Abstract:
A coating such as liquid milk chocolate is applied to articles such as confectionery bars. The bars are conveyed by conveyor under a curtain of liquid chocolate issuing through an outlet slot in a trough. A layer of air is caused to flow through the outlet slot in the trough so as to modify the flow characteristics of the curtain. The layer of air permits a curtain of even thickness to be achieved.

Description:
This application is a divisional of U.S. application Ser. No. 09/600,769 filed Nov. 9, 2000, now U.S. Pat. No. 6,730,344 which is the U.S. National Phase application of International Application No. PCT/GB99/00158, filed Jan. 18, 1999. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates, in a first of its aspects, to a method of and an apparatus for applying a coating to articles, eg an edible coating to food articles. The invention is particularly, but not exclusively, concerned with the application of edible coating materials which exhibit non-Newtonian behaviour, for example chocolate, to articles of confectionery and the like. The present invention also relates, in a second of its aspects, to an improved method of forming a curtain of material eg edible material which can be used to coat articles eg articles of food or which can be used in other ways in the formation of articles, eg articles of food. 
   2. Description of Related Art 
   It is well known to enrobe articles, of food such as chocolate assortments, confectionery bars, biscuits, cookies and cakes with a layer of chocolate. This coating process is known as chocolate enrobing and is traditionally effected by moving the articles on a mesh-type conveyor belt through a curtain of liquid chocolate whose consistency is carefully controlled. However, it is difficult to achieve the desired coating because of the high viscosity. It is therefore common practice to subject the articles to a greater than needed coating, then controlling the amount of chocolate remaining on the articles by blowing by air from fans and using vibration to remove the excess. 
   Also, the temperature of chocolate cannot be increased so as to reduce its viscosity as it will lose its temper, causing problems of incorrect fat crystallisation which can deleteriously affect the appearance and/or eating quality of the chocolate. Thus, it is common for enrobing chocolate to have a higher fat content than standard chocolate used for making chocolate bars in order to reduce its viscosity. This has adverse cost implications. 
   In order to establish the curtain of chocolate, it is known to allow chocolate to pass under the action of gravity through an outlet slot in the bottom of a trough having inclined side walls leading to the slot. This type of apparatus has a relatively low throughput because of the high viscosity of the chocolate and the relatively low rate at which it can flow through the outlet slot. The common solution to this problem is to use a wider slot, which results in a thicker curtain which then requires removal of more excess from the articles. Because of the physical characteristics of the chocolate which passes through, the slot, the curtain can “neck” to a substantial extent. In other words, the width of the chocolate curtain becomes substantially less than the length of the slot from which it issues. This means that the effective cover of the curtain over the width of the conveyor belt used to carry the articles through the curtain is reduced. 
   Another known form of apparatus for producing a chocolate curtain utilises a roller along which liquid chocolate is distributed and carried to a blade which causes the layer of chocolate on the roller to become detached and thereby establish the curtain. This can provide higher coating rates with a better control of curtain thickness, but difficulties still arise in coating. 
   With both of the above types of known apparatus, the amount of chocolate in the descending curtain is greatly in excess of that required to coat the articles. The excess drains through the mesh-type conveyor belt and needs to be recycled and its condition carefully controlled. 
   SUMMARY OF THE INVENTION 
   It is an object of a first aspect of the present invention to obviate or mitigate at least some of the above disadvantages. 
   In accordance with said first aspect of the present invention, there is provided a method of applying a coating to articles, comprising the steps of:
     (i) providing a curtain of solidifiable liquid coating material;   (ii) effecting relevant movement between articles to be coated and the curtain so as to coat the articles with the solidifiable liquid coating material; and   (iii) subjecting the solidifiable liquid coating material to the action of at least one stream of gas under pressure whereby to modify the flow characteristics of the curtain.   

