# In Vitro Simulation of Human Colonic Fermentation:
## 4 Available Colonic In Vitro Fermentation Models
### Static Colonic Fermentation Models  
Static colonic fermentation models, also called batch models, are the simplest, fastest, and least expensive way to simulate in vitro gut fermentation. Even though no specific equipment is required to perform static colonic fermentations, different ranges of complexity, from glass vessels to bioreactors, can be found. [PERSON] et al. (2020) conducted colonic fermentation in an automatic bioreactor to assay the prebiotic effect of dextran over gut microbiota and its metabolite production [26]. This bioreactor operated under anaerobic conditions at corporal temperature (37 \({}^{\circ}\)C), stirring (150 rpm), and a constant pH around 6.5 that was automatically controlled by adding NaOH. Nevertheless, most of the static colonic fermentation models consist of bottles, vessels, or tubes. They are typically placed in chambers with a gas mixture (H\({}_{2}\):CO\({}_{2}\):N\({}_{2}\)) to maintain the anaerobic conditions, in a shaking incubator, and at corporal temperature (37 \({}^{\circ}\)C) [27; 28; 29]. Regardless the complexity in the design, the fermentation process is performed with a faecal inoculum and a culture medium. Moreover, the compounds submitted to assay, or the non-absorbed compounds in the digesta resulting from previous digestion stages (oral, gastric, and small intestine), are incorporated into the reactor, leading to colonic fermentation. In some studies, the colonic fermentation is used to assay the effect of specific compounds (e.g., polysaccharides or oligosaccharides) on gut microbiota growth or the relative abundance of species and genera. In this type of study, the compounds to be assayed are added to the culture medium as the only source of carbon (i.e., as extracts or isolated compounds) [30; 31; 32]. In some other studies, the target is to assay the impact that foods (i.e., complex structural matrices) have on gut microbiota and the metabolitesproduced after a complete digestion process (including gut fermentation). Therefore, an upper gut digestion (oral, gastric, and small intestine) is performed prior the fermentation of the intestinal digesta with the non-absorbed components [33; 34]. It is worth mentioning that static models are discontinuous, which means that there are no volumetric outputs or inputs while the in vitro colonic fermentation occurs. As such, substrates are depleted with time, and the products of the microbial metabolism are accumulated. This leads to a change in the environmental conditions (i.e., oxidation-reduction potential) and eventually, the interruption of bacterial growth and activity. For this reason, the simulation length is usually short (mainly 24 h), even though they could be extended to 48 h, assuming that microorganisms need more than 24 h to degrade some complex structures, such as complex phenolic compounds [35]. These models are usually used to study the interaction between gut microbiota and different bioactive or functional compounds. Table 1 summarizes the applicability of the static colonic models explored in the food science and technology field over the last 10 years.