International Journal of Food Science and Technology 2005, 40, 885–895 Original article The effect of high pressure treatment on rheological characteristics and colour of mango pulp Jasim Ahmed,* Hosahalli S. Ramaswamy & Nikhil Hiremath Department of Food Science & Agricultural Chemistry, Macdonald Campus of McGill University, Ste. Anne de Bellevue, PQ, Canada H9X 3V9 (Received 15 December 2003; Accepted in revised form 1 March 2005) Summary The effect of high-pressure (HP) treatment (100–400 MPa for 15 or 30 min at 20
C) on the rheological characteristics and colour of fresh and canned mango pulps was evaluated. Differences were observed in the rheological behaviour of fresh and canned mango pulps treated with HP. Shear stress–shear rate data of pulps were well described by the Herschel– Bulkley model. The consistency index (K) of fresh pulp increased with pressure level from 100 to 200 MPa while a steady decrease was noticed for canned pulp. For fresh pulp the flow behaviour index decreased with pressure treatment whereas an increasing trend was observed with canned pulp. Storage and loss moduli of treated fresh pulp with HP increased linearly with angular frequency up to 200 MPa for a treatment time of 30 min while a steady decreasing trend was found for processed pulp. No significant variation in colour was observed during pressure treatment. Keywords Consistency coefficient, dynamic rheology, elastic modulus, flow behaviour, glass transition temperature, total colour difference, viscous modulus, yield stress. Introduction Mango (Magnifera indica L.) is one of the important tropical fruits. It is usually considered as a high quality fruit and has been recognized as Ôking of the fruitsÕ in the Orient. The fruit is relished for its succulence, exotic flavour and delicious taste. Mango is a rich source of carotenoids and provides high vitamin A content (Pott et al., 2003). The fruit has tremendous potential for processing and export. However, substantial quantities of the fruits are wasted because of poor post-harvest management and lack of appropriate processing facilities in developing countries. Some mango varieties like Alphonso, Duseheri, Chousa, Baganpalli and Langra experience excellent consumer demand when the fruits are marketed in the fresh form. Mango fruit is also processed and then marketed in various forms. Mango pulp and puree are the most popular mango products and are *Correspondent: Fax: 1 514 398 7977; e-mail: jahmed2k@yahoo.com doi:10.1111/j.1365-2621.2005.01026.x  2005 Institute of Food Science and Technology Trust Fund generally preserved as canned product by subjecting fruit to thermal processing. This extends the year round availability of mango (Shrikhande et al., 1976). The pulp is used to manufacture beverages, ice-cream, mango leather and other products. For such processed foods to be of consistent quality with technically and economically feasible processes, it is imperative that the physico-chemical properties of the pulp are documented and standardized. These properties include colour, flavour and texture (rheological properties) and they are of primary importance from the quality and processing point of view. High pressure (HP) processing is a novel technique in which a food experiences an elevated pressure in a quasi-instantaneous manner throughout the mass (Cheftel, 1995). HP processing has successfully been used for various specialty foods and numerous reports are available on its application to food (Oxen & Knorr, 1993; Van Camp & Huyghebaert, 1995; Basak & Ramaswamy, 1997, 1998; Mussa et al., 1999; Hsu & Ko, 2001; Ahmed & Ramaswamy, 2003). The process 885 886 Pressure rheology and colour of mango pulp J. Ahmed et al. has been considered as being either alternative or complimentary to thermal processing. HP processing retains the sensory qualities of the food better as compared with thermal processing (Hayakawa et al., 1994). HP processing has been successfully