THERMOPHYSICAL PROPERTIES OF PINK GUAVA JUICE AT 9 AND 11°BRIX

The effect of temperature on the conservation of pseudoplastic characteristics and on the physical properties of pink guava juice (Psidium guajava L.) variety Beaumont: B‐30 with two different total soluble solids (9°Brix and 11°Brix) was investigated. It was found that the juice exhibits pseudoplastic behavior in the range of shear rate between 40 to 160 s^?1 and within pasteurization temperature of 60–90C. While the flow behavior index (n) and density (p) increased, consistency coefficient (K), thermal conductivity (k) and specific heat capacity (C_p) decreased with increasing temperature. The linear regression equations or models for flow behavior index, consistency coefficient, density, thermal conductivity and specific heat capacity were determined with correlation coefficients ranging from 0.75 to 1.00.     

RHEOLOGICAL BEHAVIOR OF TOMATO PASTES IN STEADY AND DYNAMIC SHEAR

Tomato pastes (26–30 °Brix) at 40C were highly shear thinning fluids (n = 0.28) with large magnitudes of Casson yield stresses (78–212 Pa). Dynamic shear data revealed weak gel-like behavior of the pastes: magnitudes of G' were higher than those of G", and both increased with oscillatory frequencies. The Cox-Merz rule was not applicable to the steady and dynamic shear data on the pastes, but the two types of data conformed with the use of shift factor on frequency. Solids and liquid fractions in the pastes were satisfactory for correlating the dynamic properties, consistency index, and Casson yield stress.     

MODELING OF THICKNESS FOR SEMISOLID FOODS

We investigated the relationship between orally perceived thickness and calculated shear stress on the tongue for mayonnaise and custard. To this end, the applicability of the models of Kokini et al. (1977), describing the mechanical breakdown in the mouth, have been tested. Within a limited range of shear stresses (mayonnaise < 150 Pa; custard < 30 Pa), there was a linear relationship between shear stress and thickness, in accordance with the work of Kokini et al. (1977). Beyond this range, the linear relationship breaks down and the thickness levels off with shear stress for both mayonnaise and custard. The relationship over the entire range of shear stresses used in this paper can be satisfactorily described by a semilogarithmic (Fechner) relation. For both types of products, the quality of the thickness prediction by the decreasing‐height model and the constant‐height model of Kokini et al. (1977) is similar. For most mayonnaises, the contribution of the lateral movement of the tongue to the shear stress in the decreasing‐height model of Kokini et al. (1977) is orders of magnitude larger than the contribution of the squeezing or compression movement of the tongue towards the palate. This difference in magnitude is affected by the low value measured for the compression force and by the high values for material consistency K. The values for K are high because yield‐stress behavior has been neglected when the flow curves were analyzed. For custard, the models of Kokini et al. (1977) are found to be less adequate. It is proposed that this is because the models ignore interactions with saliva. Several routes to improve the modeling by incorporating viscoelastic behavior were unsuccessful. Elongational stress and yield stress were neglected in all tested models.     

HIGH TEMPERATURE RHEOLOGY OF TOMATO PUREE AND STARCH DISPERSION WITH A DIRECT-DRIVE VISCOMETER

A direct-drive concentric cylinder viscometer in a pressure chamber was used to study the flow behavior of a 11 Brix tomato puree at several temperatures in the range: 76 to 120C. The activation energy of flow of the tomato puree, based on apparent viscosity at 50 s^?1, was: 9.4 kJ mol^?1. The apparent viscosity (η) versus temperature profiles of a 4.0% waxy rice (WR) starch dispersion were determined during continuous heating from 30 to 110C at six shear rates in the shear range: 114 to 644 s^?1; beginning at about 95C, significant increase in η_a was observed at 114 and 160 s^?1. The said profiles collapsed to a single curve when η_a values were converted to reduced viscosity, values of η of the starch dispersion were lower than those of η* at about the same shear rates, γ=5.7 s^?1 and ω=6.3 rad s^?1.     

