Author:

Abstract:

In shelf life many physiological changes occur that are associated with normal ripening of fleshy fruits. In this work we wanted to explore if and how the porous tissue structure of apple (Malus × domestica Borkh.) changes during shelf life and could potentially contribute to changes of quality. The three-dimensional microstructure of different tissues (‘skin’ and ‘cortex’) of apples of two cultivars (‘Braeburn’ and ‘Kanzi’) was measured by means of X-ray micro-CT. Fruit was sourced at optimal picking date from orchards in Belgium and Italy and stored at optimal temperature conditions until the day of the experiments. Microstructure and firmness were measured after storage and after 1, 2 and 3 weeks of shelf life. At each time point, micro-CT imaging (Skyscan 1172, Bruker microCT, Kontich, Belgium) was applied to cylindrical tissue samples excised from 5 apples to obtain 3D reconstructed renderings of tissue volumes of 3 mm diameter at a pixel resolution of 5 µm. Image processing consisted of segmentation of pores and cells using a global greyscale threshold followed by calculation of structural parameters using softwares Avizo (FEI, Hillsboro, Oregon, USA) and CTAn (Bruker microCT, Kontich, Belgium). Porosity and size distributions of cells and pores were obtained and compared between fruit from different regions and at different weeks of shelf life. Over the 3 weeks of shelf life, average firmness decreased faster and to a lower level for Braeburn from Italy (from 80 to 55 N) than for Braeburn from Belgium (from 80 to 70 N). An increase in cortex porosity was observed in Italian Braeburn after 1 week (from 15 to 20 %), and after 2 weeks for the Belgian fruit (from 15 to 17 %). Shifts were also observed in the size distribution of pores towards larger diameter pores. Changes were smaller in skin tissue than in cortex tissue. Firmness and structure did not change significantly in Kanzi fruit from either location after 3 weeks. Overall, structural changes were small and hardly significant in both cultivars and between the growing regions. These results indicate that tissue structure changes may not be a major contributor to changes of apple firmness in shelf life.