Processing of oat grainstobeneficial whole grain consumer goods

Interest in the role of whole grain consumption has increased substantially over the past few years due to their health benefits. Despite the reported benefits of whole grain intake, namely stating that regular consumption of whole grain foods are associated with a reduction in the incidence of chronic disease states, consumption of whole grain foods in several Western countries is less than one serving per day.

The 2005 Dietary Guidelines for Americans and Healthy People 2010 recommends the consumption of at least three servings of whole grain (each equivalent to three ounces / 85 grams) per day. According to the Healthgrain Project, following a survey investigating whole grain consumption, it was reported in 2006 that adults over 65 years old consume a weekly average of less than five servings; adults of 24 – 64 eat only a weekly average of 2.5 servings per week with 16 – 24 year old adults only having one serving per week. These surveys not only reported these findings but stated that 95 per cent of British adults and 94 per cent of British children do not eat the recommended daily amount of three servings per day. Lack of public understanding of the health benefits, or insufficient knowledge about whole grains, has been proposed as one of the most important obstacles for increasing whole grain con – sumption. These reported results have highlighted the importance of promoting the whole grain food message. The definition of whole grains, approved and accepted by the American Association of Cereal Chemists International Board of Directors, is “consisting of intact, ground, cracked or flaked caryopsis, whose principal anatomical components, the starchy endosperm, germ and bran, are present in the same relative pro – portions as they exist in the intact caryopsis.”

What are whole grains?

Whole grains are able to deliver a unique nutrient package providing health benefits. These biologically-active compounds beyond dietary fibre include vitamins and minerals, unsaturated fatty acids, tocotrienols, toco – pherols, phytosterols, lignans, sphingolipids, phytin and many antioxidants such as phenolic acids. Grains are the seeds of plants and when they are whole they are made up of three parts: the bran, germ and endosperm, all of which contain vital nutrients. The bran forms the coarse outer layer of the seed and depending on the type of grain, can contribute between 3 – 30 per cent to the total dry weight of the seed. The bran contains concentrated amounts of several nutrients including fibre, B vitamins (thiamine, riboflavin, niacin and folic acid), minerals (zinc, copper, iron, magnesium and selenium), protein and many other phytochemicals. The starchy endosperm, also known as the kernel, makes up approximately 80 per cent of the whole grain. This part serves as the main energy storehouse for the seed. This section of the seed contains carbohydrates and proteins. Finally, the germ, although the smallest part of the grain, is packed with nutrients. The germ is the part from which a new plant sprouts, and for this reason, holds a rich supply of the following nutrients: minerals, B vitamins, vitamin E and antioxidants.

Although most of grains are similar in structure i.e. all made up of the germ, endosperm and the bran, the relative amount of these components can vary among grains. For example, in maize, bran content was six per cent whereas bran in wheat comprises of 16 per cent of the total seed. A dissected seed can be observed in Figure 1.

Figure 1Whole-grain Kernel: Major structural features of the Kernel presented as a cross-section of the grain

Although oats (Avena sativa L.) are consumed in considerably lower quantities than wheat (Triticum aestivum L., Triticum durum L.) and rice (Oryza sativa L.), this particular grain has the benefit of being generally always eaten as a whole grain cereal.

Whole grain oats

Similar to other cereal members of the Gramineae (the grass family), the oat kernel is just as complex, being organised in a similar structural pattern. The oat kernel also consists of the bran, germ and endosperm. The oat groat, consisting of the embryo, endosperm and outer layers is contained within two floral bracts (lemma and palea). These components become fibrous, dehydrated structures at maturity and are commonly known as hulls. Although this structure of the oat grain is removed for human consumption, it contributes significantly to the total kernel weight (30 per cent). During development of the oat kernel, the hulls tightly enclose the groat to provide protection. The hulls are structurally and compositionally very different during kernel development and maturity. As the kernel is being developed, hulls contain different tissues for nutrient transport such as photosynthetic and vascular tissues. These components aid in the nutrition of the groat. Following maturity of the groat, the hulls become dry and brittle and lack any metabolic activity. Following the removal of hulls, the similarities to other cereal groat morphologies are more obvious. However, a longer, hairier oat kernel is recognised in comparison to wheat and barley kernels. As discussed, the oat groat contains three morphological and chemically distinct components: the bran; the germ and the starchy endosperm (Figure 2).

