DL-Limonene: Uses, Synthesis and Toxicity

What is DL-Limonene?

Limonene is the main component of citrus oils and is mainly extracted from the fresh peel of grapefruit, lemon and orange. Limonene comes in three forms: d-limonene, which is the most abundant naturally occurring, l-limonene and the racemic isomer dl-limonene. Except for its optical activity, dl-limonene has the same physical properties as d- and l-limonene.

dl-Limonene (racemic) is reported to occur in the following foods by  the VCF:

Uses of DL-Limonene

DL-Limonene has a lemony flavour and is therefore widely used in flavours and fragrances, cosmetics, cleaning products, air fresheners and paint strippers. It is also used as a flavouring agent in food manufacture and some medicines. In addition, as an organic molecule, it is also used as a solvent for rosin, waxes, rubber, a dissolvent for gallstones and dental cement, in the manufacture of glass polymers, resins, insecticides and repellents.

Synthesis of DL-Limonene

DL-Limonene (dipentene) can be produced by pyrolysis of waste tyres. Dipentene is formed by intramolecular cyclisation of allyl radicals formed by random cleavage of the β-bond relative to the double bond in the polyisoprene chain. Under pyrolytic conditions, the dimerisation of two isoprene molecules to form dipentene seems unlikely. Especially at higher temperatures, the formed dipentene is further converted into isoprene or aromatic compounds, the latter possibly through diallyldiacetal formation. Pyrolysis operating conditions, as well as tyre type and brand, have a significant impact on dipentene yield. Of these, pyrolysis temperature is the most important variable, with the highest dipentene yields occurring at temperatures between 400 and 500 °C. However, obtaining a high concentration of dipentene fractions of sufficient quality from tyre-derived oils is not easy. In particular, the removal of sulphur-containing compounds and the separation of dipentene from 1,2,3-trimethylbenzene, m- and p-toluene and indene are future challenges.

Toxicity assessment

dl-Limonene (racemic) was evaluated for genotoxicity, repeated dose toxicity, reproductive toxicity, local respiratory toxicity, phototoxicity/photoallergenicity, skin sensitization, and environmental safety. Data show that dl-limonene (racemic) is not genotoxic.

Data on dl-limonene (racemic) provide a calculated margin of exposure (MOE) > 100 for the repeated dose toxicity, reproductive toxicity, and local respiratory toxicity endpoints. Data show that there are no safety concerns for dl-limonene (racemic) for skin sensitization under the current declared levels of use. The phototoxicity/photoallergenicity endpoints were evaluated based on ultraviolet/visible (UV/Vis) spectra; dl-limonene (racemic) is not expected to be phototoxic/photoallergenic. Data on dl-limonene (racemic) provide a calculated MOE >100 for the local respiratory endpoint. The environmental endpoints were evaluated; dl-limonene (racemic) was found not to be Persistent, Bioaccumulative, and Toxic (PBT) as per the International Fragrance Association (IFRA) Environmental Standards, and its risk quotients, based on its current volume of use in Europe and North America (i.e., Predicted Environmental Concentration/ Predicted No Effect Concentration [PEC/PNEC]), are<1.

References:

[1] A.M. API . RIFM fragrance ingredient safety assessment, dl-limonene (racemic), CAS Registry Number 138-86-3[J]. Food and Chemical Toxicology, 2022. DOI:10.1016/j.fct.2021.112764.

[2] B. DANON. A review of dipentene (dl-limonene) production from waste tire pyrolysis[J]. Journal of Analytical and Applied Pyrolysis, 2015. DOI:10.1016/j.jaap.2014.12.025.

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