Rheological Properties of Cabbage Pulp

The rheological behavior of cabbage pulp was determined using a computer controlled rotational rheometer at a temperature range of 20, 30, 40, and 50°C. The flow behavior of pulp was pseudoplastic and fitted well to the Casson model. In all the cases, the effect of soluble solids on apparent viscosity was more pronounced than the effect of temperature. The combined effect of temperature and soluble solid content on viscosity was also given. The activation energy value of 3–6°Brix cabbage puree varied from 7 to 12.6 kJ/mol when shear rate increased from 1 to 10 s^?1. The exponential model developed had an excellent fit (r² > 0.87) and would be useful for the engineering applications.     

°BRIX/ACID RATIO AS A PREDICTOR OF CONSUMER ACCEPTABILITY OF CRIMSON SEEDLESS TABLE GRAPES

ABSTRACT

Harvesting at correct time is essential for the supply of quality grapes. There is no standard method to determine the proper time of harvesting for table grapes. The applicability of objective measurements such as soluble solids concentration (°Brix), acid contents (titratable acidity) and °Brix/acid ratio of Crimson Seedless table grapes were evaluated as predictors of quality in terms of consumer acceptability. Crimson Seedless table grapes were harvested from two locations in Western Australia at weekly intervals for 5 weeks. The samples were density sorted to get berries of different maturity levels. Objective measurements and sensory evaluation (panel of 63 judges) were conducted. There were significant (P ≤ 0.05) differences in the degree of liking among grapes of 16.0–17.0, 17.1–18.0 and 19.1–20.0°Brix. Consumer liking of grapes substantially changed with a change in acidity. The data revealed correlation coefficients of determination (r²) of 0.58, 0.79 and 0.85 between overall consumer acceptability and °Brix, acidity, and °Brix/acid ratio, respectively. °Brix/acid ratio was found to be the best objective measurement that reflected the consumer acceptability and can be used as a reliable tool to determine the optimum harvesting stage of Crimson Seedless table grapes.

PRACTICAL APPLICATIONS

Crimson Seedless table grapes have been successfully grown in Australia since 1994, and they are gaining popularity with consumers day by day. However, means of determining proper time to harvest has not yet been established. This study, by examining the relationship between instrumental measurements and human perception of sensory characteristics of Crimson Seedless table grapes, will provide a simple but reliable instrumental method to predict the optimum harvest stage of Crimson Seedless table grapes. This will help in providing a quality product to the consumers while helping growers to get better return out of their produce.     

APPLICATION OF AN IN-LINE RHEOLOGICAL CHARACTERIZATION METHOD TO CHEMICALLY MODIFIED AND NATIVE CORN STARCH

The application of an in‐line ultrasonics‐based rheological characterization method for measuring the rheological properties of 6% (w/v) acid‐thinned and native corn starch suspensions and gels was studied. The measurements were performed in steady, laminar pipe flow using a 5 MHz frequency transducer to determine the radial shear rate distribution. Two pressure transducers were used to measure the pressure drop allowing the shear stress distribution to be calculated through the conservation of linear momentum. It was possible to obtain shear viscosity over a range of shear rates, 1–50 1/s, from a single velocity profile. A comparison of the shear viscosity function at different flow rates showed that this method could be a valid method of process rheometry during the manufacturing of starch‐based products. Both ultrasonics and rotational rheometry results showed that acid‐thinned and native corn starch suspensions exhibited Newtonian behavior before heat treatment and non‐Newtonian behavior after heating. Power law consistency index and flow behavior index of the acid‐thinned corn starch gel at 39°C were 0.60 Pa.s ^0.68 and 0.68, respectively; whereas those of the native corn starch were 5.90 Pa.s ^0.37 and 0.37 at 38C. Their consistency index increased and flow behavior index decreased with cooling and a thermoreversible change in the flow behavior index was observed in the native corn starch after storage.     

