Host-adapted lactobacilli in food fermentations: impact of metabolic traits of host adapted lactobacilli on food quality and human health
Introduction
A substantial proportion of the human diet consists of fermented foods, where the metabolic activity of fermentation micro-organisms determines and maintains the safety and quality of the products. Historically, non-alcoholic food fermentations aimed to improve the digestibility, nutritional value and/or the storage life of products [1]; their unique sensory properties maintained their popularity even when alternative processing methods become available. Fermented foods are not only a source of nutrients but also a major source of dietary micro-organisms if the fermentation organisms are not killed by a cooking or pasteurization step after the fermentation [2].
The microbiota of traditional food fermentations is controlled by the selection of raw materials, the product formula and the fermentation processes, and by back-slopping or the use of starter cultures. Back-slopping, the practice of inoculating a fermentation with a previous batch, profoundly alters the composition of fermentation microbiota when compared to spontaneous fermentations. In spontaneous fermentations, fermentation micro-organisms are selected from those organisms that are associated with the raw material or the processing environment [1,3,4]. In contrast, micro-organisms in back-slopped fermentations are challenged by microbiota of the raw materials in every new batch. Every time the raw material or the processing environment introduces a new strain that is more competitive than resident strains, the latter will be out-competed after a few fermentation cycles; a process that results eventually in stabilization of fermentation microbiota after a sufficient number of fermentation cycles [5••]. Once stabilization of fermentation microbiota is achieved, back-slopping maintains undefined, mixed cultures over decades or centuries with remarkable stability at the species or even strain level [5••,6••].
Section snippets
Back-slopping of food fermentations recruits host-adapted fermentation organisms
The origin of fermentation micro-organisms in back-slopped food fermentations and hence the source of ‘contamination’ or inoculation with desirable fermentation organisms is in many cases enigmatic. For example, the microbial community of surface-ripened cheeses, which includes Staphylococcus, Brevibacterium, and Corynebacterium species, is independent of the geographic location but resembles human skin microbiota [1,7,8]; experimental evidence for a human origin of cheese rind microbiota,
Species of host-adapted lactobacilli prevalent in fermented foods
Fermentation control by back-slopping is commonly used in dairy fermentations including cheese cultures, yoghurt, kefir and other fermented milk beverages, and in many cereal fermentations including sourdough fermentations, several African fermentations for production of porridges or beverages, and mash fermentations for production of vinegar or liquor in East Asia [1,16]. Owing to their importance in fermentation control, seed cultures that are used in back-slopped fermentations often have a
Metabolic properties in host-adapted lactobacilli associated with fermented food
Host-adapted lactobacilli harbour lifestyle-associated metabolic traits, including acid resistance, biofilm formation, extracellular hydrolysis of polysaccharides, bacteriocin producing and tetracycline resistance. An overview on metabolic properties of host-adapted lactobacilli that relate to their adaptation to the host is provided in Figure 1.
Acid resistance system is essential for competitiveness of vertebrate-host adapted organisms as colonization of a new host by oral or intestinal
The contribution of metabolic traits in host-adapted lactobacilli to food quality
The metabolic traits of host-adapted lactobacilli that contribute to the flavour, structure, and quality of fermented food are shown in Table 2. Glutamine and glutamate metabolism enhance bread quality by generating glutamate and γ-aminobutyric acid (GABA), respectively. The glutaminase mediated glutamate accumulation exceeds the taste threshold in bread and ripened cheese and thus contributes to the umami taste [72,73]. Dietary GABA has relaxing properties [74,75]. In baked goods, arginine
Note added in proof
The taxonomy of the genus Lactobacillus was revised at the proof stage [111]. This communication uses current nomenclature and mentions the previous classification in brackets on first mention. The generic term “lactobacilli” refers to all species that were classified as Lactobacillus species until 2020.
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Qing Li and Michael Gänzle acknowledge the China Scholarship Council and the Canada Research Chairs program, respectively, for funding. Fazer Oy, the Alberta Wheat Commission, and the Saskatchewan Wheat Development Commission are acknowledged for financial support.
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