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The interest in the link between diet and health, in particular the ability of the soluble fibre of oat to lower serum cholesterol, has increased inter-est in oats. However, most oat exports are in the form of a low value, agricultural commodity. Consumer markets for products such as porridge flakes and ready-to-eat breakfast cereal tend to be dominated by established brands. Nevertheless, there is a market for oat ingredients, such as oat bran, flour and flakes, but these products must be competitive both in quality and price.
Previous studies had shown that cultivar, environmental conditions and processing parameters all affect flake quality (Molteberg et al., 1996; Doehlert and McMullen, 2000; Lapveteläinen et al., 2001; Rhymer et al., 2005). A long-term breeding pro-gramme, combined with favourable growing conditions and a well-developed in-frastructure for the post-harvest treatment of oats, ensure high quality and a good reputation internationally. There are no widely accepted definitions of oat flake quality, or how it is to be meas-ured. Considerable variation exists in the size, thickness, colour, pasting behaviour and water absorption of flakes that are available. Furthermore, there is little knowl-edge on how to achieve a product which matches all aspects of the specification.
Two heat treatments are commonly used in oat processing. The first, kilning, is to stabilise the oat. The second, referred as steam tempering is to sof-ten it during flaking. Whilst the effect of heating on oat starch, soluble fibre, protein and nutritionally important constituents has been studied to some extent, there have been few technological studies. Molteberg et al. (1996) studied the effect of heat processing on the flavour and storage stability of oats, by steaming pre-mois-tened oats at 100°C for 10 minutes and then drying at 100°C in an oven. Oeding (1996) studied the effect of hydrothermal treatment on the physical and physi-cochemical properties of oat flakes using a response surface methodology, with three variables (steam temperature, time and product moisture). Lapveteläinen et al. (2001) reported an extensive study using industrial conditions, but some of the key data presented, such as cultivar names, were coded.
2. AIMS AND OBJECTIVE OF RESEARCH :
The aim of this study was to elucidate the effect of heat treatments on the quality of oat flakes. Since ensuring the stability of oat flakes is not a problem for industrial mills, other parameters such as flake texture and water absorption were taken as measures of flake quality.
3. LITERATURE REVIEW :
The quality of oat products is dependent on a chain of factors that starts at the farm and extends through milling and possibly other additional industrial processing (e.g. muesli or biscuit making) to the consumer. In the absence of other constraints, yield is the most important consideration for the farmer. Total yield is, however, an over simplification and the definition of yield needs to bear in mind the end use of the oats. Since the oat caryopsis (referred to as the groat) does not generally thresh free from the lemma and palea, the hull content has to be considered. Since ancient times there have been cultivars that thresh free from the hulls, and these are known as naked oats. Except in the case of naked oats, the removal of hulls is essential in oats destined for food-uses. Thus, the milling yield or the amount of unbroken groats is more important to the end user than total yield (Doehlert et al., 1999; Groh et al., 2001).
The floral structure of the oat plant is in the form of an open panicle, with spikelets at the ends of the branches. The spikelets can bear up to three kernels, the largest of which is termed the primary kernel and the smallest the tertiary kernel. Sometimes the primary kernel does not develop, giving rise to a double or bosom oat (Youngs et al., 1982; White, 1995), this seems to be mostly influenced by genetic factors but environmental factors also have a role (Doehlert et al., 2002). For industrial processing, a uniform size distribution is desirable.
Generally, cereal quality is considered in terms of chemical indicators, for exam-ple protein, starch and moisture contents. In this work, emphasis is given to the physical determinants of quality, in particular the mechanical properties. These are closely related to the structure of the groat, which has been described in detail by Fulcher (1986). Oat groats, like other cereal grains, have a cellular structure the majority of these are so called starchy endosperm cells. The starchy endosperm is surrounded aleurone layers and the pericarp. The germ is located at one end of the groat, and is separated from the endosperm by the scutellum. The endosperm cells differ from wheat, in that the starch granules are not surrounded by a continuous protein matrix, but the protein is present in the form of discrete bodies. Also the cell walls in the oat endosperm are mostly intact, and contain beta-glucan.
Genotypic variation in yield exists, however environmental factors also have a strong influence on both yield and quality and interactions between these factors (Doehlert and McMullen, 2000; Browne et al., 2003). Some environmental factors, such as the weather, are beyond the control of the farmer, but agronomic practices such as the application of nitrogen or higher seed rates can increase overall yield.
The interactions between oats and other organisms in relation to quality should not be neglected. Diseases, such as crown rust (Puccinia coronata), can affect many quality characteristics of oats, including groat breakage during hulling (Doehlert and McMullen, 2000). Some cultivars are more susceptible to infection than others (Doehlert and McMullen, 2000). Although this fungal disease is prevalent in the Americas (Leonard, 2000; Galbraith, 2004), it does not seem to be as significant in northern Europe. Weeds can also adversely affect oat quality in the field. Wild oats (Avena fatua) are difficult to separate from oats and cause an increase in hull content, thus reducing the quality and the price of the oats. The use of crown rust as a biological control agent for wild oats has been suggested (Johnston et al., 2000), but the risks to oat quality also need to be considered.
