Utilization of the Nutritional Potential of Wheat Bran Using Different Fractionation Techniques

Abstract

Wheat bran separated in the standard milling process as a by-product contains many substances of importance in livestock and human nutrition. In the Czech Republic, as in other Central European countries, a significant part of the bran is not traditionally used as a raw material for feed production and is used as a heating fuel. This means that many interesting and health-promoting components of fiber, phenolic compounds, vitamins, proteins, and minerals are lost. The bran is made up of particles of the grain outer coating and sub-coating layers, particularly the pericarp, testa, and aleurone layer. Their composition varies, but while the pericarp in particular is largely composed of cellulose and lignin, the testa and aleurone layer contain many valuable non-starch polysaccharides (hemicelluloses), as well as the macro- and micronutrients mentioned above. Wholemeal flours contain all the anatomical parts of the grain mentioned above, which brings both technological problems in terms of their bakery processing and a not always acceptable sensory impact on the products. This paper summarizes selected physical and physicochemical methods that can be used to remove those components that may cause technological and sensory problems and retain those that, on the other hand, represent a significant nutritional benefit.

1. Introduction

For several thousand years, cereals have been of indispensable importance for the nutrition of the vast majority of the inhabitants of civilized and agriculturally active cultures, both as a source of energy in the form of starch and as a source of protein, albeit nutritionally inferior. In our cultural and historical circle, wheat has been and still is the key cereal, but gradually, together with rice, it has become dominant globally. Botanically, cereals are classified as grasses (Poaceae/Gramineae).

The wheat genus includes a wide range of species, but the most commercially and productively exploited wheat genera include mainly sown wheat (Triticum aestivum L.), durum wheat (Triticum durum Desf.), and to a lesser extent spelt (Triticum spelta L.) [1].

The grains of all cereals contain carbohydrates (especially starch), proteins, lipids, vitamins, and minerals. Starch, together with other polysaccharides and oligosaccharides, is the predominant component of cereals, with protein being the other major component. The other components listed above occur at much lower concentrations. Starch accounts for approximately 60% of the weight of most cereal grains. The other polysaccharides are nutritionally fiber components. Starch is therefore a major contributor both to the energy value of cereals and to the glycemic index of cereal products [2,3].

After carbohydrates, proteins are the most abundant in their seeds, and proteins are distributed in different anatomical parts of the grain. Those found in the endosperm are mainly storage proteins (prolamins and glutelins), with a high proportion of proteins found mainly in the aleurone layer and to a lesser extent in other outer layers. The amount of protein in cereals generally ranges from 8% (w/w) to 16% (w/w). Cereal proteins have good digestibility but lack the appropriate ratio or proportion of essential amino acids. The limiting amino acid in cereals is lysine. The amount and type of these biopolymers, which are also essential nutrients, vary between cereal genera and species, thus affecting the functional (technological), nutritional, and organoleptic properties of processed products [4].

Due to their importance, the different cereal species have been the subject of many investigations and empirical application experiments, and over time their suitability for various processing techniques and uses has been discovered. The different botanical genera and species differ from each other mainly in the content and quality of starch (the complex starch–amylase system), protein, dietary fiber, and lipids.

It was on the basis of knowledge of these parameters that appropriate processing guidelines and procedures were subsequently established for each species. Wheat clearly dominates the mainstream of grain milling and baking. Wheat flour, thanks to its unique properties, can be used to form a specific gel-like structure (wheat gluten) during the kneading of dough, which makes it possible to produce a wide range of leavened and other products that are very attractive to the general consumer. This ability makes wheat unique even among very closely related cereals, such as rye or barley. Wheat protein fractions of prolamins and glutelins (gliadin and glutenin) contribute to the ability of wheat gluten to form a viscoelastic system and thus a specific light and elastic structure of dough and domed products with a characteristic crumb. Although very similar protein fractions are present in the endosperm of rye or barley, they do not have the ability to form a viscoelastic structure similar to wheat gluten [4,5,6].

Over time, wheat has become one of the three most widely grown cereals in the world (alongside maize and rice) due to its unique characteristics. It is estimated to cover 200 million hectares. In terms of its share of food processing, wheat ranks first or second along with rice. Wheat is used worldwide for the production of bread, pasta, and other bakery products and, to a smaller extent, for the production of industrial products involving starch and starch derivatives [7,8].

Wheat therefore serves in its food processing mainly as a raw material for milling into wheat flour and semolina. The entire long history of the development of the milling process for wheat, which we now refer to as standard, has been guided by the desire to primarily extract starch and protein from the wheat grain, i.e., the purest possible endosperm. The other important anatomical parts of the grain were regarded as ballast and separated in the form of a mill by-product—bran [9,10,11].

The standard mill processing of wheat is therefore still indebted to a long historical phase when it was clearly primarily a matter of obtaining energy-rich food. Standard wheat milling corresponds to a situation in which the majority of the human population lived in relative scarcity, with the energy balance shifted to the energy expenditure side. The necessity of often hard physical labor, on the one hand, and the relative scarcity of energy-rich foods on the other made wheat and other grains an excellent compensation option. This situation has passed in the last century in the developed world, and what made wheat and other cereals advantageous in this sense for many centuries has fundamentally changed.

