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Fatty Acids


 Updated 2018-09-17

 Fatty acids in foods

Fatty acids

Although fatty acids are organic acids, they are in food composition and nutrition labelling regulations treated as a distinct group of components - separated from the organic acids (see also Organic acids).

The fatty acids are a natural group of carboxylic acids in foods, and the components are listed in food composition databases and tables as sums (the sum of saturated fatty acids, the sum of monounsaturated fatty acids, the sum of polyunsaturated fatty acids, and the sum of trans-fatty acids) and/or as the individual fatty acids.

It is important to note that the fatty acid representation in food composition databases and tables by tradition is listed as the single ("free") fatty acids and not the molecules they are usually bound to in the lipid (triglycerides, etc.).

Fatty acids appear in the lipid fraction of the food usually as triglycerides, phospholipids, glycolipds, lecithin, bound in sterols, like cholesterol, etc. and only to a very small extent as free acids. In order to determine the fatty acid content of a food it is therefore necessary to recalculate the lipid content based on the knowledege of the composition of the lipid (the lipid classes) in the particular food and the analysed fatty acid profile of the food.

Lipid conversion factors

Fatty acids are normally analysed by gas-liquid chromatography (GLC) after transmethylation of the lipid extract. Therefore, the fatty acid profile, i.e. the percentage of each fatty acid in relation to the total fatty acid content, is reported as fatty acid methyl esters (FAME).

Costumarily, the food composition databases and tables report the content of the fatty acids as "free" acids (FA) in the food. It is therefore important to apply the right conversion of the reported fatty acid methyl ester content by analysis to the appropriate grams of fatty acids per 100 g food.

The most important work on lipid conversion factors for calculating fatty acid contents of foods was made by Weirauch et al [1] in the middle of the 1970's. They derived conversion factors for a range of foods by estimating how much of the lipid in these foods that was actually fatty acids. The lipid conversion factors, also called fatty acid conversion factors (FACF) indicate the weight of fatty acids (FA) in one gram of lipid, i.e. the FACF is the ratio between the part of the lipids (fat), which is fatty acids, and the total amount of lipids (fat).

Weirauch et al.'s work was a result of a comprehensive evaluation of fatty acids in foods and resulted in a series of papers by Weirauch and colleagues (see references below). The work was initiated in connection with the Nutrient Data Branch's revision of the USDA Nutrient Databank in the 1970's.

Table 1 shows the original findings mentioned by Weirauch et al [1] and its background papers [2]-[16].

 

Table 1.  General lipid conversion factors (FACF, g FA/g lipid)

Food  Conversion factor Source

Wheat, barley, rye and triticale
    whole grain 0.72 [1], [8]  
Flour 0.67 [1], [8]
starch 0.60 [1], [8]
    shorts 0.80 [8]  
bran 0.82 [1]
    germ 0.93 [1]  
Oats, whole grain 0.94 [8]
Corn
    corn starch  0.70 [8]  
    commercial corn/corn products 0.86 [8]  
Rice  
    brown rice or bran 0.92 [8]  
    polished/milled rice 0.85 [8]  
    starch 0.70 [8]  
Milk, and milk products (0.945)* [1], [3]
Eggs 0.83 [1], [5]
Fats and oils
    wheat germ oil 0.95 [1], [8]  
    corn oil 0.95 [8]  
    soybean oil 0.95 [12]  
    rice bran oil 0.94 [8]  
coconut oil and palm kernel oils (0.942)* [7]
all other fats or oils (milk lipids excluded*) 0.956 [7]
Beef
separable lean 0.916 [4]
separable fat 0.953 [4]
    beef heart 0.88 [4]  
    beef liver 0.76 [4]  
  Veal      
lean meat 0.726 [12]
separable fat 0.953 [12]
  Lamb      
lean meat 0.878 [12]
separable fat 0.953 [12]
    heart 0.763 [12]  
    kidney 0.808 [12]  
    liver 0.744 [12]  
Pork
lean meat 0.910 [9]
adipose tissue 0.953 [9]
    pig's brain 0.561 [9]  
    pigs' heart 0.789 [9]  
    pig's kidney 0.747 [9]  
    pig's liver 0.741 [9]  
Poultry  
chicken, light meat 0.81 [11]
chicken, dark meat 0.86 [11]
chicken, skin 0.94 [11]
turkey, light meat 0.81 [11]
turkey, dark meat 0.85 [11]
turkey, skin 0.95 [11]
  Seafood      
Finfish, lean (<5% total lipid) (0.700)* [1], [16]
Finfish, fat (>5% total lipid) (0.900)* [1], [16]
    Molluscs * [1]  
    Crustaceans * [1]  
Vegetables and fruits 0.800
Avocado 0.956
Nuts 0.956 [6]
  Legumes      
peanut 0.951 [13]
soybean 0.930 [12]
           

