This invention relates to the microbial decontamination
of food, especially on a small, non-industrial scale such as domestically or in
The presence of microbes on food has two possible consequences:
their growth is often the major determinant of the shelf life of the food, and/or
their growth either on the food or on ingestion can be the cause of food poisoning.
Microbes may gain access to a food at virtually any stage of the food's manufacture,
from harvest of the raw materials (a notable source of contamination by soil-borne
microorganisms, including several pathogenic types), through post-harvest storage,
processing, and distribution. Good practice throughout the food chain from raw material
to finished product is intended to ensure that the food that reaches the consumer
is wholesome and, above all, safe to eat, yet outbreaks of illnesses attributable
to food-borne pathogenic microbes still arise, implying microbial contamination
at some link in the chain before the food reaches the consumer.
Both microbial contamination and growth are likely to continue
after the food has reached the consumer, of course. Contamination can occur, for
example, during selection of food on display, or similarly during harvesting of
home-grown produce; microbial contamination to some degree is practically unavoidable
during the domestic preparation of ingredients for inclusion in any dish or meal.
Microbial growth is influenced by the entire time/temperature history of the food
from harvest to consumption, which includes the conditions between the time the
consumer selects or purchases the food and their reaching home, and domestic storage.
Consumers are increasingly concerned about food poisoning.
Understandably, food poisoning outbreaks that involve fatalities receive much attention
from the media, which heightens this concern. The concern is perhaps particularly
marked in those with responsibility for feeding people in groups of known susceptibility
to food poisoning, such as the very young or old, or those whose immune system is
There is therefore a need for a device capable of achieving
microbial decontamination of food, without materially affecting the desirable characteristics
of the food, on a small scale; most obviously in the home, but also in many catering
situations, and even, where appropriate, at the point of sale. The invention described
herein enables one to provide such a device. Preferred apparatuses capable of performing
the invention may be versatile units, being suitable for use in a variety of situations.
They may be used to decontaminate food and food ingredients in the home (for example,
prior to inclusion as ingredients of a recipe, or before transfer to a domestic
refrigerator or freezer), or decontaminating chilled foods intended for eating cold
such as salads and fruit; in short, they can be used in most situations where the
consumer is aware of, or concerned by, the possibility of contamination or spoilage.
US-A-4,233,323 discloses a small-scale apparatus for tenderising
meat. A casing contains a grid for supporting meat, and UV tubes for irradiating
meat. Irradiation periods of one hour are disclosed, using UV in the range 265-300nm.
WO94/24875 discloses industrial equipment for the UV-sterilisation
of foodstuffs. Batch processing can employ a chamber having a food-supporting grid
in a central region, and UV sources distributed around the walls. There may be means
for moving the food relative to the grid during irradiation so that portions in
contact with the grid do not escape irradiation. WO94/24875 also discloses continuous
treatment, as does US-A-4,877,964. The latter discloses a UV treatment cavity with
a roller conveyor for conveying food. Some of the UV sources are below the conveying
path, being located between conveying rollers, so that the whole surface of the
food can be irradiated.
GB-A-695 474 discloses a cabinet for drying and sterilising
crockery etc using high temperatures (e.g. over 300°C) and UV irradiation.
US-A-2674 693 concerns UV treatment of flowing liquids.
FR-A-783 519 concerns the treatment of foodstuffs with
UV or IR to extend shelf life.
US-A-4 534 282 concerns a heat sterilisation process for
liquid foods which may then be cooled and UV-irradiated.
JP 05-103610 discloses the use of UV and microwave energy
to sterilise packages containing vinegared rice.
US-A-3926 556 concerns heat treatment of liquids, e.g.
milk pasteurisation, using microwave and UV energy.
DISCLOSURE OF INVENTION
According to the invention there is provided a method of
treating food which is to be stored uncooked and/or eaten raw comprising; supporting
the food on a food support in a treatment cavity of a treatment chamber;
applying energy to the food to raise its temperature to a treatment not exceeding
irradiating the food with UV.