applied to various fruit products, giving better quality retention of colour, flavour and vitamins (Smelt, 1998) than thermal processing; however changes in texture are caused by the process (Prestamo & Arroyo, 1998). A few fruit products treated with HP such as juices and jams have been commercialized in some developed countries. However, the success of the process depends on the inactivation of enzymes and destruction of microorganisms to a safe level. Understanding the rheological properties of food products is important for product development. In particular these properties influence design and evaluation of process equipment such as pumps, piping, heat exchangers, evaporators, sterilizers and mixers. Knowledge of the fundamental rheological properties of any food can be an indication of how the food is going to behave under various processing conditions. Numerous studies have been conducted on the rheological properties of fruit pulp, purees and paste (Rao, 1977; Ibarz et al., 1995; Bhattacharya, 1999; Ahmed & Ramaswamy, 2004). The various factors affecting the rheological behaviour of fruit purees and concentrates include temperature (Holdsworth, 1971; Vitali & Rao, 1984; Oomah et al., 1999), total soluble solids (TSS)/concentration (Harper & El-Sahrigi, 1965; Ilicali, 1985), particle size (Tanglertpaibul & Rao, 1987; Pelegrine et al., 2002), addition of enzyme (Khalil et al., 1989; Bhattacharya & Rastogi, 1998) and pH (Dik & Ozilgen, 1994). Rheology of mango pulp and concentrate have been studied by various researchers (Manohar et al., 1990; Bhattacharya, 1999; Pelegrine et al., 2002) and results found have not always been consistent. Most researchers have reported that yield stress does not exist, the exception being Bhattacharya (1999). However, no information is available on the effect of HP on the rheology of fresh and/or processed mango pulp. With the advent of controlled stress/rate rheometers, it has become convenient to study the rheological characteristics of fruit pulp/puree under a wide range of conditions. Those instruments have a wide range of measurement capabilities from very low to extremely high shear rate and extreme sensitivity. Tests can also be set under steady, dynamic or oscillatory shear. Most previous reports on the rheology of fruit puree are based on experiments using a high shear rate, whereas in actuality low and medium shear rates are of greater industrial significance, especially during mixing or product development when there is a smaller degree of structural breakdown. Therefore, rheological studies based on low and medium shear rates are of much more practical significance. Another important characteristic of fruit purees/paste is the yield stress, which indicates the threshold stress to initiate the flow. The new generation of rheometers have been equipped with low shear measurement and data analysis software that routinely calculates yield stress and rheological parameters. The colour of mango pulp is capusine yellow or reddish and carotenoids contribute to the mango colour. The b-carotene constituent is the major pigment (50–64%) of the ripe mango (John et al., 1970). The maintenance of naturally coloured pigments in thermally processed and stored food is a major challenge in food processing. Ahmed et al. (2002) have studied the degradation of the colour of mango puree during thermal processing and they reported that colour degradation followed first-order reaction kinetics. However, pressure affects colour differently. It has been reported that HP causes an increased browning reaction in some vegetable products (Eshtiagi & Knorr, 1993; Arrayo et al., 1997), while Fernandez et al. (2001) reported no effect of pressurization on pigments of tomato puree. No reports are available on the colour of mango puree during HP treatment. The objective of the present work was to study the effect of high-hydrostatic pressure on flow and dynamic rheological