Abstract
The lycopene content and its stability in two commonly available tomato cultivars (Lycopersicon esculentum var. Roma VF and L. esculentum var. Ibadan Local), during processing and in-package storage were studied. Estimates of lycopene concentration in fresh tomato and tomato products were determined by measuring the absorbance of lycopene solution in hexane at 472 nm. The results indicated that Roma VF had more lycopene (5.95 mg/100 g) compared to the Ibadan Local (4.76 mg/100 g) cultivars. More lycopene was retained in the dried slices than in dried pulp and better in the presence of antioxidant (sodium metabisulphite). On storage at 29°C, the profile of lycopene degradation rate in Roma VF and Ibadan Local pulp in different packaging materials was: can<plastic bottle<laminated metalized aluminium (LMA) foil<glass bottle and glass bottle<plastic bottle< can<LMA foil. At 40°C, lycopene degraded by three to six times faster than that at 29°C and degraded faster in Ibadan Local than Roma VF pulp on storage.
Introduction
Tomato (Lycopersicon esculentum), often described as a vegetable fruit, is adapted to various culinary uses either in the fresh form in salad or as puree in gravies, stew, and soups for the diet of the diverse cultures of the world. Tomato contains 5.7% dry matter; low concentration of vitamin C, provitamin A, and minerals (especially potassium) compared to other commercially important fruit species. However, because it is consumed in quite large quantities, it is a major source of these nutrients. Of note is the more recent discovery that the carotenoid-lycopene (red pigment) in tomato is to a great extent linked in the prevention of the onset of cancer.Citation1 Citation2 Citation3 Many of the protective benefits of lycopene were suggested to be a result of its ability to protect against oxidative damage and its free radical scavenging capacity.Citation4 Citation5 Citation6 The acyclic polyene structure of lycopene (11 conjugated double bonds) increases its affinity for singlet oxygen and radical scavenging capacity beyond the other carotenoids.Citation7 Citation8 These characteristic chemical properties are also responsible for the spontaneous isomerization and oxidation of lycopene outside tomato tissue matrices, resulting in the degradation of lycopene during tomato processing.Citation9 Minimizing the loss of lycopene throughout the production process and in storage is important. The all-trans isomer of lycopene is the most stable form and most prevalent in tomato fruit,Citation10 while the cis-isomer is less stable.Citation11 Processing of tomatoes to puree or paste is an added value, as it frees lycopene from the tomato matrix, thus enhancing its bioavailability.Citation12 Citation13 However, degradation of lycopene and color loss of processed tomato products are affected by a number of factors such as high temperature, long processing time, light, oxygen, acids, and some metal ions.
Compositional variation of lycopene in tomato occurs as a consequence of varietal differences, climatic conditions, agricultural variables, stage of maturity, harvesting and post-harvest handling, conditions during storage, and transportation.Citation14 Though, the lycopene content of many tomato cultivars had been estimated,Citation11 Citation15 Citation16 the scientific literature, however, lacks information on the lycopene content in Nigerian tomato fruit, the effect of packaging materials on lycopene retention in tomato pulp and effect of drying on lycopene content of dried tomato slices and pulp. The three most commonly available tomato cultivars in Nigerian market shelves are: the indeterminate Ibadan Local (Ib. Local), determinate Roma VF, and in relatively low quantity the NHLe 158-13. Tomato production in Nigeria has more than doubled in the last 10 years, and the production in 2001 alone was about 879,000 metric tonnes.Citation17 The northern Nigerian production was more than 555,630 metric tonnes in 1995/96 alone.Citation18
The purpose of this study was to evaluate the lycopene concentration in Nigerian tomatoes and the effect of processing on lycopene in tomato products with the view to investigating stability of lycopene in tomato pulp in different packaging materials at tropical temperatures.
