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...     

Can variation in durum wheat pasta protein and starch composition affect in vitro starch hydrolysis?

Pasta was prepared with modifications designed to alter its rate of digestion, including altering the ratio of A- to B-granules, incorporating higher amylose content, and varying the glutenin:gliadin ratio and HMW:LMW glutenin subunit ratio. The effects of these modifications were investigated using an in vitro hydrolysis method. A significant reduction in parameters related to glycaemic index was established for high amylose pastas and pastas with 40% B-granule composition, while most other modifications increased the rate of starch hydrolysis. All effects observed were small in comparison to the effect on glycaemic index previously obtained by addition of soluble fibres.     

Resistant starch and “the butyrate revolution”

Early epidemiological studies indicated that populations that consume a high proportion of non-starch polysaccharide (NSP) dietary fibre (DF) in their daily diet suffer less from gastrointestinal diseases, in particular colorectal cancers, than populations that consume diets that are high in fat and protein but low in NSP fibre. In this respect, diet, by increasing the amount of vegetables and NSP DF's, has been suggested to contribute as much as 25–35% to risk reduction for colorectal cancer. A reduction of fat intake may further reduce the risk by 15–25%. Based on these observations, DF's and substances that are part of the fibre complex such as antioxidants, flavonoids, sulphur containing compounds and folate have been proposed as potentially protective agents against colon cancer. However, results from controlled prospective studies in which beta-carotene and vitamin E or isolated dietary fibres were given to high risk groups showed disappointing results. There are recent indications that the regular consumption of certain subclasses of highly fermentable dietary fibre sources result in gut associated immune and flora modulation as well as a significant production of short chain fatty acids. In vitro studies as well as animal studies indicate that in particular propionate and butyrate have the potential to support the maintenance of a healthy gut and to reduce risk factors that are involved in the development of gut inflammation as well as colorectal cancer. A suggestion put forward is that beneficial effects may be obtained in particular by the consumption of resistant starch (RS) because of the high yield of butyrate and propionate when fermented. These SCFA are the prime substrates for the energy metabolism in the colonocyte and they act as growth factors to the healthy epithelium. In normal cells butyrate has been shown to induce proliferation at the crypt base, enhancing a healthy tissue turnover and maintenance. In inflamed mucosa butyrate stimulates the regeneration of the diseased lining of the gut. In neoplastic cells butyrate inhibits proliferation at the crypt surface, the site of potential tumour development. Moreover, models of experimental carcinogenesis in animals have shown the potential to modify a number of metabolic actions and steps in the cell cycle in a way that early events in the cascade of cancer development may be counteracted while stages of progression may be slowed down. The present review highlights a number of these aspects and describes the metabolic and functional properties of RS and butyrate.     

The impact of coupled acid or pullulanase debranching on the formation of resistant starch from maize starch with autoclaving–cooling cycles

Maize starch was treated by autoclaving–cooling cycles, coupled with acid or pullulanase hydrolysis to prepare resistant starch (RS). Debranching of retrograded or gelatinized maize starch with acid or pullulanase was studied to show the corresponding impact on RS formation. When maize starch was treated with three autoclaving–cooling cycles and retrograded maize starch was hydrolyzed at room temperature, with 0.1 mol L⁻¹ citric acid for 12 h, analysis results showed that debranching of citric acid was helpful in RS formation for RS yield increased from 8.5 to 11%. Debranching of gelatinized or retrograded maize starch at 60 °C with pullulanase at addition level of 3 PUN g⁻¹ starch showed a more favorable effect on RS formation. When gelatinized maize starch was hydrolyzed by pullulanase for 12 h and then treated with two autoclaving–cooling cycles, RS yield increased to 23.5%. If retrograded maize starch subjected to one autoclaving–cooling cycle was hydrolyzed by pullulanase for 10 h and then followed by two autoclaving–cooling cycles, RS yield elevated to 32.4%. The debranching effect of pullulanase on retrograded maize starch to help RS formation is obvious and most effective, indicating this treatment is applicable in RS preparation to increase the RS yield.     

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.     

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.     

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.     

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.     

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.     

Cereal starch nanoparticles—A prospective food additive: A review

Starch is one of the most abundant biopolymers in nature and is typically isolated from plants in the form of micro-scale granules. Raw starch has limited applications due to its innate disadvantages such as poor solubility in cold water, tendency to retrograde and high viscosity once it is gelatinized. Therefore, some degree of modification is required to enhance its functionality. Starch nanoparticle is one of the products of such modification. Chemical, enzymatic, and physical treatments are used for the preparation of starch nanoparticles and to study their granular and molecular structures. Characterization of starch nanoparticles on the size distribution, crystalline structure, and physical properties in relation to the starch sources and preparation methods can be done using various characterization tools e.g. Scanning Electron Microscopy, Transmission Electron Microscopy, Atomic Florescence Microscopy, etc. Starch nanoparticles can be used as a food additive as it has adverse range of uses in food such as emulsion stabilizer, fat replacer, Thickener, or rheology modifier etc.     

