Composition and authentication of commercial and home-made white truffle-flavored oils

Highlights

• The composition of white truffle flavor used in commercial olive oils is revealed.

• White truffle flavor is impaired to oils by 4–6 sulfur containing volatiles.

• Dimethyl sulfoxide and dimethyl sulfone are markers of commercial oils.

Abstract

Truffle-flavored olive oils sell at a high premium compared to unflavored oils, yet, their rarely contain either real truffles or natural truffle aroma. Our aim here was to characterize truffle oils and explore techniques for authentication. Specifically, we characterized and compared by metabolic profiling and stable isotope ratio analysis the flavors emitted by commercial and home-made truffle-flavored oils (prepared with natural and synthetic flavors), non-flavored oils, and actual fruiting bodies of the white truffle Tuber magnatum.

Stable isotope ratio analysis (δ¹³C) of 2,4-dithiapentane, a characteristic truffle odorant detected in most flavored oil samples, could not differentiate between natural and synthetic flavors. By contrast, metabolic profiling revealed that truffle flavor was imprinted to oils by four to six sulfur containing volatiles, two of which (dimethyl sulfoxide and dimethyl sulfone) were exclusively detected in commercial oils, regardless of their synthetic or natural labeling. Overall our results also highlight inconsistencies in product labelling and question the authenticity of oils claiming to contain only natural truffle flavors.

Introduction

With thousands of euros per kilograms, truffle fungi (Tuber spp.) stand among the most expensive food items on our planet. This luxury status can be traced back to truffle's enticing aromas, which partially originate from intimate interactions between the fungus and its microbiome (Splivallo and Ebeler, 2015, Splivallo et al., 2014). Human sensed aroma of any food can typically be attributed to a blend of 3–40 odorants (Dunkel et al., 2014), and truffle are no exception. Indeed, the aroma of white and black truffles is made of 10–20 odorants per species (Culleré et al., 2010, Liu et al., 2012, Schmidberger and Schieberle, 2017, Splivallo and Ebeler, 2015).

Once harvested, truffle fruiting bodies quickly degrade and can be stored fresh for up to ten days (Splivallo & Culleré, 2016). Similarly, derived products prepared from fresh truffles (i.e. truffle cheese, truffle-flavored oil) suffer from a short shelf-life and are only available during the truffle season which typically lasts a few months in a year. To circumvent the latter shortcomings, the food industry has for long relied on synthetic flavors (i.e. odorants) for impairing truffle flavor to food (Splivallo, Ottonello, Mello, & Karlovsky, 2011). The exact composition of truffle flavors used by the food industry is unknown, even if truffle flavored-oils are said to contain more than 60 volatiles, of which 2,4-dithiapenatne is the most typical one (Pacioni et al., 2014, Torregiani et al., 2017). Indeed, 2,4-dithiapentane, has a characteristic white truffle smell, and occurs in the white truffle T. magnatum (Splivallo et al., 2011) but also in the garlic mushroom Marasmius alliaceus (Rapior, Breheret, Talou, & Bessière, 1997) and in boiled carp fillets (Cyprinus carpio L.) (Schlüter, Steinhart, Schwarz, & Kirchgessner, 1999) and in numerous microbes (Lemfack, Nickel, Dunkel, Preissner, & Piechulla, 2014). Natural 2,4-dithiapenatne (not originating from truffles) has recently appeared on the market and sells at a large premium compared to its synthetic counterpart. Important price premiums (40–200 times the synthetic (Dubal, Tilkari, Momin, & Borkar, 2008)) are typical for natural flavors, which increases the risk of falsification in food and begs for authentication and traceability methods to be developed (Van Rijswijk & Frewer, 2012).

Two techniques, which have been widely applied by the food industry towards the authentication and traceability of volatile flavors (odorants), are gas-chromatography-mass spectrometry (GC-MS) and GC-isotope ratio mass analysis (GC-IRMS). For instance, a study by GC-MS exemplified that specific volatile markers can reveal the geographical origin of honey and the identity of the botanicals that were collected by honeybees to make that honey (Radovic et al., 2001). GC-IRMS is another valuable technique for flavor authentication. It relies on the fact that the isotopic ratio of certain elements (i.e. hydrogen, carbon, nitrogen, oxygen, sulfur) can provide, similarly to a fingerprint, information about the origin of some flavors (i.e. geographical origin or a natural vs. synthetic sources) (van Leeuwen, Prenzler, Ryan, & Camin, 2014). In the food industry, authenticity studies by GC-IRMS have been used for fruits, essential and edible oils, fats, beverages and vinegars (van Leeuwen et al., 2014). The technique has also been successfully applied to distinguish synthetic from natural vanillin or strawberry and cinnamon flavors (Hansen et al., 2014, Schipilliti et al., 2011, Sewenig et al., 2003).

The aim of our study was to shed light on the differences among commercial and home-made white truffle oils using the analytical techniques mentioned in the previous paragraph. Our motivation was fueled by the recent appearance on the market of white truffle oils claiming to contain “natural truffle” aromas. In short, the volatile profiles of regular olive oils and truffle-flavored oils (commercial and home-made) were investigated by untargeted metabolic profiling using GC-MS. GC-IRMS was further employed to assess whether the carbon isotopic ratio of 2,4-dithiapentane, the major odorant identified in our truffle-flavored oils, could be further used to discriminate among natural and synthetic aromas?