characteristics as well as the visual colour of mango puree. This information will enable better understanding of how mango pulps respond to high hydrostatic pressure. Materials and methods Mango pulp Fresh mangoes (Cv. Chousa) were procured from a local store. The mangoes were produce of Pakistan and were airlifted to Canada. The International Journal of Food Science and Technology 2005, 40, 885–895  2005 Institute of Food Science and Technology Trust Fund Pressure rheology and colour of mango pulp J. Ahmed et al. mangoes were thoroughly washed, peeled and the pulp portion was sliced to separate the stone, all operations were done manually. The slices were put through a fruit strainer with a screen of 1 mm clearance to get mango pulp of uniform consistency and particle size. The pulp was immediately frozen and stored at )20
C for a maximum of 3 days prior to subjecting it HP treatment. Another lot of canned mango pulp of Indian origin (Cv Alphanso; Cedar brand marketed by Phoenica Products Inc. Montreal, Canada, canned in 2002) was purchased from a local store. High hydrostatic pressure treatment An isostatic HP machine unit (Model# CIP 42260; ABB Autoclave System, Columbus, OH, USA) with a chamber dimension of 0.56 m height and 0.1 m diameter was used to give HP treatment. Distilled water containing 2% water soluble oil (Part No. 5019; Autoclave Engineers, Columbus, OH, USA) was used as the pressure medium for pressurization. A smooth pressure rise of 2.4 MPa s)1, after an initial delay of 15 s, was characteristic of the equipment pressurization. The come-up time for pressurization ranged from 33 s to 2.8 min depending upon the pressure level and the depressurization time was
10 s. Test samples were treated at 100, 200, 300 and 400 MPa for 15 and 30 min. The chamber temperature was regulated at 20 (±1.5)
C by circulating cold water. The fruit temperature was recorded by a thermocouple (K-type) connected to a data logger during the experimentation. Pressure treatment time mentioned in the results does not include the pressure build up or release times. Prior to treatment, samples in sealed test pouches were thawed and/or equilibrated to the desired temperature (approximately 8–17
C depending on the pressure level) to accommodate the adiabatic heating of samples, which was about 3C/100 MPa. They were then placed inside the HP vessel submerged in water. Cold water just below the desired temperature was circulated through the jacket during the entire duration of the experimental runs. Duplicate samples were used for each pressure treatment. The pressure treated pouches were immediately transferred to a refrigerator (4–6
C) and subsequently the colour and rheology were evaluated.  2005 Institute of Food Science and Technology Trust Fund Rheological measurement Rheological measurements (oscillation and flow both) were made in a controlled shear rate rheometer (AR 2000; TA Instruments, New Castle, DE, USA) with its accompanying computer software (Rheology Advantage Data Analysis Program, TA). The sample was placed between parallel-plate geometry (60 mm diameter) and measurements were made using a gap size set at 1 mm. The AR 2000 Concentric Cylinder System is based on an efficient method of peltier temperature control and temperature could thus be efficiently monitored during the experiments. For both steady flow and dynamic rheological studies a sample of approximately 3 mL was placed between the plates and measurements were made at 20