Materials and Methods
Mature, ripe, and fresh tomato fruit (Roma VF and Ib. Local cultivars commonly available on the south western Nigerian market shelves) were purchased from the local market in Ile-Ife. The tin plated steel cans (300×410) were purchased from Carnaud Metal Box Nig. Plc., Ikeja, Lagos, while laminated metalized aluminum (LMA) foil (0.26 mm thick) was obtained from Chi Foods Nig Ltd., Papa Ajao-Estate, Oshodi, Lagos. The transparent glass and plastic bottles (200 mL) were purchased at Ketu market, Lagos.
Tomato Dehydration
A batch of tomato was sorted and cleaned in portable water and allowed to drain. This was divided into three lots. The first lot was cut into thin slices and charged into vacuum oven at 500–700 mmHg and 40°C until the moisture content was reduced to about 15%. The second lot of the fresh tomato was sliced to 15.0±0.5 mm with sharp knife made of stainless steel blade. The sliced tomato was then steam blanched for 120 s prior to drying in a vacuum oven at 500–700 mmHg and 40°C. The dried samples were then equilibrated in glass desiccators containing silica gel for a day. The dried samples were ground in ceramic mortar into tomato powder.
Tomato Pulping
The third lot of tomato was sorted for wholesomeness and intense red color and washed in chlorinated water (3 mg/L chlorine solution). The fruits were pulped and screened with Langsenkamp pulping machine (Model 18 SER.L 295, Indianapolis, IN). The screened pulp was salted [0.8% (w/w) NaCl] and heated to boil for 15 min, to inactivate enzymes and microorganisms. The heated pulp (juice) was poured into enamel jars. The jars were seamed and sterilized in a boiling water bath for 15 min to eliminate any post-contamination before cooling down to room temperature prior to usage. The tomato juice was hot filled into the packaging materials. The packaging materials were washed in chlorinated water (5 mg/L chlorine solution) at 70°C, and dried in hot air oven at 40°C prior to filling. The packaged tomato juice was cooled in cold-water bath to ambient temperature and stored for 12 days at ambient (29°C), 35°C, and 40°C, for bi-weekly lycopene content analysis.
Physico-Chemical Analyses
A CD 70 potable pH meter (Walden Precision Apparatus Limited, Cambridge) was used to read the pH; while the °Brix was determined using the 9446LD (40–85%) hand refractometer (Baush and Lomb incorporated, Rochester NY) previously calibrated with distilled water. Sugar content was determined using phenol/sulphuric acid reaction as described by Dubois et al.Citation19 A calibration curve was plotted by reading the absorbance of standard α-D-glucose solution in phenol/sulphuric acid mixture in spectrophotometer (Camspec M201, Sawston, Cambridge) at 490 nm. Ascorbic acid was estimated using the AOAC methodCitation20 involving titration with a standardized 2,6-dichlorophenol indophenol (dye) solution.
Acidity was determined according to the AOAC method;Citation20 0.3 mL phenolphthalein indicator was pipetted into beaker in triplicate with 3 mL of the tomato juice made up to 20 mL with distilled water and mixed in the beaker with phenolphthalein. The mixture was titrated against 0.1 N NaOH solution in burette. Results were expressed in g citric acid/100 mL, whilst the total solid was determined by gravimetric method. About 3.0 g sample was weighed in an oven dried aluminum dishes in triplicate and dried in a vacuum oven at 75°C for 24 hrs. The dishes with sample were kept in desiccator for an hour to equilibrate before recording their final weights.
Lycopene Analysis
The powdered tomato product, obtained through the drying process, was rehydrated to the consistency of the processed tomato juice prior to lycopene analysis. Approximately 12 and 16 mL of distilled water were added to 1 g of the dried Ib. Local and Roma VF type tomato powder respectively. Reconstituted tomato powders were mostly suitable for the hexane extraction. Whole tomato for lycopene analyses was ground in enamel mortar for uniform consistency. The extraction of leucopene, using a mixture of hexane–acetone–ethanol,Citation21 was according to procedure described by Sharma and LeMaguer.Citation15 Lycopene concentration was estimated by measuring the absorbance of the hexane solution containing lycopene at 472 nm on a UV-visible spectrophotometer. The values were converted into lycopene concentration using its specific extinction coefficient (E 1 cm 1%) of 3450 in hexane.
