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Fractionation diagrams are adapted according to cereal species, cereal grain structure and targeted end‐products. Harvested cereal grains can be naked or surrounded by a husk formed by the palea and lemma and mainly composed of cell wall material (cellulose, hemicelluloses and a high amount of lignin [10–15%]) associated with minerals such as hydrated silica as shown in rice (Hoije et al. 2005; Miller and Fulcher 2011; Friedman 2013). Therefore, when present, a dehulling step is needed to remove the husks, prior to the fractionation diagram. This is encountered for rice but also for barley and oat processing. In rice, the hull is not tightly stuck onto the grain and can therefore be easily removed using rubber roll huskers and an aspirator. This first step produces brown rice from paddy rice. In oat milling, dehulling is based on both impact and abrasion forces using rotors with fins or blades in the dehuller (Girardet and Webster 2011). The objective of this step in the cereal processing chain is to maximize the dehulling efficiency but to minimize the kernel breakage. The hull fraction, which accounts for about 15–25% of the initial grain weight, can be used as a high‐fibre animal feed or for biofuels or furfural production, or incinerated (Girardet and Webster 2011; Friedman 2013). When processing dehulled or naked grains, two main types of fractionation diagrams can be described in relation to the kernel shape, and more specifically with the presence of a crease, where the outer layers are invaginated (see Figure 3.1).

For cereal grains without a crease (e.g., rice), the starchy endosperm is surrounded by the peripheral layers easily removed from the outside. In that case, brown rice, issued from the dehulling step, is used in the whitening step of the process leading to the separation of the envelopes and germ tissues from the harder starchy endosperm to recover entire polished grains. Several mechanisms are used, alone or in combination, to obtain the removal of the outer layers. For example, grains can come into contact with an abrasive roll (abrasion‐type system) or rub against each other (friction‐type system). This friction processing leads to white grains with a more polished and smoother surface, but induces a higher proportion of broken grains that may have implication for later product quality. Pearling machines are equipped with a rotating abrasive stone and a surrounding screen. The degree of pearling is determined by different parameters such as the feeding rate, the distance between the stone and the screen, the retention time as well as the stone abrasiveness (Bottega et al. 2009). After rice processing, fractions consisting of about 20% of hull, 8–12% of bran and 68–72% of white rice (whole and broken) are obtained, depending on the degree of polishing. Hull, bran, germ and fine broken kernels are considered as by‐products. In such a diagram, the starchy endosperm hardness is considered as an important factor that can affect the white rice yield (Lu and Siebenmorgen 1995). Indeed, the percentage of grain breakage is commonly related to the endosperm mechanical resistance. The endosperm hardness appears dependent on genetic and environmental factors, but also on the water content of the grain.

Figure 3.1 Main fractionation diagrams according to the cereal‐grain structure.

Source: Rikard Landberg.

For cereal grains displaying a crease (i.e., barley, wheat, oat and rye) in which up to 25% of the outer layers can be invaginated, another diagram is needed. These grains are split by grinding and the starchy endosperm is recovered from the inside to the outside. The milling process, which is the most effective in the wheat industry, is made of successive controlled grinding and separation steps. At each grinding step, particles are sorted out based on their size and/or density, and further sent to an additional grinding step, leading to a complex flow scheme. In such a fractionation diagram, differences in mechanical properties between the starchy endosperm, the envelopes and germ are key factors in the grain fractionation behavior. In particular, a grain conditioning step is generally carried out to increase the moisture level and plasticize the envelopes leading to bigger bran particles that are easier to sieve. The number of grinding/sieving steps can differ according to the type of cereal and the goals in terms of endosperm extraction yield (the diagram length being directly related to the efficiency of this gradual separation). As an example, in oat milling, only two steps of size reduction are often encountered, whereas a typical wheat milling process combines at least 14 to 17 breaking/reduction grinders (with different configurations) (Wang et al. 2007). A typical rye milling consists of 5–7 breaks and 4–7 reduction steps (Lorenz 2000). The mill flow‐scheme is also different according to the targeted end‐product. For example, contrary to common wheat milling, no reduction steps using smooth roller mills are used in durum wheat milling where flour is considered as a by‐product and semolina the targeted product.

In addition to the processing step selection, depending on the crease presence, dry fractionation strategies also depend on the relative proportion of the different parts of the grain. For example, in corn grain, emphasis can be given to the germ extraction as it accounts for about 10–12% of the grain weight (Watson 1987), whereas it only represents 2–3% in wheat. Corn grain water content is thus first adjusted to about 20% and then processed in a de‐germinator where the bran and the germ are stripped away from the endosperm through abrasion. Parts of the outer layers can still remain attached to the endosperm and are further purified through the milling process, including a subsequent step of grinding and a separation step using gravity tables (Alexander 1987).

If the processing steps mainly allow the extraction of the starchy endosperm from the envelopes and germ, none of the mill streams can be considered as a pure tissue. Therefore, the degree of purity of flour and semolina, coupled to the milling yield, is evaluated to express the global milling efficiency. In the French wheat milling industry, ash content in flour or semolina is used as a conventional marker of the presence of the envelopes or germ as these tissues contain a higher concentration of minerals. However, if minerals are effectively concentrated in these tissues, they are also still present in the endosperm and their amount and distribution varies according to environmental conditions (Lempereur et al. 1997). Therefore, compounds with more specific localization have been identified to better monitor the behavior of each grain tissue during the fractionation process (Pussayanawin et al. 1988; Peyron et al. 2002; Hemery et al. 2009). These tools have been developed for wheat but generalization to other cereals is not straightforward (Barron et al. 2011).