Fruit-based functional foods I: production of food-grade apple fibre ingredients

With the increased consumer interest in fibre-enriched functional foods, industrial-scale methods for functional fibre production are demanded. The development of a food-grade fibre preparation method at lab scale that is feasible for industrial scale-up is a pre-requisite. This paper describes two lab-scale fibre preparation methods that have potential to be scaled up to industrial setting for the production of fruit fibres containing desired bioactives and functionality. The two methods, one aqueous and the other ethanolic, were used to isolate fibres from Granny Smith apples (Malus domestica Borkh cv. 'Granny Smith'). In the aqueous method, ground apple tissues were suspended in HEPES buffer (20 mM, pH 6.5), and then mechanically ruptured using an Ultra-Turrax and ring grinder. Between steps, the cell-wall materials were washed with the HEPES buffer. In the ethanolic method, ground apple tissues were stirred in 72% ethanol at 4 °C, filtered, re-suspended in 72% ethanol and then washed. Microscopic examination and chemical analysis were performed on the resultant fibres. The aqueous method produced natural and uniform dietary fibres in the form of plant cell walls containing 0.282 g uronic acid per g dried fibre. By comparisons, the ethanolic method produced crude fibres containing only 0.182 g uronic acid per g dried fibre, the lower uronic acid content indicating the presence of impurities. Thus, the aqueous method appeared to be advantageous in terms of the retained pectic polysaccharide content and cost-effectness for industrial scale-up. Further characterisation using Folin-Ciocalteu assay and high performance liquid chromatography indicates that the fibres obtained by the aqueous method contained significant amounts of phenolic compounds (2.6 mg catechin equivalent per g dried fibre). These results suggest that fibres obtained by the aqueous method may be more suitable for functional food applications where fibres with high pectic polysaccharide and beneficial phenolic antioxidants are preferred.