energy reaching the thawing zone, then the temperature of the surface region of the food will rise unacceptably high.

This effect can be reduced by cooling the surface regions of the food. This can be achieved by circulating the liquid which couples the ultrasonic transducer to the food. If the liquid is water or brine, then the surface of the food can be cooled to near 0 C. (The surface of the food must not be cooled so much that it tends to become frozen.) A further reduction of this effect can be achieved by reducing the ultrasonic power input as the thawing zone moves further though the food. This allows the cooling of the surface to remove more of the heat generated near the surface, but the rate of thawing is of course reduced.

Considering now the cavitation factor, standard counter-cavitation measures may be utilized. Thus the ultrasonic energy may be pulsed. With steady power, cavitation effects build up roughly exponentially; pulsing allows the cavitation effects to collapse during the off periods, so allowing the average power level to be increased. Also, the system may be subjected to elevated static ambient pressure, which reduces the onset of cavitation.

The use of pulsed power has the advantage that it is relatively simple, involving only the electronics of the system, but the extent to which it allows the power to be increased is limited. The use of increased static pressure is more demanding on the mechanical design and construction of the system, but is not subject to any natural limit. Indeed, if it is assumed that cavitation occurs when the peak low pressure becomes absolutely negative, then the maximum allowable amplitude is proportional to-the static pressure and the maximum allowable intensity will be proportional to the square of the static ambient pressure.

Two restrictions are imposed on the present thawing process by the cavitation limit: it imposes an upper limit on the power density in the ultrasonic wave, and it imposes a lower limit on the frequency. Thus the use of pulsed power and/or elevated static pressure allows the power density to be increased and/or the frequency to be reduced.

The intensity of the ultrasonic energy is limited by the need to avoid surface heating. We have found no evidence of cavitation at a power density of around 5 kW m-2 at atmospheric pressure for continuous ultrasonic power at 500 kHz. This corresponds to a thawing rate round 1 mm a minute (measured as the rate of advance of the thawing zone through the frozen food). We suspect that if a rate very much greater than this is desired, then anti-cavitation measures must be taken.

The frequency of the ultrasonic energy is also restricted by the need to avoid cavitation, which, for a given power, occurs below a certain frequency. As discussed above, the lower the frequency, the greater the depth to which thawing can be effectively carried out. If the onset of cavitation is delayed, the frequency can be reduced, so increasing the thickness of food which can be effectively thawed.We believe that this will permit the phenomenon of the present invention (the preferential absorption of ultrasonic energy in the thawing zone) to be utilized at frequencies very considerably (possibly up to two orders of magnitude) below the frequency limit discussed above. (One practical factor limiting the extent to which the frequency can be reduced is that it must be kept above audible limits; since the power levels involved are high, adequate sound insulation is unlikely to be feasible.) Monitoring the progress of thawing: The impedance presented by the food being thawed to the ultrasonic energy and its transmission through the food and/or reflection from inside the food will be dependent on the extent to which thawing has proceeded, and may therefore be used to monitor the progress of the thawing. For this, it may be desirable to turn off the main ultrasonic power temporarily.

It may be desirable to monitor the progress of the thawing at regular intervals, to establish parameters of the particular block being thawed; the completion of thawing can then be determined in relation to those parameters.

Practical implementations: Further aspects and features of the invention will become apparent from the following further discussion, which includes a description of a commercial thawing apparatus embodying the invention and which is given with reference to the drawings, in which: Fig. 1 is a simplified perspective view of the apparatus, Fig. 1A is a detail of a transducer assembly of Fig. 1, Figs. 2 to 6 are graphs showing the effectiveness of the present process for various values of frequency of the ultrasonic energy, and Fig. 7 is a graph showing the effectiveness of the present process for various values of pressure and of frequency of the ultrasonic energy.

