Starch gels containing mixtures of sago þ wheat of different protein content at different levels ... more Starch gels containing mixtures of sago þ wheat of different protein content at different levels of sago substitutes were prepared as follows: sago þ high protein wheat (HPW), sago þ medium protein wheat (MPW) and sago þ slow protein wheat (LPW). The rheological profile ...
Konjac glucomannan (KGM)-edible films were prepared with different amounts of glycerol or sorbito... more Konjac glucomannan (KGM)-edible films were prepared with different amounts of glycerol or sorbitol as a plasticizer. Films were characterized by moisture sorption isotherm, and following conditioning at different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbitol and glycerol) were found to plasticize KGM-based films with respect to their tensile properties. However, thermal properties and water sorption capacity (WSC) of polyolplasticized KGM films were found to vary with water activity (aw), namely at low aw (< 0.6), WSC and melting enthalpy were decreased with increasing in polyol content and the opposite was true at higher aw (>0.6). This was attributed to extensive interactions between plasticizer and KGM that reduced the available active site (-OH groups) for water adsorption. The presence of polyols at low aw appeared to suppress crystalline structures due probably to restricted molecular mobility. These effects were diminished when the moisture content was >20%.
Edible films were cast from konjac glucomannan (KGM) solutions, with or without added alkali (KOH... more Edible films were cast from konjac glucomannan (KGM) solutions, with or without added alkali (KOH) and/or sodium carboxymethylcellulose (CMC). Four types of KGM-based films (KGM, KGM–KOH, KGM–CMC and KGM–CMC–KOH) were produced and characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), moisture sorption, water vapour permeability (WVP), and tensile tests. Tensile properties were studied as a function of water activity (aw) over the range from 0.22 to 0.84. SEM revealed that films, with and without KOH, exhibited cross-sectional lamellar structures running perpendicular and parallel, respectively, to the film surface. Alkali treatment produced films with enhanced crystallinity, lower water-sorptive capacity (WSC) and WVP, and higher tensile properties. These effects were attributed to alkaline deacetylation of KGM molecules which permitted greater intermolecular interactions. The presence of CMC appeared to suppress crystallinity of native KGM films, but enhanced that of deacetylated KGM films. Films incorporating CMC exhibited higher WSC and WVP, but variable tensile properties depending on alkali treatment and aw. The tensile properties of all KGM-based films were profoundly affected by aw. Tensile modulus (TM) of all films were antiplasticized as aw was increased from 0.22 to 0.43, but tensile strength (TS) was generally plasticized by water. Tensile elongation of KGM, KGM–KOH, and KGM–CMC–KOH films was generally much less sensitive to water. However, KGM–CMC films exhibited minimum elongation, attributed to antiplasticization by water, over the intermediate aw range from 0.43 to 0.69. KGM–CMC–KOH films exhibited the highest TM and TS at any particular aw.
The effects of varying concentrations of pectin (4.5–6.5%, w/w), sucrose (40–60%, w/w) and calciu... more The effects of varying concentrations of pectin (4.5–6.5%, w/w), sucrose (40–60%, w/w) and calcium (20–60 mg/g pectin) on the viscoelastic properties of pectin dispersions at pH 3.0 were investigated. Pectin samples used were extracted from pomelo fruit peels (Citrus grandis) grown in Malaysia. The dynamic rheological parameters (G′, G″, δ and η*) of pectin–sucrose–calcium dispersion were determined at 1.5% strain from 90–20°C at a cooling rate of 3°C min−1. Plots of G′ and G″ against frequency (rad s−1) showed G″>G′ throughout the frequency range with no occurrence of crossover for most of the pectin dispersions. In addition both storage (G′) and loss (G″) moduli of the dispersions increase on cooling. Increasing pectin, sucrose and calcium concentrations increased G′ and G″ with pectin having the greatest effect. Interactions amongst the three factors were also studied. At lower pectin concentrations, addition of Ca2+ increased G′ at all temperatures. This effect was also observed at higher pectin concentrations at 20°C but not at 90°C. The opposite effect was observed with the addition of sucrose, i.e. addition of sucrose at a higher pectin concentration increased G′ whereas at a lower pectin concentration no effect was observed. Interaction between calcium and sucrose gave rise to an increase in G′ when Ca2+ was added at high sucrose concentrations, but a decrease in G′ was evident at low sucrose concentrations. Dispersions of pectin alone or in combination with sucrose exhibited a more liquid-like behaviour with G″>G′. However, in the presence of Ca2+, mechanical spectra of G′>G″ were obtained.
