Pre-cooked flours are nowadays very popular among consumers compared to native flours, because of their convenience of use, their improved nutritional and organoleptic qualities and their extended shelf life. Extrusion cooking stands out from other processes by different competitive advantages: versatility; ease of integration into complete production lines; wide adjustment and automation possibilities; high energy efficiency; high production capacity, and specific nutritional and physicochemical characteristics conferred on the flours thus treated.
Extruded flours are mainly used as complementary food, in infant nutrition or in humanitarian intervention programs. In these two cases, they are obtained by formulation of mixtures of flours and by systematic mineral-vitamin fortification. The objective is to balance the qualitative and quantitative contributions in macro- and micro-constituents. In order to get as close as possible to nutritional recommendations regarding:
For infant flours, the WHO recommends that they be used only after 6 months. At this age, infant flours can then be introduced in addition and then gradually replacing breastfeeding, after rehydration with water or milk. The rehydration rate depends on the age of the child in order to adapt to his physiological needs: slight thickening of the infant milk supplied to the bottle (2nd age) then boiled more and more consistent, administered with the spoon (children over 12 months). At this level, the extrusion cooking is of specific interest because of the highly fluidifying behavior that the dextrinization of the starch confers on the reconstituted slurries. This allows, for the same viscosity threshold, to very substantially increase the rate of incorporation of dry materials into the reconstituted slurries and therefore increase their nutritional and energy density.
Another factor to take into account is the more or less cooked and instant character of these flours. These flours are classified into three groups:
It should be noted that the digestibility of the starch is correlated with its rate of gelatinization, a process known to occur by cooking in excess of water. Regarding the extrusion cooking, this explains the advantage of favoring a treatment wet rather than dry. Moreover, it makes it possible to pre-cook the starchy base first and then, after treatment, to add the heat-sensitive vitamins as well as any protein enrichment (milk proteins) and sugar. Such a protocol prevents the development of Maillard reactions that cause, under the effect of cooking, the immobilization of lysine by reaction with sugar. Instant flours also limit energy costs and preparation time; it therefore improves practicality while optimizing the nutritional and hygienic quality (sterilizing cooking), as well as the conservation of flours.
In humanitarian contexts, pre-cooked meals can be incorporated into basic food rations distributed to the intended beneficiaries. For groups at higher risk (young children, pregnant or lactating women, patients with various pathologies, including AIDS), they can be added as additional rations to strengthen nutritional intake, prevention of malnutrition or treatment of moderate malnutrition. Between 200,000 and 300,000 tons of such flours are distributed annually by the World Food Program, many of which are produced locally and almost exclusively by extrusion cooking. As this process makes it possible to incorporate a wide range of raw materials, each country can adapt its recipes to locally available resources.
The manufacture of extruded flours goes through the steps described in the following diagram:
With regard to equipment and their behavior, the following points should be particularly monitored:
Crédit© Dominique Bounie