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Rheological properties of enzyme-treated mango pulp
Jountcrl
0 PII:
of Food
199X Elsevier
SO260-8774(98)00067-3
36 ( 1YYX) 24Y- 262 Limited. All rights reserved Printcd in Great Britain 0260-8774/9X $lYl)O + MO
Enginwring
Science
ELSEVIER
Rheological Properties of Enzyme-treated Mango Pulp Suvendu Bhattacharya” Food Engineering
Department,
Central
& N. K. Rastogi
Food Technological
Research
Institute,
Mysore 570 013, India (Accepted 5 April 19%)
ABSTRACT Mango pulp was treated with pectinase enzyme at different times (30-150 min) and temperatures (2540°C) of treatment, and at various concentrations of enzyme (0~005-0~095~0). The effect of these enzyme treatment conditions on the rheological parameters (yield stress, flow behaviour index and consistency index) and the apparent viscosity, were determined employing a second order central composite rotatable design in combination with response surjace methodology. The raw, as well as the enzyme-treated, mango pulp behaved as pseudoplastic liquid and possessed yield stress. The Herschel-Bulkley model fits well (r 2 0.991, p I 0.01) the shear rate and shear stress data. An increase in time and/or concentration of enzyme treatment is associated with an increase in the flow behaviour index. Apparent viscosity markedly decreased with an elevation in the concentration of the enzyme. 0 I998 Elsevier Science Limited. All rights reserved NOMENCLATURE
k n P x
X
Constant term in the second order polynomial Linear effects of the variables xl, x2 and x3, respectively Quadratic effects of the variables xl, _r2and x3, respectively Interaction effects of the variables in the polynomial Number of variables Consistency index (Pas”) Flow behaviour index Probability level Correlation coefficient Variable in coded level Actual level of variable
*To whom correspondence
should be addressed. 249
2.50
S. Bhattacharya, N. K. Rastogi
i Cla r7 a0 q e
Shear rate (s-l) Casson plastic viscosity (Pas) Shear stress (Pa) Yield stress (Pa) Apparent viscosity (mPas) Random error term in the polynomial
Subscript C HB P
Casson model Herschel-Bulkley Power law model
model
INTRODUCTION Rheological behaviour of liquid and semi-solid foods is important for the design of processing equipment (Saravacos, 1970), and for coating, process control, quality control and product development (Ofoli, 1990). Among the liquid foods, fruit products such as pulp/juices, concentrates, serum and clarified/filtered juices are of commercial importance. A number of studies have been reported to characterize such products including products from apple (Qiu & Rao, 1988), orange (Mizrahi & Berk, 1970), banana (Khalil et al., 1989), and pear (Ibraz et al., 1989). On the other hand, the tropical fruit mango, and its products including mango pulp have not been studied in detail. Rao et al. (1974, 1985), Gunjal and Waghmare (1987) and Manohar et al. (1990) have studied the flow properties of mango pulp at different temperatures and solid concentrations. A marked reduction in the apparent viscosity of mango pulp has been reported due to enzymatic treatment (Sreenath et al., 1987; Manohar et al., 1990; Garcia et al., 1974). The conclusions drawn from these studies indicated that mango pulp is pseudoplastic in nature without a yield stress though the rheograms cited indicate its presence. The other finding is that enzyme treatment makes the juice a Newtonian liquid. Depectinized apple and grape juices, filtered orange juices, and depectinized-clarified-filtered banana juices were reported to be Newtonian fluids (Saravacos, 1970; Khalil et al., 1989). It is necessary to mention here that the conditions of the enzymatic treatment affect the rheology of the product (Sreenath et al., 1995). Mango pulp is mainly used for juice based products, such as, nectar, jam, jelly powder, fruit bar and flakes. Some of these products possess a strong mango flavour and taste which is considered to be an added advantage, whereas, the pulpy and highly viscous nature of mango is undesirable from the point of unit operations and consumer acceptability. The quality of such products can be improved by treating mango pulp with cell wall degrading enzyme (such as pectinase) which reduces the viscosity and can increase the yield of the juice (Sreenath et al., 1987, 1995). Besides, the mango pulp, after enzymatic liquefaction, is subjected to microfiltration (MF) and the permeate/retentate thus obtained can be concentrated by a reverse osmosis (RO) technique without phase change whereby the important constituents and quality are retained. Later, the juice is either concentrated by drying or can be added directly to MF retentate to obtain aromatic mango concentrate. The enzymatic liquefaction was reported to be influenced by a number of variables including concentration of the enzyme, time of reaction with pulp, and the temperature of the treatment (Sreenath et al., 1987, 1995). It is felt that a detailed
Rheology of enzyme-treated mango pulp
251
investigation on the effect of the treatment conditions will provide an insight about the rheological status of the product due to enzymatic action. The results will be useful for the design of processing equipment. The objective of the present research is to study the effect of enzyme (pectinase) treatment (at different time, temperature and enzyme concentration) on the rheological behaviour of mango pulp using response surface methodology (RSM). MATERIALS
AND METHODS
Materials Canned mango pulp (variety 7’otupuri) was obtained from Globe Foods, Mysore, India that has a total of 18.7% solids (16” Brix). The liquid pectinase enzyme (Novo Pectinex 3XL) was procured from Novo Nordisk Ferment Ltd, Dittingen, Switzerland. The activity of pectinase enzyme is 5000 international unit (IU) per millilitre (ml). Methods Enzyme treatment