The present invention provides a method for the (typically
small-scale) microbial decontamination of a food. and subjecting at least its uppermost
surface to UV irradiation. In a preferred method, the food is located within the
cavity on an apertured or UV-transmissive support. The food may be placed directly
on the support inside the cavity, or the food may be loaded onto the support outside
the cavity and the loaded support is then transferred to the cavity. In a preferred
method, the support is located generally centrally in the cavity and the food and
support are exposed to UV radiation from a plurality of UV sources mounted on or
adjacent at least one wall of the cavity and directed inwardly towards the centre
of the cavity so as to provide irradiation from all or several directions, including
irradiating the underside of the support and the underside of the food through the
support. The food is usually exposed to UV radiation for 15-120 seconds, preferably
25-60, more preferably 30-45 seconds. In some embodiments the U.V. irradiation conditions
are selected so that the UV generates little or no increase in temperature in the
product (e.g. under 1°, preferably under 0.1°). The food may be rotated
within the cavity during irradiation. The support may rotate to effect said rotation
of the food. Alternatively or additionally the food may be displaced relative to
the support, preferably substantially radially or substantially arcuately, most
preferably substantially arcuately, e.g. during rotation of the support, so as to
expose initial contact points between the food and the support to UV irradiation.
Rotation and displacement not only help to eliminate untreated areas, they also
help to make the treatment more uniform across the food surface, helping to reduce
the required treatment times.
I have also found that there are circumstances when controlled
heating of the food is desirable. This can serve to enhance the microbicidal effect
of the UV irradiation. Generally it will be desired that, for foods which are to
be eaten raw and/or stored uncooked, this heating should be insufficient to cause
denaturation of proteins in the food. For meat, temperature as low as 27° can
cause denaturation of some proteins, so desirably the temperature will not exceed
25°. Superficial heating can be effected by the UV itself (e.g. by using broad
band UV) and/or by IR irradiation. Deeper heating can be effected by microwave irradiation.
This can be useful for a method of thawing frozen food and ensuring that it is sterile.
I have also found that a step of cooling the product surface
rapidly after UV irradiation can enhance the microbicidal action, e.g. reducing
the surface temperature to 5° or below, more preferably 0° or below, preferably
within 10 or 20 minutes, more preferably within 5 minutes, of the UV irradiation
A unit for the (typically small-scale) decontamination
of a food, may comprise a treatment cavity provided with one or a plurality of UV
sources. Preferably, the UV is provided by at least four, more preferably six or
eight UV lamps. Preferably, the UV source(s) produce radiation falling essentially
exclusively in the wavelength range 220-300 nm, more preferably 220-265nm, especially
250-265nm. "Essentially exclusively" in this context means that at least 90%, preferably
at least 95% and more preferably at least 98% of the energy of the UV radiation
is in the specified range. The surfaces of the inner walls of the treatment cavity
are preferably highly UV reflective; that is, they preferably have a refectance
to radiation of 254 nm of at least about 35%, preferably within the range 55-90%,
more preferably at least about 55%, 60%, 65%, or 70%. Suitable surfaces include
polished zinc plated steel and polished aluminium.
The UV lamps may be attached individually to walls of the
treatment cavity, or they may be at least partly recessed within the walls. In a
preferred embodiment, the lamps are supported on lamp support frames which are attached
at their lower end to the floor of the treatment cavity and at their upper end to
the upper wall of the treatment cavity. The lamp support frames, which are preferably
arcuate, are preferably of tubular construction.
UV lamps within the treatment cavity may be protected from
accidental damage or soiling by shields or covers. The shields or covers are usually
of UV transmissive material. A lamp may be protected by its own shield or cover,
or a shield or cover may protect more than one lamp. In a preferred embodiment,
a lamp cover is removably locatable on a lamp support frame.
A unit which is also capable of heating the food may also
include one or more heat sources, generally selected from broad band UV tubes, IR
tubes and microwave generators.
Units for use in the present invention may also be provided
with means for rotating the food to be irradiated during at least part of the irradiation.
A suitable means of effecting such rotation is a turntable connected to a suitable
drive mechanism via a central spindle; the turntable may be removably connected
to the central spindle and may further be shaped as to act as a drip tray, for example
it may be dished.
Units may also be provided with means for supporting the
food to be irradiated, especially for supporting the food generally centrally within
the treatment cavity. In a preferred arrangement, the means for supporting the food
is a single-shafted food support stand comprising a shaft or rod and a food support,
the rod being provided with means for attachment to the central spindle of the turntable
and for acting co-operatively with the spindle to effect rotation of the food support.