Wheat starch structure–function relationship in breadmaking: A review

Bread dough and bread are dispersed systems consisting of starch polymers that interact with other flour components and added ingredients during processing. In addition to gluten proteins, starch impacts the quality characteristics of the final baked product. Wheat starch consists of amylose and amylopectin organized into alternating semicrystalline and amorphous layers in granules that vary in size and are embedded in the endosperm protein matrix. Investigation of the molecular movement of protons in the dough system provides a comprehensive insight into granular swelling and amylose leaching. Starch interacts with water, proteins, amylase, lipids, yeast, and salt during various stages of breadmaking. As a result, the starch polymers within the produced crumb and crust, together with the rate of retrogradation and staling due to structural reorganization, moisture migration, storage temperature, and relative humidity determines the final product's textural perception. This review aims to provide insight into wheat starch composition and functionality and critically review recently published research results with reference to starch structure–function relationship and factors affecting it during dough formation, fermentation, baking, cooling, and storage of bread.     

Rheological and textural studies of fresh and freeze-thawed native sago starch–sugar gels. II. Comparisons with other starch sources and reheating effects

The viscoelastic and textural properties of freshly prepared and freeze-thawed sago starch–sugar gels were studied in comparison with other native starches from corn, wheat, tapioca, and potato. The gelatinisation and retrogradation properties of starches were studied using a DSC while the pasting properties of starch–sugar mixtures during the cooking period were studied using a starch pasting cell. The freeze-thaw stability of gels was evaluated by gravimetric measurements of the water of syneresis. The different starches gave properties which varied following to their botanical sources. High-amylose cereal starches (wheat and corn) produced harder gels, while low-amylose root starch (tapioca) produced softer gels. Sago and potato gels showed close similarities in their viscoelastic and textural characteristics. Although the freeze-thaw cycle greatly increased the viscoelasticity and hardness of these two gels, reheating at high temperature significantly reduced these negative effects and resulted in partial recovery of the gel structures. Sago starch produced gels with very low syneresis and high cohesiveness, implying its potential use as a gelling agent in the frozen food industries.     

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.     

Resistant starch—An accessible fiber ingredient acceptable to the Western palate

Dietary fiber intakes in Western societies are concerningly low and do not reflect global recommended dietary fiber intakes for chronic disease prevention. Resistant starch (RS) is a fermentable dietary fiber that has attracted research interest. As an isolated ingredient, its fine particle size, relatively bland flavor, and white appearance may offer an appealing fiber source to the Western palate, accustomed to highly refined, processed grains. This review aims to provide a comprehensive insight into the current knowledge (classification, production methods, and characterization methods), health benefits, applications, and acceptability of RS. It further discusses the present market for commercially available RS ingredients and products containing ingredients high in RS. The literature currently highlights beneficial effects for dietary RS supplementation with respect to glucose metabolism, satiety, blood lipid profiles, and colonic health. An exploration of the market for commercial RS ingredients indicates a diverse range of products (from isolated RS2, RS3, and RS4) with numerous potential applications as partial or whole substitutes for traditional flour sources. They may increase the nutritional profile of a food product (e.g., by increasing the fiber content and lowering energy values) without significantly compromising its sensory and functional properties. Incorporating RS ingredients into staple food products (such as bread, pasta, and sweet baked goods) may thus offer an array of nutritional benefits to the consumer and a highly accessible functional ingredient to be greater exploited by the food industry.     

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.     

Starch–lipid complex formation during extrusion-cooking of model system (rice starch and oleic acid) and real food (rice starch and pistachio nut flour)

The formation of starch–lipid complexes during extrusion-cooking of model system (rice starch and oleic acid) and real food (rice starch and pistachio nut flour) was evaluated. Both formulas were extruded at the same processing conditions (temperature profiles, screw speed, and water feed content). The obtained data showed that in model system and real food, the formation of starch–lipid complexes occurred under different processing conditions. In particular, the highest formation of starch–lipid complexes, that is, the highest melting enthalpy value (ΔH _m = 1.18 J/g), was obtained at the middle values of barrel temperature (100 °C) and water feed content (19%) in the model system. Yet, the only processing variable that had a significant effect on the formation of starch–lipid complexes in the real food was barrel temperature. In particular, the highest melting enthalpy of starch–lipid complexes (ΔH _m = 9.28 J/g) was obtained at the highest values of barrel temperature (130 °C). These results point out the importance of considering all components present in the raw materials submitted to extrusion-cooking in order to study biopolymer modifications, which occur during processing.     

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.     

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.