Extrusion is the process of forcing a material through a specifically designed opening. Material is continuously metered into an inlet hopper and then transported forward by the rotating screw. as the material approaches the die, there usually is an increase in pressure and temperature.

Mechanical disruption of the cell walls and starch of plant products occurs during extrusion cooking, facilitating digestion, and absorption. This mechanical breakdown of starches reduces the viscosity of gruels made from extruded cereals to enhance their caloric and nutrient density. The high-temperature heat treatment effectively pasteurizes the product. A good packaging is required to provide resistance to moisture and insects.

Extrusion cooking is the recommended method for manufacturing Fortified Blended Foods.

Other processing technique which can be applied for the thermal treatment of the raw materials is:

• Drum-drying (for SC+)

Cereal paste drying is commonly done on the surface of hot drums or by spraying the paste into heated air. The resulting dry, precooked products form smooth gruels when they are reconstituted with water. Supplementation with non-fat dried milk or soy protein and vitamin-mineral mixtures improves the nutritional quality. Proper packaging is necessary to protect the product from insects and moisture, but it is costly because of low product density.

In this operation, food slurry is contacted with a hot, revolving drum to form a thin layer on the surface. After sufficient residence time to allow the evaporation of water the product is removed from the drum by a scraper device (called a doctor knife) located usually 1/2 to 3/4 of a revolution from the point of application. Typically wheat can be made at 40% whereas rice at 30% solids. The temperature of the heat exchanger surface is 100°C, ensuring that the starch component of the cereal is fully gelatinised and reducing the microbiological load in the finished product.

Factors affecting the rate of drying and final moisture content are:

• Residence time on the drum
• Surface temperature
• Film thickness

The method of dehydration can only be applied to food slurries or liquid food systems and the product must be able to withstand high temperature-short time exposures without undergoing severe quality changes. It has been most successfully applied to drying of milk, soup mixes, and baby foods. This technique is the most expensive but also the most sophisticated to produce Fortified Blended Foods of high quality.

The validation of drum drying  is necessary to the extent of ensuring that WFP requirements for product stability, product self-life, energy density and acceptability are met.