Technical Field

The present invention relates to an apparatus for treating foods by volatile biocides.

Background Art

The preservation of perishable products continues to be the focus of considerable commercial interest. By extending the shelf life of a food product, eg, a processed or cured meat, considerable economic value can be added to that product. Approaches to this end are many and varied, eg, tight control of production and storage conditions, packaging, post and in situ applications of preservatives, and various combinations of these and other techniques are known and in practice to one extent or another.

In the context of smallgoods such as hams, bacon, sausages, cured meats, pressed chicken, turkey roll, frankfurts, etc, and baked goods all of these techniques are in use, eg, frozen or refrigerated storage, modified atmosphere packaging, and the addition of preservatives either to the starting material or mix from which the food product is prepared, or the application of a preservative to the finished food product. With respect to the latter, the application of a small amount of preservative to a finished baked good, can extend the shelf life of the baked good from a typical 6-8 days to an extended 14-16 days (all other conditions, eg, packaging, storage conditions, etc, being equal). These preservatives can include a wide variety of biocides (ie, microbiocidal substances, antimicrobial substances, etc) such as acetic acid, lactic acid, mixtures thereof, and the like.

One problem, however, in the application of a preservative to a food product is to apply the preservative in a manner that does not interfere with the natural organoleptic properties of the product, eg, taste, smell, texture, etc. In the case of applying the preservative to a finished smallgood, too much preservative can impart an unwanted tartness to the product or discolour the product.

Another problem with the application of a preservative to the finished smallgoods is consistent application of the preservative in a production line setting. Commercially distributed smallgoods, along with most other commercially manufactured and distributed perishable goods, are made in large quantities, and consistency from one item to the other is important to the commercial success of the product line. In the case of applying

preservative to smallgoods, the amount of preservative applied to the first baked good in the production cycle should be essentially the same as the amount of preservative applied to the last item in the production cycle (and all items throughout that production cycle, for that matter). This can be difficult to control over extended periods of time due to, among other things, variations in the temperature of the equipment, the preparation and delivery of the preservative to the finished product, and the like. The present applicant has found that this problem is particularly the case with gaseous bioeides applied to food products such as smallgoods.

Hitherto, conventional gaseous processes aimed at extending the shelf-life of materials prone to microbial spoilage have relied on modified atmosphere (MAP) procedures. In such procedures, the oxygen laden gas atmosphere surrounding the material is replaced with a food grade carbon dioxide and/or nitrogen atmosphere, and high barrier co-laminate packaging is used to maintain the exclusion of oxygen from the package. However, MAP processes have disadvantages. That is, whilst it has been found that an extension of the shelf-life can be achieved in respect of materials treated ^• by the procedures, the extension is often limited. Furthermore, considerable costs are involved including the cost associated with the requirement for specialised co-laminate film packaging and the necessity to slow packaging lines to ensure that totality of heat sealing is achieved. In AU 730402 and US 6224930 (incorporated herein by reference), the present applicant described an alternative method for extending the shelf-life of materials prone to microbial spoilage involving treating a material with a volatile biocidal substance(s) such as a natural food acids (eg. acetic acid) entrained in a defined carrier gas within an evacuated treatment vessel. Subsequently, the present applicant found that it was not necessary to evacuate the treatment vessel in order to achieve a satisfactory extension of shelf-life. This 'no-vacuum' method is described in AU 734421 and US 6265006 (incorporated herein by reference).

The process and apparatus described in the above mentioned patent specifications for the generation of the biocidal gas comprised the bubbling of the carrier gas through the liquid volatile biocidal substance or a solution thereof. In the case where the biocidal substance is carbonic acid and the carrier gas carbon dioxide, the carbonic acid was formed in situ by bubbling the carrier gas through water.

At present, there are no suitable commercial apparatus for batch or continuous treatment of foods by volatile biocidal substances. Although advances have been made

in the treatment process, it has been difficult to scale up this technology in suitable functioning apparatus.

The present inventor has now developed an apparatus which provides improved treatment of food using volatile biocides.

Disclosure of Invention

In a first aspect, the present invention provides an apparatus for gaseous treatment of food comprising: vessel adapted to receive a volatile biocide and food; entry port for introducing food into the vessel; treatment zone for treating food in the vessel with a volatile biocide entrained in a gas stream; gas diffuser positioned across one dimension of the vessel for introducing the volatile biocide containing gas stream into the vessel and substantially evenly distributing the volatile biocide to the treatment zone; and means positioned across one dimension of the vessel for removing treatment gas from the vessel.

