PROCESSING EFFECTS ON LYCOPENE IN TOMATOES

The main causes of lycopene degradation during food processing are oxidation and isomerization. It is widely believed that lycopene generally undergoes isomerization upon thermal treatment. Changes in lycopene content and in the distribution of trans-and cis-isomers result in modifications of its biological properties (Zechmeister, 1962). Determination of the extent of lycopene isomerization would provide better insights into the potential health effects of processed food products. In processed foods, oxidation is a complex process that depends upon many factors, such as processing conditions, moisture, temperature, and the presence of pro- or anti-oxidants and of lipids. For example, use of fine screens in juice extraction enhances the oxidation of lycopene due to the large surface exposed to air and metal. The amount of sugar, acids and amino acids also affects lycopene degradation in pro-cessed food products by leading to the formation of brown pigments (Gould, 1994).

The deterioration in color that occurs during the processing of various tomato products results from exposure to air at high temperatures during processing, causing the naturally occurring all-trans lycopene to be isomerized and oxidized. Coupled with exposure to oxygen and light, heat treatments that disintegrate tomato tissue can result in substantial destruction of lycopene.

4.7.1 HEAT

The effects of heating on total lycopene and cis-isomer content in tomato puree are shown in Figures 4.6 and 4.7 (Shi and Le Maguer, 1999a). Increasing the temperature from 90 to 150°C caused a greater loss of total lycopene. Total lycopene concentration decreased over treatment time, but cis-isomers mostly appeared within the first hour of heating. After 2 h of heating, the rate of cis-isomer accu-mulation decreased.

Temperature increase from 90°C to 150°C caused a 35% decrease in total lycopene content. The greater percentage loss of lycopene as compared to the less

Lycopene from Tomatoes 153

gain in cis-isomer suggests that oxidation of lycopene was the main mechanism for the lycopene loss when heated at temperatures greater than 100°C. Fewer cis-isomers were present in the low temperature treatment material. The cis-isomers formed during the first 1 to 2 h of heating in processed tomato samples had degraded quickly.

Thus, food processing can foster cis-isomerization in tomato-based foods if an optimum heating temperature is used.

The results of lycopene retention in tomato puree cooked at 100°C are shown in Table 4.4. The length of cooking time had little or no effect on the degradation of lycopene if the heating temperature was lower than 100°C. These results suggest that the duration of heating, rather than cooking temperature, is the critical factor affecting degradation of lycopene. High temperatures such as more than 120°C, will result in more lycopene loss.

When tomato-based products are subjected to thermal processing, the changes in lycopene content and the conversion of trans- to cis-isomers will change its bioactivity and bioavailability (Khachik et al., 1992b; Emenhiser et al., 1995;

Wilberg and Rodriguez-Amaya, 1995; Stahl and Sies, 1996). High temperature will break down lycopene molecules into small fractions (Figure 4.8).

FIGURE 4.6 The effect of heat treatment on total lycopene degradation (Shi and Le Maguer, 1999b).

FIGURE 4.7 The effect of heat treatment on cis-isomer degradation (Shi and Le Maguer, 1999b).

TX69027-ch04-Frame Page 153 Wednesday, January 16, 2002 6:18 AM

154 Functional Foods: Biochemical and Processing Aspects, Volume 2

4.7.2 LIGHT

The effects of light irradiation on the content of total lycopene and cis-isomer in tomato puree are shown in Figures 4.9 and 4.10. The increase in cis-isomers began to occur at the onset of exposure to light. Total lycopene loss as well as cis-isomer loss increased with light irradiation time.

The losses of cis-isomers were further increased by exposure to light. This suggests that the light irradiation causes losses of total lycopene content. The amount of cis-isomer formed during light irradiation appeared to decrease quickly as the intensity of light increased, suggesting that cis-isomer oxidation was the main reaction pathway. A possible explanation for this phenomenon is that the trans -isomer first -isomerizes into cis-isomers, which then preferentially follow the oxida-tive pathway. The rate of cis-isomer oxidation was much greater under heat and light treatment than the rate of cis-isomer formation.

