What causes an avocado to have a foul taste even when it looks superficially just fine? I’d like to know if there is a way of telling simply by looking at them, so I can avoid buying bad ones.
According to the book, Fruit & Nuts by Susanna Lyle (Timber Press, 2006), there are three main varieties of avocado: Mexican, Guatemalan, and West Indian. Mexican fruits tend to be smaller, with thin skins that turn a glossy deep green when ripe. Guatemalan fruits are round or pear-shaped and have pebbled skin. The largest avocados are West Indian, nearly round, with shiny green skin and flesh that is less rich in oil.
Avocados tend to take a long time (sometimes over a year) to reach maturity from the beginning of fruit-set, and sometimes they will not soften and become edible because the stems have a ripening inhibitor. For this reason, they must be picked while still hard, and then allowed to ripen at room temperature. Mexican fruits ripen fastest (6-8 months) but have a shorter storage life. Guatemalan fruits take 12-18 months to ripen and must be stored at about 10 degrees Fahrenheit for over a month.
This factsheet from U.C. Davis about post-harvest quality in avocados describes optimal storage temperatures, and mentions several variables that can lead to an unpleasant flavor:
- On-tree storage may result in development of off-flavors or rancidity with overmaturity
- Off-flavors may also develop when fruit are harvested during periods of hot weather
- >10% CO2 may increase skin and flesh discoloration and off-flavor development, especially when O2 is <1%
- Anthracnose: Caused by Colletotrichum gloeosporioides and appears as the fruit begins to soften as circular black spots covered with pinkish spore masses in later stages. Decay can penetrate through the flesh and induce browning and rancid flavor.
There are several scientific articles that discuss heat-induced off-flavors. In one example, “Isolation of Unpleasant Flavor Compounds in the Avocado (Persea americana)” by Brian I. Brown, certain varieties seem more prone to bad flavors than others, and some varieties lose the off-flavor once they are fully ripe.
I’m afraid there does not seem to be an easy way to know in advance which avocados will taste bad. Certainly, you should avoid fruits with dents or obvious discoloration in their skin. You might also complain to the market where you have purchased inedible avocados, and they can relay that information to their distributors and see if it has something to do with how the fruit is harvested or stored.