Effects on Pasting Viscosity of Starch and Flour from Different Operating Conditions for the Rapid Visco Analyser

Three wheat flours, three wheat starches, a regular maize starch and a waxy maize starch were subjected to a number of different RVA profiles. Five different initial temperatures were used, 40, 50, 55, 60, and 65°C, with different initial holding times (0–3 min), heating times (2fl–10 min), holding times at 95°C (0–6 min), cooling times (2–6 min), and final hold times (0–10 min) being applied. A range of final temperatures of 30–60°C was also utilized. Significant variations in viscosity were observed with these conditions, particularly in wheat starch and flour. The most important parameters causing these variations were the initial temperature, the heating rate, and the final holding time. Short initial holding times also resulted in a wider spread of values for peak viscosity although there was little effect on the mean value and no significant effect on the holding strength or final viscosity. The final temperature was also important in that lower temperatures gave more viscous gels. Provided that the desired cooling rate could be achieved, varying the cooling time had no effect on the peak or trough viscosities and only a very minor effect on the final viscosity. If final temperatures of 40°C or lower are to be used, the cooling conditions and final hold time would need to be adjusted so that maximum viscosity could be achieved. A proposal for a standard Rapid Visco Analyser (RVA) procedure is: at least 1 min at 50°C, heat to 95°C over 4 min, hold at 95°C for 4 min, cool to 50°C in 3 min, and hold at 50°C for 4 min. These conditions should minimize variation within samples and should allow a better comparison between samples.     

Effects of Phosphate Salts on the pH Values and Rapid Visco Analyser (RVA) Pasting Parameters of Wheat Flour Suspensions

Changes in pH and pasting properties of instant‐noodle formula dry‐mix suspensions containing each of 12 phosphate salts were investigated. The pH values of solutions alone and then solution and flour suspensions decreased as the level of phosphate salts increased, except that of trisodium phosphate, which increased the pH value. The changes in the Rapid Visco Analyser (RVA) pasting parameters of instant‐noodle formula suspensions were not consistent with the respective changes in pH, but the change trends of the RVA parameters for the two different wheat flours (hard red winter and soft white wheat) were similar. Five of the phosphate salts gradually increased the RVA peak viscosity (PV) as phosphate concentration increased. Seven other phosphate salts increased the PV at 0.05% and then decreased PV as the phosphate concentration increased. The change in trough viscosity owing to phosphate salt and concentration was similar to that of PV. The final viscosity (FV) gradually declined, to varying degrees, as the phosphate concentration increased for some of the phosphate salts. However, seven of the phosphate salts caused slight increases in FV as their concentrations increased. The response of starch gelatinization and pasting behavior, as measured by RVA, indicated that phosphate salts exert an influence on starch during heating in water. Because RVA parameters have been linked to instant‐noodle processing and textural properties, phosphate salt identity and concentration can likely be manipulated to affect end‐product quality.     

Determination of Rapid Visco Analyser Parameters in Rice by Near‐Infrared Spectroscopy

The objective of these studies was to find alternative Rapid Visco Analyser (RVA) viscoelastic parameters that are predictable by near‐infrared spectroscopy (NIRS). Currently, RVA instruments are widely used in assessing cooking and processing characteristics in rice. The ability to predict RVA parameters by NIRS would be useful in rapidly determining rice pasting qualities, but NIRS does not correlate with the traditional parameters (peak viscosity, final viscosity, breakdown, consistency, and setback). Alternative RVA parameters were sought by collecting RVA and NIRS data for a total of 86 short, medium, and long grain rice cultivars. The amylose contents were 0.41–24.90% (w/w) and protein concentrations were 8.47–11.35% (w/w). Partial least squares (PLS) regression models generated for the entire NIR spectrum against the RVA curve showed viscosity at 212–228 sec (80°C ± 1) varied linearly with NIR spectra (1,100 to ‐2,500 nm). Regression coefficient values were R = 0.961 for 212 sec and R = 0.903 for 228 sec. The PLS correlation coefficient for the prediction of amylose at 212–228 sec decreases along with the NIRS correlation to the same time frame. An opposite trend was observed for the correlation with protein at 212–228 sec. This comparison suggests the importance of amylose and protein in water absorption during this time frame.     

