Effects of Far-infrared Irradiation and Lyophilization on the Contents of Carotenoids, Tissue Structure and Bacterial Counts of Carrot

The importance and usefulness of dried food in cases of natural disaster or famine have been widely recognized. Several food drying procedures, such as lyophilization, drying in the sun, sublimation under normal atmospheric pressure, and drying by hot air have been developed. However, little work has been done on biochemical and nutritional evaluation of dried foods produced by different drying procedures. In this report, we compared a newly developed drying procedure using far-infrared irradiation (infrared at 10μm) and lyophilization, which is currently used most frequently. Dehydration of fresh carrots by the two procedures gave essentially the same results. This indicates that the far-infrared irradiation drying is as effective as lyophilization. The amounts of carotenoid and carotene components did not change after lyophilization or far infrared drying at 20°C or 40°C. The amount of carotenoid after far infrared treatment at 60°C decreased to 60% of that in the fresh sample. Microscopic observation of the tissues dried by the two procedures revealed that lyophilzation caused cracks in the tissue. Tissues dried by far-infrared showed a similar structure to the fresh sample and rehydrated tissue retained a natural texture. Additional treatment at 60°C for 1h effectively reduced bacterial numbers in the tissue without loss of carotenoid contents. These results indicate that the far-infrared drying procedure should be useful for the preparation of dried foods.