This is why I give scant regard to the instructions on the wheatbelly site. The many scientific studies I have read about L Reuteri show that growth stops after 16-24 hours, and my microbiologist friend confirms that this is typical of all bacteria - at a certain point the bugs simply run out of substrate to support further growth.
Yes, there is the typical exponential growth of bacteria that levels off when food runs out, and yes the maximum suggested incubation I've seen (other than wheatbelly) is 24 hrs. 24 hrs may well be the maximum useful ferment time. But there is one thing I've noticed researching these parameters too, and that is that in making yogurt - the recommended methods, times, temperatures, milk, additives, and just about everything done or used varies a lot from one author/chef/researcher to another. From this I gather that actually, making yogurt is generally a simple, easy DIY project, simply because this variability allows almost anyone to be successful!
For example, I've read you can't/shouldn't: use UHT milk or use lactose free milk to make yogurt, and the temperature should be at least 105-115 degrees F. And you suggest that 30+ hours is to long to ferment.
Yet I've made two batches now with UHT, lactose free milk at 97-104 degrees F and fermented for 30 hrs in each case, making firm/delicious tasting/looking yogurt. Perhaps it is the prebiotic starch, glucomannan and honey that makes this possible? From what I've read low temperature and longer ferment times give a more acid result and apparently the BioGaia strains like that, or at least the DSM 17938, so am assuming that both strains do.
I looked to this study for insight to the needs of DSM 17938, and as reason as to why I added an adjuvant culture to help nurture the DSM 17938 (and ACC PTA 6475) in the fermentation process, as both should have been chosen to grow together under similar conditions.
"A number of studies have been published concerning insufficient growth and acidification of L. reuteri in milk, and different suggestions have been made concerning the underlying reason for this. Xanthopoulos et al. (2000) showed that L. reuteri didn’t acidify milk at levels of pH 4.5 after 24 hours, which is important in the production of fermented milk."
"The traditional flora of yoghurt consists of both S. salivarius ssp. thermophilus and L. delbrueckii ssp. bulgaricus. In order to achieve the characteristic flavor of yoghurt, the two species are present in equal numbers, approximately. They have a synergistic relationship, as they both grow better together than in pure culture. L. delbrueckii provides small peptides and amino acids by its proteolytic activity, of which valine, with a low concentration in milk, is the most important one. S. thermophilus is weakly proteolytic, and the presence of L. delbrueckii will therefore promote its growth in milk. On the contrary, S. thermophilus produces formic acid and CO2, which enhances the growth of L. delbrueckii. The production of free amino acids could be the foundation of a synergistic relationship with probiotic bacteria, depending on the strain and process conditions (Oliveira et al., 2001). Both S. thermophilus and L. delbrueckii have formerly been used in probiotic cheese in order to improve technological properties and taste of the final product (Karimi et al., 2005). A drawback worth mentioning is the ability of L. delbrueckii to produce acid during cold storage of the cheese, known in the industry as postacidification. This increase in acidity could influence the survival of some probiotic bacteria. The probiotic strain L. reuteri DSM17938 used in this study"
All these (and other) references are just so many words, but the results are what counts. Without doing a bacteria count of what I've produced, this is just conjecture obviously, but it is based on some ideas gleaned from research.... and yes, even what Mr. WheatBelly has to say as well... I mean he say he too has great results!
In any event, may your yogurt ferment well however you chose to do it.
Edited by Oakman, 14 May 2019 - 02:32 PM.