Lycopene in tomatoes: chemical and physical properties affected by food processing

Lycopene is the pigment principally responsible for the characteristic deep-red color of ripe tomato fruits and tomato products. It has attracted attention due to its biological and physicochemical properties, especially related to its effects as a natural antioxidant. Although it has no provitamin A activity, lycopene does exhibit a physical quenching rate constant with singlet oxygen almost twice as high as that of beta-carotene. This makes its presence in the diet of considerable interest. Increasing clinical evidence supports the role of lycopene as a micronutrient with important health benefits, because it appears to provide protection against a broad range of epithelial cancers. Tomatoes and related tomato products are the major source of lycopene compounds, and are also considered an important source of carotenoids in the human diet. Undesirable degradation of lycopene not only affects the sensory quality of the final products, but also the health benefit of tomato-based foods for the human body. Lycopene in fresh tomato fruits occurs essentially in the all-trans configuration. The main causes of tomato lycopene degradation during processing are isomerization and oxidation. Isomerization converts all-trans isomers to cis-isomers due to additional energy input and results in an unstable, energy-rich station. Determination of the degree of lycopene isomerization during processing would provide a measure of the potential health benefits of tomato-based foods. Thermal processing (bleaching, retorting, and freezing processes) generally cause some loss of lycopene in tomato-based foods. Heat induces isomerization of the all-trans to cis forms. The cis-isomers increase with temperature and processing time. In general, dehydrated and powdered tomatoes have poor lycopene stability unless carefully processed and promptly placed in a hermetically sealed and inert atmosphere for storage. A significant increase in the cis-isomers with a simultaneous decrease in the all-trans isomers can be observed in the dehydrated tomato samples using the different dehydration methods. Frozen foods and heat-sterilized foods exhibit excellent lycopene stability throughout their normal temperature storage shelf life. Lycopene bioavailability (absorption) can be influenced by many factors. The bioavailability of cis-isomers in food is higher than that of all-trans isomers. Lycopene bioavailability in processed tomato products is higher than in unprocessed fresh tomatoes. The composition and structure of the food also have an impact on the bioavailability of lycopene and may affect the release of lycopene from the tomato tissue matrix. Food processing may improve lycopene bioavailability by breaking down cell walls, which weakens the bonding forces between lycopene and tissue matrix, thus making lycopene more accessible and enhancing the cis-isomerization. More information on lycopene bioavailability, however, is needed. The pharmacokinetic properties of lycopene remain particularly poorly understood. Further research on the bioavalability, pharmacology, biochemistry, and physiology must be done to reveal the mechanism of lycopene in human diet, and the in vivo metabolism of lycopene. Consumer demand for healthy food products provides an opportunity to develop lycopene-rich food as new functional foods, as well as food-grade and pharmaceutical-grade lycopene as new nutraceutical products. An industrial scale, environmentally friendly lycopene extraction and purification procedure with minimal loss of bioactivities is highly desirable for the foods, feed, cosmetic, and pharmaceutical industries. High-quality lycopene products that meet food safety regulations will offer potential benefits to the food industry.     

Stability of Lycopene in Oil‐in‐Water Emulsions

Lycopene can be dissolved within the oil phase of oil‐in‐water emulsions to increase bioavailability in water‐dispersible systems. It is sensitive to oxidative conditions and easily undergoes isomerization at high temperatures. Degradation kinetics and isomerization of lycopene in oil‐in‐water‐emulsions were investigated as a function of thermal treatment and oxygen content. Lycopene degradation was found to follow a first‐order kinetics and rate constants were determined. Higher temperatures are directly correlated with increasing lycopene losses. Moreover, thermal treatment leads to a significant decrease of the concentrations of all‐trans and 13‐cis isomer, while the concentration of the 9‐cis isomer increased. Oxygen‐free conditions reduce lycopene losses significantly.     

