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Oligosaccharides production from pea hull fiber using carboxylic acid-catalyzed subcritical water systems
- Author / Creator
- Valdivieso Ramirez, Carla Sofia
Production of pea into pea protein concentrates/isolates has dramatically increased due to changing dietary preferences of consumers, generating substantial amounts of pea hull as an underutilized co-product. Recently, pea hull was labeled as a ‘novel fiber’ by Health Canada due to its proven physiological laxative effect associated with its high content of insoluble dietary fiber. However, its soluble dietary fiber fraction has not been studied in terms of its extractability and utilization for the production of value-added functional ingredients like pectic oligosaccharides (POS). POS, an emerging type of prebiotic, have been obtained by enzymatic or chemical partial hydrolysis of pectin. However, the use of multi-enzyme complexes and extended reaction times as well as the low selectivity of chemical treatments are some of the challenges regarding its potential scale-up. In this thesis research, the use of aqueous di- and tri-carboxylic acids under subcritical water (sCW) conditions was investigated as a green catalytic reaction medium for pea fiber conversion into POS.
The first two studies, carried out as a proof-of-concept, investigated the catalytic effect of aqueous malic and citric acids at sCW conditions on the hydrolysis of model pectic systems of linear polygalacturonic acid (PolyGalA) and branched rhamnogalacturonan (RG). For the sCW hydrolysis, temperatures of 125-155°C and reaction times of 10 to 120 min were evaluated at constant pH 2.6 using both carboxylic acids and pressure of 100 bar. The HPSEC-RID and HILIC-ELSD analyses of the corresponding hydrolysates showed that i) PolyGalA was hydrolyzed in the selected sCW reaction medium at relatively low temperatures (125 or 135°C) and short reaction time (30 min), and that its hydrolytic pattern was characterized by poly/oligogalacturonic acid intermediates with a degree of polymerization (DP) of 8-14 that were subsequently hydrolyzed to oligogalacturonides of 2-7 DP, and ii) aqueous carboxylic acids at 125°C/100 bar favored cleavage of neutral sugar residues from the side chains of RG whereas the scission of RG backbone was evident at 135°C/100 bar where fractions of 4.7 kDa, 2.1 kDa and < 1.4 kDa were prevalent. These results demonstrated that sCW+carboxylic acid systems can catalyze the hydrolysis of pectic model substrates into oligosaccharides.
In the third study, sCW+carboxylic acid systems were used to extract the soluble polysaccharides from pea fiber and further hydrolyze them into oligosaccharides. Aqueous citric acid at 120°C/50 bar and 30 min allowed the extraction of 11.8% (g/100 g starting pea fiber) of pea fiber soluble polysaccharides with 17.4% of galacturonic acid content and esterification degree of 50.9%. Then, the addition of citric and malic acids favored the break down of the extracted pea pectic substrate into oligosaccharides at temperatures higher than 125°C and 100 bar. However, the yields of released 2 -6 DP gluco-oligosaccharides were the highest (20.4%) at 135°C/100 bar/120 min. In addition, a hydrolytic stepwise pattern was observed for pea fiber soluble polysaccharides in aqueous citric acid at sCW conditions.
The last study focused on the scale-up of the process for the sCW production of pea soluble-fiber-derived oligosaccharides at lab scale as well as on the downstream processing of pea soluble-fiber-derived oligosaccharides. Pea soluble-fiber-derived oligosaccharides were successfully produced on a large-scale (600 mL reactor) as their chemical composition and yield were not compromised. Further concentration of oligosaccharides was achieved by tangential-flow ultrafiltration followed by a conventional freeze and spray drying and pressurized gas-expanded liquid drying to obtain pea soluble-fiber-derived oligosaccharides in powder form.
Overall, the findings provide new insights about the use of carboxylic acid-catalyzed sCW systems within a potential pea hull fiber biorefinery as well as for valorization of similar pectin-rich agro-industrial co-products.
- Graduation date
- Spring 2021
- Type of Item
- Doctor of Philosophy