For each experiment, about 500 g of mango pulp was subjected to different enzyme treatment conditions as cited in Table 1. Based on preliminary experiments, the range of the variables for enzymatic treatment conditions were selected. These were the time, X, (30-150 min) and temperature, X, (25-60°C) of enzymatic treatment, and concentration of enzyme used, X, (0*005-0.095%). The temperature of enzyme treatment was adjusted to the desired level with the help of a constant temperature (kO.5”C) water bath. The pW of the pulp suspensions was maintained at 3.7. At the end of the enzyme treatment, the enzyme in the sample was inactivated by heating the suspension at 90°C for 5 min in a water bath. Rheological measurement
A rheometer (Model # RTlO, Haake GmbH, Karlsruhe, Germany) with a coaxial cylinder attachment (ratio of the external diameter of the rotating bob to the internal diameter of stationary cylinder equals 0.954) was used to determine the rheological behaviour of mango pulp (raw and treated) at a constant temperature of 25 +O.l”C up to a shear-rate of 1000 s-’ employing a controlled shear-rate (CR) measurement system. Thirty shear-rate/shear-stress data points were generated within the experimentation time of 300 s. All experiments were performed on duplicate samples. The shear-rate and shear-stress data were fitted to some of the common rheological models, such as power law [eqn (l)], Herschel-Bulkley [eqn (2)J and Casson [eqn (3)] models. The extent of fitting to a model was judged by finding the correlation coefficient (Y), and checking its statistical significance at a probability level ofp = O-01. The flow behaviour index and consistency index (for power law and Herschel-Bulkley models) were estimated by employing the technique of regression analysis (Snedecor & Cochran, 1968). For the Herschel-Bulkley (HB) model, the experimental yield stress values were used. g = k,>(f)“”
(1)
252
S. Bhattachalya, 0 = O,,H,( +
-
i-
N. K. Rastogi
(2) (3)
kHB(j)-
\‘o=I’~‘r,c+pr*cdy
r
The yield stress of the suspensions was determined experimentally using the stress relaxation technique at a shear-rate of 3 s-r (Keentok, 1982; Vitali & Rao, 1984; Bhattacharya & Bhattacharya, 1994, 1996). The experimental yield stress values were reported as a range rather than a single value. Yield stress was also calculated from the intercept using Casson model (from the plot of square root of 9 versus ii). The apparent viscosity of the suspensions at a shear-rate of 100 s-’ was determined from the experimental values of shear-stress. Experimental design and statistical analysis In the present investigation, a 3-variable (i = 3, 5 level of each variable) second order central composite rotatable experimental design (Myers, 1971; Khuri & Cornell, 1989) was employed (Table 1). The independent variables were the time (x1) and temperature (x2) of enzyme treatment, and concentration of enzyme used (x3). TABLE 1 The Central Composite Rotatable Experimental Design (in Coded Level of Three Variables) Employed for Treating Mango Pulp with Pectinase Enzyme Time (min) Serial number 1
XI (XI)
:32 14 15
54 126 54 126 54 126 54 126 30 150 90 90 90 90 90
:; 18 19 20
;: 90 90 90
32 4 5 6 ; 9 10 11
(-1) (1) (-1) (1) (-1) (1) (-1) (1) (- 1.682) (1.682) (0) (0) (0) (0) (0) I!{ (0) (0) (0)
Temp (“C) x2
62)
32 (-1) 32 (-1) 53 (1) 53 (1) 32 (-1) 32 (-1) 53 (1) 53 (1) 42.5 (0) 42.5 (0) 25 ( - 1.682) 60 (1.682) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0)
x represent the coded level of variables. X represent the actual level of variables. Figures in parentheses denote coded level of variables. *Enzyme activity = 5000 IU ml-‘.
Enzyme concentration * (%) x.3 (X.3)
0.023 (-1) 0.023 ( - 1) 0.023 (-1) 0.023 ( - 1) 0.076 (1) 0.076 (1) 0,076 (1) 0.076 (1) 0.05 (0) 0.05 (0) 0.05 (0) 0.05 (0) 0.005 (- 1.682) 0.095 (1.682) 0.05 (0) II:::: iF(] 0.05 (0) 0.05 (0) 0.05 (0)
Rheology of enzyme-treated mango pulp
253
The experimental design in the coded (x) and actual (x) levels of variables is shown in Table 1. The experimental design included star points, and five centre points (O,O,O). The response functions (y) were the rheological parameters (yield stress, flow behaviour index and consistency index) and the apparent viscosity (at a shear rate of 100 s-l). These values were related to the coded variables (xi, i = 1,2,3) by a second degree polynomial [eqn (4)] using the method of least squares (Snedecor & Cochran, 1968). y=h,,+h,x,
+b*X2+b3X2+h,,.X:+b22X~+bj~X:+b,*X,X2+b,3X,X+623X2Xj+I:...
(4)
The coefficients of the polynomial were represented by b,, (constant term), h,, h2 and b, (linear effects), b, ,, bZ2 and & (quadratic effects), and bj2, h ,3 and hz (interaction effects) and c (random error). The analysis of variance (ANOVA) tables were generated and the effect of individual linear, quadratic and interaction term has been determined (Khuri & Cornell, 1989). The significance of all the terms in the polynomial were judged statistically by computing the F-value; the significance of the F-values were judged at a probability (p) of 0.01, 0.05 or 0.25. Terms that were not significant (p 10.25) were deleted one at a time (stepwise deletion) and the polynomial was recalculated. The final polynomial, after deletion of all the non-significant terms, was reported together with the recalculated correlation coefficients (r) values. The r values were determined to know the extent of the fitting to the polynomial, and was judged at a probability level of 0.01. The level of the variables (x,, x2 and x9) (within the experimental range) to obtain a minimum apparent viscosity (ye) was determined by employing statistical canonical analysis (Khuri & Cornell, 1989; Myers, 1971). In brief, the analysis consists of the translation of the response function from the origin to the stationary point x,,. Then the response function is expressed in terms of new variables, the axes of which correspond to the principal axes of the contour system.