Comparison of Starch Pasting Properties at Various Cooking Conditions Using the Micro Visco‐Amylo‐Graph and the Rapid Visco Analyser

Pasting profiles of selected starches were compared by using a Micro Visco‐Amylo‐Graph (MVA) and a Rapid Visco Analyser (RVA). Effects of cooking (heating/cooling) rate and stirring speed on starch pasting properties were examined. The pasting viscosity of a starch suspension (8%, w/w, dsb) was measured at a fast (6°C/min) and slow (1.5°C/min) cooking rate while being stirred at either 75 rpm or 160 rpm. The pasting temperatures (PT) of all starches were higher when measured at the fast cooking rate than those at the slow cooking rate, except for wheat measured by using the RVA. PT was also higher when measured at the slow stirring speed (75 rpm) than at the fast stirring speed (160 rpm) in both RVA and MVA. When stirring speed increased from 75 rpm to 160 rpm, peak viscosity of all starch pastes except potato decreased measured by using the RVA, but increased by using the MVA. In general, amylograms of these starches obtained by using the MVA showed less breakdown, but greater setback viscosity than did that obtained by using the RVA. Differences in starch pasting properties between MVA and RVA, measured at the same cooking and stirring rates, were attributed mainly to the difference in spindle structure.     

Measuring Oxidative Gelation of Aqueous Flour Suspensions Using the Rapid Visco Analyzer

The Rapid Visco Analyzer (RVA) was investigated as a tool to measure oxidative gelation capacity (OGC) of aqueous wheat flour suspensions. One club wheat patent flour was used to determine optimal hydration time, and 33 straight‐grade flours (representing 12 hard and 21 soft varieties) were used to observe varietal differences in OGC. A 33.3% w/w flour–water suspension was tested in the RVA at 30°C and 160 rpm for 1 min to establish the flour–water baseline viscosity, and then 65 μL of 3% H₂O₂ was added and the viscosity of the suspension measured at 160 rpm for a further 5 min. Flour from the club wheat showed that 20 min of prehydration was needed to observe full OGC potential. For the 33 straight‐grade flours, final RVA water baseline viscosity was correlated with Bostwick Consistometer (BC) flow (r = −0.93, P ≤ 0.01), and RVA H₂O₂ peak viscosity was correlated with H₂O₂ BC flow (r = –0.81, P ≤ 0.01). The RVA was able to differentiate H₂O₂‐reactive from nonreactive flours. The RVA can observe phenomena not observable with the BC method (e.g., viscosity reduction over time at constant shear rate), which can provide potentially valuable additional information about the nature of OGC in wheat flour suspensions.     

Average Shear Rates in the Rapid Visco Analyser (RVA) Mixing System

The principle of mixer viscometry was utilized to determine the average shear rate in the mixing system (impeller‐cup combination) of the Rapid Visco Analyser (RVA). A relationship between the impeller Reynolds number and the power number was established with Newtonian standards. Using the matching viscosity technique and non‐Newtonian fluids consisting of various aqueous solutions of guar gum and methylcellulose, the average value of the mixer viscometer constant (k′) was 20.1/rev over speeds of 1.0–3.5 rev/sec (60–210 rpm). Hence, the average shear rate in the RVA can be estimated as 20.1 multiplied by the angular velocity given in revolutions per second. Results show the RVA system has very good potential for characterizing the rheological properties of various non‐Newtonian fluid foods.     

Genotype and Environmental Influences on Pasting Properties of Rice Flour

The pasting behavior of flour from several Australian rice (Oryza sativa L.) cultivars, differing in amylose content and grown in three different locations and three seasons, were determined using the Rapid Visco Analyser. Genotype, growth season, and growth location all affected the pasting behavior of rice flour. The amylose content of the same cultivar was significantly higher in the coolest growing season, resulting in RVA traces with lower peak viscosity and higher setback than samples with lower amylose content. When the same cultivar of rice was grown in different locations in the same season, there were no significant differences in the total starch, protein, lipid, and amylose content of the flour, but there were significant differences in the pasting behavior. This indicates that environmental as well as genetic factors influence the pasting behavior of rice flour. Flour from parboiled and quick‐cooking rice did not paste and had low viscosities compared with unprocessed rice. Results from this study showed that the pasting behavior of rice flour was related to genotype and was influenced by environmental factors that brought about subtle changes in the grains that were not picked up by chemical analyses.     

