27 Food Extrusion

Dr. Aparna Kuna

The material of which an extrusion die is made can affect the final product. Compared to stainless steel dies, a pasta machine with bronze dies produces a rougher surface. This is considered to give an improved taste, as it better retains pasta sauces. “Bronze die” pasta is labelled as such on retail packages, to indicate a premium product.

The effects of “extrusion cooking on nutritional quality are ambiguous”, as extrusion may change carbohydrates, dietary fibre, the protein and amino acid profile, vitamins, and mineral content of the extrudate in a manner that is beneficial or harmful. High-temperature extrusion for a short duration “minimizes losses in vitamins and amino acids”. Extrusion enables mass production of some food, and will “denature antinutritional factors”, such as destroying toxins or killing microorganisms. It may also improve “protein quality and digestibility”, and affects the product’s shape, texture, colour, and flavour. It may also cause the fragmentation of proteins, starches, and non-starch polysaccharides to create “reactive molecules that may form new linkages not found in nature”. This includes Maillard reactions which reduce the nutritional value of the proteins. Vitamins with heat lability may be destroyed. Nutritional quality has been found to improve with moderate conditions (short duration, high moisture, low temperature), whereas a negative effect on nutritional quality of the extrudate occurs with a high temperature (at least 200°C), low moisture (less than 15%), or improper components in the mix.

Recent research publications indicate that use of non-traditional cereal flours, such as amaranth, buckwheat or millet, may be used to reduce the glycemic index of breakfast cereals produced by extrusion. The extrudate using these cereal flours exhibits a higher bulk and product density, had a similar expansion ratio, and had “a significant reduction in readily digestible carbohydrates and slowly digestible carbohydrates”. Another research work states that replacing 5% to 15% of the wheat flour and white flour with dietary fibre in the extrudate breakfast cereal mix significantly reduces “the rate and extent of carbohydrate hydrolysis of the extruded products”, which increased the level of slowly digested carbohydrates and reduced the level of quickly digested carbohydrates.