Whole Grains and Health. Группа авторов
to their richness in starch and proteins, as well as in fibres and micronutrients, cereal grains are considered as an interesting nutritional source to promote better health (Poutanen 2012). Grains are made of numerous tissues with distinct composition, structure and physiological role for the future plant. The major part of the grain (60–85% of the dry grain mass) corresponds to the starchy endosperm (Evers and Millar 2002) and contains the storage compounds, as starch and proteins, for the plant growth. Depending on the cereal, it is covered with one or several aleurone layers (2–10% of the grain mass) containing the major part of the grain vitamins, minerals and molecules with antioxidant activities. Aleurone also plays a critical role in germination with the production of hydrolytic enzymes for the degradation of the storage compounds. It is surrounded by several tissues, called envelopes, with the nucellus, testa and pericarp acting as barriers to water and pathogens for the grain protection as well as being rich in insoluble fibres and phenolic compounds. The last part of the grain (3–7% depending on the cereal) corresponds to the germ and includes the scutellum, the plumule, the radicle and the embryonic axis that leads to the new plant.
Cereal grains are rarely consumed without prior transformation. Envelopes, especially the more external ones, and the germ have to be removed from the endosperm to maintain safety, high shelf‐life and good technological and taste properties of the cereal products. The most external grain tissues may indeed be contaminated by microorganisms (Laca et al. 2006) and undesirable compounds, such as pesticides, heavy metals and mycotoxins (Balinova et al. 2006; Rios et al. 2009; Cheli et al. 2010). Moreover, the germ, which is rich in lipids, can suffer from oxidation.
Grain processing methods are mainly focused on the isolation of the starchy endosperm to produce flour or semolina, the remaining products being recovered in the technological bran and short fractions. Technologies can differ depending on the country, cereal type, food product and industry considered. They range from small batch milling units to continuous milling of large amount of grains. They also differ according to the grain anatomy (presence or absence of a crease) and are dependent on differences between tissue mechanical properties. Semolina and flour fractions therefore mainly derive from the endosperm, whereas bran and shorts mainly come from the envelopes, aleurone layer and germ. However, each fraction also respectively contains part of the other grain tissues in proportions depending on the processing methodology and steps as well as on the sample variability inherent to genetic background and environmental conditions. This means that a comparison between fractions with the same denomination remains difficult without proper characterization of the tissue composition. Traditional methods monitoring ash content or color properties of semolina or flour are found to be limited to monitor the presence of the outer layers (Greffeuille et al. 2005). Moreover, new analytical methods have shown that bran obtained from grain debranning or milling displays a distinct proportion of grain tissues (Hemery et al. 2009).
Recently, efforts have been made to increase our understanding of the grain tissue behavior throughout the fractionation process and to answer the consumer and social demands with the aim to produce cereal fractions with desired properties but without losses of nutritionally interesting compounds. These efforts have led to the development of new tools and strategies to isolate tissues or molecules in order to better exploit the grain resource and increase the nutritional value of corresponding food products. This chapter points out and summarizes this progress, mainly focusing on dry fractionation.
3.2 Cereal technologies to obtain fractions from whole grains
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