EFFECT of PROCESSING CONDITIONS ON the PHYSICOCHEMICAL and SENSORY CHARACTERISTICS of STANLEY PLUM PASTE

Stanley plums were processed into pastes by heat concentration to 25 and 30°Brix. the soluble solids of these two pastes were increased to 40 and 45°Brix, respectively, by sugar addition. the influence of processing techniques on the physicochemical and sensory characteristics of pastes was evaluated. Heat concentration resulted in a significant decrease in titratable acidity, total anthocyanins and total pectin. Pastes showed pseudoplastic behavior within the shear rate range of 20 to 100 rpm. Sugar addition had darkening effect on color, but no noticeable effect on rheological properties of the pastes. It was not feasible to concentrate plum pastes beyond 40°Brix. Paste pH was not affected by processing conditions. Sensory evaluation indicated that preference could be adequately predicted by flavor and color under the condition that °Brix/acid ratio was suitable.     

EFFECT OF PECTIN CONTENT ON FLOW PROPERTIES OF MANGO PULP CONCENTRATES

The effect of pectin on the flow properties of mango pulp concentrates was studied using a coaxial cylindrical viscometer in the temperature (T) range 303-343°K. The variables were soluble solid content(C) 16-30°Brix, pectin content (C_pec, range 0.0323-0.0349 dry wt fraction) for the shear rate (γ) range 9.0-250 s^-1. Mango pulp concentrates behaved as pseudoplastic fluids in the entire pectin content range with a flow behaviour index of 0.286. A model relating shear stress () with the above variables is presented:     

Structural and rheological properties of potato starch affected by degree of substitution by octenyl succinic anhydride

The objective of the present study was to investigate the structural and rheological properties of octenyl succinic anhydrate (OSA) modified potato starch. Potato starch was modified with different concentrations of OSA (0, 1, 3, and 5%, v/v). X-ray results suggested that OSA modification did not disrupt the crystallinity of the native starch, and esterification occurs primarily in the amorphous regions. The use of ¹H-NMR spectra confirmed the presence of methyl protons in substituted OSA groups, which interacted with the starch molecules. In steady shear rheological analysis, OSA modified starch pastes showed a pronounced shear thinning behavior (n = 0.47–0.54). The consistency index (K) and yield stress (σ_oc) values of OSA modified starch pastes were significantly lower than those of the native starch. Dynamic shear rheological tests indicated that OSA modified starch pastes had weak gel-like behavior with storage moduli (G’) higher than loss moduli (G’’). OSA potato starch pastes were more viscous as compared to the native potato starch paste, as evidenced by their higher tan δ values.     

Concentration of Birch Sap by Reverse Osmosis

Birch sap (Betula pendula Roth, and B. pubescens Ehrh.) was concentrated by reverse osmosis in a pilot scale recirculating system using ZF-99 tubular membranes. Sap feed temperatures varying from 30-70°C and pressures from 4-5 MPa were used when concentrating the sap from about 0.7^oBrix to 13^oBrix. Optimal conditions considering both processing capacity and product quality were 50°C and 5 MPa. Retentions of glucose, fructose, sucrose, and malic acid were practically 100% when using the ZF-99 membrane, whereas with the cellulose acetate membrane the retentions fell to between 92 and 99% at 30°C and 4 MPa. The pH of the original sap varied between 6.0 and 6.6 and dropped during the run by 0.2-0.3 pH units.     

STUDIES ON THE CLARIFICATION AND CONCENTRATION OF BEETROOT JUICE

Studies on the clarification and subsequent concentration of beetroot juice were carried out. The juice was clarified using enzyme, fining agent (FA), centrifugation and ultrafiltration (UF), while reverse osmosis (RO) and thermovacuum evaporation were used for concentration. The juices were concentrated to 23–25°Brix, and the physicochemical characteristics were evaluated at different stages of processing on the basis of clarity, permeate flux, °Brix, acidity, sugars, pigments and CIE L*, a* and b* (lightness, redness and yellowness, respectively) color values. Average permeate flux during RO was found to be highest (36.08 liters per square meter of membrane per hour [L/m²h]) in the case of enzyme treatment, followed by UF juice, and lowest (30.33 L/m ² h) in the case of enzyme and FA treatments. In terms of clarity of concentrate, the highest value was obtained for juices pretreated with enzyme and UF. Pigment content and L*, a* and b* values showed that pigment loss was higher in the case of RO‐concentrated juice pretreated with enzyme. Comparative evaluation in terms of clarity, color and chemical parameters showed that concentrates obtained using both techniques were comparable.     