RESULTS
AND DISCUSSION
Rheology of mango pulp The rheogram (Fig. 1) of the raw pulp shows that it is a pseudoplastic liquid with yield stress; the latter, as determined by stress relaxation method, is in the range 3.5-3.9 Pa. The flow behaviour index and consistency index, determined employing the Herschel-Bulkley (HB) model, are 0.38 and 6.55 Pas”, respectively. The higher r value of 0.998, obtained with the HB model compared to power law and Casson models (the r values being 0.976 and 0.957, respectively) shows the suitability of the former model to correlate the shear-rate and shear-stress data for mango pulp. A similar trend was also observed with enzyme-treated pulp, and therefore, only the HB model was employed to compute the different rheological parameters of the treated pulp. The magnitude of yield stress of the untreated pulp, calculated (12.4 Pa) using Casson model (Table l), was markedly higher than that obtained through experiment (3.5-3.9 Pa). Similar results were also reported for tomato ketchup, mustard paste and apricot puree by Ofoli et al. (1987), cooked cornflour 1994, 1996), and for concentrated suspensions (Bhattacharya & Bhattacharya, orange juices (Vitali & Rao, 1984). Thus, it is suggested that the yield stress for
S. Bhattacharya, N. K. Rastogi
254
mango pulp should be determined Casson model.
from experiments
rather
than employing
the
Enzyme-treated pulp The range of the different rheological parameters (yield stress, flow behaviour index and consistency index) and the apparent viscosity (at a shear rate of 100 SK’) of the enzyme-treated pulp is shown (Table 2) along with the values for untreated pulp. Enzyme treatment markedly reduces the consistency index and apparent viscosity. Further, the increase in flow behaviour index due to enzymatic treatment indicates that the pulp shifts towards a Newtonian liquid. It is worth mentioning here that the pulp, even after the enzymatic treatment in the experimental range, maintains its pseudoplastic nature (a sample plot for enzyme-treated pulp is shown in Fig. 1). A possible reason for this can be attributed to the insoluble and suspended (crude fibre) matters and starch present in mango pulp that could not be degraded by pectinase. Effect of time, temperature and concentration The effect of the different enzyme treatment conditions (in coded level of variables) on the rheological properties (yield stress, flow behaviour index, consistency index
ooQ RAW
IO0
0
0000
ENZYME
800
400 8HEAR
Fig. 1.
OOOO
oo” PULP
ooooooo
TREATED
1200
PULP
1600
RATE (s-‘1
Rheogram of the raw and enzyme-treated mango pulp. Condition for enzyme-treated sample: Temperature: 425”C, time: 90 min and enzyme concentration: 0.05%.
Rheology of enzyme-treated mango pulp
255
TABLE 2
Parameters (Yield Stress, Flow Behaviour Index and Consistency Index) and the Apparent Viscosity (at a Shear Rate of 100 s-~‘) of Untreated and the Enzyme-treated (at Various Conditions) Pulp -
The Ranges for the Different
Rheological
Parameter
Unit
Untreated mango pulp
Yield stress Flow behaviour index Consistency index Apparent viscosity
Pa Pas” mPas
3.5-3.9 0.383 6.55 419.3
Enzyme-treated mango pulp 2.9-49 0.379-0.490 1.05-3.47 124.6-229.8
and apparent viscosity) are reported (Table 3) by the coefficients of the second order polynomials. To aid visualization, the response surfaces for these rheological properties are shown in Figs 2-5, respectively. The yield stress mostly depends on the temperature of enzyme treatment as its linear (p 10.25) as well as quadratic (p 10.01) effects are significant; hence, the overall effect is curvilinear in nature (Fig. 2). The other factors that also contribute to the yield stress include linear and quadratic effects of time, and its interaction with temperature (Table 3). The flow behaviour index (n), a criteria for judging the extent of non-Newtonian behaviour, was negatively related to linear effect of temperature @
TABLE 3
0.069 0.798
0*101*
0.101 0.003 0.098 0.249 0~001 0.256 0.069 0.783
O-256**
0.249***
O-098*
3.626 @161*
Recalculated equation
3.625 0.161
Original equation
Yield stress (Pa)
0.005 0.902
- 0.026 - 0.003 0.010 0.010 0.012 O-014 - 0.010
0.415 0.002
Original equation
0.891
0*010** 0.010** o-012** 0.014* * -0*010*
-0.026***
0.415
Recalculated equation
Flow behaviour index
-0.180* 0.839
O-083 - 0.083 0.857
0.205 * * -0.431*** -0*104* -O-189*
2.148
Recalculated equation
0.205 -0.431 -0.106 -0.191 -0.019 -0.180
2.164 0.065
Original equation
Consistency index (Pas”)
Index, Consistency
-6.075* 0.864
- 4.075 2.575 0.878
-24+396*** 6.840*
7.307*
173.679
Recalcluated equation
7.307 2.187 - 24.396 6.704 0.518 - 2.027 - 6.075
174.802
Ongina equation
Apparent viscosity at a shear-rate of 100 SK’ (mPas)
of the Second Order Polynomial for the Response Functions (Yield Stress, Flow Behaviour Index and Apparent Viscosity) in Coded Level of Variables
*Significant at p I 0.25; **significant at p IO.05; ***significant at p IO.01.
b 13 b 23 r***
2:
b 22
k
Br bz
CoefJicient of the regression equation
The Coefficients
00 -.