Figure 2 Anatomical components of a typical oat kernel. Major structural features of the oat kernel presented as a cross-section of the grain. (A), (B) and (C) are higher magnifications of portions of the bran, endosperm and germ, respectively. Reprinted from an original by Fulcher (1986) with kind permission from the author.

Major structural features of the oat kernel presented as a cross-section of the grain. (A), (B) and (C) are higher magnifications of portions of the bran, endosperm and germ, respectively.

The bran

The bran contributes approximately 30 per cent to the dry weight of the oat groat and although not the largest component of the oat groat, the properties of the bran influence the quality characteristics of the groat and contain products that are important to germinating the grain by aiding in metabolic activity. The bran is the outermost layer of the oat seed and envelopes the oat groat. This structural component is composed of the pericarp, seed coat, nucellus, aleurone layer and sub-aleurone layer.

Endosperm

The starchy endosperm may contribute approximately 55 – 70 per cent of the weight of a mature groat, depending on cultivar. Dissimilar to the aleurone layer, the starchy endosperm is considered relatively metabolically inactive. However, although the starchy endosperm is structurally very simple, only consisting of protein bodies and cell wall and containing starch, it does serve as a storehouse of reserves that provides nutrients to the growing embryo during germination.

Germ

Finally, the germ, unlike the starchy endosperm, has similar properties to the bran, being highly metabolically active. The germ is also the structural part of the oat groat whereby a mature plant arises. A large structure within the germ is the scutellum. This is constructed of two distinct tissues: the parenchyma and the epithelium; which represent approximately 80 per cent of the total germ weight.

The intact grain is considered a beneficial whole grain package however, in many circumstances cereals are not eaten as the harvested seed. Cereals need to be processed to make them consumer friendly, however, the historically designed oat milling process ensures that the oat product remains intact as a whole grain structure when converted to whole grain consumer goods.

Processing of the whole grain oat grain to whole grain oat flake

Processing is required prior to consumption, as harvested whole grains are generally not consumed directly by humans. During the refining process, the endosperm is separated from the germ and the bran, leaving refined products very nutrient poor. Importantly, this may remove significant disease preventing nutrients and phytochemicals. However, whole grains retain their nutritious bran and germ and processing to achieve whole grain products may be a beneficial way to ensure important sources of nutrients and phytochemicals are retained. When oat grains are in their intact structure, enzyme activity could be restricted. However, the majority of oats included in oat based food products are oat flakes. The processing of oats to produce these oat flakes from harvested oat grains is essential to produce oat based food products that will attract consumers. Processing is also a method to optimise flavour, colour, texture and appearance in addition to enhancing shelf life of these products. Oats contain a large amount of lipids and in an intact structure, lipids are stable. However, during and after processing these lipids can become exposed and many deteriorative reactions can take place. Therefore, not only are oat products processed to optimise flavour and appearance but the inactivation of enzymes responsible for the hydrolysis of lipids also occurs. Due to the inactivation of lipases, products can be stored for much longer periods of time. If this does not occur, a bitter rancid taste will develop rapidly, rendering oat products inedible.

The processing of oats involves several steps such as intake and cleaning, dehulling, kilning, cutting, steam tempering, flaking and cooling.

In this reports’ nomenclature, grain represents the whole seed including the hull, and groats represent the dehulled seed. Kilned oats represent the product produced by drying and toasting the oat groats. Steel-cut oats are produced by cutting the former product and finally, flaked oats represent the product produced by steaming and flaking of cut groats. The flow diagram to produce these products during the milling process is illustrated in Figure 3.

