## 4 Discussion  
The results of the current paper suggest that furan and 2-MeF are the most abundant furans detected in the analyzed foods, while the less abundant one is 2.3-dMeF (Table S2). The difference in the amounts of furan compounds could be related to the mechanism of formation of these compounds as well as to the amounts of their precursors presents in food prior to processing [41]. In addition, according to the ROC curve, the presence of five furans among the six targeted molecules can be used as a predictor to discriminate coffee as a separate group. Based on the range of total furan concentrations in the food groups shown in Figure 1, it is clear that the sum is consistently below 200 ug/kg in the majority of food groups, except in coffee and some other groups. Figure 2: The distribution of the hazard indexes in coffee sup-groups (**A**), and the relative hazard distribution for furan compounds in coffee (**B**). Legend: The centre line in the Box plot represents the median; box limits represent the upper and the lower quartiles; whiskers are 1.5\(\times\) interquartile range; and points are outliers. In the coffee group (n = 303) and particularly in solid coffee, the total concentrations are significantly higher than in the other food groups (\(p\)-value \(<\) 0.0001) due to its pattern discussed above as well as to the high contribution of each compound, in particular 2-MeF (Figure S4A). Total concentrations of furan compounds in coffee beans are higher than in ground coffee (Figure S5), which could be explained by volatilization during industrial-scale grinding compared to beans that were ground prior to analysis. In contrast, total concentrations in the other coffee sub-groups are significantly lower due to lower furan compounds content resulting from the loss of volatiles during the process (freeze-drying in instant products), the dilution factor (e.g., powdered milk), or furan precursors such as in coffee substitutes. A significant difference was observed between the total amounts of furans in coffee (in its solid state) and coffee beverages (Figure 1). This difference is due to the evaporation during coffee preparation (e.g., brewing, percolation) and the dilution with water. Indeed, the consumer behaviour for coffee preparation has an important effect on the amount of furans retained in coffee beverage [42]. The way of coffee brewing for examples and the type of coffee machine (capsules, filters etc.) seem to influence significantly the final concentration in a cup of coffee [43]. In the other food groups, the total furans concentration was relatively high in some samples, especially in seafood, composites, and bakery products as described below. The high amount in seafood group (n = 53) can be explained by the contribution of 2-EtF (Figure S4B). The amount of this compound ranged from 50 to 600 ug/kg in canned and jarred fish such as sardine, mackerel, herring, and tuna, as well as tuna salad (n = 29); the highest level being in canned fish packed with tomato sauce (up to 1372 ug/kg) (Figure S4B). This is because raw fish products generally contain high amounts of polyunsaturated fatty acid, making them sensitive to oxidation during heat treatments [44]. The 2-EtF is thought to be derived from 2-hexenal, a typical oxidation of n-3 unsaturated fatty acids [45; 46; 47], which could explain its levels in processed fish. The low pH value also seems to induce 2-EtF formation as observed in canned fish with tomato sauce. This again can be linked to the reaction mechanism by which 2-EtF is formed from 2-hexenal, requiring a dehydration of a key intermediate, which is likely to be facilitated in acidic conditions [47]. The effect of acidic pH on furan has already been observed in fish sold in lemon juice [48].