Author:

Abstract:

Wheat is worldwide one of the most cultivated crops. Its endosperm texture profoundly impacts its processing. The basic and cysteine-rich proteins puroindoline a (PINA) and b (PINB) are responsible for differences in wheat kernel hardness. Triticum aestivum L. cultivars (cvs.) containing both wild-type puroindolines (PINs) are soft, while the absence of PINA or a mutation in PINB results in a hard texture. The very hard T. turgidum L. ssp. durum cvs. lack both PINs. PINs are thought to exert their impact on wheat softness by stabilizing the amyloplast membrane during seed maturation. It is believed that the presence of a unique tryptophan-rich domain in both PINA and PINB lies at the basis of this stabilizing effect. This hydrophobic domain, located at the outside of the proteins, has a strong affinity for polar lipids and seems responsible for PINs’ interesting properties for food processing. These include high surface activity and excellent foam-forming and stabilizing properties, even in the presence of lipids. While these features have been demonstrated in vitro, it is unclear how PINs behave in wheat based end products.This doctoral work explored the functionality of endogenous PINs in wheat based end products. More in particular, the work focused on their impact in a semi-sweet biscuit and a straight-dough bread system. Biscuits are preferentially made from soft wheat flour and contain relatively high levels of wild-type PINs. PINs may influence biscuit quality due to their surface activity or by modulating gluten-starch interactions. In bread, more than in biscuits, the formation of a stable foam is desired. Surface active components enhancing foam stability can improve loaf volume and crumb structure.In a first part, the impact of the starch isolation method and gluten network formation on starch granule associated components was studied. More PINs and lipids were found at the granule surface of starch isolated with a batter method, in which little if any gluten develops, than at the surface of starch isolated with a dough ball method, during which, as the name implies, optimally developed dough is formed. The extent to which PINs adhere to the starch granule surface seems to depend on gluten network formation. Proteins and lipids at the starch granule surface impact starch water absorption and, as such, swelling, but do not affect starch granule internal phenomena such as melting of amylopectin chains. The results for flour-water model systems were confirmed for more complex food systems. Starch isolated from semi-sweet biscuit dough contained more PINs than that isolated from sugar-snap cookie dough. These two food systems are comparable as they contain, next to flour and water, considerable amounts of sugar and fat, but they differ in the extent of gluten development.The impact of PINs on starch functionality was further investigated by incubating starch (isolated with the dough ball method) with peptidases, lipases or media of different polarity and ionic strength with theaim to specifically remove or retain PINs at the granule surface. Logically, peptidases removed all PINs from the starch granule surface. They also decreased lipid levels. All lipases removed variable amounts of polar lipids from the starch granule surface and, surprisingly, also different levels of PINs. We were also able to conclude to the existence of a close relationship between PINs and polar lipids, especially phospholipids, at the starch granule surface. A solution of 50.0 mM NaCl in isopropanol:water (50:50, v/v) selectively removed PINs from the granule surface. Untreated starch and starch treated with this solvent had comparable swelling and gelatinization behavior, indicating that PINs associated with the starch granule surface do not affect starch behavior.In a second part, the impact of PINs on semi-sweet biscuit quality was investigated. To that end, biscuits were prepared with flour from wheat cvs. varying in hardness, thus containing different PIN types. Due to lower levels of water-binding components in soft wheat flour, dough made with such flour showed more lateral expansion and more gas cell expansion, yielding larger biscuits with higher porosity than did dough made from flour from hard wheat. The higher porosity of biscuits made from soft wheat flour contributed strongly to the lower fracture stress of such biscuits, but biscuit matrix strength also affected fracture stress. However, since the flour samples contained not only different PIN types but also different damaged starch and arabinoxylan levels and had different protein qualities, no straightforward conclusions on the role of PINs could be drawn. Therefore, to specifically elaborate on the role of PINs, semi-sweet biscuits were made with gluten-starch blends varying in PIN levels. This fractionation-reconstitution approach also allowed to at least partially retain interactions between PINs and starch or gluten. PINs strongly affected the dimensional, structural and textural properties. Dough made with blends having high PIN levels contained more air, showed more lateral expansion and yielded larger biscuits. Such biscuits were more porous and softer than biscuits produced with blends having low PIN levels. Furthermore, PINs also affected biscuit matrix strength, presumably by modulating gluten-starch interactions. From both approaches, it was clear that dough aeration and viscosity largely affect biscuit dimensions and porosity and are thus important parameters to steer in order to obtain large and tender biscuits.In a third part, the impact of PINs on bread quality was studied. Surface active components stabilizing gas cells are thought to be dissolved in the dough aqueous phase, which can be isolated from dough with ultracentrifugation as so-called “dough liquor” (DL). Fermentation increased the level of PINs in DL. This may well positively impact foaming properties as well as bread crumb structure. Additionally, the lower pH created by fermentation and the presence of salt increased PINs’ surface activity. Indeed, PINs were enriched more infoam from DL from fermented dough than in that from unyeasted dough. However, since only a small fraction of the total PINs in flour was recovered in DL and many other proteins were also present, it was difficult to draw straight-forward conclusions on the specific impact of PINs on bread quality. Therefore, much as for biscuits, breads were prepared with gluten-starch blends varying in PIN levels. Breads produced from blends containing high PIN levels had a more homogeneous crumb structure with finer gas cells and a thinner matrix than when breads made with blends containing medium or low PIN levels. However, the mechanism by which PINs exert this bread crumb improving effect is not clear at present. Variation in levels of PINs did not impact dough extensibility and did not result in differences in PIN levels in DL. Removal of lipids yielded bread with a less homogeneous gas cell distribution in the crumb, indicating that PINs and lipids act together to impact crumb structure.In conclusion, the results presented in this doctoral work show that PINs contribute to the quality, and, more in particular, the structure of wheat based food products. Although these proteins are present in only small levels in wheat flour, they have a tremendous effect on stabilizing foam structures. In addition, the presence of relatively high levels of endogenous wild-type PINs results in weaker gluten-starch interactions, thereby impacting the product’s texture.