Degradation Kinetics of Lycopene and Visual Color in Tomato Peel Isolated from Pomace

Abstract

Kinetics of lycopene and visual color degradation of tomato peel was studied at selected temperatures (50–100°C). Models based on lycopene and Hunter (a × b) values of fractional conversion were applied to determine the kinetic parameters. The degradation of lycopene and Hunter color values adequately followed first order reaction model with R² > 0.97. The temperature dependence of the rate constants was adequately modeled by the Arrhenius equation. The activation energies of lycopene and color parameters were 18.27 and 29.07 kJ/mol, respectively, indicating greater temperature sensitivity of visual color parameters. Correlation of lycopene content and Hunter (a × b) values showed that they can be used interchangeably with good accuracy.

Keywords:

• Tomato peel
• Lycopene
• Color parameters
• Degradation kinetics
• Processing waste

INTRODUCTION

Tomato (Lycopersicon esculentum), a member of solanaceae family, is one of the most important crops around the world. Commercial processing of tomato utilizes only juice or pulp while the pomace consisting of seed, peel, and fibrous material is discarded.[1] Pomace creates environmental pollution during the course of decomposition. The seeds can be isolated, dried, expressed, or solvent extracted to yield orangish-red edible oil,[2,3] while the meal can be used in feeds, fertilizers, or for the preparation of surfactants such as wetting agents, detergents, and soon.[4] The peel is another important component of the tomato processing waste that could be utilized for extracting the red pigment by using the organic solvents.[5–7]

The tomato peel contains many carotenoids but lycopene is the most important responsible for red color. It has a polyenic chromophore with 11 conjugated double bonds, which absorb and reflect light.[8] Lycopene exhibits the highest antioxidant activity and singlet oxygen quenching ability of all dietary carotenoids.[9–11] Use of carotenoids such as β-carotene and lycopene in the diet has been positively correlated with reduced cancer incidence.[12–14] Lycopene is susceptible to isomerization and oxidation when exposed to light and heat that decrease its biological activity.[15] It is generally presumed that isomerization resulting in conversion of the all-trans isomers to the cis-isomers. The present study was undertaken to access the thermal stability of the lycopene and the kinetics of its degradation in tomato peel isolated from the major tomato waste pomace.

MATERIALS AND METHODS

Sample Preparation

Tomato pomace was obtained from a tomato paste-manufacturing unit located in Amritsar, India. Peel was separated from pomace by a continuous flotation-cum-sedimentation system[16] and dried at 50°C in a cabinet dryer. The dried sample was kept in an airtight container stored at −20°C temperature until used.

Chemical analysis

Moisture, ash, crude protein, crude fiber and crude fat content were determined according to AOAC.[17] Carbohydrates were computed by subtracting all parameters from the sample.

Heat treatment

Triplicate dried samples (25 g) were placed in covered petridishes and heated in hot air oven adjusted to 50–100°C for 0–10 hrs.

Lycopene Estimation

Sample (2 g) was extracted using solvent (hexane: acetone: alcohol 2:1:1) containing 0.05% (w/v) butylated hydroxytoluene (BHT) till it become colorless. Cold distilled water (15 mL) was added and the suspension was agitated. The solution was allowed to stand for 15 minutes for separation of polar layer and non-polar layer containing lycopene. The total lycopene was obtained by measuring the absorbance of the lycopene solution at 503 nm using a UV visible spectrophotometer (Shimadzu Co., Ltd., Japan) and expressed as mg/100 g using extinction coefficient of 17.2 × 10⁴ mol cm⁻¹.[6]

Color Measurement

Visual color was measured using a Hunter colorimeter (Hunter associates Laboratory, U.S.A.) in terms of L (lightness), a (redness and greenness), and b (yellowness and blueness). The instrument was calibrated with a standard white tile (L = 90.55, a = −0.71, b = 0.39). A glass petri dish containing the ground peel was placed above the light source, covered with a white plate and Hunter L, a, b values were recorded.

