Post Baking Operations

The run out of the oven band and stripping biscuits from the band

A flat blade or a set of fine fingers bearing on the oven band as it curves downwards on the terminal drum is designed to lift the products clear and transfer them with minimum damage and minimum disruption to the relative positioning of the biscuits. The blade or set of fingers is known as the stripping knife. The biscuits are pushed by successive rows of biscuits onto the first cooling conveyor or fall onto a cross conveyor.The first cooling conveyor is usually quite short and may be wire mesh or cloth fabric. It is possible to retract the nose piece of this conveyor immediately behind the oven stripping knife or fingers so that burnt or bad biscuits may fall down onto a cross conveyor to be collected for scrap (see Figure 7). This whole unit is known as the oven stripper and is powered by the oven drive. Its speed is usually slightly faster than the oven band to allow some separation of the rows of biscuits.

3c-Reject-system-thumbDetail of reject system before oven stripper conveyor

Should the oven band stop during production as a result, for example, of a power failure, it is necessary to move the oven band either with an auxiliary power source or by means of a handle. Either way, there is usually a short delay before the oven band is moved again and then rather slowly. Under these conditions it is not unusual for product to emerge from the oven and to catch fire. It is important that this hot product does not fall onto any canvas cooling conveyors otherwise they may ignite and the fire spread. The oven stripper conveyor, if made of wire mesh, provides a safe buffer from which burning product can be swept onto the floor or into suitable containers.

Most short dough biscuits are soft and flexible as they leave the oven. This means that they are more difficult to strip from the band. Cooling soon sets the sugary structure so enough oven band run out after the oven is necessary to allow this to happen naturally or air must be blown down onto the biscuits to hasten the cooling/setting. In some cases the band is deliberately cooled with a unit to spray water on the underside.

Biscuit cutting

In some cases it is necessary to cut biscuits on the oven band after baking.

Examples are,

  • breaking cream crackers and Garibaldi fruit biscuits which have been baked as rows of joined pieces
  • cutting continuous extrusions such as filled bars (Fig Bars)

Biscuits baked as full rows across the oven band mean that there was significantly less cutter scrap.

Continuous extrusions must be cut by guillotining either before or after baking, the advantage of cutting after baking is that the cutting may be cleaner or will prevent the spillage of centre fill during baking.

Electronic, dielectric drying

Instead of passing from the oven band onto cooling conveyors the biscuits may be passed onto a non metallic conveyor and passed through a radio frequency drying unit.

The conveyor and biscuits pass between sets of electrodes above and below the conveyor. An alternating electronic field, at radio frequency, is established between the electrodes and this passes through the biscuits and preferentially heats the wettest parts by activating the water molecules. The spacing between the electrodes has to be critically tuned to achieve the maximum heating action from the electronic field.

By heating the water in the biscuits it is driven to the surface and lost. In this way moisture gradients which may persist after baking and which could result in checking, are significantly reduced.

Conventional baking ovens are inefficient at removing the last traces of moisture from biscuits. They are much better for developing structure and for colouring the dough pieces. Thus to use an electronic drier allows biscuits to leave the oven much wetter than usual so the baking time can be reduced and production from the oven increased. It is claimed that by using post oven dielectric drying it is possible to increase the output of a conventional oven by up to 33% but this is rather optimistic.

The limitations are that the semibaked biscuits must be rigid enough to be transferred from the oven band over a stripping knife and onto a canvas conveyor. The closer the drier is to the oven mouth the better as the hotter are the biscuits as they go into the drier the more effective is the heating from the dielectric field.

Oil sprayingArcall

Many savoury cracker biscuits, and also some other types, are given a dressing of oil immediately after leaving the oven while the biscuits are still hot. The amount of oil applied is small and this is done by spraying either from pressure nozzles, spinning discs or by electrostatic charge. All types of oil application, except the last, tend to be messy because fine droplets of oil form a fog that will drift from the spray unit unless there is positive extraction and filtering. Even the filters become saturated after a while.

The biscuits are carried through the unit on open wire conveyors. The oil dressing, applied at around 5-10% of the biscuit weight, greatly improves the appearance of the biscuit surface, enhancing the colour, and adds to the eating quality a little. The excess oil falls through the conveyor and is recycled.

In some cases flavoured oil is applied which for savoury or hard sweet types is a useful technique for applying flavour that would be lost if added in the dough before baking. The main problem with flavoured oil is that it contaminates the cooling conveyors used to hold the biscuits when they leave the oil spray unit and the smell may fill the packing area of the factory.