11.5.4 Texture and softening
Characteristic textures of different fruits and their manner of softening can be linked with anatomical features (Figure 11.14). Some fruit which are picked while hard subsequently soften markedly as a result of extensive modiﬁcations to cell wall structure. Other fruit such as apple or watermelon remain crisp and soften only slightly. Their thin cell walls remain relatively unaltered. Both types of softening occur in the pear family: Asian pear (Nashi) shows a crisp apple-like texture, whereas many European pears soften to give ripe fruit a melting texture. Interspeciﬁc crosses between the two types show that texture is heritable (Harker et al. 1996).
Many textural characteristics relate to the fate of fruit flesh when fractured and crushed in the mouth. Contributing factors include cell size, cell adhesion, turgor and packing, wall thickness, wall composition and the reaction of cells to shearing stress as they are chewed (Harker et al. 1996). For example, an apple has large (0.1–0.3 mm diameter), turgid, thin-walled cells that are loosely packed (airspace c. 20% of fruit volume). When that flesh is chewed, cells fracture and release their sugary contents as free juice. In contrast, kiwifruit has minimal airspace (c. 2% of fruit volume) and cell walls are thick and hydrophilic (Figure 11.14). Such cells tend to pull apart when the flesh is chewed, resulting in a paste moistened by liquid held in cell walls or released by damaged cells. Avocado also has cells with walls that are thick and soft and which tend to pull apart as in kiwifruit, but avocado also has a high proportion of oil that gives the pulp an oily quality in the mouth.
Cell wall softening thus plays an important part in determining fruit texture and ripening characteristics (Figure 11.15). This involves modiﬁcation and/or removal of various polysaccharides making up pectin and hemicellulose wall components. Wall softening has been the subject of much research worldwide, mostly using tomato as a model, but also other fruits in the search for common themes. Consistent changes include an increasing hydrophilic character of the cell wall as it thickens and swells, pectin solubilisation, reduction of size of individual polymers after solubilisation, and loss of galactose from individual polymers (e.g. Hobson and Grierson 1993; Redgwell et al. 1997) (Figure 11.15). Polygalacturonase (an enzyme that solubilises and degrades pectins) increased de novo 10–50-fold in tomato fruit during ripening (see Brady 1987). Understandably this enzyme was originally accorded a major role in controlling ripening. Studies with transgenic tomatoes have since shown otherwise (Section 11.7).
Kiwifruit softening has also been intensively studied (Harker et al. 1996) and chemical analyses of cell wall components show some consistent changes during early stages of ripening. These include:
- pectin solubilisation (but without further degradation), occurring independently of ethylene pretreatment;
- cell wall swelling and increased afﬁnity for water (more hydrophilic), also independent of ethylene pre-treatment;
- loss of galactose from pectins (especially of a galactan that occurs in close association with the cellulose microﬁbrils) delayed by ethylene and occurring preferentially in the absence of ethylene;
- de-esteriﬁcation of some pectins after ethylene treatment.
Once kiwifruit have begun softening to ripeness, but prior to the climacteric rise and associated dissolution of middle lamellae, these changes continue. There is a further increase in pectin solubilisation, galactose loss, and cell wall swelling as well as degradation of pectins and a reduction in the size of xyloglucan polymers. All these chemical changes reach their maximum prior to the climacteric rise. Such results (along with associated studies on the enzymes polygalacturonase, b-galactosidase, pectin methyl esterase and xyloglucan endoglycosyltransferase) indicate that pectin solubilisation and cell wall swelling are important events in the control of kiwifruit softening. However, detailed mechanisms are still unclear.