Exploration of the potential to modulate digestibility of starch in foods by preserving endosperm cell wall intactness or enzymatically modifying its structure

Starch is the main glycemic carbohydrate in human diet. High glycemic responses are an independent risk factor for developing diabetes. It is, therefore, vital to comprehend the factors that dictate starch digestion and to develop strategies to (re)formulate foods with enhanced nutritional functionality. In this context, the molecular structure of starch as well as the surrounding food matrix including the native plant cellular structure can play a decisive role on starch digestion. Targeted manipulations of these features could have a profound impact on starch digestibility and postprandial glycemic responses. Against this background, the objective of this doctoral dissertation was to explore the potential of native wheat endosperm cellular structures and enzymatic modifications of starch to modulate starch digestibility. In a FIRST PART, intact wheat endosperm cell walls were considered as a first tool to modulate starch digestibility and their role on swelling, gelatinization and digestibility of intracellular starch was studied by utilizing wheat endosperm milling fractions with different cell wall integrity levels. The swelling capacity of starch and the viscosifying potential of coarse farina (i.e. a wheat endosperm milling fraction with an average diameter of 705 µm), which contains a substantial portion of intact cells, were consistently lower than those of flour (i.e. the wheat endosperm milling fraction with an average diameter of 85 µm) and fine farina (i.e. a wheat endosperm milling fraction with average diameter of 330 µm) which contained no intact cells. This illustrates that starch present in intact cells did not swell to its full capacity. It was noted that the swelling capacity and the viscosity of only coarse farina increased upon enzymatic degradation of the cell wall polysaccharides and that, as a result of the treatment, they reached levels similar to those observed for flour and fine farina. Overall, it was demonstrated that the intact cell walls protected starch from extensive swelling upon heating in excess of water. The cell wall integrity negatively impacted in vitro starch digestibility. The extent of starch digestion was similar for all gelatinized samples, but the digestion rate constant was 42% lower for coarse farina (0.37 min-¹) than for flour (0.63 min-¹) and fine farina (0.64 min-¹). That indicated that the intact cells in coarse farina limit the diffusion of the pancreatic α-amylase and reduce the digestion rate constant. However, the similar extent of digestion suggests that the cell walls are permeable. Next, the cell walls were enzymatically degraded after hydrothermal processing to obtain starches differing in swelling behavior. In this case, the digestion rate constant became equal in all cases. This series of experiments showcased that the preservation of cell wall integrity can delay diffusion of amylolytic enzymes and lower the rate of digestion of intracellular starch irrespective of starch swelling. Coarse farina with intact cells could, hence, be used a functional ingredient to lower starch digestibility. In order to provide proof-of-concept, coarse farina containing intact cell walls was incorporated in a bread recipe and the impact on bread quality characteristics and the digestibility of its starch was studied. When substituting up to 80% of the white wheat flour with coarse farina, the bread loaf volume was decreased from 5.9 down to 4.5 mL/g and, in turn, the firmness of its crumb was increased from 0.8 up to 1.2 N. These observations were attributed to the bread having a disrupted gluten network in the presence of coarse farina particles and the unavailability of the gluten enclosed in cells to participate in the network formation. However, the quality of the bread produced from coarse farina containing recipes was acceptable. The cells in coarse farina remained seemingly intact in the bread crumb. Nevertheless, the extent and rate constant of starch digestion did not change, irrespective of the levels of coarse farina incorporation. The gluten network in regular breads may well be a sufficient barrier to starch digestion while in the breads produced from coarse farina containing recipes the contribution of starch encapsulation into the cells on starch digestibility does not compensate for the loss of the barrier effect from the disrupted gluten network. In a SECOND PART, enzymatic modifications of the molecular structure of starch were investigated as a second tool to modulate starch digestibility. The thermoactive amylomaltase (AMM) from Thermus thermophilus was employed to modify starch. This enzyme catalyzes the transfer of glucan segments from amylose (AM) to elongate the amylopectin (AP) chains