The drying behavior of yeast pellets containing a carrier was investigated. Yeast paste was mixed with different amounts of wheat flour, a solid carrier, to obtain spherical pellets with 69, 55, and 40 wt% of water. During the drying, the pellets moisture content, surface temperature, and dimensions were measured, using a balance, an infrared camera, and a stereomicroscope, respectively. A drying kinetics model was developed and successfully challenged to the experimental results. The experimental and modeling results give insights into the phenomena taking place during the drying of yeast pellets including a carrier. These results highlight that the addition of a carrier reduces the shrinkage of the pellets and increases the drying rate. Moreover, the results seem to indicate that an osmosis process may take place inside the pellets following the addition of the carrier.

Predicting and controlling the liquid dynamics in a porous medium is of large importance in numerous technological and industrial situations. We derive here a general analytical solution for the dynamics of a flat liquid front in a porous medium, considering the combined effects of capillary imbibition, gravity and evaporation. We highlight that the dynamics of the liquid front in the porous medium is controlled by two dimensionless numbers: a gravity-capillary number G and an evaporation-capillary number E. We analyze comprehensively the dynamics of the liquid front as functions of G and E, and show that the liquid front can exhibit seven kinds of dynamics classified in three types of behaviors. For each limiting case, a simplified expression of the general solution is also derived. Finally, estimations of G and E are computed to evidence the most common regimes and corresponding liquid front dynamics encountered in usual applied conditions. This is realized by investigating the influence of the liquid and porous medium properties, as well as of the atmospheric conditions, on the values of the dimensionless numbers.

The addition of a solid carrier before drying seems a promising way to improve the processability and the drying kinetics of yeast pellets. However, its influence on the quality of the yeasts during and at the end of the drying is not yet well understood and is investigated in this communication. A yeast paste was mixed with different amounts of wheat flour, a solid carrier, to obtain pellets with 69, 55, and 40 wt% moisture content. The fermentation activity of the yeasts was measured in samples taken throughout the drying process of these pellets. The experimental results highlight that the addition of a carrier does not significantly alter the quality of the yeasts before drying but has a negative impact on their final quality at the end of the drying process. Moreover, the addition of a solid carrier leads to a continuous decrease in the fermentation activity of the yeasts from the beginning of drying, in contrast to the activity of the yeasts in the absence of a carrier, which remains high during drying, as long as the moisture content of the pellets is larger than about 0.5 (d.b.).