Fractionation of pea flour with pilot scale sieving. II. In vitro fermentation of pea seed fractions of different particle sizes

Pea seeds were ground through a 4mm-screen and then dry sieved. The resulting nine particle size fractions were characterised chemically (crude protein, cell walls fractionation, starch and ash) and physically (granulometric profile, specific surface area, total porous volume and apparent density) and then fermented in vitro using an adaptation of the gas-test method. The fermentation evolutions of the fractions were studied using a gas-test method at the incubation times of 0, 2, 4, 6, 8, 12, 24 and 48h. Cumulative gas production was recorded and pH, soluble carbohydrate (SC) and ammonia (NH₃) concentrations were determined in the supernatant. As pea particle sizes became finer, the rates of gas production increased (from 2.1ml/h for the coarsest to 7.0ml/h for the finest fractions, after 8h of incubation), while pH and SC concentration decreased. Rate of production of NH₃ was higher for the smallest particles in the short term, but increased linearly for the coarsest fractions. The values of cumulative gas production, pH, SC and NH₃ at different times were better explained by chemical rather than by physical criteria. Correlation analyses indicated that lignocellulose was the best chemical predictor of cumulative gas production, pH and SC, while crude protein was the best one for NH₃. Among the physical criteria, the median diameter, and to a lesser extent the specific surface area were found to be the best related to the variations of the cumulative gas production, of the pH, of NH₃ or SC concentrations in the syringes. In the present work, pea particle size was found to have a strong influence on degradation rate and probably on microbial activity. Although the chemical and physical characteristics of the pea fractions explained a large part of the results, it was, however, not possible to separate clearly the respective influences of chemical and physical parameters on particle fermentation.