ABSTRACT
Quite recently, an interesting concept was proposed and positively verifi ed by researchers from the University of La Rochelle in France for drying heat-sensitive materials such as yeast, biotech products, and enzymes (Rakotozafy et al., 2000; Maache-Rezzoug et al., 2002). This novel technology, called dehydration by successive decompression and abbreviated as DDS (Déshydration par Détentes Successives), is based on a sequence of compression and decompression cycles during which the material is subjected to pressures of 200-1000 kPa for a certain duration followed by evacuation to low pressures of the order of several kilopascals (Figure 34.1). The product is maintained at the low-pressure level for a certain time interval. This pressure-vacuum cycling may continue over a period of time that is generally much shorter than that needed for freezedrying (F-D), vacuum drying, and convective drying of the same material at comparable temperatures (Figure 34.2). The studies focused on the effects of the various relevant parameters including the pressure levels, cycle duration, vacuum level, etc. Examination of the quality of baker’s yeast dried using this method compared with that of freeze-dried showed a better performance of the pressure cycling process (Rakotozafy et al., 2000). Clearly, F-D is a much more expensive and slower process. It is worth noting that the quality of the product (measured in terms of the viability of the yeast) is reported to be better in this process. This is likely due to the shortened duration of the total dehydration process. The cell survival rate for this process was found to be 10 times higher than that in F-D. However, this number is a function of the operating parameters tested and hence should not be generalized. Similarly, the shorter drying time and quality advantages were found for collagen gel dried by DDS over hot-air drying and vacuum drying. For example, the drying time necessary to obtain a 0.13 mm collagen fi lm at a moisture content of 0.8 kg/kg d.b. from an initial 6 mm layer using DDS was 270 min with the collagen temperature below 22°C. Regarding air drying at 27°C, a collagen fi lm of the same fi nal moisture content was obtained after 770 min. Further optimization of the process is needed to enhance its performance even better, for example, by the use of variable pressure ranges and variable cycle frequencies, which
is easy to achieve in a fully automated system. Because of the low material temperature, this process seems to have potential for applications involving highly heat-sensitive materials. The constraint could be the relatively high power needed to create a vacuum and pressurized air; hence, this new technology would better suited to high-value products.
