In vitro caecal fermentation and volatile fatty acids production from sugars in rabbits

The aim of this study was to evaluate the fermentability of selected sugar substrates using an in vitro gas production test and volatile fatty acid (VFA) analysis with a caecum inoculum from rabbits. The incubated substrates are glucose, fructose, sucrose, molasses and Molkolac® (high lactose content). In addition to determining the gas production of selected substrates, the VFA concentration was also determined after 8 h of incubation in the inoculum prepared from the caecal content of the rabbit. The gas production kinetics parameters were estimated using the Gompertz model and the maximum fermentation rate (MFR), time of maximum fermentation rate (TMFR), lag phase (Lag), amount of gas produced within 8 h (GAS8), maximum fermentation rate within 8 hours (MFR8), total amount of VFA, proportions of each VFA and the ratio between each VFA were determined. Sucrose and molasses yielded the highest gas and VFA production, with molasses showing a slower but intense early fermentation phase. Molkolac exhibited the lowest fermentability. Fermentation of sucrose and glucose was associated with a higher butyrate content, while molasses and Molkolac led to a higher acetate content. Although monosaccharides are normally absorbed in the small intestine, the results of this study confirm that if some sugars, particularly glucose, sucrose, and to a lesser extent fructose, reach the rabbit caecum, they can undergo intense fermentation comparable only to that of sugar beet pulp. In contrast, molasses and lactose (Molkolac) showed lower fermentation efficiency (lower total gas production), with molasses still making a notable contribution due to its rapid onset of fermentation, while lactose had minimal effects.