This chapter reviews the main findings on Tahitian vanilla (Vanilla ×tahitensis) over the last 10 years. It brings new insights into the hybrid origin of V. ×tahitensis and its diversification in French Polynesia. It details then the different analytical methods used to characterize the flavour properties and the aroma impact compounds of Tahitian vanilla, with a special emphasis on how they can be used to differentiate Tahitian vanilla from other vanillas. Finally, the effect of the curing process on the chemical composition and the sensory properties is discussed. These results highlight the need to include some of the key volatile compounds into a more adapted quality control, in order to describe the characteristic sensory properties of Tahitian vanilla but also those from other origins.
- V. ×tahitensis
- Tahitian vanilla
- genetic biodiversity
- aroma chemistry
- high performance liquid chromatography
- gas chromatography
- sensory properties
- quality control
- curing process
Though Tahitian vanilla production represents less than 1% of the world vanilla production , it possesses a unique aura, due to an original anise flavour, highly prized especially in gastronomy and perfumery. In order to better develop and organize the vanilla sector and to promote Tahitian vanilla specificities abroad, a dedicated institute “Vanille de Tahiti” was created in 2003 in French Polynesia . One of its main objectives is to provide support to local actors and help controlling pests and diseases threatening vanilla vines, with the sponsored supply of insect-proof shade houses for instance. A research department was also established in order to characterize Tahitian vanilla in comparison to other vanilla species, through the study of the flavour composition and genetic diversity. The institute has been part of a larger project, which focused on characterizing, protecting and promoting the sustainable use and valorisation of vanilla biodiversity (VaBiome 2012–2015). This chapter aims at capturing the most significant results that have been acquired over the last 10 years at the institute “Vanille de Tahiti” through different collaborations inside and outside French Polynesia, with a particular focus on genetic diversity and aroma chemistry.
2. Origins and diversification of
The introduction of vanilla in French Polynesia is estimated to have occurred during the mid-nineteenth century. The elucidation of the origin of Tahitian vanilla is difficult due to different reports of at least three introduction events from various origins (Philippines, France and West Indies) [3–5]. On top of that, the origin of
Even if the origin of Tahitian vanilla is still confused, it is obvious that Tahitian vanilla, as we know it today, is different from other vanillas and in particular
In 1951, Porteres suggested that Tahitian vanilla could be a hybrid offspring between
Coming back to the analysis of genetic diversity based on AFLP patterns, all Tahitian morphotypes were (i) distinct from the other
Biodiversity of Tahitian vanilla is preserved in the Genetic Resources Centre “Vanille de Tahiti” created on the island of Raiatea. About 140 accessions of
3. Tahitian vanilla flavour and differentiation from other vanillas
In French Polynesia, the quality of Tahitian vanilla is guaranteed by the training of different actors of the sector and several controls, after harvest and before exportation, are realized by sworn experts, as specified in the designation of origin “Vanille de Tahiti” recently released [16, 17]. Until 2014, there were no official analytical criteria to authenticate Tahitian vanilla in the international trade context, which would enable to differentiate it with vanilla from Papua New Guinea.
In order to determine what specifically differentiates Tahitian vanilla from other vanillas (
3.1. Authentication based on HPLC profile
To authenticate natural vanilla (i.e.
This is exemplified by a study carried out between 2005 and 2007 at the Institute “Vanille de Tahiti”. More than 300 vanilla samples, collected from vanilla curers based on the islands of Tahaa and Raiatea, where most of the vanilla was produced in French Polynesia at that time, were analysed by HPLC, together with 22 samples of
The HPLC composition of Tahitian vanilla was homogeneous between the three years of production, for compounds whose concentration was higher than 1000 ppm (relative standard deviation <15%). As a result of this, low variability shown by principal component analysis using the HPLC composition and vanilla samples from various origins, it was possible to differentiate a set of Tahitian vanilla samples from
The HPLC compositions of “Tahiti” and “Haapape” cultivars, which are the two main cultivars produced in French Polynesia, were found to be very close. The slight variations consisted in more vanillin and less p-hydroxybenzyl compounds in “Tahiti” pods. Consequently to this study, recommendations based on the contents of characteristic HPLC aroma compounds and their ratios have been made to assess Tahitian vanilla quality in French Polynesia. They have been integrated into official decrees published recently [16, 17]. These decrees define the Tahitian vanilla’s designation of origin (“Vanille de Tahiti”), its production area, the cultivars to be produced (“Tahiti” and “Haapape”) with their genetic characteristics, the production and the curing process methods, as well as the pods quality based on moisture content (45–50%), aroma content and characteristic ratios using the concentrations of anisyl, vanillyl, p-hydroxybenzyl and protocatechuyl compounds determined by HPLC. These criteria can also be used to authenticate Tahitian vanilla abroad.
