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).
3.3 The starchy endosperm fraction – a good source of energy
Starchy endosperm accounts for about 60–85% of the harvested cereal grain by weight (Hettiarachchy et al. 2000; Evers and Millar 2002; Barron et al. 2007; Miller and Fulcher 2011) and can be recovered with milling, as described briefly for wheat in the previous paragraph. The milling fractions can be used as raw material for cereal food making or directly consumed as whole grains after debranning (e.g., white rice or pearled barley). The size of the endosperm particles generated by milling clearly depends on the grain mechanical resistance and on the number of reduction steps. Flours are therefore the main expected product obtained from common wheat, whereas semolina or hominies are produced from durum wheat or corn, respectively.
The starchy endosperm fractions are mainly composed of starch (75–85%) and protein (10–15%), which make cereal food products a major source of energy. These macronutrients also display techno‐functional properties that can be exploited when making food products. In the case of wheat, and rye to a lesser extent, but not with other cereal grains, the storage proteins are able to form a network, known as gluten, when flour is mixed with water. This gluten network displays unique viscoelastic properties, which are particularly important for entrapping carbon dioxide in bread leavening (Shewry 2009). The protein content and protein nature of the starchy endosperm vary within and between cereals. Endosperm fractions are low in vitamins, minerals and total fibre (less than 3–5%) but concentrate the most part of grain soluble fibre fraction (Hemery et al. 2007). Barley and oat endosperm cell walls are rich in β‐glucans, while wheat or rye endosperm cell walls mainly contain arabinoxylans (Evers and Millar 2002). Rice is unique among the cereals in the sense of having significant amount of cellulose in its endosperm. In general, starchy endosperm fractions are also low in lipids (about 0.5–2%), except for oat where the endosperm lipid range 8–18% of the grain weight and corresponds up to 90% of the grain lipids (Barthole et al. 2012). These lipids are not encountered in individual entities (oil bodies), as observed in the aleurone layer or the embryo, but tend to fuse together into a continuous oil matrix located between starch and protein (Heneen et al. 2008).
Different quality criteria are taken into consideration to characterize the flour or semolina fractions to meet the requirements of the end‐use. Protein and ash content, the percentage of damaged starch, the particle size distribution and the product color are typical parameters considered. The flour or semolina quality does not only depend on the grain genetic background and environmental growth conditions, but also on the production parameters, process type and extraction rate. Differences in the flour particle size and starch damage are noticed depending on the mechanical solicitations (impact vs. roll mill, e.g.) during process (Posner 2000). Particle size distribution is also found to be related to the grain mechanical resistance. This character is both dependent on genetic locus Ha (Hardness) that differentiates soft and hard common wheats and the environmental conditions that affects the endosperm porosity, known as vitreousness (Greffeuille et al. 2006; Greffeuille et al. 2007). A higher degree of starch damage is observed in flours from hard common wheat in comparison with soft wheat (Brites et al. 2008). Wheat hardness also affects the flour composition due to differences in milling behavior. Thus, a greater amount of phytic acid, a cellular compound from the aleurone layer, is measured in flour from hard common wheat grains in comparison with the soft wheat (Greffeuille et al. 2005). Moreover, differences in protein content are observed according to the grain structure between the floury and vitreous part of maize or durum wheat (Alexander 1987; Samson et al. 2005).
Flours, with specific characteristics and properties, can be extracted at different stages of the milling process. All of the produced flours are generally blended to obtain the straight run grade flour, but each flour stream can also be mixed in specific proportions to obtain more dedicated flour. Their specific composition has been related to the compositional heterogeneity of the starchy endosperm. Typically, a reverse gradient of protein and starch is observed in starchy endosperm cells with higher protein concentration in the most external part (Pomeranz and Shellenberger 1961). Therefore, as the milling process progresses from the inside to the outside, starch content decreases but protein and ash content increases regardless of the cereal (Gomez et al. 2009; Gomez et al. 2011). This gradient can be accompanied by a change in the protein nature with, for example more β‐ and ϒ‐zeins in the outermost part of the starchy maize endosperm or more low‐molecular‐weight subunits of glutenin (LMW‐GS), alfa‐ and omega‐gliadins in the similar location of the wheat endosperm (Shewry and Halford 2002; Tosi et al. 2011; He et al. 2013).
Different physical post‐treatments (e.g., hydro‐thermal, micronization, air classification) of flours can be carried out in order to either stabilize or modify their functional properties. These treatments can affect both the rheological properties and the biochemical composition of the product. For example, Protonotariou et al. (2014) have demonstrated that the reduction of flour particle size by further regrinding led to an increase the starch damage level and also modify the flour rheology. Superfine powders can be obtained by jet‐milling or more classical grinding technologies, and further air classification allows enriching flour in protein content whatever the cereal species (Mattern et al. 1970; Stringfellow et al. 1976; Dick et al. 1977; Vose and Youngs 1978).
3.4 The germ fraction – the most unstable of cereal fractions
The percentage of dry grain mass corresponding to the germ, taking into account both the embryonic axis and the scutellum, differs depending on the cereal. Maize or sorghum have the highest germ mass values, in the range 12–15% (Evers et al. 1999), essentially due to the scutellum weight. The embryonic axis only represents 1–2%. However, as it is easily detached from the grain by mechanical impact (Posner and Li 1991), embryonic axis is generally called the germ in the milling industry. Germ fractions are known to be rich in proteins, lipids, sugars and minerals as well as tocopherols with vitamin E activity, B‐vitamins (mainly thiamin and riboflavin) and other bioactive compounds with antioxidant properties (Hemery et al. 2007; Brandolini and Hidalgo 2012).
Wheat germ lipids constitute around 10–15% of the germ dry mass and are rich in free fatty acids (5–25%), but are mainly composed of triglycerides with the unsaturated linoleic acid (18:2) being the most abundant fatty acid. Differences in the relative composition and amount of fatty acids are dependent on wheat cultivars, growth and storage conditions as well as methods of extraction. Germ fractions also contain compounds whose nutritional values are always under debate, like phytic acid