Development and characterization of reconstituted hydrogel from <Emphasis Type="Italic">Aloe vera</Emphasis> (<Emphasis Type="Italic">Aloe barbadensis</Emphasis> Miller) powder

Structural and rheological characterization of reconstituted hydrogels developed from A. vera non-fibrous alcohol insoluble residue (NFAIR) powder using different methods [viz., shaking (S), heating-shaking (HS), and heating (H)] and concentrations (viz., 0.2–1.6 %, w/v) was carried out. Functional group distribution by FTIR spectroscopy and Congo red (CR) method revealed the presence of acetylated acemannan in A. vera powder. Dynamic oscillation studies of A. vera (NFAIR) fluids at all concentrations of 0.2–1.6 %, w/v, showed gel strength in the order of H > HS > S method. However, in H method, increase in concentration from 0.2 to 1.6 %, w/v showed the conformational transition from semi-diluted solution to weak gel nature. Rheological models described the effect of heating temperatures (HT); 30–90 °C, and times (Ht); 15–60 min on viscoelastic behavior in reconstituted A. vera fluids. The reconstituted A. vera hydrogel prepared with a concentration of 1.6 %, w/v using 50 °C (HT) and 30 min (Ht) condition showed a good agreement with the Power law (storage modulus, G′) and Weak gel model (complex modulus, G*) fitted data (R² > 0.94) resulting higher viscoelastic moduli intercepts; G′₀ (71.5 Pa s ^n′), G″₀ (33.5 Pa s ^n″), lower slopes; n′ (0.22), n″ (0.06), higher network strength (A _F , 121.3 Pa s^1/z ) and number of network (z, 5.3) values. The obtained results suggested that heating at 50 °C/30 min can develop aqueous weak gel networks of A. vera with enhanced gel strength which may be utilized as a novel gelling agent for wide variety of targeted applications in food and pharmaceutical sectors.     

Rheological properties of batter systems containing different combinations of flours and hydrocolloids

The rheological properties of batters formulated using different combinations of wheat, corn, and rice flours with two types of hydrocolloids, namely methylcellulose (0.5%, 1% and 1.5%) or xanthan gum (0.2%), were studied. Control samples were formulated with combinations of flours without the added hydrocolloids. The effects of hydrocolloids on rheological characteristics of the batter systems were measured using a controlled stress rheometer at a temperature of 15 °C. The effects of hydrocolloids on dynamic viscoelastic parameters as functions of temperatures were evaluated. All the batters showed shear thinning behaviour with flow behaviour indices in the range 0.34–0.67. Addition of xanthan gum lowered the flow index values, imparting a higher degree of pseudoplasticity to the batter samples compared to methylcellulose. The consistency index of the control batter samples varied from 0.46 to 69.2 Pa sⁿ. Addition of xanthan gum or methylcellulose significantly increased the batter consistency index value. The gums changed the onset temperature of structure development, and the storage (G′_max) and loss moduli (G″_max) of the batter systems. However, no statistically significant effects were observed on the peak temperature of batter systems in which the G′ reached a maximum value. Xanthan gum increased both G′_max and G″_max, whereas at higher concentrations methylcellulose increased G′_max but lowered G″_max. Copyright © 2007 Society of Chemical Industry     

Rheology of Rice Starch‐Sucrose Composites

The effect of sucrose at different concentrations (0, 10, 20 and 30%) on rheological properties of rice starch pastes (5% w/w) was investigated in steady and dynamic shear. The steady shear properties of rice starch‐sucrose composites were determined from rheological parameters for power law and Casson flow models. At 25°C all the starch‐sucrose composites exhibited a shear‐thinning flow behavior (n=0.25–0.44). The presence of sucrose resulted in the decrease in consistency index (K), apparent viscosity (η_a,100) and yield stress (σ_oc). Dynamic frequency sweeps at 25°C indicated that starch‐sucrose composites exhibited weak gel‐like behavior with storage moduli (G′) higher than loss moduli (G′′). G′ and G′′ values decreased with the increase in sucrose concentration. The dynamic (η*) and steady‐shear (η_a) viscosities at various sucrose concentrations did not follow the Cox‐Merz superposition rule. G′ values as a function of aging time (10 h) at 4°C showed a pseudoplateau region at long aging times. In general, the values of G′ and G′′ in rice starch‐sucrose composites were reduced in the presence of sucrose and depended on sucrose concentration.     

