Adapted thermal imaging for the development of postharvest precision steam-disinfection technology for carrots

Postharvest carrots are brushed, hydro-cooled to 4-6 degrees C and treated with chemical fungicide before storage to prevent soft rot caused by Sclerotinia sclerotiorum (Lib.) de Bary during storage. Replacement of the fungicide with high-temperature surface heat treatment poses a dilemma: excessive heating will cause damage whereas insufficient heating will result in incomplete disinfection. This problem is further compounded by the difficulty in providing uniform surface heating. To alleviate this problem, a system for the uniform application of high-temperature short-duration steam disinfection was developed using accurate real-time temperature monitoring of individual produce segments by thermal imaging. Uniform short-duration high-temperature heat was delivered from above by steam jets combined with electric steam-drying elements and reflectors. Produce was subjected to rotational and linear motions to expose each surface segment to the same amount of heat. The novel use of thermal imaging to monitor surface temperature in steam systems was essential for determining transferred heat and heating uniformity in a treated object. The resultant, uniform application of short-duration high-temperature steam provided surface heat-disinfection with minimal internal heating and damage. Application of the steam treatment immediately after carrot hydro-cooling reduced post-storage phytotoxic color change by 60-80% and resulted in significantly reduced sensitivity to post-storage soft rots caused by S. sclerotiorum. Carrot sprouting was not increased by the steam treatment, suggesting retention of the hydro-cooling's physiological effect. These results suggest that precise heat treatment can be optimally applied after hydro-cooling to improve postharvest quality of carrots in a procedure that is harmless to man and the environment. (C) 2010 Elsevier B.V. All rights reserved.