**1. Introduction**

The production of wholemeal flours is the oldest method of mill processing of cereals. It has been replaced by the production of flours containing mostly the endosperm over the centuries. The other anatomical parts of grain (hulls and germ) separate in a different way and these by-products are often traditionally used as feed ingredients for farm animals. The most efficient procedures of endosperm separation from the coating layers of the grain and the production of white flours are achieved in the case of wheat. White wheat and bread wheat flours consisting mostly of endosperm particles are the most widespread type of flours in Europe, USA, and currently also in the majority parts of the world [1,2].

The endosperm separation reduces the spectrum of flour components to starch and storage proteins. In the case of wheat, starch (a digestible polysaccharide) accounts for about 80% of the dry matter of bakery flours and the remaining components are the proteins of gliadin and glutenin fraction. Virtually all dietary fiber and fiber accompanying substances with proven health benefits then become a component of the by-product (bran) and only a very low proportion, in the order of units of percent, remains in the standard bakery flours. The higher content of fiber, minerals, vitamins, phenolic compounds, and other bioactive compounds can be found in darker (bread) flours. This also applies to rye, another bread cereal used mainly in Central, Eastern, and Northern Europe, albeit in this case, the endosperm separation is less effective than in the case of wheat and thus rye bread flour contains higher proportions of fiber (up to 10%) [3–6].

The most important components of fiber contained in the hulls of wheat, rye, and other cereals and pseudocereals in terms of nutritional benefits are cereal beta-glucans and arabinoxylans, and the most significant accompanying substances are phenolic compounds, showing for the most part antioxidant properties [7] and also some B-group vitamins and minerals [8–10].

**Citation:** Skˇrivan, P.; Sluková, M.; Jurkaninová, L.; Švec, I. Preliminary Investigations on the Use of a New Milling Technology for Obtaining Wholemeal Flours. *Appl. Sci.* **2021**, *11*, 6138. https://doi.org/10.3390/ app11136138

Academic Editors: Silvia Mironeasa and Suyong Lee

Received: 6 May 2021 Accepted: 29 June 2021 Published: 1 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

For that reason, wholemeal flours are considerably more beneficial from the nutritional point of view than standard bakery flours. Their major problem is that for the production of basic types of bread and pastries of the Euro-American type, wholemeal flours are technologically less suitable and less sensorially acceptable [11,12]. Not to be overlooked is the fact that wholemeal flours are potentially more risky in terms of the content of contaminants, in particular mycotoxins and pesticide residues [13,14].

Standard wheat and rye wholemeal flours are produced in principle in two ways. In the first, most common case, the basis of the disintegration process is a simple impact mill that can be followed by a rolling mill for granulation treatment. The second method of production is the reconstitution of wholemeal flour from common mill products, which is used if the main production program of the mill is the standard composition of products (bakery flours) and wholemeal flour is a minor product. An example of such a product is Graham wheat flour. Even in the case of reconstituted flour, granulation treatment is usually carried out after the fractions have been assembled [1,2].

In both cases, despite the granulation treatment, insufficiently disintegrated particles of the tough outer layers remain in the flour, which are responsible for the specific sensory properties of the flour and bakery products. These particles absorb water and swell in the dough differently than the endosperm particles, making the dough denser and less homogeneous. The partially swollen bran particles then prevent the formation of a light, finely fibrous, porous dough structure during maturation and leavening, which is typical of wheat bread and pastries and is required by consumers. Even in the case of rye breads, where the structure of the dough is generally much more compact than in the case of wheat doughs, a higher proportion of common rye wholemeal flour has a negative effect. Typical Central European rye-wheat bread is as difficult to make from common wholemeal flour as wheat bread [15,16].

It turns out that granulation of particles of outer layers (bran particles) has a fundamental influence on technological behavior and sensory properties. If a sufficiently fine granulation is achieved, which is close to the average granulation of common very fine flours (below 150–200 μm), the sorption capacity (water binding) of such dough is significantly increased, but its mechanical and sensory properties change minimally. Such finely ground wholemeal flours can then replace conventional flours much better, and at the same time, thanks to the higher water binding, a significantly higher yield of dough and products is achieved.

A major problem to keep in mind in the case of production of such flours is the risk of too high a degree of damage to the starch granules. In other words, it is necessary to use a technique that reliably disintegrates the outer layers into very fine fractions, but at the same time does not cause extensive mechanical and especially thermal damage to the starch granules [15].

Damage to starch during disintegration is of two types: (a) mechanical, due to pressure, shear forces, and crash; and (b) thermal, especially due to friction. The deformation to which the grain and its internal structure are subjected during disintegration differs fundamentally depending on the type of mill and grinding parameters used. Grinding in roller mills has a different impact, depending on the parameters (advance, down pressure, specific load) and surface treatment of the rollers (grooving) and their mutual position. In the case of impact mills, other mechanical forces act, and grinding can be both very gentle and very destructive to starch granules, depending on the design of the mill and the process parameters [1,2].

When the grain is disintegrated in any way, partial deformation and damage to the starch granules always occurs. For bakery purposes, where fermentation is one of the key processes, a certain degree of starch damage is desirable because it becomes more amenable to the amylases present and a sufficient amount of substrate is created for yeast or lactic acid bacteria. In the case of flour for the production of wafers and biscuits, confectionery or pasta, damage to starch is not beneficial. However, with extensive damage, complications also arise in ordinary bakery production (stickiness of the dough, poor structure of the bread crumbs, etc.).

Starches of cereals and pseudocereals are easily broken down in the upper part of the human digestive tract. Most starches are completely resorbed in the small intestine, and only some starches may be partially resistant and are classified as indigestible polysaccharides [17,18]. Starch is gradually cleaved by amylases present in saliva and especially in pancreatic juice to oligosaccharides, which are further hydrolyzed to glucose monosaccharide. In the small intestine, glucose is actively absorbed into body fluids.

Dietary starch intake results in a significant increase in blood glucose. Glycaemia is the concentration of glucose in the blood, the value of which in fasting should not exceed 5.5 mol/L (in venous blood) according to the current approach [19]. Starch resorption is rapid, and its rate increases with the degree of damage to the native starch structure. Starch damage can occur biochemically (by enzymatic hydrolysis), chemically (by acid hydrolysis), mechanically, and thermally. The higher the starch resorption rate, the higher the glycemic index (GI) of the food in question [20].

The wholemeal flours tested in this study were produced in a special mill. Physical and chemical analyses of the flours were conducted and the impact of grinding technology on microstructure and properties of the flours was evaluated. What is essential for this work is the assessment of the extent to which the grain disintegration technique used leads to intensive comminution of the grain outer layers (bran particles) into fine granulation without significant damage to the starch granules.

#### **2. Materials and Methods**

Cereal grains (*Triticum aestivum* L., *Triticum spelta* L., *Secale cereale* L.) and a pseudocereal (*Fagopyrum esculentum* Moench) were decontaminated (cleaned) and then disintegrated in a special impact mill. Finely granulated wholemeal flours as final products were subsequently analyzed. The flours used were as follows: finely granulated wholemeal flour of wheat (wheat WM FG), rye (rye WM FG), spelt (spelt WM FG), and buckwheat (buckwheat WM FG).
