58 Produce Degradation: Reaction Pathways and their Prevention
firmness for use is also dependent on the specific requirements for a particular use.
One example where end-use parameters are important is that of pears to be used for
fresh-cut slices. Chen et al. (2003) define optimal firmness as 22 to 31 N as measured
by a Magness-Taylor probe. Gorny et al. (2000) define optimal firmness as 44 to 58 N
using a similar instrument, about twice the previous definition. The probable reason
for the difference in recommendations is that the softer fruit is intended to be stored
as slices at –1.1 to –1.7°C, while the harder fruit was intended to withstand storage
at 10°C as a sliced product. Fruit to be used for sliced product and stored at warmer
temperatures must be firmer than fruit that will be used for product that is stored at
colder temperatures since softening proceeds at a faster rate at higher temperatures
(Johnston et al., 2002a). This emphasizes the importance of defining the intended
use and storage conditions before attempting to define the optimal firmness or
ripeness of the raw product.
One principle of postharvest handling of fruit is that product that is fully mature
but not ripe will generally be firmer at harvest and will remain firmer through storage.
Certainly this has been shown to be the case for apples (Knee and Smith, 1989;
Silsby, 1993) and pears (Rousseas et al., 2001). The rate of softening in apples during
long-term storage also increases, as does the maturity of the fruit at harvest (Tu et al.,
1997). Changes in firmness in apples have been shown to have at least two compo-
nents: an earlier, slow softening phase and a later, rapid softening phase (Johnston
et al., 2002a). Fruits harvested later have a shorter duration of slow softening in
storage than do fruits harvested earlier, while fruits of both early and late maturities
have similar rapid softening phases (Johnston et al., 2002a). Also, small and large
FIGURE 3.2Texture–structure relationships in ripening tomato fruit. The graph depicts
decreasing puncture force with increasing ripening time. Note that a transition occurs in the
mode of tissue failure when it becomes easier for cells to separate or de-bond than to rupture,
corresponding to the degradation of the middle lamella to a critical level. (From Jackman and
Stanley, Trends Food Sci. Technol., 6, 187, 1995. With permission.)
Force (N)Cell relaxation
and ruptureCell
de-bondingbaRipening time