via an intermolecular transglucosylation reaction. This enzyme was chosen since its potential to decreasing the digestibility of the modified starch has not been explored in detail, despite its unique action pattern and literature based evidence that longer AP chains in retrograded starch may be digested more slowly. At low dosages of the enzyme in starch slurries, only partial shortening of the AM chains was observed which led to increased viscosity upon cooling due to the enhanced aggregation of the more mobile shorter AM chains. With increasing dosages, the peak viscosity in Rapid Visco Analysis upon heating increased from 2,825 to 3,050 mPa·s because of the quite extensive depolymerization of AM which, in its unmodified form, inhibits starch swelling. In parallel, the AP chains were gradually elongated and AM was gradually degraded to even result in its complete depletion. When the modified starches were stored at low temperature, the extent of starch retrogradation, here expressed as melting enthalpy, increased from 0.8 up to 7.7 J/g starch in line with the extent of AP chain elongation. Finally, the rate constant and extent of starch digestion were drastically reduced upon enzymatic modification of starch from 0.6 down to 0.2 min-¹ and from 54.5 down to 35.9 g digested starch per 100 g starch, respectively. The AP chain lengths were positively related with the melting enthalpies of retrograded starches, which, in turn, were negatively correlated with the extent and rate constant of in vitro digestion. Evidently, different enzyme dosages in starch slurries resulted in different molecular structures that induced different viscosity development while the considerable elongation of AP chains was found to be important for lowering the susceptibility of starch to digestion upon retrogradation. The next step was to in situ modify starch during bread making with starch-modifying enzymes [AMM and maltogenic α-amylase (MA)] and to investigate the impact on bread characteristics, starch retrogradation and digestibility. It was hoped that it would be possible to provide proof that enzymes can be used in food systems to lower starch digestibility. MA was also examined here as it is widely used in the bread making industry. The use of AMM led to partial shortening of the AM chains but not to a detectable level of AP chains elongation probably due to insufficient activity in the dough. The treatment had only a small negative impact on the rate of bread crumb firming upon storage and influenced neither starch retrogradation nor its digestibility. The addition of MA led to shortening of the AP chains. Indeed, the relative content of short chains with ≤ 8 glucose units increased from 8.8 to 13.8%. Starch retrogradation and crumb firmness were reduced upon storage of breads due to the incapacity of the shortened AP chains to re-crystallize. Interestingly, the extent of starch digestion declined from 46.7 to 38.0 g digested starch per 100 g crumb. Indeed, pancreatic α-amylase cannot bind close to the branching points of AP and when its chains are shortened, productive binding is hindered. Overall, AMM had only very limited functionality during bread making and no effect on starch digestibility, while MA presented a dual functionality in that it both decreased the crumb firmness upon storage and the extent of starch digestion. Lastly, we investigated the difference between postprandial glycemia after consumption of a cold-stored oatmeal porridge containing either unmodified or AMM modified starch. It was reasoned that oatmeal porridge is a food system the preparation of which can accommodate for sufficient AMM activity AMM. The extent of starch retrogradation, also here expressed as the corresponding enthalpy, increased from 0 (non detectable) to 5.3 J/g oats because of the enzymatic modification which probably resulted in AP chain elongation to an extent sufficient to promote re-crystallization. The postprandial glycemic responses were not significantly lowered and the enzymatic modification thus had no impact. This lack of effect may have been due to the texture-mediated effect of AM degradation on starch accessibility in the gastrointestinal tract, the potentially insufficiently increased extent of retrogradation as well as to the large inter-individual variation. In conclusion, this doctoral dissertation illustrates the potential of strategies such as starch encapsulation into the intact native wheat endosperm cells and enzymatic starch modifications for modulating starch digestibility. Further research should focus on the application of such insights in a wide range of common cereal-based food products such as other types of (whole-meal) breads, cookies or puddings as well as on validating any health benefits with randomized clinical trials.

Publication year:2022

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