Agriculture Reference
In-Depth Information
left on the tree depends on the cultivar and season. However, this 'on-tree
storage' can lead to biennial bearing or crop failure in the following year.
Generally, cultivars of the West Indian race have little or no tree-storage time,
and hybrids, particularly Guatemalan × Mexican, have a longer tree-storage
life. Other tree-storage limitations are fruit drop and development of an of -
fl avour or rancidity with over-maturity. A higher yield can also be obtained if
harvesting is staggered, with 50% of fruit harvested with 21% dry matter and
the remainder at 30%.
A major problem is the stage of maturity for harvest, especially for
cultivars that remain green upon ripening. Some avocados will start changing
skin colour and then fall from the tree when mature, and a few lines can begin
ripening on the tree. Other green-coloured avocados develop a yellowish tint
on the stem near the fruit. Maturity of cultivars that normally change skin
colour from green to reddish or purplish, such as 'Hass', is easy to ascertain.
Immature fruit, if harvested, take longer to soften and they shrivel upon
storage, with the fl esh becoming 'rubbery' rather than buttery. Most countries
with commercial avocado production have developed some sort of standards
for determining maturity. The increase in oil content is correlated signifi cantly
with maturity. As the fruit advances towards maturity, oil content increases,
the level depending upon the cultivar (Barmore, 1976). In California, oil
content was used as a standard of maturity, with a minimum of 8% oil, based
on fresh mass of fruit, exclusive of skin and seed. Oil content as a measure
of maturity was impractical in Florida, due to wide variations in oil content
among and within cultivars. Some cultivars, particularly those belonging to
the West Indian race, never reach the 8% oil content, as in California. Florida
uses minimum fruit weight and diameter as related to number of days from
fruit set, thus establishing the earliest harvest date for each cultivar (Hatton
and Campbell, 1959). Fruits originating from known bloom dates become
progressively smaller in size from the earliest bloom date to the latest (Hatton
and Reeder, 1972).
Moisture content is also used as a measure of maturity, as it does not
involve oil determination, and there is a negative correlation between oil and
moisture content. As oil content increases during maturation, there is a similar
decrease in moisture content. A 10 g sample of fl esh from a total of ten fruits is
collectively grated and spread on an open dish and dried in a microwave oven
for a few minutes. The minimum dry matter percentage for dif erent cultivars
ranges from 17 to 25%. For example, for 'Fuerte' the minimum dry matter
percentage is 19%, 'Hass' 20.8%, and 'Zutano' 18.7%. In more mature fruit
the dry matter can reach 30%. After the minimum moisture content has been
met, fruit comparable to the sample fruit can be picked and ripened at room
temperature. If the sample fruits ripen within 8-10 days without shrivelling,
they are considered to be mature, and the grower can proceed to harvest
comparable-sized fruit on the tree. Grade standards frequently set a minimum
dry-matter percentage of around 20% for 'Fuerte', 22% for 'Hass', 25% for