.2 ?T k $
z
$
9 g Ei s
Rheology of enzyme-treated mango pulp
257
Fig. 2. Response surface for yield stress of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 425°C) and (b) temperature and enzyme concentration (at a time of 90 min).
258
S. Bhattacharya, N. K. Rastogi
Fig. 3. Response surface for flow behaviour index of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 425°C) and (b) temperature and enzyme concentration (at a time of 90 min).
Rheology of enzyme-treated mango pulp
259
(4
Fig. 4. Response surface for consistency index of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 425°C) and (b) temperature and enzyme concentration (at a time of 90 min).
260
S. Bhattdmyu,
A? K. Rastogi
Fig. 5. Response surface for apparent viscosity (at a shear rate of 100 s-l) of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 42*5”C) and (b) temperature and enzyme concentration (at a time of 90 min).
Rheology of enzyme-treated mango pulp
261
Mango pulp is a pseudoplastic liquid due to discontinuous phase substances, such as, starch and fibrous materials (pectin and other fibres) when water acts as a continuous phase. The rheology of mango pulp not only depends on these suspended matters but also on the soluble solids (sugars, organic acids, etc). Further, most of the water available in the untreated pulp is bound to these substances rendering it to be a highly viscous sample. The pectinaceous substances possess a high water holding capacity (WHC) and develop a cohesive network structure. The degradation of pectinaceous material into low molecular weight compounds is a function of enzymatic treatment conditions (temperature, time and concentration) which in turn decides the rheological characteristics (flow behaviour index, consistency index, yield stress and apparent viscosity) of treated pulp. The interesting finding is that the treated pulp never becomes a Newtonian liquid. Ripe mango pulp contains about 0.6% protein, 1.1% fibre (Aykroyd, 1963) (of which 0.7% has been estimated to be in the form of calcium pectate) and considerable amount (2-4%) of starch (Pantastico et al., 1984). The action of pectinase cannot change the non-pectinaceous fibres (about 0.4%) and protein and starch; hence, the pulp remains in its non-Newtonian status. Degradation of pectin leads to a reduction of WHC, and therefore, free waters are released to the system to further reduce the apparent viscosity. The starch present in the pulp remains unaltered during enzymatic treatment and possibly responsible for the fairly constant yield stress. It is necessary to mention here that starch, even at a low concentration in water, is capable of exhibiting non-Newtonian character and yield stress.
CONCLUSIONS Mango pulp as well as enzyme-treated samples behaved as non-Newtonian liquids and exhibited yield stress. The different conditions (time and temperature of enzyme treatment, and concentration of the enzyme) for enzyme treatment revealed that all these variables markedly affect the different rheological parameters (flow behaviour index, consistency index and yield stress) and the apparent viscosity of the pulp suspensions; these can be related to the enzyme treatment conditions by second order polynomials. Apparent viscosity markedly decreased with an elevation in the concentration of the enzyme. An increase in time and/or concentration of enzyme treatment is associated with a decrease in pseudoplastic behaviour of pulp. Among the common rheological models, the Herschel-Bulkley model fits well (rr 0.991, p 10.01) the shear-rate and shear-stress data for untreated and treated mango pulp.
REFERENCES Aykroyd, W. R. (1963). The Nutritive Value of Indian Foods and the Planning of Satisfactory Diets. Special Report Series No 42, Indian Council of Medical Research, New Delhi, India. Bhattacharya, S. & Bhattacharya, S. (1994). Flow behaviour of cooked maize flour suspensions and applicability of mathematical models. Journal of Food Process Engineering, 17, 263-278. Bhattacharya, S. & Bhattacharya, S. (1996). Rheology of cooked debranned maize flour suspensions. Journal of Food Engineering, 27, 97- 105.
