The Retrogradation of Concentrated Wheat Starch Systems

A structural study of the self-association of different starches in presence of bacterial cellulose fibrils

A multi-analytical study was performed to analyse the effect of bacterial cellulose (BCF) on the self-association of starches with different amylose content (wheat, waxy-maize), assessing macrostructural properties (rheology, gel strength) and some nano and sub-nano level features (small and wide-angle X-ray scattering). Although pasting viscosities and G' were significantly increased by BCF in both starches, cellulose did not seem to promote the self-association of amylose in short-range retrogradation. A less elastic structure was reflected by a 2-3-fold increase in loss factor (G″/G') at the highest BCF concentration tested. This behavior agreed with the nano and sub-nano characterisation of the samples, which showed loss of starch lamellarity and incomplete full recovery of an ordered structure after storage at 4 °C for 24 h. The gel strength data could be explained by the contribution of BCF to the mechanical response of the sample. The information gained in this work is relevant for tuning the structure of tailored starch-cellulose composites.

Effects of Controlled Relative Humidity Storage on Moisture Sorption and Amylopectin Retrogradation in Gelatinized Starch Lyophiles

Water plays a significant role in the gelatinization and retrogradation (crystallization) of starch. Amylopectin crystalline regions can adopt several hydrated polymorphic forms; however, reports differ on the migration of water during retrogradation. The objectives of this study were to determine the moisture sorption patterns of gelatinized starch lyophiles during retrogradation in controlled relative humidity (RH) environments and document the amylopectin polymorph(s) formed. Starches from different botanical sources containing A-type and B-type amylopectin polymorphs were studied. Suspensions of starch were heated and then frozen and freeze-dried to make primarily amorphous matrices. Moisture sorption profiles of the dried samples were collected from 5% RH to 95% RH at 25 °C. To capture the retrogradation event, sample masses were also monitored at constant RHs over time (95%, 92.5%, and 90% RH). Powder X-ray diffraction was used to document the physical state of the samples, including the amylopectin polymorph formed upon retrogradation, and differential scanning calorimetry was used to determine glass transition temperatures (T_g s). In all lyophiles, water was first absorbed (mass gain), and if a critical water content was reached (at ≥92.5%RH), sample T_g s dropped below room temperature and concurrent retrogradation and water expulsion (mass loss) occurred, regardless of starch botanical source and whether A- or B-type polymorphs were formed. Overall, retrogradation and water expulsion increased as storage RH increased. These results offer further knowledge into the role of water in amylopectin retrogradation and the relationship among starch type, environmental RH, moisture sorption prior to retrogradation, and water redistribution during retrogradation. PRACTICAL APPLICATION: Starch gelatinization and retrogradation require molecular mobility, which is facilitated by water. Limited retrogradation occurred in lyophiles in the glassy state (90% RH, 25 °C), but increasing the storage RH (to ≥92.5% RH) resulted in increasing amylopectin retrogradation (note: many baked products have water activities in this range). Regardless of starch type (botanical source and amylose content), when the storage RH was high enough, the starch lyophiles first absorbed water, which depressed the T_g below the storage temperature, and then exhibited concomitant retrogradation and water expulsion. The water expelled during amylopectin retrogradation was not (fully) retained in the amorphous starch fraction, which is why samples lost weight. Water leaving the starch matrix during retrogradation could pose challenges for quality, texture, and shelf-life of starch-based products.

Investigation of the effects of starch on the physical and biological properties of polyacrylamide (PAAm)/starch nanofibers

Here, we report the development of a new polyacrylamide (PAAm)/starch nanofibers’ blend system and highlight its potential as substrate for efficient enzyme immobilization. PAAm was synthesized and blended with starch. The final blend was then electrospun into nanofibers. The response surface methodology was used to analyze the parameters that control nanofiber’s diameter. Electrospun mat was then modified either by cross-linking or phytase immobilization using silane coupling agent and glutaraldehyde chemistry. Physico-chemical properties of blends were investigated using spectroscopic and thermal studies. The evaluation of immobilized enzyme kinetics on both pure and the starch blended PAAm nanofibers was performed using Michaelis–Menten kinetic curves. Fourier transform infrared spectroscopy results along with differential scanning and X-ray diffraction confirmed that blending was successfully accomplished. TGA analysis also demonstrated that the presence of starch enhances the thermal degradability of PAAm nanofibers. Finally, it was shown that addition of starch to PAAm increases the efficacies of enzyme loading and, therefore, significantly enhances the activity as well as kinetics of the immobilized enzyme on electrospun blend mats.

