Sunday, July 18, 2010

Question: Melting points of butters?

p asks in this post: I'm really confused about one thing, though - the melting point of stearic acid is pretty high, 69 C, or so. So how can the melting point of a butter that contains stearic acid be lower than 69 C? Cocoa butter is about a third stearic acid and its melting point is 38 C. Voyageur lists the melting point of shea, which is almost half stearic acid, as 29 to 32 C (and this jibes with my experience of it melting in my pocket!). Deeply confused, help!! :)

What a fantastic question!

Here's a link on this topic at Springerlink (which means you really can't read it all without paying for it, so I'll summarize it). The article states that the melting point of oils and butters containing a mixture of fatty acids have unpredictable melting points but the presence of oleic acid in the butters reduces the melting point of a butter containing palmitic and stearic acid.

If you take something like shea butter that contains about 45% stearic acid (at 69˚C) and include about half oleic acid (13˚C), the melting point will be lower than the stearic acid alone (although we can't predict what that melting point might be). Cocoa butter with 1/3 oleic acid, 1/3 stearic acid, and 1/3 palmitic acid (62.9˚C) will have a higher melting point (38˚C) than the 45%-45% shea butter.

The melting points of our fatty acids increase with the increase in their molecular weights and saturation, so palmitic and stearic acid have higher melting points than oleic and linoleic acid (-5˚C). A combination of unsaturated, lower molecular weight fatty acids and saturated, higher molecular weight fatty acids will produce a butter with an unpredictable melting point - one with a higher melting point than the unsaturated fatty acids but lower than the saturated fatty acids.

Thanks again for the great question!

1 comment:

p said...

Wow, thank you, Susan, but what a bizarre answer, isn't it? What a strange world we live in!

Do you know, is it just oleic acid that affects the melting point of palmitic and stearic acids? I'm guessing linoleic doesn't? The Springerlink doesn't seem to work - I get an Internal Server Error. I'd love to read whatever I can about the topic (for free!).

I'm trying to think through the consequences of this effect, and it's kind of boggling my mind! I guess this means that a balm made with olive oil (high oleic), cocoa butter, and beeswax will be softer than a balm made with the same proportions except replacing olive oil with sunflower oil (low oleic)? Lotionmakers must have to take this effect into account when crafting with stearic - low oleic oils in the lotion would result in a firmer texture than high oleic oils, is that right?

And I'm trying to think through the topic of graininess in shea butter, in light of this effect - and my mind is thoroughly boggled! Is the topic (still) as simple as you described in the comments on your post on melting butters (here: http://swiftcraftymonkey.blogspot.com/2010/07/back-to-basics-aside-on-melting-butters.html)? Namely, I'm thinking that we can't say that the stearic in shea melts at 69 C, such that if you heat shea up to, say, 68 C, then the palmitic will melt but not the stearic, resulting in grains of stearic - this can't be the explanation for grains, because the melting points are all messed up by the oleic, right? But is it still the case that the stearic in the shea melts at a higher temperature than the palmitic in the shea (i.e. though the oleic acid drops the melting point of the other fatty acids, the ordering of the melting points is preserved)? In other words, the melting point of the stearic in shea is decreased to, say, 32 C, but the melting point of palmitic is lower, so if you heat shea to 31 C you'll melt the palmitic but not the stearic, resulting in grains? (This would explain why shea goes grainy - if you had to heat shea up to between the melting point of palmitic alone, 63 C, and stearic acid, 69 C, in order to form grains, well grains wouldn't form at room temperature, would they!) But if this is the case, why would you need to heat shea all the way up to 70 C to temper? Such confusion!

Thanks a bunch, Susan. Your blog is a fount of knowledge!