# A Model for Probiotic Fermented Food Production
## 1 Introduction  
Research on the gut microbiota (GM) over the past two decades has shown that it functions like another organ with important influences on human health and likely evolution [1, 2, 3, 4, 5]. A healthy GM has been shown to be responsible for improving the digestion of substances such as fiber and is also significantly involved in the production of functional metabolites and substances necessary for life, such as amino acids, vitamins, and short-chain fatty acids (SCFAs) [6, 7, 8, 9, 10]. Numerous studies have reported complex connections between diet, the GM, and human health [3, 11, 12, 13, 14, 15], including diet-mediated effects on metabolic function, pathogen resistance, immune system development, and regulation of gastrointestinal functions and associated diseases (e.g., food allergies, gastrointestinal inflammation, neurodegenerative disease, and tumors) [16, 17, 18, 19, 10]. Many of these effects are likely a result of the interaction between bacteria and host intestinal epithelial cells (IECs), which is considered central for the regulation of immunity at the host-microbe interface [19, 20], affecting the host immune response [19, 20]. Given that environmental factors such as diet can strongly influence the composition and function of the GM, there has been widespread interest in developing microbial interventions for a range of diseases and health risks. Probiotics, or \"live microorganisms which when administered in adequate amounts confer a health benefit on the host\" [21, 22, 23], are becoming more common. A variety of strains have been shown to have beneficial effects, such as the mitigation of food allergies, the prevention of diarrhea, reducing cholesterol,and immunomododulation [24; 25; 26; 27]. However, probiotics alone are often not enough to permanently shift the GM [28]. Prebiotics or fibers that feed microbes are necessary to support probiotic strains. As a result, food-based interventions with both probiotic and prebiotic traits are likely to be the most effective. Fermented foods are historically important parts of the modern human diet, and it has been hypothesized that they were a critical part of our ancestors' diets [1]. In addition to their nutritional benefits, fermented foods have both live bacteria and prebiotic properties [29; 30; 31; 32] because they are commonly produced using the fermentation pathways of lactic acid bacteria (LAB), such as _Lactobacillus_ and _Bifidobacterium_. Their consumption has been associated with numerous health benefits [33; 34; 35; 36; 37; 38]. For example, _Lactobacillus_ can reduce the duration or frequency of respiratory and gastrointestinal infections [39; 40; 41] via secretion of anti-pathogen substances, competitive exclusion of pathogens, maintenance of mucosal integrity, and stimulation of systemic or mucosal immune responses [42]. However, these health impacts can vary depending on how a given fermented food is produced, and while epidemiological evidence demonstrates broad health benefits, such as the ability to reduce disease risks and enhance longevity and quality of life [43; 44; 45], few random controlled trials (RCTs) or prospective studies have been undertaken to pinpoint specific health benefits from specific foods and bacterial strains, combined with the relatively low amount of beneficial bacteria in most fermented foods, most manufacturers cannot label their foods as 'probiotic products' [21]. The terms \"fermented food\" and \"probiotics\" cannot be used interchangeably, and \"probiotic\" should only be used when there is a demonstrated health benefit conferred by well-defined and characterized live microorganisms [21]. This distinction is challenging for most fermented food producers, given the cost of RCTs and the large number of experiments required to confirm the health benefits of the probiotic bacteria contained in their fermented foods, even without strain-specific evidence. As a result, most food manufacturers are limited to using the term \"Contains probiotics\" on their labels.