Alternatively, the means for supporting the food may comprise a frame and a food
support, the frame when in use resting on the surface of the turntable and being
rotated by friction between the turntable and the frame.
The food support is preferably a metal mesh grid, but it
may alternatively be of ceramic or plastic materials. It may be of UV transmissive
Units may be provided with one or a plurality of means
for displacing, or deflecting, the food during rotation. A preferred deflector comprises
a deflecting blade, a supporting stand, and a rotary member coupled to the blade.
Preferably, said rotary member rotates in the contrary direction to the turntable,
and preferably at a different rate of rotation. In a preferred arrangement, in use
the deflecting blade describes a generally arcuate, or sweeping, movement over but
not touching the surface of the food support. In an alternative embodiment, in use
the deflecting blade moves reciprocally generally radially over but not touching
the surface of the food support. A convenient way of effecting movement of the rotary
member is by interaction with the drive mechanism of the turntable.
A combined decontamination and sterile heating unit, suitable
for carrying out the invention, may comprise a decontamination unit as described
above and a source of microwave energy. Units of this type sterilise the air within
the treatment chamber as they decontaminate the food; such units are especially
suitable for the accelerated thawing of frozen food.
A kit may be provided for converting a microwave oven into
a unit for the accelerated aseptic thawing of frozen food, comprising UV lamp holders,
one or a plurality of UV lamps, means for locating said UV lamp holders within the
oven compartment of a microwave oven, and means for lighting and controlling said
UV lamps within the oven compartment. In a preferred embodiment, the means for locating
the UV lamps within the oven compartment are lamp support frames as described above.
The kit may also comprise a food support stand, and may further comprise one or
more food deflectors.
The invention will now be described with reference to the
BRIEF DESCRIPTION OF DRAWINGS
MODES FOR CARRYING OUT THE INVENTION
- Figure 1 is a line diagram of a preferred embodiment of the present invention;
- Figure 2 is a line diagram of a food support stand and a food deflector, suitable
for use as accessories to the unit of Figure 1: Figure 2a is a plan view, Figure
2b a corresponding side section;
- Figure 3 is a line diagram of a food support stand, suitable for use as an accessory
to the unit of Figure 1;
- Figure 4 is a schematic front elevation of the treatment cavity shown in Figure
1, with the food support stand of Figure 3 shown assembled and located within the
- Figure 5 is a schematic section correspondingly on V-V in Figure 1 of another
unit provided with an alternative food deflector;
- Figures 6 & 7 are further schematic front elevations of the treatment cavity
and show two alternative arrangements of UV lamps; and
- Figure 8 shows a detail of Figure 7 in the region of attachment of a lamp support
frame to an inner wall of a treatment cavity.
My researches have shown that the cumulative effect of
UV irradiance on micro-organisms is far more complex than conventional theory explains.
Total dosage is determined by (Irradiance level* Time of exposure). However if the
output of the device is, say, 3 watts/m2 then one continuous exposure
of 20 seconds is more effective than 2 exposures of 10 seconds each separated by
a short delay. In addition, the initial irradiance output (at the desired wavelength
of 265nm±15nm) will have a very significant effect on microcidal and microstatic
properties. A device having a 3 watt/m2 output and exposed to a product
for 20 seconds will have a considerably lower activity than a 6 watt/m2
output exposed for 10 seconds, although the total output from both is 60 joules/m2
(1 joule=1 watt* 1sec).
Heat output is also important as so-called medium and high
UV power sources have outputs which cover the whole UV spectrum and produce large
quantities of energy in the form of radiant heat. This causes rapid denaturation
of proteins and fats in many foodstuffs and is clearly undesirable and unacceptable
in foods which are to be eaten raw and/or stored uncooked. However I have found
that a small increase in product temperature (but not to denaturation point) during
treatment with UV can have a synergistic effect. Further synergism has been found
if the product is subsequently cooled in a refrigerator after treatment. These synergistic
effects are discussed more below.
The final consideration is the output of UV wavelengths
outside of the desired range. Longwave UV-A induces pigment changes and denaturation
in many foodstuffs. It is also now known that, particularly in animal proteins,
the breakdown products from pigment denaturation initiate and/or accelerate lipid
oxidation in the same product. This is further accelerated by the presence of UV-B
or very shortwave UV (220nm or less). UV-B rays are also very dangerous to human
operators and are known to be primarily responsible for conditions such as erythema,
conjunctivitis and sarcoma.
