Beta-glucan degradation induced by oxidised lipids in an emulsion model

Cereal β-D-(1→3), (1→4)-glucans are soluble polysaccharides and are known to have health and technological benefits that are highly dependent on viscosity. However, oxidative degradation mediated by radicals has been demonstrated to be critical factor that causes viscosity drop of β-glucan solutions. Radical producing compounds such as lipids commonly existing in aqueous food systems should also be taken into consideration. Lipid oxidation produces lipid hydroperoxides which decompose to produce various highly reactive radicals especially in the presence of transition metals and heat.
This research aimed to study β-glucan degradation induced by oxidized lipids in aqueous environment. Highly oxidative species formed from lipid oxidation were hypothesized to induce oxidative degradation of β-glucan. Two types of oil, purified rapeseed oil and methyl linoleate, were oxidized to produce lipid hydroperoxides which were then used to oxidize β-glucan in the presence of iron in an emulsion model system. The emulsion was made by using an emulsifier Tween 20 (0.1%). Changes in samples containing β-glucan (0.56%), oxidized lipid (1%) and iron (1mM), together with control samples were examined during storage at room temperature and 40 °C. Properties of β-glucan were followed by measuring viscosity and molecular weight (MW). Lipid oxidation status was monitored by measuring peroxide value and hexanal content.
During oxidation peroxide value and hexanal in samples increased which indicated the formation and decomposition of hydroperoxides with production of radicals in the emulsion model system. Methyl linoleate was oxidized and emulsified more easily than rapeseed oil. Samples containing β-glucan, oxidized oil and iron (BBG+OIL+Fe) showed viscosity drop (12%—26%) after 7-day storage at room temperature. Higher viscosity drop (57%—62%) was observed when BBG+OIL+Fe were stored at 40 °C. With oxidized rapeseed oil, MW decrease of BBG+OIL+Fe samples was greater when stored at 40 °C which is consistent to the high viscosity drop, indicating that heat enhanced the oxidation reactions. With oxidized methyl linoleate which had high peroxide value, MW decrease of BBG+OIL+Fe samples at room temperature was also high but the viscosity drop was not as much as expected. Two types of oils showed different behavior in degrading β-glucan but with the same type of oil, samples which contained more oxidized oil had greater molecular weight and viscosity decrease. BBG+OIL samples also showed remarkable viscosity drop when stored at 40 °C for 7 days. Therefore, β-glucan degradation occurred extensively in the presence of oxidized lipid and iron, and it proceeded gradually even in absence of added iron.