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Digital Re-print - July | August 2011 Twin screw extrusion technology to produce breakfast cereals

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Clextral pioneered twin-screw extrusion technology for breakfast cereals in the early

1970s, introducing a process that was faster, simpler and more economical than the traditional batch processes.

The advantages of the twin-screw extru-sion technology compared to the traditional processes are many and include: faster manu-facturing time (15 to 30 minutes for extrusion cooking versus six to 10 hours for traditional process) with less energy consumed; reduced space requirements; continuous and auto-mated production system allowing easier traceability; quick start up, shut down and cleaning procedures; possibility to switch eas-ily from one product to another; simplified maintenance; very large range of processed raw ingredients; and consistency of product quality.

Analyses like Rapid Visco Analyser (RVA) and texture and acoustical emission have been useful in the characterisation of com-mercial corn flakes produced by traditional process versus twin-screw extrusion technol-ogy.

Thanks to these techniques, it has been demonstrated that commercial corn flakes pro-duced by traditional process and twin-screw extrusion technology present similar profiles and sensory analyses.

Twin-screw extrusionTwin-screw extruders (TSE) are processing

machines consisting of two identical co-rota-tion, intermeshing, self-wiping screw profiles operating within a closed barrel.

During extrusion the materials is forced to flow under controlled conditions along the length of the extruder barrel and through a

shaped opening (also called die assembly) at a defined throughput. TSE can perform various functions including feeding, conveying, mixing, forming, compression, hydration, heat transfer, shaping and cooking.

Variables that influence extrusion process can be separated into three main components: raw materials composition and formulation;

thermomechanical cooking factors, including technology design (screw profile, L/D of the machine) and operating conditions (screw speed, temperature profile, water content, dry mix rate); and die texturisation factors (die design, insert shape, opening section).

Using a complex combination of all these variables allows the production of a wide vari-

ety of products in a range of shapes, sizes, bulk densities and textures.

Because of its operating mechanism, TSE offers many more advantages compared to single screw extruders. The TSE is a continu-ous mixer/cooker/former, which performs a positive pump action; it can operate at high pressures and high moisture levels and does

Twin screw extrusion technology to produce breakfast cereal

a flexible & sustainable solutionby Anne-Sophie Le Corre, Process manager at Clextral Inc R&D center, Tampa, Florida, USA

Grain&feed millinG technoloGy July - august 2011 | 25

FEATURE

tion, which combined with the screw speed variable (200-450 rpm), allows the right level of thermomechanical cooking on starch com-ponent. The time-temperature-shear history in the screw-barrel assembly determines the expansion ratio at the die as well as the textural quality of the products.

If desired, liquids may be simultaneously added through a metering pump to add specific characteristics to the final product. For example, in the case of crisp rice or corn flakes, sugar-based malt syrup is added along the barrel to reinforce the toasted taste of the end product. The temperature is accurately controlled in each section of the modular bar-rel by internal cooling channels and external heating elements.

The dough coming out of the die presents different levels of expansion depending on the targeted product and the process conditions. It can be die face cut (usually, but not exclusively, for direct expanded products) or indirect cut thanks to post cutting system (usually, but not exclusively, for indirect expanded products).

Tempering and flaking (for flakes only)

In the case of the flake process, a tempering zone may be added between the extrusion process and the flaking unit. This stage may be essential to separate the pellets that would otherwise stick together and to allow control of the dwell time before flaking.

After the tempering unit, the pellets are flaked between two chilled steel rollers. Adjusting the roll pressure on the product controls product specifications such as sticki-ness, thickness and surface characteristics.

Toasting/dryingThe direct expanded products coming from

the extruder or the flakes from the flaking rolls are fed into the dryer/toaster to reduce the

hydraulic opening of the barrel, quick access to the screw profile, quick and automated change of die assembly, and quick adjustment of ancil-lary equipments.

Due to the versatility of the twin screw extruders, flaked or expanded products made from various grains are easily produced in the same plant, using the same extruder and barrel configuration and ancillary equipment upstream (feeder, preconditioner, liquid injection) and downstream (flaker, toaster, coating unit).

The major steps in the breakfast cereal complete line process include:

Raw materials handling and premix station

Raw materials handling presents a range of options from sack tip to fully automatic bulk systems. Typical mixers for dry mixes are ribbon mix-ers, also called horizontal batch mixers.

The blender delivers the mix to a surge hopper.

Then, it passes through a metal detector, and is fed con-

tinuously and uniformly to the extruder platform through a loss-in-

weight feeder or a volumetric feeder.

PreconditioningPreconditioning essentially involves heating

and prehumidifying the raw materials to start the gelatinization of the starch.

