Wafer Baking

Plate gap setting

This is a fundamental setting which is decided when the plant is set up. It is a large engineering task to make a basic change to all the plates of an oven. The thickness of a wafer sheet is determined by the body thickness, related to the gap between the plates and also the depth of the relief or reeding in the plates. The relief cut into the plates gives not only more interest to the appearance of the wafers, but also contributes to their mechanical strength. Thus, any reeding on the top surface should be at 45° to that on the bottom.

It is interesting to note that the wafer thickness is not constantly related to the mechanical gap setting between the plates. It is always thicker. The difference is between 19-30% from older slower plants and between 5-13% for newer faster plants. Thus, it is not possible to compute the exact wafer thickness from a plate setting until production conditions have been established.

Volume of batter

The batter is applied to plates with a sparge pipe across the plate and the batter is pumped intermittently to give exactly the same volume to each plate. The batter is deposited from up to 22 holes in the pipe and the volume is adjusted to just fill the plates allowing a minimum of extrusion through the steam vents. Excess deposit gives not only excessive waste but also increased sheet weight.

Batter viscosity

As batter is pumped onto the open hot plate, gelatinisation occurs preferentially where the batter first touches. This can always be seen on the underside of a wafer sheet as a pattern of lines. The greater the number of deposit lines and the more free flowing the batter, the less pronounced is this effect and the more uniform the colouration after baking.

Plate closure speed

Major differences in process performance are apparent between modern plants, with very fast latch closure mechanisms for the plates, with older ovens where the latching operation is delayed.

Older wafer ovens were built with the short side of the plates leading on the chain. To reduce the overall length of ovens while greatly increasing the number of plates, this arrangement has been changed so that typically the long side now leads. This has necessitated a more rapid closure of the plates both in order to keep the side to side effects on the sheet uniform and also because the linear speed of the plant is higher. Unexpectedly the change in closure speed led to different wafer quality from the same batter. Faster closure gives lower weights and thinner wafers.

On some ovens the final locking position of the plates and hence the closure time can be adjusted slightly.

Steam venting

Experience suggests that type, disposition and the standard of steam vent engineering are of considerable importance to (a) the performance of the plant and (b) the between plate variation of wafer sheets. Large areas for steam venting allow excessive losses of batter but aid moisture removal, small areas control losses and batter spreading better but allow development of excessive pressures. These higher pressures cause increased wear on bearings and set up strains which have bad effects on plate settings.

Baking speed

Baking times vary between 1.5 - 3 minutes with 2 minutes being about average. On large plants there is a mechanical consideration and it is generally felt that 55 plates per minute is the fastest reasonable speed. Most ovens, even new plants, run at approximately 60% of this speed, that is only 33 plates per minute. Fast baking speeds require high plate temperatures with increased tendency to "shelling" (wafers with extremely fragile centre texture) and appreciable moisture gradients across the sheet when they are released. This is because there is not sufficient time for the moisture to move to the steam vents around the edges.

The release of the wafers from the plates is effected because some shrinkage occurs after the wafer structure sets and is the result of drying. The sheets normally fall freely as the plates open. They are caught on some sort of transfer device and placed on light weight plastic rope conveyors so that cooling may occur freely on both sides. As an aid to sheet release from the plates a small blast of compressed air may be used which impinges at the edge of the sheet to lift it from the lower plate and allow it to fall away. A sweep of air is also used just prior to the deposition of fresh batter. This is because it is quite common for a piece of "bobble" to fall from the edge of the open upper plate onto the lower which is to receive the batter. If this is not removed and it becomes incorporated into the new sheet a very hard area is formed which is not only unpleasant to eat but could also break a cutting wire after the wafers have been creamed.

The moisture content of wafer sheets will be in the range 1-2% and it is important that the variation of the moisture across the sheet is a low as possible (within ± 0.45%).