*) general factors should not be used; specific factors should be used instead, see food specific sections below.

 

Weirauch et al. emphasizes that the lipid conversion factors should be taken only as indicative. The lipid conversion factors are determined experimentally and can vary substantially (for more information, see the following sections on specific food groups).

USDA documented the FACF values used in the printed versions of USDA Agriculture Handbook No. 8 (AH 8), Volumes 1 (November 1976) through 21 (September 1988). An extract of these is listed in the table Lipid Conversion Factors documented by USDA (presently as an extract copied from the prefaces of the AH 8 volumes.).

Calculating Fatty Acid Contents of Foods

Fatty acids are the class of lipids most commonly analysed by GLC following their conversion to apolar, methyl ester derivatives. Due to the conversion of the fatty acids into fatty acid methyl esters, FAME, the direct results from fatty acid analysis may come out a weight percentages, i.e. the weight of the single FAME in relation to the total FAME content (weight).

In theory, the following conversion need to take place to convert in FAME information to fatty acid (FA, as free acid) per 100 g food (inspired by PJM Hulshof, 2007):

  Weight-% FAME (g FAME per 100 g FAME)  
  step 1
  Weight-% FA (g FA/100 g FA)  
  step 2
  Weight-% FA in food (g FA/100 g food)  

 

In practice, the first step (step 1) is often omitted - maybe inadvertently. Weirauch et al [1] indicate an equation, which covers both step 1 and step 2:

(1)             FA content (g FA per 100 g food) = weight-% FAME x FACF x lipid content (g lipid/100 g food)

where FACF: the lipid conversion factor (fatty acid conversion factor, g FA/g lipid).

An analysis of the units in this equation shows that step 1 is omitted. This means that the equation is only approximately right, if the FAME content (expressed as weight-% FAME) is the same as the FA content (expressed as weight-% FA). This only holds for lipids consisting of long-chain fatty acids, where the methyl ester group does not have much influence on the ratio of the molecular weight of the fatty acid - fatty acid methyl ester.

Further investigation of Weirauch et al [1] and the background papers [2] -[16] for this article reveals that the authors have actually provided specific information for foods containing a reasonable amount of short-chain fatty acids, e.g. milk and milk products and vegetable fats/oils containing short-chain fatty acids (in [1], [7] named lauric acid oils).

In fact, the authors give specific calculation information for the fatty acids in foods with short chain fatty acids. However, in scientific literature citing Weirauch et al. this information is often lost.

More more recent works, Sheppard [17], Hyvönen and Koivistoinen [18], and Hyvönen et al. [19] use all the steps in the conversion of the data analysed as FAME to FA content per 100 g.

