This is the fourth of seven installments covering all the facets of flat wafer processing, which discusses details regarding the very important issue of baking plates.
Wafer baking plates or books (some manufacturers like to refer to the plates as tongues) can be manufactured to any number on an oven within the module set by the respective manufacturers with 60, 45 and 30 being most common on older installations although these days in modern plants several larger ovens are also in use with up to 120 plates (72 -96 being the average).
Older units of which there are still a few around the world have the plates running length ways (longitudinally) through the oven whilst more modern or recent designs have the plates mounted in the transverse direction. This helps reduce the oven length when large numbers of plates are involved. The usual plate size is 290 mm x 470 mm although mid-sizes of 370 mm x 240 mm and 350 mm x 470 are in use. “Jumbo” plates of 700 mm x 350 mm are not uncommon.
Plates can be mounted in strong robust carriers or be self-supporting, either way, travel through the oven is achieved via a chain linked conveyor and heavy duty flanged ball bearing wheels (4 to each plate) running in guide rails. Each baking plate should ideally be consecutively numbered in a place exposed to the operator for quick and easy reference. The most common design takes the form of “V” grooves or reeding positioned at 5 or 10 grooves per 25 mm (1.0 inch) across the plate.
Apart from the numbering previously mentioned on the exterior, it is also common to engrave the plate number on the baking surface, so that the sheets can be identified with their respective plates post bake. The number is engraved discreetly in what is usually the off cut or trimming area of the sheet. For added wafer strength the grooves or reeding are engraved on the bottom plate at right angles (90°) and on the top plate diagonally (45°). The depth of reeding can range from 0.3 – 0.8 mm with 0.5 – 0.8 mm being the most common at an angle of 60-90°. The manufacturer can guarantee a variation of less than 0.002 mm in the depth of engraving to provide uniformity.
Two alternative plate finishes are usually available with the objective of giving good release to the baked wafer sheet. These would be a micro machined finish and alternatively a chrome plated finish. The latter is more expensive, but is preferred as it helps prevent sticking more than the micro machined finish. Some manufacturers believe that a chrome-finished plate produces a harder wafer, as it is said to require more heat; this is probably debatable.
Careful consideration should be given to the pitch and size of the engraving based on the anticipated end use of the wafer sheet. Too shallow or fine an engraving may highlight wide color variation caused by the difference in area of deposit compared to the rest of the sheet. This could be a problem for plain filled (naked) wafers, but acceptable for coated wafers. With coated wafers a shallow engraving is preferred, as a deep engraving could retain more chocolate on enrobing, increasing costs. Where the wafer(s) are required for a multiplicity of end uses, it is possible to have bottom and top plates engraved with fine and deep engravings respectively. Prior to creaming, the wafer, which is to make up the top sheet of the creamed book, can be programmed to be turned over automatically as required. This presents a fine or deep engraving for coating or direct wrapping as needed.
The plates have venting strips along the edges. Front and back of the top plate and both sides of the bottom plate. The venting strips have an important dual purpose often not appreciated or perhaps overlooked. To retain the batter in the closed book whilst it travels through the oven and more importantly to allow the escape of steam through notches accurately machined along their length. These “V” or “U” shaped notches (depending on the manufacturer), whilst allowing steam to escape, also allow excess batter to come from the plate in the form of bubbles.
The oven manufacturer has worked out the position and depth of these vents notches. Nevertheless, once in situ using the available batter, adjustments may be necessary to lift or lower these strips tobetter control the venting and evacuation of steam from the plate. This in turn, likewise, aids even distribution of batter across the surface of the plate. However, if the aperture is too small, the problem of high moisture in the baked wafer can result. This necessitates longer baking times and a reduction in output. If too large, excessive waste in the form of bubbles around the sheet can be expected.
An estimated loss of between 4-8% of the batter has been calculated as being typical. This may seem high to some manufacturers, but it has also been estimated that this can include in the waste some 30% water, and is consequently much less on a dry basis. Nevertheless poor baking and moisture distribution often seen as streaks or veins in the wafer can be traced back to poor venting. Some ovens incorporate ledges each side of the leading and trailing edge of the bottom baking plate to “catch” and “dry out” the bubble waste making disposal easier.
The bubble waste (also known as bobbles or a variety of other local terms) falls onto the floor or base of the oven chamber. An ideal situation is to deposit sufficient batter onto the plate to obtain a full sheet with good steam release and minimum bubble waste (this bubble waste cannot be reworked but must go to animal feed, etc.). It should be borne in mind that a heavy deposit will give a denser textured wafer as well as excessive waste.
Whilst baking the sheet, it is essential on most ovens to ensure that each plate is filled consecutively, as an omission due to depositor blocking or baked wafer sticking, etc. would require an estimated 5 or 6 circuits of the oven for the plate to reach standard condition again, i.e. provide wafer sheets of the same color, texture and moisture as previously and as the products from other plates.
Pressure between the closed plates has been estimated at between 1-1.5 bar and so depositing on an already filled plate could cause serious damage. On some latter designed ovens it is possible to take off a plate for “servicing” and program the depositor to miss the “gap” whilst still producing wafers of consistent quality.
Prior to the start of a shift the oven should be preheated for about 35 minutes before depositing starts. As soon as the batter hits the hot plate, gelatinization starts to take place. This can be seen on the baked wafer sheet as a darker color. For this reason, it is preferable to have as many nozzles depositing as possible. For a standard longitudinal plate, this would be about 12-13. The same size plate travelling transversely through the oven would have 20-22 nozzles. This aids not only better distribution of the batter across the plate but as well gives the opportunity of obtaining a more even color. This can be compared with the color of the wafer sheets made with depositors of say 6 or 8 nozzles or less. However, a compromise may often be required, as too many nozzles in turn may cause too “thin” a deposit, also resulting in discoloration.
Overall it should be considered as being better to distribute the batter over the plate via the arm rather than by it spreading due to the influence of the two plates coming together. One of the reasons is thought to be that as the plates travel around the oven individually it is not possible to have a continuous curtain deposit such as is typical in a chocolate enrober, for example. For each plate you require a cut off and ideally a suck back facility to avoid tailing and drips, etc. Compared to older plants a modern unit has plates that are almost continuous but nevertheless have a small “gap” separating each.