TWIN SCREW EXTRUSION: Producing Breakfast Cereal

Breakfast is an essential part of a well-balanced diet. This meal makes up about 20 to 25 percent of the total daily energy consumption for children, and 15 to 20 percent for adults. Breakfast cereal, in combination with a drink, a dairy product, and fruit can help consumers to meet this requirement.

Types

Cereal consists of a main carbohydrate source that has fast and/or slow absorption. Low in lipids, it can be a good source of fi ber, vitamins and minerals. In fact, as technology has improved, the range of prepared cereals has also evolved. So simple cooked oats and cooked fl aked corn have given way to sophisticated products like fl aked and directexpanded cereals. In fact, today's cereals can be vitamin-rich, high in fi ber content, sugar- and syrup-coated, chocolate-fl avored, and formed in many appealing shapes. Müsslis (a blend of fl akes, nuts, fruit, sugar, and other ingredients), co-extrudates (crisp shells with a soft fi lling), and granola bars (agglomerated and roasted cereals) belong to this large family as well.

It is estimated that over fi ve-million metric tons are consumed annually, with steady growth worldwide. Due to health and nutrition concerns, consumers are seeking products with high-added value.

Twin Screw Extruders

In the early 1970s, Clextral pioneered twin-screw extrusion technology for breakfast cereals, introducing a process that was faster, simpler and more economical than the traditional batch process-reducing production times to 30 minutes for fl akes from six to seven hours. (Twin Screw Extruders (TSE) are machines that have two identical co-rotating, intermeshing, self-wiping screw profi les operating within a closed barrel. Extruders offer many features for enhanced fl exibility including a modular barrel design, accurate temperature and shear devices, automatic barrel opening for easy access to the screw profi le, and computerized operation and control.)

The TSE is a continuous mixer/cooker/former, which performs a positive pump action. It can operate at high pressures and highmoisture levels, and does not rely on internal friction between the screws and barrel to convey the cooked mass towards the die. It can process a variety of particle sizes while maintaining uniform-product shear and production fl ow. Flaked or expanded products made from various grains are produced in the same plant, using the same extruder and barrel confi guration and ancillary equipment upstream, (feeder, preconditioner, liquid injection) and downstream (fl aker, toaster, coating unit, etc.).

Today, other benefi ts of the TSE include reduced space requirements; an ability to process a large range of processed raw ingredients; a continuous production system that can be automated; quick start-up and shut-down; and cleaning procedures which make it possible to switch easily from one product to another.

Cereal production

These are the major steps and options of breakfast-cereal extrusion.

1. Premix station: has a range of options are available for rawmaterial handling systems, from sack tip to fully automatic bulk systems. This includes a weighing, mixing and preparation area for the ingredients-partially or fully-automated-to suit the individual processor's requirements.

2. Preconditioning: introduces extra steam and water to gelatinize the raw material, making it soft and malleable. It is a gentle process that allows ingredients to retain their natural fl avor.

3. Twin screw extrusion: cooks the ingredients with a combination of heat, mechanical shear and added moisture. If desired, liquids like malt-syrup preparation may be added at the same time through a metering pump. The temperature is accurately controlled in each section of the modular barrel by internal cooling channels and external heating elements. The screw confi guration and die design are specifi c to non-expanded or expanded cereals. A post-cutting system calibrates the nibs (for fl akes) or the direct expanded products.

4. Flaking (for flakes only): after direct cutting at the exit die or through a nib cutter, the cooked but non-expanded nibs are tempered in a drum conditioner then fl aked between two chilled steel rollers. Product specifi cations, such as stickiness, and surface characteristics are controlled by adjusting the roll pressure on the product. 5. Toasting/drying: the direct expanded products coming from the extruder or the fl akes from the fl aking rolls are fed into the dryer/toaster to reduce the moisture content to around three percent. Temperatures and residence times are not the same for direct expanded cereals and fl akes. During this heat treatment, the fl akes develop their specifi c blistering, crispy texture, fl avor and color. 6. Coating: the dried or toasted products may be fed to a coating unit where a sugar-syrup preparation is applied in the required proportion. During or after coating, the products are dried then packed. 7.Packing: a multi-weighing unit is often linked to a vertical bagger connected to a vertical cartooning machine; this system offers a fully-automated and accurate packaging solution.