Soya/alginate interactions in extrusion cooking
Previous work has shown that following the inclusion of alginate high in mannuronic acid there is a reduction in the viscosity of soya melt during extrusion, this work attempts to identify the mechanism for this effect. The effect of water content and hydrocolloid inclusion on the denaturation behaviour and water binding properties of soya protein has been investigated, mainly by the use of differential scanning calorimetry. Of a range of parameters investigated, transition onset temperature and enthalpy, change in specific heat function and coopertivity of the transition all show a dependence on initial moisture content. These parameters are all changed if soya is heated in the presence of a high mannuronic acid alginate. With the exception of changes in enthalpy these effects are not seen to the same extent with other added hydrocolloids. These changes have been attributed to the production of water during heating. Measurements of freezable water and sorption isotherms suggest that alginate addition increases the water binding ability of soya isolate after denaturation. When heated at high temperatures only soya and gluten (out of 7 proteins tested) produced measurable quantities of additional water. In the presence of 2% Manucol DM the formation of water was markedly increased in the soya and gluten samples but not in the other proteins. Production of increased amounts of water after alginate addition was demonstrated to be dependent on the quantity of mannuronic acid in the alginate. The dependence of this effect on water activity suggests that it is the result of browning or condensation reactions. Glutamic acid was implicated with the specific reaction between soya and alginate, though in heated soya both loss of lysine and serine were found. Differential rates of degradation were found between a high mannuronic acid alginate and a high guluronic acid alginate after heat treatment. The alginate high in mannuronic acid was shown to depolymerise to a far greater extent. Various estimates for the molecular weight of the heat treated samples were obtained and no correlation could be found between increased reducing capability of the sample and extent of depolymerisation. Furthermore the production of volatiles that may be expected from sugar ring degradation were not found. Increased brown colour formation and water production and concomittant decreased glutamic acid content were found in soya samples extruded in the presence of 1% Manucol DM. It was concluded that the chemical reactivity of glutamic acid with alginates rich in mannuronic acid explains the well established effect of this type of polysaccharide in extrusion processing.