C. The instrument was programmed to maintain a set temperature for equilibration for 15 min followed by a two-cycle shear in which the shear rate was increased linearly from 0.1 to 100 s)1 in 5 min and immediately decreased from 100 to 0.1 s)1 in the next 5 min. All the rheological parameters were obtained from the software (Rheology Advantage, TA version 2.3, TA Instruments, New Castle, DE, USA). In order to perform a quantitative comparison of samples various rheological flow models based on shear stress–shear rate were tested (Newtonian, Bingham, Casson, power law, Herschel Bulkley) and the best fit model was selected on the basis of standard error, which is defined as: i0:5 Xh ðXm  Xc Þ2 =ðn  2Þ =Range  1000 ð1Þ Where Xm is the measured value; Xc is the calculated value; n is the number of data points and the range is the maximum value of Xm – the minimum value. For dynamic rheological studies, the linear viscoelastic range was tested and the oscillation stress was selected, it was based on the linear part of the viscoelastic range. Measurements were made during two cycles of the frequency sweep (0.1– 10 Hz and back) at 20
C after an equilibration period of 15 min. Dynamic rheological parameters storage modulus (G¢), loss modulus (G¢¢) were obtained directly from the instrument software. All rheological experiments were in duplicate. International Journal of Food Science and Technology 2005, 40, 885–895 887 888 Pressure rheology and colour of mango pulp J. Ahmed et al. Physio-chemical characteristics Visual colour was measured using a Minolta reflectance colorimeter (Minolta Corp, Ramsey, NJ, USA) in terms of L (lightness), a (redness and greenness) and b (yellowness and blueness). The colorimeter was calibrated with a white standard. A glass petri dish containing the pressure-treated puree was placed below the light source and postprocess colour L, a, b values were recorded. L, a and b measurements were evaluated from three samples and the values were averaged. A numerical total colour difference (DE), hue (H) and chroma (C) were calculated as: DE ¼ C¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðL0  LÞ2 þ ða0  aÞ2 þ ðb0  bÞ2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a2 þ b2 and h ¼ tan1 ðb=aÞ where, L0, a0 and b0 represented the readings of the control sample, and L, a and b represented the individual readings after HP treatment. The TSS and pH of both mango purees were determined by a refractrometer (Atago, Japan) and pH-meter (Accumet, USA) respectively. Proximate composition of both pulps was analysed as per the method described by AOAC (1980). Differential scanning calorimetry The glass transition temperature (Tg) of mango pulp samples was determined using a TA Q100 Differential Scanning Calorimeter (DSC) (TA Instruments, New Castle, DE, USA), equipped with a refrigerated cooling system that efficiently monitored temperature to )90
C. Nitrogen was used as purge gas at a flow rate of 50 mL min)1. Hermetic sealed aluminium pans were used to avoid any moisture loss during the analysis. In the experiment, mango pulps were sealed, cooled to )90
C, held for 15 min and then warmed to 10
C at a heating rate of 5
C min)1; equilibrated to 10
C, held for 15 min and finally cooled to )90
C at 5
C min)1. A four axis robotic device automatically loaded the sample and reference pans of the DSC. An empty aluminium pan was used as a reference. Rescans were done immediately to confirm the existence of a Tg. The DSC measurement was done in duplicate. The DSC data were analysed with the Universal Analysis Software (version 3.6C) for thermal analysis, which was provided with the instrument (TA Instruments, Newcastle, NJ, USA). Tg is a second order transition and was recognized as a change of heat flow step in the DSC thermograms (Roos, 1995). Midpoint Tg was obtained from the Universal Analysis (TA Instruments) software. Statistical analysis The influence of residence time and pressure level applied on mango pulp was determined by paired samples t-test using the Microsoft Excel software (Microsoft Office Excel, Microsoft Corporation, USA). Significance of differences was defined at P £ 0.05. Results and discussion Unprocessed control mango pulps Untreated (pressure) pulp was evaluated initially to serve as a control point for rheological and colour changes of the HP-treated pulps. The chemical and physical properties (mean ± 1 SD) of Chousa and Alphanso pulps evaluated prior to HP treatment were: pH 5.15 ± 0.02, 23.12 ± 0.03
