Fermentation – Does Natto Really Need to be Fermented at Body Temperature?

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.

If you can't find a reusable sour cream starter, you can use buttermilk starter. Some bloggers and biology/chemistry professors just use fresh active buttermilk as a starter rather than ordering some online. If you look at the various labels and product pages, you will find that both the buttermilk and sour cream starters contain the same four cultures: Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Lactococcus lactis biovar diacetylactis and Leuconostoc mesenteroides subsp. cremoris.

In the Q & A for their buttermilk starter, culturesforhealth.com also provides this tip for using it for sour cream:

It is best to maintain it in regular whole milk, rather than reculturing cream long term.

On another page, Dr. Frankhauser goes on to say:

SOUR CREAM is produced by the same bacteria as buttermilk, but the starting milk product is pasteurized light cream. Bacteria are less numerous than in buttermilk.

This may be related to the lower concentrations of lactose found in unfermented milk products that contain higher percentages of fat. My best guess is that companies don't market the sour cream starters as heirloom or reusable because starting the new batch from the last batch may not reliably provide results that are as robust over time as their other products. If you just use milk for your starter, you can reuse it over time.