Effect of protein–starch interactions on starch retrogradation and implications for food product quality

Starch retrogradation is a consequential part of food processing that greatly impacts the texture and acceptability of products containing both starch and proteins, but the effect of proteins on starch retrogradation has only recently been explored. With the increased popularity of plant-based proteins in recent years, incorporation of proteins into starch-based products is more commonplace. These formulation changes may have unforeseen effects on ingredient functionality and sensory outcomes of starch-containing products during storage, which makes the investigation of protein–starch interactions and subsequent impact on starch retrogradation and product quality essential. Protein can inhibit or promote starch retrogradation based on its exposed residues. Charged residues promote charge–dipole interactions between starch-bound phosphate and protein, hydrophobic groups restrict amylose release and reassociation, while hydrophilic groups impact water/molecular mobility. Covalent bonds (disulfide linkages) formed between proteins may enhance starch retrogradation, while glycosidic bonds formed between starch and protein during high-temperature processing may limit starch retrogradation. With these protein–starch interactions in mind, products can be formulated with proteins that enhance or delay textural changes in starch-containing products. Future work to understand the impact of starch–protein interactions on retrogradation should focus on integrating the fields of proteomics and carbohydrate chemistry. This interdisciplinary approach should result in better methods to characterize mechanisms of interaction between starch and proteins to optimize their food applications. This review provides useful interpretations of current literature characterizing the mechanistic effect of protein on starch retrogradation.     

Estimation of the free volume of starch–water barriers

The permeation of volatiles through amorphous barrier materials is affected by their size and geometry as well as by their physical–chemical affinity for the barrier. The gas barrier property of an amorphous polymer is generally a function of density, cohesive energy density and stiffness as well as of the free volume available for the diffusion of volatiles in the microstructure of polymers. The barrier property of amorphous starch is investigated by measuring the density and the cohesive energy density as a function of water contents in an extended temperature range in which the material is either a brittle to rubbery solid or a viscoelastic liquid melt. Free volumes available for the diffusion of volatiles in starch barriers are deduced numerically from a mean field equation of state providing molecular scaling parameters for starch–water mixtures from a least squares fit of experimental data. The consistency of the mean field scaling parameters is reflected in their ability to predict accurate surface tension for these systems. The adsorption of small amounts of water is found to increase the density and to reduce the free volume of starch glasses by a phenomenon called antiplasticization as it results in an increase of Young's moduli of materials whereas the opposite is true for plasticization. At higher moisture contents, an overwhelming plasticization leads to a weakening of gas barrier properties as the free volume of starch is increased during swelling by water.     

Complex formation of a 4-α-glucanotransferase using starch as a biocatalyst for starch modification

Food Science and Biotechnology - A 4-α-glucanotransferases from Thermus thermophilus (TTαGT) possesses an extra substrate binding site, leading to facile purification of the intact enzyme...     

Effect of β-cyclodextrin on the long-term retrogradation of rice starch

In this study, the impact of the addition of β-cyclodextrin (β-CD) on long-term retrogradation of rice starch was investigated by texture profile analysis (TPA), differential scanning calorimeter (DSC), and X-ray diffraction (XRD). Results showed that the addition of β-CD presented a crucial role in retarding the long-term retrogradation of starch. The present β-CD also significantly decreased crystallization rate (k), and increased avrami exponent (n). The increase in exponent value indicated that the addition of β-CD transformed the nucleation type due to the formation of amylose–lipid-β-CD complex. Further, the presence of β-CD produced an additional peak 5.2 Å of the gelatinized gel, and retarded crystalline type of the retrograded gel from V- to B-pattern, probably corresponding for the nucleation type transformation.     

