A Two-step 'in vitro' Examination of Some Carbohydrate-rich Substrates for Possible Prebiotic Potential

An 'in vitro' experiment was conducted to screen ten carbohydrate-rich substrates for the following prebiotic characteristics; 1. resistance to gastric acidity, enzymatic hydrolysis and gastrointestinal absorption, and 2. fermentation by intestinal microflora. The substrates where starches and starch by-products of different origins. In the first study, the test substrates were subjected to a 2-step digestion in pepsin/HCl solution for 1.5 hours followed by incubation in potassium phosphate buffer containing pancreatin and amylase for 1,2,3 and 6 hours while in the second study, the cumulative gas production technique was used to study the fermentabilities of the test substrates using bufferred caecal inoculum from 28-day old broiler chicks fed a standard diet free of antibiotics and any of the test substrates. Gas production, fermentation kinetics, pH, ammonia and organic matter loss (OMloss) were measured. The total starch content of the substrates ranged between 57.5% for sweet potato meal and 84.4% for sago starch. While 'in vitro' foregut digestion measured at the end study 1 showed values of 95.04 %, 50.46 %, 22.36%, 42.47%, 82.7%, 68.58%, 42.69%, 54.54 %, 50.88% and 50.8 %, 'in vitro' organic matter disappearance measured at the end of study 2 showed values of 96.58 %, 89.26%, 28.18 %, 53.42%, 92.49 %, 98.28 %, 98.28 %, 92.43 %, 81.98% and 67.13% for sago starch, sweet potato starch, arrowroot starch, rice starch, tapioca, cornstarch, cassava pulp and sweet potato meal respectively. The difference between these values represented estimates of proportion of the substrates that would be fermented in the hindgut 'in vivo'. Results from studies 1 and 2 showed that all the substrates tested were resistant to acidity and enzymatic hydrolysis in the foregut and fermentable by intestinal microflora to varying degrees. Tapioca, sweet potato starch, cassava pulp and wheat starch had the highest resistance to gastric acidity, enzymatic hydrolysis and gastrointestinal absorption and the top five fermentability by gastrointestinal microflora and thus were selected for subsequent studies.