Comparative Study of Various Pre-Treatments Coupled to Vacuum Drying in Terms of Structural, Functional and Physical Properties of Carrot Daucus Carota

Carrot is a seasonable vegetable, whose preservation has required various techniques conducted to preserve its availability. One of these most known operations has been the drying. Drying processes are carried out on most of the vegetables to extend their shelf-life (Barat and Grau, 2016; Jangam et al., 2016; Moses et al., 2014). They include convection drying using hot air (Gamboa-Santos et al., 2013), freeze-drying (Garcia-Amezquita et al., 2016), vacuum drying (Richter Reis, 2014), micro-wave drying (Cui et al., 2004), infrared technology (Kocabiyik and Tezer, 2009), etc. Compared to the other techniques, vacuum drying has the advantage to use reduced pressures and low temperatures to perform water evaporation (Richter Reis, 2014). This should decrease the thermal degradation and favor the quality of the final dried vegetables. Several pretreatment procedures are proposed and used. They differ in terms of nature and effect on the drying kinetics and the nutritional and functional quality of the final dried product. Since the natural structure of plant is inadequate for internal diffusion of water mainly because of the presence of cells whose walls act as great barrier, several processes are usually proposed as pre-treatments of drying(Arévalo-Pinedo and XidiehMurr, 2007; Niamnuy et al., 2014) They mainly aim at reducing drying time, preserving the final product quality. They include blanching (Negi and Kumar Roy, 2001), thermomechanical treatments, freezing/thawing (Kidmose and Martens, 1999). Blanching and thermal treatment are well-known as leading to the destruction and/or reduction of surface microorganisms, enzyme activity, and removal of air from surface and intercellular spaces which helps in prevention against oxidation (Rahman &Perera, 1999). Freezing/thawing is another pretreatment procedure used to accelerate drying and enhance the quality of the dried product. By using freezing/thawing as a pretreatment applied before drying, it was possible to imply some rupture of cell walls and higher stabilization of carotenoids and lipid content of the dried carrot (Albertos et al., 2015). Other industrial drying technologies have been defined through introducing a direct thermal and mechanical effect on microstructure of plant organs. This allows reducing cost, increasing both process performance and final product quality. At this respect, the very specific process of swell-drying combining conventional drying and a step of texturing by Instant Controlled Pressure Drop (DIC) technology was defined and has been developed (Mounir et al., 2014). DIC is an innovative process that is based on the thermo-mechanical effects issued from an abrupt dropping of pressure towards a vacuum(Allaf and Allaf, 2014). Thus, DIC is a high-temperature short time (HTST) treatment followed by an abrupt drop of pressure towards a vacuum of about 5000 Pa. This causes an autovaporization, implies a possible texturing of the material, and allows higher drying kinetics. The advantage of DIC treatment is that low pressure induces an instant cooling of treated products towards a temperature level of about 33 ºC, thus preventing their thermal degradation.