THERMAL COLOUR DEGRADATION KINETICS OF MANGO PUREE

ABSTRACT

Colour degradation during thermal processing of mango puree was studied at 50–90°C for 0–20 min. Post-processing Hunter colour scale values (L, a and b) were used to determine the kinetics of colour degradation. The thermal degradation of Hunter colour b value (representing yellowness) and L×a×b value (representing total colour) followed first-or der reaction kinetics. The activation energy values for yellow colour and total colour were found to be 36.26 and 36.79 kJ/mol respectively.

INTRODUCTION

Mango (Mangifera indica L.) is the most important fruit of the tropics and ranks fifth in total production among major fruit crops globally. [1] The fruit is very much relished for its succulence, exotic flavor and delicious taste. Production of mangoes during 1971 to 1993 has increased by about 50% worldwide [2] while India produces approximately 64% of the total production of mangoes in the world. Mango is processed into various forms after the extraction of pulp. Puree is one of the important intermediate mango products, which is thermally processed to enhance its shelf life and stored for the manufacture of beverage [3] and dehydrated forms.

The capusine yellow or reddish yellow colour of mango solely depends on the presence of carotenoid content in the fruit. Carotenoids are comprised of two structural groups namely hydrocarbon carotenes and oxygenated xanthophylls. The basic carotenoid structural backbone consists of isoprene units linked covalently to create a symmetrical molecule. The β-carotene constitutes the major carotenoid (50–64%) in the ripe mango. [4] Presence of 16 other carotenoids including neo-β-carotene, mono-epoxy-β-carotene, auroxanthin, zeaxanthan were also confirmed in the ripe mangoes. [5] However, high carotenoid foods showed distinct colour shift during thermal processing as heat induces cis-trans isomerization reaction. [5]

Maintenance of naturally coloured pigments in thermally processed and stored foods is a major challenge in food processing. 6-7 Various factors are responsible for the loss of pigment and colour during processing of food products. These include non-enzymatic and enzymatic browning and process conditions such as pH, acidity, oxidation, packaging material and duration and temperature of storage. Special care must be taken to produce food that retains a bright, attractive colour during food processing. Change in colour during thermal processing may therefore be used as a tool to evaluate the quality of product.

Colour degradation kinetics of food products is a complex phenomenon and dependable models to predict colour change, which can be used in engineering calculations, are limited. Numerous researchers 8-13 have studied kinetics of pigment and colour degradation of fruits and vegetables during thermal processing. The kinetic parameters namely, rate constant and activation energy provides useful information on the quality change, which occur during thermal processing. Several studies have been reported in the literature on quality aspects of mango based mainly on carotenoid loss during thermal processing and storage. However, no information is available on the colour degradation kinetics of mango puree during thermal processing.

Chroma, hue and colour difference (ΔE) are widely used parameters to specify colour of food materials. 14-15 These parameters are based on different combinations of Hunter colour values (L, a, b). Analysis of colour on the basis of a, a/b, b, Lab, La/b etc. values have also been carried out by numerous researchers. 11-13] The present study was undertaken to investigate the kinetics of degradation of visual colour using the Hunter colour (L, a, b) values during thermal processing of mango puree.

MATERIALS AND METHODS

Preparation of Puree

Dashehari variety of mangoes were procured from the local market and washed thoroughly. The skin was removed using a stainless steel knife. The pulp was separated from the stone and passed through a pulper fitted with a 40-mesh sieve to obtain the product of uniform consistency. Total soluble solids and pH of mango puree were 21° Brix and 4.87, respectively.

Thermal Treatment

It is established that an acidified food requires only pasteurization. Thermal degradation kinetics of mango puree was studied by isothermal heating at selected temperatures (50, 60, 70, 80 and 90°C) for a residence time of 20 min. Two hundred-g purees was poured into a 250 mL glass beaker and covered. The breakers containing puree were placed in a constant temperature water bath at selected temperatures (±1°C) and periodically agitated to ensure uniform temperature throughout the bulk of sample. The beakers were heated for 0, 5, 10, 15 and 20 min respectively after the puree at its geometric center attained the desired temperature.

Measurement of Visual Colour

Objective colour measurement was carried out using a Hunter colourimeter (Hunter Associates Laboratory Inc., Reston, USA) on the basis of three colour values, namely L, a and b. The L value signifies the lightness (100 for white and 0 for black), a value represents greenness and redness (−80 for green and 100 for red) while b value signifies change from blueness to yellowness (−80 for blue and 70 for yellow). The instrument (45°/0° geometry, D25 optical sensor, 10° observer) was calibrated against a standard white reference tile (L=90.55, a=−0.71, b=0.39). A glass cell (6 cm diameter) containing the heat-treated puree was placed above the light source and covered with a white plate and post-processing L, a, b values were recorded. Each experiment was repeated twice and average values were used in the analysis.