Brix and 4.01 ± 0.03, 27.1 ± 0.02
Brix, and L, a and b values (54.23 ± 0.19, 8.49 ± 0.51, 33.28 ± 0.98 and 51.05 ± 0.15, 14.32 ± 0.10, 39.69 ± 0.33) respectively. The average proximate compositions of pulps of Chousa and Alphanso were: moisture, 72.18 ± 0.12 and 68.8 ± 0.08%; protein; 0.71 ± 0.04 and 0.61 ± 0.07%; fat, 0.27 ± 0.02 and 0.52 ± 0.05%; ash, 0.44 ± 0.04 and 0.38 ± 0.09%; crude fibre, 1.27 ± 0.03 and 0.87 ± 0.02%; and carbohydrate, 25.13 ± 0.04 and 28.82 ± 0.07%. The starch contents were 2.4 ± 0.07 and 1.84 ± 0.06% for Chousa and Alphanso pulps, respectively. The pH of Alphanso mango pulps was low because of acidification for shelf-stability. The TSS and pH of test samples remained unchanged after the HP treatment. Rheological properties Flow models Various flow rheological models (Newtonian, Casson, Bingham, power and Herschel-Bulkley) were tested to see which best described the data for International Journal of Food Science and Technology 2005, 40, 885–895  2005 Institute of Food Science and Technology Trust Fund Pressure rheology and colour of mango pulp J. Ahmed et al. Table 1 Fitting of various flow models for Chousa mango pulp at 400 MPa for 30 min No. Model Yield (Pa) 1 2 3 4 5 Newtonian model Power law model Casson model Bingham model Herschel Bulkley model – – 13.46 17.14 2.27 Consistency coefficient (Pa sn) Flow behaviour index ()) SE 0.744* 15.73 0.152 0.436 13.45 – 0.252 – – 0.278 303.7 14.97 60.01 100.02 14.02 *Apparent viscosity Pa s. ð2Þ where r is the shear stress (Pa), r0 is the yield stress, c is the shear rate (s)1), K is the consistency coefficient (Pa sn) and n is the flow behaviour index (dimensionless). This model was recommended by Bhattacharya (1999) for describing the flow curves of mango pulp (TSS of 16
Brix) while Pelegrine et al. (2002) found that the Mizrahi–Berk model well described the rheograms of mango pulp with a TSS of 13.3
Brix. Effect of high pressure on flow characteristics of mango pulp 70 60 50 Shear stress (Pa) r ¼ r0 þ KðcÞn (a) 40 30 Control 100 MPa 200 MPa 300 MPa 400 MPa 20 10 0 0 20 40 60 80 100 120 100 120 Shear rate (s–1) (b) 60 50 Shear stress (Pa) shear stress–shear rate of control and mango pulp treated by pressure. It was observed that the Herschel Bulkley model was the best model based on estimated errors (Table 1). For all test cases, the magnitudes of standard errors were always <20 which was satisfactory. The Herschel Bulkley model is represented as: 40 30 Control 100 MPa 200 MPa 300 MPa 400 MPa 20 10 The rheograms for both Chousa and Alphanso mango pulps are shown in Fig. 1a and b respectively. The control was a sample without pressure treatment. Both pulps exhibited pseudoplasticity with the presence of a yield stress. The rheological parameters are presented in Table 2 for Chousa and Alphanso pulps, respectively. Yield stress is one of the important quality parameters that characterize the properties of semi-solid foods. The magnitude of yield stress for Alphanso mango pulp ranged between 3.81 and 6.24 Pa while for Chousa the magnitude varied from 1.09 to 6.14 Pa as a result of the HP treatment. The yield stress consistently increased with pressure for Alphanso pulp and followed a linear relationship (Eqn 3). However, no consistent trend was found for Chousa pulp.  2005 Institute of Food Science and Technology Trust Fund 0 0 20 40 60 80 Shear rate (s–1) Figure 1 Effect of high-pressure on rheogram of (a) Chousa mango pulp and (b) Alphanso pulp at 20
C. Bhattacharya (1999) observed a yield stress for Totapuri mango pulp to range between 2.7 and 3.6 Pa at 5–30
