Description of the process
Initially the components for the selected recipe are mixed homogeneously in a batch raw material mixer (1) and then water is added according to the required moisture content. The raw materials used for the production of extruded snacks or breakfast cereals include ground cereal products such as maize/corn, rice and wheat grits, as well as potato flakes or granules. These raw materials, which contain starch, are the main components and they can be combined with other ingredients such as common salt, sugar, cocoa or vegetable powder as well as with flavourings and emulsifiers. This mixture is then fed to an intermediate container (3) by a screw conveyor (2), from which the mixture is then conveyed by another screw conveyor (4) continuously to the hopper of the extruder dosing unit. The dosing unit is equipped with a level sensor. The constant fill level ensures that the mixture is dosed into the extruder very consistently.
The core component of the snack production plant is what is known as the cooking extruder. The key components of the single screw cooking extruder (5) are the dosing unit, the sleeve, the rotating screw, the pressure and die plates and the cutting and temperature control units. In the cooking extrusion process itself, also known as HTST (High Temperature Short Time) extrusion, raw materials containing starches and proteins are plastified at certain moisture contents and undergo structural modifications by the influence of temperature, pressure and shear forces in the barrel of the extruder. Plastification has the effect of thoroughly cooking and gelatinization of the starch in the mixure. Here the pressure and die plates generate very high pressure.
When the plastified mass emerges from the die plate, the moisture in the mixture escapes in the form of vapour, which is what leads to the typical structure of the products. At the same time the mass also expands as a result of the sudden drop in pressure to the ambient pressure level. The extruded products are then given the desired shape and texture by being cut by a rotating knife or a special cutting and moulding device.The entire cooking extrusion process is characterized through adjustable process parameters such as dosing levels, screw speeds, barrel temperatures and cutting speeds.
The pressure and temperature is permanently monitored during the process itself. The temperature in the in-feed zone can be controlled by using a water cooling unit.
The extruded products are then fed to a continuously rotating drum drier (7) via a conveyor belt (6) or pneumatic conveyor system. Drying is necessary, because the products still have a moisture content of 6 – 8% after leaving the die plate. The drying time is only a few minutes, depending on the desired texture, after which the residual moisture content is approx. 1.5 – 3%. The residence time in the drier depends on the selected drum speed and the drying temperature.
The continuously rotating drum drier can be equipped with both a hot air and an infrared drying system. The higher temperatures in infrared drying lead to a roasted taste of the kind required for breakfast cereals. The dried products are then conveyed to a continuously rotating cylindrical flavour drum (9) via a conveyor belt (8) or pneumatic conveyor system, and this drum is equipped with a system comprised of a seasoning preparation system and a dosing unit. Here the dried products are coated with a liquid mixture of seasonings and vegetable oil. Following temporary storage the flavoured products are fed to the downstream packing machines via conveyor systems (10).