3.2. Towards a better authentication based on the relevant flavour compounds
Though HPLC is a reliable analytical technique to analyse some characteristic compounds of the vanilla pods, it appears as more convenient to use complementary analytical and sensory techniques to quantify more compounds, in particular those likely to contribute/to affect the vanilla flavour. The perception of a flavour is due to the detection of a complex mixture of compounds having various physico-chemical properties. Non-volatile (or less volatile) compounds, like essential fatty acids, have the ability to fix some non-polar aroma constituents. Volatile compounds, which have various odour-detection thresholds, can sometimes have a preponderant impact on the aroma, even when present in low amounts. Tahitian vanilla aroma chemistry has been therefore investigated using additional analytical techniques, such as gas chromatography-mass spectrometry (GC-MS), sensory analysis and a hyphenated technique, gas chromatography coupled with olfactometry (GC-O) designed to detect more volatile compounds.
3.2.1. Gas chomatography-olfactometry and volatile compounds
GC-olfactometry enables to decompose the individual odours of an aroma extract and attribute these odours to specific compounds, which are called aroma impact compounds. GC-olfactometry was applied to the analysis of different vanilla extracts:
3.2.2. Sensory properties and volatile compounds
In addition to the compounds analysed by HPLC, various volatile compounds such as phenolic compounds, aliphatic aldehydes, ketones and esters contributing to the flavour of Tahitian vanilla were identified and quantified by GC-MS . Still, this method highlighted that compared to
In relation to their volatile composition, it was also possible to clearly differentiate the sensory properties of Tahitian vanilla from
As we tried to correlate the sensory properties of vanilla from different origins to their volatile composition, it remained difficult to link single volatile compounds to specific notes, as there are many interactions between volatile and non-volatile compounds. However, we were able to show that the strongest correlations were between phenolic, woody, smoky notes and a pool of phenolic compounds such as guaiacol and creosol. Such correlation between guaiacol and woody notes has also been highlighted for
4. The effect of curing process on vanilla flavour
Since we started our journey studying Tahitian vanilla characteristics, there was a question that needed to be answered: how much of the aroma is inherent to the vanilla species and what is the influence of the curing process on the aroma development? To get an answer, we first have to go back to the curing process as it is performed in French Polynesia. First, Tahitian vanilla pods are harvested when fully ripe, then they are cured following three main steps :
Shade browning: vanilla pods have 80% moisture and are exposed on the shade until being entirely brown.
Sun drying and sweating: for several weeks, pods are alternatively exposed for a short period of time (2–4 h) in the sun every day, then wrapped into a cotton fabric and leave to sweat overnight in closed wooden cases. As pods become increasingly flexible and glossy due to water loss, they are massaged to ensure seeds are spread lengthwise.
Air drying and refining: finally pods are left in the shade so that the moisture and aroma contents stabilize in order to obtain homogeneous batches (around 50% moisture).
Unlike other vanilla species,
4.1. Effect of curing on aroma composition using HPLC
The evolution of the aroma composition of Tahitian vanilla pods was monitored by HPLC analysis of “Tahiti” and “Haapape” samples collected at various steps of the curing process. It was observed that through drying, the aroma content of the pods decreased by around 30%, from around 70,000 ppm at 80% moisture content to 50,000 ppm for a final moisture content of 50–55% (Figure 9). This loss could be attributed to co-evaporation of the compounds with water while sun drying and/or loss in liquid form during pod sweating, as evidenced by the oily substance observed at the surface of the fabric wrap.
