Avocado - Crop Post-Harvest: Science and Technology - page 178

RIPENING
Avocados can be held on the tree for few weeks after
they are physiologically mature, and will ripen after har-
vest, but time to ripen decreases with increasing time on
the tree. Market research has shown that more avocados
are sold if they are offered to the consumer in a ready-to-
eat condition. This has led to a system of pre-ripening
fruit prior to stocking. Fruit is treated with ethylene and
held at 21°C before retailing. Fruit ethylene production
begins after harvest and increases greatly with ripening
to >100μl C 2 H 4 /kg·hr at 20°C. Treatment with 100 ppm
ethylene at 20°C for 48 hours (early-season fruits), 24
hours (mid-season fruits) or 12 hours (late-season fruits)
induces avocados to ripen in 3-6 days, depending on cul-
tivar and maturity at harvest. There is an increasing move
at the retail level toward 'ripe for tonight' programs that
generally result in significant increases in sales. This is
achieved by treating avocados with 10 to 100 ppm ethyl-
ene at 17°C to 20°C for approximately 48 to 72 (early-
season), 24 to 48 (mid-season) or 12 to 24 h (late-season).
This significantly reduces both the time to ripen (to 3 to
6 days, depending on cultivar and maturity), and also
fruit to fruit variability in ripening (Gazit & Blumenfeld
1972). If fruit are stored prior to ethylene treatment, the
duration of treatment required to achieve maximum rate
of ripening is reduced. For 'Hass' avocado, after 3 to 4
weeks of storage there may be relatively little benefit of
ethylene treatment (particularly for later season fruit)
since the time to ripen decreases during storage. Heat
production of avocados is much greater than many other
fruit crops, and therefore careful attention should be paid
to air flow and temperature management during ethylene
treatment and subsequent ripening. Palletized fruit may
reach temperatures of more than 30°C, with negative
effects on ripe fruit quality. For this reason, ethylene
treatment of palletized fruit should be carried out under
forced-air conditions. Ripening rate depends on
physiological maturity of the fruit, temperature and
humidity, oxygen and carbon dioxide levels (Kurlaender
1996). During ethylene treatment, CO 2 levels should be
maintained at less than 1% to 2% by either continual
venting of the atmosphere, or full venting and ethylene
re-injection if 'shot' systems are used. Avocados are best
ripened at 15°C to 20°C (Hopkirk et al . 1994). The rip-
ening rate at <15°C is relatively slow, and ripening
at >25°C may result in increased decay, uneven ripening
of the flesh and off-flavours. Avocado fruit fails to ripen
properly at 30°C, even at 25°C, the level of post-harvest
disorders was greater than in fruit ripened at 20°C, and
best final quality was obtained with fruit stored at 6°C

and then ripened at 15°C. However, if fruit are to be rip-
ened at 25°C, then final quality was better if the fruit
were not refrigerated but held at a temperature closer to
the final ripening temperature. Following ripening, fruit
should be pre-cooled to 5°C. Many consumers have trou-
ble identifying ripe ready-to-eat avocados, especially
green-skinned cultivars, and therefore stickers are now
placed on ethylene gas-ripened avocados in retail outlets
to help consumers select ripe fruits. Ripe (soft) avocados
require care in handling to minimize physical damage.

Effects of 1-methylcyclopropene (1-MCP)
Some experimental evidence have suggested that 1-MCP,
an ethylene action inhibitor, delays ripening and reduces
internal CI (flesh greying, vascular browning) which is
associated with ripening during storage where storage
times are long, or temperature management is poor (Pesis
et al . 2002; Jeong et al . 2002). The optimum treatment
conditions are 50 to 100 nL L −1 at about 6°C for 18 to
24 h. 1-MCP treatments for 'Hass' can be effective at >4
weeks, however, for other cultivars it may cause higher
levels of internal CI at even short storage times. Higher
concentrations may result in excessive delays to softening
and ripening which are likely to in turn increase disease
incidence. 1-MCP does not reduce external CI (skin
blackening) of 'Hass' avocados. 1-MCP-treated 'Ettinger'
and 'Pinkerton' avocado fruit stored at 5°C for 3.5 weeks
maintained a greener peel colour, lower chlorophyllase
activity and less chlorophyll breakdown, reduced CI
symptoms (expressed as mesocarp discoloration) and
reduced PPO and peroxidase (POD) activities
(Hershkovitz et al . 2005). Adkins et al . (2005) investigated
the potential of 1-MCP to manipulate ripening of
nonstored 'Hass' avocado fruit by treatment before or
after ethylene and at different times during ripening, and
found that ripening of fruit exposed to 100 ppm ethylene
for 24 h at 20°C could be delayed by up to 3.3 days by
applying 1-MCP. However, once the fruit started to soften
there was little effect of 1-MCP, compared with no
1-MCP treated fruit. 1-MCP treatment was associated
with increased severity of rots, caused mainly by
Colletotrichum spp. and Dothiorella spp. Significant dif-
ferences in disease severity were found between orchards,
with replicates with low disease severity being less
affected by 1-MCP treatment. These results indicate that
1-MCP can delay ripening, but careful sourcing of fruit is
required to reduce the risk of diseases in ripe fruit, and
there is some capacity to delay ripening using 1-MCP
after ethylene. There is little potential to control ripening
using ethylene after treatment with 500 nL L −1 1-MCP,