The Science of Pears – From Harvest to Application

Every year in August/September our pear trees give us a good supply of pears. They’re not the pretties, but sure are tasty. We have two types: Conference and Doyenne du Comice and once they start coming off that tree we have more than we can eat. So it’s a good time to experiment with pears! And experimentation is what we love doing best :-). Cakes, pies, jams, we put the pears in anything we can think until they’re all gone.

Along the way, we’ve learned a thing or two about these delicious green fruits. About time we dig into the science of pears!

Introducing: Pyrus

The pears that we’re referring to belong to the genus Pyrus. There are quite a few different types, some are round, like an apple, but they’re more well known for their round bottom and slightly longer elongated top. A lot of pears are green, but some varieties (e.g. the Asian pears) are more yellowish in color. They grow on trees and shrubs in mostly more moderate climates.

They very definitely aren’t prickly and thus are something very different than prickly pears. Even though prickly pears are called pears, they are not related. Instead, the prickly pear is the fruit from a cactus species and is not related to the Pyrus pears!

Importance of harvest for pears

One of the pear’s special features is when a pear should be harvested. Most fruits (though not all) are at their best when harvested when ripe (which doesn’t mean they’re always harvested when ripe, but ideally, that’s when they’d taste best). However, this is not the case for pears. Instead, pears should actually be harvested before they ripen, leaving them to ripen off the tree (it’s a climacteric fruit).

If you do harvest pears only once they’ve ripened they will have started to develop a woody structure within the pear. The plant does this to protect the pear, but it, unfortunately, gives the pear a gritty texture that is not so pleasant to eat. Once it has been formed, the woody structure will not be able to disappear. Even not during further ripening and softening either.

When pears ripen off the tree they will not form this woody texture. Instead, they’ll remain juicy and during ripening, they will turn softer and moister, even mushy, if kept for too long. Ripening happens faster at room temperature, but can be slowed down considerably in refrigerated conditions. Since pears are a climacteric fruit, ripening speeds up when they’re exposed to ethylene, a natural ripening gas. It is actually quite tricky to store pears properly and have them ripen well before selling to consumers. It’s why in the US at least, it often loses the popularity battle from the apple!

Tip: buy slightly unripe pears, this way you can control their ripening. Some people may like slightly unripe pears (myself included) whereas others prefer them very ripe and juicy. If you want to speed up ripening, place them next to bananas, they produce a lot of ethylene which helps ripen the pears.

The texture of pears

Pears and apples may seen like quite similar fruits. Not only can they be shaped similarly (some pears truly just look like apples), they also grow in similar moderate climates and both have a decently mild taste and flavor. However, pears have quite a different texture than their popular colleagues. They’re a little firmer and when you cook or bake with them they fall apart less easily.

Air content

This can mostly be explained by comparing the texture and structure of pears and apples. A first major difference is the air content. Apples tend to contain about 25% of air (the exact amount differs per variety). Pears on the other hand, only contain a few percent of air. This is a big difference! Air is released when the cells of the fruit break down. As a result, apples might lose about 25% of their volume when cooked, whereas pears will more or less maintain their volume.

Cell wall composition

Another important difference is the composition of their cell walls. Apples tend to contain a lot of pectins in their cell walls. These pectins contribute to their smoothness (just as they do for pumpkins). Pears on the other hand don’t have as many of these pectins.

What happens during cooking & baking

So texture is an important property of pears. It’s important for pears that are eaten just so, fresh, but even more so when you’re cooking and baking with pears. You’ll want to make sure that the pear disintegrates the right amount for your type of preparation method.

Loss of turgor

Remember turgor? Cells in plants, which includes fruits and vegetables!, have a special structure that ensures they are plumb and firm when fresh and moist, called turgor. When cells are in turgor, they are plump, swollen full with water. Once water starts leaving the plant (or fruit in this case) the pressure is gone and the fruit becomes softer (it’s why salad leaves wilt and pear will wrinkle over time). Compare it to a balloon, full of air it’s firm, however, when most of that air leaves the balloon it becomes soft and wrinkly.

A fresh pear’s cells will be in turgor. However, by heating the pear, cells walls are broken down and water can escape. As a result, the pear (pieces) will become softer.

Breaking down molecules

This loss of turgor is caused by large (mostly polysaccharides, such as pectins) molecules being broken down. These large molecules provide structure and strength. However, once they break down, the whole pear will soften and open up to letter water out.

Color changes

When boiling pears in water, some pears may turn from a beige to a light pink color. This color transformation is due to the earlier mentioned polyphenols. Aside from producing astringency, the procyanidins can react into an anthocyanin-like molecule. Anthocyanins are molecules with a red/pink/purple hue, causing the pear to change color. (Red cabbage also contains a lot of anthocyanins, just one of many red natural colors in food.)

Pear varieties

Even though you might only find two or three pear varieties in your local store, there are actually thousands of pear varieties. Which ones you’ll find at your store depends on the varieties that grow well in your region. For instance, in the Netherlands, we’d find ample Conference pears, whereas these aren’t common at all in the US. Asia again has slightly other common varieties.

Some pears are good to be eaten raw, you might find them being called ‘dessert pears’. Others are best eaten cooked, or they’d be too hard and dry. Yet again other varieties are mostly used for being made into drinks (e.g. perry, the pear variety of apple cider).

Taste & astringency

Which application suits a pear best, depends on various factors. First of all their taste, some pears are sweeter and more flavorful than others, making them more appealing to eat raw. When transforming a pear into perry, as is the case for apple cider, you’ll want some astringency. This comes from the polyphenols in the pears, mostly procyanidins.

Textural varietions

Secondly, their texture and cell wall composition are important. Pears that contain more pectins for instance tend to be better suited for use in butters (or jams) since they give a creamier consistency. When making poached pears, on the other hand, you mostly want to ensure that the pear keeps its shape and doesn’t fall apart. As such, it will need enough cellulose and non-degraded pectin in its structure to hold up after being heated for an extended period of time!

Unfortunately, there’s not a lot of data available on the composition of different pear varieties. As such, instead of focusing on their composition, it is best to follow recommendations in recipes and with the produce as to what they should be used for.

Sources

Marwa Brahem, Severin Eder, Catherine Renard, Michele Loonis, Carine Le Bourvellec. Effect of
maturity on the phenolic compositions of pear juice and cell wall effects on procyanidins transfer.
LWT – Food Science and Technology, Elsevier, 2017, 85, pp.380-384. link

Harold McGee, On Food & Cooking, 2004, Scribner, p.356

Joan Morgan, The Book of Pears: The Definitive History and Guide to Over 500 Varieties, 2015, Chelsea Green Publishing, link

Thyran Phaneuf, The Push to Make Pears the New Apples, 2016, link

Devin Powell, Fruit ‘lungs’ explain why pears rot faster, 15 July 2008, New Scientist, link

Catherine MGC Renard, Effects of conventional boiling on the polyphenols and cell walls of pears, J Sci Food Agric 85:310 – 318 (2005), link

Pieter Verboven, Greet Kerckhofs, Hibru Kelemu Mebatsion, Quang Tri Ho, Kristiaan Temst, Martine Wevers, Peter Cloetens, Bart M. Nicolaï, Three-Dimensional Gas Exchange Pathways in Pome Fruit Characterized by Synchrotron X-Ray Computed Tomography, Plant Physiology, Volume 147, Issue 2, June 2008, Pages 518–527, https://doi.org/10.1104/pp.108.118935 (the photos are amazing!)