Whole Grains and Health. Группа авторов
bran tissue) or even macromolecule isolation besides histological fractionation. In order to achieve dissociation at this scale size, superfine grinding is essential to disrupt the cellular structure. This can be obtained by modifying the grinding mode as done with jet milling where size reduction is the result of inter‐particle collision, or by changing the brittleness of the raw material with a strong decrease in temperature as occurred with cryogenic grinding (Hemery et al. 2007). Purity of the product can be lowered in comparison with the one obtained after wet fractionation but its structure and properties are expected to be better preserved. According to the purified macromolecule and its distribution in cereal grain, dry fractionation diagrams are carried out from different mill streams (e.g., flour for protein/starch extraction) or specific cereal species (e.g., oat or barley for β‐glucan production).
Subcellular fractionation of the aleurone tissue has been targeted taking into account its richness in compounds with nutritional interest (dietary fibres in the cell walls and micro‐nutrients and phyto‐ chemicals in the cell content). This approach allows the isolation of dietary fibres and their co‐passengers such as hydroxycinnamic acid that is known to display antioxidant properties. Coupling different steps of pin‐milling, sieving and air‐classification on wheat bran separate a fraction rich in fibrous components (coming from the aleurone and the pericarp) and a fraction rich in aleurone cell content (Antoine et al. 2004b). Subcellular fractionation has been further improved by using ultra‐fine grinding (either in cryogenic condition or at room temperature) coupled with tribo‐charging electrostatic separation (Hemery et al. 2011a; Hemery et al. 2011b). Based on different charging properties between fibrous components (aleurone cell walls vs. pericarp tissue) and cellular components, tribo‐charging electrostatic separation has successfully been carried out to recover around 65% of the initial aleurone cell walls in a unique fraction.
Macromolecular fractionation has been mainly focused on starch and protein extraction using air‐classification from cereal endosperm fractions (flours, groats) to recover a fraction enriched in proteins (up to 54%) in the finest particle class (Wu and Stringfellow 1973; Wu and Stringfellow 1992). Letang et al. (2002) have carried out jet‐milling for flour re‐grinding coupled with air classification in order to purify starch in the medium‐coarse fractions of hard and soft wheat flours. Residual protein content in the starch rich fraction has been reduced to 2% for both types of common wheat, but the workflow sheet is longer for hard wheat and the level of starch damage higher.
Considering the LDL‐cholesterol lowering effect of β‐glucan, lot of works have been carried out to extract this macromolecule either from barley flour (Knuckles et al. 1992; Wu et al. 1994) or oat flakes or bran (Wu and Doehlert 2002) to produce value‐added commercial products. Dry fractionation has the advantage to recover β‐glucan with a molecular weight close to the native one (Sibakov et al. 2014) contrary to wet extraction, which induces a significant de‐polymerization that impacts its nutritional properties negatively (Wolever et al. 2010). If conventional dry fractionation (including fine grinding, sieving and air classification) allows the enrichment of β‐glucan up to 20–25% (Knuckles et al. 1992; Wu and Doehlert 2002), higher enrichment has been achieved including in the process of bran/flakes defatting with super critical carbon dioxide (Stevenson et al. 2008; Sibakov et al. 2011) or new separation technologies such as tribo‐charging electrostatic separation (Sibakov et al. 2014). Fractions containing 34–54% of β‐glucans have then been obtained.
3.7 Conclusion
Dry fractionation processes have been developed based on know‐how mainly to isolate the starchy endosperm rich in macronutrients as starch and storage proteins but thus lead to a loss of most of the healthy micronutrients, phytochemicals and fibres present in cereal grains. In comparison with wet fractionation, it presents the advantage of reducing effluents, avoiding post‐drying and additional energy cost. Depending on the processing steps, it is possible to control the tissue structure integrity and the enrichment of molecules with functional properties.
During the last years, a huge amount of data about the characterization and localization of compounds with nutritional interest have been obtained. Different strategies have been developed to limit the germ and aleurone removal or to reintroduce part of the bran and co‐fractions or isolated compounds of interest into the endosperm fractions in order to increase the nutritional value of the product. These new fractionation diagrams are now possible due to the development of rapid methods for characterization and identification of biochemical markers to monitor the tissue behavior during processing. However, introduction of bran or germ fractions in flour or semolina cannot only alter the technological properties of the final products (i.e., texture, volume, color, shelf‐life), but also their sensory properties and thus the general acceptability of the products. Depending on the tissue introduced it is also important to re‐evaluate the products’ safety. As an example, if the increase of heath beneficial component can be reach by substituting 10% of flour by de‐branning fractions enriched in the aleurone layer, this percentage could not be further increased without dramatically altering the technological properties of the bread, and accumulating the contaminating deoxynivalenol mycotoxin (Blandino et al. 2013).
If strategies exist to enrich cereal foods with compounds with nutritional interest, recent data have also noted that the limiting factor is not only the average amount of such compounds but also their accessibility. Therefore, recent studies have shown that physical (mechanical) or biological (enzymatical) pre‐treatment of the fractions used for enrichment (bran or aleurone layer) are efficient to increase the micronutrient accessibility.
The enrichment of a target compound in the final product depends on (a) the considered cereal, as differences in amount and location exists between distinct cereal grains; (b) its accumulation in grains, which relies on the genetic background of the chosen cultivar and the environmental conditions along growth; (c) the choice of fractionation steps, which impact the compound distribution in the fractions. The health potential will depend on the molecule bioavailability depending both on its chemical form and its interactions with other components in the food matrix. Consumer acceptance of the product finally depends on a number of parameters (sensory properties, price, convenience, etc.), which are strongly affected by socio‐economical and ethnical factors.
Therefore, future research has to select the most adapted cereal grains, cultivars and samples in relation with processing steps. However, to build both the best healthy cereal food and the most suitable for consumer acceptance, it has not only to optimize the first transformation but to consider the overall chain, from the field to the fork.
3.8 References
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