Reactivity of lipids during cereal processing

摘要

The study elucidates factors affecting the reactivity of lipids in multiphase food systems, such as processed cereal food products. By using oat and oat products as model materials, both enzymatic and non-enzymatic lipid reactions were studied in aqueous suspensions and in dry flour. The information obtained can be used to improve existing cereal processing schemes and to develop new processing technologies for obtaining high quality food products with enhanced shelf stability.In aqueous cereal processes the most important factors affecting the rate of enzymatic lipid reactions were identified to be the availability of lipid substrate to lipolytic enzymes and the total lipolytic enzyme activities present in the system. The ratio between the lipid substrate and flour material was critical in determining the reaction kinetics. When the ratio of lipid to flour was low, the reaction kinetics was set by the availability of substrate whereas, in the opposite case, the reaction kinetics was controlled by lipolytic activities. Different cereals processes represent examples of these two cases. In different cereal materials, the change from substrate limited to an enzyme limited kinetics occurred at different lipid to flour ratios. A simple oat fractionation scheme was identified where the substrate limited kinetics persisted also at high lipid concentrations. Such a fraction is well suited as a processing aid for aqueous processes in which the reactivity of lipids needs to be controlled.In water activities that are encountered during storage of dry flour, two reactions of lipids were identified in processed oat: enzymatic hydrolysis of acylglycerols, and non-enzymatic oxidation of unsaturated fatty acid moieties acylated to polar lipids. The general practice to prevent enzymatic reactions by hydrothermal inactivation was noticed to actually provoke non-enzymatic lipid oxidation. This, in turn, leads to the development of oxidative rancidity upon prolonged storage of oat products. Thus, in order to prevent the unwanted reactions of lipids upon storage at low water activities, the careful control of hydrothermal treatment is crucial and should meet two requirements: elimination of the endogenous lipolytic activity, and prevention of the subsequent lipid oxidation.