Isolation and spectral characterization of thermally generated multi-Z-isomers of lycopene and the theoretically preferred pathway to di-Z-isomers

Lycopene has a large number of geometric isomers caused by E/Z isomerization at arbitrary sites within the 11 conjugated double bonds, offering varying characteristics related to features such as antioxidant capacity and bioavailability. However, the geometric structures of only a few lycopene Z-isomers have been thoroughly identified from natural sources. In this study, seven multi-Z-isomers of lycopene, (9Z,13′Z)-, (5Z,13Z,9′Z)-, (9Z,9′Z)-, (5Z,13′Z)-, (5Z,9′Z)-, (5Z,9Z,5′Z)-, and (5Z,9Z)-lycopene, were obtained from tomato samples by thermal isomerization, and then isolated by elaborate chromatography, and fully assigned using proton nuclear magnetic resonance. Moreover, the theoretically preferred pathway from (all-E)-lycopene to di-Z-isomers was examined with a computational approach using a Gaussian program. Fine-tuning of the HPLC separation conditions led to the discovery of novel multi-Z-isomers, and whose formation was supported by advanced theoretical calculations.     

Calorimetric studies of the effect of cis-carotenoids on the thermotropic phase behavior of phosphatidylcholine bilayers

Carotenoid geometry is a factor that determines their solubility and orientation in the lipid membrane as well as antioxidant capacities and bioavailability. The effects of the cis-isomers of carotenoids (zeaxanthin and β-carotene) on the thermotropic properties of lipid membranes formed with dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) were investigated by means of differential scanning calorimetry. The results were compared with the effects caused by the all-trans-isomer. Both the trans and cis isomers of zeaxanthin shifted the main phase transition temperature to lower values and decreased the cooperativity of the phase transition. The effect of all-trans zeaxanthin on the physical properties of the lipid bilayers has been shown to strongly depend on the hydrocarbon chain length of the membrane. In the case of cis-zeaxanthin this relationship is weaker.     

Isomerization of dietary lycopene during assimilation and transport in plasma

Diets of individuals were supplemented with tomatoes, either cooked or as tomato purée in order to compare uptake of lycopene from intact and homogenized fruit tissue matrices. Following a diet containing cooked tomatoes over three consecutive 7-day periods, little change in the carotenoid levels in plasma lipoproteins occurred. In contrast, a diet supplemented with concentrated tomato purée, over a 2 week period, caused a significant (p < 0.05) increase in lycopene levels in plasma, showing that the lycopene within intact cells is less bioavailable than that from processed tissue. The isomeric composition of plasma lycopene was significantly different to that of the ingested purée. A number of cis-isomers (predominantly 5-cis, 13-cis and 9-cis-) were detected in plasma, that are not present in the lycopene from purée. The significance of the increase in lycopene following dietary supplementation with respect to bioavailability and the causes of isomerization are discussed.     

Triplet state spectra and dynamics of geometric isomers of carotenoids

The observation of preferential binding of cis-carotenoids in purple bacterial photosynthetic reaction centers versus trans-isomers in antenna pigment protein complexes has led to the hypothesis that the natural selection of stereoisomers has physiological significance. In order to test this hypothesis, we have undertaken a systematic series of investigations comparing the optical spectroscopic properties and excited state dynamics of cis and trans isomers of carotenoids. The present work compares the triplet state spectra, lifetimes, and energy transfer rates of all-trans-spheroidene and 13,14-locked-cis-spheroidene, the latter of which is incapable of isomerizing to the all-trans configuration, and therefore provides a unique opportunity to examine the triplet state properties of a structurally stable cis molecule. The data reveal only small differences in spectra, decay dynamics, and transfer times and suggest there is little intrinsic advantage in either triplet energy transfer or triplet state decay arising from the inherently different isomeric forms of cis compared to trans carotenoids.     

