Elisabeth Dumoulin

Helping to choose operating parameters for a coating fluid bed process

Abstract The feasibility of solid particles coating in a fluid bed with a Wurster tube is studied for several types of particles and aqueous coating solutions. The model products are wheat semolina, beads of glass, alumina, resin polystyrene, plastic PMMA, with a size range between 125 and 1250 μm and densities between 500 and 2500 kg m−3. The chosen coatings are representative of those used for the food products, such as maltodextrin, acacia gum, and sodium chloride in aqueous solution. The air flow rate suitable for a regular circulation of particles in the reactor is determined for each particle type. For each coating solution, the flow rate leading to agglomeration is considered as the maximal limit flow rate to use for coating. Then comparative coating experiments were realized. For a similar initial load of particles, the same mass of coating was atomized (13.5 g min−1) at 50 °C. The mass of coating deposit on particle surface is increased linearly during an atomization sequence lasting 33 min. For example, for every 100 g of alumina particles, the rates are 0.48, 0.51, and 0.53 g min−1 for sodium chloride, maltodextrin, and acacia gum, respectively. We then obtain a coating efficiency between 87% and 98%. In the specific case of sodium chloride on glass beads, the deposit of crystallized salt was linear during 10 min then stopped. Addition of acacia gum (50%) to the NaCl coating solution leads again to a linear deposit over 65 min.

Prediction of Powder Stickiness along Spray Drying Process in Relation to Agglomeration

The spray drying process consists of a fast convective drying of liquid droplets by hot air. Initially, the water activity (aw) of a drop is close to 1. During drying, the drop surface aw decreases while viscosity increases until reaching a sticky rubbery state before further drying. This can be observed for products such as carbohydrates, leading to particles sticking on walls (product losses) or to adhesion between particles leading to agglomeration. In this study, particle stickiness was investigated in a cocurrent pilot spray dryer by measuring drying air properties (temperature and relative humidity) at different positions. This allowed describing the evolution of temperature and mean water content of the drying drops. Two model products (maltodextrin DE12 and DE21) were spray dried varying process parameters liquid flow rate (1.8, 3.6, and 5.4 kg/h), air temperature (144°, 174°, and 200°C), airflow rate (80–110 kg/h), and rotary atomizer speed (22,500–30,000 rpm). The two products exhibit different drying behaviors in relation to their affinity towards water (sorption isotherms) and glass transition temperature evolution with aw (stickiness). Depending on drying conditions and product, the drop stickiness was observed very rapidly, close to the atomizer, or later, along the chamber. This approach can be used to identify conditions and positions corresponding to sticky particles.

French Bread Baking

Bread has been a basic foodstuff for thousands of years. This product requires energy, time, and a very precise dexterity. Only a few simple ingredients are used. The bread maker seems to be a kind of magician. In this chapter, we describe briefly its current state of the art and then present the materials used and the methods of measurement. We highlight the links between the kinetics of mass loss from a French loaf of bread, the variation in the relative humidity of the air as it comes out of the oven, and the thickness of the product, which is linked with the internal pressure. Stress is put on use of image analysis for quantifying properties such as the distribution of the bread 3D air cells or the impact of steam on the porosity and the microstructure (e.g., size of starch grains).