Im trying to figure out the mM concentration of capric acid in coconut oil. I know that There is 8% capric acid in coconut oil, but I would like to know how much capric acid is in coconut oil in a mM ammount, not a percent. I know the molecular weight of capric acid (decanoic acid) is 172.26

The reason being: I read a study that capric acid solutions are effective against microbes and fungus at a mM concentration of 20 mM. Im wondering if coconut oil's 8% of capric acid converts to roughly around a 20mM concentration of capric acid within the oil.

Thanks very much for your answers!

  • 1
    $\begingroup$ Well, to answer the question properly, we really need to know more information. Does the 8% correspond to w/w, or w/v? And if it is w/w, then we need to know the density of the coconut oil before we can calculate the molarity of the acid. To give an unfounded answer though, whether it's 8% w/w or w/v, it sounds like more than enough to make a 20 mM solution. For comparison, a 8% w/v aqueous solution of NaCl corresponds to 1.37 M, three orders of magnitude larger than the target concentration. Btw, I think you mean microbes, not microbicides. Microbicides are the things that kill microbes :) $\endgroup$ – orthocresol Jun 3 '15 at 20:53
  • $\begingroup$ Thank you very much for your reply and for the correction in my terminology! Unfortunately the study doesnt specify if the 20mM of capric acid was based on a w/v or a w/w calculation. My guess, since capric acid is crystal like in its pure state, then turned into a powder, they are using a w/w calculation. Since I believe most aqueous/powder mixtures would be calculated by w/w. I just measured 100ml of coconut oil and it weighs exactly 90 grams. Thank you for any additional input! Your guestamation makes sense and is good enough for me if there is still not enough info to know exactly $\endgroup$ – Kevin Adams Jun 3 '15 at 21:52
  • $\begingroup$ Also, I found the density (not the weight) of coconut oil is 924.27 kg/cu.m $\endgroup$ – Kevin Adams Jun 3 '15 at 22:02

Thank you very much for a well-specified question. If 8% of coconut oil were free capric acid, then as noted in the comments the units of "%", i.e. w/w, w/v, or v/v, would affect the calculation somewhat, but since capric acid likely has a density that is very close to that of bulk coconut oil, the difference will be slight. 8% w/w, and assuming a density of 0.8 g/mL for coconut oil would yield $0.08~{\frac{\rm g~capric~acid}{\rm g~oil}}\times \frac{\rm mol~capric~acid}{172~\rm g~capric~acid}\times0.8\frac{\rm g~oil}{\rm mL}\times\frac{1000~\rm mL}{\rm L}=0.372\frac{\rm mol~capric~acid}{\rm L}=372~\rm{mM}$.

However, I suspect that the capric acid in coconut oil is not free acid. It is part of triglycerides. Instead of containing capric acid, the oil contains an ester of capric acid with glycerol. Triglycerides are molecules where all of the available 3 hydroxyl groups of glycerol are esterified with a fatty acid. Because triglycerides are large complex molecules, they were historically difficult for chemists to analyze directly. The method of choice for analysis of plant oils was transesterification or ester hydrolysis, converting the triester triglyceride molecules into three smaller monoester molecules that were easier to detect by analysis. The 8% figure you are using is probably the result of such an analysis.

The bottom line is that coconut oil that is "8% capric acid" actually contains very little capric acid, but contains a large amount of glycerol-esterified capric acid. But only free capric acid is likely to be antimicrobial. So the capric acid content of coconut oil is unlikely to make it antimicrobial.

The presence of free fatty acids in plant oils is usually a sign of degradation. In olive oil, an acidity above 0.5% (i.e. of all the fatty acid chains in the olive oil, 1 in 200 are in the free form and 199 in 200 are in the esterified form) is a sign of lower quality. I would imagine the for coconut oil the situation is similar, although the specific numbers used as quality thresholds may be different.

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