baking walnuts

What happens when roasting walnuts?

If you’ve ever eaten both a freshly roasted as well as a raw nuts of any type, you will have noticed how different the two are from one another. An unroasted walnut can be kind of boring and flat in flavour. A freshly roasted one of the other hand seems to have picked up so many new characteristics (and is a lot crunchier)!

Roasted walnuts go great with one of my personal favorite lunch sandwiches. A freshly baked slice of bread, drizzled with a little honey, covered with goat’s cheese, a few slices of cucumber, and, last, but not least, some freshly roasted walnuts on top.

The flavours go well together (test it with a flavour pairing tool) but what really finishes it off is that rich flavour of the roasted walnuts. As with most nuts, roasting really improves the flavour of the nuts! But what really happens when you do this? And how come the nuts undergoes this wonderful transformation?

Walnuts & their composition

Most nuts grow within a protective shell when they’re still on the plant. This shell protects the nuts from animals, but also from harsh sunshine or excessive rain. Therefore any type of processing will start by removing the shell, whether that’s by hand (and a hammer) or done in large scale processes. Once you start looking around you might be surprised to see how many foods go through this process. Not just nuts but also foods like chocolate and coffee!

Once you have the nut itself, you might be surprised to learn that they contain a considerable amount of fat. Most nuts contain more than 40% fat and walnuts can contain about 50-70%. (This fat, and more specifically the melting of the fat is why it’s so much easier to chop hot nuts versus cold ones!) The rest of the walnut consists of about equal quantities of protein and carbohydrates. Nuts, including walnuts, don’t contain a lot of water, just a few percent.

walnuts in shell

What happens when roasting walnuts?

When we roast nuts, the colour & flavour of the nuts change. The nuts turn a darker brown (take care it doesn’t turn black!). Most of this is due to the Maillard reaction, the proteins and sugars in the nuts react together to form all these flavoursome and brown molecules.

We already know that during roasting the flavour & colour of walnuts changes. This is all caused by the Maillard reaction which is the reaction between sugars & proteins. However, more things happen.


Remember that most nuts, including walnuts, contain a lot of fat. This is a mixture of different fats which each their won melting points. Most nut oils are liquid at room temperature, however, some are solid at room temperature. When you roast nuts and heat them these fats will also melt. This makes nuts softer (and easier to chop!). It is one of the reasons why warm nuts don’t have such a crispy bite compared to room temperature nuts!

So why do the nuts still maintain their shape and not melt into a puddle? That’s because of their plant cell texture, this will help them maintain their structure. These cells don’t break down during roasting so the nut keeps its shape nicely.


Last but not least, roasting dries out the nuts. The high temperatures cause the moisture in the nuts to evaporate. This happens in all methods mentioned and makes the nuts more crunchy. Remember, they are soft when coming straight from the oven due to their liquid fats. Upon cooling down the fats will solidify again and make the nut crunchy crunchier than it was before roasting.

Effect of roasting on shelf life

Once nuts have been roasted their shelf life goes down, in other words, you can’t store them for as long anymore. The heat treatment has initiated various reactions such as oxidation of fats which will continue to occur, leading to spoilage.

At the same time, roasting does kill off micro organisms. When these are pathogenic it makes the nuts safer to eat.

cross section of buckwheat walnut bread

Using roasted walnuts

Feeling like some freshly roasted walnuts now? Try this easy (and lazy) risotto recipe, bake some bread with roasted walnuts, bake a pumpkin bread/cake or make a caramel nut tart! If it’s lunch time, give the honey, goat’s cheese, cucumber, walnut sandwich a try.

Roasting isn’t just nice for nuts, marshmallow roasting actually works very similarly!

Haven’t had enough yet of roasting walnuts? Read this scientific article.

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