The thawing may be either a batch or a continuous flow process. For a batch process, the size of the zone of ultrasonic energy will be matched to the size of the blocks to be thawed. For a continuous flow process, the zone will be moved relative to the food to be thawed, e.g. by carrying the food on a conveyor, and the width of the zone will be chosen to match the width ofthe blocks of frozen food; its length (in the direction of motion) will be chosen in dependence on the desired rate of thawing.

For a continuous flow process, the conditions will normally be held constant. For a batch process, it may be desirable to vary the conditions as thawing proceeds (e.g. by decreasing the ultrasonic power or increasing the flow of coolant as the depth of the thawing zone from the surface increases, to prevent excessive heating of the surface of the block).

The apparatus of Fig. 1 is primarily intended for thawing food frozen in blocks of roughly predetermined size; typical surface areas vary in the region of from 150 by 150 mm to 400 by 400 mm, and typical thicknesses in the region of 50 to 150 mm.

The food is typically meat or fish, with little or no surrounding ice. The apparatus is designed primarily for commercial purposes, and provides a continuous flow process, with the ultrasonic energy being coupled to the blocks from both sides by water as the couplant.

The apparatus comprises a water bath 10 with a corrugated conveyor belt 11 passing through it, driven and guided by drive and guide wheels and rollers 12. A pair of transducer assemblies 13 and 14, energized by a drive unit 16, are arranged opposite each other, as shown, such that blocks 15 of frozen food placed on the conveyor are passed through the zone-between the two transducer assemblies. Power for the drive unit 16 is supplied at 19. Frozen blocks of food are placed on the conveyor at the right-hand side of the apparatus, and emerge thawed at the left-hand side, where they can be removed. The conveyor includes means such as clips (not shown) for holding the frozen blocks on it as they are submerged.The two transducer assemblies direct ultrasonic energy onto opposite faces of the blocks, so doubling the thickness which can be effectively thawed (or, if the blocks are relatively thin, halving the thawing time).

Each of the assemblies 13 and 14 consists of an array of individual ultrasonic transducers arranged as shown in Fig. 1A, which shows the face of assembly 13. As shown, the assembly includes fourteen transducers 17. To minimize the loss of intensity at the ends of the transducer assemblies, the transducer density is increased at their ends. (Alternatively, the intermediate transducers may be energized with lower power than the end ones.) One of the limits on the power of the system is, as discussed above, is the requirement that cavitation should not occur. This limits the power intensity (power per unit area); so, to achieve maximum power, the faces of the transducers 20 should be large and should be packed closely together.To increase the power of the apparatus further, more rows of transducers can of course be used; the transducers in adjacent rows can conveniently be either side by side or staggered (i.e.

on either a square or a hexagonal lattice). To increase the cavitation threshold the pressure can be increased, at least in the spacec between the transducers. If it is not necessary to minimize the thawing time, the power intensity can of course be reduced. This can be done either by scanning or oscillating a transducer to and fro across the food to be thawed (mechanically or electronically), or by using a transducer (or transducer assembly) which generates a substantially uniform beam of ultrasonic energy, whichever is preferable as a matter of engineering convenience.

If the progress of thawing is to be monitored ultrasonically, the transducer assemblies can be periodically de-energized for this. One of the transducers can be used as a transmitter and another as a receiver for the monitoring; alternatively, a separate transducer (or pair thereof) can be provided therefor.

Other forms of apparatus can also be used to implement the present principles.

A second thawing apparatus for rectangular blocks comprises a pair of plate-like transducer assemblies, similar to those of the Figure 1 apparatus, but of a size matching the blocks to be thawed, and mounted so that they can be moved towards each other to contact the top and bottom faces of the block. This provides a batch process using direct contact. It may however be desirable to immerse the transducer assemblies in a water bath to prevent air cavities.

A third thawing apparatus consists of a container acting as a water bath and having a convenient number of transducers mounted on it to produce a "sea" of ultrasonic energy throughout the bath. This apparatus is suitable for blocks of irregular shape, which are merely held immersed in the bath and exposed to the ambient ultrasonic energy in it, somewhat like an ultrasonic cleaning bath. It will be realized that the ultrasonic energy will be absorbed with reasonable efficiency by the ice-containing portions of the food whatever its size, shape or position, somewhat like the absorption of microwaves in a microwave oven. It may however also be desirable to adapt microwave distribution techniques, such as the use of a "stirrer".