Influence of β-cyclodextrin on the short-term retrogradation of rice starch

The effect of β-cyclodextrin (β-CD) on short-term retrogradation of rice starch was studied. Retrogradation of normal rice starch was reduced more by β-CD than by glycerol monostearate (GMS). β-CD reduced the rate of retrogradation of amylose and normal rice starch but not waxy rice starch. Differential scanning calorimetry (DSC) detected a potential amylose-β-CD complex formation. DSC data were analysed using the Avrami equation. Results showed that β-CD significantly lowered the crystallizing rate (k) and increased the Avrami exponent (n) of amylose recrystallisation (P < 0.05). Molecular dynamics (MD) simulation indicated that the stability of the complex was primarily due to non-bonded interactions, such as Van der Waals (Vdw forces), electrostatic force, and hydrogen bonds formed in the presence of β-CD.     

Mechanical and microstructural properties of soy protein – high amylose corn starch extrudates in relation to physiochemical changes of starch during extrusion

Mechanical and microstructural properties of expanded extrudates prepared from blends of high amylose corn (Zeamays L. ssp. Mays) starch (HACS) and soy protein concentrate (SPC) were studied in relation to the physicochemical changes in starch. Effects of screw speed (230 and 330 rpm) and SPC level (10%, 20%, 30% and 50%) on expansion and mechanical properties were determined. Compared with 230 rpm, screw speed at 330 rpm resulted in increased specific mechanical energy, expansion ratio, water absorption and water solubility indices and decreased bulk density and piece density. Varying screw speeds did not significantly affect the mechanical strength of extrudates or starch molecular weight distribution. Bulk and piece densities, and water absorption index (WAI) only slightly increased or exhibited no significant trends as SPC level increased to 20%. A substantial increase in bulk and piece densities and decrease in expansion ratio and WAI were observed as SPC level increased from 20% to 30%. The trends were either reversed or moderated as SPC increased to 50%. These results in combination with average crushing force and water solubility index data provided a significant insight into the interactions between HACS and SPC during extrusion processing. As compared to an earlier baseline study by our research group on normal corn starch – SPC extrudates, results from the current study indicated that the expansion of extrudate containing HACS alone was lower than that of extrudates containing normal corn starch. However, expansion of the HACS–SPC blends was not significantly impacted at 10–20% SPC levels, whereas the expansion of normal corn starch was significantly reduced.     

Time-dependent flow properties of starch–milk–sugar pastes

The time-dependent flow properties of starch–milk–sugar (SMS) pastes have been studied. The flow properties were assessed from the measurement of the shear stress versus time of shearing at constant shear rate. Corn and wheat starches were used in this study, while the sugars were glucose, sucrose, and fructose. The Weltman model was used to evaluate the flow properties of SMS pastes prepared under different conditions. SMS pastes heated at 95 and 85 °C exhibited a thixotropic behavior, while pastes heated at 75 °C behaved like a rheopectic fluid. It was noted that the thixotropy occurred at high shear stress (above 50 Pa), and the rheopexy occurred at low shear stress (below 45 Pa). The degree of thixotropy, as assessed by the Weltman model parameters, increased significantly with starch concentration, and with less pronounced effect with sugar concentration. The effect of sugar type on the degree of thixotropy of SMS pastes heated at 95 °C decreased in the following order: fructose>sucrose>glucose. The type of starch played a role in the time-dependent flow properties of the SMS paste, with a general conclusion that wheat starch had a greater degree of thixotropy than corn starch.     

Effect of different β-glucans on the gelatinisation and retrogradation of rice starch

Four β-glucan preparations, i.e., curdlan (CL), oat (OG), barley (BG) and yeast (YG) β-glucans, were compared for their effects on the gelatinisation and retrogradation of rice starch (RS). Rapid visco-analysis (RVA) showed that addition of any of these β-glucans significantly increased the peak, breakdown, final, and setback viscosities of RS, whereas the pasting temperatures were significantly decreased by OG or CL addition, but were unaffected by BG or YG addition. Differential scanning calorimetry (DSC) demonstrated that all the β-glucans had a negligible effect on the onset (T_o), peak (T_p), and conclusion (T_c) temperatures but slightly decreased the gelatinisation enthalpy (△H₁) of RS. Storage of all the gels at 4 °C resulted in a marked decrease in the T_o, T_p, T_c, and melting enthalpy (△H₂) values. The retrogradation ratio (△H₂/△H₁) and the phase transition temperature range (T_c–T_o) of all the gels increased with storage time. Dynamic viscoelastic measurements revealed weak gel-like behaviour of all the gels, in which their storage modulus (G′) increased and their loss tangent (tan δ) decreased during storage. Steady flow tests illustrated time-dependent shear-thinning (thixotropic) behaviour of all the gels. The hysteresis loop area and the gel hardness increased with storage time. However, the rate and extent of retrogradation and the rheological and textural changes of the RS gels were reduced by addition of any of these β-glucans. The extent of the aforementioned effects differed among the different β-glucan preparations, generally in the order OG ≈ BG > CL ≈ YG.     