Referring first to Figure 1, sterilising unit 10 comprises
a treatment cavity 20 defined by end wall 30, inner side walls 40, 50, upper wall
60 and floor 70, and (when appropriate) inner wall 75 of door 80. UV irradiation
is provided within the treatment cavity 20 of unit 10 by four high intensity UV
tubes 90, 100, 110, 120 (See Fig. 4). Floor 70 houses turntable 130 having a raised
central spindle 135. The surfaces of walls 30, 40, 50, 60, 75 and floor 70 are generally
highly UV-reflective (for example, they may be constructed from polished zinc plated
steel or polished aluminium) in order to enhance the efficacy of irradiation, and
to help ensure that all exposed surfaces of the food receive comparable amounts
of radiation. Optional door window 85 may have a mirrored surface to maintain the
reflectivity of inner wall 75. Observation window 85 is substantially UV-opaque,
and typically will be of safety glass, perspex, etc.
The food to be irradiated is preferably located within
treatment cavity 20 on a stand 470. When assembled in situ, stand 470 (see Fig 2)
comprises an extension rod 460 and a food support 450, which is a mesh grid. Extension
rod 460 has a tubular region 465, 475 at either end. At one end of rod 460, the
internal diameter of tubular region 465 is slightly larger than the external diameter
of turntable spindle 135 so that rod 460 can be located and retained coaxially around
spindle 135. At the other end of rod 460, tubular region 475 is designed to receive
and house shank 478 of locating pin 480. At the centre of food support 450 is a
locating hole 455. Food support 450 is held in position on rod 460 by means of head
485 of connecting pin 480 when shank 478 is housed coaxially in hole 455 and tubular
The nature of the connection between spindle 135 and tubular
region 465 can be any of a variety of types, as can the corresponding connection
between shank 478 and tubular region 475: either or both connections may be simple
friction or interference fits, or interlocating splines, screw threads, or bayonet-type
fittings, etc, may be provided.
Optimum spacing of the interstices of the mesh of food
support 450 prevents food falling through the grid while minimising the area of
contact between food and grid; overall, the grid has to be sufficiently rigid as
to support the product.
The food to be irradiated is placed on upper surface 440
of food support 450 and cavity 20 is enclosed by shutting door 80. Most types of
food covering currently available, and especially those used in small-scale operations
(eg aluminium foil, self-clinging polyethylene films, etc), are substantially or
completely UV opaque. Even relatively transmissive wrapping materials can increase
the likelihood of folds and crevices in the food which act as 'dead spots' for UV
irradiation. Consequently the food to be irradiated will not usually be covered
or wrapped. Once door 80 has been securely shut, irradiation can begin. Exposure
time is usually a fixed cycle, dependent upon tube radiance output and required
radiance exposure levels. The duration of irradiation is controlled via control
panel 180, which can also be provided with audible and visible indications of the
cycle, elapsed time, equipment status, etc. Typically, irradiation occurs in a single
exposure of 30-45 seconds duration, causing little or no detectable rise in the
surface temperature of the food. Turntable spindle 135 rotates food stand 470 throughout
the radiation cycle to help ensure that the exposed food surfaces (ie all food surfaces
not in contact with food support 450) receive substantially similar amounts of radiation.
In a preferred embodiment, the grid will complete one rotation in about 15 seconds.
Safety features, familiar to manufacturers of electrical
goods for domestic or catering use, will usually be provided. An example of such
features would be to prevent operation of unit 10 without door 80 being closed (for
example, a door safety lock system can be incorporated such that radiation will
not occur unless door catch 190 is securely located in socket 200: further modifications
can, of course, be included, such as electronic locking of door 190 in socket 200
which releases in the event of power failure). Sensors can be included capable of
detecting the presence and integrity of observation window 85. Opening of the unit
during irradiation will immediately abort the operation, and an alarm signal becomes
both audible and visible. The alarm signal has to be cancelled manually before operation
is allowed to continue.