Emulsion films prepared from konjac glucomannan (KGM), carboxymethylcellulose (CMC), with and wit... more Emulsion films prepared from konjac glucomannan (KGM), carboxymethylcellulose (CMC), with and without alkali (KOH), were characterised for selected properties such as morphological characteristics, water vapour permeability (WVP), water sorptive capacity and tensile properties. It was found that smaller lipid globules, with a homogeneous distribution, increased the apparent hydrophobicity and “tortuosity” of an emulsion film for water molecule transmission, thereby resulting in reduced WVP. This was achieved when alkaline deacetylated KGM and CMC were served as the base polymers for the emulsion, with the latter functioning as an emulsifier. Interactions between deacetylated KGM and CMC resulted in a stable emulsion.
Foams were prepared from starfruit (Averrhoa carambola L.) puree by adding various concentrations... more Foams were prepared from starfruit (Averrhoa carambola L.) puree by adding various concentrations of methyl cellulose (Methocel 65HG). All foams were prepared by whipping using a kitchen mixer. Overrun of foams from various concentrations of methyl cellulose were compared. Average bubble diameter and bubble size distribution were calculated based on microphotography. Overrun and stability of foams increased with increasing methyl cellulose concentration until maximum value was obtained at methyl cellulose concentration of 40 g/kg. At this concentration, the overrun value of 653.2% and foam density of 0.153 g cm−3 was obtained. Higher concentration of methyl cellulose produced larger air bubbles. The size of the air bubbles showed a wide distribution as the methyl cellulose concentration was increased, with the average diameter ranged from 176 to 381 μm.
Edible films prepared from partially acid-hydrolyzed konjac glucomannan (KGM) were characterised ... more Edible films prepared from partially acid-hydrolyzed konjac glucomannan (KGM) were characterised with respect to the influence of altered chain length and molecular weight distribution on water sorptive capacity (WSC), water vapour permeability (WVP), thermal, and tensile properties. Acid treatment produced films with higher WSC and WVP, and lower melting enthalpy than that of an untreated one. These effects were attributed to an increase in the number of shorter chains, which contributed to more active sites for moisture sorption but disfavoured the formation of junction zones. The elastic modulus (EM) and tensile strength (TS) increased initially with decreasing molecular weight and increasing polydispersity, and then decreased with further reduction in molecular weight. Conversely, tensile elongation (TE) exhibited the opposite trend.
The effects of water and glycerol as diluents on physical properties of glassy tapioca starch fil... more The effects of water and glycerol as diluents on physical properties of glassy tapioca starch films were studied. Water and glycerol depressed the calorimetric glass transition temperature (Tg) of the tapioca starch films. In glycerated and non-glycerated starch films, the tensile modulus (E) was plasticized by moisture. Glycerol acted as an antiplasticizer on film modulus when present at a low concentration (2.5%) in drier films (aw≤0.22). Both strain-at-break and toughness of starch films increased to a maximum as aw was increased to a critical level (0.22–0.32) before decreasing with further humidification. In contrast, minima in those two properties were observed in films containing 10–15% glycerol at aw=0.11 and 0.22. The presence of small amounts of glycerol was able to exert classic antiplasticizing effects on possibly all tensile properties studied, but only in relatively drier systems. On the other hand, antiplasticization by water manifested itself only in mechanical properties determined at high deformation (i.e. tensile strength and toughness).
