Studying oxidative stability of lipids in rye bran during extrusion and storage using volatile compounds

Rye bran contains dietary fiber, and lipids. Unsaturated fatty acids make rye bran products vulnerable to oxidation and formation of off-flavor. High level of dietary fiber gives rye bran a rough feel in the mouth. Therefore, extrusion was chosen to make rye bran products more appealing to consumers. The literature review covered flavor of rye, lipids in rye, lipid oxidation in cereals, extrusion of cereals, and principles of the HS-SPME-GC-MS method. The aim of this thesis was to study stability of lipids of rye bran during the extrusion process using different extrusion parameters, and the stability of extrudates during storage. Four experiments were carried out. The first three were to analyze the effect of extrusion die temperature (80, 100, 120 and 140 °C), water content of the rye bran feed (13, 16, 22, and 30%) and particle size of the rye bran (633 ± 13 and 15 ± 1 ?m). In the last experiment, five most interesting samples were selected from the previous three experiments and reproduced. A stabilization of relative humidity was conducted. HS-SPME-GC-MS analysis was run after stabilization to determine effects of the parameters on the initial quality of extrudates, and then every two weeks till the 10-week time point. In total of 88 (max.) volatile compounds were detected. Among them, 30 volatiles were selected for further study based on peak areas at first, mainly lipid oxidation and Maillard reaction products (MRPs): aldehydes, furans and pyrazines. PCAs analyzing the extrudates were run with the 30 compounds as variables. Ten volatiles appeared to have a significant effect on the PCA model. From the first experiment, extrusion temperature of extrusion die had a small effect on the oxidation stability. The indicator of oxidation, hexanal, of all samples had almost the same level at initial point (around 3.0 * 107 counts/s). The increasing tendency during storage was also similar. MRPs were more likely to appear in the extrudates produced at higher temperature (140 °C). From the second experiment, water content had great impact on the oxidation and MRPs. MRPs were better released at 2-week time point and then decomposed or reacted with other compounds. The initial state of oxidation was similar, but during the storage, the lower the water content was, the better the oxidative stability the extrudates had. Comparing the results of the experiment 2 and 3, the level of MRPs was much higher when the particle size of the bran was small, while the oxidative stability was improved in the third experiment, especially in the 16% water content samples. In conclusion, water content and particle size of rye bran had significant influence on the initial state of the extrudates. Samples produced with lower water content and bran with finer particle size had better oxidative stability and higher level of MRPs. This will be important in the developing of extruded rye bran products