Oat Flavours

Oat flavour and aroma is instantly appealing to consumers and have a unique profile.

Oats (Avena sativa L.) are important raw materials for food, animal feeds and even industrial raw materials, they are also used in the preparation of Scottish regional foods such as porridge and oatcakes. Any processing contributes greatly to the development of aroma and flavour which become important contributors to the overall pleasantness of the product.

Raw oats have a hay-like flavour (Klensporf and Jelén, 2008). Oat flavour is mainly formed during processing when volatiles appear after particular stages such as rolling and flaking,  and are a complex mixture of precursor- and heat-dependent compounds (Sides et al., 2001; Heiniö et al., 2001).

Oat Flavour And Processing

Processed oats are especially characterised in descriptive sensory studies by a sweet, nutty, toasted cereal aroma and flavour that is invariably described as creamy. The oatiness becomes extremely important in cooked oatmeal dishes (Zhou et al., 2000). Oat flakes have the most prominent creamy nutty notes with bread aroma (Klensporf and Jelén, 2008). Headspace solid phase microextraction (SPME) and solvent assisted flavour evaporation (SAFE) techniques have revealed hexanal in oat flakes to have most significant aroma impact and the levels vary 10 fold from 170 to 1670 μg/kg depending on processing. The other key aroma compounds of oat flakes identified using gas chromatography – olfactometry (GC-O) and aroma extract dilution analysis (AEDA) were 2-methyl-3-furanthiol with a roast or cooked oatmeal flavour together with methional, dimethyl trisulfide, 1-octen-3-ol, 2-methyl-3,5-diethylpyrazine.

Oat flakes with their cereal-like, sweet aroma were later analysed using AEDA of a distillate prepared by solvent extraction and vacuum distillation. Nine aroma-active compounds were detected by gas chromatography-olfactometry (GC-O) with AEDA. The most significant contributor of creamy-oaty flavour was  (E,E,Z)-2,4,6-nonatrienal with an extremely low odour threshold of 0.2 pg/L in air. The other underlying and pertinent odours with a low flavour dilution (FD) factor were (E)-β-damascenone, (Z)-3-hexenal, and butanoic acid. Generally, all these flavours are contributed by hydrolysis of lipids (Schuh and Schieberle, 2005).

Problematic flavours such as metallic, bitter, yeast-like and musty usually occur when processing has not been controlled or products have been improperly stored.

Oat flavour is determined by oat source, the degree and intensity of processing, storage of raw and final products, any further end-product processing and preparation in intermediates of the final product. Growing site and cultivation conditions are not as significant in the creation of flavour as is the case with many cereals. Genotypic variations are known as well as environmental impacts on genotype in a number of unpublished reports (Sansome-Smith, 2013 unpubl. data).

The characteristic features of oats are its relatively high fat content compared to other grains such as barley and rye. It also produces a good quality protein and a high soluble fibre content – beta-glucan which has made it an attractive nutritional food source (Welch, 1995). These features of the grain also have a strong influence on oat flavour.

Unfortunately, oats also have a high fat content which makes it prone to oxidation and thus the degradation of various non-volatile free fatty acids leads to production of rancid flavours (Welch, 1995). As well as high levels of fat, they also have large concentrations of lipid degrading enzymes, mainly lipoxygenases which cause severe sensory losses. There is also a significant amino acid content which generate important flavour precursors in Maillard browning reactions with sugars whether natural or added and some interesting phenolic acids. The non-volatile types have a high antioxidant potential.

Roasting with hot air and steaming are tried and tested methods for  inactivating lipoxygenases but do little to improve oat flavour. Infra-red and microwave heating generally reduce lipase and peroxidase activity without causing much loss of either glucans or lipids (Ruge et al., 2012).

The oat phenolics range from simple structures to more complex types which are unique to this grain. In isolation they could act as food additives because they are potent antioxidants that can scavenge reactive oxygen and nitrogen species and chelate various minerals like copper and iron that promote oxidation reactions (Emmons and Peterson, 1999). It must be borne in mind that these phenolics are located in the bran layer of the grains so removal of this portion reduces the nutritional impact that oats can have as well as some more important functional benefits (Slavin, 2000).

A number of oat phenolics are known, ferulic acid, caffeic acid, p-hydroxybenzoic acid, p-hydroxyphenylacetic acid, tricin, apigenin, luteolin, vanillic acid, protocatechuic acid, syringic acid, p-coumaric acid, quercetin, sinapic acid and  kaempferol. These oat phenolics are present as free or simple soluble esters and, to a greater extent, as complex insoluble esters with polysaccharides, proteins, or cell wall constituents. In addition, Collins (1989) isolated and characterized a group of cinnamoylanthranilate alkaloid oat polyphenols, called avenanthramides, which appear to be unique to oats. These are possibly what gives this cereal their unique nutritional properties.

