Oats are widely regarded as a very healthy food and widely consumed across the planet. There are mainly two commercially grown types called hulless and covered oats. What is significant here is the effect on them when they are turned into rolled oats or oat flakes. Oats generally contain starch which is not the same as starches from other cereals such as wheat and barley.
In this article we not only look at types of oats but also their flavour as it does have a bearing on the quality of the various types.
Whole Oats Or Oat Groats
The basic oat type is the whole oat or oat groat. This oat is the whole kernel with the outer hull removed. They have the longest cooking time. In some cases they need soaking overnight before cooking. A typical cooking time is somewhere between 45 and 60 minutes long.
Steel-Cut Oats Or Groats
Whole oats that are reduced in size to about a third using steel knives. Being smaller, they take less time to cook but longer than rolled oats.
Rolled Oats/Oat Flakes
The oat flake is an oat grain which has been dried in a kiln, steamed and then flattened. They come in a variety of thicknesses. These oat grains can be derived from whole or steel-cut groats. Oat flakes are often used in a variety of foods which makes them extremely versatile and also nutritious. Not only are they found in muesli but also bread and muffins, various baked goods and other cereals.
There is considerable confusion between oat flakes and rolled oats. The distinction comes from the flake thickness. We also have in the market place, quick oat flakes and bavy oat flakes. For many oat flakes are often marketed as just old fashioned oats but this is normally reserved for rolled oats. Oat flakes are a good source of vitamins and minerals, and oat based proteins.
Rolled oats are traditionally known as old fashioned oats/groats and they are also traditionally known as oat meal. These oats which can be whole or split are dehusked and steamed before rolling into flat flakes using heavy metal rollers. The whole oat flakes are usually called ‘jumbo’. The processing stabilises the healthy oil in the oats. They keep longer and the oats cook quicker because of their higher surface area to volume ratio. In fact you can buy rolled oats in restaurant-sized metal cans which are designated as nr. 10 with a reduced-oxygen atmosphere. This helps retain their sensory and nutritional attributes longer. They are one of the foodstuffs designated by the US Dept. of Homeland Security for storage in the event of a natural disaster or conflict.
Quick-cooking rolled oat were actually invented by the Quaker Oat Company in the USA in 1877.
Instant Oat Flakes
These are a form of quick oats that have been precooked, so all you need to do is add hot water. They are derived from instantized steel-cut groats. Before cutting, these oats are given a special proprietary pre-cooking process that makes them rapid cooking. These flakes are typically 0.011 to 0.013 inches or 0.28mm to 0.33mm in thickness.
In commercial cereals, usually there is some sugar pre-added to them as well for flavor. There is almost no cooking time required for instant oats, just add hot water and they are ready. They are also a healthy replacement for breadcrumbs.
The Process Of Producing Oat Flakes
Producing oat flakes involves two rotating rollers. These groats which leave kiln drying are further susceptible to crushing to a powder occurs because of their reduced, low moisture content. To prevent this, the groats are exposed to steam during agitation before rolling with the intention of increasing the moisture content to between 3 and 5% moisture. The process takes up to 20 to 30 minutes and increases the temperature of the groats. In an optimal process, moisture equilibration is achieved with the smallest temperature increase over the shortest time to minimise nutrient degradation (Decker et al., 2014).
The thickness of the flakes is altered by controlling the distance between the rollers. A thinner flake is produced by minimising the distance between the rollers. The thinner the flakes, the more rapidly they cook. What are termed quick-cooking flakes are rolled to between 0.36 and 0.46mm whilst whole-oat flakes are a width of 0.51 to 0.76mm. The thickest flakes are used for muesli and other breakfast cereals.
For quick-cooking oats, the thickness of the flakes varies from 0.7 to 1.2 mm but in some cases will be less than 0.4mm according to Decker et al., (2014).
The flakes are usually passed through an air stream to decrease both their temperature and moisture content which is claimed to return the flakes to 10 or 12% by weight of water. The flakes are passed over a shaking shifter to force and break apart the clumps of flakes and remove fines and smaller flakes.
Steaming the flakes followed by drying causes the starch in the oat flakes to become partially pre-gelatinised. This pre-gelatinised starch rapidly absorbs water a bit more readily than unprocessed starch which also decreases cooking time (BeMiller & Huber, 2008).
Nutrition Of Raw Oats
A 1-cup (81-gram) serving of raw oats contains (USDA Nutrition Site):
- Calories: 307
- Carbs: 55 grams
- Fiber/Fibre: 8 grams
- Protein: 11 grams
- Fat: 5 grams
- Magnesium: 27% of the Daily Value (DV)
- Selenium: 43% of the DV
- Phosphorus: 27% of the DV
- Potassium: 6% of the DV
- Zinc: 27% of the DV
Lipids are important in oats. A raw oat contains between 6% and 11% lipid, of which 80% are unsaturated (Ganbmann & Vorwerk, 1995).
Another question often asked is what is the difference between naked or hulless oats and covered oats. Each is often turned into either oat flakes or rolled oats.
Covered Oats And Naked Oats: What’s The Difference Here?
Most of us in the West are familiar with the species called Avena sativa L. which is the covered oat. In China, the naked or hulless oat predominates and is a species called Avena nuda. Both species belong to the Poaceae family. One of the main advantages is that when threshed, the hull separates easily from the kernel.
Varieties of naked oat include As 78, Caesar, Manu, Nuprime and AJ86/2/1.
The covered oat (Avena sativa) is the most common of the two and consists of a two-part hull which encloses the caryopsis or groat. This hull is made up of two floral bracts called the lemma and palea. These parts enclose and protect the groat.
