Salting and drying are two of the earliest methods of treating foods to help preserve freshness and improve flavour. Over the years, improved techniques for processing foods have resulted in the expansion of our food supply by prolonging keeping times, preventing spoilage and increasing the variety of food products available. This is the first in a series of articles in which Food Today will look at various technologies used today and the benefits they offer in improving the food supply.

Extrusion - new shapes and textures

Snack foods, breakfast cereals, confectionery and even some pet foods have been produced from a method of food processing known as extrusion. It basically involves compressing food into a semi-solid mass, and then forcing it through a small aperture to increase the variety of texture, shape, and colour obtainable from a basic food ingredient. The technique has given rise to products with hitherto unknown shapes and textures. Extrusion can form and sometimes even cook raw ingredients into finished products.

A typical extruder consists of a power supply to operate the main screw, a feeder to meter in the raw ingredients, and a barrel, which surrounds the screw. The screw conveys the raw material through towards a shaped hole, the die, that shapes the product. Extrusion can take place under high temperatures and pressures or can be simply a non-cooking, forming process.

One of the potential benefits of using extrusion in food production is to help preserve food products. Extrusion can be used to control the water activity of ingredients, which in turn determines microbial growth and hence, spoilage. It is therefore useful in producing, shelf-stable foods and increasingly important in producing a variety of things like some snackfoods, certain breakfast cereals and types of confectionery.

New and creative products

Snacks are one of the fastest-growing segments of the food industry, and extrusion is already established as a means of producing new and creative products. Most cereals can be extruded, and cereal-based products, such as breads, breakfast cereals and cakes can be processed in this way. Extrusion can also be used to make pet foods.

A particularly promising application of extrusion is in the processing of Textured Vegetable Protein (TVP). This is basically soya flour that has been processed and dried to give a substance with a sponge-like texture that may be flavoured to resemble meat. Soya beans are dehulled and their oil extracted before being ground into flour. This flour is then mixed with water to remove soluble carbohydrate and the residue is textured by extrusion. This involves passing heated soya residue from a high-pressure area to a reduced pressure area through the die, resulting in expansion of the soya protein. It is then dehydrated and may either be cut into small chunks or ground into granules. With extrusion techniques good quality meat analogues can be produced from TVP and mycoprotein (protein obtained from fungi). TVP is also being used in the development of some functional foods where the potential health benefits of soy protein are sought.

Extrusion processing has been used in the preparation of camping and military field rations, foods for special dietary needs and for feeding during disaster and famine recovery. It has even been proposed in the design of a food processing system to be stationed on Mars! The use of extrusion to produce new and innovative food products holds much promise for future food production.

FOOD TODAY 02/2002