The avenanthramides are antipathogens (phytoalexins), which are produced by the plant in response to exposure to pathogens such as fungi (Matsukawa et al., 2000; Okazaki et al., 2004).

Generally, the free phenolic acids, the alkenyresorcinols and the avenanthramides were extracted into fractions with methanolic acetic acid, 80% methanol and 100% methanol before quantification by HPLC. The total phenolic acids of oats are not as high as in brans of wheat for example. Oat flakes contain about 26 to 27 mg/kg of avenanthramides which is almost double that in the bran at 13 mg/kg (Mattila et al., 2005).

Storage of oats is perhaps more significant than processing because of what can happen to the creation of rancid off-notes. The sweetness disappears (Molteberg et al., 1996). Hexanal, which is always the most abundant volatile in any samples analysed, is commonly used as an indicator for lipid oxidation in cereals (Ekstrand et al., 1993; Pigott et al., 1991; Shin et al., 1986). In combination with other volatile aldehydes, predicting oxidation changes during storage is one of the best routes forward in monitoring storage conditions.

The ideal drying temperature range is between 40 and 50 °C whilst the moisture should be lower than 13% (Marini, 2005).

Oatcakes (like porridge) are a traditional Scottish dish but prone to the generation of ageing flavours – they don’t have as long a shelf-life as they should. It might seem surprising as oats have a high antioxidant level which should protect and stabilise fats and oils from becoming rancid. The types and chemistry of the oat derived antioxidants is well reviewed (Peterson, 2001).

Ways to overcome flavour loss have been extensively covered by the former SCRI (Scottish Crop Research Institute, now The James Hutton Institute). One method tried with little success was the addition of rosmarinic acid which is touted as a natural preservative especially of flavour because it acts as an antioxidant (Cognat et al., 2014). One component, phytate is not a positive contributor to health preventing absorption of minerals in the gut. So, malting is a method to increase mineral bioavailability. By malting the oats for 5 days at 11 °C with subsequent incubation for 17 hours at 37-40 °C, the phytate content was reduced by 98% (Larsson and Sandberg, 2006)- an effective technique indeed !

References

Cognat, C., Shepherd, T., Verrall, S. R., & Stewart, D. (2014). Relationship between volatile profile and sensory development of an oat-based biscuit. Food Chem. 160, pp. 72-81.

Heiniö, R. L., Oksman‐Caldentey, K. M., Latva‐Kala, K., Lehtinen, P., & Poutanen, K. (2001). Effect of drying treatment conditions on sensory profile of germinated oat. Cereal Chemistry, 78(6), pp. 707-714 (Article).

Mattila, P., Pihlava, J. M., & Hellström, J. (2005). Contents of phenolic acids, alkyl-and alkenylresorcinols, and avenanthramides in commercial grain products. J. Agric. Food Chemistry, 53(21), pp. 8290-8295

Moltenberg E.L., Magnus E.M., Bjorge J.M., Nilsson A. (1996) Sensory and chemical studies of lipid oxidation in raw and heat-treated oat flours. Cereal Chem., 73, pp. 579–587.

Okazaki, Y., Isobe, T., Iwata, Y., et al. (2004)  Metabolism of avenanthramide phytoalexins in oats. Plant J. 39 pp. 560–572.

Peterson, D.M. (2001) Oat Antioxidants. J. Cereal Sci., 33(2) pp. 115-129

Piggott J.R., Morrison W.R., Clyne J., (1991) Changes in lipids and in sensory attributes on storage of rice milled to different degree. Int. J. Food Sci. Technol., 26, pp. 615–628.

Shih M.G., Yoon S.H., Rhee J.S., Kwon T.W., (1986) Correlation between oxidative deterioration of unsaturated lipid and n-hexanal during storage of brown rice. J. Food Sci., 51, pp. 460–463.

Slavin, J. L. (2000) Mechanisms for the impact of whole grain foods on cancer risk. J. Am. Coll. Nutr. 19: 300S–307S.

Welch, R. W. (1995). The Chemical Composition Of Oats. In: The Oat Crop, Production And Utilization. Springer Netherlands. Or Chapman & Hall, UK pp. 279-320.

Zhou, M., Robards, K., Glennie-Holmes, M. and Helliwell, S. (2000), Contribution of volatiles to the flavour of oatmeal. J. Sci. Food Agric., 80 pp. 247–254. doi: 10.1002/(SICI)1097-0010(20000115)80:2<247::AID-JSFA525>3.0.CO;2-0