(105d) Gentle Drying of Sensitive Food Products

Natural extracts are of special interest for the food industry. They are often gained by extraction using conventional solvents followed by solvent separation and powderization, because solvent free and powderous products are desired. The most important techniques therefore are spray and freeze drying. These techniques offer the possibility for high quality powders production, but also are characterised by high energy consumption and the risk of oxidation.

In this investigation a product-gentle drying process for natural food products (called PGSS-Drying) will be presented. The process uses the solvent power of supercritical gases for the solvent removal in a continuous process step. No long extraction times like in batch extraction processes are needed. The solution or dispersion to be dried is continuously mixed (often in a static mixer) under pressure with a supercritical gas (often used is carbon dioxide) and depressurized over a nozzle into a spray tower. Fine atomization is achieved. No indirect heat transfer is necessary. The drying is achieved through combined extraction and CO₂-driven flash evaporation. The condensation of solvent drops in the spray tower has to be avoided by choosing the right spray tower conditions (e.g. temperature and pressure). By changing the amount of used gas and spray tower conditions the created powder particles properties can be changed (e.g. in shape, size, porosity and residual moisture). By using a multi-component solution encapsulation of liquids are possible. Typically low to moderate temperatures can be used in the spray tower (30-60°C), so that temperature sensible substances can be dried in a product-gentle way.

The versatility of the described process has been demonstrated in the last years with many different systems as e.g green tea extract. Dry and free flowing powders were achieved. The presented investigation is focused on process design, possibilities and costs. Particle with different morphologies, generated via PGSS-Drying are shown in the following figure 1.

Figure 1: Particles dried using the PGSS-Drying process