Patent Application
20060286220
The present invention relates to a method and apparatus for the coating of products and is applicable in particular, though not necessarily, to a method and apparatus for coating food products.
There exist a wide variety of products which require to be coated with a substance (e.g. powder, liquid, suspension, etc) during their preparation. For example, snack foods such as potato crisps and corn chips are usually coated with a flavouring. It is generally desirable to be able to achieve a uniform and controllable coating over the entire surface of a product. It is also desirable to be able to minimise wastage of the coating, and to reduce the dispersion of the coating into the environment around the products.
A traditional method used for coating snack foods involves the use of a rotating drum which has its axis at a slight tilt to the horizontal. Products introduced into one end of the drum tend to travel to the other end as the drum rotates. Flavouring in the form of a powder is delivered via an inclined chute to a location within the drum so that the flavouring is sprinkled onto the products to be coated. Whilst this technique tends to result in an acceptable level of coating uniformity, it is not ideal, with flavour concentration varying between product pieces and across the surfaces of individual pieces. Further, the drum itself is an expensive piece of equipment, and environmentally unfriendly. When it is required to change the coating substance to be used, it is likely that the drum will have to be cleaned or replaced due to contamination with the first coating substance. This results in decreased productivity due to downtime.
It has been recognised that coating uniformity may be improved by placing a charge on the powder grains, using for example a wire electrode located in the transit path of the product. If the product delivery mechanism, e.g. the rotating drum, is grounded, the products to be coated will also tend to be grounded as they pass under the flavouring delivery mechanism. Charged flavouring grains will be attracted to the products in a generally uniform manner, in turn resulting in a generally uniform coating. Such a process is described in our co-pending British Patent Application No. GB 0204786.8.
A known electrostatic coating apparatus comprises a venturi tube through which the coating substance is forced by a pressurised gas. Upon emerging from an exit nozzle, the substance passes over a charging electrode. A blockage problem can arise as the coating substance builds up within the venturi tube, particularly where the surrounding environment is hot and possibly humid (this is often the case in a food production facility). A further potential problem is that the coating substance is not broken down into sufficiently small grains by the delivery system. Such blockage results in production downtime, increasing operating costs and decreasing productivity.
GB 2,177,585 describes an apparatus for coating food products in which the falling product is subjected to a stream of charged coating from an electrostatic sprayer. The product is collected in a wire basket connected to an earthed funnel, the funnel carrying away excess coating. The funnel may, over time, become blocked with excess coating, leading to production downtime whilst the funnel is cleaned or changed. As the product falls through the stream of coating and impacts upon the basket, there is the possibility of damage to the product, especially if the product is fragile. Once the basket has been filled with coated product, the basket has to be emptied, requiring a pause in production. Further, the wire basket is earthed whilst the falling product is isolated, resulting in the charged coating being attracted to the basket in preference to the product, causing it to adhere to the basket.
According to a first aspect of the present invention, there is provided a method of coating a product, the method comprising delivering the product to a top end of an inclined chute, the chute comprising at least one mesh portion provided in its conveying surface; allowing the product to fall along the conveying surface; and when the product passes over the at least one mesh portion during its fall, spraying the product with a coating.
The present invention is applicable in particular to the coating of snack food products with a dry, powdered flavouring. However, the invention may also be used to coat other products including, but not limited to, pharmaceuticals. The invention may also be used to coat products with substances other than dry powdered substances including, for example, liquids, suspensions and slurries.
An advantage of the present invention is that the food product comes in direct physical contact with the earthed wire mesh, the food product consequently also being earthed. The charged coating material will then be attracted to the food product directly, in contrast with the described prior art.
Preferably, the coating is subjected to at least one pressurised gas stream, whereby the coating is sprayed towards the at least one mesh portion. More preferably, the method further includes the step of applying an electric field in order to charge the coating.
The at least one mesh portion may be coupled to a potential, whereby the charged coating is attracted to the mesh.
Preferably, the conveying surface is vibrated, thereby encouraging the product to fall.
The conveying surface may be provided with corrugations along its length, thereby encouraging the product to fall along the conveying surface.
