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If I roast a selection of mixed nuts, does it significantly change their nutritional content?

Also: Are there any potential food safety issues around roasting nuts (e.g. does it damage the oils)?

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As per the faq, questions about health and nutrition are off topic here. –  rumtscho Jul 24 '11 at 22:04
    
In general cooking any food make it more digestible, and make more nutrients available. Fats don't magically convert on cooking unless you burn it –  TFD Jul 25 '11 at 1:18
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@rumtscho: Despite the awkward wording, there is one specific and testable claim being asked about, which is similar to (for example) this one about oil smoke points. This could do with some editing but I think it's within our guidelines for nutritional questions (i.e. that they need to relate to cooking somehow). –  Aaronut Jul 25 '11 at 3:06
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@Aaronut I would probably have edited the title too, if I had thought of that, because in its current wording, it makes a bad precedent. But yes, good edit and good answer. +1 from me. –  rumtscho Jul 25 '11 at 5:42
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@rumtscho: Thanks, I meant to edit the title too. This one should be better. –  Aaronut Jul 25 '11 at 13:36

2 Answers 2

Here's a good article addressing your question:

http://www.whfoods.com/genpage.php?tname=george&dbid=104

Edit: To paraphrase the article... In a nutshell, yes and no... Roasting nuts over 170 degrees Fahrenheit (75 degrees Celsius) can damage the fats, causing the the fat to break down (and potentially release free radicals)...

The article advises against commercially roasted nuts because you don't know at what temp they were roasted

Roasting nuts under that temperature should maintain their nut integrity :)

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Fair enough... I was in a hurry... –  Rikon Jul 25 '11 at 2:39
    
+1 for the 'nutshell' pun –  BaffledCook Jul 25 '11 at 8:02

Discussion of the long-term health effects would be little more than speculation; however, there is a specific nutritional claim which can be tested, and has been tested in the commercial food-processing setting.

See: Formation of Lipid Oxidation and Isomerization Products during Processing of Nuts and Sesame Seeds:

The aim of the present study was to quantify some nutritional and safety quality parameter changes that take place in nuts (roasting) and sesame seeds (dehulling, roasting, milling, and sterilization) during processing.

...

All these parameters were significantly affected by the different processing stages, especially by roasting and sterilization (tahina). Nut roasting and sesame heat treatment increased the primary (hydroperoxides) and secondary (aldehydic compounds) lipid oxidation products, with the p-anisidine value reaching 6−11.5 and thiobarbituric acid reactive substances 3−5 mg/kg (equiv of malondialdehyde) in the different end products. In addition, roasting led to the formation of CML (between 12.7 and 17.7 ng/mg) and tFAs (between 0.6 and 0.9 g/100 g) in nuts and tahina, which were absent in the raw material. Roasting parameters appear as the critical factor to control to limit the CML and tFA formation in the final product.

You'd have to read the whole study for a detailed analysis of how roasting affects the lipid oxidation products, which are the same products seen in rancid or overheated fats, but there is going to be some effect.

This is really more of an issue in a food plant because they use big machines designed to roast huge quantities of nuts (we're talking 200 kg/h for a low-end, $5000 gas-fired machine). The heat characteristics of one of these machines is going to be completely different from the comparatively slow roasting of a tiny quantity of nuts in a home oven, so don't panic.

To be any more specific, you'd have to specify the kind of nut. The oils from different nuts peroxidize at different temperatures; some unrefined nut oils start to smoke at around 160° C / 320° F (peanut/walnut), others go well into the 200° C / 400° F range (e.g. almonds). In any case, it's very rare that you'd actually be heating the nuts up to these temperatures unless you left them roasting too long, and they'd smell very burnt at that point.

Just don't eat hundreds of pounds of roasted nuts every month, and don't burn them when you roast them, and you'll be fine.

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P.S. I would like to say that roasting improves nutrient bioavailability as TFD mentions in an earlier comment - however, I was unable to find any studies relating to nuts specifically. There are several around beans and legumes, which are somewhat similar nutritionally (in fact peanuts are a legume), but it would be irresponsible to make inferences based on that. –  Aaronut Jul 25 '11 at 3:10

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