Rheological Properties of Starch and Whey Protein Isolate Gels

The paste viscosity of starches Amioca (~0.5% amylose), native corn (~27% amylose), Hylon VII (~70% amylose), tapioca (~19% amylose), and their whey protein isolate (WPI; 50/50) mixtures at varying solid concentration of 2.5, 5, 10, 15 and 20% were determined. At higher gel concentration (20%) during the cooling cycle, WPI paste measured by rapid visco-analyser (RVA) showed a tremendous increase in viscosity over that of the micro visco-amylograph (MVA), 5784 and 184 cP, respectively. RVA gels had a more uniform network of coarse particles (~250—500 nm) than the fine structures (~50 nm) in those of the MVA. Inclusion of WPI reduced the paste viscosity of all starches by at least 50%, except for Hylon VII at 20% solid concentration. The strength of Hylon VII and corn starch gels was reduced by WPI. In contrast, gels of WPI/tapioca ( G' = 45.4 Pa) and WPI/amioca ( G' = 18.3 Pa) had similar rheological properties as their pure starch control ( G' = 47.4 Pa and G' = 15.3 Pa for Tapioca and Amioca, respectively).

Monitoring Wood Degradation during Weathering by Cellulose Crystallinity

The degree of crystallinity of cellulose was used for assessing the degradation level of coated and uncoated samples of pine wood after weathering. X-ray diffraction (XRD) and Fourier Transform Infrared (FT-IR) spectroscopy measured the changes in the surface crystallinity of cellulose resulting from weathering, both natural and artificial. Both techniques revealed an increase in the crystallinity index (CI) of cellulose when wood was subjected to weathering. An increase in the size of crystallites was also observed by XRD measurements. These results were related to the reduction of the amorphous fractions of wood, and, consequently, to the enrichment of the relative crystalline content. Thanks to FT-IR analysis, the degradation of hemicellulose was observed for uncoated samples after exposure to artificial weathering. The effect of weathering was less evident on coated samples because of the protective action of the coating. A good correlation between the crystallinity indexes obtained from FT-IR and XRD was found. The experimental results proved that the proposed method may be a very useful tool for a rapid and accurate estimation of the degradation level of wood exposed to weathering. This methodology can find application in the field of conservation and restoration of wooden objects or in the industry of wood coatings.

In vitro starch digestibility and predicted glycemic index of microwaved and conventionally baked pound cake

The present study compares the effect of baking process (microwave vs conventional oven) on starch bioavailability in fresh pound cake crumbs and in crumbs from pound cake stored for 8 days. Proximal chemical analysis, resistant starch (RS), retrograded starch (RS3) and starch hydrolysis index (HI) were evaluated. The empirical formula suggested by Granfeldt was used to determine the predicted glycemic index (pGI). Pound cake, one of Mexico's major bread products, was selected for analysis because the quality defects often associated with microwave baking might be reduced with the use of high-fat, high-moisture, batted dough. Differences in product moisture, RS and RS3 were observed in fresh microwave-baked and conventionally baked pound cake. RS3 increased significantly in conventionally baked products stored for 8 days at room temperature, whereas no significantly changes in RS3 were observed in the microwaved product. HI values for freshly baked and stored microwaved product were 59 and 62%, respectively (P > 0.05), whereas the HI value for the conventionally baked product decreased significantly after 8 days of storage. A pound cake with the desired HI and GI characteristics might be obtained by adjusting the microwave baking process.