Ocimum basilicum: is basil carcinogenic when harvested too soon?
Introduction
Ocimum basilicum, commonly called basil, or sweet basil, is an annual plant of the Ocimum genus belonging to the Lamiaceae family. This particular genus in the family of the mints has been selected for his characteristics, particulary for his leaf size, flower color, physical appearance, and top of them aroma. It’s usage is part of many culinary traditions, whether if consumed fresh, dried, or processed, since its foliage adds a distinctive flavor to many foods. First of all, in its processed form, basil is world known for being employed in the production of pesto, a typical Italian sauce.
It is an economically important crop in several countries, particularly in Italy, where its cultivation covers over 250 acres. There are over 160 named commercially available types of basils and the aromatic character of each type of basil is determined by its genotype, and so depends on the chemical compounds of essential oils that come together in different proportions. Those are primarily consisting of Monoterpenes and Phenylpropanoids .
Chemical constituents
The strong clove scent of sweet basil is derived from eugenol, the same chemical as actual cloves. The citrus flavour of lemon basil and lime basil reflects their higher portion of citral, whether, for instance, Cinnamon basil or Licorice basil have higher levels of cinnamate or anethole, giving them a different scent.
Other chemicals that help to produce the distinctive scents of many basils, depending on their proportion in each specific breed, include:
The main components are:
molecule of Methyleugenol:
Methyleugenol is the prevalent component (58.7%) of the erb. Methyleugenol belongs to the phenylpropanoid group, an important natural constituent of a large number of herbs, spicee, and vegetables. Although methyleugenol is approved for commercial use as a flavoring agent in food and as fragrance in perfumes, creams, and detergents, human exposure to this compound is of toxicological concern because of its structural resemblance to known carcinogenic phenylpropanoids such as estragole and safrole.
NTP Technical Report on the 3-Month Toxicity Studies of Estragole
Intake of methyleugenol with the human diet is usually considered to be very low, but the consumption of pesto made with very short basil may lead to the intake of non-negligible doses of methyleugenol.
Chemical components in plants of different age or height
Some studies found Ocimum basilicum, the basil cultivar used the most in the production of pesto, having different aromatic composition of plants at different growth stages. Plants from different areas of northwestern Italy were analyzed at 4 and 6 weeks after sowing and showed methyleugenol and eugenol as the main components. The content of these compounds was correlated with plant height rather than plant age. Particularly, methyleugenol was predominant in plants up to 10 cm in height, whereas eugenol was prevalent in taller plants.
Methyleugenol in Ocimum basilicum L. Cv. genovese Gigante, 2001
Those tables compare the percentage of methyl-eugenol and eugenol from plants from varius samples. Each sample is a vase with some plants, half of which are harvested 4 weeks after sowing and the other half 2 weeks later.
Mean height and essential oil composition of basil plants analyzed 4 weeks after sowing:
| sample n° | mean height (cm) | methyl-eugenol (%) | eugenol (%) | other components (%) |
|---|
| 1 | 12.0 | 39.4 | 51.7 | 8.9 |
| 2 | 7.3 | 78.1 | 21.9 | 0.0 |
| 3 | 12.8 | 55.5 | 43.2 | 1.3 |
| 4 | 13.8 | 10.6 | 75.0 | 14.4 |
| 5 | 5.5 | 75.0 | 18.1 | 6.9 |
| 6 | 3.7 | 100.0 | 0.0 | 0.0 |
| 7 | 8.8 | 80.2 | 16.4 | 3.4 |
| 8 | 7.7 | 52.0 | 46.0 | 2.0 |
| 9 | 5.2 | 20.6 | 74.8 | 4.6 |
| 10 | 10.2 | 44.9 | 55.1 | 0.0 |
| 11 | 3.0 | 91.4 | 2.4 | 6.2 |
Mean height and essential oil composition of basil plants analyzed 6 weeks after sowing:
| sample n° | mean height (cm) | methyl-eugenol (%) | eugenol (%) | other components (%) |
|---|
| 1 | 12.0 | 5.5 | 73.0 | 21.5 |
| 2 | 7.3 | 79.5 | 20.5 | 0.0 |
| 3 | 16.7 | 11.8 | 88.2 | 0.0 |
| 4 | 13.8 | 10.7 | 74.3 | 15.0 |
| 5 | 5.6 | 86.6 | 10.3 | 3.1 |
| 6 | 2.7 | 100.0 | 0.0 | 0.0 |
| 7 | 6.0 | 99.8 | 0.2 | 0.0 |
| 8 | 6.0 | 100.0 | 0.0 | 0.0 |
| 9 | 5.3 | 16.5 | 80.3 | 3.2 |
| 10 | 12.3 | 16.6 | 78.9 | 4.5 |
| 11 | 3.0 | 89.3 | 10.7 | 0.0 |
The chromatograms below show the composition of essential oil from samples 5 (6 weeks old), 3 (4 weeks old), and 4 (6 weeks old) and are representative of all analytical profiles.
Results show how different is the composition of essential oils based on the height of each plant. Studies demonstrate that methyleugenol derived from eugenol by a specific methylation involving an
S-adenosylmethionine dependent O-methyltransferase.
