Apples - Crop Post-Harvest: Science and Technology - page 93

The techniques and methods described so far for assessing
fruit maturity are imprecise and only measure one attrib-
ute. A range of indices and techniques has been developed
to integrate these maturity measures. For example, the
Streif Index was developed in Germany to combine three
maturity measures (firmness, SSC and starch conversion)
to estimate the optimum harvest time (Streif 1996). It is
calculated as {firmness / (soluble solids content × starch
index)}. This approach has been successfully applied to a
range of apple varieties in many countries, but adaptations
to specific cultivates in each region are needed to validate
this approach (DeLong et al . 1999).

fruit maturity allows growers to spread their harvest window,
and this can be an important commercial management tool
if there are large volumes of fruit that need harvesting
at one time.
There are also postharvest benefits of the preharvest
application of ReTain™. Research has shown that the
application of ReTain™ delays the postharvest fruit
ripening (Greene 2003). Apples treated with ReTain™ have
been shown to soften more slowly than untreated fruit when
stored in air or CA storage. The treatment effect was most
dramatic for fruit stored in air at 0°C (Golding et al . 2005).
Research has also shown that ReTain™ applied at seven
days before harvest to 'Gala' and 'Pink Lady™' apples had
little impact on the maturity indices of fruit but delayed the
postharvest ripening of the fruit and reduced the rate of
postharvest softening (Halder-Doll & Bangerth 1987).
The relative benefit of the application of ReTain™ has
been shown to be dependent on environmental factors, the
time of application, harvest date and variety (Greene 2003).

Regulation of maturity and ripening
by manipulating ethylene
Genetic manipulation of ethylene biosynthesis has been
shown to significantly affect fruit maturity, ripening and
fruit quality (Defilippi et al . 2004). However recent
research has provided two chemical treatments that
can regulate fruit maturity and ripening. These chemicals,
aminoethoxyvinylglycine (AVG) and 1-methylcyclopro-
pene (1-MCP), both work by blocking ethylene activity
and as a result delay fruit ripening.

1-MCP
A major development in apple storage technology has been
the postharvest use of 1-methyl-cyclopropene (1-MCP,
SmartFresh SM ). This compound has only been available
for the last decade but is not only revolutionising our
understanding of fruit ripening (Blankenship & Dole 2003)
but is also changing apple storage practices. 1-MCP has
been successfully integrated into commercial use around
the world to maintain apple fruit quality during storage.
1-MCP is applied as a gas to fruit in airtight cool rooms
after harvest. It acts by irreversibly binding to ethylene
receptors (Sisler & Serek 1997) where the affinity of
1-MCP for the receptor is approximately ten times greater
than that of ethylene. Eventual recovery is thought to be via
the synthesis of new ethylene receptors. The blocking of
ethylene perception with 1-MCP has a variety of effects
on apple respiration, ethylene production, volatiles produc-
tion, chlorophyll degradation and other colour changes,
protein and membrane changes, softening, disorders and
diseases, acidity and sugars (Blankenship & Dole 2003).
In general, the correct postharvest application of 1-MCP
has been to shown to dramatically reduce the rate of
softening in apples during storage, resulting in firmer fruit
after storage (Watkins et al . 2000). 1-MCP has also been
shown to slow the decline in acidity of the fruit during
storage (Watkins et al . 2000). In addition, 1-MCP reduces
the occurrence of some common storage disorders such as
soft scald, core flush and greasiness (Fan et al . 1999). One
of its most useful benefits is its inhibition of superficial
scald. However, 1-MCP has been shown to induce 'new' or

AV G
Aminoethoxyvinylglycine is a plant growth regulator that
inhibits ethylene production in plants by competitively
inhibiting 1-aminocyclopropane-1-carboxylase (Halder-
Doll & Bangerth 1987). The inhibition of ethylene produc-
tion has some important postharvest benefits for managing
apple maturity.
The preharvest application of AVG to apple trees has
been shown to reduce pre-harvest fruit drop, delay ripening
and maturation and reduce the loss of quality of long-term
stored apples (Bramlage et al . 1980; Halder-Doll &
Bangerth 1987; Greene 2003; Phan-Thien et al . 2004;
Silvermann et al . 2004; Jobling et al . 2005). As a result of
these benefits, AVG became commercially available in the
1980s. A commercial product is sold as ReTain™ (Valent
BioSciences Pty Ltd).
Research has shown that ReTain™ applied 28 days
before first harvest (where harvest is defined as the earliest
possible pick for long-term storage) delays fruit maturity
(Greene 2003; Phan-Thien et al . 2004). The delay in
maturity is reported to be between five and 15 days
depending on the variety and the seasonal weather
conditions (Greene 2003, 2005). Research by Phan-Thien
et al . (2004) showed that the maturity of 'Gala' apples
was delayed by nine to 12 days and the maturity of 'Pink
Lady™' apples was delayed by five days. This delay in