Kinetic Data Analysis
The change in lycopene at a constant temperature over a period of time was evaluated by methods of analysis of kinetic data as adapted by Ariahu and Ogunsua.Citation22 The relationship between percentages of lycopene remaining vs. time (weeks) was estimated from the differential equation:
where (C L)0 is the initial lycopene concentration; C L is lycopene concentration at time (t); k is the reaction rate constant; and (T i) is initial temperature of degradation. The dependence of the reaction rate constant (k) on temperature was investigated using the Arrhenius equation:
where E a is activation energy, R is the universal gas constant, and k 0 is the frequency factor. Activation energies and frequency factors were determined from the slopes and intercepts respectively of the lines generated by regression of ln(k) vs. 1/T using the least square linear regression.
Statistical Analysis
Wherever applicable, the data generated were analyzed statistically using the general linear model (GLM) procedure (Excel for windows, 1998) for ANOVA, t-test and least square differences tests (LSD) according to Stoodley et al.Citation23
Results and Discussion
The results presented in Table show the chemical composition and lycopene concentrations in tomato Roma VF and Ib. Local cultivars before packaging. The total solid, soluble solid, ascorbic acid content, and pH value were significantly higher (p>0.05) for the Roma VF cultivars, while Ib. Local had more titratable acidity and reducing sugars. Total solids in Roma VF (6.85 g/100 g) and Ib. Local (5.42 g/100 g) were within the range of values (5.39–6.84 g/100 g) reported by Sharma and LemaguerCitation15 in tomato pulp of different cultivars. The °Brix (soluble solid) determined for Roma VF (6.70) and Ib. Local (5.10) pulp could be used to categorize concentration of tomato juice. Cassia and DeliaCitation24 reported the °Brix value for tomato juice to range between 5.0 and 6.0. The content of reducing sugars in Ib. Local (1.16 g/100 mL) was more than that in Roma VF (0.94 g/100 mL). The usual long haulage distance and time, and the indigenous storage practice tend to promote the break down of sugars in the Roma VF type tomato fruit. The tomato fruit continues to respire aerobically, converting the available sugar to energy, carbon dioxide (CO2), and water, thus depleting the sugar content.
Table 1 Composition of whole and pulp from Roma VF and Ib. Local tomato cultivars
The ascorbic acid (vitamin C) contents were 10.35 and 8.95 mg/100 mL for Roma VF and Ib. Local, respectively. These values are similar to the average ascorbic acid content (10 mg/100 mL) reported for tomato juice by Ensminger et al.Citation25 Titratable acidity was higher in Ib. Local (0.50 g citric acid/100 mL) than in Roma VF (0.39 g citric acid/100 mL). The average titratable acidity measured as citric acid was reported by WillsCitation26 to be 0.43 g/100 g. The Ib. Local tomato could be termed high acid tomato cultivars, therefore, it has the advantage of better keeping quality as a high acid fruit.