4.7.3 OXYGEN

Monselise and Berk (1954) first reported on the oxidative destruction of lycopene during the processing of tomato purée. Cole and Kapur (1957b) showed that more than 30% of lycopene was degraded upon heat treatment at 100°C for 3 h in the FIGURE 4.8 Reaction sequence for the formation of volatile compounds during heat treatment of lycopene. [Kanasawud, P. and Crouzet, J.C. 1990, J. Agric. FoodChem., 38: 1238–1242.

Copyright 1990 American Chemical Society. With permission.]

O

O

CHO

O

CHO Lycopene

2-methy-2-hepten-6-one

pseudo-ionone

geranial

6-methyl-3.5-heptadien-2-one neral

Lycopene from Tomatoes 155

presence of oxygen, whereas only 5% was lost in the presence of CO₂ after 3 h of treatment (Table 4.5).

4.7.4 ISOMERIZATION DURING PROCESSING

The contents of various lycopene isomers in some commercial tomato products are shown in Table 4.6. Detailed studies of the degree of isomerization resulting from food processing are limited. Color changes, which are usually used as a quality index, are only partly due to lycopene isomerization to cis-isomers. It is generally accepted that the all-trans form of lycopene has higher stability, and the cis-isomers have lower stability. Biological activity depends on the extent of isomerization and oxidation as well as the stability (Zechmeister, 1962; Khachik et al., 1992a; Stahl and Sies, 1992; Emenhiser et al., 1995; Wilberg and Rodriguez-Amaya, 1995).

Trans-cis isomerization leads to faster degradation of lycopene. Although the pro-cessing of tomatoes by cooking, freezing, or canning does not usually cause a serious loss in total lycopene content, it is widely assumed that lycopene undergoes impor-tant isomerization during processing. Heat, light, acids and other factors have been FIGURE 4.9 The effect of light irradiation on total lycopene degradation (Shi and Le Maguer, 1999b).

FIGURE 4.10 The effect of light irradiation on cis-isomer degradation (Shi and Le Maguer, 1999b).

0 1 2 3 4 5 6 7 8 9 10

0 2 4 6

Days

Lycopene Content mg/100

400 µmol 500 µmol 600 µmol

1 3 5

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

0 2 4 Days6

Cis-isomer content mg/ 100 g

400 µmol 500 µmol 600 µmol

1 3 5

TX69027-ch04-Frame Page 155 Wednesday, January 16, 2002 6:18 AM

reported to cause isomerization of lycopene (Zechmeister, 1962; Wong and Bohart, 1957; Lovric et al., 1970; Boskovic, 1979; Schierle et al., 1996; Nguyen and Schwartz, 1998; Shi and Le Maguer, 1999a,b). A true assessment of the relationship between nutritional quality and health benefits of dietary lycopene depends not only on the total lycopene content, but also on the distribution of lycopene isomers. Better characterization and quantification of lycopene isomers would provide better insight into the potential nutritional quality and health benefits. The control of lycopene isomerization during production and storage can be of benefit in improving the retention of product color, overall quality and biological activity.

Lycopene isomerization increases as a function of processing time using heat.

The results in Table 4.7 show that food processing can enhance cis-isomerization in tomato-based foods. Heating tomato-based foods in oil caused increased lycopene isomerization vs. heating in water. This would indicate that not only the duration and temperature of heat treatment, but also the food matrix components (such as oil or fat) further influence the lycopene isomerization. An outline of the lycopene degradation pathway has been proposed by Boskovic (1979) (Figure 4.11).