On the other hand, it is irrational that wheat in particular has become an object of distrust (often even a kind of dishonesty) not only on social networks but also in professional circles in recent decades. Wheat, like rye or barley, and to a lesser extent oat, contains protein fractions (mainly prolamins) that trigger the manifestations of celiac disease and some other forms of intolerance. These proteins are referred to as gluten, which leads to some confusion of terms, since gluten originally only referred to the specific structure mentioned above, which is formed when wheat dough is kneaded. Indeed, people who suffer from celiac disease need to completely exclude wheat and the other cereals mentioned from their diet. This risk does not apply for the rest of the population. What is generally true is the above-mentioned energy benefit and glycemic index [12,13].

One solution is to significantly increase the proportion of wholemeal wheat flours in the production of bread, pastry, and pasta. This not only reduces the two unfavorable factors (energy and glycemic index) but also substantially increases the proportion of dietary fiber components, especially those with nutritional benefits. These will be discussed in more detail below. But the problem remains that the coarse particles in the grain outer layers interfere with the formation and maturation of wheat dough, causing both technological and serious sensory problems and often altering the character of the product [14,15].

Our work focuses on the possibility of producing not whole grain flours but wheat flours enriched with defined fractions of the outer layers and especially sub-coat layers of the grain, which contain the highest concentrations of β-glucans, arabinoxylans, and adjuvants with proven health benefits. At the same time, such flours should, on the other hand, contain a significantly lower proportion of lignocellulosic complexes than wholemeal flours, which are mainly responsible for technological and sensory problems.

2. Cereals and Their Potential Nutritional Benefits

Globally, cereals contribute to 60–70% of the human diet. They are also an important source of food for livestock and, last but not least, a small amount is processed industrially, for example, for biofuel production [16]. The table below (

Table 1. Approximate annual global production of major cereals for the period 2013–2022 [17].

Annual Production (Million Tonnes)
Maize	>1000
Rice	730–750
Wheat	730–750
Barley	130–150
Sorghum	60–70
Millet	20–30
Oat	20–25
Rye	10–15

) shows the global annual production of the most important cereals.

All cereals share the same common origin, which is reflected in the similarity in the structure of their grains and in their chemical composition (Figure 1).

The table below (

Table 2. Chemical composition of selected cereals (per 100 g edible sample) [19].

Cereals	Moisture	Proteins	Lipids	Carbohydrates	Dietary Fiber
Wheat	14.0	12.7	2.2	63.9	12.6
Maize	12.0	8.7	0.8	77.7	11.0
Rice	11.8	6.4	0.8	80.1	3.5
Barley	11.7	10.6	2.1	64.0	17.3
Sorghum	14.0	8.3	3.9	57.4	13.8
Millet	13.3	5.8	1.7	75.4	8.5
Rye	15.0	8.2	2.0	75.9	14.6
Oat	8.9	12.4	8.7	72.8	10.3

) compares the contents of the basic chemical constituents in the different cereals.

The most important cereal for human nutrition in our cultural and historical circle is wheat, as was already mentioned in the Introduction of this paper. Wheat is processed primarily in the milling process into flour and secondarily for the production of bread, bakery products, and pasta.

The wheat kernel consists of three main anatomical parts: the outer (and sub-coat) layers, the endosperm, and the germ. All three, but especially the outer layers and sub-coat layers, are a complex of several morphologically and histologically specific sub-parts. The wheat outer and sub-coat layers can be further sub-divided into three mains morphologically and functionally distinct layers, namely the pericarp, the testa, and the aleurone layer [18]. The aleurone layer is botanically the outer part of the starchy endosperm, which is separated from the testa by a thin hyaline layer to which the cells of the aleurone layer are closely adjacent. Therefore, during milling processing, the aleurone layer is separated from the endosperm and separated together with the outer layers as part of the bran [20]. Wheat bran is rich in minerals, dietary fiber, B vitamins, and bioactive compounds, many of which have been shown to have health-promoting properties [18].

Bran is a by-product of the standard wheat milling process and has a global production of more than 150 million tonnes per year [16]. Bran has traditionally been a valuable component of livestock feed mixtures, but its surplus production is processed in different ways, often as a raw material for the production of alcohol or biogas, some of which is also used as fuel. Meanwhile, bran has considerable nutritional potential [11,21,22,23]. Bran contains about 13–18% protein, 3–4% lipids, 3–8% ash, and more than 57% various carbohydrates (mainly hemicelluloses, cellulose, and 14–25% starch) [24].

If we compare the composition of the bran with the other parts of the grain, we obtain the following: the starchy endosperm (besides available carbohydrates) consists of 15% lipids, 13% proteins, dietary fiber (0–5%), and ash (1–5%) [25], and wheat germ is rich in proteins (25%), lipids (8–13%), and ash (4–5%).

