During recent years, numerous studies have shown the ability of dietary fiber to
protect the colon from disease and to improve intestinal health. Beneficial health
effects, such as the lowering of blood cholesterol levels and a reduction in the risk of coronary heart disease, are often directly related to cereal β-glucan, one of the major components of dietary fiber found in oat and barley. No definitive theory on the mechanisms of β-glucan during human digestion has yet been established, although it has been suggested that its solution and aggregation behavior, as well as its ability to form viscous slurries in the gut, are important and have the biggest impact.
The aim of the first part of this work was to develop an extraction method for
mixed-linkage β-glucan from oat and barley, in which the effects of extraction on its structure were minimized, and to improve current knowledge concerning the solution and aggregation behavior of β-glucan found in food products.
Relatively high purities were obtained with the extraction method developed. β-
Glucan was characterized using asymmetric flow field-flow fractionation (AF4) in
combination with UV, multiangle light scattering (MALS), differential refractive
index (dRI) and fluorescence detectors to obtain information on molecular size,
molar mass distribution and conformation. The β-glucan analyzed contained high molar masses and large sizes. Cryo-transmission electron microscopy (cryo-TEM) showed the presence of highly aggregated structures in a widely distributed and loose arrangement in both oat and barley β-glucan, whereas barley β-glucan also showed surprisingly well-defined and dense aggregates. No proteins were found to be covalently bound to the β-glucan. However, as potential proteinaceous moieties in the extracts might affect the solution behavior of β-glucan, a study was carried out to investigate the relation between β-glucan and proteins. Aggregation patterns were found suggesting that electrostatic interactions play an important role during aggregation. In further studies, a direct relationship between the two macromolecules was confirmed. β-Glucan and proteins seem to aid each other during passage through the digestive system, enhancing the other’s beneficial health effects. In an in vitro gastric and gastrointestinal digestion study, the digestive enzymes were not found to have any effect on the conformation of oat β-glucan. However, bile acids showed a molecular interaction with oat β-glucan able to cause a reduction in blood cholesterol. The results presented in this thesis shed light on the solution and aggregation behavior of β-glucan and its association with
other biomolecules, which will help in gaining a better understanding of its
beneficial health effects.
The second part of this thesis deals with the utilization of AF4-MALS for molar
mass and size determination of macromolecules, with special attention on
interference resulting from co-elution. Co-elution of only low quantities of very large analytes during AF4 fractionation was shown to cause disturbances in the MALS data, which can lead to errors in the size determination. Pre-injection filtering of samples was investigated to improve the results. A phenomenon often observed when determining the molar mass of highly polydisperse samples with AF4-MALS is a downturn (reduction) in the molar mass with elution time. This is usually regarded as an artifact and not further considered. However, it was shown in this work that this phenomenon might be a correct result arising from the inherent properties of the analyte.