C. so ¼ 0:006P þ 3:61ðR2 ¼ 0:95and SE ¼ 0:26Þ ð3Þ The HP treatment resulted in an increase in shear stress of Chousa pulp at any shear rate (Fig. 1a) while a reverse trend was observed for Alphanso pulp (Fig. 1b). It was observed that the International Journal of Food Science and Technology 2005, 40, 885–895 889 Pressure rheology and colour of mango pulp J. Ahmed et al. Table 2 Effect of pressure on rheological parameters of (a) Chousa pulp and (b) Alphanso pulp Yield Consistency Pressure Duration stress coefficient (min) (MPa) (Pa) (K), Pa sn Flow behaviour index (n) SE (a) 0.101 100 100 200 200 300 300 400 400 – 15 30 15 30 15 30 15 30 6.242 1.151 2.219 1.085 2.657 1.348 2.956 3.779 2.273 8.82 11.91 12.09 14.81 18.55 13.26 15.97 11.73 13.45 0.338 0.327 0.270 0.260 0.245 0.268 0.267 0.293 0.278 11.12 7.83 9.47 13.05 16.28 11.79 16.12 14.31 14.02 (b) 0.101 100 100 200 200 300 300 400 400 – 15 30 15 30 15 30 15 30 3.805 3.642 4.085 4.008 4.707 5.407 5.125 5.260 6.238 11.33 10.44 7.613 8.263 8.856 9.163 8.630 7.295 7.681 0.309 0.318 0.354 0.345 0.343 0.334 0.345 0.363 0.362 18.83 17.38 16.44 16.17 17.26 16.22 18.30 15.77 17.76 70 60 Shear stress (Pa) Chousa pulp was fresh and not heat treated while the canned Alphonso pulp was obviously heat treated. The flow behaviour index (n) indicated an increase in pseudoplasticity for Chousa pulp (n decreasing from 0.34 to 0.25) while it was found that pseudoplasticity decreased for Alphanso pulp (n increasing from 0.31 to 0.36). The flow behaviour index reported in the literature for mango pulp ranged between 0.39 and 0.48 (Bhattacharya, 1999; Pelegrine et al., 2002). This difference in the magnitudes of the K and n values of pulps could be caused by the presence of high molecular weight carbohydrates, such as sugar and starch. In the case of the acidified Alphanso pulp, the maximum conversion of sugar and starches and inactivation of enzymes would have occurred during thermal treatment, and therefore no significant effect was observed. It is obvious from Table 2a that the highest magnitude of K and the lowest n for Chousa pulp was observed at pressure–time combination of 200 MPa and 30 min. Therefore, it could be a critical pressure level that contributes to the alteration of the flow characteristics of pulp. Thixotropy 50 Mango pulp treated with HP exhibited thixotropy (Fig. 3). The area enclosed by the hysteresis loop signifies the degree of structural breakdown during steady shearing. The upper curve (0.1–100 s)1) of the rheogram was found to be higher compared to the down ward curve (100–0.1 s)1) which indicated 40 30 Chousa 15 min Chousa 30 min Alphanso 15 min Alphanso 30 min 20 10 0 0 20 40 60 80 100 120 Shear rate (s–1) 70 60 Figure 2 Effect of residence time of applied of 200 MPa on rheogram of mango pulps at 20
C. consistency index (K) for Chousa increased significantly (P £ 0.05) with pressure treatment while K was found to decrease for Alphanso pulp (P > 0.05). However, the trend was not consistent. The effect of the residence time of pressure treatment on mango pulps is illustrated in Fig. 2. An increase in K value with time has been found for Chousa pulp whereas K was much more constant for canned Alphanso mango pulp. The difference was possibly due to the fact that the Shear stress (Pa) 890 50 40 30 Chousa (up) Chousa (down) Alphanso (up) Alphanso (down) 20 10 0 0 20 40 60 80 100 120 Shear rate (s–1) Figure 3 Thixotropy of pressurized pulps at 300 MPa for 30 min at 20
C. International Journal of Food Science and Technology 2005, 40, 885–895  2005 Institute of Food Science and Technology Trust Fund Pressure rheology and colour of mango pulp J. Ahmed et al. Table 3 Effect of high pressure on thixotropy of mango pulps at 20