Post-harvest handling and in particular drying and storage influence quality before the oats leave the farm. In Finland oats are typically harvested with moisture con-tent around 21 - 23% (Aaltonen et al., 1999). Unless they are dried, they are suscep-tible to both sprouting and microbial spoilage.
The oat flaking process consists of a series of unit operations that are shown in Figure 3. As with any industrial process, mills will adapt this general flow to suit their needs.
Bulk density is used by the miller to control the flaking process, as this relates to many quality parameters. As well as affecting pack filling, bulk density measures flake thickness. The principle method of controlling flake thickness is by adjusting the roll gap. The size of the flakes, or granulation, is determined by the groat size and flake thickness. Flake pieces passing through a 2 mm sieve are an indication of poor flake integrity (Rhymer et al., 2005). Flake breakage is a problem because it results in an inhomogeneous product that causes difficulties in industrial processes and is unattractive to consumers. Certain cultivars produce flakes that are more resistant to breakage (Rhymer et al., 2005) and the smaller flakes obtained from cut groats are also more resistant. In addition, as has been noted earlier, steaming and tempering conditions during flaking also influence flake breakage. Breakage is closely linked to the texture of the flake.
Oat flakes are commonly mixed with cold water or milk, for example during manu-facture of biscuits or snack bars. In these industrial processes, the influence of wa-ter absorption on the consistency of batters and doughs is important, as variations in consistency can affect the spreading of biscuits. This has consequences on pack weight and may present problems during packaging (Hsu, 1984; Machado and Oliveira, 1998).
Flavour is also considered an important quality factor and has been extensively studied (Haydanek and McGorrin, 1986). Of particular relevance is the absence of the rancid, soapy and bitter flavours associated with lipid degradation and oxidation (Hutchinson, 1953) and the presence of a toasted odour and flavour (Lapveteläinen and Rannikko, 2000).
Apart from muesli, oat flakes are seldom eaten raw. Thus the behaviour during cooking is relevant to quality. As was mentioned earlier, oats contain a considerable amount of soluble fibre, and this together with the heat treatments during process-ing has a major influence on the pasting properties. The onset of gelatinisation of native oat flour was found to vary between 59.5 - 62.2°C and to the gelatinisation peak temperature was at 64.4 - 67.1°C (Colleoni-Sirghie et al., 2004). Pasting is the increase in viscosity during cooking and peak viscosity was not observed until the slurry was cooked for some time at 95°C (Colleoni-Sirghie et al., 2004). Onset of pasting for freshly ground, native flour was reported as 68.5 - 84.8°C (Zhou et al., 1999). Kilning and the steam tempering associated with flaking stabilised the past-ing properties during storage as well as prolonging the time to reach peak viscosity and increasing peak viscosity (Zhou et al., 1999).
Quality is dependent on end-use, and porridge remains the main food use of oats in both Europe and the USA, followed by cold breakfast cereals (Pullinen, 1998). Various types are of flakes are available to consumers, from thick flakes prepared from whole groats to small, thin flakes prepared from cut groats that receive special steam treatments to render them instant (Ranhotra and Gelroth, 1995). Industrially produced snacks, such as biscuits and bars also represent a major use for oat flakes. In the European Union (EU) there was considerable growth in this sector in the 1990s (Pullinen, 1998). According to that study oat usage was predicted to increase, but this trend was not observed from food consumption data.
4. Materials and Methods:
4.1. Effect of kilning on the strength of oat flakes (I)
The effect of kilning was tested by preparing oat flakes at three levels of thick-ness from kilned and unkilned groats at a commercial mill (Myllyn Paras Ltd., Hyvinkää, Finland). Kilning was for 2.5 hours at 85°C, and all samples were steamed and tempered for 45 minutes before flaking.
4.2. Dynamic rheological behaviour of oat groat (II)
The effect of heating and cooling on the mechanical properties of oat were tested from samples of regular thickness (0.8 mm) that were machined from individual groats. The machining technique was adapted from Haddad et al. (1998).
4.3. Mechanical properties of steamed groats (III)
Kilned and unkilned groats were prepared from a single batch of oats by Myllyn Paras Ltd. (Hyvinkää, Finland). The samples (about 20 kg) were stored in cool, dry conditions (10°C, 40% RH) in paper sacks. Small sub-samples were taken, as needed and warmed to room temperature before use (21 - 23°C).
Preliminary tests were performed to establish suitable steaming and tempering protocols. Two steaming times (15 and 30s), two oven temperatures (80 and 100°C) and two tempering times (30 and 90 min.). On the basis of these tests, a full fac-torial experimental design was used with a single steaming time (30 s) and three levels of oven temperature (80, 95, 110°C) and tempering time (30, 60, 90 min.).
4.4. Effect of steaming and tempering on flake quality (IV)
Groats were tempered as described above and flakes were produced using a labora-tory scale flaking machine (Ames and Rhymer, 2003). The flaking machine was adapted to suit whole groats. Industrial flaking rolls are typically 400-500 mm in diameter compare with 70 mm for the experimental flaking machine.
4.4.1. Flake quality
Inactivation of enzyme activity was verified by extracting ground oat flakes (0.5 g) in water (100 ml) for 30 min. Peroxidase activity was determined by observing the formation of red colour in guaiacol, in the presence of hydrogen peroxide.
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