## 3 Results  
### SCFAs Production during Simulated Colonic Fermentation  
After the in vitro colonic fermentation, the composition of SCFAs was analyzed. SCFAs were produced due to the three substrates, but a small amount of SCFAs was also produced throughout the four vessels due to the carbohydrate components such as starch, inulin, guar gum etc. contained in the simulation medium. Overall, total SCFA production was between 43 to 176 mM, with some variation among different substrates (Figure 1). The fermentation of simulation control (SC), PDX, and lacitol produced mainly acetate, while xylitol fermentation produced mainly butyrate, and this pattern was constant throughout V1-V4.  
Figure 1: Overall SCFA production profile during in vitro fermentation. Polydextrose (PDX), lacitol and xylitol was fermented in vitro by the EnteroMix fermentation model, simulation medium was used as simulation control (SC) (n = 2). The production of acetate, propionate, butyrate and minor SCFAs (iso-butyrate, 2-methylbutyrate, isovalerate, valerate, lactate) was measured. V1–V4, vessel 1 to vessel 4, represent the ascending, transverse, descending and sigmoid colon. Data are presented as mean \(\pm\) SEM (mM). For detailed statistical analysis, see Figure 2.  
Figure 2: Production of total and major SCFAs during in vitro fermentation. Polydextrose (PDX), lacitol and xylitol was fermented in vitro by the EnteroMix fermentation model, and simulation medium was used as simulation control (SC). V1–V4, vessel 1 to vessel 4, represent the ascending, transverse, descending and sigmoid colon. The production of acetate, propionate, butyrate and total SCFAs is presented as mean \(\pm\) SEM of two fermentations (n = 2). Within each vessel (V1–V4), bars with different letters differ significantly at \(p<0.05\) according to Tukey’s test.  
When compared with SC, all three substrates increased total SCFA production significantly except PDX in V1 (\(p\) = 0.106, Figure 2). The total SCFAs production was tripled by latcitol fermentation (\(\sim\)176 mM) and doubled by PDX and xylitol fermentation (\(\sim\)134 mM) from V1 to V3, but it was not significantly different between the 3 substrates in V4 (133 mM, 107 mM, and 109 mM, respectively). For individual SCFAs, although PDX fermentation doubled acetate and butyrate concentrations compared with SC, they were not statistically different. Lactitol fermentation resulted in the highest acetate production (\(\sim\)101 mM, \(p\) < 0.05) in V1-3 among all simulations. Xylitol fermentation did not increase acetate concentration as compared with SC, but resulted in the highest butyrate production (\(\sim\)94 mM, \(p\) < 0.01) except V1. Propionate production was almost constant among substrates and vessels, but only PDX fermentation led to a small increase in V2 and V3 compared with SC (\(\sim\)32 mM, \(p\) < 0.05).  
Despite the differences in actual concentrations, the proportion of each individual SCFA during the fermentation of SC, PDX, and lacttidol did not differ. Xylitol fermentation resulted in significantly lower acetate percent except V1 (14-22%, \(p\) < 0.01) and higher butyrate percent (62-70%, \(p\) < 0.01) due to the production of butyrate (Table 1).