Preparation of aqueous dispersion of β-carotene nano-composites through complex formation with starch dextrin

Aqueous dispersions of β-carotene nano-composite were prepared through a blending process with a dextrin of high amylose corn starch in immiscible phases of aqueous dextrin and organic β-carotene solutions and water solubility and stability against oxidation of β-carotene were examined. Gradual evaporation of organic solvent during the mechanical stirring allowed the formation of aqueous solutions in which β-carotene was homogeneously dispersed with dextrin. Proper control of the solid concentrations effectively induced the complex formation between dextrin and β-carotene whereas an excess amount of β-carotene (>8 mg) in a dextrin solution (200 mg in 50 mL) induced crystallization and precipitation of β-carotene. The complex particles showed an irreversible endothermic transition of V-amylose (via DSC). X-ray diffraction analysis, however, revealed that the complex had no typical V-type crystalline arrangement. Hydrodynamic size of the complex particles in the aqueous solution could be decreased to nano-scale (16–30 nm) by a mild ultrasonication. The nano-composite contained β-carotene at approximately 30% on a weight, with improved stabilities against oxidation and enzymatic digestion.     

Properties and sorption studies of polyethylene oxide–starch blended films

A series of polyethylene oxide (PEO) and starch blends were prepared by extrusion and their films prepared by compression molding. The mechanical, barrier, optical, thermal properties and surface morphology characteristics of PEO–starch blended films were studied. The tensile strength of the blended films decreased whereas the haze values increased with an enhanced starch concentration in PEO–starch blends. The inter-molecular interaction between PEO–starch blends were investigated with FTIR. The thermal properties were also analyzed by differential scanning calorimetric (DSC) and thermo-gravimetric analysis (TGA) techniques. Moisture-sorption characteristics of PEO–starch blended films were carried out at 27 °C for water activity (a_w) ranged from 0.1 to 0.9. The sorption data at different a_w were used to fit different sorption isotherm models as proposed in the literatures. The model constants were determined by linear fitting of the sorption equations. The high coefficient of determination R² (ranged from 0.7 to 1) confirms the applicability of the equations employed. The study on the application of such water activity data of PEO/starch blended films on different model equations will be helpful in prediction of durability and functional behavior of moisture sensitive biopolymeric films.     

Reaction kinetics study of α-amylase in the hydrolysis of starch size on cotton fabrics

A study of tension surging and of the properties of the resultant yarns is reported. Tension profiles are discussed and assigned to three modes of operation, namely, pre-surging, surging, and post-surging. The critical conditions for these three modes and the wavelengths and amplitude of tension surging are investigated. The insertion of twist is examined, and it is shown that, not only is the surging mode unstable, hut there is also a higher-frequency instability in the post-surging mode. On the basis of the results obtained from routine tests for textured yarns, it is confirmed that the quality of the yarns resulting from both surging and post-surging modes is not acceptable.     

