Sulphite and oxidative—reductive depolymerization reactions

Addition of sulphite at levels between 0.004 and 1% helps to protect the water soluble neutral polysaccharide guar gum from thermal degradation. The greater the amount of sulphite incorporated the more viscous the guar is post-retorting. Starch granules, from a wide range of sources, swell when heated in excess water. The amount of polysaccharide released from the swollen granule after pasting for 60 min at 95 °C is increased by the inclusion of low levels (~0.01%) of sulphite. For cassava starch, a pronounced minimum in swelling volume is observed at this sulphite level. This minimum has been related to the pro-oxidant activity of the sulphite in contrast to the antioxidant activity shown for the guar. Intrinsic viscosity data indicates that the action of the sulphite is to degrade the polysaccharides within the starch granule and this can be reduced by scavenging oxygen from the system or by the addition of propyl gallate. It therefore appears that low levels of sulphite protect guar gum from degradation but can cause degradation of the starch polysaccharides.