Simultaneous quantification of retinol, retinal, and retinoic acid isomers by high-performance liquid chromatography with a simple gradiation

A new method of high-performance liquid chromatography (HPLC) analysis to quantify isomers of retinol, retinal and retinoic acid simultaneously was established. The HPLC system consisted of a silica gel absorption column and a linear gradient with two kinds of solvents containing n-Hexane, 2-propanol, and glacial acetic acid in different ratios. It separated six retinoic acid isomers (13-cis, 9-cis, all-trans, all-trans-4-oxo, 9-cis-4-oxo, 13-cis-4-oxo), three retinal isomers (13-cis-, 9-cis-, and all-trans) and two retinol isomers (13-cis- and all-trans). Human serum samples were subjected to this HPLC analysis and at least, all-trans retinol, 13-cis retinol, and all-trans retinoic acid were detectable. This HPLC system is useful for evaluating retinoic acid formation from retinol via a two-step oxidation pathway. Moreover, it could be applied to monitoring the concentrations of various retinoids, including all-trans retinoic acid in human sera.     

Membrane phospholipids and the dark side of vision

The key step in the visual pigment regeneration process is an enzyme-catalyzedtrans tocis retinoid isomerization reaction. This reaction is of substantial general interest, because it requires the input of metabolic energy. The energy is needed because the 11-cis-retinoid reaction products are approximately 4kcal/mol higher in energy than their all-trans congeners. In the retinal pigment epithelium a novel enzymatic system has been discovered which is capable of converting all-trans-retinol into all-trans retinyl esters, by means of a lecithin retinol acyl transferase (LRAT), followed by the direct processing of the ester into 11-cis-retinol. In this process the free energy of hydrolysis of a retinyl ester, estimated to be approximately –5kcal/mol, is coupled to the endothermic (+4kcal/mol) isomerization reaction, resulting in an overall exothermic process. The overall process is analogous to ATP-dependent group transfer reactions, but here the energy is provided by the membrane phospholipids. This process illustrates a new role for membranes: they can serve as an energy source.     

Carotenoid differences in isogenic lines of tomato fruit colour mutants

The main pigment present in fruits of tomato lines isogenic with the cultivar ‘Ailsa-Craig’, but with different fruit colours, is all-trans-β-carotene. Most of the tomato lines also contain cis-phytoene, all-trans-phytofluene, all-trans-ζ-carotene, all-trans-neurosporene, all-trans-lycopene and all-trans-α-carotene. Delta-del fruits accumulate all-trans-δ-carotene as the major pigment, and Tangerine coloured fruits contain massive amounts of the intriguing di-cis-ζ-carotene, tri-cis-neurosporene and tetra-cis-lycopene (also known as ‘prolycopene’); smaller amounts of cis-phytoene and di-cis-phytofluene are also found in Tangerine tomato fruits.     

Light-dependent switch from formation of poly-cis carotenes to all-trans carotenoids in the Scenedesmus mutant C-6D

The Scenedesmus obliquus mutant C-6D forms untypical poly-cis carotenes and lacks cyclic carotenoids when grown heterotrophically in the dark. After transfer to light, a very fast formation of usual all-trans carotenoids is observed leading to a phenotype very similar to the wild-type. Carotene analysis and quantitative kinetics of carotenoid changes after illumination in the presence of inhibitors which inhibit the three enzymes involved in carotene conversion showed isomerization of poly-cis carotenes, especially pro--carotene, to all-trans isomers. This result and consequently photoisomerization of pro--carotene in solution demonstrated that photoisomerization is the light-dependent mechanism leading to all-trans carotenoids in illuminated cultures. Now the different phenotypes of Scenedesmus C-6D can be explained by a single mutation of phytoene desaturase when photoisomerization of poly-cis carotenes is considered.Non-common abbreviations CPTA 2-(4-chlorophenylthio)-triethylamine HCL     

Transmembrane localization of cis-isomers of zeaxanthin in the host dimyristoylphosphatidylcholine bilayer membrane