Effect of the incorporation of antioxidants on physicochemical and antioxidant properties of wheat starch–chitosan films

Blend edible films were prepared from wheat starch (WS) and chitosan (CH) with glycerol as plasticizer. Four active ingredients (antioxidants) were added, namely basil essential oil, thyme essential oil, citric acid and α-tocopherol. The starch:antioxidant mass ratio was 1:0.1. Prior to characterisation, the films were conditioned at 25 °C–53%RH as to their structural, mechanical, optical and barrier properties. The antioxidant capacity of the active ingredients was determined by means of a spectrophotometric method. The incorporation of antioxidants led to a heterogeneous film microstructure, mainly in those containing α-tocopherol, which affected the surface roughness. Yellowness was induced in films when α-tocopherol was added and no notable colour changes were observed in the other cases, although all the antioxidants increased the transparency of the films. Despite of the fact that the mechanical properties were barely affected by the incorporation of antioxidants, citric acid promoted an increase in the elastic modulus but a decrease in film stretchability. The water vapour barrier properties of the films were only slightly improved when citric acid and α-tocopherol were added, whereas the oxygen barrier properties were significantly improved in all cases. The greatest antioxidant capacity of the films was reported for films containing α-tocopherol, which exhibited the highest antioxidant power.     

Potentiometric titration for determination of amylose content of starch – A comparison with colorimetric method

Amylose, the amount of which varies significantly depending on the source, is one of the key components of starch. The proportion of the essentially linear amylose and the highly branched amylopectin greatly influences the physico-chemical properties of starch. In this study, potentiometric titration using an auto-titrator and colorimetric methods were compared for determining amylose contents of a variety of starch samples. Potentiometric titration results for starches from a variety of botanical sources were within the reported literature ranges while the colorimetric method seemed to overestimate amylose content.     

Studies on sweet potatoes—III. Distribution of unit chains of branched and unbranched molecules of starch

Freshly prepared sweet potato (SP) starch was separated quantitatively into amylose and amylopectin by a concanavalin A precipitation method. Amylose was further fractionated into two subfractions containing essentially linear (Am) and moderately branched (Ax) molecules, by hot-butanol extraction. The molecular weight determined by GPC of Am was 3 × 10⁶ Da, the values for Ap and Ax were 1.5 × 10⁷ and 5 × 10⁶ Da, respectively. The β-amylolysis limit values were 57, 70 and 92%, respectively, for Ap, Ax and Am. The native, as well as the derived β-limit dextrins of Ap and Ax, were debranched with isoamylase and their unit chain lengths, measured by GPC and SE-HPLC methods, showed Ap (C̄L, 21.0) to have a trimodal distribution of unit chains with D̄P values of 52, 37 and 20, whereas Ax had tetramodal unit chains with D̄P values of 40, 30, 21 and 9. Am was more or less a linear molecule. In vitro SP starch was more digestible than legume and cereal starches.     

Long-term storage stability of selected potato starch – Non-starchy hydrocolloid binary gels

All 3, 4 and 5% potato starch gels containing either κ-carrageenan, guar gum or xanthan gum were prepared and their stability in terms of rheological and textural properties was checked on 30 day storage. It was shown that rheology of these binary potato starch – non-starchy hydrocolloid mixtures could be controlled by anionic character of the admixed hydrocolloid. Depending on the hydrocolloid added, gels made of the tuber starch can either retain their pattern typical for tuber starches or turn into that typical for gels of cereal starches. Hydrocolloids added plasticity to the binary gels but it was lost to a significant extent within the first day of storage. Resulting gels with dominating elastic character are rheologically and texturally fairly stable for subsequent 30 days of storage. Generally, hydrocolloids stabilized potato starch gel on long-time storage to the extent dependent on the gel concentration.     

The effect of freeze–thaw cycles on microstructure and physicochemical properties of four starch gels