The rheological properties of agar and κ-carrageenan mixtures (with a total polysaccharide concen... more The rheological properties of agar and κ-carrageenan mixtures (with a total polysaccharide concentration of 1.5% w/w) were investigated using dynamic oscillatory measurements, creep compliance tests and compressive deformation measurements. The effects of different agar/κ-carrageenan ratios (100/0, 80/20, 60/40, 40/60 and 20/80) on the formation and properties of the gel mixtures were investigated at different pH values (pH 3.5, 4.5 and 5.5). Temperature dependence of G′ of mixtures showed one step like change during cooling, however, two-steps like change was observed on heating trace. Significant thermal hysteresis was observed in all mixtures, moreover, the observed hysteresis was influenced by those characteristics of κ-carrageenan. The gel point as determined by oscillatory measurements of storage and loss modulus (G′ and G″) of these mixtures show a monotonic decrease in temperature as the proportion of κ-carrageenan increased. Additionally, the gelling temperature of the mixtures resembled that of pure agar system. Incorporation of κ-carrageenan caused a large reduction in gel rigidity, which arise from sharp drop in G′ determined by mechanical spectra. Reduction in gel rigidity has also been demonstrated by a marked increased in instantaneous compliance evaluated by creep compliance tests. Young's modulus, stress and strain at failure were also monitored. In comparison with agar gel, the mixed gels were much more deformable, with a higher failure strain, but had lower strength indicated by a marked decrease in Young's modulus and failure stress. On the other hand, a decrease in pH below 4.5 caused a sudden drop in the gelling temperature, G′ and instantaneous compliance. In general, reduction in pH caused gel to be weaker and more brittle, as shown by lower values for Young's modulus, failure stress and failure strain. These effects were attributed to an increase in the number of shorter chains, which disfavored the formation of junction zones. The results of this study suggest that the gelation of the mixtures appeared to have occurred through a segregative phase separation where agar-rich phase formed a continuous phase and κ-carrageenan rich-phase formed a discontinuous gelled phase.
Viscoelastic properties of sago starch and wheat flour, and sagowheat flour mixtures at differen... more Viscoelastic properties of sago starch and wheat flour, and sagowheat flour mixtures at different percentages of sago substitution were studied. Swelling characteristics of high protein wheat (HPW), medium protein wheat (MPW) and low protein wheat (LPW) flour, and sago starch ...
The wealth of current knowledge on starch retrogradation is due in large measure to the wide arra... more The wealth of current knowledge on starch retrogradation is due in large measure to the wide array of analytical methods at the disposal of food scientists. Since retrogradation is a complex process affected by many factors, it is unlikely that any single method would be able to give a complete picture of the retrogradation properties of starch gels at both the macroscopic and molecular levels. Independent evidence derived from two or more methods allows cross comparisons that can provide a fuller understanding of this phenomenon. For quantitative measurement of rates of retrogradation, the “ideal” method should be simple, rapid, non-destructive, precise, and inexpensive. Comparisons of kinetic data from different sources should be made with caution; various factors (thermal history, in particular) that can lead to unjustifiable comparisons and erroneous conclusions should be carefully considered first. This review covers the general principles, capabilities, advantages, and limitations of various methods available to study starch retrogradation.
The effects of Na2CO3 and NaOH (at a concentration of 1 g alkalising agent/100 g dry starch) on t... more The effects of Na2CO3 and NaOH (at a concentration of 1 g alkalising agent/100 g dry starch) on the gelatinisation properties of 1:4 and 2:3 starch/water mixtures were studied using differential scanning calorimetry (DSC). The starches studied included wheat, rice, waxy rice, corn and waxy corn. The addition of Na2CO3 or NaOH resulted in significant increases in transition temperatures (onset, peak and conclusion), associated with starch gelatinisation, suggesting a stabilisation of the starch granules, probably through electrostatic interactions between Na+ ions and starch hydroxyl groups. The presence of Na2CO3 generally caused a significant reduction in gelatinisation enthalpy (ΔHG) of the higher moisture (1:4) systems, but had no effect in the less hydrated (2:3) systems. NaOH generally had no effect on ΔHG of 1:4 systems, but gave rise to inconsistent effects in the 2:3 systems. Heating of alkali-containing samples above the gelatinisation temperature range gave rise to an intriguing exothermic peak, the origin of which was attributed to repolymerisation of the products of alkaline hydrolysis of starch. A separate experiment, conducted to investigate the influence of increasing Na2CO3 concentration (up to 3 g/100 g starch) on thermal properties of 1:4 waxy rice starch/water systems, showed a gradual increase in gelatinisation transition temperatures, a general decline in ΔHG, and a marginal decrease in peak temperature of the repolymerisation exotherm.