Investigating rennet coagulation properties of recombined highly concentrated micellar casein concentrate and cream for use in cheese making

Highly concentrated micellar casein concentrate (HC-MCC) contains ～18% casein with ～70% of whey proteins removed by microfiltration and diafiltration of skim milk, followed by vacuum evaporation for further concentration. When blended with cream, HC-MCC forms recombined concentrated milk (RCM), which could be used as a starting material in cheese making. Our objective was to investigate the rennet coagulation properties of RCM while varying parameters such as casein level, pH, rennet level, and coagulation temperature. The HC-MCC was mixed with cream using low shear at 50°C for 10 min, followed by cooling to 31, 28, or 25°C and adding rennet, and rheological properties were determined. Rennet coagulation time [RCT, the time at which storage modulus (G′) = loss modulus (G″)] decreased from 8.7 to 7.4 min as casein level increased from 3.2 to 5.7%, without a significant additional difference in RCT at casein levels >5.7%. The initial G″ (G″₀) increased about 10-fold when casein levels were increased from 3.2 to 10.9%, whereas no change in initial G′ (G′₀) was observed. When G′ was measured relative to RCT (i.e., 1, 1.5, or 2 times RCT after RCT was reached, and expressed as G′₁, G′_1.5, and G′₂), log relationship was found between relative G′ and casein level (R² > 0.94). Lowering coagulation temperature from 31 to 25°C increased G″₀ by 6 fold and extended RCT from 7.4 to 9.5 min. After coagulation, relative G′ was initially higher at the lower temperature with G′₁ of 3.6 Pa at 25°C and 2.0 Pa at 31°C, but delayed in further development with G′₂ of 0.8 kPa at 25°C and 1.1 kPa at 31°C. Lowering pH of RCM from 6.6 to 6.2 resulted in reduced RCT from 11.9 to 6.5 min with increased relative G′ after coagulation. When less rennet was used, RCT increased in a linear inverse relationship without changes in relative G′ or G″. The microstructure of RCM coagulum (～11% casein), observed using transmission electron microscopy, confirmed that RCM curd had a rigid protein matrix containing extensively cross-linked protein strands.     

Effect of Freezing/Thawing Temperature on the Viscoelastic and Nutritional Qualities of Carrots

This study evaluated and correlated the viscoelastic and nutritional properties of carrots after freezing (–20, –70, and –196^oC) and thawing (4 and 18^oC) treatments. Results showed that all samples exhibited a solid behavior (storage modulus G′ > loss modulus G″) dominating the viscoelastic response. After treatments, G′, G″, hardness, fracturability, springiness, and chewiness of carrots significantly decreased, whereas loss tangent (Tanδ) increased. Compared with other treatments, fast freezing (–196^oC) and thawing (18^oC) better maintained G′, G″, hardness, and fracturability of carrots, and intermediate freezing (–70^oC) better retained springiness, cohesiveness, and chewiness. For nutritional parameters, fast freezing and thawing conditions also contributed to preserving the contents of carotenoids of carrots. Partial least squares regression analysis revealed that β- and α-carotene, lycopene, and soluble sugars were positively correlated with hardness, fracturability, G′, and G″, and negatively with Tanδ in carrots. The variation of viscoelastic parameters could well predict the changes at nutritional levels.     

