sliced avocado

The Science of Growing, Storing & Ripening Avocados

Are you one of those people who squeeze every single avocado in a store before deciding on one to buy? Or do you buy them unripe and leave them to ripen at home? Or do you eat them fresh from the tree?

I sure hope you’re not opting for the last version. Avocados do not ripen on the tree and they’ll be rock solid when harvested. It’s best to leave them to ripen first and we’ll discuss just how that happens.

Growing avocados

The major part of the world’s avocados, about a third, comes from Mexico. The Dominican Republic, Peru and the state of California are other major avocado producers. The climate in these regions is optimal for growing the fruit (yes, an avocado is a fruit!). Avocados grow on a tree and can be picked for several months of the year.

Growing your own

If you want to see an avocado plant, you can try to grow one yourself. If you don’t live in a suitable climate it likely won’t turn into an actual tree or ever start bearing fruit, but you can get a a decent sized plant.

All avocado trees also out as an avocado pit. That large seed has all the nutrients for it to start growing into a little plant. You can grow an avocado from this pit by removing the outer layer and leaving it to float in a bowl of water. You can use toothpicks, or a fancy device such as an Avoseedo, to help the pit stay afloat.

avoseedo with pit
Avocado pit floating in an avoseedo

After three weeks or so, you can start to see cracks in the pit. This is a good sign, it means a root is trying to get through. A few more days of waiting and you should be able to see a root growing out of the bottom of the avocado. The root will continue to grow down and might start branching out after a few more weeks.

Not a lot later and you will be able to see your first stem with leaves appear from the top of the seed. It’s strong enough to be transferred to a pot once the root is quite sturdy and it has a few leaves, probably after about four weeks.

avocado plant in pot, 4 months old
4 months old!

Types of avocados

If you do not live in an avocado growing country, chances are you’ll mostly see just one, maybe two types of avocado. By far the most common avocado is the Hass avocado. A big advantage of the Hass avocado is its sturdy skin, this makes it easy to transport these otherwise delicate fruits around the world.

Just as is the case for apples, there are a lot of avocado varieties. Some of the other varieties are Bacon, Lamb Hass, Fuerte, Pinkerton, Reed and Zutano. For farmers, growing different varieties can help them deliver a year-round supply of avocados since not all of them will ripen at the same time of year.

Maturing of avocados

Long before an avocado ripens, it needs to mature on the tree. First the avocado grows into its final size. When it’s at or close to its final size the avocado fruit will start to produce fats. During maturation the fat content of an avocado goes up considerably. This is very important for getting a high-quality avocado. Only when an avocado contains enough fat will it be able to ripen into a soft tasty avocado during ripening. As immature avocado, won’t ever ripen properly.

Farmers can measure the maturity of their avocados by measuring the oil content or dry matter content. The dry mass is the weight of an avocado without any of the moisture. This includes the fat content as well as the carbohydrates, structural molecules such as cellulose and several more. A well-matured avocado has a dry matter content of about 20-40%. Farmers know that about 11% of the avocado is non-oil dry mass. As such, a mature avocado contains about 9-29% of oils and is said to need at least 8% upon harvest to ripen properly.

16,5 month avocado plant
1,5 years old!

Ripening of avocados

Interestingly, avocados do not ripen on the tree, even if they have matured fully. Ripening only starts once the avocados have been harvested, as such, they are climacteric fruits.

Climacteric fruits ripen because of exposure to a gas called ethylene. The fruits will actually produce this gas themselves, but don’t need their ‘own’ gas to ripen. As such, placing an avocado in an area with a lot of ethylene, or placing it next to a fruit that produces a lot of ethylene (bananas for instance), you will speed up ripening. By capturing the ethylene, by wrapping the avocado in a paper bag for instance, you can speed it up further.

Avocados ripen fully within 5-7 days after harvest without any special acceleration measures taken. They ripen faster at warmer temperatures and of course if more ethylene is present.