Changes in pasting and thermal properties of non-waxy rice flour and its isolated starch during s... more Changes in pasting and thermal properties of non-waxy rice flour and its isolated starch during storage at 25, 35 and 45°C were compared. Aging had no apparent effect on the pasting behaviour of the isolated starch, but markedly affected that of the flour. Peak viscosity (Vp) of rice flour pastes generally increased with both temperature and time of storage, but reached a plateau within 4 weeks of storage at 45°C. Fresh flour paste exhibited lower Vp, a slower rise in apparent viscosity, and much better stability than an isolated starch paste. The addition of isolated oryzenin to isolated rice starch resulted in pasting behaviour which more closely approximated that of an extensively aged flour. DSC scans of fresh flour at ∼13% moisture revealed a weak heat-irreversible endothermic event over the temperature range from 47 to 66°C which was attributed to the denaturation of oryzenin. This transition was shifted to higher temperatures with increasing storage temperature and time. It also became increasingly skewed and broadened until it was no longer detectable after the flour had been stored at 45°C for 8 weeks. Starch gelatinization characteristics of both flour and isolated starch, as determined by DSC, were apparently unaffected by aging. However, rice flour exhibited significantly higher onset temperature of gelatinization, but lower gelatinization enthalpy, than isolated rice starch. Similarly, pulsed NMR studies showed no apparent effect of aging on retrogradation behaviour of rice flour or starch gels. These results suggest that modification of the protein component, rather than starch, was primarily responsible for rheological changes associated with aging of rice flour.
Foams were prepared from starfruit (Averrhoa carambola L.) purée by adding various concentrations... more Foams were prepared from starfruit (Averrhoa carambola L.) purée by adding various concentrations of methocel. Overrun and density of the foams from various concentrations of methocel were compared. Relative stability of the foam was determined by comparing amounts of juice separated from foam at 70°C. Drying rates at two drying temperatures (70 and 90°C) were also compared. Quality of the dry and reconstituted powder was evaluated with simple sensory evaluation and Hunterlab instrument. Overrun and stability of foams increased with increasing methocel concentration until maximum value was obtained at a methocel concentration of 0.4% w/w. Falling rate was observed for foam dried at both temperatures. Drying time could be shortened by as much as 30 min when drying temperature was increased from 70 to 90°C. However, obvious colour and flavour changes were observed in the product dried at 90°C. This study has indicated that, under the experimental conditions employed, reasonably good powder characteristics can be obtained.
Starch gels containing mixtures of sago þ wheat of different protein content at different levels ... more Starch gels containing mixtures of sago þ wheat of different protein content at different levels of sago substitutes were prepared as follows: sago þ high protein wheat (HPW), sago þ medium protein wheat (MPW) and sago þ slow protein wheat (LPW). The rheological profile ...
Konjac glucomannan (KGM)-edible films were prepared with different amounts of glycerol or sorbito... more Konjac glucomannan (KGM)-edible films were prepared with different amounts of glycerol or sorbitol as a plasticizer. Films were characterized by moisture sorption isotherm, and following conditioning at different relative humidities, by differential scanning calorimetry and tensile tests. Moisture and polyols (sorbitol and glycerol) were found to plasticize KGM-based films with respect to their tensile properties. However, thermal properties and water sorption capacity (WSC) of polyolplasticized KGM films were found to vary with water activity (aw), namely at low aw (< 0.6), WSC and melting enthalpy were decreased with increasing in polyol content and the opposite was true at higher aw (>0.6). This was attributed to extensive interactions between plasticizer and KGM that reduced the available active site (-OH groups) for water adsorption. The presence of polyols at low aw appeared to suppress crystalline structures due probably to restricted molecular mobility. These effects were diminished when the moisture content was >20%.
Edible films were cast from konjac glucomannan (KGM) solutions, with or without added alkali (KOH... more Edible films were cast from konjac glucomannan (KGM) solutions, with or without added alkali (KOH) and/or sodium carboxymethylcellulose (CMC). Four types of KGM-based films (KGM, KGM–KOH, KGM–CMC and KGM–CMC–KOH) were produced and characterized by scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), moisture sorption, water vapour permeability (WVP), and tensile tests. Tensile properties were studied as a function of water activity (aw) over the range from 0.22 to 0.84. SEM revealed that films, with and without KOH, exhibited cross-sectional lamellar structures running perpendicular and parallel, respectively, to the film surface. Alkali treatment produced films with enhanced crystallinity, lower water-sorptive capacity (WSC) and WVP, and higher tensile properties. These effects were attributed to alkaline deacetylation of KGM molecules which permitted greater intermolecular interactions. The presence of CMC appeared to suppress crystallinity of native KGM films, but enhanced that of deacetylated KGM films. Films incorporating CMC exhibited higher WSC and WVP, but variable tensile properties depending on alkali treatment and aw. The tensile properties of all KGM-based films were profoundly affected by aw. Tensile modulus (TM) of all films were antiplasticized as aw was increased from 0.22 to 0.43, but tensile strength (TS) was generally plasticized by water. Tensile elongation of KGM, KGM–KOH, and KGM–CMC–KOH films was generally much less sensitive to water. However, KGM–CMC films exhibited minimum elongation, attributed to antiplasticization by water, over the intermediate aw range from 0.43 to 0.69. KGM–CMC–KOH films exhibited the highest TM and TS at any particular aw.