Rheology of Mixed Systems of Sweet Potato Starch and Galactomannans

The effect of galactomannans (guar gum and locust bean gum) at different concentrations (0, 0.2, 0.4 and 0.6%, w/w) on rheological properties of sweet potato starch (SPS) was studied. The flow behaviors of SPS‐galactomannan mixtures were determined from the rheological parameters of power law and Casson models. The SPS‐galactomannan mixtures had high shear‐thinning fluid characteristics (n = 0.30‐0.36) exhibiting yield stress at 25°C. The presence of galactomannans resulted in the increase in consistency index (K), apparent viscosity (η_a,100) and Casson yield stress (σ_oc). In the temperature range of 25‐70°C, the mixtures followed the Arrhenius temperature relationship. Dynamic rheological tests at 25°C indicated that the SPS‐galactomannan mixtures had weak gel‐like behavior with storage moduli (G′) higher than loss moduli (G") over most of the frequency range (0.63‐62.8 rad/s) with frequency dependency. The magnitudes of dynamic moduli (G′, G" and η*) of the SPS‐galactomannan mixtures were higher than those of the control (0% gum), and increased with an increase in gum concentration. The tan δ (ratio of G"/G′) values (0.41‐0.46) of SPS‐guar gum mixtures were much lower than those (0.50‐0.63) of SPS‐locust bean gum mixtures, indicating that there was a more pronounced effect of guar gum on the elastic properties of SPS.     

Effect of staling on viscoelastic properties of pastes prepared from arabic bread

Dynamic testing detected differences in the rheological properties of pastes prepared from progressively aged Arabic bread. The storage (G') and loss (G″) modulii decreased at different rates upon storage. The progress of textural changes responsible for the generation of the sensory ‘stale’ response was marked by the transition of Arabic bread pastes from viscoelastic solids (G′>G″) to elastoviscous liquids (G″>G′).     

Pectin–sucrose–Ca2+ interactions: effects on rheological properties

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.     

Thermal gelation of globulin from Phaseolus angularis (red bean)

The heat-induced gelation properties of red bean globulin (RBG) were studied under the influence of salts and several protein structure perturbants. The viscoelastic properties of the gels were evaluated by small amplitude oscillatory tests and creep experiments. Gel structure developed progressively when a protein dispersion (≈10% w/v) was heated from 25 to 95 °C, and both the storage modulus (G′) and loss modulus (G″) increased rapidly during cooling. The addition of NaCl at lower concentrations led to increases in G′ and G″ values, with decreased creep compliance, suggesting higher viscoelasticity. At higher salt concentrations, viscoelasticity was decreased, and the optimum NaCl concentration to produce maximum gel rigidity was 0.2 M. Sodium dodecyl sulfate and urea caused more pronounced reduction of the gel moduli than dithiothreitol and N-ethylmaleimide. The data suggest that hydrophobic interactions and hydrogen bonding play an important role in heat-induced gelation, while electrostatic interactions and a balance of attractive–repulsive forces are also important. The gels formed at 95 °C exhibited a homogenous microstructure with extensive cross-links and sheet-like network structures.     

BRAZILIAN HULL-LESS AND MALTING BARLEY GENOTYPES: II. THERMAL AND RHEOLOGICAL PROPERTIES OF STARCH

Thermal and rheological properties of starch isolated from two Brazilian barley experimental lines, malting and hull‐less, are reported. Gelatinization's thermal properties were T_o 53.3 and 58C, T_p 59.4 and 62.7C, and ΔH 6 and 6.8 J/g for the malting and hull‐less starch, respectively. A trend to slightly higher retrogradation rates were obtained for the malting (73.3%) compared with the hull‐less (72.1%) starch, while lower hot‐paste peak and final viscosity were observed in the former (125 and 114 RVU versus 169 and 179 RVU, respectively). Changes in storage (G′) and loss (G″) moduli during heating and cooling showed higher values for the starch from the hull‐less line. During heating, G′was higher than G″and developed at higher temperatures. A trimodal distribution pattern of G″was observed in the starch from the hull‐less barley line. Two G′peaks at 75 and 87.5C were 6.2 and 2.4 times higher, respectively, in the starch from the hull‐less line than that from the malting line.     