Albumin Significantly Affects Pasting and Textural Characteristics of Rice Flour

The influence of albumin on the pasting and rheological properties of rice flour was investigated. Albumin was removed from the flour of three rice cultivars (Amaroo, Opus, and Langi) by water extraction and the pasting profile of the albumin‐depleted flour was analyzed using the Rapid ViscoAnalyser (RVA). Removal of albumin resulted in a significant (P < 0.5) decline in all the pasting parameters measured. When the extracted albumin was added to pure rice starch, exactly opposite trends occurred. The concentration of albumin in rice starch had a positive linear relationship with all pasting parameters measured. When the gels formed after RVA analyses were analyzed using the TA‐TX2 texture analyzer, the concentration of albumin had a positive linear relationship with hardness, but a near linear negative relationship with adhesiveness. The presence of albumin in rice starch slowed the uptake of water by starch in the initial stages of cooking, but the water uptake accelerated in later stages, and the final water absorption was higher in the samples containing albumin than in pure starch. The water‐soluble nature of albumin suggests that protein‐water‐starch interactions could be responsible for its effect on the physical properties of rice.     

Milling—A Further Parameter Affecting the Rapid Visco Analyser (RVA) Profile

Rapid Visco Analyser (RVA) profiles were recorded for raw maize grits and two extruded nonexpanded pellets based on wheat and maize. Large differences were found between the profiles obtained when an impeller mill was used to prepare the samples compared with a disk mill. The differences were related to differences in particle properties of the ground products (particle‐size distribution, particle shape, and protein content). Generally, milling the samples with the impeller mill resulted in greater starch conversion than with a disk mill. For raw maize grits, this was shown by X‐ray diffraction, differential scanning calorimetry (DSC), and alkaline viscosity measurements. Several other laboratory mills were tested and all produced particulates with a sieve range of 125–212 μm that had substantially differing RVA profiles. Cooling the sample during milling did not nullify the milling effects. All the laboratory mills produced <20% of the particulates of the size range required for the RVA analysis. The mill used for sample preparation can exert a significant effect on the RVA for both raw and processed cereal samples, even if measurements are made on a defined sieve fraction.     

Shortened Temperature Program for Application with a Rapid Visco Analyser in Prediction of Noodle Quality in Wheat

The use of the Rapid Visco Analyser (RVA) for application in the screening of wheat breeding lines for starch quality and potential noodle quality has been limited by relatively low sample throughput. Current methods generally enable only 20–30 samples to be tested each day. This study sought to develop a more rapid time‐temperature profile that could be applied to whole meal samples. A profile that involved a total analysis time of 7.5 min/sample gave measurements of peak viscosity (PV) and breakdown (BD) on whole meal that were highly correlated with corresponding measurements obtained using a more conventional profile that had been applied to low‐extraction flours. BD and PV were also highly correlated with the total texture score of ramen (Chinese‐style alkaline noodles as manufactured in Japan), but only when 1 mM AgNO₃ was used to eliminate the effects of α‐amylase.     

Effect of Stabilized Rice Bran Fractions on the Formation of Rice Flour Pasting Properties

Rice flour composition played a key role in determining the changes in pasting properties of rice flour. The influence of incorporating defatted rice bran (DFRB), rice bran fiber (RBF), rice bran protein (RBP), and stabilized rice bran (SRB) fractions on the mechanism of rice flour pasting viscosities was investigated. Pasting properties of long‐ and medium‐grain rice flour substituted with 5, 10, 15, 20, and 100% bran fractions resulted in a significant decrease (P < 0.05) in rice flour pasting property values. Flour substituted with RBP had the lowest pasting property measurements compared with other fractions, and the greater the percentage substituted, the lower the pasting property values. DFRB and RBF were least affected properties when used as a replacement. Results were attributed to the contribution of rice starch in the mechanism of rice paste formation, in which decreasing starch in a rice flour sample, as a result of substituting with fractions of SRB, may have resulted in faster swelling of starch granules to the maximum extent and increased their susceptibility to be disrupted by shear, resulting in low paste viscosities. Results also suggested that protein structural integrity and the nature of starch–protein bonding affected rice flour pasting mechanism formation.     

Prediction of Specific Japanese Sponge Cake Volume Using Pasting Properties of Flour

In Japanese soft wheat (Triticum aestivum L.) breeding programs, protein content (PC), and specific surface area (SSA) of flour have been used as important factors for the baking quality of Japanese sponge cake. We proposed batter pasting viscosity (BPV) as a parameter to predict the baking quality of Japanese sponge cake. BPV was measured using a Rapid Visco‐Analyser (RVA) with a modified heating profile. Twenty soft wheat samples from the 2006‐07 season and 22 from the 2007‐08 season, including Japanese soft wheat cultivars, advanced breeders' lines, and Western White (WW) imported from the United States, were milled and evaluated for solvent retention capacity (SRC) values of four solvents, batter pasting properties, flour pasting properties, PC, SSA, and specific cake volume (SCV) to investigate their relationships. BPV was the most strongly correlated of the parameters to SCV (r = –0.90, P < 0.001). Stepwise multiple regression analysis selected BPV and minimum viscosity (MV) of flour pasting as significant independent variables to predict SCV (corrected R² = 0.848). The variability in BPV related to cake batter expansion was highly explained by PC and sucrose SRC (corrected R² = 0.854, P < 0.001). MV was correlated to SSA (r = 0.56, P < 0.001) and might be related to the prevention of sponge cake shrinkage during baking.     

