AbstractsChemistry

The consumption of whole grain foods high in fibre is of interest because of the health-promoting effects associated with dietary fibre. Therefore, there is interest in developing new fibre-rich cereal foods. However, these kinds of foods also contain polyunsaturated lipids, which are prone to oxidation. Further, lipids are dispersed in a heterogeneous matrix of starch, proteins and fibre, which increases their tendency to oxidize because of a large surface area and possible contact with prooxidants. The oxidation of lipids decreases nutritional quality and causes the formation of undesirable flavours. Knowledge of the oxidation behaviour of dispersed lipids in solid cereal foods, and of how factors like process parameters, structural features of the products and storage conditions affect lipid oxidation, is limited. In this thesis, the oxidative behaviour of foods with dispersed lipids was studied using two model systems. The first model system was a spray-dried emulsion containing sunflower oil encapsulated in a Na-caseinate-maltodextrin matrix, with either non-cross-linked or cross-linked proteins. The stability of the total and surface lipid fractions was determined during storage under different relative humidities (RHs). Further, the effect of RH on the amount of volatiles released from oxidized spray-dried emulsions was studied. The second model system consisted of extruded cereals produced from either whole grain oats or rye bran (coarse or fine) using different extrusion parameters. Their oxidative stability was studied during storage at 40 ºC, after milling and standardization to RH 33%. The primary oxidation was measured by peroxide values in the spray-dried emulsions and by losses of tocopherols and tocotrienols in the spray-dried emulsions and rye bran extrudates. Secondary oxidation was determined based on volatile secondary lipid oxidation products analysed by static head space (SHS-GC-FID) in the spray-dried emulsions and by head space solid-phase micro extraction (HS-SPME-GC-MS) in the extruded cereals. In addition to the oxidation parameters, enzymatic hydrolysis of lipids in the oat extrudates and the fatty acid composition of all models were studied by measuring the neutral lipid and fatty acid profiles, respectively. Increasing the RH improved the oxidative stability of both the total and surface lipid fractions of the stored spray-dried emulsions. This behaviour was mainly linked to the loss of individual powder particles upon caking and collapsing of the matrix at RH 75%. In addition, excess protein may have delayed oxidation via its radical scavenging activity. At RH 54%, cross-linking of the protein slightly improved the oxidative stability. The profiles of the volatile oxidation products from the spray-dried emulsions analysed by HS-SPME were also influenced by the RH. The effect was related to water-induced changes in hydrophilicity, structure and binding ability of the matrix, and to partitioning and solubility of the volatiles. The highest overall amount of volatiles released was…