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Most (all) recipes I know talk about 50/50 butter/flour for roux. However, at home I'm using something like a 40/60 butter/flour mix. It's thicker, but it works.

Does anybody know what the absolute minimum butter content is for roux, so it will thicken with a low fat content. As far as I understand, the fat is there to align the starch molecules.

Cheers

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Are you measuring by volume or weight? Many people use 1:1 by volume, but others use 1:1 by weight, which is much less fat (I've always gone with a heaping Tbsp of flour for each Tbsp of fat). Also, butter isn't fully fat, so would cut it down even more. I don't know exact proportions it all works out to, though. (it makes a rather thick glob ... almost looks like clay, or like after you've mixed tile grout correctly) –  Joe Nov 20 '10 at 12:21
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The minimum amount is whatever it actually takes to coat the flour. If you've got loose flour that won't incorporate, you haven't used enough. I'm not sure exactly what ratio that is, but I never even measure, personally... –  Aaronut Nov 20 '10 at 18:01
    
@Aaronut - your comment is the closest thing to an answer I've seen so far. –  justkt Nov 21 '10 at 2:17
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I am not sure if you are interested in substitutions, but I have used Earth Balance (Soy-based, 11g total fat per tablespoon) for gravies and have been very satisfied with the results. A tablespoon of butter is also 11g, but Earth Balance markets itself with non-GMO, and better-than-normal-fat-is hooks. –  mfg Nov 21 '10 at 14:34

2 Answers 2

up vote 10 down vote accepted

The purpose of using a roux, as opposed to just plain flour, is to improve the dispersal of starch molecules in the sauce. If you just toss a bunch of flour into a simmering sauce - or do the reverse, pour hot liquid onto dry flour - then you'll immediately start getting gross glutinous lumps and will find it nearly impossible to smooth them out.

Starch is not actually soluble in fat; instead the fat coats the starch molecules, making it difficult for them to clump together when incorporated into a sauce. Making a roux out of the flour (and making sure to give the sauce a really good whisking after incorporation) basically helps to ensure an even distribution of starch so that your sauce ends up smooth, not lumpy.

So to answer your question, the minimum amount of butter (or other fat) you need is however much it takes to fully incorporate the flour. If you've still got dry clumps, you haven't used enough fat. More fat is okay, but if you don't use enough then you'll suffer some of the same lumpiness as if you didn't use a roux at all.

I'm not sure if there's a magic number - it's going to depend on the specific flour you're using and the specific fat. Even if you narrow it down to, say, all-purpose flour and butter, it can depend on the brand of flour, the fat content of the butter, and whether or not the butter has been clarified (which, if you have the time and patience, it should be).

The 1:1 ratio (by volume or mass) gives you a pretty decent safety margin; you won't need more than that, but it's also not so much that it'll make your sauce greasy. But you really don't need to measure it out at all; I don't, I just add fat while stirring until the flour is fully incorporated. Sometimes I might be more generous with the butter simply for flavour purposes.

Bottom line, it's really pretty obvious when you haven't got enough fat. If it looks good, it is good, and after a few sessions without the measuring spoons, you should very quickly get a feel for how much fat you really need.

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I was looking for the 'magic number', but your answer is comprehensive and actually what I do in practice, so thumps up! –  BaffledCook Nov 21 '10 at 8:57

The fat is there to more easily incorporate the flour and add flavour. You can thicken liquids without added fat, it's just more difficult to avoid lumps (and less flavourful). In addition to a smoother mouthfeel, the butter can help brown the flour (for a dark roux) and adds salt and other tasties.

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