Flavour Fragr J. 2019;34:63–69. wileyonlinelibrary.com/journal/ffj | 63 © 2018 John Wiley & Sons, Ltd. 1 | INTRODUCTION The flash-point temperature of fragrance and flavour oils is a prop- erty of critical importance to regulatory compliance, particularly for safety considerations concerning the handling, shipping, and stor- age of potentially flammable products. By definition, the flash-point temperature is the lowest temperature at which the vapour phase of a liquid material will ignite in air in presence of a spark or a flame. 1,2 Perfumers and flavorists are required to finalize their formula- tions with a flash-point measurement, which is time and resource consuming, and slows down the creation process. Reliable flash- point prediction for perfume and flavour oils is therefore a critical business need in our industry. Yet, to our knowledge, the assessment of flash-point predictions for complete perfumes and flavours, which can easily encompass dozens or even hundreds of compounds with diverse chemical functionalities, is not available in the current scien- tific literature. The flash point of a liquid mixture is dependent upon the flash points of the individual ingredients, the vapour pressure–temperature relationship for each ingredient, molecular interactions between species in the mixture (captured via activity coefficients), 3,4 and the homogeneity of the solution (miscibility gap). 5 Assuming that vapour pressure–temperature relationships and flash points for individual ingredients are available, the key to reliable flash-point prediction is being able to capture molecular interactions in multi-component, non-ideal solutions with reasonable accuracy. 6–9 In cases where the number of compounds in the mixture is lim- ited (<10), two approaches have been reasonably successful in pre- dicting the flash points of mixtures. 10 One approach is empirical, based on the quantitative structure–property relationship (QSPR), and is constructed from reliable experimental data (e.g. boiling point, the composition range of flammable components in the mixture, molecular descriptors, etc.), but depends on the quality of the avail- able data and the quantity and type of molecular descriptors. 11,12 The other approach is drawn from thermodynamics, and combines Le Chatelier's rule and a vapour–liquid equilibrium (VLE) model. 13–19 The latter approach is theoretically more reliable because every pa- rameter in the model has a physical meaning, although it can also Received: 22 June 2018 | Revised: 13 November 2018 | Accepted: 20 November 2018 DOI: 10.1002/ffj.3485 RESEARCH ARTICLE Flash-point prediction of fragrances or flavours accounting for non-ideality of the liquid phase Denis Shcherbakov 1 | Silvère Massebeuf 2 | Valery Normand 1 1 Firmenich Inc., 250 Plainsboro Road, Plainsboro, NJ 08536, USA 2 ProSim SA, Immeuble Stratège A, 51 rue Ampère, 31670 Labège, France Correspondence Valery Normand, Firmenich Inc., 250 Plainsboro Road, Plainsboro, NJ 08536, USA. Email: valery.normand@firmenich.com Abstract The flammability classifications of flavour and fragrance oils are determined from their flash-point temperatures, which are of critical importance for safety and regula- tory considerations. The reliable prediction of flash points for our products is a busi- ness need challenged by the sheer number of ingredients present in a typical formulation, ranging from dozens to hundreds of ingredients, and their chemical di- versity. To assess the predictability of flash points for the complex mixtures relevant to our industry, we consider five model mixtures of 15 perfume and flavour raw ma- terials, with a variety of chemical functionalities, and apply Le Chatelier's rule to- gether with vapour pressure curves from the Design Institute for Physical Properties (DIPPR) database and activity coefficients predicted from the Modified UNIFAC (Dortmund 1993) group contribution model. Flash-point temperatures of the 15-compound model mixtures were predicted with an accuracy of ±2°C, compared with experimental data, when taking into account the non-ideality of the liquid phase. KEYWORDS flash-point temperature, ignition, lower flammable limit, mixtures, safety