Effect of Alkaline‐Soluble Proteins on Pasting Properties of Nonwaxy Rice Flour

The pasting properties of rice flours and reconstituted rice flours from mixing a common starch with proteins extracted from different rice cultivars at different total protein content levels were studied. Results showed that not only the total protein content but also the protein composition had an effect on the pasting properties of the rice flours. Among the different strands of rice proteins, globulin had the strongest influence on the pasting properties, followed by glutelin, whereas prolamin had the least influence. At the subunit level of the proteins, proteins with a molecular weight of 17,000, most likely from globulin, had the strongest effect on the peak viscosity of the rice flour, followed by those of 33,000. In comparison with that of the rice starch, the influence of proteins in rice was limited. The effect of interactions between the rice proteins and the starch, such as the role of starch‐granule‐associated proteins, was not isolated in this study, and further investigation is required to quantify this effect.     

Impact of Rice Flour Cold‐Water‐Soluble Fraction Removal on Gelatinization and Pasting Properties

The influence of the cold‐water‐soluble fraction on gelatinization and pasting properties of rice flour was investigated. The cold‐water‐soluble fraction was removed by water extraction under room temperature. The gelatinization properties of untreated and treated flour were analyzed with a differential scanning calorimeter, and pasting profiles were measured with a rapid viscosity analyzer. The removal of the cold‐water‐soluble fraction resulted in the formation of a loosened starch granule structure, a morphological alteration of protein bodies, a markedly lower gelatinization temperature, and a significantly higher pasting enthalpy. The impact on paste viscosity followed different trends. In some cultivars that had lower endogenous amylase activity, the paste viscosity was greatly reduced by the removal of the cold‐water‐soluble fraction. In others, the higher level of endogenous amylase activity led to more soluble saccharides being released through starch hydrolysis. Removing the soluble fraction caused a remarkable increase in peak viscosity. The overall effect on paste viscosity of removing the cold‐water‐soluble fraction was attributed to multiple factors, involving loosening of the starch granule structure, alteration of morphology of protein bodies, and the release of saccharides by endogenous amylase activity.     

Effect of Storage on Fat Acidity and Pasting Characteristics of Wheat Flour

Two cultivars of wheat (Triticum aestivum L.), Sunco and Sunsoft, were used to study the influence of storage time and temperature on the formation of starch-lipid complexes in flour pastes. Untreated and fat-reduced whole meal flours were stored separately for up to 12 months at 4, 20, and 30°C. The stored samples were analyzed for fat acidity, pasting properties, and iodine binding values. Fat acidity increased significantly in the untreated flour samples stored at 30 and 20°C compared with 4°C. Starch pasting properties, as measured using a Rapid Visco Analyser (RVA) indicated that the final viscosity of untreated flour samples of both cultivars increased significantly with storage time and elevated temperature, and correlated positively with increased fat acidity. Iodine binding values of the RVA pastes decreased with storage time and elevated temperature, and correlated negatively with fat acidity and final viscosity. The fat-reduced Sunco and Sunsoft flours showed less pronounced changes compared with untreated flours, whereas small changes in the RVA parameters were noted in grains stored over 12 months. The results indicate that free fatty acids are released during storage and that they increase the potential for starch-lipid complex formation when stored whole meal wheat flours are pasted in the RVA. These changes were evident after two to three months of storage at 20 and 30°C.     

Near‐Infrared Spectroscopy for Determination of Protein and Amylose in Rice Flour Through Use of Derivatives

The use of the derivative method for near‐infrared (NIR) calibration was investigated to determine protein and amylose content in rice flour. Samples for two years, 1996 and 1999, were combined to give a wide range of the constituents for development of the calibration model. The NIR spectral data were transformed with Savitzky‐Golay derivative with multiplicative scatter correction. To develop the best derivative models, the polynomial fits (quadratic, cubic, and quartic), convolution intervals (3–11 points for protein, 3–17 points for amylose), and derivative orders (1st derivative D1; 2nd derivative D2) were investigated. For the protein analysis, all polynomial fits with 3–11 points were acceptable to develop both the D1 and D2 models. However, the three‐point quadratic and five‐point quartic fits were not acceptable for the D1 model, and the three‐point quadratic fit was not acceptable for D2. For the amylose analysis, the D1 model produced generally better results than D2. Higher convolution intervals were required for the D2 model, whereas the D1 model was not affected by convolution intervals. A quadratic (or cubic) fit with 17‐point convolution interval was acceptable for the amylose D2 model, and the quadratic fit with 5–11 points and cubic (or quartic) fit with 7–17 points were suitable for the D1 model. Based on the standard error of cross‐validation (SECV), the calibration models developed using data for two years resulted in good precision with an SECV of 0.23% for protein using four factors and an SECV of 1.0% for amylose using 10 factors.     

