Model-based design of reactor-separator processes for the production of oligosaccharides with a controlled degree of polymerisation

Abstract

High molecular mass β−glucans are a valuable functional food ingredient, scientifically proven to offer beneficial effects in terms of lowering cholesterol and attenuating the postprandial glycaemic response of the host upon consumption. The production of its constituents, such as non-digestible oligosaccharides, is scarce. However, they were found to have several positive effects, such as higher fat and bile-binding capacities and significant improvements in antioxidant and antibacterial activities. Therefore, the present work aims to develop and provide reliable mathematical models, which enable the production of non-digestible oligosaccharides with controlled molar mass from oat β−glucan at laboratory and industrial scales.

A novel structured kinetic model for the homogeneous acid-catalysed hydrolysis of oat β−glucan considering the difference in the reactivity of β−(1,4) and β−(1,3) glycosidic bonds in a non-random structure, as well as their positions in the polysaccharide chain, was developed. New kinetic data, including the molar mass distributions and the formation of oligosaccharides of oat β−glucan degradation, were collected. It is shown that the structured kinetic model accurately describes the reaction kinetics. The results reveal that the reactivity of glycosidic bonds decreases with the distance from the nearest chain end. In addition, the β−(1,3) bonds in oat β−glucan were found more susceptible to acid-catalysed hydrolysis than the β−(1,4) bonds.

Size-exclusion chromatography was chosen as the separation method as it offers more degrees of freedom in a fractionation scheme towards a narrower molecular size distribution of the target components. An efficient model describing a size-exclusion chromatographic separation was developed based on a discrete convolution. The developed model allows fast simulation and satisfactorily describes the experimental chromatograms of oat β−glucan hydrolysates comprising a complex mixture of thousands of molecules with different molecular sizes.

An intermittent recycle-integrated reactor-separator system was studied experimentally and by numerical simulations for the production of non-digestible oligosaccharides derived from oat β−glucan with a controlled degree of polymerisation ranging from 15 to 30. The reactor was operated intermittently at 80 °C. Batch chromatography with Sephadex G–25 size-exclusion gel was found suitable for separating the product from reactants and impurities. Part of the reaction mixture was periodically withdrawn and fed to the separation column. Dimensionless operating parameters and equipment design parameters were introduced for analysing the performance of the intermittent reactorseparators. The numerical simulations were performed by combining the reactor and separator models.

The experimental data show that the intermittent recycle-integrated reactor-separator system provides approximately 2.0- and 2.5-times higher yield and purity of the target oligosaccharides than a batch reactor with the same mean residence time of 4 hours. The simulations show that intermittent operation offers higher product yield and purity than continuous operation when the mean residence time in the reactor is long.

The present work is expected to serve as a theoretical guide to produce polysaccharides or oligosaccharides with a controlled degree of polymerisation from oat β−glucan. The models developed in this work are flexible enough to enable predicting the formation of degradation molecules of any size. In light of the results, it is demonstrated that an intermittent process might be preferred over a continuous process when high yield and purity are required.