Steady and oscillatory shear behaviour of semi-concentrated starch suspensions

The viscoelastic moduli G’ and G” of aqueous suspensions with 40% (w/v) normal corn starch (NCS) and waxy corn starch (WCS) were determined by oscillation rheometry. The oscillatory shear flow experiments at heating from 30° to 75 °C and maintaining at this temperature showed changes from a behaviour predominant viscous (G”>>G’) to predominant elastic (G’>G”) for both starches at 60.5 °C for WCS, respectively 70,85 °C for NCS, WCS having higher values of G’ and G” as NCS. After the gelatinisation temperature was attired, NCS showed no significant changes, both moduli remaining relatively constant. Peaks of both moduli G’ and G” were obtained for WCS at its maintaining at 75^oC, these changes being attributed to the changes in the amylopectin structure in the absence of amylose for this starch type. The frequency influenced the results; analysis at constant low frequency (10 s^-1) gave big oscillations during the measurements and made the analysis impossible, whereas frequencies as 50 s^-1 or 100 s^-1 gaves reproducible and similar results. The shear flow measurements realised at angular frequencies ω from10^-1 to 10³s^-1 at 25^oC showed that changes from a behaviour predominant elastic (G’>G”) to predominant viscous (G”>>G’) occurred when ω attired the values 10 s^-1 for WCS and 3 s^-1 for NCS. The calculation of the ‘Power-Law’ parameter B showed that NCS forms a physical gel structure, whereas WCS behaves as a covalent gel in the frequency domain 10^-1 to 10 s^-1 and as physical gel in the frequency domain 10 to 10² s^-1.     

Dynamic Rheological Properties of Mozzarella Cheese During Refrigerated Storage

Storage (G′) and loss (G″) moduli of low-moisture, part-skim Mozzarella cheese were determined at 10 and 20°C during 1 mo of refrigerated aging. At both temperatures, G′ was always greater than G″. Averaged over aging, G′ increased from 90 to 630 and G″ from 44 to 52 kPa at 10°C, and at 20°C G′ increased from 28 to 190 and G″ from 14 to 53 kPa for the frequency range 0.005-20 Hz. Averaged over frequency, both G′ and G″ decreased about 20% at 10°C and 25% at 20°C during aging. Relaxation spectrum, computed from shear relaxation data, was used to calculate the G. The calculated values of G′ were in good agreement with those determined experimentally. These data help predict and compare melting behaviors of such cheeses.     

Structure and Viscoelastic Properties of Starches Separated from Different Legumes

A comparison between the morphological, structural, thermal and viscoelastic properties of starches separated from pigeon pea, chickpea, field pea, kidney bean and blackgram was made. The shape of the starch granules in the different legumes varied from oval to elliptical or spherical. X-ray diffraction of the legume starches indicated a typical C-pattern (mixture of A- and B-type). Granules of blackgram and pigeon pea starch had a higher degree of crystallinity than those of field pea and kidney bean starches. Apparent amylose content of field pea, kidney bean, chickpea, blackgram and pigeon pea starch was 37.9%, 36.0%, 34.4-35.5%, 32.9-35.6% and 31.8%, respectively. Distribution of isoamylase-branched materials among the starches revealed that the proportions of long and short side chains of amylopectin ranged between 13.6-18.5% and 41.7-46.5%, respectively. Field pea and kidney bean starch had the highest apparent amylose content and the lowest amount of long side chains of amylopectin, respectively. Blackgram and pigeon pea starch possessed higher proportions of both long and short side chains of amylopectin than field pea and chickpea starches. The onset, peak and conclusion temperatures of gelatinization (T_o T_p and T_c, respectively) were determined by differential scanning calorimetry. T_o and T_c ranged from 59.3 to 77.3°C, 66.8 to 79.6°C, 55.4 to 67.6°C and 68.3 to 69.3°C, respectively, for chickpea, blackgram, field pea and kidney bean starch. The enthalpy of gelatinization (ΔH_gel) of field pea, kidney bean, chickpea, blackgram and pigeon pea starches was 3.6, 3.0, 2.6-4.2, 1.6-1.7 and 2.6 J/g, respectively. Pastes of blackgram and pigeon pea starches showed lower storage and loss shear moduli G′ than field pea, kidney bean and chickpea starches. The changes in moduli during 10 h at 10°C revealed retrogradation in the order of: field pea> kidney bean> chickpea> blackgram> pigeon pea starch. In blackgram and pigeon pea starches, the lower proportion of amylose plus intermediate fraction and higher proportion of short and long side chains of amylopectin are considered responsible for the higher crystallinity, gelatinization temperature and enthalpy of gelatinization.     

