This turned into more of an article than a short reply, but it’s definitely worth reading.
Fermentation, especially when working with specific bacterial cultures such as L. reuteri and L. salivarius, is often misunderstood as a fixed, time-based process. In reality, it is a dynamic biological system shaped by interactions between microorganisms, environmental conditions, and the stage of the culture itself.
One of the most common questions is why these two species are combined, rather than using L. reuteri alone. While single-strain approaches may appear cleaner or more “targeted,” they do not reflect how fermentation works in natural systems. In practice, bacteria rarely function in isolation. They interact, support each other, and create a more stable and resilient environment in yogurt, just as they do in our gut. L. reuteri on its own can be effective, but it is also more sensitive to variations in temperature, milk composition, and handling. By introducing L. salivarius, the culture becomes more robust, more adaptable, and more consistent across batches. This mirrors traditional fermentation, where diversity is not a drawback, but a strength.
L. salivarius itself is far from a secondary addition. From a fermentation perspective, it is a highly capable organism. It adapts well to various types of milk, produces lactic acid efficiently, and contributes to a stable, predictable fermentation process. It also tends to produce a milder and more balanced flavour profile. From a biological standpoint, it is particularly interesting because it is naturally present in the human microbiome, both in the gut and the oral cavity. It has been studied for its ability to produce bacteriocins—compounds that help regulate competing microorganisms—as well as for its interaction with the immune system. Like all probiotics, however, its effects are not universal. The outcome depends heavily on the individual’s existing microbiome, diet, and overall health. No single strain can be expected to act the same way in every person.
Another point that often causes confusion is fermentation time, particularly the widely discussed 36-hour method. This approach originates largely from Dr. William Davis’ work, where fermentation was often done using an oven as an incubation method. Ovens, however, are not designed for fermentation. They tend to fluctuate in temperature and lack the stability required for optimal bacterial activity. As a result, fermentation progresses more slowly, and longer incubation times are needed to reach the desired level of acidity and transformation.
In contrast, modern yogurt makers provide a controlled and stable temperature environment. Under these conditions, bacterial activity is more efficient, and the same fermentation endpoint can often be reached in a significantly shorter time. This highlights a fundamental principle: fermentation is not governed by time alone. It is influenced by multiple factors, including milk composition, temperature stability, inoculation strength, and the current stage of the culture.
Because of this variability, time is only a rough guideline. A far more reliable measure is pH. As bacteria ferment lactose into lactic acid, the acidity of the milk increases, lowering the pH. This acidification is what transforms milk into yogurt, affecting both its texture and its preservation. Typically, yogurt forms when the pH drops to around 4.5 to 4.0. Beyond this point, increasing acidity can begin to stress the bacteria and alter the balance of the culture.
Understanding bacterial growth also requires distinguishing between different starting points. When using a freeze-dried starter, both L. reuteri and L. salivarius must go through the full sequence of fermentation stages: an initial adaptation phase, followed by active growth, and eventually reaching their peak. This process takes time, often extending toward 12 to 24 hours depending on conditions.
In contrast, when using already prepared yogurt for reculturing, the bacteria are active, metabolically engaged, and already adapted to the milk environment. In this case, both L. reuteri and L. salivarius effectively bypass the longest initial phase and move directly into active fermentation. Under stable conditions—especially in an electric yogurt maker—this can reduce fermentation time significantly, often to around three to six hours.
This distinction is critical. A freeze-dried starter represents the beginning of the fermentation cycle, while recultured yogurt represents a continuation of it. The same organisms behave differently depending on where they are in that cycle.
Ultimately, fermentation should not be approached as a rigid formula, but as a responsive and evolving system. If a 36-hour fermentation produces good results in a particular setup, it simply reflects the conditions under which it was carried out—often lower or less stable temperature. However, for consistency and repeatability, especially when using controlled equipment, shorter fermentation times are typically more appropriate once the culture is established.
The goal is not to follow a fixed number of hours, but to understand the process itself—how L. reuteri and L. salivarius grow, interact, and respond to their environment. With that understanding, fermentation becomes not only more predictable, but also more adaptable and consistently successful.
It is also important to understand that the goal is not to reach the maximum possible concentration of any single bacterium. Health is not driven by the dominance of one strain, but by the diversity and balance of the overall microbiome. A reduced number of bacterial species—often referred to as dysbiosis—is associated with a wide range of conditions. Simply increasing the quantity of one organism, regardless of how beneficial it may appear, does not correct this imbalance.
Dr. William Davis places strong emphasis on L. reuteri and has described its effects in very positive terms, even referring to it as “remarkable” or “transformative” in certain contexts. However, even within that framework, the broader principle remains: no single bacterium operates in isolation within the human body. The microbiome is an ecosystem, and its function depends on interactions between many species rather than the dominance of one.
In this sense, while L. reuteri may play an important role, it should be seen as part of a wider microbial network rather than a standalone solution. Supporting diversity and balance remains fundamental to improving and maintaining gut health.