Rheological properties of blueberry puree for processing applications

Rheological properties of purées made from small fruits like blueberries are important for application in handling and thermal processing where their physical and/or chemical attributes can be altered. Flow of purées made from highbush blueberry (Vaccinium corymbosum L.) was investigated in the 10-1000 s⁻¹ shear rate range with the objective of determining the influence of temperature and solids content on the rheological properties. The rheological behavior was well described by the three-parameter Sisko model. The activation energy of flow (E_a) calculated with respect to apparent viscosity at 100 s⁻¹ increased from 11.4 to 17.1 kJ/mol for purée with 10% and 25% total soluble solids, respectively. When evaluated in terms of consistency coefficient, the activation energy varied between 10.7 and 21.7 kJ/mol within the same range of solid contents. For the conditions investigated, a mathematical model that is suitable for describing the influence of temperature and dissolved solids on the apparent viscosity of blueberry purée was obtained. The rheological behavior of 10-20 Brix blueberry purée was well predicted (R²=0.99). Further investigation is needed to improve the model to cover a wider Brix range.     

FLOW BEHAVIOR OF GUAVA PUREE AS A FUNCTION OF TEMPERATURE AND CONCENTRATION

Flow behavior of guava puree was determined with a tube viscometer. The puree samples (9.8-16.0° Brix) exhibited shear-thinning behavior. The flow behavior index of the power law model was nearly constant for all the samples over a temperature range of 25-60°C; its mean value was 0.43 with a standard deviation of 0.031. The apparent viscosity decreased with increase in temperature and the activation energy offlow was found to be 3.7 kcallg mole. The apparent viscosity increased as the concentration of the purees was increased; the increase was proportional to concentration raised to a power of 2.7. The Mizrahi-Berk model described well the flow data of the purees. The magnitude of the shear rate exponent was about 0.48, indicating that the purees were suspensions in shearthinning media.     

Influence of Fluid Concentration on the Elevation of Boiling Point of Blackberry Juice

The rise in boiling point of blackberry juice was experimentally measured at soluble solids concentrations in the range of 9.4 to 58.4°Brix and pressures between 4.9?×?10³ and 9.0?×?10⁴ Pa (abs.). Different approaches to representing experimental data, including the Dühring's rule, a model similar to Antoine equation and other empirical models proposed in the literature were tested. In the range of 9.4 to 33.6°Brix, the rise in boiling point was nearly independent of pressure, varying only with juice concentration. Considerable deviations of this behavior began to occur at concentrations higher than 39.1°Brix. Experimental data could be best predicted by adjusting an empirical model, which consists of a single equation that takes into account the dependence of rise in boiling point on pressure and concentration.     

RHEOLOGICAL BEHAVIOR OF POMEGRANATE (PUNICA GRANATUM L.) JUICE AND CONCENTRATE

The rheological behavior of pomegranate juice (Punica granatum L.), prepared from fresh pomegranates, was studied as a function of solids concentration in the range 17.5–75°Brix at 10–55C, using a controlled stress rheometer. Concentration methods did not influence flow behavior. There were no significant differences (P > 0.05) between viscosity and activation energy values for the pomegranate juices concentrated by different methods. The juices exhibited Newtonian behavior regardless of the concentration method. The effect of temperature was described by an Arrhenius‐type equation with an activation energy in the range 5.34–32.2 kJ/mol depending on concentration. An exponential model described better the effect of the soluble solids on the viscosity and E_avalues. A simple equation was proposed for describing the combined effect of temperature and soluble solids content on the juice viscosity.     