Freeze‐Thaw Stability of Bualoy Thai Dessert Using Waxy Rice Flour

Frozen food products are gaining acceptance in Thai food industry and frozen bualoy dessert is a good opportunity for marketing in domestic and for exports. One important factor affecting quality of frozen starchy foods is retrogradation of starch gels. Thus freeze‐thaw stability of a frozen bualoy made from total waxy rice flour was studied and compared among the samples modified by 20 and 30% cross‐linked tapioca starch (CTS) derivatized with phosphorylation and 0.25% propylene glycol alginate (PGA). The waxy rice flour was pregelatinized by adding boiled water before shaping as a ball, then boiled and mixed with coconut syrup. All samples were subjected to five freeze‐thaw cycles over 60 days in a conventional freezer (–18°C). Texture analysis firmness and stickiness of the nonfrozen gels substituted with 20% CTS (382 ± 43, 20.5 ± 7.1 g·f) and 30% CTS (493 ± 37, 31.1 ± 7.0 g·f) were significantly different as compared with the control (329 ± 22, 14.8 ± 3.1 g·f). Similar results were observed for the samples continuously frozen for 60 days. The effects of freeze‐thaw stability to the frozen gels of the control, CTS, and PGA substituted samples appeared after two cycles and exhibited a large increase in firmness and stickiness at the fourth cycle. The firmness values obtained from the control and the samples substituted with 20% and 30% CTS were 2,397 ± 197, 2,182 ± 203, and 2,104 ± 200 g·f, respectively. This evidence was also observed with the samples containing PGA, but the effect was slightly less. This might account for the recrystallization of amylopectin molecules induced by freeze‐thawings. With DSC, the waxy rice gels showed a significant increase in the melting enthalpy (2.39 ± 0.23 J/g) at the fifth cycle from the nonfrozen gels (0.11 ± 0.02 J/g). The sensory tests of the bualoys were correlated with textural qualities that were acceptable to the panelists when the freeze‐thawing went no further than the second cycle.     

Effect of Surfactant Structure on the Pasting Properties of Wheat Flour and Starch Suspensions

Systematic studies were performed on the effect of the surfactant alkyl chain length (10–16 carbon atoms) and the head group charge/structure (anionic, cationic, nonionic) on the pasting properties of wheat flour and starch aqueous suspensions by means of a Rapid Visco Analyser (RVA). An excellent agreement was observed between the effect of surfactants on the onset temperature of the pasting process (PT) and the time to reach peak viscosity (t_peak) of wheat flour and wheat starch suspensions. Moreover, a correlation was found between the effect of different surfactants on these two parameters. With the exception of the cationic surfactants (alkyl trimethyl ammonium bromides), the effect of surfactants (alkyl sulfates, maltosides, monoglycerides, and sucrose esters) was found to be strongly dependent on the surfactant chain length. Shorter chain surfactants (C10–C12) induced an earlier pasting, while longer chain surfactants (C14–C16) had the opposite effect. The effect of surfactants on PT and t_peak of flour suspensions was enlarged when the surfactant concentration was increased from ≈1% to 15% (w/w) on a dry starch basis.     

Key Proteins Causing Changes in Pasting Properties of Rice During Aging

Key components that cause changes in pasting properties of rice during storage aging were investigated in this work. The main nonstarch components in rice were sequentially removed from fresh and aged rice, the aging effect of the component was separated, and thus the aging contribution rate of the component (CACR) on rice aging could be deduced. The results showed that the largest contributor to rice aging was albumin with a CACR of 65%, followed by globulin and prolamin with the CACRs of 38 and 14%, respectively, and the CACR of glutelin was small (1%). In contrast, the CACRs of fat and crude starch were –7 and –11%, respectively. These findings suggest that albumin and globulin are predominantly responsible for changes in pasting properties of rice during storage aging. This conclusion directs future researchers to the changes occurring in albumin and globulin for disclosing the mechanisms of rice aging.