Mixtures and condensation products
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Components that consist of two or more constituent parts that can be individually identified using existing thesaurus terms can be represented by a combination of the constituent codes. For example, some compositional values are reported for mixtures that were not separated analytically, as can be the case for a mixture of the carotenoids lutein and zeaxanthine. Also, the general term condensation product can be used for various types of compound that are made up of more than one constituent part, e.g. salts and esters. Similarly to mixtures these might be represented by the identifiers of their constituent parts.
Mixtures
Reported components may be mixtures either because their constituent components were not resolved during analysis or because the constituent components are aggregated into a single value. Where the result represents the total for a group of components, this will normally be defined as a separate descriptor term in the thesaurus, e.g. "total saturated fatty acids" [FASAT]. However, for mixtures of more arbitrary sets of components, the composite component can be represented as the constituent components linked by the plus sign. For example, the unresolved mixture "lutein plus zeaxanthine" is identified as LUTN+ZEA and the sum of a selection of cis monounsaturated fatty acids as F14:1CIS+F16:1CIS+F18:1CIS. Use of the previous code for "lutein plus zeaxanthine" [LUTEZEAX] is now deprecated.
The proposed format uses the plus sign without surrounding spaces, i.e. "A+B", not "A + B". However, this does give rise to a possible conflict with proposed future codes representing metal oxidation states, e.g. for "molybdenum 4+" [MO4+] and "nickel 2+" [NI2+]. Further discussion of possible conflicts in the use of the plus sign is needed. Although oxidation state will normally be defined through a terminal plus sign, on rare occasions both conventions might be used, e.g. FE2++FE3+.
Condensation products
A condensation products results when two or more moieties combine to form a single molecule, accompanied by the loss of small molecules, often water. Food components that can be considered as condensation products include esters, other types of glyceride, and higher sugars. Their constituent parts may already be represented in the thesaurus. For example, several retinyl esters have been requested as various combinations of a retinol stereoisomer with a fatty or other organic acid. Requiring separate entries when data are reported may form a considerable burden for thesaurus maintenance.
An alternative would be to allow the combination of descriptors for condensation products in a similar way to that described above for mixtures. Above, X+Y was defined to represent "and" for mixtures and analogously a symbol might be defined that represents the expression "combined with". Possibly this could be a pair of parentheses, i.e. (). The double character would introduce further possibilities. For example, it could enclose coding to specify further information about the combination such as whether an acyl group was linked to an oxygen or nitrogen atom, i.e. X(O)Y or X(N)Y. This approach might provide useful extra flexibility in reporting components without necessarily having to add a large number of new terms to the thesaurus.
In the case of retinyl esters, terms for all-trans-retinyl palmitate, all-trans-retinyl oleate, all-trans-retinyl stearate, all-trans-retinyl linoleate and all-trans-retinyl acetate have been requested. However, it is possible that the reason the palmitate, oleate, stearate and linoleate esters were analysed and reported was that they were the commonest fatty acids, rather than the only ones. To avoid the need to predefine identifiers for retinyl esters, and possibly to avoid the need to add them to the thesaurus at all, the esters could be represented by, for example, the identifier RETOL()F16:0 for retinyl palmitate. This approach would also be clearer and avoid any problems of ambiguity in a code such as RETOLLIN. It should be discussed for introduction when version 2.0 of the Component Thesaurus is implemented.
These initial proposals for combining component identifiers to represent mixtures and condensation products used several punctuation characters to indicate the relationship between the constituent parts. However, not all of these characters are available for INFOODS tagnames. For mixtures, FAO have proposed that mixtures are represented by connecting the two parts by the string underscore-A-underscore, for example using the code CYS_A_MET for a mixture of cystine and methionine (with the "A" representing AND). The use of "_A_" for mixtures would allow harmonisation between the EuroFIR and INFOODS representations. For EuroFIR, the combined codes for mixtures are data created from separate terms in the thesaurus when necessary; they will not themselves be added to the EuroFIR Component Thesaurus. Further discussions may decide that it should be acceptable for EuroFIR to allow the alternative formats of X+Y (as in the original proposal) and X_A_Y.
It would be beneficial both to harmonisation and software development to standardise on the use of the three-character sequence "_x_" as a separator in combined component identifiers. For condensation products, the original proposal could be modified to replace the use of parentheses with the standard sequence. The separator should always include a specific indication of the relationship between the two parts, for example using "_E_" for esters. Thus retinyl palmitate would have the identifier RETOL_E_F16:0, rather than RETOL()F16:0 as in the original posting.
Please post your comments to the alternative proposals for identifying mixtures, esters and similar components as soon as possible so that EuroFIR can discuss the issues further with INFOODS and finalise the rules as part of the work on the Component Thesaurus version 2.0.
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