Preferably, the treatment zone if formed substantially throughout the vessel.

The apparatus according to the first aspect of the present invention is adapted to treat food on a batch basis.

In a second aspect, the present invention provides an apparatus for gaseous treatment of food applied in a continuous mode comprising: vessel adapted to receive a volatile biocide and food; entry port for introducing food into the vessel; exit port for removing food from the vessel; means for conveying food through the vessel from the entry port to the exit port; treatment zone for treating food in the vessel with a volatile biocide entrained in a gas stream; gas diffuser positioned across one dimension of the vessel for introducing the volatile biocide containing gas stream into the vessel and substantially evenly distributing the volatile biocide to the treatment zone; and

means positioned across one dimension of the vessel for removing treatment gas from the vessel.

The apparatus according to the second aspect of the present invention is adapted for continuous treatment of food. The apparatus may further include: biocide measurement and control means to ensure correct amount of volatile biocide is provided and maintained in the vessel or in the treatment zone during treatment of food.

The apparatus may further include: means to recirculate the biocide containing gas stream from the removing means to the gas diffuser and adding additional biocide to the gas stream to maintain the concentration of the biocide in the gas stream at a desired level.

A proportional chemical dosing pump may be used to meter the biocide into the circulating gas stream to establish and replenish the circulating biocide concentration. The vessel can be made of any suitable material such as stainless steel, food approved plastic or other suitable materials for food contact. Typically, the vessel is rectangular in cross section which can assist in the even treatment of food. It will be appreciated that the vessel can have any suitable shape or cross section.

The apparatus can be adapted for batch treatment or continuous treatment of foods using a treatment gas containing a volatile biocide.

Preferably, the gas diffuser is a tunable gas diffuser in which the pressure differential across the diffuser can be adjusted to ensure that the flow of gas through the diffuser is uniform across its whole area. Biocide is preferably directed to the gas diffuser via a distribution plenum. In an apparatus for bach processing, the gas diffuser is preferably positioned in the top of the vessel substantially equidistance from opposing sides of the vessel. Preferably, the gas diffuser is positioned substantially across the width the vessel.

In an apparatus for continuous processing, the gas diffuser is preferably positioned in the bottom of the vessel substantially equidistance from the entry and exit ports of the vessel. Preferably, the gas diffuser is positioned across the width the vessel and substantially transverse to the direction of movement of the food through the vessel.

Preferably, the means for removing volatile biocide is a gas extraction duct positioned in the vessel opposite the gas diffuser.

Preferably, volatile biocide is added from the top or bottom of the vessel and removed via the duct from below or above the vessel. During use, there is typically a slight pressure differential between the gas diffuser and the removing means.

The removed volatile biocide can be recycled through the system and further volatile biocide is added to the treatment gas to maintain the required volumes of gas in the treatment zone. Control of addition of the biocide to the recirculating gas stream can be done automatically using data acquired by online analysis of the biocide concentration in the circulating air (or carrier gas) and passing this data to a Process Logic Controller, or similar device, programmed to regulate the rate of dosage by the proportioning chemical dosing pump. Several methods are available for this analysis including solid state sensors and Near Infra Red (NIR) spectrophotometry. The present inventor has found that NIR is the preferred method and that both vaporous organic acids and vaporous hydrogen peroxide can be detected for this purpose by the application of NIR spectrophotometry. A suitable analyser is a Servomex Xendos 2500 or similar analyser. For example, sensors or sampling points can be positioned to measure concentration of volatile biocide at various positions in the system to assist in the control of treatment.

In a third aspect, the present invention provides a process for gaseous treatment of food in a batch mode comprising: providing an apparatus according to the first aspect of the present invention; placing food in the vessel; and distributing a volatile biocide entrained in a gas stream to the treatment zone in the vessel so as to evenly expose the food to the biocide for a desired period.

In a fourth aspect, the present invention provides a process for gaseous treatment of food in a continuous mode comprising: providing an apparatus according to the second aspect of the present invention; passing food through the treatment zone of the vessel; and distributing a volatile biocide entrained in a gas stream to the treatment zone in the vessel so as to evenly expose the food to the biocide for a desired period. Typically, the food is a substantially solid material for human consumption which is susceptible to microbial spoilage and which has an exposed surface having a water activity of greater than or equal to about 0.85. Preferably, the food is selected from

smallgoods, baked goods, cheese, fresh fruit, dried fruit and nuts, fresh pasta, fresh poultry and fish and other manufactured or processed foods.

