Agriculture Reference
In-Depth Information
during storage (despite being classed as nonclimacteric),
and are more firm than those harvested fully ripe. However,
maximum flavour and aroma (volatile emission) is rarely
achieved when fruit are picked before proper horticultural
maturity. Overripe strawberries should not be picked for
storage as both external and internal colour darkens with
off-flavours being produced.
Maintaining soft fruit at low temperature not only
decreases physiological deterioration (e.g. loss in firmness,
calyx discoloration, loss in glossiness) but also suppresses
post-harvest disease incidence. Storage of soft fruit at
< 5°C generally controls leak rot caused by
Rhizopus
stolonifer
, but only suppresses grey mould development
(Terry & Joyce 2000) as certain
B. cinerea
isolates have a
minima cardinal temperature of between −2°C to 5°C.
Cooling
Once soft fruit are harvested they are more sensitive to
water loss and temperature changes than when attached to
the plant. Post-harvest low temperature management is
the most important method for maintaining soft fruit
quality. However, temperature abuse during the cold
chain is common. Delays between harvest and cooling
significantly hasten fruit deterioration (e.g. water loss/
shrivelling, physiological deterioration, colour loss and
time to disease incidence; Nunes
et al
. 1995). For every
3 h delay in cooling to 5°C, time to disease incidence has
been found to double (Maxie
et al
. 1959). Thus, rapid
removal of field heat immediately after harvest is
paramount as strawberry respiration rate increases
ca
.
fivefold from 28 to 127 ml CO
2
kg
−1
h
−1
between 5 and
20°C, respectively (Robinson
et al
. 1975). Soft fruit are
amongst fruits with the highest respiration rates. Rapid
heat removal is commonly achieved either passively by
placement in a refrigerated room or using forced-air
tunnel cooling (FAC) (Olías
et al
. 2001; Anderson
et al
.
2004). FAC may lead to excessive weight loss and
possibly reduction in fruit aroma. Ferreira
et al
. (1996)
showed that hydro-cooling of strawberry fruit by
immersion in chlorinated water extended post-harvest life
without significant fruit weight loss. Hydro-cooling time
depends on fruit size and water flow rate, but has not been
accepted commercially for soft fruit. Recent research
suggests, however, that while the best temperature for
long-term storage of strawberry cv. Jewel fruit is 0.5°C,
nutritional quality can be improved at 10°C if only a few
days storage are required (Shin
et al
. 2007).
Under ideal storage conditions strawberry quality can be
maintained for 2-10 days at 0-2°C and 90-95% relative
humidity (RH; Snowdon 1990;
cf
. Thompson 1996).
However, strawberry fruit quality was maintained for up to
18 days when control cvs. Elsanta or Andana fruit were
held in the dark at 5°C and 95-100% RH in individually
closed but vented polystyrene containers (Plate 11.4; Terry
& Joyce 2000). Recent results have demonstrated that
strawberry cv. Elsanta can be stored at 5°C for up to 40
days without a detrimental result on appearance (Terry &
Macnish unpublished, 2004).
Protection from mechanical damage
Soft fruit are intrinsically vulnerable to physical damage
after harvest. The thin fragile epidermis of soft fruit
provides little protection against injury. Compression,
impact and vibrational bruising increase respiration rate
and disease incidence. Strawberry fruit are especially
prone to puncturing from the cut pedicel of neighbouring
fruit in the punnet (or clam shell container). Susceptibility
to puncture injury may be cultivar dependent. In order to
reduce physical damage throughout the supply chain, soft
fruit are generally only handled once (i.e. by the picker).
The 'one punnet system' has been universally adopted.
Bubble film (wrap) is often inserted within a punnet to
give added protection. Low temperature storage increases
fruit firmness and, thus, reduces incidence of mechanical
damage. Other pre- and post-harvest treatments, such as
foliar sprays, CA storage and hot water immersion also
retard fruit softening (Table 11.7; Civello
et al
. 1997).
Cheour
et al
. (1990; 1991) demonstrated that foliar
applications 3-4 days before harvest of 20 kg CaCl
2
ha
−1
reduced fruit softening and decay during seven days
storage at 5°C.
SELECTIVE GASEOUS ATMOSPHERE
STORAGE
Controlled atmosphere (CA) storage and modified
atmosphere packaging (MAP) usually imply elevation of
CO
2
and/or reduction of O
2
concentration. Proper low-
temperature storage and management of relative humidity
should always be pre-requisites to CA and/or MA of soft
fruit. In contrast to CA, MAP relies on either active or
passive alteration of gaseous concentrations within pack-
aging or within shrouded pallets and, thus, is a function
of fruit respiration rate versus packaging permeability.
Strawberry fruit, unlike many fresh produce types, are
relatively tolerant of elevated CO
2
concentrations.
Consensus suggests that 15-20% CO
2
and 5-10% O
2
are
ideal levels for successful strawberry fruit CA storage
(Table 11.7). Similarly, CA and/or MAP storage
significantly suppressed decay and maintained quality
on other berry fruit (Agar
et al
., 1997), e.g. blueberry fruit