The effects of the 9-cis and 13-cis isomers of zeaxanthin on the molecular organization and dynamics of dimyristoylphosphatidylcholine (DMPC) membranes were investigated using conventional and saturation recovery EPR observations of the 1-palmitoyl-2-(14-doxylstearoyl)phosphatidylcholine (14-PC) spin label. The results were compared with the effects caused by the all-trans isomer of zeaxanthin. Effects on membrane fluidity, order, hydrophobicity, and the oxygen transport parameter were monitored at the center of the fluid phase DMPC membrane. The local diffusion-solubility product of oxygen molecules (oxygen transport parameter) in the membrane center, studied by saturation-recovery EPR, decreased by 47% and 27% by including 10 mol% 13-cis and 9-cis zeaxanthin, respectively; whereas, incorporation of all-trans zeaxanthin decreased this parameter by only 11%. At a zeaxanthin-to-DMPC mole ratio of 1:9, all investigated isomers decreased the membrane fluidity and increased the alkyl chain order in the membrane center. They also increased the hydrophobicity of the membrane interior. The effects of these isomers of zeaxanthin on the membrane properties mentioned above increase as: all-trans < 9-cis ≤ 13-cis. Obtained results suggest that the investigated cis-isomers of zeaxanthin, similar to the all-trans isomer, are located in the membrane interior, adopting transmembrane orientation with the polar terminal hydroxyl groups located in the opposite leaflets of the bilayer. However, the existence of the second pool of cis-zeaxanthin molecules located in the one leaflet and anchored by the terminal hydroxyl groups in the same polar headgroup region cannot be completely ruled out.     

Simultaneous determination of all-trans-, 13-cis- and 9-cis-retinoic acid, their 4-oxo metabolites and all-trans-retinol in human plasma by high-performance liquid chromatography

All-trans-retinoic acid (all-trans-RA) and 13-cis-retinoic acid (13-cis-RA), due to their effects on cell differentiation, proliferation and angiogenesis, improved treatment results in some malignancies. Pharmacokinetic studies of all-trans-RA and 13-cis-RA along with monitoring of retinoic acid metabolites may help to optimize retinoic acid therapy and to develop new effective strategies for the use of retinoic acids in cancer treatment. Therefore, we developed a HPLC method for the simultaneous determination in human plasma of the physiologically important retinoic acid isomers, all-trans-, 13-cis- and 9-cis-retinoic acid, their 4-oxo metabolites, 13-cis-4-oxoretinoic acid (13-cis-4-oxo-RA) and all-trans-4-oxoretinoic acid (all-trans-4-oxo-RA), and vitamin A (all-trans-retinol). Analysis performed on a silica gel column with UV detection at 350 nm using a binary multistep gradient composed on n-hexane, 2-propanolol and glacial acetic acid. For liquid-liquid extraction a mixture of n-hexane, dichloromethane and 2-propanolol was used. The limits of detection were 0.5 ng/ml for retinoic acids and 10 ng/ml for all-trans-retinol. The method showed good reproducibility for all components (within-day C.V.: 3.02–11.70%; day-to-day C.V.: 0.01–11.34%. Furthermore, 9-cis-4-oxoretinoic acid (9-cis-4-oxo-RA) is separated from all-trans-4-oxo-RA and 13-cis-4-oxo-RA. In case of clinical use of 9-cis-retinoic acid (9-cis-RA) the pharmacokinetics and metabolism of this retinoic acid isomer can also be examined.     

Photoisomerization of lycopene during carotenogenesis in mutant C-6D of Scenedesmus obliquus

Mutant C-6D of the unicellular green alga Scenedesmus obliquus has lost the ability to form cyclic carotenoids during heterotrophic growth in the dark. In the dark it accumulates acyclic intermediates, i.e. lycopene, neurosporene and ζ-carotene. The lycopene and two neurosporene forms were identified to be cis-isomers. Upon transfer to light, intermediates decrease and a normal set of carotenoids is synthesized. Inhibition of the cyclization reaction by nicotine reveals a lightinduced isomerization of cis-lycopene to trans-lycopene. Since the spectral characteristics of these two isomers differ drastically the isomerization can be followed in vivo by measuring a light-induced absorbance change. This absorbance change has its maximum at 520 nm and shows fast kinetics under high light intensities reaching a saturation level after about 2 min. Fluence-response curves for this absorbance change were performed for different wavelengths of actinic light. From the linear parts of these curves an action spectrum was caculated showing maxima at 670, 630 and 440 nm originating from chlorophyll and a maximum at shorter wavelengths (400–510 nm) which is interpreted to derive from ζ-carotene. A model for the light regulation of carotenogenesis in mutant C-6D is presented and the relation to the so-called 520-change observed in many plants is discussed.     

