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Short version: How can you minimise the fructan content of a spelt sourdough loaf? Is using a freshly rebuilt starter (low acidity, low sourness) ok? Is 9-10 hours of proofing enough, or should I retard the proof?

Long version:

Hi guys, my sister is on a low FODMAP diet, which includes reducing the intake of fructose and fructans. Wheat is quite high in fructans, so it's on the banned list in the FODMAP diet. Spelt, however is much lower in fructans and can be tolerated by more people. My sister can't tolerate "regular" spelt bread, but is fine with high % (33%+) sourdoughs which have fermented for a long time.

I'm just starting out making sourdough breads, but I've gotten to the point where I'm concerned about the impact that my handling of the starter has on the taste of the bread. "Fresher" starters which have been recently rebuilt from about a tablespoon of starter + wheat & water have a great, fruity, mellow taste. Is this fresher starter still effective at breaking down the fructans in the dough, or should I opt for an older, more acidic starter.

Also, does the proofing temperature (room temperature vs fridge temperature) and therefore the length of the proofs impact the levels of frutans in the final loaf, or am I OK as long as the loaf doesn't resist a poke too much at the end of the proofing?

Thanks for your help, Jeremy.

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There isn't really any scientific information available about the fermentation of fructans in sourdough. I did find this page which says that there are some lactobacilli that are effective at breaking down fructans, but the effectiveness varies by strain of bacteria and type of fructans. (There are also many lactobacilli that don't break down fructans: of 712, only 16 were effective.)

While a longer fermentation time will mean more of the sugars in your dough are digested, since you are aiming to ferment a specific type of sugar, the establishment of your starter may be more important. According to this study, a couple of the strains of lactobacilli shown to be effective at breaking down fructans are found in well-established starters.

In short, rather than giving your dough a long fermentation time, it may be more effective to make dough with a high proportion of a well-established starter instead. The starter will have had a longer cumulative fermentation period to break down fructans, and if your starter has the appropriate culture, it will be more effective at breaking down the fructans in the added flour of your final dough. There is little research on this specifically, so this is largely conjecture and entirely dependent on the culture of your starter.

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    Reading this answer, I think it might be worth creating the starter in a high-fructan environment first. "long-established starters" means ones where the bacteria have evolved over many generations. If you want to have a natural selection which favors fructan-eating bacteria, you should let breed them in a high-fructan environment. So, it probably has to be a starter which has fed on wheat for generations. – rumtscho Feb 1 '14 at 12:13
  • The thing about sourdough is, however, that you are not in a sterile, highly controlled environment. Even if you start a culture with a specific strain of lactobacilli and yeasts, your sourdough culture will be "contaminated" with other strains as soons as it comes into contact with flour, water or air. Ultimately, only the strains that are best suited for the food supply, temperatures and other environmental conditions will be left after some time. Chances are those are not the strain(s) you started out with. – Anpan Feb 1 '14 at 22:46
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    @Anpan As you'll see in the study linked, the lactobacillus strain it refers to is one most likely to be found in starters after several days of feedings (ie, it is well suited to the environment of a starter). As rumtscho pointed out, keeping your starter a high-fructan environment will favor the development of microbes suited to digest fructans. – SourDoh Feb 3 '14 at 4:56
  • I was mostly referring to "specially" selected strains you might be able to buy somewhere to start your sourdough with. In the end, you still cannot be sure what strains are in your sourdough unless you frequently sent it in for laboratory analysis. It's likely yes, but not guaranteed, which is probably important in this case. – Anpan Feb 3 '14 at 10:10
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You cannot ferment sourdough indefinitely.

The safest way to minimize the different sugar contents in the dough is just ferment as much of it as possible (though I never ferment more than 50% of the flour). While retarding exposes the dough to longer fermentation times it also exposes the microbes to lower temperatures (which slows fermentation). So unless by retarding you mean to let the dough "ferment to death" and just use it to improve the taste of a yeast bread, retarding will make no big difference in terms of sugar contents.

However, I'd like to tell you that starch is essentially a long chain of sugar molecules which get broken down in your intestines in any case (even by the same mechanism microbes in sourdough do it: using enzymes). So by ingesting starch you will - always - also get non-trivial amounts of fructose as well.

The big advantage to sourdough is that it can break down some sugars (or rather carbohydrates) that we (humans) cannot break down that well. If that's your goal: ferment 50% of the flour, knowing that it will also give the bread a strong taste.

Regarding your starter: As soon as your starter is strong enough to rise the dough on it's own again, the microbe composition and therefore the metabolism is not much different from your original starter. So in your case, it doesn't make a difference.

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Personal experience: longer fermentation times allows me to eat sourdough without problems. Too short: 7–12 hours, and I feel the effects pretty quickly. About right: ~24 hour range, and I can eat and feel good. Just beginning to test the limit of what I can eat in a sitting before I feel the effects, or even if there is a (reasonable) limit.

Monash University's take: they concur with my experience. They say their tests indicate that longer fermentation of wheat bread, which in my understanding would always require a retarding step (correct?), is/can be okay for a low-FODMAP diet. They say a 2 slice max in a sitting. (Source.)

In reading about the process described by some of the baker's Monash has certified, it looks like what might be needed is a fermentation time in the 22-30 hour range. For example: Morpeth Sourdough Bakery says they have a 22 hour ferment time. You can find a link to their site by clicking here for a list of Monash Certified Bakeries.

Differences in process/ingredients should affect the needed time. Perhaps even a much shorter time could be achieved (?) if doing things a different way, such as a higher percentage of levain to begin with.

Just a thought: there are enzymes at work too, so while a cold rise slows down yeast/bacterial activity, enzymes may be the key factor in the longer time reducing the fructans.

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