ISO/WD
10
© ISO 2013 – All rights reserved
Case 1 - Absence of C18:3 TFA isomers
No peak (if only one peak is detected see the comment above regarding the presence of another C20:1 isomer in
milk). The presence of only one C18:3 TFA isomer is not possible.
Case 2 - Presence C18:3 TFA isomers (a minimum two isomers, C18:3 c,c,t and C18:3 t,c,c.)
The C18:3 t,c,c peak area is approx. 80% of the C18:3 c,c,t peak area (ratio 5:4). This ratio is always constant
also when other C18:3 TFA isomers are present.
Case 3 - Presence C18:3 TFA isomers (three isomers, C18:3 c,c,t C18:3 c,t,c and C18:3 t,c,c)
The same as described above for case 2, but with the C18:3 c,t,c. The peak area of this isomer is always small
and is sometime lower than LOQ. In case of co-elution with C20:1 n-9 (or another C20:1 isomer), its contribution
on total C18:3 TFA is negligible.
Case 4 - Presence C18:3 TFA isomers (four isomers, C18:3 t,c,t C18:3 c,c,t C18:3 c,t,c and C18:3 t,c,c)
The same as described above (case 2 and 3), but with the C18:3 t,c,t. This isomer is formed by the degradation
of C18:3 c,c,t and C18:3 t,c,c (the two principal C18:3 TFA isomers). When its amount is high, the presence of
other C18:3 TFA isomers could be suspected possible. The presence of other C18:3 isomers can be confirmed
with the qualitative standard mixture (5.15). See also Annex B Figure B.6 (real example of Infant Formula
containing C18:2 TFA and C18:3 TFA from deodorized vegetable oils mixture)
Use the following terms to express TFA results:
C18:1 TFA : the sum of
trans
positional isomers from C18:1.
C18:2 TFA : the sum of
trans
isomers from C18:2 n-6 in deodorized oils (tt,ct and tc) and in milk fat (C18:2
c9,t13 + C18:2 t8,c12 and C18:2 t11,c15).
C18:3 TFA : the sum of
trans
isomers from C18:3 n-3 in deodorized vegetable oils (tct, cct, ctc and tcc).
Total TFA : Sum of C18:1 TFA, C18:2 TFA and C18:3 TFA
NOTE
The present method is not intended to determine TFA having non ruminant origin (
i.e.
industrial TFA) in foods
containing milk fat and hydrogenated and/or deodorized vegetable oil(s), but an estimation can be made using C18:1
trans
isomers distribution/ratio (
i.e.
C18:1 ∆9
trans
and C18:1 ∆11
trans
), the presence of C18:2 TFA and C18:3 TFA originated
from the vegetable oil deodorization process, and also considering the aboundance of some fatty acids in the full profile. The
detection/quantification of TFA in oils and fats used as ingredients in finish product is more precise and accurate.
NOTE
Branched-chain fatty acids (i.e. saturated iso- and anteiso-Methyl-Branched fatty acids) are also present in milk
fats and they elute with other current fatty acids. In pure dairy fats branched fatty acids most often encountered have 14 to 17
carbons in the chain and could represent between 1-2% of the milk fat profile. Thus this category of fatty acids is not
considered in the standard as labelled fatty acids, nevertheless corresponding peak areas of these fatty acids and/or other
unidentified fatty acids, can be added into “other fatty acids “(OFA). A response factor (mean of response factors from C12:0
to C24:0) can be applied for their quantification. For more information, please see the figure 6 in enclosures.
10 Calculation and expression of results
10.1 Calculation
10.1.1 Calculation of response factor
Inject into the gas chromatograph 1,0 µl of the calibrating solution (5.19 or 5.20). Determine the area of peaks
attributable to each fatty acid methyl esters present in the calibration standard mixture and calculate their
respective response factors (Rf
i
) relative to the internal standard (C11:0) by using Equation:
