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Investigation of the antioxidant properties of tomatoes after processing
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JOURNAL OF FOOD COMPOSITION AND ANALYSIS
Journal of Food Composition and Analysis 17 (2004) 635–647 www.elsevier.com/locate/jfca
Original article
Investigation of the antioxidant properties of tomatoes after processing E. Sahlina,b, G.P. Savagea,*, C.E. Listerc b
a Food Group, AFSD, Lincoln University, P.O. Box 84, Canterbury, New Zealand Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden c Nutrition and Health Team, Crop & Food Research Ltd., Christchurch, New Zealand
Received 13 January 2003; received in revised form 30 September 2003; accepted 2 October 2003
Abstract A study was made of the antioxidant content, activity and colour of two New Zealand commercially grown tomatoes (Lycopersicon esculentum L. var. Excell and Aranca) cultivated in glasshouses using hydroponic techniques. Excell tomatoes were harvested and stored individually while cultivar Aranca was harvested by cutting the vine, which consisted of a group of eight tomatoes. Both cultivars of tomatoes were stored in the dark for 4 days at 15 C to simulate normal pre-purchase storing conditions. The antioxidant content of the raw tomatoes after 4 days of storage were markedly different while the CIE LAB colour values of the cut inner surfaces of the two cultivars were similar. After 4 days storage, subsamples of each cultivar were either boiled, baked or fried, and analysis of CIE colour, ascorbic acid, total phenolics, lycopene and antioxidant activity (using the ABTS assay) was undertaken. Boiling and baking had a relatively small effect on the ascorbic, total phenolic, lycopene and antioxidant activity of the two cultivars while frying significantly reduced (Po0:001) the ascorbic, total phenolic and lycopene contents of the two cultivars. Chromatic colour analysis showed that both cultivars became significantly (Po0:001) darker and less red after cooking by all methods. In a following experiment, the two cultivars of tomatoes were sliced and allowed to soak for 20 min in a mixture of olive oil and white vinegar, or olive oil and white vinegar separately. CIE colour of the two cultivars showed no change after processing but the treatments of oil and vinegar separately and together reduced (Po0:05) the red component of the colour. Treating the two cultivars of tomatoes with the oil and vinegar mixture resulted in a significant reduction (Po0:001) in the ascorbic acid, total phenolic and antioxidant activity of the tomatoes. Treatment with oil significantly reduced the amount of lycopene that could be extracted from the tomatoes while treatment with vinegar had no effect. r 2003 Elsevier Inc. All rights reserved. Keywords: Tomato; Lycopersicon esculentum L; Antioxidant activity; Lycopene; Total phenolics; Ascorbic acid; Lab colour; Storage
*Corresponding author. Tel.: +64-33253-803; fax: +64-33253-851. E-mail address: [email protected] (G.P. Savage). 0889-1575/$ - see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2003.10.003
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1. Introduction Tomatoes are one of the most widely used and versatile vegetable crops. They are consumed fresh and are also used to manufacture a wide range of processed products (Madhavi and Salunkhe, 1998). Tomatoes and tomato products are rich in health-related food components as they are good sources of carotenoids (in particular, lycopene), ascorbic acid (vitamin C), vitamin E, folate, flavonoids and potassium (Beecher, 1998; Leonardi et al., 2000). Other constituents are protein and dietary fibre (Davies and Hobson, 1981). The chemical composition of the tomato fruit depends on such factors as cultivar, maturity and the environmental conditions in which they are grown (Davies and Hobson, 1981; Giovanelli et al., 1999; Abushita et al., 2000; Thompson et al., 2000). It has been shown that ripening processes and storage temperatures can severely affect the final nutrient composition of the fruit (Madhavi and Salunkhe, 1998). However, even though many researchers comment that tomato quality depends on the cultivar, growing conditions and ripening position on the vine, limited data is available to support these assertions, particularly in greenhouse-grown tomatoes. Regular consumption of tomatoes has been correlated with a reduced risk of various types of cancer (Franceschi et al., 1994; Gerster, 1997; Weisburger, 1998) and heart diseases (Lavelli et al., 2000; Pandey et al., 1995). These positive effects are believed to be attributable to the antioxidants, particularly the carotenoids, flavonoids, lycopene and b-carotene (Lavelli et al., 2000). Furthermore, recommendations to increase daily intake of fruits and vegetables rich in nutrients such as carotenoids and vitamins C and E to lower the risk of cancer and cardiovascular diseases have been made by the American Cancer Society (1984), Steinmetz and Plotter (1991), Block et al. (1992) and the World Cancer Research Fund (1997). Giovannucci (1999) reviewed a number of epidemiological studies and concluded that the intake of tomato products was consistently associated with a lower risk of a variety of cancers and in particular prostate cancer. The main antioxidants in tomatoes are carotenoids, ascorbic acid and phenolic compounds (Giovanelli et al., 1999). The overall antioxidant activity of tomatoes varies considerably according to the genetic variety, ripening stage and growing conditions (Leonardi et al., 2000). Lycopene is responsible for the red colour of tomatoes (Nguyen and Schwartz, 1999) and is regarded as an antioxidant with high biological activity in the body (Stahl and Sies, 1996). The lycopene content of tomatoes varies considerably between cultivars, stage of maturity and growing conditions. Field-grown tomatoes appear to contain higher levels of lycopene, ranging ! from 5.2 to 23.6 mg/100 g wet matter (WM) (Abushita et al., 2000; Gomez et al., 2001; Takeoka et al., 2001), than greenhouse-grown tomatoes, which are reported to contain between 0.1 and 10.8 mg/100 g WM (Leonardi et al., 2000). Arias et al. (2000) showed that the lycopene content of vine-ripened tomatoes was 33% higher than when ripened off the vine. Bramley (2000) reports that lycopene values for fresh tomatoes range from 8.8 to 42.0 mg/100 g WM. Tomatoes also contain a considerable amount of ascorbic acid; the New Zealand Food Composition Tables (Burlingame et al., 1993) give a value of 23.7 mg/100 g WM while a value of 20.0 mg/g WM is recorded in the United States Department of Agriculture data base (USDA, 2001) though many authors record values both above (Oliver, 1967; 25 mg/100 g WM), and below (Davey et al., 2000; 16 mg/100 g WM) this reference value. Ascorbic acid is relatively stable in tomatoes because of the acidic conditions found in the tissue (Davidek et al., 1990). Levels of
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ascorbic acid change according to the maturity of the fruit and the level is highest in the periods of active growth, i.e., during spring and early summer (Fox and Cameron, 1995). Abushita et al. (2000) showed that salad tomatoes grown in field conditions contained between 15 and 21 mg/ 100 g WM, while a range of industrial grades of tomatoes had a mean value of 19 mg/100 g WM. Vine-ripened tomatoes have been shown to contain more ascorbic acid than post-harvest-ripened fruits (Giovanelli et al., 1999). Significant losses of ascorbic acid can occur during any post-harvest storage period and losses can also occur during preparation and cooking of foods, partly due to oxidation and partly to leaching into the water used for cooking (Fox and Cameron, 1995). Davey et al. (2000) suggest that the milder the treatment and the lower the temperature, the better the retention of vitamin C. Tomatoes contain phenolic compounds which also have antioxidant activity (Shahidi and Wanasundara, 1992). Flavonoids are potent antioxidant compounds found in plants that have been found, for instance, to inhibit tumour development (Shahidi and Naczk, 1995). Flavonoids also have a wide range of other potential benefits (Hollman et al., 1996; Nijveldt et al., 2001). Tomato and tomato products are a rich source of flavonols and Stewart et al. (2000) have shown that tomato flavonols were able to withstand industrial processing methods, being detected in a variety of tomato-based products. Flavonol contents have been found to vary according to fruit variety, size, and country of origin, with cherry tomatoes originating from warm, sunny climates containing the highest concentrations. Stewart et al. (2000) also found that 98% of the flavonols detected in tomatoes were found to occur in the skin. So it might be expected that small sized tomatoes will contain higher levels when compared to larger tomatoes. Light has also been shown to be one of the major environmental controls on the synthesis of flavonols (Parr and Bolwell, 2000). Van Boekl and Jongen (1997) suggest that as foodstuffs are a complex matrix of interacting factors, it has become more accepted to measure the antioxidant capacity of foods to give an index of the healthiness of foods. There are a wide range of assays that can be used for assessment of antioxidant activity. The ABTS+ Assay (Miller and Rice-Evans, 1997) measures the ability of the water-soluble antioxidants in a food to decolourize the ABTS+ radical (i.e. scavenge a free radical). Lister et al. (1999) used this assay to measure the antioxidant activity of a range of vegetables including tomatoes. Tomatoes are usually stored to some extent and frequently cooked prior to consumption. Processing usually involves heat treatment and/or homogenization, both of which can disrupt the cellular matrix of tomatoes (Van het Hof et al., 2000a, b). The intactness of the cellular matrix determines the bioavailability of different nutrients. Published data on the effects of heat treatment are not consistent. Van het Hof et al. (2000a, b) found that heat treatment can have a deleterious effect on the micronutrient content of vegetables but at the same time the bioavailability of some nutrients can increase. The production of tomato paste from fresh tomatoes is an example where both homogenization and heat treatment are used and where the bioavailability of carotenoids is enhanced. However, at the same time as bound antioxidants are released by processing (Stahl and Sies, 1992; Tonucci et al., 1995) other, labile antioxidant compounds are being destroyed (Abushita et al., 2000). Data on the effects of processing on the carotenoid content of foods are conflicting owing to several factors. These include the type of carotenoid assessed, the processing conditions used and the constituents of the tomato matrix. Furthermore, processing conditions and modes of
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calculating ‘retention’ during cooking are frequently not comparable from one study to another (Rodriguez-Amaya, 1997). Rodriguez-Amaya (1997) reported that thermal processing increased carotenoid concentration which could be attributed to enzymatic degradation weakening protein– carotenoid aggregates (Stahl and Sies, 1992; Johnson, 2000) and the effect of reducing the moisture content of the processed product. Chen et al. (2000) showed that the antioxidant activity of cultivar Black Persimmon increased during storage at 15 C, while frying in oil or steaming the tomatoes led to a fall in vitamin C content but the overall antioxidant activity increased following these thermal treatments. Home processing of tomatoes can involve soaking slices for a short length of time in a marinade made from oil and vinegar. The tomatoes are consumed raw but the nutrients in the tissue may well be modified by this treatment even though no heat treatment is involved. No data on the effect of this commonly used home processing method is available in the literature. This study was carried out to measure the antioxidant components, and activity and the LAB colour of two cultivars of tomatoes grown commercially in New Zealand and to determine the effect of cooking or treatment with oil and vinegar of these two tomato cultivars on both the colour and the antioxidant content and activity of the processed tomatoes.
2. Materials and methods 2.1. Sample preparation Uniformly sized cultivars of tomatoes were harvested at maturity stage 5 (California Tomato Commission, 2002) in early September 2001. The tomatoes were commercially grown in Canterbury, New Zealand in glasshouses using a flood hydroponic system. Cultivar Excell was harvested and stored individually while Aranca tomatoes were harvested by cutting the vine and harvesting the branch, which consisted of a group of eight tomatoes together. Aranca tomatoes are commonly sold in bunches while Excell are always sold loose. These tomatoes were kept together during storage but were randomized for later processing. Following sampling, the tomatoes were stored in the dark for 4 days in a temperature-controlled incubator at 15 C before processing, to simulate normal storage conditions and the usual time it takes for tomatoes to reach the consumer. Before processing, the tomatoes were washed with distilled water and dried thoroughly. The tomatoes were divided into groups of six and then treated in two different ways. Experiment 1 included boiling 500 g tomatoes with 500 mL water for 15 min, baking (18 min), and frying (4 min in olive oil), with raw tomatoes as the reference sample. In experiment 2, 1 kg of tomatoes was cut into 5 mm slices and these were allowed to soak for 20 min at 20 C in a mixture of 256 mL of marinade consisting of 75% olive oil (Signature Range, 100% Pure Italian Olive Oil, SR Brands Ltd., New Zealand) and 25% white vinegar (4% acetic acid) (D.Y.C., Bluebird Foods Ltd., New Zealand). Further, 1 kg samples of sliced tomatoes were soaked in 195 ml of olive oil or 63 mL white vinegar alone. Colour measurements using a Minolta Chroma Meter CR-200 (Minolta Camera Co. Ltd., Osaka, Japan) were taken from the cut inner surface of the tomatoes before and after processing. The chromameter consisted of an 8 mm diameter measuring area and diffuse illumination/0 viewing was utilized. The CIE L a b readings were calibrated against a standard white tile.
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After processing, the tomato samples were placed on aluminium trays and immediately frozen at 23 C and then freeze-dried. The untreated reference tomatoes were sliced before freezedrying. The freeze-dried powder was stored desiccated at 23 C in sealed plastic bags until analysis could commence. All analyses of the dried tomato powder were carried out in triplicate. Dry matter content of the samples was determined using AOAC method 925.10, total fat content was determined using the Soxhlet extraction method using petroleum ether AOAC method 963.15 (AOAC, 2002). 2.2. Extraction and analysis of antioxidants Freeze-dried tomato powder (1 g) was extracted with 10 mL 80% acetone in a rotary mixer for 4 h in a cool dark room set at 5–7 C. The extracts were centrifuged at 3800 rpm for 10 min and the total phenolic content and antioxidant activity were measured in the supernatant. Total phenolic content was measured using the Folin–Ciocalteu method (Spanos and Wrolstad, 1990). The absorbance was measured at 765 nm and calculated as gallic acid equivalents (mg GAE/100 g DM). The antioxidant activity of the acetone extracts was measured using the modified ABTS+ method (Miller and Rice-Evans, 1997). Acetone extract (100 mL) was added to a diluted solution of ABTS+ and the absorbance at 734 nm 1 min after addition of the extract to ABTS+ was recorded and compared to a known Trolox (a water-soluble vitamin E analogue) standard curve. The results were then expressed as Trolox equivalent antioxidant activity (mmole TEAC/100 g DM). Ascorbic acid was measured by titration with phenolindo-2, 6-dichlorophenol (DPIP) using a 670 titroprocessor (Metrohm, Switzerland). Freeze-dried tomato powder (0.2 g) was mixed with 40 mL of a buffer solution made up of 1 g/L oxalic acid and 4 g/L sodium acetate anhydrous. This was titrated against a solution containing 295 mg/L DPIP and 100 mg/L sodium bicarbonate. The autotitrator was calibrated using standard ascorbic acid and the results expressed as mg/100 g DM. Lycopene was extracted in the dark from 100 mg of freeze-dried powder using 3 mL of heptane for 10 min at 45 C, then a further 30 min on a rotary mixer at 5–7 C. After removal of the supernatant, a further 3 mL heptane was added and mixed vigorously for 1 min before this supernatant was pooled with the first. A further 3 mL of heptane was added and the procedure performed once again. The absorbance of the combined heptane extract was measured at 470 nm and the concentration of lycopene was calculated using the extinction coefficient (3450 mol/cm) (Van het Hof et al., 2000a, b). 2.3. Statistical analysis The data are presented as the mean of three determinations7standard deviation. Analysis of variance was used to determine significance of differences between cultivars and treatment effects using Minitab (Version 13.1).
3. Results Both tomato cultivars were grown in green houses using a commercial hydroponic system to feed the plants. The cultivar Aranca tomatoes were harvested by cutting the vine and allowing
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them to ripen for 4 days in the dark while attached to a small piece of vine. Aranca tomatoes had a mean weight of 52.4 g and a dry matter content of 8.3 g/100 g WM after 4 days storage on the vine. In contrast, cultivar Excell tomatoes, which were harvested and stored separately, had a mean weight of 70.7 g and a lower dry matter content (5.3 g/100 g WM). 3.1. Cooking The two cultivars of tomatoes were cooked in three different ways, boiled, baked or fried. The cooking time, cooking temperatures and the resulting dry matter and fat contents of the two cultivars of tomatoes are shown in Table 1. Cooking resulted in a loss of moisture from both cultivars and for the fried tomatoes a large increase in the total fat content from the cooking oil. The concentrations of ascorbic acid, total phenolics, lycopene and antioxidant activity of the cooked tomatoes are shown in Table 2 compared to the values for the raw tomato. There was a significant difference (Po0:001) in the ascorbic acid, total phenolics and antioxidant activity between the two cultivars in their raw state. Aranca, the smaller tomato, contained much higher levels of ascorbic acid and total phenolics when compared to the larger tomato cultivar, Excell. The lycopene content of the raw Excell tomatoes was significantly higher (Po0:01) than the raw Aranca tomatoes. Boiling, baking and frying both cultivars of tomatoes resulted in a significant reduction (Po0:01) in the ascorbic acid, total phenolic and lycopene contents when compared to the respective raw cultivar. Although a reduction in the antioxidant activity appeared to occur as a result of cooking the tomatoes, statistical analysis showed that the changes were not significantly different from the original raw tomato activity. Cooking both cultivars of tomatoes resulted in a significant (Po0:05) loss of the red colour (a value); overall there was no significant change in
Table 1 Cooking parameters and composition of the two cultivars of tomatoesa Cooking time (min)
External temperature ( C)
Internal temperature ( C)
Raw Aranca Excell
— —
— —
— —
8.3 5.3
2.2 3.2
Boiled Aranca Excell
15 15
100 100
83 83
11.7 5.2
1.9 3.8
Baked Aranca Excell
18 18
200 200
87 87
11.0 6.2
2.0 3.5
Fried Aranca Excell
4 4
110 110
75 75
11.8 6.7
33.0 39.7
a
Dry matter content (g/100 g wet matter)
Values are the means of three measurements (except for the cooking time and temperatures).
Fat content (g/100 g wet matter)
Cultivar/cooking method
a =b
Aranca Raw Boiled Baked Fried
300.879.2 227.6711.6 276.873.0 171.070.6
438.675.8 400.7731.5 441.5738.8 307.4717.3
45.672.5 35.572.1 43.671.4 23.673.1
1909.6792.4 1709.5757.6 1760.1792.9 1201.3738.3
42.771.5 44.471.7 43.072.3 43.871.8
24.172.6 24.872.3 23.573.3 17.671.2
20.571.5 23.571.1 24.671.2 24.371.6
1.2 1.1 1.0 0.7
Excell Raw Boiled Baked Fried
176.2713.0 134.171.4 93.274.7 71.272.3
354.83721.5 319.90733.9 312.17730.0 245.38721.3
47.970.4 44.871.8 43.173.1 24.871.0
1400.0737.7 1263.2727.0 1151.3726.1 892.7724.5
40.771.6 46.572.3 43.771.7 44.771.7
23.171.8 19.172.7 22.671.4 15.772.7
16.871.2 19.372.1 21.172.1 16.672.5
1.4 1.0 1.1 0.9
NS
NS
NS
NS
NS
NS NS NS
Source of variation
df Probability
Cultivar 1 Cooking 3 Cultivar cooking 3
NS NS
The results are presented on a dry weight basis as means7standard deviation (n ¼ 3). Chromatic coordinates of the CIE LAB system (L ; brightness; a ; red-green component; b ; yellow-blue component and a =b ratio) are also shown for the two cultivars of tomatoes. Significance: NS=not significant; Po0:05; Po0:01; Po0:001:
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Ascorbic acid Total phenolics Lycopene Antioxidant activity CIE LAB coordinates (mg/100 g DM) (mg GAE/100 g DM) (mg/100 g DM) (mmole TEAC/ 100 g DM a b L
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Table 2 Concentration of ascorbic acid, total phenolics, lycopene and antioxidant activity for raw, boiled, baked and fried cultivars of Aranca and Excell
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the a =b ratio, except when the tomatoes were fried. By far, frying seemed to cause the largest loss of nutrients and antioxidant activity and this might be expected as higher temperatures are used for this process. The effect of boiling was similar to the effect of baking. Cooking water was not discarded when the tomatoes were boiled so no leaching losses of nutrients occurred. The largest loss of antioxidant compounds was that of ascorbic acid which occurred when the tomatoes were fried (a 43% and 60% reduction, respectively, for Aranca and Excell). When the tomatoes were boiled the level fell by only 24% for both cultivars. Baking gave a decrease of 8% and 47%, respectively. 3.2. Marinade treatment The two cultivars of tomatoes were soaked in oil and vinegar or vinegar and oil separately. The dry matter content of the two cultivars of tomato was not affected by these treatments but soaking in oil or in oil and vinegar resulted in a large increase in the fat content of the tissue of both cultivars of tomatoes (Table 3). The data presented in Table 4 are presented on the original dry matter content of the tomato tissue allowing for the increase in oil from the olive oil in the marinade treatment. Overall soaking in oil, vinegar and oil/vinegar significantly decreased (Po0:001) the antioxidant content and antioxidant activity of the processed tomato samples (Table 4). The losses were most evident for the oil/vinegar treated tomato samples. In particular, the ascorbic acid content of these samples was lower when compared to the untreated samples. Of the three different treatments, soaking in vinegar showed the smallest decrease in all analyses and these values were similar to the untreated tomato samples. Soaking tomatoes in vinegar alone for 20 min did not significantly reduce the lycopene content of the treated tomatoes.
Table 3 Composition of the two cultivars of tomatoes following treatment in oil and vinegara Dry matter content (g/100 g wet matter)
Fat content (g/100 g wet matter)
Raw Aranca Excell
8.3 5.3
2.2 3.2
Oil/vinegar Aranca Excell
7.6 4.8
50.2 42.0
Oil Aranca Excell
8.7 5.1
33.3 40.7
Vinegar Aranca Excell
7.6 5.2
2.2 3.1
a
Values are the means of three measurements.
a =b
Aranca Raw Oil/vinegar Oil Vinegar
300.879.2 142.371.4 195.577.1 244.475.4
438.675.8 230.6711.3 296.178.7 359.7746.0
45.672.5 24.076.9 27.078.5 49.872.0
1909.6792.4 996.7747.9 1354.1763.4 1747.6755.7
42.771.5 45.172.9 44.172.0 44.873.2
24.172.6 22.373.0 23.672.3 21.472.6
20.571.5 19.571.2 19.771.4 21.371.4
1.2 1.1 1.2 1.0
Excell Raw Oil/vinegar Oil Vinegar
176.2713.0 84.172.5 104.974.9 154.576.7
354.8721.5 228.8728.1 234.0716.9 344.3713.5
47.970.4 32.270.5 29.870.3 44.371.7
1400.0737.7 859.5715.7 922.5719.8 1395.2723.3
40.771.6 43.071.4 42.771.2 40.771.9
23.171.8 29.571.2 28.172.6 28.771.7
16.871.2 19.570.8 17.470.8 19.771.4
1.4 1.5 1.6 1.5
NS
NS NS NS NS NS NS
NS NS NS NS NS NS
NS NS NS NS NS NS
Source of variation Cultivar Oil Vinegar Oil vinegar Cultivar oil Cultivar vinegar
df Probability 1 1 1 1 1 1
NS NS
NS NS NS NS
NS
NS NS NS NS NS
The results are presented on a dry weight basis as means7standard deviation (n ¼ 3). CHROMATIC coordinates of the CIE LAB system (L ; brightness; a ; red-green component; b ; yellow-blue component and a =b ratio) are also shown for the two cultivars of tomatoes. Significance: NS=not significant; Po0:05; Po0:01; Po0:001:
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Ascorbic acid Total phenolics Lycopene Antioxidant activity CIE LAB coordinates (mg/100 g DM) (mg GAE/100 g DM) (mg/100 g DM) (mmole TEAC/ 100 g DM) L a b
Cultivar/treatment
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Table 4 Concentration of ascorbic acid, total phenolics, lycopene and antioxidant activity after soaking cultivars Aranca and Excell in either oil or vinegar
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The colour readings by the CIE LAB system on the soaked tomato samples of both cultivars showed little changes in L and a =b values with only small changes in the a values when the raw tomato cultivars were soaked in oil and vinegar separately (Table 4).
4. Discussion The tomatoes used in this experiment were harvested at the beginning of September, which is early spring in New Zealand. The same cultivars, Aranca and Excell, were harvested and analysed by Molyneux (2001) in autumn (April–May). Although the tomatoes were taken from the same source for both trials and picked at the same maturity, they showed different antioxidant activity. Molyneux’s values were 26% and 57% higher, respectively, for Aranca and Excell, which could be due to different picking-times. The earlier study by Molyneux (2001) did show that cultivars of tomatoes which are smaller at the same stage of ripeness contained higher levels of antioxidant nutrients. This study has confirmed that Aranca does contain higher levels of ascorbic acid, total phenolics and lycopene, as well as having a higher antioxidant activity overall. The present study went on to show that cooking does have a deleterious effect on ascorbic acid, total phenolics, lycopene content and antioxidant activity for both cultivars. These results are in agreement with studies by Van het Hof et al. (2000a). Frying caused the largest loss of antioxidants and antioxidant activity. However, not all processed tomatoes contain less antioxidants, as several studies have shown that lycopene becomes more extractable following processing and cooking. A number of studies on the thermal stability of carotenoids in processed fruits and vegetables found that lycopene and b-carotene were relatively heat-resistant (Khachik et al., 1992). Moreover, Nguyen and Schwartz (1999) argue that, in the case of lycopene, food processing to make sauce and pastes is in fact a value-added step as more lycopene becomes available following thermal treatment. The intactness of the cellular matrix determines the availability of different nutrients and increased availability could be due to the fact that the nutrients get detached or extracted from their structures. Homogenization and heat treatment disrupt cell membranes, and heat treatment also has been suggested to further disrupt the protein– carotenoid complexes (Erdman et al., 1988). The production of tomato paste from fresh tomatoes is an example where both homogenization and heat treatment are used and where the availability of carotenoids is enhanced (Van het Hof et al., 2000a). Moreover, all foods are complex mixtures of components that have the potential to react and interact with each other and thereby change the nutritive value of the food (Davey et al., 2000). Food processing and storage can make these interactions even more complicated (Nicoli et al., 1999). Nicoli et al. (1999) went on to suggest that heat treatment could lead to the formation of new compounds with increased antioxidant properties. Lycopene levels for Aranca and Excell (3.9 and 2.6 mg/100 g WM) agreed with Leonardi et al. (2000), who reported greenhouse-grown tomatoes to contain between 0.1 and 10.8 mg lycopene/ 100 g WM. Aranca contained 25.5 and Excell 9.6 mg ascorbic acid/100 g WM and these are reasonably comparable to previously reported values for the same cultivars of tomatoes harvested in autumn (Molyneux, 2001). Since lycopene is responsible for the red colour of tomatoes and colour is an index of quality for tomato products, the loss of lycopene throughout the production process and during storage has
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always been important (Abushita et al., 2000). Oxidation is the main cause of lycopene loss during processing (Anguelova and Warthsen, 2000). Lycopene levels decreased the most for the fried samples while boiling and baking did not cause an appreciable change in these samples. Raw Aranca had a similar red colour (a ) to Excell even though the level of lycopene in Excell was significantly higher (Po0:01). The L and b values for Aranca increased in the cooked samples whereas the a value decreased. This is not in agreement with Arias et al. (2000) who found that tomatoes ripened on the vine, like Aranca, have significantly higher a and lower L values. An increase was also seen on the L value for Excell, and a small decrease in the a values. A higher L value means a darker colour. 5. Conclusions This study has confirmed that tomatoes contain significant amounts of ascorbic acid, lycopene and phenolics, and that they have high levels of antioxidant activity. Processing tomatoes in different ways tends to reduce the nutrients compared to the raw tomato, but they still contain useful amounts when consumed in this way. Further studies are required to fully understand the role that fresh and processed tomatoes have on the diet and their role in promoting health and preventing human disease. Acknowledgements The authors wish to acknowledge Dr. Roger Andersson, Plant Products, Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden for his assistance with statistical analysis of the data and the support and encouragement given by Dr. Paresh Dutta, Food Chemistry, Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden. References Abushita, A.A., Daood, H.G., Biacs, P.A., 2000. Change in carotenoids and antioxidant vitamins in tomato as a function of varietal and technological factors. Journal of Agricultural and Food Chemistry 48, 2075–2081. American Cancer Society, 1984. Nutrition and cancer: caution and prevention. An American Cancer Society Special Report. CA: A Cancer Journal for Clinicians 34, 5–10. Anguelova, T., Warthsen, J., 2000. Lycopene stability in tomato powders. Journal of Food Science 65, 67–70. AOAC, 2002. Official Methods of Analysis of AOAC International, 17th Edition. AOAC International, Gaithersburg, MD, USA. Arias, R., Lee, T.-C., Logendra, L., Janes, H., 2000. Correlation of lycopene measured by HPLC with the L, a, b colour readings of a hydroponic tomato and the relationship of maturity with colour and lycopene content. Journal of Agricultural and Food Chemistry 48, 1697–1702. Beecher, G.R., 1998. Nutrient content of tomatoes and tomato products, Nutrient content of tomatoes. Proceedings of the Society of Experimental Biology and Medicine 218, 98–100. Block, G., Patterson, B., Subar, A., 1992. Fruit, vegetables, and cancer prevention: A review of the epidemiological evidence. Nutrition and Cancer 18, 1–29. Bramley, P.M., 2000. Is lycopene beneficial to health? Phytochemistry 54, 223–236.
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JOURNAL OF FOOD COMPOSITION AND ANALYSIS
Journal of Food Composition and Analysis 17 (2004) 635–647 www.elsevier.com/locate/jfca
Original article
Investigation of the antioxidant properties of tomatoes after processing E. Sahlina,b, G.P. Savagea,*, C.E. Listerc b
a Food Group, AFSD, Lincoln University, P.O. Box 84, Canterbury, New Zealand Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden c Nutrition and Health Team, Crop & Food Research Ltd., Christchurch, New Zealand
Received 13 January 2003; received in revised form 30 September 2003; accepted 2 October 2003
Abstract A study was made of the antioxidant content, activity and colour of two New Zealand commercially grown tomatoes (Lycopersicon esculentum L. var. Excell and Aranca) cultivated in glasshouses using hydroponic techniques. Excell tomatoes were harvested and stored individually while cultivar Aranca was harvested by cutting the vine, which consisted of a group of eight tomatoes. Both cultivars of tomatoes were stored in the dark for 4 days at 15 C to simulate normal pre-purchase storing conditions. The antioxidant content of the raw tomatoes after 4 days of storage were markedly different while the CIE LAB colour values of the cut inner surfaces of the two cultivars were similar. After 4 days storage, subsamples of each cultivar were either boiled, baked or fried, and analysis of CIE colour, ascorbic acid, total phenolics, lycopene and antioxidant activity (using the ABTS assay) was undertaken. Boiling and baking had a relatively small effect on the ascorbic, total phenolic, lycopene and antioxidant activity of the two cultivars while frying significantly reduced (Po0:001) the ascorbic, total phenolic and lycopene contents of the two cultivars. Chromatic colour analysis showed that both cultivars became significantly (Po0:001) darker and less red after cooking by all methods. In a following experiment, the two cultivars of tomatoes were sliced and allowed to soak for 20 min in a mixture of olive oil and white vinegar, or olive oil and white vinegar separately. CIE colour of the two cultivars showed no change after processing but the treatments of oil and vinegar separately and together reduced (Po0:05) the red component of the colour. Treating the two cultivars of tomatoes with the oil and vinegar mixture resulted in a significant reduction (Po0:001) in the ascorbic acid, total phenolic and antioxidant activity of the tomatoes. Treatment with oil significantly reduced the amount of lycopene that could be extracted from the tomatoes while treatment with vinegar had no effect. r 2003 Elsevier Inc. All rights reserved. Keywords: Tomato; Lycopersicon esculentum L; Antioxidant activity; Lycopene; Total phenolics; Ascorbic acid; Lab colour; Storage
*Corresponding author. Tel.: +64-33253-803; fax: +64-33253-851. E-mail address: [email protected] (G.P. Savage). 0889-1575/$ - see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2003.10.003
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1. Introduction Tomatoes are one of the most widely used and versatile vegetable crops. They are consumed fresh and are also used to manufacture a wide range of processed products (Madhavi and Salunkhe, 1998). Tomatoes and tomato products are rich in health-related food components as they are good sources of carotenoids (in particular, lycopene), ascorbic acid (vitamin C), vitamin E, folate, flavonoids and potassium (Beecher, 1998; Leonardi et al., 2000). Other constituents are protein and dietary fibre (Davies and Hobson, 1981). The chemical composition of the tomato fruit depends on such factors as cultivar, maturity and the environmental conditions in which they are grown (Davies and Hobson, 1981; Giovanelli et al., 1999; Abushita et al., 2000; Thompson et al., 2000). It has been shown that ripening processes and storage temperatures can severely affect the final nutrient composition of the fruit (Madhavi and Salunkhe, 1998). However, even though many researchers comment that tomato quality depends on the cultivar, growing conditions and ripening position on the vine, limited data is available to support these assertions, particularly in greenhouse-grown tomatoes. Regular consumption of tomatoes has been correlated with a reduced risk of various types of cancer (Franceschi et al., 1994; Gerster, 1997; Weisburger, 1998) and heart diseases (Lavelli et al., 2000; Pandey et al., 1995). These positive effects are believed to be attributable to the antioxidants, particularly the carotenoids, flavonoids, lycopene and b-carotene (Lavelli et al., 2000). Furthermore, recommendations to increase daily intake of fruits and vegetables rich in nutrients such as carotenoids and vitamins C and E to lower the risk of cancer and cardiovascular diseases have been made by the American Cancer Society (1984), Steinmetz and Plotter (1991), Block et al. (1992) and the World Cancer Research Fund (1997). Giovannucci (1999) reviewed a number of epidemiological studies and concluded that the intake of tomato products was consistently associated with a lower risk of a variety of cancers and in particular prostate cancer. The main antioxidants in tomatoes are carotenoids, ascorbic acid and phenolic compounds (Giovanelli et al., 1999). The overall antioxidant activity of tomatoes varies considerably according to the genetic variety, ripening stage and growing conditions (Leonardi et al., 2000). Lycopene is responsible for the red colour of tomatoes (Nguyen and Schwartz, 1999) and is regarded as an antioxidant with high biological activity in the body (Stahl and Sies, 1996). The lycopene content of tomatoes varies considerably between cultivars, stage of maturity and growing conditions. Field-grown tomatoes appear to contain higher levels of lycopene, ranging ! from 5.2 to 23.6 mg/100 g wet matter (WM) (Abushita et al., 2000; Gomez et al., 2001; Takeoka et al., 2001), than greenhouse-grown tomatoes, which are reported to contain between 0.1 and 10.8 mg/100 g WM (Leonardi et al., 2000). Arias et al. (2000) showed that the lycopene content of vine-ripened tomatoes was 33% higher than when ripened off the vine. Bramley (2000) reports that lycopene values for fresh tomatoes range from 8.8 to 42.0 mg/100 g WM. Tomatoes also contain a considerable amount of ascorbic acid; the New Zealand Food Composition Tables (Burlingame et al., 1993) give a value of 23.7 mg/100 g WM while a value of 20.0 mg/g WM is recorded in the United States Department of Agriculture data base (USDA, 2001) though many authors record values both above (Oliver, 1967; 25 mg/100 g WM), and below (Davey et al., 2000; 16 mg/100 g WM) this reference value. Ascorbic acid is relatively stable in tomatoes because of the acidic conditions found in the tissue (Davidek et al., 1990). Levels of
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ascorbic acid change according to the maturity of the fruit and the level is highest in the periods of active growth, i.e., during spring and early summer (Fox and Cameron, 1995). Abushita et al. (2000) showed that salad tomatoes grown in field conditions contained between 15 and 21 mg/ 100 g WM, while a range of industrial grades of tomatoes had a mean value of 19 mg/100 g WM. Vine-ripened tomatoes have been shown to contain more ascorbic acid than post-harvest-ripened fruits (Giovanelli et al., 1999). Significant losses of ascorbic acid can occur during any post-harvest storage period and losses can also occur during preparation and cooking of foods, partly due to oxidation and partly to leaching into the water used for cooking (Fox and Cameron, 1995). Davey et al. (2000) suggest that the milder the treatment and the lower the temperature, the better the retention of vitamin C. Tomatoes contain phenolic compounds which also have antioxidant activity (Shahidi and Wanasundara, 1992). Flavonoids are potent antioxidant compounds found in plants that have been found, for instance, to inhibit tumour development (Shahidi and Naczk, 1995). Flavonoids also have a wide range of other potential benefits (Hollman et al., 1996; Nijveldt et al., 2001). Tomato and tomato products are a rich source of flavonols and Stewart et al. (2000) have shown that tomato flavonols were able to withstand industrial processing methods, being detected in a variety of tomato-based products. Flavonol contents have been found to vary according to fruit variety, size, and country of origin, with cherry tomatoes originating from warm, sunny climates containing the highest concentrations. Stewart et