Step 1 includes the conversion of FAME to FA. This conversion is described in detail by Sheppard in [17]. The factors for the conversion of each fatty acid methyl ester, FAME, to the free form of the fatty acid, FA, are in some literature named after the author and called the Sheppard factors. The factors include conversion of FAME to FA, FAME to TAG (triacylglycerols, more about this expression below), fatty acid butyl esters (FABU) to FA, etc. The conversion from FAME to FA is

(2)               FA (g FA per 100 g FA)  =  FAME (g FAME) x Sheppard factor (g FA per g FAME)*100/Sum of FA (g FA)

where the Sheppard factor is determined by the molecular weights of the fatty acid (FA) and the corresponding fatty acid methyl ester (FAME). The factors are listed in Møller [20]. In this step the FAME content (g FAME per 100 g FAME) is recalculated to a normalised FA content (g FA per 100 g FA).

Step 2 is the actual step where the foods FA content (g FA per 100 g food) is calculated using the FACF and the foods total lipid content (often abbreviated to TL):

(3)              FA content (g FA per 100 food)   =   FA (g FA per 100 g FA) * FACF (g FA per g total lipid) * TL (g lipid per 100 g food)

As mentioned above, equation (1) is a shortcut that is only valid under certain circumstances. It is therefore recommended to go through both step 1 (equation 2) and step 2 (equation 3) when calculating the fatty acid content of foods.

Specific Lipid Conversion Factors

According to Weirauch et al [1], a single lipid conversion factor (FACF) should not be used for foods with a significant amount of short-chain fatty acids. Milk and milk products and lauric acid oils are mentioned in particular.
 

Milk and milk products

Weirauch et al [1] and Posati et al [3] mentions that using a single FACF of 0.945 g FA/g lipid, which is the generally accepted average weight percent of fatty acids in milk fat, would lead to an overestimation of about 9% for butyric acid and smaller errors would be introduced into the other fatty acid figures. The authors recommend a procedure similar to the procedure with step 1 including Sheppard factors above, they suggest using specific FACFs for each fatty acid methyl ester.

The conversion factors suggested by Weirauch et al and Posati et al for fatty acids of milk lipids are shown below together with the Sheppard factors for the same fatty acids:

 

 
 
  Fatty acid Conversion factor Sheppard factor (from [20])  
 
 
  C04:0  0.867 0.863  
  C06:0 0.897 0.892  
  C08:0 0.916 0.911  
  C10:0 0.929 0.924  
  C12:0 0.939 0.935  
  C14:0 0.947 0.942  
  C16:0 0.953 0.948  
  C18:0 0.958 0.953  
  C14:1 0.946 0.941  
  C16:1 0.953 0.948  
  C18:1 0.958 0.953  
  C18:2 0.957 0.952  
  C18:3 0.957 0.952  
  other 0.952    
 
 

 

The difference between the suggested conversion factor and the Sheppard factor can be explained by the "short cut" made by Weirauch et all/Posati et al to normalise the fatty acid distribution. In all cases, the conversion factor is a factor 1.005 (1.0046) higher than the Sheppard factor.

The factor 1.005 is the ratio of 3 x the molecular weight of FAME to the average molecular weight of lipid (expressed as triglyceride).

The table below shows the impact of the FAME to FA conversion on the fatty acid profiles for fatty acids in milk fat. It can bee seen from the relative profiles for FAME (g FAME/100 g FAME) compared to FA (g FA/100 g FA) that the FAME to FA conversion especially has an impact on the short chain fatty acids whereas the impact on the long chain fatty acids is neglible. This is the reason why Weirauch and Posati makes the exeption for foods with short chain fatty acids.

 