RHEOLOGICAL BEHAVIOR AND PHYSICOCHEMICAL CHARACTERISTICS OF GOLDENBERRY (PHYSALIS PERUVIANA) JUICE AS AFFECTED BY ENZYMATIC TREATMENT

Goldenberry (Physalis peruviana) juice was prepared and enzymatically treated with two dosages (300 and 600 ppm) of Pectinex Ultra SP‐L, then, the juice was concentrated to 30 and 40°Brix. Rheological properties of juices were studied at a wide range of temperatures (5 to 100C) and shear rate range from 0.3 to 100/s using Brookfield Digital Viscometer. The Herschel–Bulkley, Casson and Bingham rheological models were applied to describe the rheological properties of different juices. These properties are key parameters required to solve food industry problems in numerous areas such as quality control, evaluation of consumer acceptance and texture. The results indicated that juice concentrates behave as non‐Newtonian fluids (pseudoplastic) and have a definite yield stress. The Bingham and Casson, plastic viscosity, yield stress, consistency index and flow index decreased with increasing enzyme dosage and temperature. The effect of temperature on the viscosity described by means of an Arrhenius‐type equation. The activation energy for viscous flow depends on the total soluble solids. Moreover, sensory evaluation for juices was carried out. In consideration of industrial utilization, detailed knowledge on the rheological and physicochemical characteristics of cape gooseberry juice is of major importance.     

OPTIMIZATION OF VACUUM PULSE OSMOTIC DEHYDRATION OF CANTALOUPE USING RESPONSE SURFACE METHODOLOGY

The optimum levels of vacuum pressure, concentration of osmotic solution and dehydration time for vacuum pulse osmotic dehydration of cantaloupe were determined by response surface methodology (RSM). The response surface equations ( P < 0.05 and lack of fit > 0.1) explain the 97.6, 88.0 and 97.1% of the variability in weight loss, water loss and °Brix increase, respectively, at 95% confidence level. The canonical analysis for each response indicated that the stationary point is a saddle point for weight loss and °Brix increase, and a point of maximum response for water loss. The region that best satisfied all the constraints (low values in weight loss and °Brix increase, and high value in water loss) is located within the intervals from 49.5 °Brix to 52.5 °Brix for concentration and from 75 min to 84 min for dehydration time at a vacuum pulse of 740 mbar.     

THIXOTROPY OF ORANGE CONCENTRATE AND QUINCE PUREE

Orange juice with pulp and pectins is thixotropic at soluble solids concentrations of 55 and 60 Brix in the range of temperatures from 0 to 20C and shear rate from 7.2 to 57.6 s⁻¹. Quince purée is thixotropic at soluble solids concentrations of 12.3 to 28 °Brix in the range of temperatures between 0 to 20C and shear rate from 7.2 to 57.6 s⁻¹. The thixotropic behaviour of orange juice and quince purée increases with increasing concentration and decreasing temperature and they can be described by the kinetic model proposed by Figoni and Shoemaker (1983):

The thixotropic structure of orange juice was destroyed by applying a shear rate of 57.6 s⁻¹ for 5 min, and for quince purée for 10 min. Quince purée shows a greater thixotropic character than orange juice, because it has a higher content of fiber, pulp and pectins and also because it shows a microscopic structure consisting of long particles and heterogeneous fibers.     

GRANULE SIZE DISTRIBUTION AND RHEOLOGICAL BEHAVIOR OF HEATED MODIFIED WAXY AND UNMODIFIED MAIZE STARCH DISPERSIONS

The effect of granule size distribution, supported by microscopic analysis, on the steady shear and dynamic shear rheological behavior of 5% gelatinized modified waxy maize starch (MWMS) dispersions heated at 80C for time periods from 0.5 to 30 min was examined and data obtained with 5% unmodified maize starch (UMS) dispersions was used to provide additional insight. The MWMS dispersions showed a faster and more extensive swelling than the UMS dispersions at 80C. They also exhibited rheopectic (anti-thixotropic) time-dependent and shear thinning with yield stress behavior. UMS dispersions showed shear thickening characteristics after short heating times that gradually turned to shear thinning as the heating time was increased. Dynamic tests on MWMS dispersions showed G' was about ten times higher than G". G' showed a small dependence on frequency over 0.6–63 rad/s while G", after an increase at low frequencies, became less sensitive. The gelatinized MWMS dispersions can therefore be classified as 'weak gels.' The flow behavior index of UMS dispersions was strongly