The effects of varying concentrations of pectin (4.5–6.5%, w/w), sucrose (40–60%, w/w) and calciu... more The effects of varying concentrations of pectin (4.5–6.5%, w/w), sucrose (40–60%, w/w) and calcium (20–60 mg/g pectin) on the viscoelastic properties of pectin dispersions at pH 3.0 were investigated. Pectin samples used were extracted from pomelo fruit peels (Citrus grandis) grown in Malaysia. The dynamic rheological parameters (G′, G″, δ and η*) of pectin–sucrose–calcium dispersion were determined at 1.5% strain from 90–20°C at a cooling rate of 3°C min−1. Plots of G′ and G″ against frequency (rad s−1) showed G″>G′ throughout the frequency range with no occurrence of crossover for most of the pectin dispersions. In addition both storage (G′) and loss (G″) moduli of the dispersions increase on cooling. Increasing pectin, sucrose and calcium concentrations increased G′ and G″ with pectin having the greatest effect. Interactions amongst the three factors were also studied. At lower pectin concentrations, addition of Ca2+ increased G′ at all temperatures. This effect was also observed at higher pectin concentrations at 20°C but not at 90°C. The opposite effect was observed with the addition of sucrose, i.e. addition of sucrose at a higher pectin concentration increased G′ whereas at a lower pectin concentration no effect was observed. Interaction between calcium and sucrose gave rise to an increase in G′ when Ca2+ was added at high sucrose concentrations, but a decrease in G′ was evident at low sucrose concentrations. Dispersions of pectin alone or in combination with sucrose exhibited a more liquid-like behaviour with G″>G′. However, in the presence of Ca2+, mechanical spectra of G′>G″ were obtained.
Emulsion films prepared from konjac glucomannan (KGM), carboxymethylcellulose (CMC), with and wit... more Emulsion films prepared from konjac glucomannan (KGM), carboxymethylcellulose (CMC), with and without alkali (KOH), were characterised for selected properties such as morphological characteristics, water vapour permeability (WVP), water sorptive capacity and tensile properties. It was found that smaller lipid globules, with a homogeneous distribution, increased the apparent hydrophobicity and “tortuosity” of an emulsion film for water molecule transmission, thereby resulting in reduced WVP. This was achieved when alkaline deacetylated KGM and CMC were served as the base polymers for the emulsion, with the latter functioning as an emulsifier. Interactions between deacetylated KGM and CMC resulted in a stable emulsion.
Foams were prepared from starfruit (Averrhoa carambola L.) puree by adding various concentrations... more Foams were prepared from starfruit (Averrhoa carambola L.) puree by adding various concentrations of methyl cellulose (Methocel 65HG). All foams were prepared by whipping using a kitchen mixer. Overrun of foams from various concentrations of methyl cellulose were compared. Average bubble diameter and bubble size distribution were calculated based on microphotography. Overrun and stability of foams increased with increasing methyl cellulose concentration until maximum value was obtained at methyl cellulose concentration of 40 g/kg. At this concentration, the overrun value of 653.2% and foam density of 0.153 g cm−3 was obtained. Higher concentration of methyl cellulose produced larger air bubbles. The size of the air bubbles showed a wide distribution as the methyl cellulose concentration was increased, with the average diameter ranged from 176 to 381 μm.
