Sara Gomand

Fate of Starch in Food Processing: From Raw Materials to Final Food Products

Starch, an essential component of an equilibrated diet, is present in cereals such as common and durum wheat, maize, rice, and rye, in roots and tubers such as potato and cassava, and in legumes such as peas. During food processing, starch mainly undergoes nonchemical transformations. Here, we focus on the occurrence of starch in food raw materials, its composition and properties, and its transformations from raw material to final products. We therefore describe a number of predominant food processes and identify research needs. Nonchemical transformations that are dealt with include physical damage to starch, gelatinization, amylose-lipid complex formation, amylose crystallization, and amylopectin retrogradation. A main focus is on wheat-based processes. (Bio)chemical modifications of starch by amylolytic enzymes are dealt with only in the context of understanding the starch component in bread making.

Presence of Amylose Crystallites in Parboiled Rice

Mildly, intermediately, and severely parboiled Jacinto [16% free amylose (FAM) content] and Puntal (26% FAM content) rice samples were submitted to differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). DSC thermograms revealed ungelatinized starch only in mildly parboiled rices and retrograded amylopectin in all parboiled samples. Amylose crystallites were present in intermediately and severely parboiled samples but could not be detected due to their high melting temperature. Nonparboiled and parboiled rice DSC profiles showed only type I and type II amylose-lipid complexes, respectively. Intermediately and severely parboiled rice showed a clear V(h)-type (crystalline amylose-lipid complexes) with a superimposed B-type (retrograded amylopectin and/or amylose crystallites) pattern. The mildly parboiled samples showed a mix of A- (native starch crystallites) and V(h)-type patterns (Puntal) and A-, V(h)-, and B-type patterns (Jacinto). Mild acid hydrolysis destroyed the acid labile retrograded amylopectin crystallites and increased the relative abundance of amylose crystallites. Indeed, acid-hydrolyzed intermediately and severely parboiled samples of both cultivars showed a clear B-type diffraction pattern conclusively showing, for the first time, the presence of amylose crystallites. The melting temperature of the amylose crystallites was ca. 135 degrees C, and melting peaks were visible in the DSC thermograms of the intermediately and severely parboiled samples. Their levels depended on the degree of parboiling and FAM content.

Systemic availability and metabolism of colonic-derived short-chain fatty acids in healthy subjects: a stable isotope study

The short‐chain fatty acids (SCFAs) are bacterial metabolites produced during the colonic fermentation of undigested carbohydrates, such as dietary fibre and prebiotics, and can mediate the interaction between the diet, the microbiota and the host. We quantified the fraction of colonic administered SCFAs that could be recovered in the systemic circulation, the fraction that was excreted via the breath and urine, and the fraction that was used as a precursor for glucose, cholesterol and fatty acids. This information is essential for understanding the molecular mechanisms by which SCFAs beneficially affect physiological functions such as glucose and lipid metabolism and immune function.

Characterisation of three starch degrading enzymes: thermostable β-amylase, maltotetraogenic and maltogenic α-amylases.

Maltogenic α-amylase from Bacillus stearothermophilus (BStA) is widely used as bread crumb anti-firming enzyme. A maltotetraose-forming α-amylase from Pseudomonas saccharophila (PSA) was recently proposed as alternative, hence the need to compare both exo-acting enzymes with some endo-action component. A purely exo-acting thermostable β-amylase from Clostridium thermosulfurogenes (CTB) was included for reference purposes. Under the experimental conditions used, temperature optima of the enzymes are rather similar (60-65 °C), but temperature stability decreased in the order BStA, PSA and CTB. The action of the enzymes on different substrates and their impact on the rheological behaviour of maize starch suspensions demonstrated that, while CTB acts exclusively through an exo-action mechanism, BStA displayed limited endo-action which became more pronounced at higher temperatures. PSA has more substantial endo-action than BStA, which is rather temperature independent. This is important for their impact in processes such as breadmaking, where temperature is gradually increased.

Quantification of in Vivo Colonic Short Chain Fatty Acid Production from Inulin

Short chain fatty acids (SCFA), including acetate, propionate, and butyrate, are produced during bacterial fermentation of undigested carbohydrates in the human colon. In this study, we applied a stable-isotope dilution method to quantify the in vivo colonic production of SCFA in healthy humans after consumption of inulin. Twelve healthy subjects performed a test day during which a primed continuous intravenous infusion with [1-13C]acetate, [1-13C]propionate and [1-13C]butyrate (12, 1.2 and 0.6 μmol·kg−1·min−1, respectively) was applied. They consumed 15 g of inulin with a standard breakfast. Breath and blood samples were collected at regular times during the day over a 12 h period. The endogenous rate of appearance of acetate, propionate, and butyrate was 13.3 ± 4.8, 0.27 ± 0.09, and 0.28 ± 0.12 μmol·kg−1·min−1, respectively. Colonic inulin fermentation was estimated to be 137 ± 75 mmol acetate, 11 ± 9 mmol propionate, and 20 ± 17 mmol butyrate over 12 h, assuming that 40%, 10%, and 5% of colonic derived acetate, propionate, and butyrate enter the systemic circulation. In conclusion, inulin is mainly fermented into acetate and, to lesser extents, into butyrate and propionate. Stable isotope technology allows quantifying the production of the three main SCFA in vivo and proved to be a practical tool to investigate the extent and pattern of SCFA production.

