Using Sourdough Breads to reduce Fructans 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 &amp; 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.
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.