Packaging

Process with vast potential for the manufacture of new products with functional characteristics

Friday 06. March 2009 - Trend report on extrusion technology at Anuga FoodTec 2009

Extrusion is one of the key technologies in today’s food processing industry. It is used in a wide range of areas, including the manufacture of snack products, breakfast cereals, health bars, crispbreads, ice cream, confectionery and animal foods. There are extrudates in a whole variety of shapes, colours and tastes on today’s supermarket shelves. No other process has quite the same potential to recast the product matrix and develop innovatively textured foods. The crucial parameters in this process – which should be operated continuously in a closed system – are pressure, temperature and shear.

Molecular food design
Extrusion is one of the processes used in the “molecular” design of food. Taking into account the physical and technical properties of the raw materials and additives used, extrusion can be used to create new products with defined functional characteristics. In recent times, for example, projects have examined the use of wet extrusion to produce meat texturates and frozen extrusion to produce ice cream.

An extruder is basically just a continuous high-viscosity mixer. Different types of extruder are used for different processes in food production. The single-screw extruder, for example, is very robust and simple in construction. A twin-screw extruder, however, offers particular advantages, which frequently make it the favoured system. These include a narrower residence time distribution compared to single-screw extruders and more flexible processing by means of variations in loading ratio, throughput, machine speed and direction of rotation. With products such as cold-extruded chewing gum and liquorice, for example, the degree of expansion and the shape of the extrudate are largely determined by the design of the extrusion die. As the example of snack items shows, product design plays – alongside the factors of texture and sensory experience – a crucial role in market success. Such products are processed in a relatively dry state, with moisture contents of around 30 per cent.

A flexible and continuous process
At the same time, there are also many other examples of the use of extrusion in the manufacture of products with specific structural properties. Most recently, for example, a continuous extrusion process that is both flexible and time-saving has been developed for the production of breadcrumbs containing functional ingredients. After the ingredients have been mixed, they are forced into the extruder in just a few seconds by two parallel screws and mixed to a dough in the process. The input of mechanical and thermal energy, and the temperature and pressure inside the machine, lead to a modification of the starch particles in the dough. The product expands at the die as a result of the differences in temperature and pressure. In contrast to the traditional manufacturing method, extrusion therefore combines kneading and baking in one single process. The fermentation or “proofing” phase is completely omitted. The extrudate is cut into pellets at the die and can be dried to the requisite moisture content, as in conventional breadcrumb production. The pellets are then ground to the required degree of granulation, sifted and packed.

When set up in the appropriate way, this process can be used to produce more than just breadcrumbs in all sorts of varieties. Other products that can be manufactured with the same piece of equipment include fried batter pearls, croutons and bread chips.

Breakfast cereal flakes made of wheat bran
Another important application of extrusion technology in the food industry is in the processing of flour and starch. The raw materials are cooked up in a process reactor, which gelatinizes the starch and alters its structural properties. The resulting extrudate can be used as a natural binding agent in a host of foodstuffs. Even the byproducts of flour milling, which traditionally are only fit for animal fodder, can be upgraded by means of extrusion. For example, extrusion will transform wheat bran into breakfast cereal flakes.

One of the most modern multipurpose cereal plants in Europe, which can be used to expand products both directly and indirectly, belongs to a food manufacturer in Italy. The plant has a maximum output of 1,600 kg/h and a product spectrum that ranges from multigrain flakes and cornflakes to rice crispies, maize balls and even chocolate puffs. It is installed in a building 200 metres long, 75 metres wide and 17 metres high. The plant is unusual in that it can produce breakfast cereals both in a traditional batch cooker and extruder.

It is also possible – and, furthermore, economical – to continuously produce extrudates in a variety of colours from just one system. For this process, it is necessary to attach an auxiliary module consisting of colorant tanks, each of which is equipped with a pump and a connector system to the extruder. The process is then controlled in such a way as to avoid any blurring when changing between different colorants. The dosing pumps are regulated directly by the control system for the extruder.

Encapsulation of functional substances
In a further example of the diverse applications of extrusion, it can also be used to encapsulate functional substances. First of all, the base material in powdery form – usually a hydrocolloid or a suitable premix with film-forming properties – is added to the process chamber of the extruder. If required, water or a softener is added downstream and, later, the product for encapsulation. A fine emulsion of core and shell material is generated in the extruder and then forced under pressure through appropriate dies. The process of micro-encapsulation in a twin-screw extruder produces a combination of a thick, mechanically stable shell and a practically unlimited choice of recipes, which in turn enables a host of release mechanisms.

A highly interesting application in the field of creative product development is the process of coextrusion, which can be used to produce filled foods. This involves the simultaneous processing of several materials – as a rule, flour and protein for the casing and, for the filling, a soft mass that can be pumped. In this area, too, there is plenty of scope for experimentation.

More detailed information on the vast potential of extrusion for the purposes of product diversification and differentiation is available from a whole range of exhibitors at this year’s Anuga FoodTec, which is being staged by Koelnmesse GmbH in cooperation with the DLG (German Agricultural Society) from 10th to 13th March in Halls 4 to 10 of the Cologne Exhibition Centre.