Kinetics of Lycopene and Visual Color Degradation

The kinetics of degradation of both pigment and visual color has been reported to follow first order reaction adequately.[18–22] The first order kinetic model based on pigment concentration is

(1)

where, L = amount of lycopene content at time t (mg/100 g), L_o = initial amount of lycopene (mg/100 g), k₁ = reaction rate constant (h⁻¹), and t = heating time (h). Fractional conversion is a convenient variable and often used in place of concentration[23] and has been reported to increase the accuracy of the calculation.[22]

First order reaction in terms of the fractional conversion may be represented as

(2)

where f₁ = (L_O – L)/(L_O – Lα), k₂ = Reaction rate constant (h⁻¹), and Lα = lycopene content at infinite time (mg/100 g). Different combinations of hunter values (a/b, a × b and b/a) were plotted against time to find out the variables that vary in linear fashion. Hunter (a × b) values was analyzed for color degradation following model 1 and 2 after replacing lycopene concentration with Hunter (a × b) values, respectively.

(3)

(4)

where f₂ = [(a_o × b_o) – (a × b)/(a_o × b_o) – (a_α × b_α)], k₃ and k₄ = Reaction rate constant for Eqs. (3) and (4), respectively (h⁻¹), (a_o × b_o) = initial value, (a × b) = value at time (t), and (a_α × b_α) = value at infinite time.

Effect of Temperature on Lycopene and Color Degradation

The Arrhenius model was applied to describe the temperature dependence of lycopene degradation.

(5)

where k_o = frequency factor (h⁻¹), E_a = activation energy (kJ/mol), R = universal gas constant (8.314 J/(mol. K), and T = absolute temperature (K).

Relationship Between Lycopene and Visual Color

Change in visual color is a direct manifestation of change in pigment content. The Hunter (a × b) values were therefore correlated with pigment concentration of tomato peel heated for 10 hours at selected temperatures. The relationship between visual color and lycopene content:

(6)

where k_a and k_b are the coefficients.

Statistical Analysis

Regression analysis and Pearson's correlation coefficient was completed using the statistical package, Statistica, 5.0 version (Stat Soft Inc, USA).

RESULTS AND DISCUSSION

Chemical Characteristics of Tomato Peel

The chemical analysis of tomato peel isolated from pomace is shown in Table 1. The dried peel contained 5.74% moisture, 14.27% protein, 3.72% crude fat, 1.28% ash, 71.25% crude fiber, and 3.46% carbohydrate. Earlier studies have reported 6.69–10% moisture, 10–10.7% crude protein, 1.7–3.96% crude fat, 1.13–5.6% ash, 46.1–55.9% crude fibre, and 26.69% carbohydrate.[1,24,25] Therefore, moisture, ash, and crude fat values of present study were in the range of previously reported values. The high protein content of the peel might be due to the differences in agronomical practices where as higher fiber content of peel might be due to hot break treatment that resulted in leaching of soluble material.

Table 1 Chemical composition of dried tomato peel (n = 3).

Constituents	Contents
Moisture (%)	5.74 ± 0.109
Protein (%)	14.27 ± 1.57
Crude fat (%)	3.72 ± 0.049
Ash (%)	1.28 ± 0.071
Crude fiber (%)	71.25 ± 1.91
Carbohydrate (by difference)	3.46
Lycopene (mg/ 100g)	1.75 ± 0.05

The lycopene content of the waste tomato peel was 1.75 mg/100 g peel. In the common variety of tomatoes, lycopene is found in the concentration of 3.1–7.7 mg/100 g.[15] Lower lycopene content of the tomato peel might be due to the hot break process (85°C/ 5 min) that reduced the pigment content.

Lycopene Degradation

Heat treatment resulted in accelerated color degradation in tomato peel. At 50°C lycopene content decreased from 1.75 to 1.38 mg/100 g while at 100°C, it decreased from initial value to 0.92 mg/100 g after 10 hours. The lycopene content decreased in tomato pulp as the temperature was increased from 29 to 40°C.[26] Decline in lycopene content was probably due to the destruction by heat and oxidation resulting in fragment products like acetone, methyl-heptenone, laevulinic aldehyde, and glyoxal.[27,28]

The fractional conversion factor was computed from the lycopene and was used in first order model to explain degradation.[22,29] The fractional conversion gave good results (Fig. 1) and its reaction rate constants ‘k₂’ increased from 0.024743 to 0.054076 h⁻¹. Anguelova and Warthesen[30] reported ‘k’ in the range of 0.02116–0.01823 h⁻¹ for lycopene, 0.01487–0.01447 h⁻¹ for β-carotene, and 0.00923–0.00751 h⁻¹ for α-carotene at 37°C. The coefficients of determination (R²) were 0.9967–0.9988, and the standard error ranged from 0.004 to 0.008 (Table 2). Thus predicted values were in good agreement with experimental value. However, the values reported earlier were lower than the present values.