Edible films prepared from partially acid-hydrolyzed konjac glucomannan (KGM) were characterised ... more Edible films prepared from partially acid-hydrolyzed konjac glucomannan (KGM) were characterised with respect to the influence of altered chain length and molecular weight distribution on water sorptive capacity (WSC), water vapour permeability (WVP), thermal, and tensile properties. Acid treatment produced films with higher WSC and WVP, and lower melting enthalpy than that of an untreated one. These effects were attributed to an increase in the number of shorter chains, which contributed to more active sites for moisture sorption but disfavoured the formation of junction zones. The elastic modulus (EM) and tensile strength (TS) increased initially with decreasing molecular weight and increasing polydispersity, and then decreased with further reduction in molecular weight. Conversely, tensile elongation (TE) exhibited the opposite trend.
The effects of water and glycerol as diluents on physical properties of glassy tapioca starch fil... more The effects of water and glycerol as diluents on physical properties of glassy tapioca starch films were studied. Water and glycerol depressed the calorimetric glass transition temperature (Tg) of the tapioca starch films. In glycerated and non-glycerated starch films, the tensile modulus (E) was plasticized by moisture. Glycerol acted as an antiplasticizer on film modulus when present at a low concentration (2.5%) in drier films (aw≤0.22). Both strain-at-break and toughness of starch films increased to a maximum as aw was increased to a critical level (0.22–0.32) before decreasing with further humidification. In contrast, minima in those two properties were observed in films containing 10–15% glycerol at aw=0.11 and 0.22. The presence of small amounts of glycerol was able to exert classic antiplasticizing effects on possibly all tensile properties studied, but only in relatively drier systems. On the other hand, antiplasticization by water manifested itself only in mechanical properties determined at high deformation (i.e. tensile strength and toughness).
The rheological properties of agar and κ-carrageenan mixtures (with a total polysaccharide concen... more The rheological properties of agar and κ-carrageenan mixtures (with a total polysaccharide concentration of 1.5% w/w) were investigated using dynamic oscillatory measurements, creep compliance tests and compressive deformation measurements. The effects of different agar/κ-carrageenan ratios (100/0, 80/20, 60/40, 40/60 and 20/80) on the formation and properties of the gel mixtures were investigated at different pH values (pH 3.5, 4.5 and 5.5). Temperature dependence of G′ of mixtures showed one step like change during cooling, however, two-steps like change was observed on heating trace. Significant thermal hysteresis was observed in all mixtures, moreover, the observed hysteresis was influenced by those characteristics of κ-carrageenan. The gel point as determined by oscillatory measurements of storage and loss modulus (G′ and G″) of these mixtures show a monotonic decrease in temperature as the proportion of κ-carrageenan increased. Additionally, the gelling temperature of the mixtures resembled that of pure agar system. Incorporation of κ-carrageenan caused a large reduction in gel rigidity, which arise from sharp drop in G′ determined by mechanical spectra. Reduction in gel rigidity has also been demonstrated by a marked increased in instantaneous compliance evaluated by creep compliance tests. Young's modulus, stress and strain at failure were also monitored. In comparison with agar gel, the mixed gels were much more deformable, with a higher failure strain, but had lower strength indicated by a marked decrease in Young's modulus and failure stress. On the other hand, a decrease in pH below 4.5 caused a sudden drop in the gelling temperature, G′ and instantaneous compliance. In general, reduction in pH caused gel to be weaker and more brittle, as shown by lower values for Young's modulus, failure stress and failure strain. These effects were attributed to an increase in the number of shorter chains, which disfavored the formation of junction zones. The results of this study suggest that the gelation of the mixtures appeared to have occurred through a segregative phase separation where agar-rich phase formed a continuous phase and κ-carrageenan rich-phase formed a discontinuous gelled phase.
Viscoelastic properties of sago starch and wheat flour, and sagowheat flour mixtures at differen... more Viscoelastic properties of sago starch and wheat flour, and sagowheat flour mixtures at different percentages of sago substitution were studied. Swelling characteristics of high protein wheat (HPW), medium protein wheat (MPW) and low protein wheat (LPW) flour, and sago starch ...
The wealth of current knowledge on starch retrogradation is due in large measure to the wide arra... more The wealth of current knowledge on starch retrogradation is due in large measure to the wide array of analytical methods at the disposal of food scientists. Since retrogradation is a complex process affected by many factors, it is unlikely that any single method would be able to give a complete picture of the retrogradation properties of starch gels at both the macroscopic and molecular levels. Independent evidence derived from two or more methods allows cross comparisons that can provide a fuller understanding of this phenomenon. For quantitative measurement of rates of retrogradation, the “ideal” method should be simple, rapid, non-destructive, precise, and inexpensive. Comparisons of kinetic data from different sources should be made with caution; various factors (thermal history, in particular) that can lead to unjustifiable comparisons and erroneous conclusions should be carefully considered first. This review covers the general principles, capabilities, advantages, and limitations of various methods available to study starch retrogradation.