   Also in accordance with said first aspect of the present invention, there is provided apparatus for applying a coating to articles, comprising:
     (i) supply means arranged to provide a curtain of solidifiable liquid coating material;   (ii) means for effecting relative movement between articles to be coated and the supply means whereby in use the articles are coated with the solidifiable liquid coating material in the curtain; and   (iii) means arranged to subject the solidifiable liquid coating material to the action of at least one stream of gas under pressure whereby to modify the flow characteristics of the curtain.   

   Preferably, the articles are articles of food and the coating material is an edible coating material. 
   In the case where the present invention is used in a coating apparatus of the type in which the coating material is caused to flow along a surface of the supply means (eg a trough) towards an outlet slot through which the coating material flows under the action of gravity to form the curtain, it is preferred for said at least one stream of gas to be introduced between the coating material and the surface of the supply means. In this way layer of gas can be introduced between the surface and the coating material as it flows towards the outlet slot. Preferably, in the case of a trough where the coating material flows over opposed surfaces towards the outlet slot, a layer of gas is introduced between the coating material and each of the opposed surfaces. This can not only substantially reduce the resistance to flow of the coating material over the surfaces, but can also reduce the viscosity of the coating material if it is of a type whose viscosity is reduced when subjected to shear. 
   It is particularly preferred to cause said at least one stream of gas under pressure to become attached to the surface of the supply means so as to assist in establishing the layer of the gas between the surface and the coating material. This may be achieved by positioning one or more gas-admission slots in such a way as to direct the gas against the surface of the supply means. The spacing between the outlet slot and the or each gas-admission slot depends upon the nature of the coating material and the geometry of the supply means, which may comprise a trough having V-shaped walls defining opposed surfaces which converge towards the outlet slot. If the or each gas-admission slot is disposed too close to the outlet slot, then the flow of gas through the outlet slot may actually restrict the flow of coating material therethrough. On the other hand, if the or each gas admission slot is disposed too far away from the outlet slot, the coating material flowing over the surface may become re-attached to the surface of the supply means before it reaches the outlet slot. 
   In the case where the above-mentioned trough is employed, it is within the scope of the present invention to provide said at least one stream of gas under pressure at one or both convergent opposed surfaces of the trough leading to the outlet slot. 
   Said at least one stream of gas under pressure may be applied to the coating material after the curtain has been established in order to change the direction of the curtain and/or a physical property of the coating material forming the curtain. It is also within the scope of the present invention to subject the coating material to the action of at least one stream of gas both before and after the curtain has been established. 
   A curved surface may be provided adjacent to part of the curtain, and means may be provided for causing a stream of gas to flow over the curved surface by virtue of the Coanda effect and to use this to induce a change in the direction of travel of the curtain. In this way, it is possible to control the direction of flow of the curtain from any angle from vertical to substantially horizontal. This effect can be used whether or not the coating material is subjected to the action of at least one stream of gas under pressure before the curtain is established. In this regard, the curtain may be established by flow of the coating material through an outlet slot or by distributing the coating material over the length of a roller and causing it to be transported to a blade which removes the coating material from the roller and thereby establishes the curtain. 
   It is within the scope of the present invention to use one or more curtains of coating material to coat the articles and to control the direction of movement of these curtains simultaneously or independently in such a way as to ensure maximum coverage. For example, one of the curtains may be controlled so that its direction of movement is an acute angle (e.g. 45°) relative to the direction of relative movement between the articles and the curtain, whilst the other curtain can be controlled so that its direction of movement is at an obtuse angle (e.g. 135°) with respect to said direction of relative movement. In this way, coating of upstream and downstream ends of the articles may be facilitated. 
   In certain embodiments, the control of the pressure of the stream of gas can be employed to control the speed of descent of the curtain. Thus, by controlling the rate of descent of the curtain and the rate of relative movement between the articles and the curtain, a variety of different effects can be achieved. For example, if the rate of descent of the curtain is matched with the rate of relative movement, then a smooth coating can be achieved. If the rate of curtain descent is greater than the rate of relative movement, then a surface patterning effect can beg achieved by the resultant folding of the applied-curtain onto the articles. On the other hand, if the rate of descent is less than the rate of relative movement, a degree of stretching of the coating material as it becomes attached to the articles may be achievable with resultant thinning of the layer of coating material applied to the articles. The effects achieved will also depend upon the physical properties of the coating material. 
