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For blueberries, the softening induced by ripening is concentrated up to fully ripe stage, as discussed in section 1.2.1. However, firmness in blueberries has also been associated with other fruit features such as moisture loss (Forney et al., 1998), skin toughness and presence of stone cells (Allan-Wojtas et al., 2001; Bunemann et al., 1957). Since blueberries are harvested fully ripe, firmness evolution at postharvest may be influenced by a combination of factors rather than restricted just to cell wall changes, as is often inferred. Postharvest softening is one of the major problems for the blueberry industry (Slaughter and Rohrbach, 1985), and therefore to review the aspects affecting firmness during storage becomes essential to understand the postharvest behaviour of this commodity.

1.2.3.1 Relationship between water loss and firmness

Water provides physical support to plant tissues since it is attracted into cells producing turgor pressure within the plasma membrane which is contained by the cell wall (Taiz and Zeiger, 2006). Water loss affects textural attributes of many fruits and vegetables, by decreasing product firmness and crispness due to loss of turgor (Ku et al., 2000; Paull, 1999; Wills et al., 2007). For blueberries, turgor loss has been suggested to induce softening during storage, although this parameter has not been directly evaluated (Allan-Wojtas et al., 2001; Chiabrando and Giacalone, 2011; Forney et al., 1998). Nevertheless, weight loss during postharvest has shown

interesting correlations with blueberry firmness. Tetteh et al. (2004) found that different weight loss magnitudes from 1% to 9% correlated with higher firmness loss from 3% to 33% in ripe rabbiteye blueberries (cv. Tifblue) during 4 d of storage at 4, 21, 27 and 32°C, whereas increasing weight loss from 4% to 7% in rabbiteye blueberries (cv. Bonita) stored for four weeks at 1°C correlated with decreasing firmness from 1.28 N to 0.65 N (Ferraz et al., 2001).

Very low levels of moisture loss seem to generate blueberry firming rather than softening. Miller et al. (1993) obtained an increase of sensory evaluated firmness in ripe highbush and rabbiteye blueberries when weight loss was lower than 1% after 3 weeks of storage at 1°C, while weight loss from 4-5% correlated with berry softening in the same study. Similarly, weight loss of 1% and 2% in highbush blueberries (cv. Burlington) correlated with 50% and 80% of fruit firming, respectively, after 3 to 9 weeks of storage at 3°C, whereas weight loss from 4-14% showed a trend to induce fruit softening (Forney et al., 1998). Explanations for the observed firming response at low moisture loss were not provided in these studies. In contrast, weight loss as high as 22% was correlated with an increase of firmness of 9% during the storage for 6 weeks at 5°C of two cultivars of rabbiteye blueberries, whereas weight loss of 3- 9% produced fruit softening (Basiouny, 1988). From these experiences, the relationship between moisture loss and postharvest firmness rises as a possible key factor for blueberry quality, whose understanding could provide relevant information for the postharvest management of this fruit.

1.2.3.2 The influence of skin and microstructure on firmness

The textural properties of the epidermis, such as thickness and toughness, are known to affect to overall fruit firmness (Jackman and Stanley, 1995). The skin’s resistance to compression force may produce similar firmness outputs when different internal fruit textures are assessed (Jackman and Stanley, 1995). The blueberry fruit epidermis, commonly referred as skin, comprises a single layer of cells forming a tough tissue which provides physical resistance against mechanical damage (Allan- Wojtas et al., 2001; Fava et al., 2006). Experiments relating skin characteristics to blueberry firmness are not coincident. Bunemann (1957) reported that the skin

toughness of ripe highbush blueberries stored for 6 weeks at 5°C correlated positively with fruit firmness, although both parameters were measured subjectively. In contrast, Saftner et al. (2008) concluded that sensory evaluated skin toughness did not correlated with compression firmness in ripe blueberries for twelve different highbush and rabbiteye cultivars.

The presence of wall lignified cells, known as stone cells, as well as the overall cell arrangement of fruit parenchyma influences the mechanical properties of fruit and can contribute to the texture variability between species and cultivars (Smith, 1935; Yarbrough and Morrow, 1947). Stone cells, also called sclereids, are found in the pericarp of many species such as pear, apple, guava and blueberry (Smith, 1935; Weinert and Vanwyk, 1988; Yarbrough and Morrow, 1947). Stone cells are present in blueberry mesocarp tissues, concentrated mainly in the zone between the epidermis and 1.4 mm into the flesh, in densities varying from 1.5 to 19.5 per µm2 (

Figure 1-6) (Gough, 1983). Sclereids are disposed as individual cells or form clusters of two units or more (Gough, 1983). Sclereids in ripe blueberry have a heavily lignified and thick cell wall of 13.5 µm compared to 1.5 µm of parenchyma cells (Gough, 1983). The cell wall thickness of stone cells, their disposition to form conglomerates and to associate to parenchyma cells, as well as their size and orientation, have all been suggested to influence firmness in blueberry (Allan-Wojtas et al., 2001; Gough, 1983), although these relations have not been confirmed. However, in these studies the total number of stone cells was shown to not be related to blueberry firmness.

Figure 1-6. Diagrammatic distribution of stone cells in blueberry fruit represented as a longitudinal cross-section. Adapted from Gough, 1983

Modifications affecting the cell wall anatomy have been also related to blueberry firmness at postharvest. Allan-Wojtas et al. (2001) found that thickening of the cell

Stone cells Calyx end

wall of parenchyma cells correlated with increasing firmness (35%) during storage of ripe highbush blueberries (cv. Burlington) for 6 weeks at 0°C, although these results were not consistent in a second year of study under similar conditions. In another work, microscopically observed corrugation of the cell walls of epidermal and hypodermal cells developed during storage for 8 weeks at 10°C generated an increase of sensory evaluated firmness in air stored ripe highbush blueberries (Bunemann et al., 1957).