Nutritionally attractive bran components include proteins or the aforementioned polysaccharides (dietary fiber components). The extraction of fiber components and their use and incorporation into recipes are currently receiving increased attention, not least because of the desire to reduce waste from conventional food and agricultural production. Fiber components are of particular interest because of their effects on the human body and its gut microbiome and the associated reduction in the risk of cardiovascular disease. This attractiveness is supported by health claims approved by the European Commission [23,26].

Next, we shall look at the chemical composition of the various anatomical parts that become part of the bran as a by-product in the standard milling process.

The pericarp, which forms the uppermost outer layer of the grain, is composed mainly of non-water-extractable fiber components such as cellulose, some hemicelluloses, and lignin. It also contains minerals, vitamins, and phenolic acids in bound form. Overall, this layer represents about 3–5% of the weight of the whole grain [18].

The seed coat is composed of non-starchy polysaccharides, mainly hemicelluloses, which are partly water-extractable. Furthermore, phenolic compounds, carotenoid dyes, alkylresorcinols, minerals, and vitamins are present at significant concentrations. This layer accounts for about 1% of the weight of the whole grain [11,18]. The arabinoxylans of the hyaline layer, which lies between the awn and the aleurone layer, are usually sparsely cross-linked and therefore more easily extractable [20].

The aleurone layer, as noted, is located at the interface between the outer layers and the endosperm and constitutes about 6–9% of the grain weight [18]. It is essentially composed of a single layer of large cells with a diameter of 20–75 μm. The aleurone cell walls contain 29% β-glucans, small amounts of proteins, and 65% arabinoxylans. They have a low ratio of arabinose to xylose and contain high amounts of the ester monomer ferulic acid. Inside the aleurone cells are spherical particles, called aleurone granules, that are 2–4 μm in diameter. These granules can be composed of phytate inclusions or niacin and protein inclusions and are surrounded by a thin lipid layer. Overall, the aleurone layer is rich in nutrients. The cells forming the aleurone layer contain large amounts of proteins, minerals, phytates, B vitamins (especially niacin), and plant sterols. The aleurone layer contains 15% of the total amount of wheat protein and also 30% of the total amount of lysine, which is the first limiting essential amino acid in wheat. In addition, at least 80% of the total amount of niacin is present, as well as a significant amount of other B vitamins. Approximately 40–60% of the total mineral content of wheat is found here, making the aleurone layer a rich source of minerals [20].

The cell walls of cereal endosperm cells are important in germination and grain structure and are one of the sources of fiber. The endosperm occupies on average 80–85% of the grain and is composed mainly of starch, protein, and on average 2% dietary fiber. The predominant cell wall polysaccharides in wheat endosperm are mainly arabinoxylans, a mixture of β-glucans, and then a smaller amount of cellulose [18,27].

The wheat germ makes up about 2.5–3.5% of the grain. The germ contains relatively high amounts of protein (25%), sugars (18%), lipids (13–16%), and ash (5%). It contains no starch but is relatively high in B vitamins and contains many enzymes. The sprouts also have a relatively high vitamin E content, reaching up to 500 ppm. Simple carbohydrates are mainly represented by sucrose and raffinose [5].

The main non-starch polysaccharides in wheat bran are arabinoxylans, cellulose, and β-glucans, which account for 73%, 24%, and 6% of starch-free bran, respectively. Arabinoxylan is classified as a hemicellulose. In cereals it is one of the main cell wall polysaccharides. The amount and structure of arabinoxylans vary depending on the cereal species, variety, anatomical location, and environmental conditions during grain growth and maturation [28,29]. Furthermore, small amounts of xyloglucans are found in wheat bran, which are mainly derived from the pericarp. Glucomannans and arabinogalactans, which originate from aleurone and endosperm cells, are also present in minute amounts [28]. De Brier et al. reported that wheat bran contains 18.8–21.4% arabinoxylans on a dry weight basis [30].

Arabinoxylans are formed by the main chain of β-linked (1 ⟶ 4)-d-xylopyranose and are mainly substituted with the side chains of α-l-arabinofuranose. Arabinoses are attached to the O-2 and/or O-3 position of the xylose units Figure 2 [31]. The rate of arabinose/xylose (A/X) substitution of arabinoxylans increases from the center of the endosperm towards the outer layers of the wheat kernel. The most substituted arabinoxylans are in the pericarp, where on average one xylose carries one arabinose [28]. Some arabinose units are ester-linked to O-5 by ferulic acid [32]. Edwards et al. reported that 31% of unsubstituted xylose and 24% of monosubstituted and 39% of disubstituted xylose are found in wheat bran [33].