Within a same compound series, acids concentrations (in red) remained stable during the curing process; while alcohols concentrations (in blue) tended to decrease linearly over time. The fate of aldehydes (in black) varied across the series, as anisaldehyde concentration remained stable, while vanillin concentration decreased linearly. These different trends were related to many interdependent factors such as the oxidation state of the compounds (alcohols can be oxidized into aldehydes then into acids within a series), their physico-chemical properties (volatility and lipophilicity), and the fact that the compounds can be potential substrates or metabolites of intrinsic enzymes or colonizing microorganisms. The relatively high variability of the compounds concentration, especially vanillin, observed at the beginning of the curing process progressively decreased, resulting in more homogeneous vanilla batches at the end of the curing process.
4.2. Effect of curing on sensory properties and volatile compounds
In order to get a better understanding of how the pod aroma was developed, we studied the evolution of sensory properties by quantitative descriptive analysis and of relative volatile compounds concentration by GC-MS at different steps during the curing process: (i) step S1—shade browning (80% moisture), (ii) step S2—sun drying (60–65%, pods are wrinkled); (iii) step S3—air drying (50–55% moisture); (iv) step S3*—enhanced drying: sun drying was extended to mimic curing methods of vanillas from other origins and obtain vanilla pods at 40% moisture.
Some of the sensory notes of “Tahiti” cultivar varied significantly during the curing process (Figure 10), while the variations for “Haapape” were not significant (data not shown). Regarding the “Tahiti” cultivar, vanilla, anise, rum and caramel notes remained as intense as they were initially or slightly decreased while woody, phenolic, smoky, spicy and fruity notes progressively (and significantly for most of them) built up during the curing process. It is noteworthy that even when
The curing method induced variations in the volatile composition, which impacted greatly the final flavour. Despite a relatively low concentration in the Tahitian vanilla extracts, some aliphatic aldehydes and ketones played a pivotal role by GC-olfactometry  due to a low odour threshold . They originate from the oxidation of essential fatty acids such as linoleic and oleic acids, the major fatty acids in Tahitian vanilla pods . Indeed, the fatty acid content was found to decrease from 2.7% of dry matter to 1.6% during the curing process (cultivar “Tahiti”). The increase in the concentration of saturated aldehydes (hexanal, heptanal, octanal and nonanal) linked to fruity notes was seen as positive, contrary to the increase of monounsaturated (heptenal, octenal, nonenal and decenal) and diunsaturated aldehydes (2,4 decadienal) (Figure 11a), which displayed less pleasant notes like leather, olive, wax and cooking fat by GC-O . Thus, drying the pods until 40% moisture is not recommended. Similar variations of aliphatic aldehydes were observed during the curing process of
The overall content of odour-active anisyl compounds was stable during the curing process of Tahitian vanilla, even though individual compounds contents varied (Figure 11b). When comparing the evolution between steps S1 and S3*, anisyl alcohol and methyl anisate contents were found to decrease, while anisaldehyde, anisyl acetate and anisyl formate contents slightly increased. The overall content of phenolic compounds (guaiacol, p-cresol, creosol, phenol, p-vinyl-phenol and p-vinyl guaiacol) increased during the curing process, in tune with the development of phenolic notes. Particularly, there was a dramatic increase of p-vinyl guaiacol by five-fold (Figure 11c). The increase of such phenolic compounds was also observed when Tahitian vanilla was stored for a long period of time (five years, data not shown). Overdrying vanilla pods to 40% moisture, as usually performed in other countries, was not beneficial to the Tahitian vanilla aroma, due to the concomitant increase of phenolic compounds concentration and the development of phenolic notes. The higher levels of p-vinylguaiacol and other phenolic compounds detected in vanilla from Papua New Guinea can find its roots in the way the curing process is performed.
Single compounds variations such as anisaldehyde, methyl anisate, guaiacol or p-vinyl guaiacol could not explain all the variations observed while monitoring sensory properties during the curing process, as aroma compounds also interact with less volatile components and between them. However, they were found to be overall good indicators of the development of the targeted sensory properties and it would be advisable to monitor their concentrations.
Tahitian vanilla has come a long way to be as it is known today. Various vanilla vines from different species have travelled around the world and been introduced in French Polynesia, to give the hybrid
New authentication criteria of Tahitian vanilla based on HPLC profiles and specific anisyl compounds have been published recently in French Polynesia and could be used to certify its origin. In order to maintain Tahitian vanilla originality, quality control should also be orientated towards odour-active compounds, which impact definitely the aroma. This will help protecting the specificities of this unique spice.