56 Produce Degradation: Reaction Pathways and their Prevention
2003; Kader, 2003). Maturity at harvest is a very important factor in determining
storage life and final quality of a fruit or vegetable product (Kader, 2003). Most
fruits and mature-fruit vegetables achieve optimal eating quality if they are allowed
to ripen on the plant. However, in order to withstand the rigors of shipping, some
of these are harvested mature, but unripe. There is a compromise made between
picking for optimal quality and picking for the best shipping condition. In most
nonfruit and immature-fruit vegetables the optimum quality is reached before full
maturity, but harvesting at progressively more mature stages leads to progressively
more rapid deterioration after harvest (Kader, 2003). The variety of plant parts used
as food adds significant complexity to the definition of optimal maturity (Figure
3.1); this unfortunately adds to the difficulty in understanding the impact of maturity
on quality degradation through the postharvest continuum. The difficulty in defining
maturity in unripened plant parts (leaves, immature floral parts and fruits, stems,
and roots) is reflected in the lack of in-depth information on these commodities
(Reid, 2002). It will be clear from the following discussion that virtually all under-
standing of the issue of maturity and quality is focused on ripening fruits and fruit-
vegetables. It may be that handling of unripening plant parts (leaves, immature floral
parts and fruits, stems, and roots) may not have required any further understanding
of maturity in the past. However, because all horticultural commodities are becoming
candidate components in new products such as fresh-cut salads, the need for better
understanding of maturity will arise.
3.2 EFFECTS OF MATURATION AND RIPENING
3.2.1 TEXTURE
Plant growth and development is intimately related to the growth and development
of the plant cell and wall, so the texture of foods derived from plant parts is ultimately
determined by the plant cell, its wall, and its relationships with the surrounding cells.
Jackman and Stanley (1995) view texture change in plant foods as a consequence
of decreasing shear force as the fruit ripens (Figure 3.2). As the fruit progresses
through the ripening phase, it eventually reaches a stage where cellular cohesiveness
is failing; this stage is termed de-bonding (Jackman and Stanley, 1995), and fruit at
this stage is normally not acceptable for use or consumption. The changes in cell
wall cohesiveness with maturity have been well documented (Tu et al., 1997) and
are somewhat understood at the gene expression level (Giovannoni, 2001).
Storage life is often defined by the time period over which a product can maintain
a minimal acceptable firmness level (Harker et al., 2002a). The definition of the
term acceptable, to date, has been generally more of a pragmatic one (relating to
what can be measured by available instrumentation), rather than one that actually
relates to human perception of quality (Harker et al., 2002a). There are exceptions
to this general pattern of softening; for instance, peas, snap beans, lima beans,
eggplant, broccoli, and okra actually get tougher (fibrous) or firmer with advancing
maturity (Pantastico, 1975; Sams, 1999; Gross et al., 2002).
Firmness, one of the most measured textural characteristics, is quite often used
as an objective criterion to define maturity level in many fruits (Pantastico et al.,