al. (2000) also found that 98% of the flavonols detected in tomatoes were found to occur in the skin. So it might be expected that small sized tomatoes will contain higher levels when compared to larger tomatoes. Light has also been shown to be one of the major environmental controls on the synthesis of flavonols (Parr and Bolwell, 2000). Van Boekl and Jongen (1997) suggest that as foodstuffs are a complex matrix of interacting factors, it has become more accepted to measure the antioxidant capacity of foods to give an index of the healthiness of foods. There are a wide range of assays that can be used for assessment of antioxidant activity. The ABTS+ Assay (Miller and Rice-Evans, 1997) measures the ability of the water-soluble antioxidants in a food to decolourize the ABTS+ radical (i.e. scavenge a free radical). Lister et al. (1999) used this assay to measure the antioxidant activity of a range of vegetables including tomatoes. Tomatoes are usually stored to some extent and frequently cooked prior to consumption. Processing usually involves heat treatment and/or homogenization, both of which can disrupt the cellular matrix of tomatoes (Van het Hof et al., 2000a, b). The intactness of the cellular matrix determines the bioavailability of different nutrients. Published data on the effects of heat treatment are not consistent. Van het Hof et al. (2000a, b) found that heat treatment can have a deleterious effect on the micronutrient content of vegetables but at the same time the bioavailability of some nutrients can increase. The production of tomato paste from fresh tomatoes is an example where both homogenization and heat treatment are used and where the bioavailability of carotenoids is enhanced. However, at the same time as bound antioxidants are released by processing (Stahl and Sies, 1992; Tonucci et al., 1995) other, labile antioxidant compounds are being destroyed (Abushita et al., 2000). Data on the effects of processing on the carotenoid content of foods are conflicting owing to several factors. These include the type of carotenoid assessed, the processing conditions used and the constituents of the tomato matrix. Furthermore, processing conditions and modes of
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calculating ‘retention’ during cooking are frequently not comparable from one study to another (Rodriguez-Amaya, 1997). Rodriguez-Amaya (1997) reported that thermal processing increased carotenoid concentration which could be attributed to enzymatic degradation weakening protein– carotenoid aggregates (Stahl and Sies, 1992; Johnson, 2000) and the effect of reducing the moisture content of the processed product. Chen et al. (2000) showed that the antioxidant activity of cultivar Black Persimmon increased during storage at 15 C, while frying in oil or steaming the tomatoes led to a fall in vitamin C content but the overall antioxidant activity increased following these thermal treatments. Home processing of tomatoes can involve soaking slices for a short length of time in a marinade made from oil and vinegar. The tomatoes are consumed raw but the nutrients in the tissue may well be modified by this treatment even though no heat treatment is involved. No data on the effect of this commonly used home processing method is available in the literature. This study was carried out to measure the antioxidant components, and activity and the LAB colour of two cultivars of tomatoes grown commercially in New Zealand and to determine the effect of cooking or treatment with oil and vinegar of these two tomato cultivars on both the colour and the antioxidant content and activity of the processed tomatoes.
2. Materials and methods 2.1. Sample preparation Uniformly sized cultivars of tomatoes were harvested at maturity stage 5 (California Tomato Commission, 2002) in early September 2001. The tomatoes were commercially grown in Canterbury, New Zealand in glasshouses using a flood hydroponic system. Cultivar Excell was harvested and stored individually while Aranca tomatoes were harvested by cutting the vine and harvesting the branch, which consisted of a group of eight tomatoes together. Aranca tomatoes are commonly sold in bunches while Excell are always sold loose. These tomatoes were kept together during storage but were randomized for later processing. Following sampling, the tomatoes were stored in the dark for 4 days in a temperature-controlled incubator at 15 C before processing, to simulate normal storage conditions and the usual time it takes for tomatoes to reach the consumer. Before processing, the tomatoes were washed with distilled water and dried thoroughly. The tomatoes were divided into groups of six and then treated in two different ways. Experiment 1 included boiling 500 g tomatoes with 500 mL water for 15 min, baking (18 min), and frying (4 min in olive oil), with raw tomatoes as the reference sample. In experiment 2, 1 kg of tomatoes was cut into 5 mm slices and these were allowed to soak for 20 min at 20 C in a mixture of 256 mL of marinade consisting of 75% olive oil (Signature Range, 100% Pure Italian Olive Oil, SR Brands Ltd., New Zealand) and 25% white vinegar (4% acetic acid) (D.Y.C., Bluebird Foods Ltd., New Zealand). Further, 1 kg samples of sliced tomatoes were soaked in 195 ml of olive oil or 63 mL white vinegar alone. Colour measurements using a Minolta Chroma Meter CR-200 (Minolta Camera Co. Ltd., Osaka, Japan) were taken from the cut inner surface of the tomatoes before and after processing. The chromameter consisted of an 8 mm diameter measuring area and diffuse illumination/0 viewing was utilized. The CIE L a b readings were calibrated against a standard white tile.
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After processing, the tomato samples were placed on aluminium trays and immediately frozen at 23 C and then freeze-dried. The untreated reference tomatoes were sliced before freezedrying. The freeze-dried powder was stored desiccated at 23 C in sealed plastic bags until analysis could commence. All analyses of the dried tomato powder were carried out in triplicate. Dry matter content of the samples was determined using AOAC method 925.10, total fat content was determined using the Soxhlet extraction method using petroleum ether AOAC method 963.15 (AOAC, 2002). 2.2. Extraction and analysis of antioxidants Freeze-dried tomato powder (1 g) was extracted with 10 mL 80% acetone in a rotary mixer for 4 h in a cool dark room set at 5–7 C. The extracts were centrifuged at 3800 rpm for 10 min and the total phenolic content and antioxidant activity were measured in the supernatant. Total phenolic content was measured using the Folin–Ciocalteu method (Spanos and Wrolstad, 1990). The absorbance was measured at 765 nm and calculated as gallic acid equivalents (mg GAE/100 g DM). The antioxidant activity of the acetone extracts was measured using the modified ABTS+ method (Miller and Rice-Evans, 1997). Acetone extract (100 mL) was added to a diluted solution of ABTS+ and the absorbance at 734 nm 1 min after addition of the extract to ABTS+ was recorded and compared to a known Trolox (a water-soluble vitamin E analogue) standard curve. The results were then expressed as Trolox equivalent antioxidant activity (mmole TEAC/100 g DM). Ascorbic acid was measured by titration with phenolindo-2, 6-dichlorophenol (DPIP) using a 670 titroprocessor (Metrohm, Switzerland). Freeze-dried tomato powder (0.2 g) was mixed with 40 mL of a buffer solution made up of 1 g/L oxalic acid and 4 g/L sodium acetate anhydrous. This was titrated against a solution containing 295 mg/L DPIP and 100 mg/L sodium bicarbonate. The autotitrator was calibrated using standard ascorbic acid and the results expressed as mg/100 g DM. Lycopene was extracted in the dark from 100 mg of freeze-dried powder using 3 mL of heptane for 10 min at 45 C, then a further 30 min on a rotary mixer at 5–7 C. After removal of the supernatant, a further 3 mL heptane was added and mixed vigorously for 1 min before this supernatant was pooled with the first. A further 3 mL of heptane was added and the procedure performed once again. The absorbance of the combined heptane extract was measured at 470 nm and the concentration of lycopene was calculated using the extinction coefficient (3450 mol/cm) (Van het Hof et al., 2000a, b). 2.3. Statistical analysis The data are presented as the mean of three determinations7standard deviation. Analysis of variance was used to determine significance of differences between cultivars and treatment effects using Minitab (Version 13.1).