Biosynthesis of estragole and methyl-eugenol in sweet basil (Ocimum basilicum L). Developmental and chemotypic association of allylphenol O-methyltransferase activities, 2000
The strict correlation between methyleugenol and eugenol found in basil could be explained by a pathway in which the activity of the invoked enzyme ( EOMT ) decreases when plants grow.
These results are important in the evaluation of risk to human health posed by dietary ingestion of methyleugenol contained in pesto.
Carcinogenicity
Methyleugenol is reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in experimental animals.
Report on Carcinogens, Twelfth Edition, 2011
FINAL Report on Carcinogens Background Document for Methyleugenol, 2000.
Cancer Studies in Experimental Animals
Oral exposure to methyleugenol caused tumors in two rodent species and at several different tissue sites. Methyleugenol administered by stomach tube caused benign or malignant liver tumors (hepatocellular adenoma or carcinoma) in rats and mice of both sexes. In rats, methyleugenol also caused benign or malignant stomach tumors (neuroendocrine tumors) in both sexes and tumors of the kidney (renal-tubule adenoma), mammary gland (fibroadenoma), and skin (fibroma or fibrosarcoma) in males. Malignant neuroendocrine tumors of the stomach in male mice also were considered to be related to methyleugenol exposure.
Studies on Mechanisms of Carcinogenesis
Mechanistic studies indicate that liver tumors induced by methyleugenol and structurally related allylbenzenes result from metabolism of these compounds to DNA-reactive intermediates.
Methyleugenol may be bioactivated by three different pathways:
1. Hydroxylation at the 1′ position of the allylic side chain to yield 1′-hydroxymethyleugenol, followed by sulfation of this intermediate to form 1′-hydroxymethyleugenol sulfate.
2. Oxidation of the 2′,3′-double bond of the allylic side chain to form methyleugenol-2,3-oxide.
3. O-demethylation followed by spontaneous rearrangement to form eugenol quinone methide.
The most important reaction is the first one since the formation of protein adducts and DNA adducts in the livers of animals (and in cultured human hepatocytes) exposed to Phenylpropanoids and induction of liver tumors by these compounds in animals have been attributed to activation via the hydroxylation pathway, because similar effects were produced by the 1′-hydroxy metabolites and because these effects were inhibited by pretreatment with sulfotransferase inhibitors.
So the current studies have shown that 1’-hydroxylation of methyleugenol have been implicated in formation of DNA and protein adducts and thereby in the rodent and human hepatotoxicity and carcinogenicity for this kind of compound,which is catalyzed in vitro by distinct enzymic components in microsomes.
Cytochrome P450 mediated bioactivation of methyleugenol to 19-hydroxymethyleugenol in Fischer 344 rat and human liver microsomes, 1997
Estimated human dietary exposure
The Flavor Extract Manufacturers Association (FEMA) estimated that methyl eugenol consumption from food is about 5–6 μg/kg/day.
FEMA complete report GRAS FLAVORING SUBSTANCES 22 n° FEMA 4069-4253
The FEMA group assumed that the top 10% of people consuming methyl eugenol consumed all of the methyl eugenol in commerce. In addition, use of essential oils as flavoring agents has resulted in a total methyl eugenol exposure of no more than about 10 μg/kg/day.
Safety assessment of allylalkoxybenzene derivatives used as flavouring substances - methyl eugenol and estragole, 2002
However, ethnic or cultural dietary habits may result in substantially higher exposures to methyl eugenol; one estimate indicates as much as 250 μg methyleugenol/kg/meal from a pesto meal.
Methyleugenol in Ocimum basilicum L. Cv. Genovese Gigante, 2001.
This is because Pesto is traditionally prepared with Ocimum basilicum cv. Genovese Gigante that is 10–12 cm in height, when the percentage of methyleugenol in the essential oil is generally more than 40%. Considering that, at this stage of growth, the amount of essential oil in O. basilicum cv. Genovese Gigante corresponds to ~0.5% of his net dry weight and supposing that one portion of pesto contains ~10 g of basil, the resulting dietary exposure to methyleugenol could reach 250 μg/kg of body weight per meal in adults and 500 μg/kg of body weight per meal in children.
Conclusion
Further studies are necessary to evaluate whether a real risk is associated with the consumption of pesto prepared with basil rich in methyleugenol. The results would not be of only local significance because the worldwide consumption of pesto has increased due to recent interest in Mediterranean cuisine, which is considered to be healthful and valuable even if it is sometimes lacking of suitable controls.
Anyway, since is dimonstrated that plants 3.5-6.5 cm high contain methyleugenol as the prevalent component, and taller plants contain lesser and lesser methyleugenol, which is replaced by eugenol, is here suggested to prepare pesto, which is the predominant sauce containing this spice, with basil leaves coming from plants taller than 16 cm, when eugenol is the main component and methyleugenol is greatly reduced.
National and International harvester and producer of any kind and form of basil should modify their growing time, adjusting the harvesting time to be when the plant is at its higher level, until further studies will confirm or less the cancerogenic properties of methyleugenol at this concentrations on humans, hoping international regulations on this topic will be modified.