Varieties of covered or hulled oat include Maris Oberon, BDMY-6, BDMY-7, PD2-LV65, Sargodha-81, HFO-114, Weston-11, FOS-1/29 Che-Chois and Y-2330.
The structure of oats generally and their composition is well described in a review by Miller and Fulcher (2011).
The hull is composed of cellulose and hemicellulose with smaller amounts of lignin and other related phenolic compounds. The groat itself is composed of many different chemical constituents including proteins, starch, lipid, and β-glucans, as well as smaller amounts of vitamins, minerals, phenolic compounds, and enzymes. The concentration and distribution of each components is often specific to particular regions of the groat.
The outer layer of the groat, the bran, includes the pericarp, the testa or seed coat, the nucellus, the aleurone layer, and a portion of subaleurone starchy endosperm.
The bran contains protein bodies, neutral lipids, β-glucans, phenolics, and significant concentrations of niacin, phytin, and aromatic amines.
The starchy endosperm is the primary source of starch, protein, and β-glucans, as well as some lipid.
The germ is typically high in protein and lipid and contains phytin as well.
The hulless or naked oats are thought to originate in China and have been grown there for many centuries. Hulless oats were probably brought from China to Europe. In Britain, they were grown in early Elizabethan times in the mid-1500s.
Covered oats are grown in colder regions of China such as Tibet for fodder.
Naked oat flakes have significantly higher lipid and sodium ion contents, a greater degree of whiteness but lower levels of beta-glucan and iron compared to hulled oat flakes from Western sources which are hulled or covered oat flakes. Naked oat flakes showed significantly higher water absorption index at room temperature when compared with hulled oat flakes (Hu et al., 2014).
The Taste Of Oat Flakes
Oats themselves are perhaps the first port of call when it comes to development of aroma and flavour. Various precursors and enzymes all contribute the initial flavour and are then modified during processing. Native oat contains odour-active compounds like 3-methylbutanal, 2,4-decadienal, and benzaldehyde which are associated with raw oat, weed-hay, and grass-like flavors (Heydanek & McGorrin, 1986).
What ever happens to oats, whenever any heat is applied then similar compounds found in all cereals are usually generated. Their production often depends on the type of drying. It can be fairly certain that various heterocyclic Maillard reaction products such as pyrazines, pyrroles, and furans are generated. These have all been identified with roasted notes and have been found in a range of oat based products which have been dried, rolled and heated (Pfannhauser, 1993; Parker et al., 2000). If the temperature of processing is low enough and high moisture levels are employed these Maillard browning reactions do not produce the roasted aromas. Correspondingly, low humidity and very high temperature is required to product such toasted aromas so there is an important consideration here.
Oatflakes from around the world have been studied from a chemical composition and taste point of view. The best way to evaluate them is as cooked oatmeal. The main sensory properties of oatmeal is usually to examine the thickness of the product, does it adhere to the spoon, what the actual size of the swollen flake particles, the colour – especially how dark it is, level of slipperiness and roughness. Generally aroma and flavour score quite weakly but most people look at sweetness, a cereal note, the presence of any chemical notes and finally the cooked aromas like toastiness. It seems oat flake thickness and cooking time have the biggest impact on taste in an oatmeal form (Lapveteläinen & Rannikko, 2000).
Naked oats contain more protein and lipid but less beta-glucan than covered oats.
Covered oats are claimed to score better on sensory evaluation than naked oats (Hu et al., 2014).
Phenolic compounds in oats are important contributors to flavour. As the drying temperature during production of any flaked oat rises, there is a tendency for these types of compounds to rise. Compounds such as avenanthramides are associated with the fresh flavour of oats (Molteberg et al., 1996b) but these disappear with processing. During drying, the sensory profile of oats really starts to develop. The phenolic compounds in oats apparently accounts for nearly 30% of the variation in the sensory profile. That indicates there is a significant desire to either use oats with different measurable phenolic contents depending on how much of a note is desired.
Lipids are surprisingly important in flavour but they are really only significant when processed oats are stored according to various industrial suppliers. Oats are rich in lipids as we’ve already mentioned when examining their nutrition profile. They are sensitive to lipolysis by enzymes before processing because they possess an active lipase (Elkstrand et al., 1993). So, raw oats tend to deteriorate mainly because of lipid hydrolysis.
To reduce rancidity, the lipase needs to be denatured by heating. However, there is always the possibility of retention of some activity. There is also the likely possibility of lipid breakdown through non-enzymatic means (Elkstrand et al., 1993; Moteberg et al., 1995). This can be through moisture ingress and generally poor storage.
If left at ambient storage, raw oats produce rancid off notes extremely quickly which occurs with all cereals (Heiniö et al., 2002). Heat processing should alleviate this issue. However, it’s also possible to detect bitter notes due to lipid oxidation (Biermann and Grosch, 1979). In some cases, the triacylglycerols which are fat molecules are hydrolytically degraded to release free fatty acids. These free fatty acids are converted to hydroperoxides especially those that are polyunsaturated and then on to secondary oxidation products. The oxidation of the FFAs contributes some of the worst off flavours.
According to most processors, oat flour is claimed to be the least susceptible to rancidity.
One compound that is problematic is hexanal. This is a secondary oxidation chemical produced by lipid oxidation and one of the final compounds to be generated. It has always been linked to rancidity (Heiniö et al., 2002). Hexanal appears to accumulate in heat-treated rolled oats during storage but fortunately only produces small sensory issues. There is some thought that there is an intrinsic mechanism that removes hexanal by binding it to less volatile compounds such as hexananoic acid but why this should be significant given that increasingly hexanal is noticeable is not ever really explored.
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