262
S. Bhattacharya, h! K. Rastogi
Garcia, R., Rivera, J. & Rolz, C. (1974). Rheological properties of some tropical fruit products and their enzymic clarification. Proceedings of the 4th International Congress on Food Science and Technology, 2, 18-26. Gunjal, B. B. & Waghmare, N. J. (1987). Flow properties of pulp, juice and nectar of ‘Baneshan’ and ‘Neelum’ mangoes. Journal of Food Science and Technology, 2420-23. Ibraz, A., Pagan, J., Gutierrez, J. & Vicente, M. (1989). Rheological properties of clarified pear juice concentrate. Journal of Food Engineering, 10,57-63. Keentok, M. (1982). The measurement of the yield stress of liquids. Rheologica Acta, 21, 325-332. Khalil, K. E., Ramakrishana, P., Nanjundaswamy, A. M. & Pahvardhan, M. V. (1989). Rheological behaviour of clarified banana juice: effect of temperature and concentration. Journal of Food Engineering, 10,231-240. Khuri, A. I. & Cornell, J. A. (1989) Response Surfaces: Designs and Analyses. Marcel Dekker, Inc., New York. Manohar, B., Ramakrishna, P. & Ramteka, R. S. (1990). Effect of pectin content on flow properties of mango pulp concentrates. Journal of Texture Studies, 21, 179-190. Mizrahi, S. & Berk, Z. (1970). Flow behaviour of concentrated orange juice. Journal of Terture Studies, 1, 342-355. Myers, R. H. (1971). Response S&ace Methodology. Allyn and Bacon, Inc., Boston. Ofoli, R. Y. (1990). Interrelations of rheology, kinetics, and transport phenomena in food processing. In Dough Rheology and Baked Product Tature , eds H. Faridi & J. M. Faubion, pp. 497-512. Van Nostrand Reinhold, New York. Ofoli, R. Y., Morgan, R. G. & Steffe, J. F. (1987). A generalized rheological model for inelastic fluid foods. Journal of Texture Studies, 18,213-230. Pantastico, E. B., Lam, P. F., Kesta, S., Yuniarti & Kosittrakul, M. K. (1984). Post harvest physiology and storage of mango. In Mango, eds D. B. Mendosa & R. B. H. Wills, pp. 39-52. ASEAN Book Handling Bureau, Kuala Lumpur, Malaysia. Qiu, C. G. & Rao, M. A. (1988). Role of pulp content and particle size in yield stress of apple sauce. Journal of Food Science, 53,1165-1170. Rao, M. A., Palomino, L. N. 0. & Bernnardt, L. W. (1974). Flow properties of tropical fruit purees. Journal of Food Science, 39, 160-161. Rao, K. L., Eipeson, W. E., Rao, P. N. S., Patwardhan, M. V. & Ramanathan, P. K. (1985). Rheological properties of mango pulp and concentrates. Journal of Food Science and Technology, 22, 30-33. Saravacos, G. D. (1970). Effect of temperature on viscosity of fruit juices and purees. Journal of Food Science, 35, 122-125. Snedecor, G. W. & Cochran, W. G. (1968). Statistical Methods, 6th edn. Oxford and IBH Publ. Co., Calcutta, India. Sreenath, H. K., Nanjundaswamy, A. M. & Sreekantiah, K. R. (1987). Effect of various cellulases and pectinases on viscosity reduction of mango pulp. Journal of Food Science, 52, 230-231. Sreenath, H. K., Sudarshana Krishna, K. R. & Santhanam, K. (1995). Enzymatic liquefaction of some varieties of mango pulp. Lebensmittal-Wissenschafi-und-Technologie, 28, 196-200. Vitali, A. A. & Rao, M. A. (1984). Flow properties of low-pulp concentrated orange juice: effect of temperature and concentration. Journal of Food Science, 49, 882-888.
0 PII:
of Food
199X Elsevier
SO260-8774(98)00067-3
36 ( 1YYX) 24Y- 262 Limited. All rights reserved Printcd in Great Britain 0260-8774/9X $lYl)O + MO
Enginwring
Science
ELSEVIER
Rheological Properties of Enzyme-treated Mango Pulp Suvendu Bhattacharya” Food Engineering
Department,
Central
& N. K. Rastogi
Food Technological
Research
Institute,
Mysore 570 013, India (Accepted 5 April 19%)
ABSTRACT Mango pulp was treated with pectinase enzyme at different times (30-150 min) and temperatures (2540°C) of treatment, and at various concentrations of enzyme (0~005-0~095~0). The effect of these enzyme treatment conditions on the rheological parameters (yield stress, flow behaviour index and consistency index) and the apparent viscosity, were determined employing a second order central composite rotatable design in combination with response surjace methodology. The raw, as well as the enzyme-treated, mango pulp behaved as pseudoplastic liquid and possessed yield stress. The Herschel-Bulkley model fits well (r 2 0.991, p I 0.01) the shear rate and shear stress data. An increase in time and/or concentration of enzyme treatment is associated with an increase in the flow behaviour index. Apparent viscosity markedly decreased with an elevation in the concentration of the enzyme. 0 I998 Elsevier Science Limited. All rights reserved NOMENCLATURE
k n P x
X
Constant term in the second order polynomial Linear effects of the variables xl, x2 and x3, respectively Quadratic effects of the variables xl, _r2and x3, respectively Interaction effects of the variables in the polynomial Number of variables Consistency index (Pas”) Flow behaviour index Probability level Correlation coefficient Variable in coded level Actual level of variable
*To whom correspondence
should be addressed. 249
2.50
S. Bhattacharya, N. K. Rastogi
i Cla r7 a0 q e
Shear rate (s-l) Casson plastic viscosity (Pas) Shear stress (Pa) Yield stress (Pa) Apparent viscosity (mPas) Random error term in the polynomial
Subscript C HB P
Casson model Herschel-Bulkley Power law model
model
INTRODUCTION Rheological behaviour of liquid and semi-solid foods is important for the design of processing equipment (Saravacos, 1970), and for coating, process control, quality control and product development (Ofoli, 1990). Among the liquid foods, fruit products such as pulp/juices, concentrates, serum and clarified/filtered juices are of commercial importance. A number of studies have been reported to characterize such products including products from apple (Qiu & Rao, 1988), orange (Mizrahi & Berk, 1970), banana (Khalil et al., 1989), and pear (Ibraz et al., 1989). On the other hand, the tropical fruit mango, and its products including mango pulp have not been studied in detail. Rao et al. (1974, 1985), Gunjal and Waghmare (1987) and Manohar et al. (1990) have studied the flow properties of mango pulp at different temperatures and solid concentrations. A marked reduction in the apparent viscosity of mango pulp has been reported due to enzymatic treatment (Sreenath et al., 1987; Manohar et al., 1990; Garcia et al., 1974). The