Oats are a unique cereal due to its relatively high protein content and its distinct protein composition compared to other cereals. The protein composition and concentrations according to Osborne classification in oat is given in Table 1. While prolamins are the main storage protein in other cereals such as wheat, rye and barley, in oats, the major storage proteins are globulins and prolamins constitute only a minor fraction. Moreover, instead of the more typical cereal prolamines gluten (in wheat) and also zein (in maize), oat prolamins are mainly avenins rendering them relatively safe for sufferers of gluten allergy.

Globulins are the major storage protein in oats. Identification of salt (1 M NaCl) extracts of oat grain revealed existence of 3, 7, and 12S globulins, 12S being the major fraction. In the oat grain, globulins are mainly found in the endosperm, whereas the lysine-rich fractions of the oat globulins are distributed in and around the embryo. Oat prolamins (avenins) belong to the S-rich prolamins and contain low amounts of basic amino acids and high in glutamic acid and proline. Avenins are generally extracted with 70% (v/v) ethanol without reduction of disulphide bonds. In oat grain, prolamin inclusions are present within a globulin matrix in the endosperm but not in the aleurone layer.

Most of the metabolically active proteins, mostly enzymes, in oats are in the water-soluble albumin fraction. Among the enzymes, presence of proteases, maltase, a-amylase, lichenase, phenoxyacetic acid hydroxylase, phosphatase, tyrosinase, and lipases were reported. Also proteinase inhibitors which are considered to be anti-nutritional are present. Albumins contain high levels of lysine, in fact, the highest among the cereal albumins and they contain the lowest levels of glutamine-glutamic acid among the other oat proteins. Their high levels of lysine indicate that albumins are located around the embryo as lysine is mostly found in the embryo.

Table 1. Oat protein fractions according to Osborne protein solubility¹

¹Table remodelled from references Klose & Arendt 2011, Ma & Harwalkar 1984, Robert 1985

*shows wide differences depending on the cultivar and growth conditions (climate, soil quality etc.)

Nutritional quality of oat proteins

When compared with amino acid requirements for children, oats exceed the requirements for all essential amino acids except lysine and threonine (Table 2). However, they fall short when compared with egg and milk proteins. Nutritional value of oat protein products can be improved by supplementation of lysine, threonine and methionine.

Table 2. Essential amino acid (EAA) composition of oats compared with animal proteins and estimated human requirements¹

¹Table remodelled from reference Peterson 2011

Overall, oats provide proteins with high nutritive quality with digestibility higher than 90%, biological value around 75% and net protein utilization of 70%.