The smallgoods are preferably hams, bacon, sausages, cured meats, pressed chicken, turkey roll, frankfurts or other processed meats. Preferably, the smallgoods are ham, bacon, chicken or turkey. More preferably, the smallgoods are ham or similar processed meat.

Preferably, the baked goods are crumpets, bread loaves, or the like. Preferably, the cheese is grated or shredded.

Preferably, the volatile biocide is selected from the group consisting of natural food acid, volatile chemical biocide, and mixtures thereof.

Preferably, the natural food acid is acetic acid, propionic acid or mixtures thereof. More preferably, the natural food acid is acetic acid.

Preferably, the concentration of acetic acid in air or carrier gas is between 0.0025 and 0.1 gm/litre. Similarly for other suitable food acids a value of 0.002 to 0.08 gm/litre is usually required for optimum results.

In order to overcome or minimise the risk of explosion in the case that the carrier gas is air, the amount of volatile biocide should be less than about 80% of the Lower Explosive Limit in air (LEL). For acetic acid, the LEL is quoted in the published literature at various levels, the lowest referenced by the present inventor is about 4% v/v or about 0.14 gm/litre.

For propionic acid, the LEL is quoted in the published literature at various levels, the lowest referenced by the present inventor is about 2.9 % v/v or about 0.09 gm/litre.

The application of acetic acid as a preservative to a food product typically begins with the conversion of liquid acetic acid to gaseous acetic acid. This conversion is accomplished by any one of a number of different procedures such as flash evaporation, atomisation, heating, etc, and the gaseous acetic acid is then transported, typically by a carrier gas to a treatment vessel.

Preferably, the volatile chemical biocide is hydrogen peroxide.

The volatile biocide can be in air or in a defined carrier gas such as nitrogen or carbon dioxide. Preferably, the volatile biocide is in air.

The present inventor has shown that in order to obtain consistent treatment of food, it is important to ensure that there is substantially even distribution of the treatment

gas in the form of a volatile biocide to the treatment zone in the vessel. To achieve this aim, a gas diffuser evenly distributes the volatile biocide to the treatment zone.

Preferably, the process according to the present invention results in an increase of the shelf life of a food product by about 100 to 400%, more preferably about 300%. With processed hams, for example, the shelf life can be extended to greater than 6 months compared with normally expected 2 month shelf life.

The present inventor has found that the sufficient contacting period is up to about 10 minutes. Preferably, the contacting period is about 5 minutes or less. More preferably, the contacting period is less than about 1 minute. It will be appreciated that the contacting period can depend on the food being treated, the type and size of the vessel, and the viable microbial content or bioburden of the food. From the findings of the present inventor, a suitable contacting period can be ascertained for a given food product.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia prior to development of the present invention. In order that the present invention may be more clearly understood, preferred embodiments will be described with reference to the following drawings and examples.

Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this specification.

In order that the present invention may be more clearly understood, preferred embodiments will be described with reference to the following drawings and examples.

Brief Description of the Drawings

Figure 1 is a schematic of a treatment facility suitable for the present invention.

Figure 2 is schematic of a gas diffuser which can sit on top or bottom of the treatment vessel .

Figure 3 is a schematic of a system to facilitate the delivery of the process of this invention to a small food product in a continuous manner.

Figure 4 is a schematic of a system to facilitate the delivery of the process of this invention to larger food product, such as hams, in a continuous manner. Figure 5 is a schematic of a biocide metering system for batch mode operation.

Figure 6 is a schematic of vaporiser suitable for the present invention. Figure 7 is a schematic of a vaporiser in a much simpler form,

Mode(s) for Carrying Out the Invention Batch Mode

In the treatment facility which has been constructed to reduce this invention to practice, the process is preferably fully automatic. Once the parameters for the treatment are entered, the treatment proceeds under the control of a Process Logic Controller (PLC), equipped with data capture to ensure that batch-to-batch variation in parameters does not occur outside set limits.

The facility is depicted schematically in Figure 1 and consists of a food grade clean room (CR) with direct access to the treatment vessel (3). The vessel is of sufficient size as to accommodate a full meat processing trolley (2).