These second and third form of apparatus can conveniently be constructed so that they can be pressurized to reduce cavitation onset.

The third form of apparatus is suitable for domestic as well as commercial use. For domestic purposes, it will usually be convenient to use a power level which can be supplied by normal domestic mains supplies; that is, something not significantly above 3 kW.

When the cavitation threshold is increased, for example by subjecting the thawing system to elevated static ambient pressure, significant reductions in thawing times may be achieved. The usual care to avoid overheating is needed, although the temperature rise is often lower at elevated pressure. Other methods of increasing the cavitation threshold, for example pulsing the energisation of the transducers, may be applied.

The exact relationships of elevated pressure, frequency, efficiency and thawing time are not entirely clear. However elevated pressures of 300 kPa and 600 kPa both reduce the thawing time by a factor of between 5 and 10, increased pressure reducing the time. The variation of thawing time with frequency once the pressure is elevated is not great although the lower the frequency in the range up to 520 kHz the shorter the thawing time, at a given pressure. The efficiency appears to rise with elevation of pressure towards some 600kPa and with reduction of frequency-towards some 300 kHz but measurements of this aspect are not easy.

The power used to thaw small samples of frozen meat was about 1 watt/cm2, values in excess of -1 watt/cm2 could be used with lower frequencies (around 300 kHz) and higher pressure (around 600 kPa). At normal ambient pressure the power could not exceed about 0.5 watt/cm2 without risk of overheating.

General operating conditions: Figs. 2 to 6 are graphs which illustrate some of the conditions under which the present procedure is effective.

These graphs show behaviour for a continuous ultrasonic signal at atmospheric pressure.

Fig. 2 is a graph showing the attenuation A of ultrasonic energy in meat, plotted against temperature t (this graph is based on C.A. Miles & D. Shore, "Changes in the attenuation of ultrasound in meat during freezing", 24th meeting of Meat Research Workers, Kulmbach, vol 1, D 4:3 to D 4:6, 1978). It will be seen that the attenuation is small for thawed meat and is substantially independent of temperature. At the point where the meat is just beginning to freeze (just below 0 C) the attenuation rises sharply to a peak. As the temperature is decreased further, so the attenuation drops from the peak, and by the time the meat is substantially completely frozen, the attenuation has become small (about twice that for thawed meat), and temperature effects are also small.

This graph is valid for low energy levels. The absolute values of attenuation are dependent on the nature of the food; for example, the attenuation in muscle with its fibres aligned parallel to the direction of propagation of the ultrasonic energy is about twice that for muscle with the fibres aligned perpendicular to the direction of propagation. The absolute values are also roughly proportional to the frequency.

Fig. 3 is a graph showing the depth x to which a block of frozen food can be thawed before the temperature at its surface reaches a predetermined limit (say 20"C), plotted against frequency f. It is assumed that a uniform ultrasonic field is incident normally on the block.

Fig. 4 is a graph showing the temperature t of the block plotted against distance x into the block. The thawed/frozen interface is at point 23. The portion 24 to the left of this point shows that the temperature is highest at the surface (x = 0) and falls to just below 0 C at the interface. The temperature then falls sharply (portion 25) just beyond this interface, to the original frozen temperature (portion 26).

Fig. 5 is a graph showing the attenuation A of the block plotted against distance x into the block. This can be obtained from the Fig. .2 curve by horizontally compressing the left-hand (high temperature) part and expanding the right-hand (low-temperature) part. The similarity between the Fig. 2 and Fig. 5 curves arises because both the left-hand and right-hand parts of the Fig. 2 curve are substantially level, and its shape is therefore unchanged by such expansion and compression.