Lycopene in Fresh Tomato Cultivars
Roma VF and Ib. Local tomato cultivars contained 7.00% and 5.42% total solid in the fresh fruit with 5.95 mg and 4.76 mg/100 g lycopene, respectively. These values fall within the range (3.1–7.7 mg/100 g) reported for different tomato cultivars by other researchers.Citation11 Al-Wandani et al.,Citation27 however, reported that the whole matured tomato fruits contain 3.35 mg/100 g lycopene. Whereas, the lycopene concentration (6.46–10.70 mg/100 g) reported by Sharma and LeMaguerCitation15 and (6.00–15.00 mg/100 g) reported by Camora et al.Citation16 for whole fresh tomato fruit were higher than the results of this investigation. The lycopene content on (Table ) supports the finding by Sharma and LeMaguerCitation15 that tomato cultivars with higher content of fibre and insoluble solids have more lycopene than those with low level of total solid. They reported that, the total solid in five tomato varieties was in the range 5.39%–6.84%. WillsCitation26 gave tomato dry matter content range between 5% and 7%. These values were similar to the values obtained in this investigation. It has been established by Liu and LuhCitation28 that lycopene accumulation in tomato increases with the stage of maturity on the field before harvesting. Moreover, the high tropical climate temperature will inhibit the accumulation of lycopene in tomatoes leading to the conversion of lycopene to β-carotene.Citation29 Citation30 Citation31 The relatively lower lycopene content of Nigerian tomatoes would be expected since tomato for the fresh market are usually not allowed to attain full ripening before they are plucked for onward transmission to the market stalls where they are ripened for sale. The pH of the fruit affects lycopene content directly as low pH red tomato accumulates more lycopene.Citation32 The Ib. Local tomato is a low pH (4.23) tomato, compared to Roma VF (pH 4.45). These pH values fall within the range suitable for processing tomato to discourage microbial spoilage in the products.Citation33
Lycopene Content in Processed Tomato
The lycopene contents 5.84 and 4.63 mg/100 g in whole blanched Roma VF and Ib. Local tomato, respectively, were not significantly different (p<0.05) from the lycopene content of the fresh tomato of both cultivars (Table ). The lycopene retention after blanching was 96.94% and 98.35% in Roma VF and Ib. Local tomato, respectively. The tomato skin probably serves to prevent autoxidation during heating by protecting the native lycopene from oxygen.
Table 2 Lycopene concentration (mg/100 g) in processed Roma VF and Ib. Local tomato
The fresh pulp had lycopene concentration of 5.85 and 4.59 mg/100 g in Roma VF and Ib. Local tomato, respectively. About 85.30% and 89.34% of the lycopene in Roma VF and Ib. Local tomato juice/pulp were retained after boiling the juice for 15 min. There was high content of serum and seed in Ib. Local compared to those of Roma VF but the latter contained more skin. This could be responsible for the difference in behavior of the two cultivars in the fate of lycopene during heat processing. The tomato skin probably serves to prevent autoxidation during heating by protecting the native lycopene from oxygen.Citation11 Citation34
Fruits and vegetables are usually sulphited to prevent discolouration during sub-sequent drying. The values 5.29 and 4.47 mg/100 g lycopene concentration were determined for sulphited dried slices of Roma VF and Ib. Local tomato, respectively. However, values of 5.16 mg and 3.93 mg/100 g were determined for non-sulphited dried slices of Roma VF and Ib. Local tomato, respectively. The dried tomato juice/pulp had the least lycopene retention; 4.01 and 2.95 mg/100 g were obtained for sulphited and 3.66 and 2.84 mg/100 g for non-sulphited Roma VF and Ib. Local tomato juice/pulp. NobleCitation35 reported that heat concentration of tomato pulp resulted in up to 57% loss of lycopene which was higher than 34.14%, 37.25% and 39.85%, 39.51% loss of lycopene recorded for sulphited and non-sulphited dried Roma VF and Ib. Local tomato, respectively, in our finding. Tomato powders produced from Roma VF had a darker red color than Ib. Local powder. The high titratable acid in Ib. Local tomato could be responsible for inhibiting the formation of hydroxymethylfurfural, a product of thermal damage in dehydrated tomato that causes browning.Citation36 Danziger et al.Citation37 reported that low pH and increasing level of SO2 inhibit browning and the effect is pronounced as the water activity (a w) decreases. The trend in the lycopene retention is an indication that the harsher the thermal processing on tomato, the lower the lycopene retained.
Fate of Lycopene in Tomato Pulp on Storage
The tomato pulp prior to heating had a bright red color that changed to dark red after heat processing. This change in color intensity may be due to the isomerization of lycopene from the usually all trans form to cis isomers.Citation9 Citation10 Citation11 Citation12 Citation13 Citation38 Citation39 Figures and show the percent of lycopene remaining over time in different packages on storage at 29°C, 35°C, and 40°C. The result of this investigation shows that there was loss of lycopene in all the tomato pulp in the different packages and at all the storage temperatures. The loss of lycopene was more pronounced in Ib. Local tomato pulp than in Roma VF pulp. GouldCitation33 reported that color loss of tomato juice on storage was materially accelerated by high temperature (>31°C). The reaction rate constant (k) was determined from the slope of the line obtained by least regression analysis of Eq. (Equation2). The kinetic parameters are presented in Table for both first order and zero order. Anguelova and WarthesenCitation8 reported that the kinetics of lycopene degradation reaction at 37°C followed a first order kinetic model. Sharma and LeMaguerCitation40 reported the kinetics of lycopene degradation in tomato pulp solids to be a pseudo first order reaction.