Arabinoxylans are divided into water-extractable (WE-AX) and non-water-extractable arabinoxylans (NW-AX) [32]. Approximately 30% of the 2.5% of all arabinoxylans present in wheat flour endosperm are water-extractable. In contrast to the non-water-extractable arabinoxylans, they are not maintained in the cell wall by covalent and non-covalent interactions with other arabinoxylan molecules or with cell wall components such as proteins, lignin, or cellulose. It can be assumed that WE-AXs are loosely bound to the cell wall surface. Several factors are involved in the ability to extract arabinoxylans with water, including incomplete cross-linking with other components, small structural differences, or initial enzymatic degradation in the grain [34].

WE-AXs from wheat bran are less substituted with arabinose than those from the inner endosperm. The lower degree of substitution is due to a higher proportion of unsubstituted xylose residues and a lower proportion of disubstituted xylose residues. Since wheat bran is composed of different grain tissues, the actual structure of arabinoxylans in wheat bran is very heterogeneous. The arabinoxylans in wheat bran are mainly low-substituted ones, with an arabinose/xylose (A/X) ratio of 0.2, and then highly substituted ones, with an A/X ratio of 1 [35].

Arabinoxylans also serve as an important reservoir of antioxidant phenolic compounds, including alkylresorcinols and phenolic acids. Ferulic acid is very often bound directly to arabinoxylan. Various technological treatments have been investigated to facilitate the release of ferulic acid and other bound phenolic compounds from the arabinoxylan structure to exploit their antioxidant potential. These treatments include thermal, alkaline, enzymatic, ultrasonic, and fermentation treatments. In addition, recent studies have shown that the degree of substitution and molecular weight have a significant effect on the antioxidant potential of arabinoxylans. Lower degrees of substitution led to higher antioxidant capacity of arabinoxylans. Water-extractable arabinoxylan fractions treated with endoxylanase also exhibited higher antioxidant capacity [31].

From a technological point of view, arabinoxylans play an important role in the preparation of bread. Both WE-AXs and NW-AXs contribute to an increase in water absorption, but their effect on baking properties seems to differ. The insolubility of NW-AXs causes them to trap water, which limits the amount of water available for the development of the gluten complex. WE-AXs, on the other hand, increase the viscosity of free water in the dough during absorption [36]. Arabinoxylans have interesting functional properties for use as food additives. They have applications as viscosity enhancers, as they have a high water retention capacity. At the same time, they are able to stabilize protein foams [34]. Furthermore, they can be used as texture-forming agents, especially water-soluble arabinoxylans. They have been used to produce oligosaccharides with physiological functions and also demonstrate the potential to reduce glucose and lipid absorption [28]. They have also been found to have a positive effect on the proper function of the intestinal tract, and their regular daily intake has been shown to improve immunoregulatory activity [29].

Cereal β-glucans are one of the components of dietary fiber and have positive effects on human health attributed to them. These positive effects are related to their structure and solubility. Their polysaccharide structure is mainly composed of blocks of three cellulose units linked by β-(1 ⟶ 4) bonds or blocks of four cellulose units linked by β-(1 ⟶ 3) bonds. The ratio of these two individual blocks of cellulose units with different bonding affects solubility, with very high or very low ratios resulting in lower solubility [37]. For the final polysaccharide, β-(1 ⟶ 3)-linked cellotriosyl units (58–72%) and cellotetraosyl units (20–34%) predominate, but a small number of sequences with β-(1 ⟶ 4) linkages are also present, with up to 14 glucose units linked in sequence Figure 3 [38]. The final structure of these polysaccharides is an important factor in determining their physical properties, which include water solubility, viscosity, and gel-forming ability. At the same time, their structure is responsible for their physiological action in the gastrointestinal tract [39].

Structural differences in cereal β-glucans are indicated by the ratio of trisaccharides to tetrasaccharides. Differences can be found within different cereal genera, but not within the same genus. Cereal β-glucans are present in large amounts, mainly in oats (4.5%), but also in wheat, mainly in bran, where they are found at around 2.5%. In oats and barley, β-glucans are found throughout the starchy endosperm, whereas in wheat, the highest concentration is in the aleurone layer, and only a small fraction is in the rest of the starch endosperm [38].

Cereals are an important source of dietary fiber for humans, even with standard processing. They account for approximately 40% of total intake, of which half of the total cereal intake is consumed in the form of bread. Although β-glucan is a minor component in white wheat flour, approximately 0.5% on a dry matter basis, the high consumption of bread, pasta, and other wheat products makes it the main source of β-glucan in the diet of the majority of the population. The intake from wheat therefore far exceeds that from barley or oats, despite the high concentration of β-glucan in these two cereals.

Wheat, and in particular white wheat flour, is therefore suitable for enrichment with the important fiber components, β-glucans. These polysaccharides reduce the risk of cardiovascular disease and type 2 diabetes. The health benefits associated with β-glucans are thought to be related to their physicochemical properties, in particular solubility and viscosity. Both soluble and insoluble β-glucans have a positive effect on cardiovascular health and glycemia attributed to them. In addition, insoluble β-glucan benefits intestinal motility [37].

