Transfers to solid foods — Simplified calculation tools

The aim of this article is to use the concepts presented in the "Physical and mathematical models" section to develop simple calculation methods that can be used to roughly predict the progress of industrial solid food treatment processes. In line with the general topic of transfers in solid foods, only processes whose progress over time is controlled by internal heat and matter transfers (at least for certain operating conditions and during one of the process phases) are concerned. These include traditional surface treatment technologies based on contact, convection and infrared radiation (for opaque products):

• purely "thermal" food preservation (sterilization, freezing) and cooking processes;

• mixed processes in which, in addition to heat treatment, a difference in pressure (freeze-drying) or in the concentration of a given species (air-drying) is applied between the product and its environment;

• purely "mass" processes for preservation (brining, salting) and separation (solvent extraction).

Process engineering literature often uses the term kinetics to describe the time evolution of a variable characterizing the state of the product (e.g. temperature or average water content), and the term velocity to describe the flow (of material or heat) leaving or entering the product. In this article, only the expressions kinetics and velocity (from which we can deduce the duration of an operation) will be developed, as they are the key elements in the implementation (design, dimensioning) of a process.

The article is not organized according to the various unit processes, as is customary in process engineering literature, but according to the various internal transfer models (transfer equations, boundary and initial conditions) that correspond to them, thus highlighting the similarities in the physical approach and mathematical modeling between the various processes. If we analyze all the above-mentioned processes from this point of view, simplifying the physical model and its mathematical representation for each one, we can group them into two categories.

The first category covers processes whose degree of advancement is controlled by the unsteady diffusion of heat or a material species within the product, while the second covers processes controlled by the progression of a water state change front (evaporation, solidification, sublimation) within the product. Whether or not a given process falls into one of these categories depends on both the nature of the food and the operating conditions. A separate part of the article is devoted to each of these two categories, in which the expressions kinetics, speed and process duration...