Influence of molecular structure on the susceptibility of starch to α-amylase.

The effect of the molecular structure of sweet potato (SPS), cassava (CAS) and high amylose maize (HAS) starches on the susceptibility to fungal and maltogenic α-amylases was investigated. The logarithm of the slope (LOS) and non-linear least-squares (NLLS) methods were used for fitting hydrolysis kinetics data. The malto-oligosaccharides released during hydrolysis were quantified and the hydrolysis residues were analyzed. The hydrolysis kinetic curves were well fitted to the LOS and NLLS models. SPS, CAS and HAS were hydrolyzed in one single phase by fungal α-amylase while two hydrolysis phases were identified for the root starches and a single phase for HAS, when maltogenic α-amylase was used. The lowest percentage of residual starch was found for CAS, independent of enzyme source, due to the high proportion of amylopectin short chains in this starch. On the other hand, the high proportion of HAS long chains contributed to its increased starch degradation rate coefficient during fungal α-amylase hydrolysis, while the high amylose content favored the endo-action pattern of maltognic α-amylase. Independent of starch source, malto-oligosaccharides of different sizes, especially G2-G5, were released after the fungal α-amylase action which hydrolyzes mainly inner and long amylopectin chains. Mainly maltose was produced in the maltogenic α-amylase hydrolysis which breaks the outer amylopectin chains by exo-action and amylose chains by endo-action. The starch molecular structure strongly interferes in both enzyme susceptibility and the action mechanism, as well as in the distribution and amount of products released.

Resistant starch: effect on rheology, quality, and staling rate of white wheat bread.

The effect of the partial substitution (0, 10, 15, and 20%) of wheat flour with resistant starch (RS) on dough rheology and structure, and on the quality and staling rate of bread was evaluated. The results from farinograph, extensograph, alveograph, oscillatory rheological tests, and from confocal laser scanning microscopy, indicated that the substitution up to 15% of flour with RS slightly affected the dough structure, weakening it through dilution of gluten protein. Bread made with 15% of RS had specific volume, crumb moisture, and firmness values similar to those of the control bread (without RS), indicating very good quality. During storage, the RS breads had higher crumb moisture, lower firmness, and a lower retrogradation rate than the control bread. The lower retrogradation rate, in conjunction with the higher crumb moisture and high water-retention capacity of RS, was responsible for lower crumb firmness in bread containing up to 15% RS. Using wheat flour of high quality helped to minimize the deleterious effect of RS on the dough and provided high-fiber bread with high quality and low staling.

Physico-chemical properties of starches isolated from potato cultivars grown in soils with different phosphorus availability.

BACKGROUND: Starch is the major component of potato tubers, amounting approximately to 150-200 g kg (-1) of the tuber weight. Starch is considered to be a major factor for the functionality of the potato in food applications. This study evaluated the physical characteristics of potato starches isolated from tubers of different potato cultivars grown in soil with three levels of phosphorus (P) availability. All potatoes were growing according the same method. The starches were isolated by physical methods and the samples were analyzed for the amylose, P content, paste properties (RVA) and thermal properties of gelatinization and retrogradation (DSC). Experimental data were analyzed considering the potato cultivars and the three soil P availability. RESULTS: For all measured parameters significant impact of cultivar and soil P availability was determined. Phosphorus contents in potato starches ranged from 0.252 to 0.647 g kg(-1) and amylose from 27.18 to 30.8%. Starches from different potato cultivars independent of soil showed a small range of gelatinization temperature. All starches showed low resistance heating and shear stress. CONCLUSION: The results showed the influence of growing conditions (soil P availability) and also of the differences between the potato cultivars on important characteristics of applicability of starches.

Effect of heat-moisture treatment on the structural, physicochemical, and rheological characteristics of arrowroot starch.

The effect of heat-moisture treatment on structural, physicochemical, and rheological characteristics of arrowroot starch was investigated. Heat-moisture treatment was performed with starch samples conditioned to 28% moisture at 100 ℃ for 2, 4, 8, and 16 h. Structural and physicochemical characterization of native and modified starches, as well as rheological assays with gels of native and 4 h modified starches subjected to acid and sterilization stresses were performed. Arrowroot starch had 23.1% of amylose and a CA-type crystalline pattern that changed over the treatment time to A-type. Modified starches had higher pasting temperature and lower peak viscosity while breakdown viscosity practically disappeared, independently of the treatment time. Gelatinization temperature and crystallinity increased, while enthalpy, swelling power, and solubility decreased with the treatment. Gels from modified starches, independently of the stress conditions, were found to have more stable apparent viscosities and higher G' and G″ than gels from native starch. Heat-moisture treatment caused a reorganization of starch chains that increased molecular interactions. This increase resulted in higher paste stability and strengthened gels that showed higher resistance to shearing and heat, even after acid or sterilization conditions. A treatment time of 4 h was enough to deeply changing the physicochemical properties of starch.

Structural characterization of Peruvian carrot (Arracacia xanthorrhiza) starch and the effect of annealing on its semicrystalline structure.

Structural characteristics of native and annealed Peruvian carrot (Arracacia xanthorrhiza) starches were determined and compared to those of cassava and potato starches. Peruvian carrot starch presented round and irregular shaped granules, low amylose content and B-type X-ray pattern. Amylopectin of this starch contained a large proportion of long (DP > 37) and short (DP 6-12) branched chains. These last ones may contribute to its low gelatinization temperature. After annealing, the gelatinization temperatures of all starches increased, but the ΔH and the crystallinity increased only in Peruvian carrot and potato starches. The annealing process promoted a higher exposure of Peruvian carrot amylose molecules, which were more quickly attacked by enzymes, whereas amylopectin molecules became more resistant to hydrolysis. Peruvian carrot starch had structural characteristics that differed from those of cassava and potato starches. Annealing affected the semicrystalline structure of this starch, enhancing its crystallinity, mainly due to a better interaction between amylopectin chains.

Production of crude xylanase from Thermoascus aurantiacus CBMAI 756 aiming the baking process.

In recent years, the baking industry has focused its attention on substituting several chemical compounds with enzymes. Enzymes that hydrolyze nonstarch polysaccharides, such as xylanase, lead to the improvement of rheological properties of dough, loaf specific volume, and crumb firmness. The purpose of this study was to find a better solid-state fermentation substrate to produce high levels of xylanase and low levels of protease and amylase, which are enzymes involved in bread quality, from Thermoascus aurantiacus CBMAI 756. Wheat bran, corncob, and corn straw were used as energy sources. The enzyme extract of corncob showed high xylanase activity (130 U/mL) and low amylase and protease activity (<1 and 15 U/mL, respectively). This enzyme profile may be more profitable for the baking industry, because it results in a slower degradation of gluten. Our results confirm this finding, because the enzyme obtained by fermentation in corncob resulted in a gluten with a higher specific volume than all the other substrates that were tested. The crude xylanase presented maximum activity at a pH of 5, and the optimum temperature was 75 °C. It was stable up to 70 °C for an hour and at a pH range from 4 to 10.