TABLE 4.5

Effect of Oxygen on Rate of Loss of Lycopene on Heating Tomato Pulp at 100°C

Condition Heating Time, Hours Loss (%)

Dark and CO₂ 0

Daylight and CO₂ 0

1 Source: Data from Cole, E.R. and Kapur, N.S. 1957a. “The stability of lycopene. I. Degradation by oxygen.” J. Sci.

Food Agric., 8: 360–365. Copyright Society of Chemical Industry. Reproduced with permission. Permission is granted by John Wiley & Sons Ltd. on behalf of the SCI.

Lycopene from Tomatoes 157

TABLE 4.6

Lycopene Isomers in Various Commercial Tomato Products

Sample

(“Tomatenmark,” Panocchia, Italy)

52 96 4 <1 <1 <1

(“Miracoli,” Kraft, Germany) 3.7 91 5 1 2 <1

Tomato ketchup

(“Hot Ketchup,” Del Monte, Italy)

9.5 88 7 2 3 1

(“Hot Ketchup,” Heinz, U.S.) 3.0 77 11 5 7 1

Instant meal

(“Eier-Ravioli,” Hero, Switzerland)

0.6 76 8 5 6 5

Sauce

(“Hamburger Relish,” Heinz, The Netherlands)

3.0 93 5 <1 3 <1

(“Sauce Bolognaise,” Barilla, Italy) 9.2 67 14 6 5 8

Canned tomatoes (“Chris,” Roger Sud, Italy)

7.1 84 5 3 5 3

Data from Schierle et al. (1996).

TABLE 4.7

Effect of Heating Treatment on Lycopene Trans-Cis Isomerization in Aqueous and Oily Dispersions of Tomato Paste (70°C) Source: Data from Schierle et al. (1996).

TX69027-ch04-Frame Page 157 Monday, January 21, 2002 9:54 AM

158 Functional Foods: Biochemical and Processing Aspects, Volume 2

4.7.5 STABILITY DURING STORAGE

The fate of lycopene in processed tomato products is notably influenced by storage factors. Lycopene content of tomato powder decreased in the presence of oxygen during storage (Table 4.8). A study on vacuum-dried tomato powder showed that in-package desiccation and packaging in an inert atmosphere (e.g., nitrogen) favored the retention of color, while the presence of air caused a loss of lycopene and color FIGURE 4.11 Fate of lycepene in situ during production and storage of tomato powder.

Propose outline of reaction pathway. (Boskovic, 1979. With permission.)

TABLE 4.8

Retention of Total Lycopene in Tomato Powder Stored under Different Atmospheres for Different Periods of Time

Storage Period (days) Storage Conditions Retention of Total Lycopene (%)

0 Fresh Tomato Powder 100

30 N₂, 20°C

Source: Data from Lovric, R., Sablek, Z. and Boskovic, M. 1970. “Cis-trans isomerization of lycopene and color stability of foam-mat dried tomato powder during storage.” J. Sci.

Food Agric., 21: 641–647. Copyright Society of Chemical Industry. Reproduced with permission. Permission is granted by John Wiley & Sons Ltd. on behalf of the SCI.

All trans-lycopene (ATL) Very low A_w/moistures, low

temperatures

Lycopene from Tomatoes 159

fading due to oxidation (Kaufman et al., 1957). Analyses of lycopene content in stored tomato powder samples by Wong and Bohart (1957) showed that air-packed samples retained the lowest levels of lycopene; all air-packed samples showed a progressive decrease in lycopene content during the storage period. The most impor-tant factor contributing to lycopene degradation was oxygen availability during storage. With careful selection of storage conditions to protect the products from air (e.g., by storing in an inert atmosphere or under vacuum), it is possible to protect initial color during storage. The mechanism of lycopene loss depends upon many parameters during food processing and storage. The main cause of damage to lycopene during food processing and storage is oxidation. Application of suitable antioxidants (e.g., ethoxyquin, ascorbic acid, sodium acid pyrophosphate) at appro-priate levels can be beneficial (Granado et al., 1992; Clinton et al., 1996; Porrini et al., 1998). Low storage temperature, low oxygen contents and avoidance of light exposure in storage will also limit the extent of the oxidation of lycopene.