How an avocado softens

An important transformation that happens while the avocado ripens is that it turns softer. You’ve probably also lightly squeezed an avocado to test whether it’s ready to use for your guacamole or avocado toast. An unripe avocado is hard and won’t give in at all, an overripe avocado feels very soft. A ripe avocado sits within those two extremes, upon gently pushing the avocados you feel resistance and it gives in just slightly.

This softening process is part of the ripening of the avocado. It is caused by several processes going on simultaneously. Most of those are related to the break down of large complex and hard molecules such as pectins and cellulose. Enzymes, called cellulases, break down cellulose. Pectin methylesterase acts on the pectins (pumpkin is also full of these) to break those down and soften the avocado fruit.

Skin color change of an avocado

The most common avocado variety, the Hass, changes skin color during ripening. An unripe avocado has a dark green color. This green color then turns into a purplish hue until turning very dark brown/black on color. This loss of green color is actually very common for other fruits, certain apple varieties do the same when ripening.

The color change in avocados is initially caused by literally a loss of green color. The chlorophyll molecule that colors avocados, and many other green vegetables, green breaks down. After a few days another reaction sets in resulting in the formation of very dark-colored anthocyanins.

Confusingly for consumers, the softening of the flesh and the color changes of the skin don’t always go hand in hand. Some still unripe avocados can be a dark color already whereas some ripe avocados might still have a greenish skin. These two different mechanisms are both slightly differently affected by storage temperature and maturity upon harvest.

avocados
Ripe avocados

Stringy avocados

You may have encountered an avocado that was very stringy, with fibrous bands running through the avocado. If you left a cut avocado open, these strings will also be the part to turn brown first.

These strings are vascular bundles within the avocado, the system that’s used to transport nutrients and water throughout the fruit. This stringiness can occur in immature fruits and are a sign that the avocado was harvested too soon.

What’s in an avocado?

Keep in mind that every avocado is slightly different because of differences in maturity. And those differences can easily be several percentages, especially when also comparing between different varieties.

Generally speaking (based on USDA nutritional data) an avocado consists of more than 70%. It contains just a few percent of protein and around 8% of carbohydrates of which the majority are fibers. The second major ingredients after water though is fat, you can expect an average fat value of about 15%.

Of all those fats, a little less than 15% are saturated fatty acids, most of which are palmitic acid. Another 15% are polyunsaturated fatty acids, mostly linoleic acid. The remainder of the fats, some 70% are monounsaturated fatty acids, most of which are oleic acids.

Why does a cold avocado taste different than a warm one

These fats are what make the avocado so creamy and smooth. But have you ever eaten an avocado straight from the fridge and thought it wasn’t as creamy as it usually is? This effect is also caused by these fats. Some of the fats in avocados are liquid at room temperature, like olive oil, whereas others are solid. This is because different fats have a different melting temperature. When you place an avocado in the fridge, more of the fats in the avocado will turn solid. As a result, your avocado is harder and feels less creamy. Luckily, it’s easy to resolve just take a few minutes for it to warm up and the creaminess is back!

avocados and toast
Warm up your avocado on hot toast and it’s even softer!

References

Avoseedo, link

Bower, J.P., Cutting, J.G., Avocado fruit development and ripening physiology, 1988, J. Janick (ed.) Horticultural Reviews. Volume 10:229-271. Timber Press, Portland, OR, link

Californias Avocados, Avocado 101, link

Katy A. Cox, Tony K. McGhie, Anne White, Allan B. Woolf, Skin colour and pigment changes during ripening of ‘Hass’ avocado fruit, Postharvest biology and technology, 31, 2004, 287-294, link

EcoFarms, Fruit availability, link

How does it grow, Avocado, S2 E6, May-2, 2017, True Food TV, link

Pesis, E., Fuchs, Y., Zauberman, G., Cellulase activity and fruit softening in avocado, Plant Physiology, 1978, 61, 416-419, link

Schaffer, B.A., Wolstenholme, B.N., Whiley, A.W., The avocado: botany, production and uses, CABI, 2013, p.494, link

USDA, Avocados, raw, California, FoodCentral Search Results, April-1, 2019, link

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