The preconditioner has two shafts fitted with variable pitch blades that stir the material and move it towards the outlet. The shafts are usually contra-rotating. The configuration of the blades and the speed of rotation play an important role in the quality of the mixture dispersion. During preconditioning, the mix is humidified by spraying water into the precon-ditioner via nozzles.

Other types of liquids (colors or malt syrup, for example) can be added via different injection ports, at different steps into the preconditioner. The mix is pre-heated at the same time by steam injection via multiple injectors.

Extruding, forming and shaping

After preconditioning, the mix continuously enters the twin screw extruder.

Twin screw extru-sion cooks the ingredi-ents with a combination of heat, mechanical shear and added moisture. Water is added to reach the appropriate moisture content (15-25 percent depending of the product).

The extruder is equipped with a high shear screw configura-

not rely on internal friction between screws and barrel to convey the cooked mass towards the die. It can process a variety of particle sizes and variety of raw materials composition while maintaining uniform product shear and produc-tion flow.

In addition to the basic features the TSE can offer, some extruders manufacturers offer many advantages to enhance flexibility including

a modular barrel design, accurate temperature and shear devices, automatic barrel opening for easy access to screw profile, and computerized operation and control.

Direct expanded and indirect expanded

The products obtained with twin-screw extrusion technology can be divided into two sections: the direct expanded products and the indirect expanded products.

The direct expanded product is an extrud-ed product that expands just after the die, due to moisture flash-off. Configuration of direct expanded implies high shear level screw profile, high temperature and low water content. The indirect expanded product does not expand after the die, but later during post operations. Extruder configuration for indirect expanded product usually includes a first step that allows a perfect cooking of the starch components and

then a cooling section that controls the expansion at the end of the machine.

The change of configura-tion between direct expand-ed and indirect expanded

products can be done very easily and quickly while performing other maintenance require-ments on the line.

It usually requires a slight change to the screw profile, the die and a few ancillary devices. These set-up changes are made very easily thanks to equipment features such as the

Grain&feed millinG technoloGy26 | July - august 2011

FEATURE

Due to the versatility of the twin-screw extruders, flaked or expanded products made from various grains are easily produced in the same plant, using the same complete line from the mixer of raw materials to the packaging units.

Simple clip-on modules can be easily and quickly added to this unique line, allowing the launch of innovative products such as multi-colors, multiple shapes, co-extruded, or multigrain products.

Complete lines using twin-screw extrusion bring reactivity and flexibility to cereal manufac-turers and thus open the door to short term and reactive innovation.

added simultaneously to sugar coating to fine-tune product characteristics.

Coating of product is usually associated with additional drying of the product (and particularly drying of the thin surface of sugar-syrup applied on the product). This drying can take place at the same time of the coating (Coater-Dryer) or continuously in an additional

belt dryer, just after the coating.

PackagingBefore entering the

packing units, the prod-ucts are cooled down

to approximately room temperature by convective heat transfer. They are then packed in the desire pack-age, thanks to multi-weighing units linked to ver-tical baggers and vertical cartooning machines.

ConclusionAccording to

a breakfast cereal manufacturer using twin-screw extrud-ers, “We wanted a very flexible and multi purpose line as we provide high quality direct expanded cereals, flakes cereals and co-extruded prod-ucts.”

moisture to three percent and to give toasted taste to specific products.

With toasted products an air-impingement dryer is usually chosen, which aims to fluid-izing the product, and optimises the contact between hot air and cereal.

Temperature and residence time can be adjusted for each product. During this heat

treatment, the flakes develop their specific blistering, crisp texture, flavor and color. Direct expanded products are traditionally dried in belt drying technology, but can be also dried with an air-impingement drying technique.

For some specific products, such as the crisp rice, this technique allows a quick Maillard reaction to develop and give the right quality profile to the final product in terms of texture, taste and surface color.

Then, dried cereals are conveyed to a sifter to eliminate fines, and to the coating/cooling unit when necessary.

CoatingThe dried or tasted products may be fed to

a coating unit where a sugar-syrup preparation is applied in the required proportion (10-35 percent of final product, depending on the country and type of products).

The spraying system of the coating equip-ment is designed to ensure optimum con-tact between product and sugar syrup. The temperature and Brix degree are adjusted to control the surface aspect of the coating (from shiny to glassy surface appearance, according to market requirements). Dry additives can be

“We wanted a very flexible and multi purpose line as we provide high quality direct expanded cereals, flakes cereals and co-extruded products”

Grain&feed millinG technoloGy July - august 2011 | 27

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www.clextral.com (+33) 04 77 40 31 31Twin Screw Extrusion and Drying Technologies

FEATURE

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In this issue:

• Cultura Technologies’ MillMaster

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August 2011

• Packaging for the future

• Taking the first step: How to help yourself and others develop the love and understanding of flour milling

• Cooking cerealswith extrusion

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