Description of rolling as unit operation during the couscous process

Couscous agglomerates are made from durum wheat semolina by the succession of 4 unitoperations: agglomeration, rolling, cooking and drying. Rolling stage is conducted to give aspherical shape to the agglomerates and to separate them according to size criteria.Continuous rolling drum is used by industry, and for some of them it is composed of aninclined perforated cylinder rotating about its central axis. The rolling stage is one criticaloperation during the couscous grain processing, as rolling mainly determines the process yield,by generating significant mass flows of too small and too large agglomerates that could reachmore than 60% of the total mass flow. The objective of the present work is to investigate therolling stage as a unit operation, to determine the rolling process efficiency and itscontribution on the structuring mechanisms of the couscous agglomerates.We used specific rolling equipment at pilot scale. Two metal grids allow separatingthe agglomerates in three fractions: fine, medium, and large agglomerates. Durum wheatsemolina was used as raw materials to generate the wet agglomerates by a standard procedure.We investigated (i) the screening efficiency by assessing the three fractions yield, (ii) therolling efficiency by measuring the circularity of agglomerates after rolling; (iii) thesecondary agglomeration mechanisms by comparing the size distributions of the agglomeratesbefore and after rolling; and (iv) the impact of rolling conditions on the hydro-texturalcharacteristics of agglomerates.Results demonstrated that the rolling operation induces slight changes in the structureof the agglomerates and generates secondary agglomeration mechanisms. The change insetting conditions of the rolling operation has no significant impact on the screeningefficiency and on the hydro-textural characteristics of agglomerates (diameter, compactness,and water content). A physical approach was conducted to determine both the sieving andpassing speeds of agglomerates and showed that classification function of the rotating drumcan be greatly improved.