For each batch product (1) for treatment is carefully removed from their packaging and hung from the wheeled sanitary constructed trolley. Once loaded with product for treatment, the trolley is rolled into the treatment vessel and the door closed in preparation for treatment.

Room air, contained in the vessel (3) and the system was heated by a heat exchanger (6) supplied with steam from boiler (5). This heated air stream is recirculated by a fan (4) running at about 10,000 M ³ /hour, so that the system is equilibrated to the treatment temperature, 30 - 55 ⁰ C, in 1 to 3 minutes. During this step, product surface temperature has been shown by the inventor to rise by less than about 4 ⁰ C.

The calculated dose of biocide, for example 75% Food Grade Acetic Acid, is delivered by calibrated peristaltic pump (9), or other suitable method such as pressure displacement, from a reservoir (8) through a spray jet (10) onto the evaporation media (7A) in the evaporator (7). Counter-current, up-flowing room air, heated to 30 - 55 ⁰ C by the heat exchanger (6), entrains the volatile Acetic Acid and conveys it, via a 3 way valve (11) and recirculation ducting (12) to the product in the treatment vessel and equilibration is facilitated by continuance of this recirculation for a period of 1-5 minutes, as found by experimentation with the particular product under test to ensure complete equilibration.

The airflow with the biocide is directed to a distribution plenum (16) via a continuation of the ducting (12) and is evenly distributed by a tuneable gas diffuser (17). An example of a suitable tunable gas diffuser is described more fully below. ,

Uniform distribution of the biocide in the carrier gas is a preferred requirement of the process. Numerous unsuccessful attempts were explored prior to achieving satisfactory uniformity of distribution of the biocide in the treating vessel. One means for achieving even distribution was achieved an arrangement for an air diffuser set out in Figure 2 which sits above the treatment vessel (1) in Figure 1.

This sub-assembly comprises a distribution plenum (21) which allows the uniform distribution of the air or carrier gas (22), laden with biocide and is further enhanced by a plug filter (23). The air diffuser is equipped with a means of compressing the filter media - a top perforated mesh (24) is pulled towards the supporting perforated mesh (25) by tightening nuts (26) on threaded rod or captured bolts (27). By increasing the compression on the filter, slight but subtle increases in back pressure can be imparted to the treatment gas to ensure even distribution of the biocide to the treatment zone in the vessel. Optionally, the entire air diffuser can be removed to facilitate sanitation of the system via a sealed and hinged port (28).

After passing through the gas diffuser, the air (or carrier gas) and contained biocide enters the vessel. To assist in the even distribution of the volatile biocide to the treatment zone of the batch mode apparatus, the air diffuser is positioned substantially across the top of the vessel. It has been found that good results can be achieved when the air diffuser is positioned across the width the vessel. To assist in the even distribution of the volatile biocide to the treatment zone of the continuous mode apparatus, the air diffuser is positioned substantially equidistant from the entry and exit ports of the vessel. It has been found that good results can be achieved when the air

diffuser is positioned across the width the vessel and substantially transverse to the movement of the food through the vessel.

Immediately after exposure, HEPA filtered room air is again admitted from HEPA filter (13) via valve (14) and the un-absorbed Acetic Acid remaining in the system is conveyed, via the redirected 3 way valve (11) and ducting (15) to a water scrubber tower (16) where it is removed from the air stream by water (17) sprayed in through a nozzle (18) onto scrubber media (16). The moistened air, freed of Acetic Acid, is then released to atmosphere via an outside vent (19). Water, containing the unabsorbed Acetic Acid, is released to waste, after neutralisation, through a waste line (20). Once safe to open, the trolley with the treated product is removed from the treatment vessel into the clean room and each unit is repacked into point-of-sale plastic bags and vacuum packed.

Treated product is then stored at 3-4 ⁰ C prior to entering the supply chain.

Continuous Mode

Rotary Lock In-feed & Out-feed

Figure 3 is a schematic representation of a system to facilitate the delivery of the process of this invention to small piece or sliced food product in a continuous manner.

A vaporiser (29), as taught herein and depicted in Figure 6 for example, establishes and maintains a desired concentration of the liquid vaporous biocide within an airtight cavity (30). The carrier gas and entrained vaporous biocide is circulated through distribution ducts at high rate by a fan (31) to ensure even application of the biocide to all product treated by passage through the system.

