Bruising Profile of Fresh Apples Associated with Tissue Type and Structure

Bruising of apples results in millions of dollars in loss annually. To reduce economic loss, a greater understanding of the mechanisms involved is necessary. Our objective was to visualize damage due to bruising, using different methods of microscopy, as an aid to substantiating which mechanisms were most viable. 'Golden Delicious' apples at different maturity levels were similarly bruised with an artificial silicon finger attached to an Instron machine. Bruising was induced on freshly harvested fruit and examined after 48 h at room temperature. We used fluorescence microscopy with Calcofluor (to identify cell walls) and CDFA (to identify intact cell membranes) in the bruised and discolored tissue. Together with scanning electron microscopy (SEM), different breakage mechanisms were observed in the bruised volume. These techniques revealed that bruised tissue was comprised of both live cells, and dead cells that appeared burst, crushed or without apparent damage. The greater the amount of intercellular space present in the tissue, the more tissue damage from bruising occurred. Because airspaces weakened the tissue, damage initiated close to these sites. As apples matured, there was an increase in damaged cells surrounding larger intercellular spaces.