baking walnuts

Science of Roasting Walnuts

One of my favorite lunches is 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. Perfetto!

The flavours go well together (test it with a flavour pairing tool!) but what really finishes it off is the rich flavour of the roasted walnuts. As with most nuts, roasting really improves the flavour of the nuts which is why you’ll see it being used in a lot of processes and recipes. Therefore, it’s time for a deep dive into the science of roasting walnuts (and most of this applies to all other nuts as well)!

Walnuts & their composition

Wanuts grow inside a shell. This shell protects the nut inside but isn’t suitable for eating, it is way too hard. Therefore any roasting process will start by removing the shell, whether that’s by hand (and a hammer) or done in large scale processes.

The remaining walnut is high in fat (50-70%). The rest of the nut consists of about 50:50 protein and carbohydrates. Nuts, including walnuts, don’t contain a lot of water.

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.

Tips for deshelling

Don’t have a special tool for deshelling your walnuts? Don’t worry, you don’t need one. We generally wrap the walnuts in shell in an old, but clean, towel. We then use a hammer (often a meat hammer) to break the shells. Next you have to remove the nut from the shell. It will probably not come out whole but in most cases that shouldn’t be a problem at all.

walnuts in shell

How to roast walnuts

Once you’ve got your walnuts, whether deshelled yourself or bought, it’s time to roast them. You roast nuts by heating them to temperatures well above 100C for some extended period of time. You can do this in various ways, here’s just four as an example:

1. Air roasting – An oven

Method one is to use hot air to roast your walnuts, using an oven for instance. Air isn’t the most efficient transporter of heat (as we discussed here). As a result, this method takes some patience. The nuts need to be in the hot air for at least 20 minutes before something really changes.

Something else to keep in mind is that in the oven you’d roast your nuts on a rack. The heat transfer from the rack to your nuts will be more eficient than through the air surrounding the nuts. As a result they brown (and burn) more quickly on the bottom, hence the instruction to shake up your nuts sometime halfway. The type of tray you use will influence how well heat gets transferred, darker sheets for instance causes browning more quickly.

Using the oven for roasting walnuts has one great advantage: you can control the temperature easily an you have enough time to decide whether you want them browner or not. If you want them browner, just leave them in longer.

2. Pan roasting – Frying pan

My preferred method is to bake them, without oil, in a frying pan. It’s a lot quicker than using an oven (especially if you plan on having lunch in 5 minutes from now and don’t have time to pre-heat an oven). The heat all comes from the bottom, so you’ll need to shake the pan a little more often. However, because the nuts are in direct contact with the hot surface (which is quite above a hot stove), it all goes very quickly.

The great disadvantage is that it’s pretty easy to burn the walnuts. But that can be solved by being patient (don’t turn the heat to max) and staying around the pan. Also, it’s best to use a thinner pan (not cast iron). Cast iron takes a lot time to heat up. By the time you’re pan is finally hot, your nuts are almost down. A thinner pan works better.

Pre-heat the pan on a medium-high heat. Do not add any oil or the like, just add the walnuts. Shake them regularly and turn them around to assure even heating. They should be done within 5 minutes.

baking walnuts
A simple but effective method: roast your walnuts in a frying pan. Simple and quick but cumbersome for large quantities.

3. Oil roasting – Deep-frier

Next up is to fry your nuts in hot oil. Oil is a really good heat transporter so heats up your nuts a lot faster than the oven. Since the oil sits all around the nut, it will also give the most evenly browned/roasted nut.

Generally, this method isn’t used for roasting walnuts. Walnuts are almost always roasted using one of the other methods. This probably has to be with the types of fat present in walnuts which make it susceptible to oxidation.

4. The microwave

Yes, it is possible to roast walnuts using a microwave, although I must admit I haven’t tried it myself (yet). Feel like giving it a try, here are two guides to help you along: method 1 and method 2.

What happens when roasting walnuts?

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.

Softening

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. 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.

Drying

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

Deciding whether to roast your walnuts

In some cases walnuts don’t have to be roasted in advance. Often, that is the case when the rest of your recipe or process serves as a replacement. For instance, nuts on top of a cake batter will be roasted while baking the cake. Not sure whether to roast the nuts on forehand? Think about these following steps:

  • Will the walnuts receive quite some heat treatment during the rest of the recipe? If so, do not roast them in advance. An example would be a walnut bread or a pumpkin bread. The walnuts tend to sit on the outside and roast during baking. If they are pre-roasted, they might actually burn.
  • Does the rest of the recipe contain a lot of moisture in which the walnuts will sit during a heat treatment? Roast the walnuts in advance. The high moisture content will greatly slow down the Maillard reaction so the walnuts won’t brown that much.
  • Won’t there be any further heat treatment? Roast the walnuts in advance for optimal flavour.

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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