3. Results Both tomato cultivars were grown in green houses using a commercial hydroponic system to feed the plants. The cultivar Aranca tomatoes were harvested by cutting the vine and allowing
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them to ripen for 4 days in the dark while attached to a small piece of vine. Aranca tomatoes had a mean weight of 52.4 g and a dry matter content of 8.3 g/100 g WM after 4 days storage on the vine. In contrast, cultivar Excell tomatoes, which were harvested and stored separately, had a mean weight of 70.7 g and a lower dry matter content (5.3 g/100 g WM). 3.1. Cooking The two cultivars of tomatoes were cooked in three different ways, boiled, baked or fried. The cooking time, cooking temperatures and the resulting dry matter and fat contents of the two cultivars of tomatoes are shown in Table 1. Cooking resulted in a loss of moisture from both cultivars and for the fried tomatoes a large increase in the total fat content from the cooking oil. The concentrations of ascorbic acid, total phenolics, lycopene and antioxidant activity of the cooked tomatoes are shown in Table 2 compared to the values for the raw tomato. There was a significant difference (Po0:001) in the ascorbic acid, total phenolics and antioxidant activity between the two cultivars in their raw state. Aranca, the smaller tomato, contained much higher levels of ascorbic acid and total phenolics when compared to the larger tomato cultivar, Excell. The lycopene content of the raw Excell tomatoes was significantly higher (Po0:01) than the raw Aranca tomatoes. Boiling, baking and frying both cultivars of tomatoes resulted in a significant reduction (Po0:01) in the ascorbic acid, total phenolic and lycopene contents when compared to the respective raw cultivar. Although a reduction in the antioxidant activity appeared to occur as a result of cooking the tomatoes, statistical analysis showed that the changes were not significantly different from the original raw tomato activity. Cooking both cultivars of tomatoes resulted in a significant (Po0:05) loss of the red colour (a value); overall there was no significant change in
Table 1 Cooking parameters and composition of the two cultivars of tomatoesa Cooking time (min)
External temperature ( C)
Internal temperature ( C)
Raw Aranca Excell
— —
— —
— —
8.3 5.3
2.2 3.2
Boiled Aranca Excell
15 15
100 100
83 83
11.7 5.2
1.9 3.8
Baked Aranca Excell
18 18
200 200
87 87
11.0 6.2
2.0 3.5
Fried Aranca Excell
4 4
110 110
75 75
11.8 6.7
33.0 39.7
a
Dry matter content (g/100 g wet matter)
Values are the means of three measurements (except for the cooking time and temperatures).
Fat content (g/100 g wet matter)
Cultivar/cooking method
a =b
Aranca Raw Boiled Baked Fried
300.879.2 227.6711.6 276.873.0 171.070.6
438.675.8 400.7731.5 441.5738.8 307.4717.3
45.672.5 35.572.1 43.671.4 23.673.1
1909.6792.4 1709.5757.6 1760.1792.9 1201.3738.3
42.771.5 44.471.7 43.072.3 43.871.8
24.172.6 24.872.3 23.573.3 17.671.2
20.571.5 23.571.1 24.671.2 24.371.6
1.2 1.1 1.0 0.7
Excell Raw Boiled Baked Fried
176.2713.0 134.171.4 93.274.7 71.272.3
354.83721.5 319.90733.9 312.17730.0 245.38721.3
47.970.4 44.871.8 43.173.1 24.871.0
1400.0737.7 1263.2727.0 1151.3726.1 892.7724.5
40.771.6 46.572.3 43.771.7 44.771.7
23.171.8 19.172.7 22.671.4 15.772.7
16.871.2 19.372.1 21.172.1 16.672.5
1.4 1.0 1.1 0.9
NS
NS
NS
NS
NS
NS NS NS
Source of variation
df Probability
Cultivar 1 Cooking 3 Cultivar cooking 3
NS NS
The results are presented on a dry weight basis as means7standard deviation (n ¼ 3). Chromatic coordinates of the CIE LAB system (L ; brightness; a ; red-green component; b ; yellow-blue component and a =b ratio) are also shown for the two cultivars of tomatoes. Significance: NS=not significant; Po0:05; Po0:01; Po0:001:
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Ascorbic acid Total phenolics Lycopene Antioxidant activity CIE LAB coordinates (mg/100 g DM) (mg GAE/100 g DM) (mg/100 g DM) (mmole TEAC/ 100 g DM a b L
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Table 2 Concentration of ascorbic acid, total phenolics, lycopene and antioxidant activity for raw, boiled, baked and fried cultivars of Aranca and Excell
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the a =b ratio, except when the tomatoes were fried. By far, frying seemed to cause the largest loss of nutrients and antioxidant activity and this might be expected as higher temperatures are used for this process. The effect of boiling was similar to the effect of baking. Cooking water was not discarded when the tomatoes were boiled so no leaching losses of nutrients occurred. The largest loss of antioxidant compounds was that of ascorbic acid which occurred when the tomatoes were fried (a 43% and 60% reduction, respectively, for Aranca and Excell). When the tomatoes were boiled the level fell by only 24% for both cultivars. Baking gave a decrease of 8% and 47%, respectively. 3.2. Marinade treatment The two cultivars of tomatoes were soaked in oil and vinegar or vinegar and oil separately. The dry matter content of the two cultivars of tomato was not affected by these treatments but soaking in oil or in oil and vinegar resulted in a large increase in the fat content of the tissue of both cultivars of tomatoes (Table 3). The data presented in Table 4 are presented on the original dry matter content of the tomato tissue allowing for the increase in oil from the olive oil in the marinade treatment. Overall soaking in oil, vinegar and oil/vinegar significantly decreased (Po0:001) the antioxidant content and antioxidant activity of the processed tomato samples (Table 4). The losses were most evident for the oil/vinegar treated tomato samples. In particular, the ascorbic acid content of these samples was lower when compared to the untreated samples. Of the three different treatments, soaking in vinegar showed the smallest decrease in all analyses and these values were similar to the untreated tomato samples. Soaking tomatoes in vinegar alone for 20 min did not significantly reduce the lycopene content of the treated tomatoes.