Irrespective of the general design of the treatment cavity,
the contact points between the food and mesh grid 450 will not receive irradiation
and will therefore harbour microbes. One or more food deflectors, positioned and
angled around the periphery of mesh grid 450 but without contacting upper surface
440 may be incorporated to impart varying degrees of lateral displacement to the
food during rotation. An example of this is shown in Figure 2. Deflector 550 comprises
a blade 560, a support stand 570, and a rotary member or wheel 580 mounted on an
upright shaft 590. The shaft 590 is housed coaxially within support stand 570. One
end of the blade 560 is pivotally mounted (597) to an outer region of the wheel
580. The blade extends through a narrow slot in a slider 600 to overlie the grid
450. Its outer end portion 601 is angled. The slider travels on a guide rail 575
which extends transversely to the blade. Thus when the wheel 580 is rotated by the
shaft 590, the blade is carried with it, remaining approximately parallel to its
initial direction owing to the action of the slider and guide rail. The end portion
601 of the blade is caused to move generally arcuately across but above upper surface
440 thereby imparting lateral displacement to the food.
Fig 2a shows the arrangement between food support 450 and
food deflector 550 in plan view. The choice of the way in which support and deflector
interact to effect food displacement is quite wide, but in a preferred embodiment
the wheel 580 rotates in the opposite direction to food support 450; the arrangement
shown in Fig 2a is best suited for a clockwise rotation of the wheel 580 and an
anticlockwise rotation of support 450. Deflector 550 is shown, in outline, in two
further rotationally related positions (X', X'') to illustrate the sweeping action
described by blade 560 under the combined influence of the wheel 580 and a guide
575. Other suitable geometries of wheel, guide, etc can be devised empirically.
Rotation of the wheel 580 and its shaft 590 can be effected
in a number of ways, of course, but a convenient way is via pulley 615 connected
by drivebelt 620 to the drive mechanism (not shown) of turntable spindle 135. Provided
the wheel 580 rotates at a different rate to food support 450 the use of a single
food deflector 470 is usually found to be satisfactory; however, a plurality of
deflectors may be provided and positioned variously circumferentially around mesh
grid 450 to act co-operatively to displace the food backwards and forwards across
upper surface 440 thereby continually altering contact points, thereby further ensuring
all surfaces of the food are exposed to radiation.
Food support 450 may be made from materials other than
metal mesh, for example ceramics or plastics. The support need not be apertured
if a UV-transmissive material is used for its manufacture.
An alternative stand 170 is shown in Figure 3. Stand 170
comprises a frame 160 and a food support 150 which is detachable from frame 160
for facilitating both loading with food and cleaning. Frame 160 should be strong
enough to hold the loaded support 150 in position but should also be of a design
that does not unduly hinder the amount of upwardly directed UV radiation reaching
the underside of the food. The food to be irradiated is placed on upper surface
140 of food support 150. The loaded food support 150 is then located on or in frame
160 and assembled stand 170 is transferred to turntable 130. As shown in Figure
4 with an unloaded food support 150, when assembled stand 170 is positioned inside
treatment cavity 20 the upper surface 140 of food support 150 is located generally
centrally within treatment cavity 20. When loaded stand 170 has been positioned
in cavity 20, cavity 20 is enclosed by shutting door 80 and treatment proceeds as
In Figure 5, stand 170 is shown in association with an
alternative type of food deflector 250. Food deflector 250 comprises blade 260,
support stand 270, and a cam 280 mounted on shaft 290. The shaft 290 and cam 280
are rotated by pulley 315 which is connected by drivebelt 320 to the drive mechanism
(not shown) of turntable 130. The shaft 290 and cam 280 are linked to deflector
blade 260 via follower 300 and spring 310 such that the rotation of cam 280 causes
deflector blade 260 to move reciprocally radially across but above upper surface
140 in the directions shown by the double-headed arrow, thereby imparting the lateral
displacement to the food.
Turntable spindle 135 of Figures 1, 2, & 7 performs a second
function in those Figures as a connection between a single-shafted stand and the
drive mechanism. This function is not required with a stand such as that shown in
Figure 3 which sits on turntable 130 and rotates by virtue of friction between the
turntable and the base of the stand, in which case the spindle 135 may be covered
by turntable 130 (see especially Fig 5). In either case, turntable 130 may be removably
coupled to spindle 135, and may be designed to serve a secondary function as a drip
tray; for example, it may be dish-shaped.