   The present invention is applicable to the use of non-Newtonian liquids such as chocolate or Newtonian liquids such as caramel as coating materials. 
   In the case of chocolate (or other non-Newtonian liquid, a liquid whose viscosity reduces when subjected to shear), a very high degree of control is achievable because the stream of gas under pressure can be caused to contact the surface of the chocolate in such a way as to reduce its viscosity by application of a shear force. This has the particular advantage that the chocolate flows much more easily but then rapidly thickens once the shear force has been removed. The application of the stream of gas under pressure to the chocolate before the curtain has been established can enable the previously mentioned necking problem to be mitigated and can also enable a much higher throughput to be achieved for a given size of coating apparatus. Because of the viscosity reduction achieved, it is possible to coat with a much higher viscosity chocolate than has heretofore been considered possible. For example, it, is possible to coat with relatively viscous tempered chocolate, rather than having to coat with a chocolate composition having an increased fat content and subsequently lowered tempered viscosity in order to establish and maintain the desired liquid curtain. The need to effect air blowing and/or vibration on the coated articles may be obviated or mitigated. The present invention permits a curtain of even thickness to be achieved and may also enable a thinner curtain to be produced than has heretofore been possible. 
   Conveniently, the gas is air. The temperature of the gas may be substantially the same as that of the solidifiable liquid coating material. This is particularly advantageous in the case where the material is liquid chocolate. 
   It will be appreciated that the present invention in its first aspect involves the control of a curtain of solidifiable liquid coating material for the purpose of coating or enrobing articles. However, it will be appreciated that similar techniques can be employed for controlling a curtain of a solidifiable liquid material for use in the production of other articles. For instance, the curtain can be controlled for the purpose of enabling a layer of the material to be deposited into a mould or moulds (e.g. to produce shells of solidified material which can then be used to contain filling material), or onto a conveyor for solidification as a layer thereon which can be subsequently cut to size or otherwise shaped. The fact that the curtain can be very accurately controlled in terms of the thickness of the curtain and/or its angle/speed of descent means that a close control over the thickness and/or texture of the deposit can be obtained which can be difficult to achieve with standard confectionery shell technology. 
   Thus, in its second aspect, the present invention resides in a method of controlling a curtain of a solidifiable liquid composition comprising the steps of:
     (i) providing a curtain of solidifiable liquid material; and   (ii) before, during and for after step (i), subjecting the solidifiable liquid material to the action of at least one stream of gas under pressure whereby to modify the characteristics of the curtain.   

   Also in accordance with said second aspect of the present invention, there is provided apparatus for controlling curtain of a solidifiable liquid composition comprising:
     (i) supply means arranged to provide a curtain of solidifiable liquid coating material; and   (ii) means arranged to subject the solidifiable liquid coating material to the action of at least one stream of gas under pressure whereby to modify the characteristics of the curtain.   

   The method and apparatus may further include provision for (a) depositing the modified curtain of solidifiable liquid composition to form a layer in a mould or on a surface, and (b) solidifying the deposited composition. 
   The solidifiable liquid composition is preferably an edible composition. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic view of one embodiment of an apparatus according to the present invention, where, for ease of demonstration, only one longitudinal trough surface is shown with an air supply, 
       FIG. 2  is a side view of a trough forming part of the apparatus of  FIG. 1 , and 
       FIG. 3  is a schematic view of a second embodiment of apparatus according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIGS. 1 and 2  of the drawings, the apparatus illustrated therein is for enrobing confectionery bars with a layer of chocolate. The apparatus comprises a mesh-type conveyer belt  10  carrying confectionery bars  12  to be coated horizontally from right to left as viewed in  FIG. 1 . A liquid chocolate supply trough  14  is spaced above the conveyor  10  and comprises V-shaped walls  14   a  and  14   b  leading to a downwardly opening outlet slot  14   c . The trough  14  contains liquid chocolate  16  which is to be used for coating the confectionery bars  12  to form coated confectionery bars  18 . 