Molecular and Morphological Aspects of Annealing-Induced Stabilization of Starch Crystallites

A unique series of potato (mutant) starches with highly different amylopectin/amylose (AP/AM) ratios was annealed in excess water at stepwise increasing temperatures to increase the starch melting (or gelatinization) temperatures in aqueous suspensions. Small-angle X-ray scattering (SAXS) experiments revealed that the lamellar starch crystals gain stability upon annealing via thickening for high-AM starch, whereas the crystal surface energy decreases for AM-free starch. In starches with intermediate AP/AM ratio, both mechanisms occur, but the surface energy reduction mechanism prevails. Crystal thickening seems to be associated with the cocrystallization of AM with AP, leading to very disordered nanomorphologies for which a new SAXS data interpretation scheme needed to be developed. Annealing affects neither the crystal internal structure nor the spherulitic morphology on a micrometer length scale.

Distribution of Minerals in Wheat Grains (Triticum aestivum L.) and in Roller Milling Fractions Affected by Pearling.

The distribution of minerals in (pearled) wheat grains was measured by synchrotron X-ray fluorescence, and the impact of pearling (0, 3, 6, 9, and 12% by weight) on the mineral composition of flour, shorts, and bran was identified by ICP-MS. The xylem mobile elements (Mn, Si, Ca, and Sr) dominated in the outermost bran layers, while the phloem mobile elements (K, Mg, P, Fe, Zn, and Cu) were more concentrated in the aleurone. Pearling lowered the concentrations of xylem mobile elements and increased the concentrations of most phloem mobile elements in the pearled grains. Molybdenum, Cd, and especially Se were more evenly distributed, and pearling affected their concentrations in milling products less. Pearling (3%) increased the concentration of several nutrients (P, Zn, Cu) in the flour because the bran fractions reaching the flour are enriched in aleurone. The correlations of concentrations of Mg, Fe, Zn, and Cu with that of P suggested their association with phytate.

Structural and thermal transitions during the conversion from native to granular cold-water swelling maize starch

Native maize starch was gradually converted into granular cold-water swelling starch (GCWSS) by aqueous ethanol treatments at elevated temperatures. At a treatment temperature of 95°C, decreasing ethanol concentrations from 68 to 48% (v/v) led to decreased post-treatment gelatinization enthalpies in excess water, reflecting remaining original A-type crystals. Concomitantly to native A-type crystal melting, VH-type crystals appeared. At an ethanol concentration of 48%, a granular cold-water swelling maize starch was successfully produced. All crystals in its intact granules were of the VH-type and appeared birefringent when studied in ethanol under polarized light. Removal of all residual solvent by high temperature drying did not influence swelling power, proving that a high temperature drying step is not necessary to induce cold-water swelling capacity. Based on in situ calorimetric measurements, the thermal requirements to produce GCWSS from different ethanol:water mixtures were elucidated. This work is the first to demonstrate that the amylose fraction contributes almost exclusively to VH-type crystal formation in GCWSS.

Moisture distribution during conventional or electrical resistance oven baking of bread dough and subsequent storage.

Electrical resistance oven (ERO) baking processes bread dough with little temperature gradient in the baking dough. Heating of the dough by means of an ERO is based on the principles of Joules first law and Ohms law. This study compared the changes in moisture distribution and physical changes in starch of breads conventionally baked or using an ERO. The moisture contents in fresh ERO breads are generally lower than those in conventional breads. During storage of conventionally baked breads, water migrates from the crumb to the crust and moisture contents decrease throughout the bread crumb. Evidently, less moisture redistribution occurs in ERO breads. Also, the protons of ERO bread constituents were less mobile than their counterparts in conventional bread. Starch retrogradation occurs to similar extents in conventional and ERO bread. As a result, the changes in proton mobility cannot be attributed to differences in levels of retrograded starch and seem to be primarily determined by the overall lower moisture content.

Structural and physicochemical characterisation of rye starch

The gelatinisation, pasting and retrogradation properties of three rye starches isolated using a proteinase-based procedure were investigated and compared to those of wheat starch isolated in a comparable way. On an average, the rye starch granules were larger than those of wheat starch. The former had very comparable gelatinisation temperatures and enthalpies, but slightly lower gelatinisation temperatures than wheat starch. Under standardised conditions, they retrograded to a lesser extent than wheat starch. The lower gelatinisation temperatures and tendencies of the rye starches to retrograde originated probably from their higher levels of short amylopectin (AP) chains [degree of polymerisation (DP) 6-12] and their lower levels of longer chains (DP 13-24) than observed for wheat starch. The rapid visco analysis differences in peak and end viscosities between the rye starches as well as between rye and wheat starches were at least partly attributable to differences in the levels of AP short chains and in average amylose molecular weight. The AP average chain lengths and exterior chain lengths were slightly lower for rye starches, while the interior chain lengths were slightly higher than those for wheat starch.