The effect of repeated freeze–thaw (FT) cycles (up to seven) on microstructure, thermal and textural properties of four starch gels from various botanical origins (gingko, Chinese water chestnut, potato and rice) was investigated and compared by scanning electronic microscope, differential scanning calorimetry and texture analyzer. The chemical composition and molecular structure of four starches were also examined. The Chinese water chestnut, potato and rice starch gels formed a honey-comb structure after 7 FT cycles, while gingko starch gel exhibited lamellar structure. The 7 FT cycles decreased the transition temperatures and enthalpies of four starches in comparison with each native starch, and the retrogradation percentage followed the order: rice > gingko > Chinese water chestnut > potato. The 7 FT cycles increased the hardness of all the evaluated starch gels and decreased springiness and cohesiveness. Results showed that the molecular structure of starches caused notable differences to the microstructure and textural properties of starch gels. The higher amount of longer branch chain (degree of polymerization (DP) > 18) might benefit the formation of the lamellar structure of gingko starch. The percentage of branch chains (DP 18–23) was negatively related with the springiness and cohesiveness of native starch gels, while the percentage of medium chains (DP 12–17) was positively related to the springiness of starch gels after 7 FT cycles.     

New source of the thermostable α-glucosidase suitable for single step starch processing

Thermus thermophilus contains α-glucosidase that is located in the cell cytoplasm. The highest enzyme activity was achieved after 25–30 h of microorganism cultivation at 70 °C in a medium containing 0.8% peptone, 0.4% yeast extract and 0.2% Na Cl. Addition of 2% of starch, maltose or glucose to the medium enhanced the enzyme activity by 83, 72 and 31%, respectively. The enzyme was purified 7.2-fold by combination of ammonium sulphate precipitation and ion-exchange chromatography on Sepharose CL-6B. The enzyme preparations exhibited highest specific activity for pNPG hydrolysis (1.94 U mg⁻¹) at pH 6.2 and 85 °C. The activity of partially purified enzyme was almost unchanged during 5 h of incubation at 75 °C in phosphate-citrate buffer (pH 6.2) and the half-life of this enzyme incubated at 85 °C was 2 h. Rate of pNPG cleavage catalysed by the α-glucosidase was 9.5-times higher than that in the case of maltose hydrolysis. Furthermore, this enzyme shows remarkable activity toward maltose and maltotriose. With maltotetraose, maltopentaose and maltohexaose the reaction rate decreased with increase in molecular weight of the substrate.     

Preparation and characterization of glycerol plasticized (high-amylose) starch–chitosan films

The chitosan–starch (high amylose) blend film (1;1), via microfluidization, was prepared by casting with different glycerol concentration (0%, 2.5%, 5% and 10%). The films were characterized for their mechanical, thermal and morphological properties. The addition of glycerol at 5% (w/w) and higher concentrations resulted in decrease in tensile strength, increase in elongation at break due to plasticization. The well-known antiplasticization was observed in the polymer films with 2.5% of glycerol. The addition of glycerol promoted the interactions among chitosan, starch and glycerol through hydrogen bonding as reflected on the shifting of main peaks of the glycerol-free film to higher wavenumbers as shown by FTIR spectra. The decrease in intensity of glycerol-related peaks in starch–chitosan–glycerol films in both ¹H NMR and ¹³C NMR spectra proved the strong interactions (decrease in glycerol mobility) occurring among starch, chitosan and glycerol in glycerol-plasticized films.     

Mechanisms of starch digestion by α-amylase—Structural basis for kinetic properties

Recent studies of the mechanisms determining the rate and extent of starch digestion by α-amylase are reviewed in the light of current widely-used classifications for (a) the proportions of rapidly-digestible (RDS), slowly-digestible (SDS), and resistant starch (RS) based on in vitro digestibility, and (b) the types of resistant starch (RS 1,2,3,4…) based on physical and/or chemical form. Based on methodological advances and new mechanistic insights, it is proposed that both classification systems should be modified.

Kinetic analysis of digestion profiles provides a robust set of parameters that should replace the classification of starch as a combination of RDS, SDS, and RS from a single enzyme digestion experiment. This should involve determination of the minimum number of kinetic processes needed to describe the full digestion profile, together with the proportion of starch involved in each process, and the kinetic properties of each process.

The current classification of resistant starch types as RS1,2,3,4 should be replaced by one which recognizes the essential kinetic nature of RS (enzyme digestion rate vs. small intestinal passage rate), and that there are two fundamental origins for resistance based on (i) rate-determining access/binding of enzyme to substrate and (ii) rate-determining conversion of substrate to product once bound.