   The trough  14  contains an additional wall  14   d  which is inclined at an acute angle relative to the wall  14   b  and which terminates about 10 mm above the outlet slot  14   c  which, in this embodiment, has a width of about 2 mm. The walls  14   b  and  14   d  together define a downwardly convergent plenum chamber  20  terminating in an air-admission slot  22 . The slot  22  has a width of 0.2 mm and extends for the whole length of the outlet slot  14   c , and is disposed about 10 mm above the outlet slot  14   c  of the trough  14 . The plenum chamber  20  is connected with a source of pressurized air, the pressure of which is controlled by a controller  26 . Although not shown in the drawings, the wall  14   a  is likewise provided with an additional wall defining an identical air-admission slot like slot  22 . 
   Disposed below the conveyor  10  is a tank  24  which also contains liquid chocolate  16 . A pump (not shown) serves to pump the liquid chocolate  16  from the tank  24  to the trough  14  via pipework  28 . 
   In use, the conveyor belt  10  is operated to move the confectionery bars  12  in a path which carries them under the outlet slot  14   c  of the trough  14 . The chocolate  16  in the trough  14  is typically maintained at a temperature in the range of 28 to 31° C. and travels through the slot  14   c  under the action of gravity so as to produce a curtain  30  of liquid chocolate through which the confectionery bars  12  pass. The curtain  30  extends perpendicularly across the conveyor belt  10  which carries a plurality of rows of the confectionery bars  12 , although only one row of bars  12  is illustrated in the drawings. The resultant coated bars  18  are carried by the conveyor  10  and excess chocolate drips through the holes in the screen conveyor  10  and back into the tank  24  for recirculation. 
   During this time, air under pressure is supplied to the plenum chamber  20  so that it is ejected through the air-admission slot  22  so as to become attached to that portion of the wall  14   b  which lies between the slots  22  and  14   c . The result of this is that a layer of air is inserted between the chocolate  16  passing towards the outlet slot  14   c  and the wall  14   b  immediately upstream of the slot  14   c . The same occurs at the wall  14   a . This substantially reduces the friction between the chocolate and the walls. Additionally, the pressurised air exerts a shear force on the adjacent surface of the chocolate, thus reducing the viscosity of the chocolate in the region of the outlet slot  14   c . This improves the flowability of the chocolate so that it substantially increases the flow rate through the slot and also mitigates the necking problem whereby the width of the curtain  30 , i.e. the dimension perpendicular to the plane of the drawing, can be maintained substantially the same as the length of the slot  14   c.    
   In one experiment, it was found that no less than a 70% increase in the flow rate through the slot  14   c  could be achieved using an air supply pressure of 2 psig, as compared to the situation where no air is supplied through the gas admission slot  22 . Although, substantially improved results could be achieved with air pressures as low as 0.5 psig and up to about 3 psig. It will be understood that the shear effect is produced because the air is moving at a greater rate than the chocolate with which it is in contact. 
   The above-described method enables improved control to be achieved, which can lead to the following advantages:
     (a) lighter chocolate coatings,   (b) more precise coating with less excess deposit,   (c) less variation in deposit across the conveyor belt,   (d) faster enrobing speeds,   (e) lower proportion of chocolate to be recycled,   (f) less build-up of chocolate on the edges of the bars, leading to better edge definition,   g) selective production of textured or plain coatings by adjustment of air pressure and conveyor belt speed.   (h) avoidance of the need to use enrobing chocolate (which has a high fat content to decrease its viscosity when liquid), thereby enabling the use of regular or even lower fat chocolate.   

