Next figure summarizes the smart control concept of continuous real-time quality control applied to the freeze-drying process of LAB.
During the freeze-drying process, a raw material composed of concentrated bacteria obtained from a fermentation process and protective molecules undergoes the three steps of the freeze-drying process and a subsequent storage of several months. The freeze-drying process involves three successive steps: freezing of the aqueous solution, followed by primary drying to remove ice by sublimation and finally secondary drying to remove unfrozen or sorbed water by desorption. Only three process variables govern the whole process: the temperature of the cooling/heating source (the shelf temperature), the chamber pressure and the duration.
The final product should be characterized by a high biological activity recovery of the bacteria (just after the freeze-drying process and after several months of storage) and an elegant visual aspect of the freeze-dried cake (non collapsed and non sticky). Since the direct on-line measurement of the biological activity of the bacteria is impossible, critical process parameters (CPP) for quality need to be identified and quantitative relationships between these CPP and the viability or the acidification activity of bacteria need to be established. By integrating the model of the bacteria quality degradation in a model describing the drying kinetic, it becomes possible to develop an in-line control policy of the process maximising the productivity and the quality.
The quality of lactic acid starter is of great importance in the dairy industry for the manufacture of cheeses and fermented milk and is defined by a concept of “biological activity”. The biological activity includes bacterial viability and physiological state and corresponds to its ability to acidify a certain medium and to the enzymatic activities resulting in the production of aroma and thickening agent.
In the CAFÉ project, two indicators were selected for defining the biological activity of the bacteria all along the process and the storage:
- The bacteria viability (or survival) measured by the capacity of cells to form colonies on agar medium;
- The acidification activity. The Cinac system (Corrieu et al. 1988) allows the acidification activity of lactic acid bacteria to be characterized by measuring the time necessary to reach the maximum acidification rate in milk (tm or tmax in min). The higher the tmax, the longer the latency phase and, thus, the lower the acidification activity. This automatic real time pH measurement is more meaningful and easier to carry out and interpret than viability measurement. The dimensionless acidification activity recovery Ai was defined as:
With tgeneration, the time necessary to duplicate a bacterial population, and
dtm = Tmax after freeze-drying or storage – Tmax before freeze-drying.
Another important quality parameter is the visual/physical aspect of the freeze-dried cake. During the process, if the product temperature is higher than a critical value, the material will undergo viscous flow, resulting in loss of the pore structure obtained by freezing, which is defined as the collapse phenomenon. Collapsed dried products generally have a high residual water content and lengthy reconstitution times and may also present a loss of functional properties. Furthermore, primary drying is the most time-consuming stage of the process, and it is well known that raising the temperature significantly increases productivity. Since a small variation of product temperature can greatly modify the drying time as well as the dried product structure, an accurate determination of the collapse temperature is critical for the process design and optimization.
Since the direct on-line measurement of the biological activity of the bacteria is impossible, critical process parameters (CPP) for quality need to be identified and quantitative relationships between these CPP and the viability or the acidification activity of bacteria need to be established. The product temperature was identified as a critical process parameter for maintaining the physical structure of the dried cake during the process and the subsequent storage. Another important CPP for the product biological stability during the storage is the residual water content or water activity of the freeze-dried product. The effect of the water activity of the freeze-dried samples on the stability of lactic acid bacteria was investigated.