Influence of extrusion on material properties

LIKE other heat treatments of food, extrusion processes can alter the characteristics of raw materials favourably or unfavourably, and may have both beneficial and undesirable effects on nutritional value. On the one hand, extrusion cooking can destroy anti-nutritional factors and gelatinise starch. On the other hand, Maillard reactions between protein and sugars reduce the nutritional value of the protein and heat-labile vitamins may be lost to varying extents.

Here, we choose to focus on possible positive effects that the combination of heat and mechanical pressure can have on three types of ingredients: peanuts, lentil flour and oat beta-glucan.

Reduction of peanut aflatoxin

Extrusion cooking may lower aflatoxins in peanuts, according to a study1 to be published in the July 2011 of LWT - Food Science and Technology.

Researchers, led by Firibu Saalia from the University of Ghana, found that when contaminated peanut meal was extruded in the presence of calcium chloride with lysine and methylamine, levels of aflatoxins may be reduced by up to 84 per cent.

Aflatoxins are naturally occurring toxins that are known to contaminate corn, peanuts and other food crops including nuts. High-level aflatoxin exposure produces an acute hepatic necrosis and can lead to liver cancer.

Lysine was as effective as methylamine in mediating aflatoxin reduction during extrusion cooking, they noted. "It is hypothesised that extrusion cooking of contaminated peanut meal in the presence of lysine might have the potential to reduce aflatoxin levels, similar to that reported for methylamine and ammonium hydroxide," the authors said.

They also noted that the efficacies of such reagents in reducing aflatoxins depend on treatment variables. Feed moisture, pH, and temperature of the extruder barrel were the most influential variables in reducing aflatoxins, the authors reported.

They found that extrusion of moistened peanut meal at pH 9.5, in addition to nucleophiles (lysine, methylamine) improved reduction in aflatoxins. Under these conditions, extrusion cooking reduced aflatoxins in the peanut meal by 84 per cent from an initial 417.72 micrograms per kg to 66.87 micrograms per kg.

The authors added that the binding of aflatoxins to the molecules in the peanut meal, and their subsequent protection from destruction during extrusion cooking may be reduced via the addition of calcium chloride.

Improving sensory profiles of lentil-based snacks

Manipulation of the process conditions during extrusion of corn-and-lentil based snacks can lead to products with desired sensory attributes, a study2, published in Journal of Sensory Studies reports. Researchers investigated the changes in appearance, flavour, texture and overall acceptance of corn and corn-lentil extrudates as affected by differing extrusion conditions using a twin screw extruder.

Dr. A. Lazou from the National Technical University of Athens and colleagues investigated the sensory characteristics of corn and corn-lentil extrudates as affected by extrusion conditions and legume-to-corn ratio. Extrusion cooking technology has been used to develop various types, shapes and textures of snack foods from corn meal, rice, wheat flour or potato flour, the authors said, but the application of extrusion to legume flours is a relatively new area. "Sensory evaluation is important in product development, product improvement and optimization. Extrusion conditions, as well as raw material properties affect the sensory characteristics of products," said the researchers.

The authors further noted that legumes are an important source of food proteins and dietary fibres that could be incorporated in extruded corn snacks givAdidas