Furan fatty acids (FuFA)

FuFA are heterocyclic lipid components with an eponymous furan moiety in the alkyl chain. They can also be regarded as epoxy-bridged fatty acids [1].

Fig. 1: Structure of (a) furan and (b) the furan fatty acid 9-(3,4-dimethyl-5-propylfuran-2-yl)-nonanoic acid (9D3)

Unfortunately data on FuFA is scarce although they are considered to be valuable food ingredients. FuFA are occurring as minor components virtually in all samples where polyunsaturated fatty acids (PUFA) are present [2]. FuFA are excellent radical scavengers which can protect PUFAs from lipid peroxidation [1]. Therefore, it is important to study their role in nutrition and their occurrence in our food. The lack of comprehensive scientific data is due to difficulties in the analysis of FuFA. Their determination is hardly possible without previous enrichment and has to be based on GC/MS analysis (while conventional fatty acids are frequently analyzed by means of GC/FID). In addition, reference standards are barely available. These problems hamper the investigation of their real relevance. With our research in the field we aim at developing methods suited for routine analysis and the generation of reference standards in order to support such research which we deem important.

General structures of FuFA

The central part of these acids is the furan moiety which can be substituted on three or all four carbons (Fig. 1 + 2).

Fig. 2: Structure and nomenclature of the furan fatty acids

The 2-position (α-position) is substituted with a straight carboxyalkyl chain with typically 7, 9, 11 or 13 carbon atoms. The second α-carbon (5-position) usually bears a propyl- (Fig. 1) or pentyl chain (Fig. 2). The β-positions are either substituted with one (then in 3-position) or two methyl groups (3- and 4-Position) (Table 1). Accordingly, the most relevant FuFA in food are substituted with odd carbon numbers in all positions (Table 1). In recent years we have detected further FuFA in food which do not fit in this scheme (see below). Moreover, some unsaturated FuFA (e.g. four in fish [5]) were also described in the literature.

Nomenclature of FuFA

The chemical names of FuFA are derived from furan substituted carboxylic acids. For instance, the FuFA shown in Fig. 2 is a nonanoic acid which is substituted in 9-position with a furan derivative. The furan ring in Fig. 2a is methyl-substituted in 3-position and propyl-substituted in 5-position. Accordingly its chemical name is 9-(3-methyl-5-propylfuran-2-yl)-nonanoic acid (Table 1). The chemical name of the FuFA in Fig. 2b is 9-(3,4-dimethyl-5-propylfuran-2-yl)-nonanoic acid (Table 1). These examples show that the chemical names of FuFA are rather complicated. For this reason more convenient short terms have been developed for daily use.
We have recently introduced short terms derived from the system of Rahn et al. based in form of “number-letter-number” [3]. The first number lists the length of the carboxyalkyl chain (i.e. “9” for nonanoic acid in the examples of Fig. 2), the last number addresses the carbon number of the alkyl chain in 5-position of the furan ring (i.e. “5” for pentyl tails as in Fig. 2) [3]. Between these numbers is a letter which i seither “D” (for 3,4-dimethylfuran, Fig. 2b), “M” for 3-methylfuran (Fig. 2a) or “F” if there are no methyl groups at all in 3- and 4-position [3]. In our system the short term of 9-(3-methyl-5-propylfuran-2-yl)-nonanoic acid is 9M3. Advantages of this system are that (i) the chemical structure of a FuFA can be directly deduced from short term and that the short terms are good to spell („nine-emm-five”) [3]. Scarcely occurring unsaturated FuFA are termed 9:1M3 („nine-one-emm-three”) or 9M3:1 („nine-emm-three-one”), similarly to the naming of conventiona fatty acids.

Occurence of FuFA acids in food

Since 2012, we are developing methods for the determination of FuFA in food by application of one FuFA standard synthesized by David Knight from University of Cardiff (UK) and further self-prepared FuFA. These methods are based on transesterification of lipid extracts (generation of methyl esters) and selective enrichment of FuFA by silver ion chromatography [2-4]. Subsequent analysis by GC/MS-SIM allows us to determine virtually all FuFA (as methyl esters) in different food matrices (Fig. 3).

Fig. 3: Daily intake of FuFA in Germany [mg] with mean values (lines in the bars) and minimum and maximum values

In this context we noted that organic milk from the German market is richer in FuFA than conventional milk (Fig. 3) [4]. In addtion, concentrations were higher in summer than in winter (Abb. 3). Interesting as well, crude soybean oil is also a rich source of FuFA. However, refining of soybean oil removes a significant portion of FuFA in the products. Since soybean oil used in food is generally refined, its contribution to the uptake of FuFA is relatively low. In Germany, the most relevant food source for FuFA ist fish, despite the comparably low consumption [4]. Application of a specific GC/MS-SIM method enabled the detection of up to 23 different FuFA [5]. In this context we developed a method [2,5] which allows to assign the structure to unknown FuFA. In this way we could identify 8D4 in fish [5]. In collaboration with Jaap van Rijn (Rehovot) we received and analyzed fish from a zero-discharge-aquaculture system and found that a high quality of fish feed is important in order to prevent formation of artefacts in form of 8F6 and other non-methylated FuFA which can be accumulated in fish [2]. A further study showed that FuFA contribute up to 64% to the fatty acids in the cholesteryl ester fraction and very little PUFAs [6].

References