β-glucans constitute only 20–25% of the total polysaccharides of the cell wall of the starchy endosperm and aleurone layer of wheat. Some studies indicate that they are poorly extractable in aqueous solvents, probably due to entrapment in arabinoxylan, which is the predominant cell wall polysaccharide in wheat. During wheat grain development, β-glucans are deposited in the endosperm cell walls earlier than arabinoxylans. Improved extraction of β-glucans occurs under alkaline conditions, under which arabinoxylans are also extracted, and/or during xylanase cleavage [37].

In summary, the nutritional potential of cereals can be roughly summarized as follows: they are energy-dense raw food materials (mainly due to their high starch and protein content). Starch is responsible not only for the energy value but also for the high glycemic index values of many cereal products. Furthermore, cereals are a source of non-protein, with lysine as the limiting amino acid. Fiber is concentrated in the outer layer and sub-coat layers of cereals, and some of its components, in particular cereal β-glucans and arabinoxylans, can confer significant nutritional and health benefits. These and other fiber components are accompanied by other substances, in particular phenolic compounds, which also have interesting nutritional potential.

Wheat is the most widely used, and by far the most important, raw material for the production of bread, pastry, and pasta, foods that are extremely important and are consumed in large volumes and therefore contribute a great deal to human nutrition. The standard processing of wheat leads to the separation (extraction) of the pure endosperm in the form of baking and pastry flours, and nutritionally important components with proven health benefits become part of the bran [40].

What are the realistic pathways to increasing the content of these compounds in bakery and pasta products?

3. Variability of Wheat Milling and Technological Potential

The aim of the standard milling process is to separate the grain outer layers, sub-coat layers, and the germ from the starchy endosperm, i.e., to remove the pericarp, the awns, and also the aleurone layer, in order to obtain the white flour used in baking. The standard milling of wheat is a typical example of refining. The desired fraction is extracted from the raw material, in this case the purest possible endosperm alongside by-products, and waste. The aim is, as in any refining process, to extract the maximum amount of the desired fraction with the highest possible efficiency but, of course, while maintaining its quality. The modern milling process of standard wheat processing fully meets this requirement in well-equipped production lines. If the wheat grain contains approximately 80% endosperm by weight, the yield of standard baking and pastry flours also reaches this value [5,41].

The basic parameter characterizing baking and pastry flours is the mineral content or ash content. The ash content of bread flours is around 1% (weight % in dry matter of the flour), and the cumulative ash content of all standard baking flours is around 0.65% (weight % in dry matter of the flour), which corresponds to the average ash content of the endosperm. The endosperm is covered by an aleurone layer, but its ash content is many times higher, so the standard milling process needs to lead to its removal if the requirements of standard baking and pastry flours are to be fulfilled. This results in the virtually complete removal of all the layers of grain that may contain nutritionally important fiber components, since these are, as mentioned above, in the aleurone layer and the layers above it [5].

On the other hand, the layers above the aleurone layer towards the grain surface clearly contain higher and higher proportions of those fiber components (cellulose, lignin, non-extractable hemicelluloses) that are responsible for the deterioration of the technological properties of doughs and the sensory properties of the final products [42,43].

There are basically three ways to achieve a substantial increase in the content of desirable fiber components while avoiding undesirable effects on technological and sensory properties [41]:

• The use of wholemeal flours containing all the ingredients of the outer layers, but sufficiently finely granulated to minimize their destructive effect on the dough structure. This cannot be achieved with the standard milling process. It is therefore necessary to choose an alternative milling method which achieves fine granulation of the pericarp particles while avoiding deeper destruction of the starch in the endosperm. This requires the use of special milling machines, which are not included in the standard milling process and can also be very expensive.
• The use of flours that are not wholemeal but contain a substantial proportion of mainly sub-coat layers, where more of the required fiber components and less of the lignocellulosic complex are present. This can be achieved by surface treatment (see below).
• A third possibility is to classify the bran particles obtained in a conventional milling process (after possible modification of their granulation) on the basis of the different physicochemical properties of the particles with different contents of the substances under discussion.

Bran obtained by the standard milling process can also be treated without fractionation. Currently, several approaches exist to treat wheat bran without separating any of its components for further use, and some of them have even been patented for the commercial sphere. These are either by modifying the properties of the bran, for example, by extrusion or hydrothermal treatment, or using enzymatic processes or fermentation [21,22,23,44,45].

4. Grain Finishing Options—Peeling

Surface treatment has another major importance besides the issues we have discussed. The outer layers of the wheat grain are most exposed to contamination by natural and synthetic contaminants. These contaminants include mycotoxins, in wheat most commonly Fusarium fungal products, especially deoxynivalenol or zearalenone, synthetic contaminants such as heavy metals cadmium and lead, and especially pesticides [46].

One of the fractionation processes is the so-called “peeling” or “debranning” of the grain. This is a process in which the outer layers of the grain are removed by rubbing and abrasion [30]. The degree of peeling can be effectively controlled to separate those fractions of the outer layers of the grain that are technologically problematic and may also be a safety hazard. Thus, the middle fraction of wheat grain outer layers obtained by repeated peeling after removal of the uppermost layers could be used as a functional ingredient in order to enrich wheat-flour-based products with bioactive substances but also to rid them of the risk of contaminants associated with the use of all outer grain layers [46].