Comparison of the effect of sugars on the viscoelastic properties of sweet potato starch pastes

Viscoelastic properties of sweet potato starch (SPS) pastes (5% w/w) were studied in the presence of various sugars (sucrose, glucose, fructose, and xylose) at different concentrations (0, 10 and 20%) by small‐deformation oscillatory measurements. Dynamic frequency sweeps at 20 °C indicated that all SPS–sugar mixtures were more elastic than viscous with storage moduli (G′) higher than loss moduli (G′′) at all values of frequency with a frequency dependency. Dynamic moduli (G′ and G′′) values increased with the increase in sugar concentration from 10 to 20%. Changes in the dynamic moduli were more pronounced with xylose in comparison to the control (no sugar) and other sugars. G′ values as a function of ageing time (10 h) at 4 °C continuously increased with time during ageing without a plateau region. In general, G′ values of SPS–sugar mixtures during ageing decreased in the following order: pentose (xylose) > hexose (glucose and fructose) > control > disaccharide (sucrose), indicating that the xylose had the greatest ability in retarding retrogradation of SPS.     

State diagram of salmon (Salmo salar) gelatin films

BACKGROUND: A state diagram presents different physical states of a biomaterial as a function of solid content and temperature. Despite their technological interest, little information is available on protein systems such as gelatin/water mixtures. The objective of this work was to develop state diagrams of salmon gelatin (SG) and bovine gelatin (BG) in order to determine maximal freeze concentration parameters (T′_g, T′_m and X_s′) and to relate possible differences to their biochemical characteristics. RESULTS: Biochemical characterisation of SG showed lower molecular weight and iminoacid concentration compared with BG. Likewise, the glass transition temperature (T_g) was lower for SG at X_s > 0.8, which was associated with its lower molecular weight. Unexpectedly, the depression of freezing temperature (T_f) was greater for SG at X_s > 0.1, which was associated with its higher ash content. Isothermal annealing produced effective values of T′_g ≈ ? 52 °C, T′_m ≈ ? 46 °C and X′_s ≈ 0.6 for both gelatins. Interestingly, the enthalpy change associated with T′_m (ΔH) was significantly higher for SG than for BG after annealing, indicating a higher proportion of ice present at about ? 50 °C. CONCLUSION: Maximal freeze concentration parameters were similar between the two gelatins, though differences in biochemical properties were evident. The results show that there are likely different ways of interaction of SG and BG with water. Copyright © 2011 Society of Chemical Industry     

Steady and dynamic shear rheology of glutinous rice flour dispersions

The steady and dynamic shear rheological properties of Korean glutinous rice flour dispersions were evaluated at different concentrations (4, 5, 6, 7 and 8%). Glutinous rice flour dispersions at 25 °C showed a shear‐thinning behaviour (n = 0.487–0.522) with low magnitudes of Casson yield stresses (σ_oc = 0.056–0.339 Pa). The magnitudes of σ_oc, consistency index (K) and apparent viscosity (η_a,100) increased with the increase in concentration. The power law model was found to be more suitable than the exponential model in expressing the relationship between concentration and apparent viscosity. The apparent viscosity over the temperature range of 25–70 °C obeyed the Arrhenius temperature relationship, with high determination coefficients (R² = 0.982–0.998), indicating that the magnitudes of activation energies (E_a) were in the range of 9.05–11.89 kJ mol^?1. A single equation, combining the effects of temperature and concentration on η_a,100, was used to describe the flow behaviour of glutinous rice flour dispersions. Magnitudes of storage (G′) and loss (G′′) moduli increased with the increase in concentration and frequency. Magnitudes of G′ were higher than those of G′′ over most of the frequency range.     