   Referring now to  FIG. 3  of the drawings, there is illustrated an apparatus also in accordance with the present invention for altering the direction of descent of chocolate curtain  30 . The chocolate curtain  30  may be one which has been produced as described hereinabove with reference to  FIGS. 1 and 2 . Alternatively, it may be a conventionally produced chocolate curtain which has been formed without introduction of an air-stream into trough  14 . As a further alternative, it may be a conventionally produced curtain formed by distributing a layer of liquid chocolate onto the surface of a roller and then detaching the layer from the roller by means of a blade. 
   In, the apparatus of  FIG. 3 , there is provided a cylindrical plenum chamber  40  which is horizontally disposed to one side of the curtain  30  above the conveyer  10  and which extends for greater than the full width of the curtain  30 . The plenum chamber  40  may conveniently be mounted on the trough  14  so that it can be positioned close to the outlet  14   c . The plenum chamber  40  has a pressurised air inlet  42  and a row of upwardly directed air outlets  44  extending over the length of the plenum chamber  40 . An angled cap  46  is secured to the outer periphery of the plenum chamber  40  along a longitudinal side edge thereof which is remote from the curtain  30 . The opposite longitudinal side edge of the strip  46  is spaced from the peripheral surface of the plenum chamber  40  so as to define an air-exit slot  48 . The slot  48  has a width of typically about 0.2 mm. The plenum chamber  40  can be moved laterally horizontally relative to the curtain  30  so as to enable the gap between it and the curtain  30  to be adjusted. Likewise, the plenum chamber  40  can be turned about its longitudinal axis to enable the position of the slot  48  relative to the curtain  30  to be adjusted. 
   In use, air is supplied through the inlet  42  into the plenum chamber  40  from whence it issues through the outlets  44  and thence through the outlet slot  48 . It is thus caused to adhere to the curved peripheral surface of the plenum chamber  40  by virtue of the Coanda effect whereby it follows the external periphery of the plenum chamber  40  for a considerable distance. The effect of this curved flow of air is to draw the curtain  30  towards the plenum chamber  40 , thus altering the angle of descent of the curtain  30 . The angle of descent can be varied by varying the pressure of the air and/or by varying the positioning of the slot  48  relative to the curtain  30 . The pressure of the air can be varied by controller  26 . 
   In  FIG. 3 , the plenum chamber  40  is shown on the downstream side of the curtain  30  relative to the conveying direction of the confectionery bars  12  through the curtain  30 . Thus, the effect is to incline the direction of descent of the curtain  30  at an acute angle relative to the direction of movement of the confectionery bars  12 . This can enable improved effects to be achieved. It is possible to lay the curtain  30  of chocolate gently onto the surfaces of the bars  12  by appropriately matching the rate of descent of the curtain  30  to the speed and movement of the bars  12 . It is also considered that, because of the angling of the curtain  30 , it will be possible to improve coating of the leading ends of the confectionery bars  12 . 
   However, it will be appreciated that it is possible to locate the plenum chamber  40  on the opposite side of the curtain  30 , ie on the upstream side thereof so as to cause the direction of descent of the curtain  30  to extend at an obtuse angle relative to the conveying direction of the confectionery bars  12 . In this way, it is considered that an improved coating of the trailing ends of the confectionery bars  12  may be achievable. Also, further control over the effects produced can be achieved by altering the conveyor rate relative to the rate of descent of the curtain  30 . 
   If desired, the confectionery bars  12  may be taken through more than one curtain  30  with the curtains being disposed of the same or different angles depending upon the effects required. 
   If desired, the confectionery bars may be passed through the curtain twice, the first time for the purpose of effecting a main coating operation and a second time for the purpose of ensuring that the coated bar is of the specified weight. 
   The curtain of chocolate (or another solidifiable liquid material) produced as described with reference to either or both of the illustrated embodiments may, instead of being used to enrobe articles such as confectionery bars, be used to form a layer of controlled properties (eg thickness) into moulds as an alternative to conventional depositing technology, followed by cooling to solidify the layer. Such a technique can be used to form shells for subsequent filling with a filling material. Alternatively, the curtain may be laid onto a conveyor to form a controlled Layer thereon which is subsequently solidified and cut to size to the desired shape.