Orange pulp and peel fibres: pectin-rich by-products from citrus processing for water binding and gelling in foods

Pulp and peels are the main by-products during juice extraction of citrus fruits. Typically, they contain large amounts of cell wall polysaccharides and can be a source of dietary fibres which are rich in pectin. In the past, numerous studies have focussed on hydration properties of fruit derived dietary fibres but very limited information is available on their gelling properties. The aim of the present study was to investigate both water binding and gelling properties of two orange fibre preparations from pulp and peel with a galacturonan content >27% and a degree of methoxylation >65%. The water retention capacity (WRC) of these fibres was significantly increased by mechanical pre-treatment but decreased by high temperatures (80 °C) during soaking. Moreover, intrinsic factors of the soaking solution significantly affected the water binding properties: a raising pH increased the WRC whereas the incorporation of calcium decreased it. Pulp fibre showed superior water binding properties compared to peel fibre. Gelling properties were evaluated by means of oscillation measurements as well as texture analyses and revealed that pulp and peel fibres were able to form sugar-acid gels at low pH < 2.7 and soluble solids loads of 55 and 60%, respectively. Pulp fibre exhibited similar gel points like isolated citrus pectin. Fibre gels were, however, less elastic than citrus pectin gels. Possibly this is a consequence of the swollen fibre matrix which might interfere with the pectin gel network.