Onions are a great source of fructooligosaccharides that are a natural prebiotic that beneficial bacteria of our gut microbiome feed on.

Having read about this I have started eating raw onions but they can be nasty, as for boiled onions - that's kind of easier for me yet nobody likes them anyway, some people actually find it easier to eat onions raw and just find boiled onions disgusting.

So I have ended up seeking a healthy-tasty ways to cook them.

I've found a scientific papers by Roberto Vega and Zuniga-Hansen (http://dx.doi.org/10.1016/j.foodchem.2014.10.119) that says the percent of retention of sc-FOS is greater than 95% at temperatures above 95 °C yet it doesn't seem to make it clear how far above 95 °C can we go.

Olive oil smoke point is 160°C. Is sautéeing onions at near this temperature ok if saving the fructooligosaccharides is what I want?

  • Perhaps you should fry them gently in a low smoke point fat (like butter).
    – Chris H
    Jul 16, 2018 at 6:35
  • 5
    I'd like to remind the close voters that the "no nutrition advice" rule means that we don't answer questions where the answerer's opinion on what is healthy matters. When an OP has decided what measurable substances they want to eat, then asking how to cook food which contains them is allowed, as long as nobody discusses whether the OP's decision is correct. See cooking.meta.stackexchange.com/questions/1217.
    – rumtscho
    Jul 16, 2018 at 8:57

1 Answer 1


To give some perspective on FOS (fructooligosaccharides) denaturation characteristics to sucrose (for a baseline since sucrose is a common molecule),

FOS are water-soluble and their sweetness is 0.3-0.6 times that of sucrose, depending on the chemical structure and the degree of polymerization of the oligosaccharide. FOS are highly hygroscopic and their water holding capacity is greater than that of sucros. The viscosity of a FOS solution is higher than that of sucrose at the same concentration because the greater molecular weight of FOS. The enhanced viscosity of the gastrointestinal content may delay the rate of gastric emptying and the digestion and absorption of nutrients. Their thermal stability also is greater than of sucrose. FOS are highly stable in the normal range of food pH (4.0-7.0).

Source: https://www.ukessays.com/essays/biology/chemical-structure-of-fructooligosaccharides-biology-essay.php

This is comparing the FOS (fructooligosaccharides) to normal sucrose. Specifically, this mentions that the thermal stability is greater than sucrose.

If sucrose has a decomposition temperature of 186°C (367 °F), when it starts to form caramelized onions, then it is safe to assume that you will be able to fry your onions in olive oil with a smoke point of 160°C without denaturing the FOS since FOS has a greater thermal stability.

Source for decomposition temperature of sucrose: https://en.wikipedia.org/wiki/Sucrose

  • 2
    This is a good find! A bit too free with the assumptions - sucrose starts to form caramel at much lower than 186, it is a continuous process, and people regularly take their oil above the smoke point without realizing it - so I wouldn't be firmly convinced of the conclusion. But it definitely has good information, +1.
    – rumtscho
    Jul 17, 2018 at 7:58
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    @rumtscho I guess I should edit my answer to be a little more clear. Sucrose on its own definitely has a much lower caramelization point. But in the context of onions, there are some environmental characteristics which can change the behavior of sucrose. pH will have a significant effect on pyrolysis which browns the onions at a lower temperature, like you mentioned. Since onions have a pH of 5.5 to 6.5 and FOS has stability in that range, we can make the assumption that FOS is stable in the smoke point since cooking temp is below the degradation point sucrose under normal conditions. Jul 17, 2018 at 14:02

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