Effect of freezing rate and frozen storage on starch–sucrose–hydrocolloid systems

Model food systems based on starch (100 g kg⁻¹), sucrose (150 g kg⁻¹) and water (750 g kg⁻¹) with and without the addition of a low proportion of hydrocolloid (xanthan gum, guar gum or sodium alginate) were gelatinised, frozen at different rates and stored to analyse textural changes by oscillatory rheometry. Differential scanning calorimetry (DSC) was used to analyse gelatinisation, amylopectin retrogradation and glass transition temperatures. Sucrose had a significant effect on the increase in the gelatinisation temperature as well as on the decrease observed in glass transition values. The onset temperature of the second step of the glass transition, corresponding to the heat capacity change close to ice melting (denoted Tg_im in the present work), ranged between −23.0 and −22.2 °C. Rheological viscoelastic tests showed an increase in the dynamic moduli G* and G ′ after slow freezing and during storage at −19 °C (T > Tg_im) in starch–sucrose systems that is related to sponge formation due to amylose retrogradation. DSC studies confirmed that also amylopectin retrogradation occurs during storage; however, samples containing gums did not develop the spongy appearance. Storage at the usual commercial temperatures (close to −18 °C, slightly above Tg_im) affects the quality of aqueous starch–sucrose pastes without gums owing to amylose and amylopectin retrogradation. However, when hydrocolloids are included in the formulations, the usual storage conditions allow the maintenance of acceptable textural attributes. © 2000 Society of Chemical Industry     

Studies on the morphological,thermal and rheological properties of starch separated from some Indian potato cultivars

The starches separated from five different Indian potato cultivars (Kufri Chandermukhi, Kufri Badshah, Kufri Jyoti, Kufri Sindhuri and S1) were investigated for morphological, thermal, rheological, turbidity and water-binding properties. The starch separated from all the five potato cultivars had a granule size ranging between 15–20 μm and 20–45 μm. The shape of starch granules varied from oval to irregular or cuboidal. Starch isolated from cv. Kufri Badshah had largest irregular or cubiodal granules while starch from cv. Kufri Chandermukhi had small and oval granules. The transition temperatures and enthalpy of gelatinization (ΔH_gel) were determined using differential scanning calorimetry (DSC). The enthalpy of retrogradation (ΔH_ret) of gelatinized starch was also determined after 14 days of storage at 4°C using DSC. Kufri Chandermukhi starch showed the lowest ΔH_gel and ΔH_ret while Kufri Badshah starch showed the highest values. ΔH_gel and ΔH_ret values of 12.55 J/g and 6.42J/g, respectively, for Kufri Chandermukhi starch against 13.85 J/g and 8.61 J/g, respectively, for Kufri Bhadshah starch were observed. Rheological properties of starches from different potato cultivars, measured using the Dynamic Rheometer during heating and cooling, also differed significantly. The starch from cv. Kufri Badshah showed the highest peak G′ and G″ and lowest tan δ. The starches having higher peak G′(G′ at gelatinization temperature) showed higher breakdown in G′ and vice versa. The turbidity of gelatinized aqueous starch suspensions from all potato cultivars increased with increase in storage period. Starches with low water binding capacity had higher G′ and G″ and lower tan δ values.     

