Het slijpen van tarwe (Triticum aestivum L.) als voorbehandeling op het vermalingsproces: de impact op de nutritionele en technologische eigenschappen van de bekomen slijpsel- en zemelfracties

Wheat (Triticum aestivum L.) bran is a fraction of wheat grain which is separated from white flour by milling. Millers’ bran mainly consists of the outer layers of the grain (pericarp, seed coat, nucellar epidermis and aleurone) while the white flour mainly comprises the starchy endosperm. Wheat bran contains a large and diverse group of nutritional compounds such as dietary fiber, protein, minerals, vitamins, methyl donors, and phenolic acids. However, wheat bran is only incorporated in cereal-based human food products to a limited extent because doing so deteriorates their texture, organoleptic profile and shelf life. This is to be regretted since consumption of food containing wheat bran is associated with different health benefits. Indeed, numerous epidemiological and clinical studies have provided strong evidence that consumption of whole meal foods significantly reduces the risks of several chronic diseases. This thesis is devoted to the potential of using pearling prior to milling as a way to improve the functionality and nutritional value of wheat bran. Pearling is a mechanical process in which the outer layers of wheat grains are removed by abrasion. This process can enhance the bread and pasta making quality of flour and semolina, respectively and may reduce potential contaminants present at the grain kernel surface. At the same time, it offers novel bran co-products of which the potential is largely unknown. Against this background, this doctoral thesis aimed to provide insights in the nutritional profile of protein and minerals present in pearling and bran fractions produced by pearling and/or milling, and to investigate the quality of bran-rich breakfast flakes when using these fractions. In a first part, a study was made of the impact of different degrees of pearling (0, 3, 6, 9 and 12% by weight) on the structure, composition and enzymatic load of pearlings and bran obtained when subsequently roller milling pearled wheat kernels. Pearling does not remove the kernel outer tissues homogeneously as abrasion affects especially the accessible parts of the kernels. Nevertheless, the first 3% removed consisted of mainly pericarp. The regular bran and that obtained by subsequent roller milling of pearled wheat kernels had similar compositions but the latter had a lower average particle size and a relative increase in aleurone content. Moreover, removal of the outermost kernel layers substantially decreased the activities of especially α-amylase and lipase in the bran. This may be a benefit for product making and storage. In a second part, wheat bran protein was characterized. Albumin/globulin (33% of the protein) was the most abundant Osborne protein fraction. The amino acid composition (high levels of proline and glutamine/glutamic acid) and molecular weight distribution of both bran prolamin (11% of the protein) and glutelin (16% of the protein) suggested the presence of starchy endosperm gluten protein. Ball milling fractured the walls of aleurone cells, released albumin/globulin and glutelin but not prolamin and, hence, increased the total protein extraction yield from 60 to 80%. As a substantial part was only released following fracture of the aleurone cells, not all glutelin originated from the starchy endosperm. Enzymatic in vitro digestibility analysis showed that 70% of the bran protein is solubilized by human digestive enzymes. When the cell wall material was broken down by either ball milling or pearling, the in vitro digestibility increased up to 85-90%. The (regular) bran residue isolated after incubation with digestive enzymes probably would reach the colon in vivo. The fractions with relative increase in aleurone content were slightly more susceptible to colonic fermentation than regular bran. Their higher degree of fermentation could be attributed to the low arabinose to xylose ratio of the arabinoxylan, the smaller particle size, the more degraded cell wall structures, and, thus, to the better accessibility by fecal bacteria. Because of the importance of minerals in wheat and their bio-accessibility, their concentrations, speciation, and spatial distribution were studied in intact and pearled wheat grains. The xylem mobile elements manganese, silicon, calcium and strontium were dominant in the most outer bran layers while phloem mobile elements such as potassium magnesium, iron, zinc and copper were rather concentrated in the aleurone and co-localized with phosphorus. Most of the latter in wheat grains is stored as phytate structures which are only poorly digested or absorbed by humans due to lack of intestinal phytase enzymes. The nucellar projection in the crease region had very high concentrations of iron which is not phytate-bound. Abrading the most outer bran layers lowered the concentrations of the xylem mobile elements in the bran fractions obtained by roller milling. However, pearling does not reduce flour or bran cadmium concentration because of its homogeneous distribution in the grain. In a last part, the potential of the fractions obtained by pearling prior to milling as food ingredient was investigated by including them in the ingredient bill of bran-rich breakfast flakes. Flakes containing residual bran, obtained by removing 4 and 6% of the grain by weight prior to milling, were more crispy and better maintained hardness during soaking in milk, especially for 40% bran-rich breakfast flakes, than their regular bran containing counterparts. In conclusion, this work shows that removing the outermost bran layers prior to milling offers bran with relative increase in aleurone content which has high potential for incorporation in wheat-derived food products as a source of protein, minerals and (fermentable) dietary fiber. Furthermore, these fractions exhibit good potential for incorporating in the ingredient bill of bran-rich breakfast flakes. Modifying the structure of the bran matrix by either pearling or ball milling enhances the bio-accessibility or intestinal release of protein, which can be related to a higher bio-availability and health.status: publishe