C Area under the curve (1 s)1 Pa) Thixotropy (Pa s)1) Pressure Duration Chousa Alphanso Chousa Alphanso (min) (MPa) 0.101 100 100 200 200 300 300 400 400 – 15 30 15 30 15 30 15 30 16.23 17.20 17.77 18.76 17.63 17.82 19.06 16.28 20.88 2.40 5.04 5.65 3.63 4.31 3.12 2.62 2.58 1.42 284 323.1 373.8 356.5 332.6 323.9 373.1 259.8 336 89.5 131.0 132.5 107.7 112.6 119 108.7 107.9 81.36 a thinning of mango pulp with time of shearing. Similar behaviour was observed by Bhattacharya (1999) for mango pulp. The rate of thixotropy and the hysteresis loop area shown by Chousa pulp treated by at 200 MPa for 30 min was higher than the control indicating more thixotropy (Fig. 3). Pressure treatment of canned Alphanso pulp exhibited limited thixotroic behaviour, possibly because of previous breakdown of texture due to heat treatment during canning (Table 3). However, pressure treatment time had no effect on thixotropy. Effect of high pressure on dynamic rheology of mango pulp The dynamic rheological characteristics of mango pulps are presented in Fig. 4a–d. The dynamic rheological modulii values (G¢ and G¢¢) of both mango pulps increased significantly with angular frequency and G¢ values were much higher than G¢¢ at all values of frequency employed. There was no crossover of G¢ and G¢¢ indicating no gel formation occurred during HP treatment of mango pulp. Both G¢ and G¢¢ values increased with frequency for Chousa pulp as a result of HP treatment at 100 and 200 MPa (both 15 and 30 min) while at, or above, 300 MPa both parameters decreased. On the contrary, a steadily decreasing trend of G¢ and G¢¢ was found for Alphanso pulp treated with pressure. No significant differences (P > 0.05) were observed between HP treatment times of 15 and 30 min for both pulps.  2005 Institute of Food Science and Technology Trust Fund The experimental data of G¢ and G¢¢ fitted a polynomial equation (Rheology Advantage, TA version 2.3) adequately with standard errors <3. The coefficients to the third order are systematic with the experiments and therefore rheological parameters to the third order are presented in Table 4. Rheological coefficients support the idea that there was dynamic rheological behaviour of mango pulp with variation of rheological parameters above 200 MPa. Effect of high pressure on visual colour of mango pulp The colours of mango pulps treated with HP are presented in Table 5 for Chousa and Alphanso, respectively. The changes in the values of the three-colour parameters (L, a and b) of Chousa pulp with pressure treatment were not significant (P > 0.05). The quality parameter (a/b), C and h remained almost constant indicating minimal effect on pigments. The total colour difference (DE), which takes into account the evolution of the three colour parameters, decreased with an increase in pressure intensity with Chousa (fresh) pulp. Higher pressures decreased the magnitude of DE while treatment time had no effect. For Alphanso pulp the changes in colour b values (represents carotenoids), parameters a/b and C and h values were not significant. The magnitude of (DE) for Alphanso pulp increased following a 30-min treatment at ‡300 Mpa, this indicated an initiation of browning. This could be because of browning of thermal processed pulp during pressurization at higher level. The browning is evident from the increase of colour ÔaÕ value and decrease of ÔbÕ values at ‡300 MPa. This retention of colour of mango pulp during HP processing was supported by earlier work of Fernandez et al. (2001) on tomato puree. They found no effect of pressurization on pigments (b-carotene and lycopene) of tomato puree during HP treatment (Fernandez et al., 2001). However, contradictory results were reported by other researchers (Arrayo et al., 1997; Prestamo & Arroyo (1998). These researchers reported that browning occurred after HP treatment of some vegetable products and the products become unacceptable to the consumers because International Journal of Food Science and Technology 2005, 40, 885–895 891 Pressure rheology and colour of mango pulp J. Ahmed et al. (a) (b) 1000 1000 G// (Pa) G/ (Pa) Control 200 MPa 30 min 300 MPa 30 min 400 MPa 30 min 100 Control 200 MPa 30 min 300 MPa 30 min 400 MPa 30 min 100 100 x 10–3 10 1 x 100 10 x 100 100 x 100 0.1 1.0 Angular frequency (rad s–1) (c) (d) 1000 100.0 1000 Control 200 MPa 30 min 300 MPa 30 min 400 MPa 30 min Control 200 MPa 30 min 300 MPa 30 min 400 MPa 30 min G// (Pa) 100 0.1 10.0 Angular frequency (rad s–1) G/ (Pa) 892 100 10 1 10 Angular frequency (rad 0.1 100 1.0 s–1) 10.0 Angular frequency (rad 100.0 s–1) Figure 4 Effect of pressure on (a) storage modulus of Chousa pulp, (b) loss modulus of Chousa pulp, (c) storage modulus of Alphanso pulp (d) loss modulus of Alphanso pulp. of retention of peroxidase and polyphenol oxidase enzymes. Effect of high pressure on glass transition of mango pulp The glass transition temperature of mango pulp was determined for both fresh and processed mango pulps. The Tg of both samples remained constant ()87.45 and )88.73