conclusions drawn from these studies indicated that mango pulp is pseudoplastic in nature without a yield stress though the rheograms cited indicate its presence. The other finding is that enzyme treatment makes the juice a Newtonian liquid. Depectinized apple and grape juices, filtered orange juices, and depectinized-clarified-filtered banana juices were reported to be Newtonian fluids (Saravacos, 1970; Khalil et al., 1989). It is necessary to mention here that the conditions of the enzymatic treatment affect the rheology of the product (Sreenath et al., 1995). Mango pulp is mainly used for juice based products, such as, nectar, jam, jelly powder, fruit bar and flakes. Some of these products possess a strong mango flavour and taste which is considered to be an added advantage, whereas, the pulpy and highly viscous nature of mango is undesirable from the point of unit operations and consumer acceptability. The quality of such products can be improved by treating mango pulp with cell wall degrading enzyme (such as pectinase) which reduces the viscosity and can increase the yield of the juice (Sreenath et al., 1987, 1995). Besides, the mango pulp, after enzymatic liquefaction, is subjected to microfiltration (MF) and the permeate/retentate thus obtained can be concentrated by a reverse osmosis (RO) technique without phase change whereby the important constituents and quality are retained. Later, the juice is either concentrated by drying or can be added directly to MF retentate to obtain aromatic mango concentrate. The enzymatic liquefaction was reported to be influenced by a number of variables including concentration of the enzyme, time of reaction with pulp, and the temperature of the treatment (Sreenath et al., 1987, 1995). It is felt that a detailed
Rheology of enzyme-treated mango pulp
251
investigation on the effect of the treatment conditions will provide an insight about the rheological status of the product due to enzymatic action. The results will be useful for the design of processing equipment. The objective of the present research is to study the effect of enzyme (pectinase) treatment (at different time, temperature and enzyme concentration) on the rheological behaviour of mango pulp using response surface methodology (RSM). MATERIALS
AND METHODS
Materials Canned mango pulp (variety 7’otupuri) was obtained from Globe Foods, Mysore, India that has a total of 18.7% solids (16” Brix). The liquid pectinase enzyme (Novo Pectinex 3XL) was procured from Novo Nordisk Ferment Ltd, Dittingen, Switzerland. The activity of pectinase enzyme is 5000 international unit (IU) per millilitre (ml). Methods Enzyme treatment
For each experiment, about 500 g of mango pulp was subjected to different enzyme treatment conditions as cited in Table 1. Based on preliminary experiments, the range of the variables for enzymatic treatment conditions were selected. These were the time, X, (30-150 min) and temperature, X, (25-60°C) of enzymatic treatment, and concentration of enzyme used, X, (0*005-0.095%). The temperature of enzyme treatment was adjusted to the desired level with the help of a constant temperature (kO.5”C) water bath. The pW of the pulp suspensions was maintained at 3.7. At the end of the enzyme treatment, the enzyme in the sample was inactivated by heating the suspension at 90°C for 5 min in a water bath. Rheological measurement
A rheometer (Model # RTlO, Haake GmbH, Karlsruhe, Germany) with a coaxial cylinder attachment (ratio of the external diameter of the rotating bob to the internal diameter of stationary cylinder equals 0.954) was used to determine the rheological behaviour of mango pulp (raw and treated) at a constant temperature of 25 +O.l”C up to a shear-rate of 1000 s-’ employing a controlled shear-rate (CR) measurement system. Thirty shear-rate/shear-stress data points were generated within the experimentation time of 300 s. All experiments were performed on duplicate samples. The shear-rate and shear-stress data were fitted to some of the common rheological models, such as power law [eqn (l)], Herschel-Bulkley [eqn (2)J and Casson [eqn (3)] models. The extent of fitting to a model was judged by finding the correlation coefficient (Y), and checking its statistical significance at a probability level ofp = O-01. The flow behaviour index and consistency index (for power law and Herschel-Bulkley models) were estimated by employing the technique of regression analysis (Snedecor & Cochran, 1968). For the Herschel-Bulkley (HB) model, the experimental yield stress values were used. g = k,>(f)“”
(1)
252
S. Bhattachalya, 0 = O,,H,( +
-
i-
N. K. Rastogi
(2) (3)
kHB(j)-
\‘o=I’~‘r,c+pr*cdy
r
The yield stress of the suspensions was determined experimentally using the stress relaxation technique at a shear-rate of 3 s-r (Keentok, 1982; Vitali & Rao, 1984; Bhattacharya & Bhattacharya, 1994, 1996). The experimental yield stress values were reported as a range rather than a single value. Yield stress was also calculated from the intercept using Casson model (from the plot of square root of 9 versus ii). The apparent viscosity of the suspensions at a shear-rate of 100 s-’ was determined from the experimental values of shear-stress. Experimental design and statistical analysis In the present investigation, a 3-variable (i = 3, 5 level of each variable) second order central composite rotatable experimental design (Myers, 1971; Khuri & Cornell, 1989) was employed (Table 1). The independent variables were the time (x1) and temperature (x2) of enzyme treatment, and concentration of enzyme used (x3). TABLE 1 The Central Composite Rotatable Experimental Design (in Coded Level of Three Variables) Employed for Treating Mango Pulp with Pectinase Enzyme Time (min) Serial number 1
XI (XI)
:32 14 15
54 126 54 126 54 126 54 126 30 150 90 90 90 90 90
:; 18 19 20
;: 90 90 90
32 4 5 6 ; 9 10 11
(-1) (1) (-1) (1) (-1) (1) (-1) (1) (- 1.682) (1.682) (0) (0) (0) (0) (0) I!{ (0) (0) (0)
Temp (“C) x2
62)
32 (-1) 32 (-1) 53 (1) 53 (1) 32 (-1) 32 (-1) 53 (1) 53 (1) 42.5 (0) 42.5 (0) 25 ( - 1.682) 60 (1.682) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0) 42.5 (0)
x represent the coded level of variables. X represent the actual level of variables. Figures in parentheses denote coded level of variables. *Enzyme activity = 5000 IU ml-‘.