According to a second aspect of the present invention, there is provided apparatus for coating a product, the apparatus comprising an inclined chute comprising at least one mesh portion provided in its conveying surface; and means for spraying a coating onto the product when the product is in contact with the at least one mesh portion.
The means for spraying may comprise a gas jet nozzle; means for coupling a source of pressurised gas to the gas jet nozzle; and means for introducing the coating adjacent to the gas jet nozzle; wherein in use gas ejected from the nozzle impinges on the coating, thereby creating a coating spray.
The means for spraying may further comprise an electrode attached to or located adjacent to the nozzle; and means for charging the electrode; wherein in use the electrode provides an electric field which is used to charge the coating.
Preferably, the apparatus further comprises means for vibrating the inclined chute.
The conveying surface may be provided with corrugations along its length
There is illustrated in
Located on top of a support frame (not shown in
The lower or exit end of the supply chute 5 projects towards the wire mesh 16, the exit end having a width of about 25 to 75 mm. A nozzle mounting 7 is located beneath the lower end of the supply chute 5, and is illustrated in more detail in
Fixed into the mounting 7 is a short needle electrode 48, located in the passage just beneath the mouth of the nozzle. The electrode 48 is coupled via a conductor 49 to a high voltage charging circuit (for example a cascade arrangement). In use, the electrode is charged up to a voltage of around 85 KV. An end of the nozzle 44 is coupled via a hose 46 to a source of pressurised air (not shown in the figures). The nozzle mounting and the various components attached thereto are designed to withstand the high ambient levels of heat and moisture, and to be suitable for use on a food production line. A suitable control unit is used to control the pressure of air supplied to the nozzle and the voltage applied to the electrode. The nozzle mounting is substantially as described in the above-referenced co-pending British Patent Application.
Having explained the structure of the apparatus with reference to
As the coating 2 reaches the exit end of the supply chute 5, it will spill over the edge and will begin falling under gravity. Directly beneath the exit end of the supply chute, pressurised air is forced through the nozzle 7 and passes through the electric field created by the charged electrode 48. The air becomes charged as it passes through this field. The charged air then impacts on the coating 2, tending to disperse the powder and to break up particles into micro sizes (i.e. 45-75 microns in diameter). The coating particles also become charged when subjected to the electric field created by the charged electrode 48. It is noted that smaller particles (i.e. of micron sizes) accept a charge more easily than larger particles.
In the case where the coating is a liquid or slurry, it may be preferable to use a pump and a misting nozzle to generate the coating spray rather than using pressurised air from the gas jet nozzle as described above.
Products 8 to be coated are conveyed along the first chute 10 and move onto the second chute 12, where they fall under gravity and with the aid of vibration from the motor 18, passing over the wire mesh 16. The wire mesh 16 is connected to earth potential and therefore when the products 8 come into electrical contact with the mesh either directly or indirectly, the products 8 are also connected to earth potential. The dispersed, charged coating 2 accelerates towards the wire mesh 16 and the products in contact with the wire mesh. As the charged coating meets those products in contact with the mesh, it becomes attached to the surface of the products. Any excess coating will pass through the wire mesh and be collected in the hopper 20 and may be conveyed back to the hopper 1 for reuse without disrupting the coating process. Preferably, the chutes 10, 12, 14 are made from an insulating material, for example, plastic, to prevent them from attracting the coating. Coated products continue to pass along the second chute 12, and then continue to pass along the third chute 14, from which they are removed and packaged. The method described above is further illustrated in the flow diagram of
A further embodiment of the present invention is illustrated in
A further embodiment of the present invention is illustrated in
It will be appreciated that the apparatus and method described above do not require any small aperture components, e.g. Venturi tubes, slots, or holes, through which the flavouring powder must pass. There is therefore little likelihood of components becoming blocked, resulting in less downtime and higher productivity.
It will be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention. For example, where larger amounts of flavouring must be delivered to the product to be coated, a wider chute or multiple chutes, arranged side by side, may be used. A corresponding set of nozzles may be employed to charge and disperse the flavouring. Alternatively, a wide mouthed knife nozzle may be used.
