By John Leech

Artisanal fermented foods such as milk kefir, water kefir, kombucha and sauerkraut, have re-emerged as popular food choices across Western society. This is due in part to their perceived health benefits and an increased appreciation of microbes, including the human microbiota, as a key player in human health. This renewed popularity of fermented foods has led to predictions that the corresponding global market will reach $40 billion USD by 2024 (all fermented foods). The fact that artisanal fermented foods can also be made at home with minimum equipment or experience is another factor pushing their popularity.

Making fermented foods at home, including pickled carrots, pickled onions and garlic, and sauerkraut

Advances in technology and medicine have increased the lifespan of humans, mainly by eradicating or controlling infectious diseases. As a result, chronic, rather than infectious, diseases are now the biggest killers in the industrialized world. It has emerged in recent years that the development and severity of many chronic illnesses are associated with the differences in the composition and activity of the gut microbiome of those individuals. However, in many cases it is not yet clear if such differences reflect cause or effect. Regardless, it is clear that for many disease states, individuals have an altered gut microbiota, including in some cases a lower microbial diversity, relative the healthy controls. As a result, microbiota diversity has attracted attention as a potential biomarker of gut health.

Interestingly, chronic diseases are experienced to a lesser extent by populations that live a more traditional, less Westernised, lifestyle. Examples include the Yanomami of Venezuela and populations in Burkina Faso. It has also been noted that individuals from these societies have higher microbial diversity in their guts. There are several reasons proposed as to why these lifestyles facilitate higher gut diversity. For a start, their diets contain much more fibre than Westernised diets. Secondly, they do not add preservatives to their foods to prevent microbial growth. They also do not sanitize their environment with cleaning agents. A high fibre diet coupled with this increased exposure to microbes, both through diet and environment, is perhaps how these populations maintain a high level of gut diversity. It comes at a cost though, and traditional populations have a higher chance of succumbing to infectious diseases.

It could be argued that artisanal fermented foods offer a solution to problems faced by both industrial and traditional populations. Fermented foods can mitigate some of the loss of exposure of environmental and food microbes experienced by Western consumers, while ensuring that there is not an increased risk of foodborne disease from food pathogens.

To determine if fermented foods can indeed be used to enhance human health through exposure to specific microorganisms, it is first necessary to study what microorganisms are present in the foods. Such research has been performed ever since Pasteur and Lister set out to show that microbes were responsible for fermentation in the late 19th century. Ever since, scientists have been studying the microbiology of food fermentations with a view to optimising the finished products. In more recent years, molecular investigations have shed new light on these ecosystems to unprecedented depths. Although not widely adopted yet, industry has taken an interest in the novel information about the microbiota of foods that is provided by high throughput sequencing approaches. More specifically, these insights have the potential to inform producers of how to alter the flavour or texture of their products or how to improve the health promoting properties of certain foods.

To improve health promoting properties, it is first necessary to determine what these health benefits are and if, for example, fermented foods that contribute to reducing the incidence of some of these chronic diseases? Many large-scale prospective studies have shown a correlation between consuming fermented foods and positive health outcomes. Some intervention studies have also shown a positive health outcome from consuming these foods. However, it is still very early days for human fermented food studies and we need far more placebo controlled, intervention trials. Choosing appropriate controls, and performing blinded studies (where participants are not aware if they are consuming the test or placebo food), also poses a potential problem. Indeed, in order to fully tease apart whether it is the presence of live microbes, or the altered chemistry due to fermentation, might be bringing about a potential health benefit, both a sterilised control, and a control containing all the unfermented raw materials, is necessary.

My own homemade fermented foods, including pickled carrots, pickled onions and garlic, and sauerkraut

One of the biggest drivers of the artisanal fermented food market is their perceived benefit to “gut health”. Fermented foods contain a myriad of lactic acid bacteria, acetic acid bacteria and yeast. While lactic acid bacteria (LAB) are widely used as probiotics, and LAB probiotics have been isolated from fermented foods, not all LAB are probiotic and it cannot be claimed that a specific fermented food contains a probiotic unless the specific microbes found within it can be characterised as such. Indeed, artisanal fermented foods display large variation in their microbial composition and, thus, while one form of a specific fermented food, such as milk kefir, may contain a microorganism that is probiotic, it does not imply that all milk kefirs contain this (or, indeed, any) probiotic. Hence, the merit of in-depth analyses of different foods is clear.

The specific impact of different fermented foods on the human gut microbiota, as a means of improving health, is unclear. However, a handful of studies have shown promising results. Studies have demonstrated that consuming specific forms of fermented dairy, kimchi or sauerkraut, can have a gut microbiota modulating effect. These studies employed less discerning 16S rRNA based analysis and thus were not able to identify if any specific strains were transferred from the food to the gut. Interestingly, in the sauerkraut study, pasteurised sauerkraut had a similar effect on gut composition as non-pasteurised sauerkraut. This could indicate that, in some instances, the substrate (cabbage) or metabolites produced by the fermentation process (postbiotics) are more important than the presence of live bacteria in terms of “gut health”. It was also noted that while microbiota changes occurred, microbial diversity was not shown to significantly increase in any of these studies. Ultimately, far more research is needed in this area.

Overall, it is a very exciting time to be involved with fermented food research. Interest in these foods is growing and initial studies show promising results, but there is much more to be done.

John Leech is a PhD student based in Cork, Ireland. Funded by Teagasc and APC Microbiome Ireland