Enzyme concentration * (%) x.3 (X.3)
0.023 (-1) 0.023 ( - 1) 0.023 (-1) 0.023 ( - 1) 0.076 (1) 0.076 (1) 0,076 (1) 0.076 (1) 0.05 (0) 0.05 (0) 0.05 (0) 0.05 (0) 0.005 (- 1.682) 0.095 (1.682) 0.05 (0) II:::: iF(] 0.05 (0) 0.05 (0) 0.05 (0)
Rheology of enzyme-treated mango pulp
253
The experimental design in the coded (x) and actual (x) levels of variables is shown in Table 1. The experimental design included star points, and five centre points (O,O,O). The response functions (y) were the rheological parameters (yield stress, flow behaviour index and consistency index) and the apparent viscosity (at a shear rate of 100 s-l). These values were related to the coded variables (xi, i = 1,2,3) by a second degree polynomial [eqn (4)] using the method of least squares (Snedecor & Cochran, 1968). y=h,,+h,x,
+b*X2+b3X2+h,,.X:+b22X~+bj~X:+b,*X,X2+b,3X,X+623X2Xj+I:...
(4)
The coefficients of the polynomial were represented by b,, (constant term), h,, h2 and b, (linear effects), b, ,, bZ2 and & (quadratic effects), and bj2, h ,3 and hz (interaction effects) and c (random error). The analysis of variance (ANOVA) tables were generated and the effect of individual linear, quadratic and interaction term has been determined (Khuri & Cornell, 1989). The significance of all the terms in the polynomial were judged statistically by computing the F-value; the significance of the F-values were judged at a probability (p) of 0.01, 0.05 or 0.25. Terms that were not significant (p 10.25) were deleted one at a time (stepwise deletion) and the polynomial was recalculated. The final polynomial, after deletion of all the non-significant terms, was reported together with the recalculated correlation coefficients (r) values. The r values were determined to know the extent of the fitting to the polynomial, and was judged at a probability level of 0.01. The level of the variables (x,, x2 and x9) (within the experimental range) to obtain a minimum apparent viscosity (ye) was determined by employing statistical canonical analysis (Khuri & Cornell, 1989; Myers, 1971). In brief, the analysis consists of the translation of the response function from the origin to the stationary point x,,. Then the response function is expressed in terms of new variables, the axes of which correspond to the principal axes of the contour system.
RESULTS
AND DISCUSSION
Rheology of mango pulp The rheogram (Fig. 1) of the raw pulp shows that it is a pseudoplastic liquid with yield stress; the latter, as determined by stress relaxation method, is in the range 3.5-3.9 Pa. The flow behaviour index and consistency index, determined employing the Herschel-Bulkley (HB) model, are 0.38 and 6.55 Pas”, respectively. The higher r value of 0.998, obtained with the HB model compared to power law and Casson models (the r values being 0.976 and 0.957, respectively) shows the suitability of the former model to correlate the shear-rate and shear-stress data for mango pulp. A similar trend was also observed with enzyme-treated pulp, and therefore, only the HB model was employed to compute the different rheological parameters of the treated pulp. The magnitude of yield stress of the untreated pulp, calculated (12.4 Pa) using Casson model (Table l), was markedly higher than that obtained through experiment (3.5-3.9 Pa). Similar results were also reported for tomato ketchup, mustard paste and apricot puree by Ofoli et al. (1987), cooked cornflour 1994, 1996), and for concentrated suspensions (Bhattacharya & Bhattacharya, orange juices (Vitali & Rao, 1984). Thus, it is suggested that the yield stress for
S. Bhattacharya, N. K. Rastogi
254
mango pulp should be determined Casson model.
from experiments
rather
than employing
the
Enzyme-treated pulp The range of the different rheological parameters (yield stress, flow behaviour index and consistency index) and the apparent viscosity (at a shear rate of 100 SK’) of the enzyme-treated pulp is shown (Table 2) along with the values for untreated pulp. Enzyme treatment markedly reduces the consistency index and apparent viscosity. Further, the increase in flow behaviour index due to enzymatic treatment indicates that the pulp shifts towards a Newtonian liquid. It is worth mentioning here that the pulp, even after the enzymatic treatment in the experimental range, maintains its pseudoplastic nature (a sample plot for enzyme-treated pulp is shown in Fig. 1). A possible reason for this can be attributed to the insoluble and suspended (crude fibre) matters and starch present in mango pulp that could not be degraded by pectinase. Effect of time, temperature and concentration The effect of the different enzyme treatment conditions (in coded level of variables) on the rheological properties (yield stress, flow behaviour index, consistency index
ooQ RAW
IO0
0
0000
ENZYME
800
400 8HEAR
Fig. 1.
OOOO
oo” PULP
ooooooo
TREATED
1200
PULP
1600
RATE (s-‘1
Rheogram of the raw and enzyme-treated mango pulp. Condition for enzyme-treated sample: Temperature: 425”C, time: 90 min and enzyme concentration: 0.05%.