As the depth of the thawing zone increases, so the total attenuation which the ultrasonic energy experiences in reaching that zone rises, and the amount of energy reaching that zone thus falls. The rate at which that zone advances thus decreases steadily. The absorption of energy near the surface of the meat is steady, and this surface thus heats up steadily as the thawing zone gets deeper into the block. The temperature in the block falls from the surface to the thawing zone, partly because the parts near the surface have been thawed for longer and so have had longer to absorb energy than the deeper parts, and partly because the intensity of the energy decreases steadily with depth because of the attenuation in the thawed regions.

The portion 24 of the graph of Fig. 4 shows a substantial temperature rise towards the surface of the food being thawed, even though the attenuation, and hence energy absorption, in the thawed portions of the food is low compared to the absorption in the thawing zone. This is because the specific heat of the thawed food (which is roughly that of water) is small compared to the latent heat required for thawing from ice to water in the thawing zone.

If the surface of the food is cooled, e.g. by passing water at or near 0 C over it, then the temperature/distance curve follows branch 27, diverging downwards from branch 24 towards the surface of the food. Such cooling thus reduces the peak temperature attained in the food, and so allows the food to be thawed to a greater depth for a given limiting peak temperature.

As noted above, substantially all the ultrasonic energy reaching the interface is absorbed there and converted into heat. Some of this energy goes into melting the ice at the interface (latent heat), and some is conducted away from the interface, into the thawing zone on the cold side of the interface, where it goes into melting some of the ice content of this zone and warming it towards the thawed state.

Returning to Fig. 3, line 20 is for muscle with its fibres aligned parallel to the direction of propagation of the ultrasonic energy, i.e. normal to the surface of the block.

This line shows that the depth of thawing decreases with increasing frequency. This downward slope of line 20 with increasing frequency is due to the increasing rate of attenuation of the ultrasonic energy in thawed meat with increasing frequency. The depth x has fallen below a useful value at a frequency of some 740 kHz.

Line 21 is for muscle with its fibres in "perpendicular" alignment. This line is similar to but higher than line 20.

The attenuation of this alignment is less than for the "parallel" alignment, and the depth of thawing is thus greater.

Line 22 shows the sharp cut-off, which occurs between 430 and 460 kHz at normal ambient pressure. At this frequency and pressure very large surface heating occurs. It is essential that the frequency should be above this point for effective thawing to occur at normal ambient pressure.

Fig. 6 is a graph showing the efficiency E of thawing, plotted against frequency; the efficiency is defined as the ratio of the ultrasonic energy absorbed at the thawing zone to the total ultrasonic energy absorbed. Two lines are shown, 30 for the "perpendicular" orientation and 31 for the "parallel" orientation. The two lines share a common cut-off line 32 at the critical low frequency. The position of this line is dependent on the intensity of the ultrasonic energy, moving slightly to the right as the intensity is decreased. This is because the point at which cavitation occurs is dependent on intensity as well as frequency.

Figure 7 shows the thawing of test samples of frozen beef semitendinosus muscle tissue of 30 millimetres diameter cut perpendicular to the muscle fibres. The samples were 75 millimetres long. Thermocouples were inserted into the sample under test at various depths (d). Various transducers were used, energised from a radio frequency power amplifier.

Measurements of power supplied were made. The test sample was held in an insulated block and a transducer pressed onto the sample. The sample in the block was enclosed in a pressure vessel so that elevated pressure could be applied if required.

The elevated pressure was achieved by pressurizing the air in the pressure vessel.

The time (T) in hours for each thermocouple to reach -1 C (considered to be the thawing zone position) is shown on the graph for various frequencies and ambient pressures. The curve labelled Th is the theoretical behaviour on the assumption of steady state thermal conduction through a thawed region of conductivity K and depth d from the surface at a temperature n'e above the initial freezing point. AH is the initial enthalpy of the frozen meat relative to that at 0 C in kJ/kg and p the specific gravity. When T is the time for the thawed zone to reach depth d T = AHpd2 2kQ9 and this gives curve Th.

