Relationships between physical characteristics of sugar-beet fibre and its fermentability by human faecal flora

Abstract

To determine the role of physico-chemical characteristics (gross porosity, microporosity, particle size, crystallinity) of dietary fibres rich in primary cell walls in their degradability by human flora, two sources of sugar beet fibres were submitted to various chemical and then dehydration treatments. Chemical extraction involving hot dilute acid followed by cold dilute alkali treatments resulted mainly in the removal of pectic polysaccharides (9–49% recovery) at the expense of cellulose (80–100% recovery). Harsh drying, when applied to simply hydrated fibre, did not result in dramatic changes of the pore volume (from 14.9 to 12.7 ml g −1 for the total pore volume, and from 10.4 to 7.8 ml g −1 for the pore volume accessible to bacteria). The presence of pectins in such materials may prevent pore changes at high drying temperatures. In contrast, when following chemical extraction, harsh drying induced a noticeable decrease in the total pore volume (from 14.9 to 6.1 ml g −1) and especially in the pore volume accessible to bacteria (from 10.4 to 3.2 ml g −1). This dramactic changes was ascribed to the distortion and shrinking of cells during drying. Harsh drying conditions following chemical extraction did not affect the crystallinity of cellulose in the fibre. Neither the particle size, nor the crystallinity of cellulose were major determinant factors in degradability of sugar-beet fibres. In contrast, pore volume accessible to bacteria in sugar beet fibres was highly correlated ( R=0.88) with its fermentability. This was consistent with polysaccharides degrading enzymes being closely associated with bacteria and illustrates the importance of matrix physical structure in the control of the physicochemical behaviour of fibre.