Experimental characterization and modeling of Baker's yeast pellet drying

- A phenomenological model is proposed for the drying of Baker's yeast.
- There are three types of water in Baker's yeast: “free”, “bond” and residual.
- There is an isotropic linear shrinkage of the Baker's yeast pellets during drying.
- Pellet temperature falls between the dry and wet bulb temperatures of the gas.
- Baker's yeast activity decreases only below a critical moisture content.

A contribution to a better understanding and modeling of the drying of a Baker's yeast rod-shaped pellet is proposed. An original experimental setup is developed allowing the characterization of the drying of a Baker's yeast pellet by using a precision balance, a microscope and an infrared camera. Using this experimental setup, the time evolution of the pellets moisture content, dimensions and surface temperature can be obtained in a large set of operating conditions (air temperature, air velocity and initial pellet diameter). The evolution of the fermentative activity of the Baker's yeast during drying as a function of its moisture content can also be investigated through the measurement of its gassing power. In this study, a set of drying experiments of Baker's yeast pellets in this original setup and with a data treatment procedure is presented.A new phenomenological model of the drying of a cylindrical Baker's yeast pellet is built on the basis of heat and mass transfer laws. It includes a description of the shrinkage of the material and it provides a link between the initial drying rate and the Baker's yeast activity at the end of the drying. The unknown parameters are identified by a comparison with experimental results. The combined use of the model and the experimental results leads to four main conclusions: three types of water exists in Baker's yeast pellets (a residual water, a water whose extraction is limited by mass transport phenomena in gas phase inside or outside the pellet, and a third type of water whose extraction is significantly slower); there is an isotropic linear shrinkage during drying which has no significant influence on the drying rate but which appears to be drying rate dependent as, at a same moisture content, a larger drying rate leads to a smaller shrinkage; the time evolution of the pellets surface temperature presents a change of regime at a moisture content of about 0.6 (d.b.); the Baker's yeast activity remains high when its moisture content is larger than 0.75 (d.b.) and the activity at the end of the drying is directly related to the initial Baker's yeast drying rate.