Influence of processing on the flavour formation of oat and rye: Dissertation

The use of whole grain foods, such as oat and rye, beneficial to one's health, could be substantially extended if their flavour properties and flavour stability were better known, and they could be modified in a controlled way. The aim of this study was to specify how the flavour of oat and rye is formed, and which processing factors influence it. The understanding underlying the perceived flavour formation of grain was considerably improved through this thesis. This study showed that the relatively mild flavour of native grains was considerably adjusted by applying different processing techniques, such as milling fractionation, sourdough fermentation and baking, germination and subsequent heat treatment as well as extrusion cooking. The flavour components were unevenly distributed in the rye grain with the innermost endosperm being the mildest, and the outer bran layers being the most bitter and intense in flavour. The shorts fraction in the middle of the rye grain that had high bioactivity proved to be most interesting in further applications of new products by having a cereal-like flavour without any obvious bitterness. The importance of the grain fractions on the flavour was still obvious when the fractionated rye flour was used in baking bread. Sourdough baking and germination were used to increase the amounts of phenolic compounds, most of which are beneficial to one's health. In addition, the phenolic compounds considerably influenced the perceived flavour. The sourdough fermentation of rye resulted in sour and intense flavour notes. Heat treatments, such as baking or extrusion cooking of sourdough-fermented rye, further modified the flavour without losing the sourness. The identity and overall acceptance of sour rye bread, when evaluated by consumers, were affected by the acidity and the salt content, even though the effect actually depended on the wheat to rye flour ratio used in the bread. The determination of the various simultaneous interactions between the process parameters was a novel approach. A new, short germination procedure was applied to oat and rye. The germinated oat dried at high temperatures was perceived to have flavour characteristics of roasted, sweet and nutty, and in this study these sensory characteristics were clearly related to particular volatile compounds, such as dimethyl sulphide and isobutanol. An important discovery was that the roasted flavour in oat was gained without any apparent Maillard reaction. Germination and drying extended the shelf-life of crushed oat in comparison with native oat, and the development of the undesired sensory attributes, such as bitterness and rancidity, were closely related to the accumulation of free fatty acids and volatile compounds originating from lipid oxidation. Studies on the flavour formation of germinated rye are exceptional. The statistical data treatment used to relate the perceived flavour of processed oat and rye to the flavour-active volatile substances proved to be successful in linking the results of these techniques, and useful information for developing new cereal products was obtained. In conclusion, the flavour attributes of grain were highly varied. To tailor these attributes according to consumer expectations, appropriate processing techniques need to be chosen to attain the desired flavour characteristics, e.g. roasted or sour. New tools for developing novel palatable whole grain foods with high bioactivity, such as breakfast and snack products, were introduced in this thesis.