Rheological and differential scanning calorimetry studies on structural and textural changes in frozen Atlantic mackerel (Scomber scombrus)

The viscoelastic behaviour and thermal stability of Atlantic mackerel fillets stored at ?20 and ?30 °C for up to 2 years were investigated. An increase in elastic (G′) and viscous (G″) modulus values, reflecting protein aggregation, was observed in samples stored at ?20 °C compared with those stored at ?30 °C, as well as with storage time. The results indicate that toughening on frozen storage is not just limited to lean gadoid fish but also occurs in fatty fish, leading to texture deterioration. Differential scanning calorimetry of fillets stored at ?20 °C showed a shift to a lower transition temperature (T_m) and a decrease in enthalpy (ΔH) compared with control fillets stored at ?30 °C; this change was enhanced when fillets were stored for a longer period of time, confirming protein denaturation and the formation of aggregates reported previously by the authors (J Sci Food Agric 82: 579–586 (2002)). The contribution of lipid oxidation to protein aggregation was shown by storing minced mackerel with or without the antioxidant vitamin E at ?10 °C. The G′ and G″ values were higher in samples stored without vitamin E than in samples stored with vitamin E; thus antioxidants may be used to minimise protein aggregation in fatty fish. The role of lipid oxidation in promoting protein aggregation and deterioration in the texture of fatty fish has not been reported hitherto. Antioxidants such as vitamin E may be used not only to prevent lipid oxidation but also to minimise protein damage in order to prolong the shelf‐life of fatty fish. Copyright © 2004 Society of Chemical Industry     

Fine Structure,Thermal and Viscoelastic Properties of Starches Separated from Indica Rice Cultivars

Starches were separated from indica rice cultivars (PR‐113, Basmati‐370, Basmati‐386, PR‐115, IR‐64, and PR‐103) and evaluated using gel permeation chromatography (GPC), X‐ray diffraction, differential scanning calorimetry (DSC) and dynamic viscoelasticity . Debranching of starch with isoamylase and subsequent fractionation by GPC revealed 9.7–28.3% apparent amylose content, 3.7–5.0% intermediate fraction (mixture of short amylose and long side‐chains of amylopectin), 20.6–26.6% long side‐chains of amylopectin and 45.8–59.4% short side‐chains of amylopectin). IR‐64 starch with the highest crystallinity had the highest gelatinization temperatures and enthalpy, T_o, T_p, T_c, and ΔH_gel being 71.8, 75.9, 82.4°C and 5.1 J/g, respectively, whereas PR‐113 starch with lower crystallinity showed the lowest gelatinization temperatures (T_o, T_p, T_c, of 60.8, 65.7 and 72.2°C, respectively). Basmati‐386 starch exhibited two endotherms during heating, the first and second endotherm being associated with the melting of crystallites and amylose‐lipid complexes, respectively. T_o, T_p, T_c and ΔH_gel of the second endotherm of Basmati‐386 starch were 99.0, 100.1, 101.1°C and 2.0 J/g, respectively. During cooling, Basmati‐386 also showed an exotherm at a peak temperature of 87°C. PR‐113 starch with the highest amylose content and the lowest content of short side‐chains of amylopectin had the highest peak storage modulus (G′= 1.6×10⁴ Pa). The granules of PR‐113 starch were the least disintegrated after heating. The effects of heating starch suspensions at different temperatures (92°C, 130°C and 170°C) on intrinsic viscosity [η], transmittance and viscoelasticity were also studied to evaluate the extent of breakdown of the molecular structure. The intrinsic viscosity of starch suspensions heated at 92, 130 and 170°C ranged between 103–114, 96–110 and 28–93 mL/g. Transmittance value of starches cooked at 92°C decreased with increase in storage duration. All starches except PR103, cooked at 130°C also showed decrease in transmittance during storage, however, at lower rate. PR103 starch heated at 130°C did not show any change in transmittance up to a storage time of 48 h. The changes in viscoelasticity of starch pastes cooked at different temperatures during cooling and reheating were also evaluated. G′ and G′′ increased with decrease in temperature during cooling cycle. Starches heated at 130°C with apparent amylose content ≤ 21.2% showed an improvement in G′ and G′′ in comparison to the corresponding starches heated at 92°C, this improvement was observed to be higher in starches with lower amylose content. All starches heated at 170°C had a higher proportion of breakdown in molecular structure as indicated by lower G′ and G′′ than the same starches heated at 130 and 92°C.