X-ray computed (micro) tomography for detecting internal defects in fruit

Proceedings of the International Conference “Environmentally friendly and safe technologies for quality of fruit and vegetables”, held in Universidade do Algarve, Faro, Portugal, on January 14-16, 2009. This Conference was a join activity with COST Action 924.Internal disorders in horticultural products are not revealed by external visual symptoms. Non-destructive and non-invasive monitoring techniques are required to detect the occurrence and investigate development of internal disorders. X-ray CT allows visualization and analysis of plant materials with a resolution down to a few micrometers, and without sample preparation or chemical fixation. X-ray tomography is based on X-ray radiography: an X-ray beam is sent on a sample and the transmitted beam is recorded on a detector. The level of transmission of these rays depends mainly on the mass density and mass absorption coefficient of the material. The resulting image is superimposed information (a projection) of a volume in a 2D plane. The classical way to retrieve 3D information is to perform a large number of radiographs while rotating the sample between 0° and 180°. The filtered back-projection algorithm can then be used to reconstruct the volume of the sample from these radiographs. Compared to two-dimensional radiography used in medicine and linescan radiography applied on grading machines, X-ray computer tomography (CT) is the most powerful technique from the horticultural research point of view, since two and three dimensional images can be reconstructed from the accumulated data to study internal physical and physiological processes. Most internal disorders like woolliness in nectarines, hollow heart in potato, core breakdown in pears, watercore in apples and spongy tissue in mango affect the density and water content of the internal tissue and, hence, are detectable by means of X-ray measurements. For in vivo observations, high resolution submicron tomography has up to recently only been achieved on relatively dry -or hard- biological samples, such as plant seeds. We have shown, however, that by means of synchrotron radiation X-ray sources, it has now become feasible to image plant tissues at a resolution of less than one micrometer