Product for treatment is admitted to the closed vessel through a rotary in-feed lock (32), which is commercially available, comprising a inner rotating set of blades which wipe against the outer stationary body of the valve so that containment of the biocide within the vessel is maintained by at least two of the blades at all times. Product is swept through these sealed compartments formed by the lock and admitted to the vessel without release of the contained atmosphere. The admitted product falls onto a horizontal conveyor (33) made of woven stainless steel mesh or similar food approved conveyor belting which travels to a drop point and delivers the food product to a lower conveyor (34) travelling in the reverse direction, thus inverting the product to facilitate contact with its under side.

After treatment of the product while in the vessel, this second conveyor delivers the treated product to a rotary lock (35) similar to the in-feed rotary lock, through which the product leaves the vessel with limited release of the enclosed atmosphere. Any escaped biocide is swept away by an exhaust duct and fan (36).

The speeds of the rotary locks and the conveyors are matched to ensure that the delivery and conveyance of product through the system is continual and bank ups don't occur.

Interlocked Gated In-feed & Out-feed Figure 4 is a schematic representation of a system to facilitate the delivery of the process of this invention to large piece food product, such as hams, etc in a continuous manner.

A vaporiser (29), as taught herein and depicted in Figure 6 for example, establishes and maintains a desired concentration of the liquid vaporous biocide within an .airtight cavity (37). The carrier gas and entrained vaporous biocide is circulated through distribution ducts at high rate by a fan (38) to ensure even application of the biocide to all product treated by passage through the system.

Product for treatment (39) is admitted to the closed vessel through an interlocked pair of knife action sliding doors (40) powered pneumatically, so that containment of the biocide within the vessel is maintained by at least one of the doors being closed at all times. The door slides and seals are food grade high-density polyethylene and are sufficiently closely toleranced to prevent escape of biocide laden carrier gas.

Product is moved through the sealed compartment formed by these doors and moved into the vessel by the in-feed conveyor (41) in the in-feed lock (42) without release of the contained atmosphere. Sealing of the compartments is facilitated by contact of the doors to their respective seals through gaps in the conveyors over which product is handed from one moving conveyor to the next.

The admitted product is carried through the treatment vessel (37) on a horizontal conveyor (43) made of woven stainless steel mesh or similar food approved conveyor belting. The choice of this material must ensure that the product sits on a minimum

1 number of high points as possible to minimise shielding of the surface of the product from contact with the biocide laden carrier gas. A woven 1-3 mm thick stainless steel wire mesh with 25 mm centres was found to be preferred for this purpose.

This conveyor delivers the treated product to a third vessel (44), similar to the in- feed vessel (42), equipped with an interlocked pair of knife action sliding doors (45), so that containment of the biocide within the vessel is maintained by at least one of the doors being closed at all times. Product is moved through the sealed compartment formed by these doors and moved out of the vessel by the out-feed conveyor in the out- feed vessel (44) with minimal release of the contained atmosphere. Any escaped biocide is swept away by an exhaust duct and fan (46).

The speeds of the conveyors are matched and synchronised with the doors by action of a controlling PLC to ensure that the delivery and conveyance of product through the system is continual and bank ups don't occur.

Biocide Metering System for Batch Mode

It was found by the inventor that reliance on a calibrated peristaltic pump to accurately deliver the required dose of biocide was problematic. Variations in pump segment elasticity and a lack of sensitivity in the available timers within the PLC contributed to errors in the volume of biocide delivered to repeat batches of product. Reproducibility and certainty of biocide delivery is an essential criterion for food processing equipment, to ensure the attainment of HACCP compliance, so a more robust sub-assembly design was sought. Figure 5 shows one embodiment of such a metering system. It is structured to volumetrically meter a measured dose of biocide, repeatedly, while having the ability to vary the dose from batch to batch on demand from the system control panel, via PLC control. Similarly, all other parameters, timing as well as pump and valve control are achieved by PLC programming. Made from stainless steel and food grade plastic wetted parts, this sub-assembly comprises a cylinder (1) and piston (2), with O-ring seals. The piston is connected to a hollow rod (3). A linear actuator (4), which is an item of commerce, comprising a low voltage motor driving a nut and Acme thread arrangement, is used to position the piston according to an analogue output from the PLC (not shown), proportional to the desired volume set by the operator.

The fluid path provided through the hollow rod (3) passes a solenoid valve (6) and is connected to a flexible return to reservoir line (7). The reservoir (8) contains the liquid biocide which is pumped to the cylinder by a peristaltic pump (9) through an in-feed control valve (10).