This peeling technology is well known for its use in rice milling and is sometimes used before milling durum wheat, but peeling has rarely been used for common wheat. Of particular interest for food purposes are bran fractions enriched with the aleurone layer, as this tissue is rich in soluble fiber and biologically active substances such as vitamins, minerals, and antioxidants, while being free of possible exogenous contaminants, as these are removed together with the peripheral layers of the grain [30]. De Brier et al. also report that the enrichment of flour with a selected fraction obtained by peeling (containing an aleurone layer) leads to an improvement in the nutritional profile of bread, with less adverse effects on bread volume and texture than bread made from flour with the addition of conventional wheat bran [30].

The table below (

Table 3. Distribution of selected grain components in wheat fractions obtained by peeling (content of compounds is expressed in weight %) [42].

Peeling Fraction	Protein (%)	Fiber (%)	β-Glucans (%)	Free Phenolic Acids (μg/kg)
0–5%	11.7	58.3	1.4	788
5–10%	17.4	37.8	1.9	572
10–15%	19.6	24.4	1.5	500
15–20%	18.2	19.7	1.1	395
20–25%	17.5	12.9	0.9	261
Remaining 25–100%	12.0	5.6	0.3	23

) compares the distribution of certain components in the outer layers of wheat grain. The different fractions (always constituting 5% of the original grain weight) were obtained by successive abrasion of the wheat grain from the outer layers to the starchy kernel of the grain using the technological method of “peeling” [42].

Peeling or partial debranning, i.e., surface treatment of varying intensity, is a direct and relatively simple way to significantly improve the nutritional properties of flours and to compensate for the negative technological and sensory effects of using wholemeal flours. However, it is at the same time a relatively inaccurate method. The grain is irregular in shape, and the size of individual grains varies slightly. As a result, the efficiency of separating technologically problematic layers and, conversely, preserving nutritionally important grain layers is always rather imperfect [30,41,43].

5. Bran from the Standard Mill Process and Possibilities for Its Fractionation

The most important separation methods potentially applicable for industrial practice are presented below.

5.1. Enzymatic Fractionation Methods

Enzymatic methods use bran without prior separation or sorting. Often a single preparatory step is present, namely grinding the bran into homogeneous particles.

5.1.1. Extraction of Arabinoxylans

Currently, base-catalyzed extraction in aqueous media is used for the extraction of arabinoxylans from wheat bran. This method consists of two steps. The first is the removal of starch and protein from the bran by an enzymatic method in an aqueous environment under ideal conditions for enzyme activity. Via this route arabinoxylans lose attached functional groups such as ferulic acid [47].

5.1.2. Separation of the Aleurone Layer

For a more gentle procedure to obtain a nutritionally interesting aleurone layer, a patented method can be used (Bohm et al., 2006) [48]. In this case, the bran is suspended in water, and the aleurone layer particles are transferred from the bran particles to the aqueous phase using enzymes. After application, the material is pressed, filtered, or decanted. The aqueous phase can then be centrifuged to extract the aleurone layer components or filtered by micro- or ultrafiltration. Alternatively, the aqueous phase can be concentrated in an evaporator and then spray dried to obtain the aleurone feedstock.

5.2. Electrostatic Separation

Electrostatic separation is a “dry method” outside of an aqueous environment. It uses the triboelectric effect. Its manifestation in everyday life is static electricity. The method is based on the charging of particles by collisions with the device wall and subsequent separation in an electric field. The polarity and strength of the charge vary according to the material used and the temperature, pressure, or even the speed at which the material is kept. Nitrogen is often used as the carrier gas [49].

The first step is the charging of the particles entrained by the carrier gas, which occurs when the particles rub against each other or against the inner wall of the charging device. The magnitude and polarity of the resulting charge can be estimated from the so-called triboelectric series of materials, which has been developed experimentally. When two particles come into contact, the one that is higher up (at the positive end of the series) becomes positively charged. The resulting charge is greater the further apart the materials are in the series. The most commonly used material for the production of bran separation tubes is Teflon (polytetrafluoroethylene), which is located at the negative end of the triboelectric series. Other materials used to separate bran and other plant materials are nylon, aluminium, or stainless steel; unlike Teflon, these are located closer to the positive end of the triboelectric series [50,51].

Before the actual processing of the bran by this method, the bran needs to be milled into smaller particles in order to achieve a greater number of collisions with the wall of the charging device. Nevertheless, there is a size limit below which the particles begin to adhere to the equipment wall and the number of particle collisions increases—particles that are not effective at creating a charge [52]. This phenomenon was not seen in pericarp particles, where particles of different sizes achieved the same average charge, thus the authors believe that the surface composition of these particles, in addition to their size, has an effect on the size of the charge [53].