The group of curves labelled OkPa are the actual times for four different frequencies at ambient pressure with a power level held to the level to avoid overheating of the sample. The points on the curve for 300 kHz are marked with a circle, for 350 kHz with a square, for 415 kHz with a triangle and for 520 kHz with a diamond.

The group of curves labelled 300/600 kPa shows the actual times for the above frequencies at these elevated pressures.

These results indicate that a commercial frozen beef block 150 millimetres thick could be thawed in some 45 minutes when ultrasound is applied to both sides. The power level would be low enough to avoid excessive heating provided an elevated pressure is used to control cavitation at the lower frequency required for this thawing rate. A typical temperature maximum is about 7"C. Suitable frequencies would be around 300 kHz to 350 kHz with an elevated pressure of some hundreds of kilo Pascals, typically 300 to 600. It is expected that an elevated pressure as little as 100 kPa, that is approximately 1 atmosphere above ambient pressure, will produce an effective increase in the cavitation threshold even at frequencies below 300kHz. A proportion of the power supplied is not coupled to thaw the meat so some cooling may be needed to remove waste heat.

The significant point is that the increase in the pressure above ambient permits the use of the lower frequencies found to be more effective but liable to cause harmful cavitation at normal ambient pressure.

Claims (22)

1. A method of thawing frozen food characterized by directing ultrasonic energy into frozen food subject to the conditions that (a) the frequency is high enough that the energy is not attenuated excessively at the surface of the food, and (b) the frequency is low enough that the energy is not attenuated excessively in the body of the thawed food, whereby the energy effecting the thawing results primarily from the absorption of the ultrasonic energy in the zone between any thawed portion and the substantially frozen portion, the thawing zone.

2. A method according to claim 1 characterized by the further condition that the cavitation threshold is increased.

3. A method according to claim 1 characterised in that the frequency is below 740 kHz.

4. A method according to claim 3 characterized in that the frequency is above 430 kHz.

5. A method according to claim 1 characterised in the further condition that the ultrasonic energy is pulsed.

6. A method according to claim 1 characterised in the further condition that the thawing is carried out at a static pressure elevated above ambient pressure.

7. A method according to Claim 6 characterised in that the static pressure is elevated up to 600 kPa above atmospheric pressure.

8. A method according to- Claim 7 characterised in that the static pressure is elevated to at least 300 kPa above atmospheric pressure.

9. A method according to Claim 6 characterised in that the frequency is reduced when elevated pressure is applied.

10. A method according to any previous claim, characterised by means for determining the completion of thawing by detecting the passage of ultrasonic energy completely through the food.

ll.A method according to any previous claim, characterized in that the food is subjected to a directed beam of ultrasonic energy.

12. A method according to any one or claims 1 to 10 characterized in that the food is subjected to two oppositely directed beams of ultrasonic energy.

13. A method according to any one of claims 1 to 10 characterized in that the food is subjected to a substantially random sea of ultrasonic energy.

14. A method according to either of claims 11 and 12 characterized in that the ultrasonic energy is applied to the food by means of direct contact with the at least one source of the ultrasonic energy.

15. A method according to either of claims 11 and 12, characterized in that the food is moved transversely through the at least one beam of ultrasonic energy.

16. A method according to any one of claims 11 to 13, characterized in that the ultrasonic energy is coupled to the food by means of aqueous liquid.

17. A method of thawing frozen food according to any of claims 1 to 16 characterized in that the food is contained in a conforming jacket.

18. A method according to claim 17 characterized in that the jacket is arranged to be evacuated at least initially.

19. A method according to Claim 17 characterized in that the evacuation of the jacket is continued during thawing.

20. Apparatus to thaw frozen food in accordance with the method in any of claims 1 to 19.

21. A method of thawing frozen food substantially as herein described with reference to the accompaning drawings.

22. Apparatus to thaw frozen food substantially as herein described with reference to the accompaning drawings.