Agriculture Reference
In-Depth Information
more rapidly than those that mature in the later part of the
harvest season. However, for some apple varieties it is
important not to cool apples so quickly as they can develop
low temperature injuries such as low temperature break-
down and core flush (Little & Holmes 2000). Step-wise
cooling can be used on varieties of apple that are suscepti-
ble to these disorders such as 'Jonathan' and 'Bonza'. This
involves four weeks at 2°C, four weeks at 1°C then storage
at 0°C. However, as expected, this technique can advance
ripening and reduce storage and shelf life
Due to its sensory importance, the effect of CA on aroma
production is of enormous commercial interest. The
characteristic aroma production starts after the climacteric
rise of ethylene production and typical flavour compounds
are only produced after ripening has been initiated by
ethylene. CA has been shown to suppress aroma
production in apples, although with time volatile produc-
tion rates return to acceptable limits (Fellman
et al
. 2000).
For example the aroma levels in CA stored 'Fuji' apples
decrease over time and fruit stored in ultra-low oxygen
(ULO) atmospheres (1% O
2
) show the lowest aroma
production (Echeverria
et al
. 2004). This may be due to the
action of O
2
on ethylene action, where low O
2
suppresses
ethylene action. But this may also be likely to be via action
of O
2
on oxidative processes including respiration where
the suppression of volatiles production is a result of
lowered respiration rate during CA storage via lower
adenosine triphosphate (ATP) concentrations to decreased
fatty acid biosynthesis (Saquet
et al
. 2003).
If the CA conditions are too severe and beyond the
fruit's tolerance, it will shift from aerobic to anaerobic
respiration. This is detrimental to fruit quality and
causes accumulation of acetaldehyde and ethanol (Beaudry
1999). However, a nondestructive measure of chlorophyll
fluorescence of the apple peel in CA storage can assist
in managing low O
2
storage environments (DeLong
et al
.
1999). Specific responses to CA depend upon variety,
maturity and ripeness stage, storage temperature and
duration.
The commercial use of CA for long-term storage is the
basis of year around apple marketing. Although increasing
international trade will see the decline in reliance of long
term CA, there will still be a need to maintain fruit quality
using this well-established technology.
Cool storage and CA storage can only maintain fruit
quality, they do not improve it. As a result the fruit must
be in the best condition before being placed in storage.
The following sections describe some of the physical,
physiological and pathological disorders that can occur in
storage to the detriment of fruit quality.
Relative humidity
To minimise water loss and shrivel during cold storage
the ideal relative humidity is around 98%. This is difficult to
achieve in most cool stores that run at about 80% relative
humidity. Nonetheless, under high humidity, several physi-
ological disorders such as low temperature breakdown are
enhanced (Little & Holmes 2000). In addition, storage
problems associated with free water on the fruit surface such
as lenticel discolouration and skin blotchiness can occur.
Controlled atmosphere storage
Along with temperature management, controlled atmos-
phere (CA) storage has been the basis of apple storage
technology for the last 50 years. CA storage of apple fruit
involves the regulation of the storage atmosphere at low
temperatures where the level of oxygen is reduced to 1-3%
and carbon dioxide is increased to 1-3%. The advantage of
CA storage is a 30-40% increase in storage life over
regular air storage, slower rate of fruit softening, few
storage disorders (such as superficial scald and greasiness)
and better shelf life following storage (Beaudry 1999;
Little
et al
. 1973).
The basic principles of modifying the storage
environment to increase storage life have been in practice
for thousands of years. However, it was not until the 1920s
and 1930s that Kidd
et al
. (1927) demonstrated the effect
of reduced O
2
and elevated CO
2
in the storage environment
would prolong the storage life of apple fruit. This is the
basis of CA technology that is used around the world.
The physiological basis of CA relies on the reduced rate
of respiration and reduction in ethylene sensitivity with the
concomitant reduction in the rate of ripening and other
ethylene-mediated events such as chlorophyll breakdown.
Controlled atmospheres also slow down the activity of cell
wall degrading enzymes involved in softening, therefore
slowing the rate of firmness loss in CA. Low O
2
and/or
high CO
2
atmospheres influence flavour by reducing loss
of acidity, starch to sugar conversion, sugar interconver-
sions and biosynthesis of flavour volatiles (Beaudry 1999).
Bruising
Bruising can be a major problem in apple handling, storage
and transport. Physical impacts during harvesting, sort-
ing and packing can result in bruises that significantly
reduce apple quality. Such injuries may not always be
obvious immediately or even after storage, but develop
during marketing and shelf life. Such injuries not only
reduce returns to growers but also affect customer satis-
faction and re-purchases. A thorough review of impact