Another fiber parameter that influences the obtained charge and thus the success of the separation is the moisture content. The best separation occurs at moisture contents below 10%, but no significant differences in separation success have been observed between moisture contents below 10% [54]. But it is possible that moisture content does not affect the size of the recovered bran charge overall but predominantly that on the pericarp particles [55].

It has been shown experimentally that when the appropriate material is used during charging, particles originating from the aleurone layer and those originating from the pericarp acquire opposite charges and gravitate towards different electrodes, presumably due to the different cellular structure and the structure of the fiber components (branched or linear) in each outer layer [54,56,57].

The shape and length of the charging device then also have an impact on the success of the charging and particle separation. Cells that are spiral shaped are preferable, because they increase the delay time of the material in the cell and increase the likelihood of material colliding with the cell wall. Thus, while there is an advantage of higher particle charges at the exit when using longer devices, there is a risk of aggregation of particles with opposite charges, which then do not separate in the electric field [49].

Another parameter that can be controlled is the flow velocity of the carrier gas that carries the whole particles. A higher flow velocity is preferred, and a slightly turbulent flow is recommended for wheat bran, which increases the separation efficiency and is also capable of breaking up particle aggregates [49].

The concentration of material entering the cell also affects the success of the separation. If the particle concentration is too high, there is a higher probability of particle–particle collisions; moreover, particles with opposite charge may again aggregate [58]. There are studies that show that the selectivity of the separation can increase with an increase in the input concentration of the material. But this study investigated the separation of a mixture of starch and proteins [59,60].

After charging, the particles enter the separation cell, in which the charged particles are transported to the electrodes using an electric field. The construction of this cell also affects the separation yield. The two basic types of separation cells are horizontal and vertical. Horizontal cells generally have lower separation efficiency due to the gravitational force on the particles [49,59,61]. The main factor is the electric field strength, which depends on the electrode potential and electrode distance [58]. The positively charged fractions mainly yield particles originating from the aleurone layer and are thus enriched in β-glucans. In contrast, in the negatively charged fractions, the pericarp particles end up containing mainly insoluble fiber and arabinoxylans [56]. As for enzymatic methods, a specific use of this method has been patented to obtain aleurone layer particles [48]. Electrostatic separation yielded 25% (w/w) arabinoxylans from wheat bran. The fractions rich in arabinoxylans were used to produce bread [61].

5.3. Combination of Methods

It is possible to use multiple separation methods on a single sample to achieve better yields. The use of electrostatic separation followed by sieving through a 50 μm mesh sieve yielded a fraction with a higher content of arabinoxylans, additionally depleted of starch and proteins [49].

5.4. Air Screening and Sieving

Another method that takes advantage of the different physical properties of the packing layers and the different fiber components is the combination of sieving and float grading [41]. This is a combination of sorting by dimensions but also by aerodynamic properties and specific gravity. The use of this method, known as the elusive process, has been most thoroughly described for dried distillery stillage from ethanol production [48]. Similar processes are generally used in grain cleaning plants, but in which different material dimensions are involved. The incoming material is divided into two final parts; the first is enriched with neutral detergent fiber, and the second has more suitable parameters for use as feed than the incoming meat. The neutral detergent fibers are cellulose, hemicelluloses, and lignin. Some components of soluble fiber are not determined by this method, as they dissolve during the determination and are not captured [62]. However, fiber values determined as neutral detergent fiber and fiber values determined by the gravimetric method correlate strongly with each other for pure wheat products and especially for milling fractions.

The first step is to screen the materials through several sieves with progressively smaller meshes. The fractions thus obtained are further separated by elutriation. The particles that are carried by the air stream to the top of the elutriation column are designated as the lighter fraction, and the particles that remain at the bottom are designated as the heavier fraction [63]. This method yields a lighter fraction with a neutral detergent fiber content above 50% and a heavier phase with a reduced neutral detergent fiber content slightly above 30%.

Different physical properties of the particles contribute to the difference in the behavior of the different particles in terms of buoyancy. The key ones are density, size, shape, and their combined effects. To optimize the separation, Srinivasan and Singh determined the density of the fiber and non-fiber fraction particles, as well as their equivalent spherical particle diameter, particle sphericity, and terminal velocity [64]. The fibrous particles were also entrained at lower air velocities due to their lower density.

The same technique was used for the separation of pericarp particles from corn flour for ethanol production, in which the higher fiber content fraction accounted for nearly 4% of the entering mass and contained over 60% neutral detergent fiber [64,65].

This method did not appear to be as effective for a mixture of by-products of flour production. The separation factor of the neutral detergent fiber is lower than that of the other intermediates studied [66].

Another study used a slightly different approach and came up with results more interesting for use in human diets. The aim here was not to separate the fiber but to obtain a bran fraction with increased protein content. The bran, after drying to a moisture content of 5%, was ground in an impact mill with a resulting median particle size of 131 μm. This was followed by air jet sorting. For the lighter fraction, with increased protein content, the insoluble fiber content was reduced. There was no change in the soluble fiber content, which raised the soluble to insoluble fiber ratio from 0.22 to 0.85. This change suggests that, as in the previous study, there may have been a reduction in the content of particles belonging to the pericarp, and thus containing higher levels of cellulose and lignin. The lighter fraction also has a higher ash content than the heavier fraction, which could indicate a transition of particles from the aleurone layer, the outer layer most rich in ash, to the lighter fraction [67,68].