Intake and cleaning

The arrival of oats from suppliers/farmers is the start of the milling process. Only oats of satisfactory quality will be accepted by the mill.

Incoming oats are tested for bushel weight, moisture content, absence of foreign material, level of seeds and non oat cereals, insects, ergot and taints/odour. Any parameter outside specification limits results in rejection and removal from site. Within this process, sieves are used to remove contaminants on the basis of size. These machines incorporate wire meshes whereby oats are sieved through by a rotary motion. Oats are also passed through a rotary magnet, colour sorter and a de-stoner to remove any metal present, discoloured oats and stones/glass, respectively. The cleaning process also involves the use of a single-slotted cylinder to remove small chips and weed seeds and also a double-slotted cylinder to remove unwanted stock such as wheat and barley. Following these quality checks, oat grains are moved to the impact huller to be dehulled.

Dehulling

As hulls lack flavour, are tough and unpalatable for human consumption, they must be removed by a process called dehulling. Dehulling occurs using an impact huller which completely replaces the stone huller. Oats are fed through a spout into the impact huller which consists of an inner rubber ring. The rotating disc spins at approximately 1850 revolutions per minute and throws oat grains against a rubber ring, removing the outer oat shell, the hull. Detached hulls are removed from the impact dehuller by aspiration. The resultant groats are then polished to remove any remaining pieces of hull material. In many mills, removed hulls are used as a by-product. Polished groats are sent to the next step in the process, kilning.

Kilning

As explained earlier, kilning is a step in the process that aids in the destruction of enzyme systems that can attack lipids within oats, making them susceptible to rancidity. During this process, oats are passed through a kiln slowly where they are dried and partially toasted by a current of hot air. Kilning is normally performed for 90 – 120 minutes at a temperature of 80-105ºC. Despite the primary aim of kilning being to inactivate enzymes, kilning is also important in reducing moisture content of the groats to levels below 13 per cent and also contributes to the development of flavour. Following kilning, kilned oats are either sent to be flaked or sent to be steel-cut prior to being flaked.

Steel-cutting

The requirements of the end product can determine the minimum number of pieces the oat groat is cut into. Steel-cutting involves cutting of the groat by stationary knives within rotating drums. The cut groats are now ready for the flaking process. The size of the groats determines the thickness of flake. Therefore, in many circumstances, many products require a large, thick flake whereby the whole groat is flaked and not cut prior to this. Similar to cut groats, uncut groats move on to the steaming and flaking stage.

Steaming, flaking and cooling

To condition the groats for flaking, a steam process is required to soften the groats. This process allows the groats to be flaked with minimum breakage in addition to completing enzyme inactivation. During flaking, groats are passed between two rolls, with a diameter typical of the flake size required for that product. Flaking of intact groats produce rolls in the region of 0.5 mm-0.8 mm, whereas cut groats allow thinner flakes to be produced (0.25 mm- 0.45 mm). Following flaking, oats are passed to the cooler and the temperature is brought down to approximately 45ºC. This final process allows the final product to be shelf ready and ensures acceptable shelf life.

Conclusion

Whole grain oat kernels are a complex structure similar to all cereal members of the Gramineae family. Within this structure, the composition of the endosperm, germ and bran must stay in the same relative proportion when produced into flakes from the intact caryopsis. The historical, well defined oat milling process, consisting of intake, cleaning, dehulling, kilning, steel cutting and flaking ensures that the beneficial aspects of whole grain cereal remain intact to benefit consumer consumption of these products.

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About the Author

Laura Stenhouse has a degree in Biological Sciences from the University of Edinburgh in addition to a PhD in Nutritional Sciences from the University of Nottingham. Within her PhD, Laura specialised in oat processing and subsequent to completion, she joined PepsiCo International, working for Quaker Oats. During Laura’s four years in Quaker, she worked as a Project Manager on the cereal portfolio, responsible for managing the launching of NPD to market, brand maintenance in addition to Oat Agro. Laura is now the Seasoning Manager with the Developed Markets Team.