Table 3 Composition of the two cultivars of tomatoes following treatment in oil and vinegara Dry matter content (g/100 g wet matter)
Fat content (g/100 g wet matter)
Raw Aranca Excell
8.3 5.3
2.2 3.2
Oil/vinegar Aranca Excell
7.6 4.8
50.2 42.0
Oil Aranca Excell
8.7 5.1
33.3 40.7
Vinegar Aranca Excell
7.6 5.2
2.2 3.1
a
Values are the means of three measurements.
a =b
Aranca Raw Oil/vinegar Oil Vinegar
300.879.2 142.371.4 195.577.1 244.475.4
438.675.8 230.6711.3 296.178.7 359.7746.0
45.672.5 24.076.9 27.078.5 49.872.0
1909.6792.4 996.7747.9 1354.1763.4 1747.6755.7
42.771.5 45.172.9 44.172.0 44.873.2
24.172.6 22.373.0 23.672.3 21.472.6
20.571.5 19.571.2 19.771.4 21.371.4
1.2 1.1 1.2 1.0
Excell Raw Oil/vinegar Oil Vinegar
176.2713.0 84.172.5 104.974.9 154.576.7
354.8721.5 228.8728.1 234.0716.9 344.3713.5
47.970.4 32.270.5 29.870.3 44.371.7
1400.0737.7 859.5715.7 922.5719.8 1395.2723.3
40.771.6 43.071.4 42.771.2 40.771.9
23.171.8 29.571.2 28.172.6 28.771.7
16.871.2 19.570.8 17.470.8 19.771.4
1.4 1.5 1.6 1.5
NS
NS NS NS NS NS NS
NS NS NS NS NS NS
NS NS NS NS NS NS
Source of variation Cultivar Oil Vinegar Oil vinegar Cultivar oil Cultivar vinegar
df Probability 1 1 1 1 1 1
NS NS
NS NS NS NS
NS
NS NS NS NS NS
The results are presented on a dry weight basis as means7standard deviation (n ¼ 3). CHROMATIC coordinates of the CIE LAB system (L ; brightness; a ; red-green component; b ; yellow-blue component and a =b ratio) are also shown for the two cultivars of tomatoes. Significance: NS=not significant; Po0:05; Po0:01; Po0:001:
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Ascorbic acid Total phenolics Lycopene Antioxidant activity CIE LAB coordinates (mg/100 g DM) (mg GAE/100 g DM) (mg/100 g DM) (mmole TEAC/ 100 g DM) L a b
Cultivar/treatment
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Table 4 Concentration of ascorbic acid, total phenolics, lycopene and antioxidant activity after soaking cultivars Aranca and Excell in either oil or vinegar
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The colour readings by the CIE LAB system on the soaked tomato samples of both cultivars showed little changes in L and a =b values with only small changes in the a values when the raw tomato cultivars were soaked in oil and vinegar separately (Table 4).
4. Discussion The tomatoes used in this experiment were harvested at the beginning of September, which is early spring in New Zealand. The same cultivars, Aranca and Excell, were harvested and analysed by Molyneux (2001) in autumn (April–May). Although the tomatoes were taken from the same source for both trials and picked at the same maturity, they showed different antioxidant activity. Molyneux’s values were 26% and 57% higher, respectively, for Aranca and Excell, which could be due to different picking-times. The earlier study by Molyneux (2001) did show that cultivars of tomatoes which are smaller at the same stage of ripeness contained higher levels of antioxidant nutrients. This study has confirmed that Aranca does contain higher levels of ascorbic acid, total phenolics and lycopene, as well as having a higher antioxidant activity overall. The present study went on to show that cooking does have a deleterious effect on ascorbic acid, total phenolics, lycopene content and antioxidant activity for both cultivars. These results are in agreement with studies by Van het Hof et al. (2000a). Frying caused the largest loss of antioxidants and antioxidant activity. However, not all processed tomatoes contain less antioxidants, as several studies have shown that lycopene becomes more extractable following processing and cooking. A number of studies on the thermal stability of carotenoids in processed fruits and vegetables found that lycopene and b-carotene were relatively heat-resistant (Khachik et al., 1992). Moreover, Nguyen and Schwartz (1999) argue that, in the case of lycopene, food processing to make sauce and pastes is in fact a value-added step as more lycopene becomes available following thermal treatment. The intactness of the cellular matrix determines the availability of different nutrients and increased availability could be due to the fact that the nutrients get detached or extracted from their structures. Homogenization and heat treatment disrupt cell membranes, and heat treatment also has been suggested to further disrupt the protein– carotenoid complexes (Erdman et al., 1988). The production of tomato paste from fresh tomatoes is an example where both homogenization and heat treatment are used and where the availability of carotenoids is enhanced (Van het Hof et al., 2000a). Moreover, all foods are complex mixtures of components that have the potential to react and interact with each other and thereby change the nutritive value of the food (Davey et al., 2000). Food processing and storage can make these interactions even more complicated (Nicoli et al., 1999). Nicoli et al. (1999) went on to suggest that heat treatment could lead to the formation of new compounds with increased antioxidant properties. Lycopene levels for Aranca and Excell (3.9 and 2.6 mg/100 g WM) agreed with Leonardi et al. (2000), who reported greenhouse-grown tomatoes to contain between 0.1 and 10.8 mg lycopene/ 100 g WM. Aranca contained 25.5 and Excell 9.6 mg ascorbic acid/100 g WM and these are reasonably comparable to previously reported values for the same cultivars of tomatoes harvested in autumn (Molyneux, 2001). Since lycopene is responsible for the red colour of tomatoes and colour is an index of quality for tomato products, the loss of lycopene throughout the production process and during storage has
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always been important (Abushita et al., 2000). Oxidation is the main cause of lycopene loss during processing (Anguelova and Warthsen, 2000). Lycopene levels decreased the most for the fried samples while boiling and baking did not cause an appreciable change in these samples. Raw Aranca had a similar red colour (a ) to Excell even though the level of lycopene in Excell was significantly higher (Po0:01). The L and b values for Aranca increased in the cooked samples whereas the a value decreased. This is not in agreement with Arias et al. (2000) who found that tomatoes ripened on the vine, like Aranca, have significantly higher a and lower L values. An increase was also seen on the L value for Excell, and a small decrease in the a values. A higher L value means a darker colour. 5. Conclusions This study has confirmed that tomatoes contain significant amounts of ascorbic acid, lycopene and phenolics, and that they have high levels of antioxidant activity. Processing tomatoes in different ways tends to reduce the nutrients compared to the raw tomato, but they still contain useful amounts when consumed in this way. Further studies are required to fully understand the role that fresh and processed tomatoes have on the diet and their role in promoting health and preventing human disease. Acknowledgements The authors wish to acknowledge Dr. Roger Andersson, Plant Products, Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden for his assistance with statistical analysis of the data and the support and encouragement given by Dr. Paresh Dutta, Food Chemistry, Department of Food Science, Swedish University of Agricultural Sciences, Uppsala, Sweden. References Abushita, A.A., Daood, H.G., Biacs, P.A., 2000. Change in carotenoids and antioxidant vitamins in tomato as a function of varietal and technological factors. Journal of Agricultural and Food Chemistry 48, 2075–2081. American Cancer Society, 1984. Nutrition and cancer: caution and prevention. An American Cancer Society Special Report. CA: A Cancer Journal for Clinicians 34, 5–10. Anguelova, T., Warthsen, J., 2000. Lycopene stability in tomato powders. Journal of Food Science 65, 67–70. AOAC, 2002. Official Methods of Analysis of AOAC International, 17th Edition. AOAC International, Gaithersburg, MD, USA. Arias, R., Lee, T.-C., Logendra, L., Janes, H., 2000. Correlation of lycopene measured by HPLC with the L, a, b colour readings of a hydroponic tomato and the relationship of maturity with colour and lycopene content. Journal of Agricultural and Food Chemistry 48, 1697–1702. Beecher, G.R., 1998. Nutrient content of tomatoes and tomato products, Nutrient content of tomatoes. Proceedings of the Society of Experimental Biology and Medicine 218, 98–100. Block, G., Patterson, B., Subar, A., 1992. Fruit, vegetables, and cancer prevention: A review of the epidemiological evidence. Nutrition and Cancer 18, 1–29. Bramley, P.M., 2000. Is lycopene beneficial to health? Phytochemistry 54, 223–236.
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