Kinetics Model

Degradation of visual colour has been shown to follow first-order reaction kinetics. [11], 13-14, 16-17 Following this evidence, it was reasoned that the thermal colour degradation of mango puree follows the first-order reaction kinetics, represented by Eq.1. [11], [13]

Dependence of the degradation rate constant on temperature is represented by the Arrhenius equation

where, C is measured Hunter colour value (L, a, b) or a combination of these at time t (dimensionless); C₀ is measured colour value(s) at zero time (dimensionless); k is rate constant (min⁻¹); t is heating time (min); k₀ is frequency factor (min⁻¹); E is activation energy (kJ/mol); R is universal gas constant (8.314 J/(mol.K)) and T is absolute temperature (K).

The samples were transferred to an ice water bath immediately after the thermal treatment. The come-up time for the samples heated to different temperatures (50–90°C) were obviously different. The difference in come-up time was however taken care of by incorporating Hunter colour initial values (L ₀,a ₀,b ₀) in Eq.1.

RESULTS AND DISCUSSION

Visual Colour Degradation Kinetics

Since the major colour of mango puree is yellow, Hunter b value was considered as a physical parameter to describe the yellow colour degradation of mango puree during thermal processing. Equation1 can berewritten as:

where, b is Hunter colour b value at time t (dimensionless); b ₀ is Hunter colour b value at zero time (dimensionless) and k₁ is rate constant for yellow colour degradation(min⁻¹).

Equation 3 described adequately the degradation of yellow colour of mango puree over the entire temperature range (Fig. 1). The coefficient of determination values (R²) were greater than 0.90 while the standard error values were less than 0.0008 for all the cases.

During thermal processing of mango puree, it was observed that apart from Hunter b value, both L and a values also decreased with time at a given temperature as the puree turned brown. Therefore, different combinations of tristimulus L, a, b colour values were tested to describe the total colour change of mango puree (Table 1). These combinations were subjected to linear regression with respect to time as represented by Eq.1 and the coefficients were determined. Standard error and R² values were used as the basis to select the combination which best described the first-order reaction for the entire temperature range. It was found that L×a×b was the most appropriate combination, which described closely, the first-order reaction kinetics of colour degradation of mango puree (Fig. 2). The R² values were between 0.938 and 0.995 while the standard error values were less than 0.0009. Several authors [11], 17-18 have reported similar observation while working on thermal processing of pureed foods. They reasoned that all the three parameters should be combined together and found L×a/b or L×a×b as the optimum combination to describe the total colour degradation during thermal processing.

Figure 1. Temporal variation of Hunter colour b ratio of mango puree at selected temperatures.

Table 1. Regression Coefficients of Eq.1 for Selected Combinations of Hunter Colour Values for Mango Puree at 50°C

No.	Combination	k(min^−1)	R^2	Standard Error
1	L	0.0004	0.954	0.00003
2	a	0.0015	0.879	0.00001
3	b	0.0008	0.980	0.00004
4	La	0.0019	0.938	0.00013
5	Lb	0.0012	0.976	0.00006
6	a/b	0.0007	0.139	0.00017
7	ab	0.0024	0.973	0.00011
8	L/a	0.0011	0.710	0.00015
9	L/b	0.0004	0.972	0.00002
10	Lab	0.0028	0.983	0.00010
11	La/b	0.0011	0.674	0.00016
12	Lb/a	0.0003	0.480	0.00019
13	L/ab	0.0003	0.480	0.00019

Figure 2. Temporal variation of Hunter colour Lab ratio of mango puree at selected temperatures.

Effect of Temperature on the Rate Constant

Effect of temperature on visual yellow colour (Hunter b value) and total colour (L×a×b) degradation rate constants is shown in Fig. 3. Dependence of the rate constant on temperature obeyed the Arrhenius relationship (Eq.2) (R ²>0.90). The computed values of activation energies were 36.26 and 36.79 kJ/mol for Hunter colour b and L× a×b value respectively. The magnitudes of activation energy indicate that practically there was no difference in the heat sensitivity of both yellow colour and total colour. Since total colour includes the effect of both non-enzymatic browning and carotenoid loss, it could be used as a quality indicator from a processing viewpoint.

Figure 3. The Arrhenius relationsh ip between colour degradation rate constant (Lab and b) and temperature of mango puree.

Colour is an important attribute because it is usually the first property the consumer observes. The tristimulus colourimetry has been accepted as rapid and simplest instrumental method of specifying visual perception of the food products. [19] Results of this study can therefore be used to compute the extent of colour degradation during thermal processing of mango puree.

CONCLUSIONS

Degradation of visual colour during thermal processing of mango puree followed first-order reaction kinetics. Variation of degradation rate constants with temperature followed an Arrhenius relationship. The computed values of the activation energy for yellow colour (b) and total colour (L×a×b) were 36.26 and 36.79 kJ/mol respectively. Total colour may be used to describe the colour change during thermal processing of mango puree.

Acknowledgments