Relationship between food structure and drying processes

Drying has been used for thousands of years as a form of food preservation. Dried foods may give a more desirable product in terms of extended product life, easier handling and improved food processing. However, traditionally methods such as solar, spray, convective and drum drying have tended to decrease the quality of the dried product due to the high temperatures and long processing times required for drying. Recent developments in the drying of foodstuffs have introduced techniques that operate at comparatively low temperatures for short times. The drying techniques used in this project, osmotic dehydration and fluidised bed drying, are both examples of gentler techniques. Using Fick’s Second Law and characteristic curve modelling it was possible to predict accurately the moisture contents and, hence, drying curves for fruit and vegetables undergoing osmotic dehydration and fluidised bed drying. The predicted moisture contents allowed further analysis of the samples to be carried out to discover how a foodstuff physically dries below the surface; an area with little previous understanding. The structure of carrot and its effect on the drying process was investigated in two ways: (i) texture analysis and (ii) NMR imaging. NMR imaging was used in the novel application of obtaining in-depth pictures of what happens to structure during drying. These images indicated that the central structure of the carrot, the xylem and phloem, dried at a faster rate than the outer structure, the cortex. The results of the texture analysis supported the findings that different sections of carrot do not dry at the same rate. Hence drying is effected by food structure, in that different biological structures dry at different rates, and this knowledge needs to be taken into account in the future development of drying processes, ultimately improving the quality of dried foods.