Table 3 The rate constants and regression coefficient for lycopene degradation in packaged Roma VF and Ib. Local tomato pulp
Figure 1. Changes in lycopene retention with storage time of tomato pulp in can and glass bottle at different temperatures. Key: ♦, 29°C; ▪, 35°C; ▴, 40°C.
Figure 2. Changes in lycopene retention with storage time of tomato pulp in plastic bottle and LMA foil at different temperatures. Key: ♦, 29°C; ▪, 35°C; ▴, 40°C.
Comparison of the k-data at common reference temperatures reveals that lycopene in Roma VF tomato could be more stable in cans at 29°C than in other packaging materials. At 40°C, lycopene degraded two to eight times as fast as at 29°C in the various packages. The results of this finding show that lycopene though relatively stable in canned Roma VF pulp at 29°C, it was less stable in Ib. Local pulp and even at higher temperatures. The highest lycopene loss was recorded for Ib. Local tomato pulp in cans at 40°C. Pacheco et al.Citation41 showed that lacquer failure was, to a large extent, responsible for color deterioration in canned tomato pulp. Lycopene was reported by Sun and DingCitation42 to have poor stability in solutions containing high acid content. Adachi et al.Citation43 also reported that tomato purees, which included tomato serum, showed a color change proportional to the amount of serum included, indicating that the serum was the principal cause of color change. The Ib. Local tomato contains higher serum and acid content than Roma VF tomato this could be responsible for the higher loss of lycopene in the stored Ib. Local pulp.
The temperature dependence of lycopene degradation is shown in Table . The activation energies (E a) for lycopene degradation in cans were higher for both Roma VF and Ib. Local tomato pulp at first order reaction kinetics, while the lowest activation energies were recorded in LMA foil and plastic bottles. Tomato pulp stored at ambient temperature gave the lowest lycopene destruction rate. Ensminger et al.Citation25 have established that tomato pulp in transparent glass bottles is unstable to sunlight. When shielded from sunlight, the lycopene content in tomato pulp stored in glass bottle was relatively stable at ambient temperature as shown by its degradation reaction rate. Plastic bottles also retained lycopene better at room temperature in Roma VF and Ib. Local tomato pulp respectively. Laminated metalised aluminium foil was also effective in retaining lycopene at 29°C in Roma VF pulp. However, at higher temperatures, it became less effective as shown by the degradation rate constants in Table .
Table 4 The activation energy for lycopene degradation in packaged tomato pulp
Conclusion
On wet basis, the lycopene concentration in Roma VF tomato was higher than in Ib. Local tomato. The Roma VF tomato has more total solid than the Ib. Local tomato, therefore confirms that lycopene is associated with the fiber and the insoluble fraction in tomato fruit. Tomato powder produced from sliced blanched tomato retained more lycopene than powders obtained from the dried juice/pulp.
Kinetic studies showed that lycopene degradation was faster in Ib. Local juice/pulp than in Roma VF juice/pulp and this color deterioration was more pronounced at 40°C and highest in cans. Lycopene instability in cans was due to high temperature and high acidity in the Ib. Local tomato serum. However, the lowest lycopene degradation rate was recorded for Roma VF in cans at 29°C. Lycopene pigment retention was good in plastic and glass bottle when shielded completely from light at ambient (29°C) for both tomato cultivars' pulp. However, the Roma VF tomato cultivar was more suitable for juice/pulp production as leucopene had been more stable, especially at 29°C.
Related Research Data
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