When the piston has been positioned as described above to set the desired volume for the process, valves (11) and (13) are closed and valve (6) opened. The pump (9) runs for a set time equivalent to the time necessary to displace 125% of the total free volume of the cylinder to ensure complete filling of the void and the removal of all air pockets. Pump (9) stops and simultaneously valve (6) and (10) close.

Regulated and filtered compressed air (12) is admitted by the opening of valve (11) with simultaneous opening of valve (13). The volume of liquid biocide contained in the cylinder is displaced by the pressure regulated compressed air through a rotary pulse counter (14) and is delivered to a spray head (not shown) in the vaporiser at fixed flow and pressure to enable an even spray pattern and reproducible atomisation, not possible with the variables experienced when using direct injection via a peristaltic pump.

Compressed air flows for sufficient time to evacuate 125% of the total free volume of the cylinder to ensure all liquid biocide is evacuated. The pulse counter chosen is a type which only respondsjo liquid and is not activated by the flow of compressed air. The pulses generated by the passage of liquid biocide are totalised by the PLC and compared to the dose selected by the operator. The program responds with an out of specification alarm if there is a deviation of more than 5% from the set pint for each batch, allowing the operator to reject a batch if inappropriately treated.

Gas Generation Components for Continuous Mode Vaporiser

To facilitate the establishment and maintenance of the vaporous biocidal concentration in the carrier gas under continuous mode operation differs from batch mode in various ways.

In batch mode it suffices to deliver an accurate and reproducible single dose of the biocide and then vaporise this into the fixed volume of the system. In continuous mode, however, preferably the system should monitor the concentration of biocide and the pressure within the system and take corrective action to maintain equilibrium for both parameters. In this way biocide absorbed onto product will be replaced and the pressure within the system will not become excessive should the interlock doors fail to open. Figure 6 is a schematic representation of an example of such a vaporiser. Carrier gas (46) with entrained biocide is drawn from the vessel applicator by the action of a fan (47. The concentration of biocide is sensed by a sensor (48), such a sensor for acetic acid is an item of commerce. The sensor output is fed as an analogue input to the PLC (49) which responds with an analogue signal (50) thus controlling a proportioning valve

(51) which controls the flow of regulated and filtered compressed air from a suitable supply (52).

This compressed air powers an atomising valve (53), also available commercially, which is supplied with liquid biocide from a reservoir (54) via an in-feed line (55). The supply of liquid biocide is maintained through an in-feed line (56) from a reservoir as required. The biocide is entrained into the compressed air and forms a fine mist of biocide with particles mostly in the range of 20 to 50 microns. This fine mist (56A)is conveyed by the escaping compressed air upward to a heat exchanger (57) supplied with heat energy (15) which may be in the preferred form of contained steam, although other heating methods could be envisaged. In this heat exchanger (14) which runs at temperatures between 50 to 75 ⁰ C, the mist is vaporised. Any larger aerosol droplets are coalesced and the condensate on a screen (59) drips back to the reservoir (54) from whence it is re-atomised.

The vaporised biocide (60) is carried back to the vessel in the carrier gas to be sampled, sensed and made up in a feedback loop, which establishes and maintains the biocide concentration in continuous mode.

A pressure relief valve (61) is also controlled by the PLC which senses the pressure within the system from a remote pressure sensor located in the central cavity (not shown) and is backed by a safety pressure relief valve also in the central cavity. Figure 7 is another example of such a vaporiser in a much simpler form. A duct

(62) carries air (64) from a system representative of the invention and returns it to the system. A fan (63) induces high rate air flow though the duct (62), which contains a heater(s) (65) and misting nozzle(s) (76).

A sample of the circulating air (64) is passed via side-stream line (66) to a vaporous biocide (67) which may be similar to the NIR Analyser already described, and after analysis the sampled air is returned via return line (68) to the duct (62).

Readings (69) from the analyser are passed to a PLC (70) which analyses this input data (69) and responds with a signal (71) to control a piston, or similar, dosing pump (72). The pump (72) is supplied with liquid biocide from a reservoir (73) via in-feed line (74) and the dosed biocide is conveyed to the misting nozzle (76) by line (75). Thus a feedback loop is developed where-in the concentration of biocide in the circulating air (64) can be controlled and made-up on demand.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments

without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.