As shown in Fig 5, floor mounted lamps 90, 100 may be covered
along substantially their entire length by lamp shields 350, 360 to protect tubes
90, 100 from breakage by impact with food, etc, that might occur while loading or
in operation. Lamp shields 350, 360 must, of course, be apertured or preferably
of UV-transmissive material (eg PTFE - "Teflon") to allow irradiation. If shields
350, 360 are of UV-transmissive material they can also prevent debris and juices
from the food falling onto the surfaces of the lamps, which would otherwise result
in staining, loss of irradiating efficacy, and earlier replacement.
It will be appreciated that simpler - and therefore usually
cheaper to manufacture - units than those described here can be designed that would
be expected to achieve at least some reduction in microbial numbers. For example,
unit 10 can be used without food stand 170 or 470, in which case the under surface
of the food will receive little or no radiation. Nevertheless, use of the unit without
the stand is still in principle capable of achieving at least about 50% reduction
of microbial numbers on the surface of the food, because at least the upper surface
of the food can be sterilised. The efficacy of such a reduction in microbial numbers
in enhancing shelf life and/or reducing the risk of food poisoning will, of course,
depend on a variety of factors and circumstances, for example the nature of the
food, the state of microbial spoilage and growth before treatment, the nature of
the microflora, and the subsequent handling and storage of the food after irradiation.
An alternative to the use of a stand is to provide the unit with one or more food
deflectors, similar to those described above but located as to deflect food on a
support placed directly on the turntable, and intended to cause complete or partial
inversion of the food with each rotation of the turntable. A very basic sterilisation
unit would be one in which the turntable was omitted (although rudimentary, it can
be imagined that such a unit, when used in conjunction with a food stand such as
170, might achieve greater reduction in bacterial numbers than a unit with a turntable
but without stand or deflector, since in the former unit only the contact points
and shadows cast by the frame onto the food are not irradiated but in the latter
unit the entire underside of the food is not irradiated).
The number of lamps used in a sterilisation unit of the
present invention, and their location within the treatment cavity, is a matter of
choice to the manufacturer determined mainly by the output of the lamps. Suitable
UV tubes include especially those which produce radiation falling essentially exclusively
in the wavelength range 220-300 nm. Ballasts and starters can be replaced by an
electronic lighting and control circuit to help overcome weight and space constraints.
The units shown in Figures 1, 3, & 4 have four lamps 90, 100, 110, 120 which are
shown in paired parallel horizontal arrangement. A similar number of lamps can be
used in paired parallel vertical arrangement, as shown for lamps 390, 400, 410,
420 in Figure 6, or fewer or greater numbers of lamps may be used in either arrangement
or a combination of both. In Figure 6, lamps 390, 400, 410, 420 are shown attached
to inner side walls 40, 50 by lamp holders 430 but they may alternatively be recessed
within any suitable wall or walls of the cavity.
Figures 7 & 8 illustrate a further approach to providing
treatment chamber 20 with UV-irradiation. UV lamps 700, 702 clip into lamp connectors
705 which are supported on lamp support frames 710, 720. Lamp support frames 710,
720 are preferably of tubular construction as this design conveniently provides
neat trunking to receive the wiring 730 associated with lamp connectors 705. Lamp
support frames 710, 720 may be further provided at suitable locations with clips
740 for receiving protective cover 750. Cover 750, which is intended to be easily
detachable from clip 740 for cleaning by the consumer, performs a similar function
to the lamp shields 350, 360 of Fig 5, but because of its location on the framework,
and its preferred one-piece design, protects all lamps 700 or 702 on frame 710 or
720, respectively. Cover 750 is constructed of UV -transmissive material.
Lamp support frames 710, 720 are each fixed to upper wall
60 and floor 70 by means of threaded holes 760.
Since the combined use of turntable 130, stand 170, deflectors
250 (as shown in Fig 5) and high reflectivity generally within the treatment cavity
should ensure that all surfaces of a food are able to be exposed to irradiation
originating from anywhere within the cavity, it follows that a unit having a single
UV lamp can effect adequate sterilization of food surfaces given a sufficiently
long treatment cycle.