Figure 1 Lycopene degradation kinetics of tomato peel at selected temperatures.

Table 2 The kinetics parameters of lycopene and visual color degradation (n = 3).

	Lycopene			Hunter color value (a × b)
Temperature (°C)	k2 (h^−1)	R^2	Standard error	k4 (h^−1)	R^2	Standard error
50	0.02438	0.9985	0.004	0.00825	0.9949	0.001
60	0.03119	0.9972	0.005	0.01106	0.9915	0.002
70	0.03550	0.9988	0.003	0.01706	0.9942	0.005
80	0.04381	0.9976	0.007	0.02125	0.9952	0.003
90	0.05113	0.9967	0.008	0.02769	0.9954	0.005
100	0.06244	0.9985	0.007	0.03450	0.9962	0.007

Color Loss

Various combinations of Hunter color values (a/b, a × b and b/a) were selected to describe the visual color change. The combination (a × b) was found to describe the first order degradation kinetics adequately. Previous studies have also stated that Hunter (a × b) values to describe the visual color degradation as it measures the color changes at the surface.[22,29] Hunter (a × b) decreased with increase in temperature (50°–100°C) and time (0–10 h). Fraction conversion factor was computed and plotted versus time (Fig. 2). The reaction rate constant ‘k₄’ values varied from 0.008678 to 0.03368 h⁻¹. Ahmed et al.,[29] reported ‘k’ in the range of 0.131–0.378 h⁻¹for visual color of papaya puree. The coefficients of determination were 0.9915–0.9962, and the standard errors were 0.001 to 0.007 (Table 2).

Figure 2 Hunter value (a × b) degradation kinetics of tomato peel at selected temperatures.

Temperature Dependence

The dependence of rate constant for both lycopene (k₂) and visual color (k₄) on temperature followed the Arrhenius equation. An Arrhenius plot between the ln k versus 1/T followed a linear relationship with R² more than 0.97 (Fig. 3). The activation energies obtained for lycopene and visual color were 18.27 and 29.07 kJ/mol, respectively. The lower activation energy of lycopene was probably because dry heating was used for lycopene extraction and decreased stability might occur.[31] Higher activation energy signified greater heat sensitiveness of visual color at selected temperature. The activation energy for lycopene degradation in tomato pulp was ranged from 19.9 to 27.74 kJ/mol at −20.5 and 25°C.[29] Nakagawa et al.,[32] reported activation energy value from 30.80 to 37.08 kJ/mol for lycopene degradation in organic solvents heated at 100–130°C. Thus, the previous findings support the current study of lycopene degradation.

Figure 3 Dependence of degradation rate constant for lycopene and Hunter color of tomato peel on temperature using Arrhenius model.

Relationship Between Visual Color and Lycopene Content

The plots of Hunter (a × b) and lycopene content of peel exhibited linear relationship with R² 0.995–0.998 and S.E was 0.55–1.85 (Table 3). Typical relationship between visual color and lycopene content of tomato peel heated at 100°C is shown in Fig. 4. Similarly, Ahmed et al.,[29] found typical relationship between visual color and carotenoids of papaya puree at 70°C. There is a direct relationship between visual color and lycopene content in tomato peel. So visual color parameters may adequately be used in place of lycopene content in tomato peel.

Table 3 Regression parameters for the correlation of visual color and lycopene content (Eq. 6).

Temp (°C)	ka	kb	R^2	Standard error
50	213.62	73.97	0.9983	0.55
60	201.35	80.44	0.9979	0.88
70	164.80	102.45	0.9952	1.85
80	160.86	102.98	0.9982	1.50
90	136.48	117.93	0.9984	1.45
100	119.16	128.29	0.9985	1.12

Figure 4 Correlation between lycopene and visual color of tomato peel at 100°C.

CONCLUSION

Lycopene content in tomato peel decreased during heating at selected temperatures resulting in decreased Hunter (a × b) values. The kinetics of lycopene and visual color degradation followed first order reaction model. Lycopene degradation and color loss followed Arrhenius model, while higher activation energy of visual color indicated to be more heat sensitive as compared to lycopene. Visual color and lycopene content can be used interchangeably with coefficient of determination more than 0.995.