The effects of Na2CO3 and NaOH (at a concentration of 1 g alkalising agent/100 g dry starch) on t... more The effects of Na2CO3 and NaOH (at a concentration of 1 g alkalising agent/100 g dry starch) on the gelatinisation properties of 1:4 and 2:3 starch/water mixtures were studied using differential scanning calorimetry (DSC). The starches studied included wheat, rice, waxy rice, corn and waxy corn. The addition of Na2CO3 or NaOH resulted in significant increases in transition temperatures (onset, peak and conclusion), associated with starch gelatinisation, suggesting a stabilisation of the starch granules, probably through electrostatic interactions between Na+ ions and starch hydroxyl groups. The presence of Na2CO3 generally caused a significant reduction in gelatinisation enthalpy (ΔHG) of the higher moisture (1:4) systems, but had no effect in the less hydrated (2:3) systems. NaOH generally had no effect on ΔHG of 1:4 systems, but gave rise to inconsistent effects in the 2:3 systems. Heating of alkali-containing samples above the gelatinisation temperature range gave rise to an intriguing exothermic peak, the origin of which was attributed to repolymerisation of the products of alkaline hydrolysis of starch. A separate experiment, conducted to investigate the influence of increasing Na2CO3 concentration (up to 3 g/100 g starch) on thermal properties of 1:4 waxy rice starch/water systems, showed a gradual increase in gelatinisation transition temperatures, a general decline in ΔHG, and a marginal decrease in peak temperature of the repolymerisation exotherm.
Changes in pasting and thermal properties of non-waxy rice flour and its isolated starch during s... more Changes in pasting and thermal properties of non-waxy rice flour and its isolated starch during storage at 25, 35 and 45°C were compared. Aging had no apparent effect on the pasting behaviour of the isolated starch, but markedly affected that of the flour. Peak viscosity (Vp) of rice flour pastes generally increased with both temperature and time of storage, but reached a plateau within 4 weeks of storage at 45°C. Fresh flour paste exhibited lower Vp, a slower rise in apparent viscosity, and much better stability than an isolated starch paste. The addition of isolated oryzenin to isolated rice starch resulted in pasting behaviour which more closely approximated that of an extensively aged flour. DSC scans of fresh flour at ∼13% moisture revealed a weak heat-irreversible endothermic event over the temperature range from 47 to 66°C which was attributed to the denaturation of oryzenin. This transition was shifted to higher temperatures with increasing storage temperature and time. It also became increasingly skewed and broadened until it was no longer detectable after the flour had been stored at 45°C for 8 weeks. Starch gelatinization characteristics of both flour and isolated starch, as determined by DSC, were apparently unaffected by aging. However, rice flour exhibited significantly higher onset temperature of gelatinization, but lower gelatinization enthalpy, than isolated rice starch. Similarly, pulsed NMR studies showed no apparent effect of aging on retrogradation behaviour of rice flour or starch gels. These results suggest that modification of the protein component, rather than starch, was primarily responsible for rheological changes associated with aging of rice flour.
Foams were prepared from starfruit (Averrhoa carambola L.) purée by adding various concentrations... more Foams were prepared from starfruit (Averrhoa carambola L.) purée by adding various concentrations of methocel. Overrun and density of the foams from various concentrations of methocel were compared. Relative stability of the foam was determined by comparing amounts of juice separated from foam at 70°C. Drying rates at two drying temperatures (70 and 90°C) were also compared. Quality of the dry and reconstituted powder was evaluated with simple sensory evaluation and Hunterlab instrument. Overrun and stability of foams increased with increasing methocel concentration until maximum value was obtained at a methocel concentration of 0.4% w/w. Falling rate was observed for foam dried at both temperatures. Drying time could be shortened by as much as 30 min when drying temperature was increased from 70 to 90°C. However, obvious colour and flavour changes were observed in the product dried at 90°C. This study has indicated that, under the experimental conditions employed, reasonably good powder characteristics can be obtained.
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