C and )85.34 and 84.74
C for fresh and processed) during pressure treatment. It is concluded that HP did not alter the sugar composition of mango pulp significantly. Conclusion Rheological properties of HP treated fresh mango pulp were more complex than those of heat processed pulp. The effect of pressure level was International Journal of Food Science and Technology 2005, 40, 885–895  2005 Institute of Food Science and Technology Trust Fund Pressure rheology and colour of mango pulp J. Ahmed et al. Table 4 Dynamic rheological parameters of (a) Chousa mango pulp and (b) Alphanso mango pulp at residence time of 30 min at 20
C Storage modulus Pressure (MPa) (a) 0.101 100 200 300 400 (b) 0.101 100 200 300 400 Loss modulus a0 a1 a2 a3 SE a0 a1 a2 397.0 410.5 424 394.3 358.9 138.8 141.2 144.5 129.2 117.3 33.98 29.25 27.11 36.44 29.51 )24.03 )5.47 )4.47 )9.32 )7.57 0.84 1.12 1.74 0.98 1.58 96.29 101.2 105 95.53 84.62 64.98 67.55 70.90 67.02 57.02 25.91 17.50 12.63 26.61 18.2 17.32 13.90 16.46 13.16 13.55 )3.52 )11.19 )6.98 )7.16 )9.88 0.89 0.60 0.76 0.84 0.61 52.23 43.08 50.05 48.40 46.28 40.51 34.61 40.04 39.11 38.49 22.33 19.06 21.53 22.66 20.86 192.7 160.7 181.7 178.2 156.6 66.10 52.22 58.84 56.93 57.78 a3 2.54 3.76 4.67 5.15 8.015 12.44 3.52 )1.13 2.89 0.63 SE 1.81 1.57 4.66 2.82 3.46 1.58 2.71 0.70 1.48 1.47 Table 5 Effect of pressure on tristimulus colour of (a) Chousa pulp and (b) Alphanso pulp Pressure (MPa) (a) 0.101 100 100 200 200 300 300 400 400 (b) 0.101 100 100 200 200 300 300 400 400 Duration (min) L – 15 30 15 30 15 30 15 30 54.23 49.11 49.69 51.01 49.38 51.62 51.61 51.28 51.33 – 15 30 15 30 15 30 15 30 51.05 51.19 51.46 51.81 50.60 51.07 51.02 51.51 51.55 a b a/b C H DE 8.49 7.37 7.44 7.76 8.01 7.69 8.22 8.18 8.37 33.28 28.95 27.55 29.92 33.10 32.33 33.14 32.75 32.55 0.255 0.255 0.270 0.260 0.242 0.238 0.248 0.250 0.257 34.34 29.87 28.53 30.92 34.06 33.24 34.15 33.76 33.61 75.7 75.70 74.89 75.43 76.38 76.61 75.12 76.70 75.24 6.80 7.39 4.70 4.88 2.89 2.64 3.01 2.99 14.32 14.12 14.20 13.98 14.16 14.35 13.95 14.13 14.28 39.69 36.16 36.27 36.00 36.67 35.93 35.13 34.97 34.87 0.361 0.390 0.392 0.388 0.386 0.399 0.397 0.404 0.410 42.19 38.82 38.95 38.63 39.30 38.69 37.80 37.71 37.68 70.15 68.89 68.62 68.78 69.58 65.55 65.94 66.32 66.08 3.58 3.44 3.77 3.06 3.76 4.57 4.75 4.84 more significant than that of the treatment time on the rheology and colour of fresh mango puree. Moderate pressures (100–200 MPa) increased the rheological parameters of fresh pulp while a decreasing trend was found at higher pressure levels (300–400 MPa). A steady decrease was noticed with heat treated mango pulp. Colour was retained in mango pulp treated by HP and this was found to be the best example of an unchanged  2005 Institute of Food Science and Technology Trust Fund quality attribute during HP processing. More studies are necessary on the inactivation of enzymes in the pulp, namely polyphenol oxidase and peroxidase. A microbiological study would reveal the safety of the process and the suitability of the process for the industry. Studies using scanning electron microscopy could provide data on the exact textural variation during HP treatment. International Journal of Food Science and Technology 2005, 40, 885–895 893 894 Pressure rheology and colour of mango pulp J. Ahmed et al. References Ahmed, J. & Ramaswamy, H.S. (2003). 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