A combination of milling, sieving, and airflow sorting was used to obtain a fraction of defatted oat bran enriched in β-glucans [69]. In this case, oat bran was defatted, milled in a punch mill, and then airflow sorted by size. Elevated β-glucan contents were already present in particles larger than 30 μm. The highest β-glucan contents were reached by fractions with particle sizes between 100 and 200 μm.

5.5. Comparison of Methods

In separation processes using enzymes, energy is consumed for mixing with water, maintaining the temperature if necessary, and in the final drying process. In aerodynamic separation, the majority of the energy is consumed by pneumatic transport of the material and mechanical sieving. In electrostatic separation, energy is again spent on transporting the material and also on creating and maintaining the electric field.

Enzymatic methods also result in the loss of functional properties of compounds bound to the fiber, particularly acids with antioxidant properties. In electrostatic separation and bran fractionation by sieving and air screening, the process involves grinding the bran into smaller particles, which does not significantly impair the functional properties of the fiber components. Conversely, grinding the bran into smaller particles increases the accessibility of the functional fiber components when consumed [55].

In addition, air screening and sieving have an advantage over electrostatic separation, due to the fact that they use procedures and technologies used in conventional milling. Mills commonly use pan sifters and the pneumatic conveying of raw materials.

In addition, there are already efforts to reformulate some products with the addition of a lighter fraction obtained purely by sieving with air screening, since the fractions obtained still contain a quantity of protein. In addition, these products may have nutritional claims of “reduced fat” and “source of fiber” [70].

6. Conclusions

Wheat flours, or semolina, which are used for the production of bread and pastries, as well as pasta and many other traditional products, are predominantly produced in the standard milling process, which is aimed at extracting the endosperm as perfectly as possible and converting it to the desired granulation. The outer and sub-coat layers (pericarp, pericarp, hyaline layer, and to a large extent the aleurone layer) become part of the mill by-product (bran), which traditionally serves as a valuable ingredient in livestock feed. Nevertheless, in the event of a lack of sales to the animal feed industry, they are processed in a different way, often as a raw material for the production of alcohol or biogas, some of which is also used as fuel. The bran has considerable nutritional potential and is capable of adding very valuable fiber and other additives to cereal products. At the same time, the fiber is also able to partially compensate for the high energy value represented by starch in particular and also partially compensate for the associated high glycemic index of conventional bakery products and other foodstuffs.

The easiest and most indirect route leading to the use of grain outer and sub-coat layers for human nutrition is the production and use of wholemeal flours. It is known that these flours, due to their content of cellulose, lignin, and non-extractable hemicelluloses, complicate the technological properties of these flours and adversely affect the sensory properties of bread and bakery products or pasta. A way of partially eliminating these negative characteristics is to achieve very fine granulation of the lignocellulosic complexes, which cannot be achieved by standard milling processes but requires the use of special milling machines. Even so, there is always a risk of deeper damage to the starch granules, which is ultimately counterproductive both technologically and nutritionally.

However, it is possible to significantly increase the fiber content and other nutritionally important substances found in the outer layers and, in particular, in the sub-coat layers, while reducing the negative impact on the technological and sensory properties, by means other than the production of wholemeal flour.

The first (and easier) way is by surface treatment of the grain in the form of peeling or partial debranning. In this way, significant separation of the technologically and sensorially unfavorable pericarp components from the remaining layers can be achieved. But its accuracy and efficiency are limited. In intensive peeling, even valuable parts of the sub-coat layers go into the by-products, whereas in a less intensive process, pericarp particles remain in the final product.

Another, albeit less well studied and technically not yet easy, way is to take standard bran (bran from the standard milling process for wheat flour production), disintegrate it if necessary, and sort it into fractions according to the content of the components being targeted. The components rich in nutritionally important fiber components, and possibly other substances, can then be returned to the flours and the other fractions separated into a by-product or waste.

There has not been much work published on this route, but the studies that have been performed suggest several possible principles that can enable effective fractionation to be achieved. As we have shown, these include physical procedures as well as physicochemical and biochemical ones. Some of these are clearly applicable to laboratory fractionations or to procedures directed to the isolation and preparation of certain substances from bran in special manufacturing. For wider use in mill production practice, which should aim at enriching flours not with preparations, but with not fully prepared but defined bran fractions, the combination of sieve sorting and airstream sorting seems most promising. The bran particles would then be sorted by size, and also by aerodynamic properties and specific gravity, after possible prior adjustment of the granulation. This is a challenge for future research and development, which will focus on the one hand on applicable sieving and aerodynamic separation techniques and on the other on a thorough analysis of the fiber components and other nutritionally important substances in the separated fractions.