Rheology of enzyme-treated mango pulp
255
TABLE 2
Parameters (Yield Stress, Flow Behaviour Index and Consistency Index) and the Apparent Viscosity (at a Shear Rate of 100 s-~‘) of Untreated and the Enzyme-treated (at Various Conditions) Pulp -
The Ranges for the Different
Rheological
Parameter
Unit
Untreated mango pulp
Yield stress Flow behaviour index Consistency index Apparent viscosity
Pa Pas” mPas
3.5-3.9 0.383 6.55 419.3
Enzyme-treated mango pulp 2.9-49 0.379-0.490 1.05-3.47 124.6-229.8
and apparent viscosity) are reported (Table 3) by the coefficients of the second order polynomials. To aid visualization, the response surfaces for these rheological properties are shown in Figs 2-5, respectively. The yield stress mostly depends on the temperature of enzyme treatment as its linear (p 10.25) as well as quadratic (p 10.01) effects are significant; hence, the overall effect is curvilinear in nature (Fig. 2). The other factors that also contribute to the yield stress include linear and quadratic effects of time, and its interaction with temperature (Table 3). The flow behaviour index (n), a criteria for judging the extent of non-Newtonian behaviour, was negatively related to linear effect of temperature @
TABLE 3
0.069 0.798
0*101*
0.101 0.003 0.098 0.249 0~001 0.256 0.069 0.783
O-256**
0.249***
O-098*
3.626 @161*
Recalculated equation
3.625 0.161
Original equation
Yield stress (Pa)
0.005 0.902
- 0.026 - 0.003 0.010 0.010 0.012 O-014 - 0.010
0.415 0.002
Original equation
0.891
0*010** 0.010** o-012** 0.014* * -0*010*
-0.026***
0.415
Recalculated equation
Flow behaviour index
-0.180* 0.839
O-083 - 0.083 0.857
0.205 * * -0.431*** -0*104* -O-189*
2.148
Recalculated equation
0.205 -0.431 -0.106 -0.191 -0.019 -0.180
2.164 0.065
Original equation
Consistency index (Pas”)
Index, Consistency
-6.075* 0.864
- 4.075 2.575 0.878
-24+396*** 6.840*
7.307*
173.679
Recalcluated equation
7.307 2.187 - 24.396 6.704 0.518 - 2.027 - 6.075
174.802
Ongina equation
Apparent viscosity at a shear-rate of 100 SK’ (mPas)
of the Second Order Polynomial for the Response Functions (Yield Stress, Flow Behaviour Index and Apparent Viscosity) in Coded Level of Variables
*Significant at p I 0.25; **significant at p IO.05; ***significant at p IO.01.
b 13 b 23 r***
2:
b 22
k
Br bz
CoefJicient of the regression equation
The Coefficients
00 -.
.2 ?T k $
z
$
9 g Ei s
Rheology of enzyme-treated mango pulp
257
Fig. 2. Response surface for yield stress of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 425°C) and (b) temperature and enzyme concentration (at a time of 90 min).
258
S. Bhattacharya, N. K. Rastogi
Fig. 3. Response surface for flow behaviour index of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 425°C) and (b) temperature and enzyme concentration (at a time of 90 min).
Rheology of enzyme-treated mango pulp
259
(4
Fig. 4. Response surface for consistency index of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 425°C) and (b) temperature and enzyme concentration (at a time of 90 min).
260
S. Bhattdmyu,
A? K. Rastogi
Fig. 5. Response surface for apparent viscosity (at a shear rate of 100 s-l) of mango pulp as a function of (a) time and enzyme concentration (at a temperature of 42*5”C) and (b) temperature and enzyme concentration (at a time of 90 min).
Rheology of enzyme-treated mango pulp
261
Mango pulp is a pseudoplastic liquid due to discontinuous phase substances, such as, starch and fibrous materials (pectin and other fibres) when water acts as a continuous phase. The rheology of mango pulp not only depends on these suspended matters but also on the soluble solids (sugars, organic acids, etc). Further, most of the water available in the untreated pulp is bound to these substances rendering it to be a highly viscous sample. The pectinaceous substances possess a high water holding capacity (WHC) and develop a cohesive network structure. The degradation of pectinaceous material into low molecular weight compounds is a function of enzymatic treatment conditions (temperature, time and concentration) which in turn decides the rheological characteristics (flow behaviour index, consistency index, yield stress and apparent viscosity) of treated pulp. The interesting finding is that the treated pulp never becomes a Newtonian liquid. Ripe mango pulp contains about 0.6% protein, 1.1% fibre (Aykroyd, 1963) (of which 0.7% has been estimated to be in the form of calcium pectate) and considerable amount (2-4%) of starch (Pantastico et al., 1984). The action of pectinase cannot change the non-pectinaceous fibres (about 0.4%) and protein and starch; hence, the pulp remains in its non-Newtonian status. Degradation of pectin leads to a reduction of WHC, and therefore, free waters are released to the system to further reduce the apparent viscosity. The starch present in the pulp remains unaltered during enzymatic treatment and possibly responsible for the fairly constant yield stress. It is necessary to mention here that starch, even at a low concentration in water, is capable of exhibiting non-Newtonian character and yield stress.
CONCLUSIONS Mango pulp as well as enzyme-treated samples behaved as non-Newtonian liquids and exhibited yield stress. The different conditions (time and temperature of enzyme treatment, and concentration of the enzyme) for enzyme treatment revealed that all these variables markedly affect the different rheological parameters (flow behaviour index, consistency index and yield stress) and the apparent viscosity of the pulp suspensions; these can be related to the enzyme treatment conditions by second order polynomials. Apparent viscosity markedly decreased with an elevation in the concentration of the enzyme. An increase in time and/or concentration of enzyme treatment is associated with a decrease in pseudoplastic behaviour of pulp. Among the common rheological models, the Herschel-Bulkley model fits well (rr 0.991, p 10.01) the shear-rate and shear-stress data for untreated and treated mango pulp.
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