Thus far, the invention has been described by reference
to examples of dedicated units. In a further embodiment, a combined microwave oven/UV
-sterilization unit comprises a sterilization unit as disclosed herein and a magnetron
for providing microwave radiation within the same treatment cavity. In a preferred
embodiment, the location and number of UV tubes is preferably paired parallel vertical
conformation, as shown for example in Figure 6, although the number and location
of tubes will frequently be influenced by the characteristics and location of the
magnetron, and vice versa. In a more preferred embodiment, UV tubes are supported
on support frames as shown in Figure 7.
Many existing domestic microwave ovens are convertible
to combined units: for example, the components shown in Figures 7 & 8 can be provided
in kit form. Such a kit therefore includes a pair of support frames 710, 720, each
frame being provided with one or more sets of lamp connectors 705 (arranged in pairs
to receive the lamps), and a suitable number of UV lamps. For convenience, wiring
would be provided to and from the connectors as required and running substantially
within the tubes of the support frames, with the ends of the wires suitably finished
for ease of connection with appropriate circuitry of the microwave oven. Suitable
electronic lighting and control circuitry would normally be included in the kit,
preferably at least part preconnected and located within, or provided with, appropriate
housing. Means for connecting the frames to an inner wall would normally be included
in such a kit, for example the stands may be provided at either or both ends with
flattened extensions or feet for attachment to a wall by adhesive. Attachment may
also be as shown in Figure 8, in which case threaded housings may be included in
the kit for locating in suitably cut holes made through inner walls of the oven.
(The conversion would need to be effected by a suitably qualified technician, for
obvious reasons of safety).
A particular advantage of such a combined unit would be
for improved accelerated thawing of frozen foods. A perceived beneficial use of
microwave ovens domestically is as a means of accelerating the defrosting of frozen
foods. Notwithstanding detailed instruction manuals, it is a potentially hazardous
operation domestically as it is very easy in practise to allow the frozen food to
heat up too rapidly: bacterial growth can restart at temperatures only slightly
above the melting point of the food, especially on the outer surfaces (despite microwaves'
theoretical potential for heating frozen food evenly throughout the mass of the
food, it is common experience - both commercially and domestically - that uneven
heating does occur). A combined unit for use in the invention, by irradiating the
food with ultra violet throughout microwave-mediated thawing, can virtually eliminate
bacterial regrowth especially on the food's surface.
Generally the thawing is effected with a very low intensity,
as is understood by those skilled in the art. After the food has thawed it may be
cooked. This may also be effected by the microwave oven, using a higher intensity
of irradiation. UV irradiation may not be necessary during cooking.
I have indicated in Fig 1 that the illustrated unit may
incorporate the features of a conventional microwave oven, including a grille G
through which microwave radiation can be fed into the cavity, and controls C for
operating the microwave generator.
As mentioned above, I have found that heating and cooling
can potentiate the microbicidal effects of the UV irradiation. This will now be
EXPERIMENT 1: HEATING
Food articles (raw pork chops) initially at 4°C were
divided into two groups. A portion ("A") of the chops of the first group were irradiated
with UV (narrow band, 265nm peak) at 5w/m2 for 45 seconds. Microbiological
testing was then carried out to determine their total plate count ("TPC") and the
number of coliform bacteria. As a control ("A(c)") the chops which had not been
irradiated were also tested.
The second group of chops were heated to raise their surface
temperature to 25°. A portion ("B") were then irradiated under the same conditions
as the A chops whereas the remainder ("B(C)") were not irradiated. Microbiological
testing was carried out as before. The results are shown in Table 1.
EXPERIMENT 2: COOLING
As in Experiment 1, pork chops were subjected to four different
regimes (A, A(c), B and B(c)) and then subjected to microbiological testing as before.
Chops A and A(c) had a surface temperature of 20°C,
The chops A received UV irradiation as used in Experiment 1 whereas the chops A(c)
were merely maintained at the same temperature.
Chops B were irradiated in the same manner as the chops
A, and then cooled to reduce their surface temperature to 0°. The chops B(c)
were merely cooled. Cooling was effected by placing the chops in a chilled cabinet.
The results of the microbiological testing are shown in
Similar effects have been induced in other red meats, white
meats, prepared and processed foods, fruits and vegetables. It is most dramatic
on uneven wet surfaces such as meat and less dramatic on smooth dry surfaces such
as fruits where the UV treatment alone is sufficient to establish near aseptic conditions.
There is no reason to suppose that a similar effect would not be induced in food
groups not yet examined.