Sensory and Chemical Studies of Lipid Oxidation in Raw and Heat-Treated Oat Flours

Cereal Chem. 73(5):579-587 The storage stability of oat flours (Avena sativa L.) was investigated by sensory and chemical methods. Raw and heat-treated flour samples of three cultivars of oats (Kapp, Mustang, and Svea) were stored at 23°C and 50% relative humidity for 0, 5, 18, and 42 weeks before analyses. Descriptive sensory analysis and analyses of total fatty acids (FA), free fatty acids (FFA) and volatile lipid oxidation products were performed after each storage period. Storage of raw flours for five weeks resulted in 66% FFA but stable levels of flavors and volatile compounds. After 18 weeks, the level of volatiles and FFA was higher, while the samples at 42 weeks had an intense paint flavor, high levels of several volatiles, and reduced levels of FA and FFA. The major volatiles in stored oat flours were hexanal and 2-pentyl-furan. The other carbonyls were mainly aldeCompared to wheat, the lipid content of oats is =5 times higher (Percheron and L6liger 1990) and the lipolytic enzymes are 10-15 times more active (Matlashewski et al 1982). Milling to flours allows lipids and enzymes to react and release free fatty acids (FFA), which are much more susceptible to lipid oxidation than the original lipids. Oats contain a variety of antioxidants, including tocopherols and phenolic acids (Kalbasi-Ashtari and Hammond 1977), and are thus considered to be fairly stable towards nonenzymatic oxidation (Percheron and Loliger 1990). However, high levels of unsaturated FFA and the presence of lipoxygenase favor lipid oxidation. High moisture levels may further enhance the enzymic oxidation, and exposure to heat, oxygen, catalysts, or light accelerates nonenzymic oxidation (Galliard 1994). While most oats for feed are enzyme-active, most commercially processed oats for human consumption are stabilized by heat-treatment. An adequate heat process inactivates lipolytic enzymes and develops the characteristic, pleasant flavor that is associated with highquality commercial oat products. Optimum stability is expected after a heat treatment strong enough to inactivate lipolytic enzymes but mild enough to protect the natural antioxidants in oats and prevent excessive oxygen exposure due to drying (Galliard 1994). The storage stability of cereals may be studied by sensory or chemical methods. Studies of milled, rolled and whole cereals by use of descriptive sensory analysis and chemical methods have been reported in rice (Paule and Powers 1989, Piggott et al 1991), pearl millet (Lai and Varriano-Marston 1980), and oats (Dahl et al 1989; Molteberg et al, in press). Oat oil stability has also been studied (Fors and Schlich 1989). Most of the studies investigated the effects of processing and storage conditions and related the sensory attributes related to various lipid oxidation products. In rice, removal of the bran and aleurone layer increased the storage stability and reduced the levels of FFA, hexanal, and carbonyls