cis–trans isomerization of carbon double bonds in monounsaturated triacylglycerols via generation of free radicals

We investigated the heat-induced cis/trans isomerization of double bonds in monounsaturated lipids. When triolein (9-cis, 18:1) was heated around 180 °C, small amounts of isomerization products were obtained depending on the heating period. The heat-induced isomerization of triolein was considerably suppressed by the addition of different antioxidants or under nitrogen stream, and these additives simultaneously inhibited the thermal oxidation of double bonds in triolein. Therefore, an intermediate of the thermal oxidation reaction might be responsible for the heat-induced isomerization of the double bonds in triolein. The thermodynamics of the heat-induced isomerization of triolein (9-cis, 18:1) and trielaidin (9-trans, 18:1) were investigated using Arrhenius plot. The Arrhenius activation energies of cis double bonds in triolein and trans double bonds in trielaidin were 106 kJ/mol and 137 kJ/mol, respectively. The calculated internal rotational barrier heights of these double bonds were similar to those of the double bond of 2-butene radical and significantly lower than those of non-radicalized double bonds in 2-butene. These results suggest that heat-induced cis/trans isomerization of triolein and trielaidin occurs mainly through the formation of radical species, which are the intermediates produced during thermal oxidation. The activation energy difference between the two forms suggests that trans trielaidin radicals are more stable than cis triolein radicals. The high thermodynamic stability of the trans double bonds in lipid radicals would influence the population of cis and trans isomers in edible oils and contribute to slight accumulation of trans-18:1 isomers during heating or industrial processing.     

Factors influencing lycopene content of foods, and lycopene intake of Hungarian population

Lycopene is an acyclic, biologically active carotenoid that constitutes foods, its preventive role in several cancerous diseases have been proved by epidemiological and experimental data. Its beneficial role in maintenance of human health is related to its significant antioxidant properties. Data of dietary lycopene intake of the Hungarian population is not available. The aims of the present complex study were 1) to measure the lycopene content of foods frequently consumed in Hungary, 2) to investigate the effect of agrotechnological procedures and food processing on lycopene content of tomatoes, 3) to estimate the lycopene intake in two groups of the Hungarian population with the use of a three-day dietary record. The best lycopene sources are the raw (5.0-16.0 mg/100 g) and processed tomatoes and tomato products (3.0-80.0 mg/100 g), and also watermelon (3.6-6.2 mg/100 g). The variety of the plants, the growing circumstances, and the weather conditions significantly influence the lycopene content of freshly consumed and processed tomato fruits. Mild technological processes can preserve a considerable amount of the original lycopene content in tomato. The estimated average dietary intakes of the Hungarian children (n=521) and adults (n=205) were 2.98 +/- 4.71 mg/day/capita, and 4.24 +/- 8.47 mg/day/capita, respectively. Optimal climate conditions of Hungary makes possible to produce tomato fruits with high dietary value including significant amount of health protective lycopene. Increased consumption of tomato and tomato products with high concentration of lycopene may improve the antioxidant capacity of human body, and the risk of several cancerous diseases may be reduced.     

