What is behind cereal flavour: Case studies on linking sensory and instrumental data

Cereal products are important in the daily diets of consumers. Especially consumption of wholegrain products is health beneficial by reducing the risk of several chronic diseases. However, cereal flavour needs to be tailored to gain hedonic acceptability. Grain varieties and different cultivars have their characteristic flavour, depending indirectly on the growing environment, genotype, differences in harvesting conditions or season. The flavour of native, untreated grain is mild and bland. Generally, the cereal flavour forms in processing, especially in heat treatments. By applying different bioprocessing techniques, such as sourdough fermentation, germination, extrusion or enzymatic treatment, the flavour and texture of the products may be adjusted in desired direction. The knowledge of flavour-active chemical compounds is crucial in understanding the flavour formation. Volatile and non-volatile compounds, such as sugars, amino acids and small peptides, free fatty acids and lipids, and phenolic compounds, compounds influence directly the perceived flavour. Several non-volatile compounds also influence the flavour indirectly through reactions that form totally new flavour compounds in the product, e.g. in Maillard reaction. Descriptive profiling is used to form a general view of the sensory characteristics in the product. This information may be related to flavour-active chemical compounds by using statistical multivariate techniques, PLS regression. However, all chemical compounds are not flavour-active, and the odour and taste thresholds of the substances need to be considered carefully. Some case studies: • Due to the high fat content, a rancid and bitter flavour develops easily in oat during storage. A germination-drying process adjusts effectively the perceived flavour and increases the flavour stability. Stored, deteriorated oat has musty, earthy odour and bitter, rancid flavour; these attributes are closely correlated to free fatty acids and volatile compounds related to lipid oxidation. By contrast, phenolic compounds and volatile compounds derived from protein degradation are related to favourable roasted flavor. • In mechanical milling fractionation rye kernel is separated into fractions, each of them having their characteristic flavour: between the mild tasting innermost, endospermic part of rye grain and the bitter tasting outermost bran fraction, a rye-like flavour without any obvious bitterness is observed. This fraction contains significant amounts of bioactive compounds, such as alk(en)ylresorcinols, lignans and phenolic acids, which may have their input to the perceived flavour. • Different volatile compounds evaporate from the same raw material depending on the applied bioprocessing technique. Thus, sourdough fermented, germinated and milled rye extrudates deviate from each other both in their sensory attributes and their volatile compounds. • Enzymatic treatment is a new approach for modifying the flavour. The intensive, bitter flavour of rye may be caused by small peptides and phenolic compounds, and can be studied by enzyme-aided processing.