Cyclo(D-Pro-L-Pro-D-Pro-L-Pro): Structural properties and cis/trans isomerization of the cyclotetrapeptide backbone

Combinations of L - and D -proline residues are useful compounds for finding new structures and properties of cyclic peptides. This is demonstrated with one striking example, the cyclic tetrapeptide c(D -Pro-L -Pro-D -Pro-L -Pro). For this molecule composed of strictly alternating D - and L -configurated residues, a highly symmetrical structure is expected, which should be an optically inactive meso-form. Cyclization of the enantiomeric pure linear precursor D -Pro-L -Pro-D -Pro-L -Pro, however, yields a racemic mixture of two enantiomeric cyclotetrapeptides, both with twofold symmetry and a cis–trans–cis–trans sequence of the peptide bonds. Remarkably, this formation of a racemate was not caused by racemization, but by cis/trans isomerization of all peptide bonds in the ring. This process may occur in the linear precursor during the ring formation (cyclization of conformers with trans–cis–trans or cis–trans–cis arrangement of the amide bonds) as well as in the enantiomeric pure cyclic tetrapeptide at higher temperature. In the latter case, an all-cis structure should exist as the intermediate, which can form a cis–trans–cis–trans sequence in two equivalent ways, leading finally to two enantiomeric cyclotetrapeptides. In the first one, the cis peptide bonds are attributed to the L -residues and the trans peptide bonds to the D -residues; in the second one, the cis bonds belong to the D and the trans bonds to the L -residues. The mixture of these two enantiomers does not crystallize in the racemic form, but in enantiomeric pure separate crystals. The structural properties could be proved by ¹H- and ¹³C-nmr spectroscopy and x-ray analysis. The cis/trans isomerization process was confirmed by optical rotation measurements and CD spectroscopy, as well as DREIDING model studies. Calorimetric measurements in the solid state suggest the existence of the expected all-cis intermediate. The backbone conformation of the 12-membered medium-sized ring shows only slight deviations—up to 6° —from the planarity of the peptide bonds. On the other hand, the four pyrrolidine rings show different types of puckering of the C_γ or the C_β atoms.     

Steric Enhancement of Imidazole Basicity incis-Urocanic Acid Derivatives: Models for the Action of Chymotrypsin

To test the hypothesis that substrate-induced steric compression between His 57 and Asp 102 at the active site of chymotrypsin can increase the basicity of His 57, we have synthesized thecis- andtrans-isomers of 2-bromo-3-(N-tritylimidazole)-2-propenoic acid and 2-chloro-3-(N-tritylimidazole)-2-propenoic acid and compared selected properties with those ofcis-andtrans-urocanic acids. Thecis-isomers display low field¹H NMR signals at 17 ppm in dimethylsulfoxide, similar tocis-urocanic acid; whereas thetrans-isomers do not show strong hydrogen bonds. Increasing the size of the C2 substituent (H < Cl < Br) in thecis-isomers increases the pK_aof the imidazolium group from 6.78 for H to 7.81 and 9.10 for Cl and Br, respectively; whereas the pK_as of thetransisomers are all 6.0 ± 0.1. The results indicate that thecis-urocanic acid derivatives with large substituents at C2 act as proton sponges in water, and they support the concept that steric compression in the catalytic triad of chymotrypsin can increase the basicity of His 57.     

Is lycopene beneficial to human health?

Since humans cannot synthesise carotenoids de novo, we depend upon the diet exclusively for the source of these micronutrients. Although the necessity for beta-carotene, as the precursor of vitamin A has been recognised for many years, it is lycopene that has attracted substantial interest more recently. Lycopene is the red-coloured carotenoid predominantly found in tomato fruit, but in few other fruits or vegetables. It has claimed that it may alleviate chronic diseases such as cancers and coronary heart disease. This possibility has been studied extensively, by epidemiological studies and biochemical investigations of its properties and its bioavailability from tomato-based diets. This article summarises the current state of knowledge of the properties of lycopene, its possible role in human health and areas for future research.     

β-Phenylmercapto-cis- and trans-cinnamic Acids

β-(p-Chlorophenyl) mercaptocinnamic acid and β-(p-acetamidophenyl) mercaptocinnamic acid were synthesized and separated into cis- and trans-isomers of each. Their configurations were determined by ring closure to be thioflavons under mild conditions. trans-Acids have an absorption band at 8.3μ while cis-acids do not have such band. Other physical methods generally used to determine the geometrical configuration were of no avail for these compounds carrying a bulky group at β-position. cis-Acid was produced via isomerization from the trans-ester during the procedure of saponification.