  
Fatty Acid g FAME
/100 g FAME
Sheppard factor g FA/
100 g FAME
g FA/
100 g FA
   

  
  C 4:0 4.2 0.862662 3.6 3.8    
C 6:0 4.3 0.892257 3.9 4.1    
  C 8:0 1.9 0.911358 1.7 1.8    
  C 10:0 3.0 0.924706 2.7 2.9    
  C 12:0 3.4 0.934561 3.2 3.4    
  C 14:0 10.3 0.942134 9.7 10.3    
  C 15:0   0.945299        
  C 16:0 28.6 0.948136 27.1 28.8    
  C 17:0 0.950694    
  C 18:0 12.0 0.953010 11.4 12.1    
  C 20:0 0.3 0.957047 0.3 0.3    
  C 22:0 0 0.960445 0 0    
  C 24:0 0 0.963345 0 0    
  C 14:1 1.2 0.941653 1.1 1.2    
  C 16:1 1.7 0.947750 1.6 1.7    
  C 18:1, n-9 25.9 0.952693 24.7 26.2    
  C 18:1, cis n-7 0 0.952693 0 0    
  C 20:1, n-11 1.0 0.956782 0.9 1.0    
  C 22:1, n-9 0 0.960221 0 0    
  C 22:1, n-11 0 0.960221 0 0    
  C 24:1, n-9 0 0.963152 0 0    

  


Seafood

Due to the extreme variations of the lipid content between seasons, it is not possible to use one conversion factor for finfish, molluscs and crustaceans. Exler et al. [16] have investigated the lipid classes and found means of relating the lipid conversion factor to varying lipid levels.

For finfish, the relationship between the conversion factor and the total lipid, TL, is

     FACF (g FA/g lipid) = 0.933 (g FA/g lipid) - 0.143 (g FA/100 g food)/TL (g lipid/100 g food)

Similarly, the relationships have been found for molluscs and crustaceans:

     FACF (g FA/g lipid) = 0.956 (g FA/g lipid) - 0.296 (g FA/100 g food)/TL (g lipid/100 g food)

     FACF (g FA/g lipid) = 0.956 (g FA/g lipid) - 0.273 (g FA/100 g food)/TL (g lipid/100 g food)

All three relationships are shown in the following diagram:

  FACF   
Lipid Conversion Factors of Mixed Foods

Fatty acid content and lipid conversion factors should be calculated from the proportion of fat contributed by the ingredients.

This means that the resulting lipid conversion factor for mixed food is the weighted average of the lipid conversion factors for each of the lipid contributing ingredients.

 


 References

  1. Weirauch JL, Posati L, Anderson BA, Exler J: Lipid Conversion Factors for Calculating Fatty Acid Contents of Foods. Journal of the American Oil Chemists' Society, Vol. 54, No. 1, pp. 36-40, 1977.
    [DOI: 10.1007/BF02671370]
     
  2. Kinsella JE, Posati L, Weirauch JL, Anderson BA: Lipids in Foods: Problems and Procedures in Collating Data. CRC Critical Reviews in Food Technology, Vol. 5, Issue 3, pp. 299-324, 1975.
    [DOI: 10.1080/10408397509527177]

  3. Posati LP, Kinsella JE, Watt BE: Comprehensive evaluation of fatty acids in foods - I. Dairy products. Journal of the American Dietetic Association, Vol. 66, May 1975.

  4. Anderson BA, Kinsella JA, Watt BK: Comprehensive evaluation of fatty acids in foods - II. Beef products. Journal of the American Dietetic Association, Vol. 67, No. 1, July 1975.

  5. Posati LP,  Kinsella JA, Watt BK: Comprehensive evaluation of fatty acids in foods - III. Eggs and Egg Products.  Journal of the American Dietetic Association, Vol. 67, No. 2, August 1975.

  6. Fristrom GA, Stewart BC, Weirauch JL, Posati LP: Comprehensive evaluation of fatty acids in foods - IV. Nuts, Peanuts and Soups.  Journal of the American Dietetic Association, Vol. 67, No. 4, October 1975.

  7. Brignoli CA, Kinsella JE, Weirauch JL:  Comprehensive evaluation of fatty acids in foods - V. Unhydrogenated fats and oils.  Journal of the American Dietetic Association, Vol. 68, No. 3, March 1976.

  8. Weirauch JL, Kinsella JE, Watt BK:  Comprehensive evaluation of fatty acids in foods - VI. Cereal products. Journal of the American Dietetic Association, Vol. 68, No. 4, April 1976.

  9.  Anderson BA:  Comprehensive evaluation of fatty acids in foods - VII. Pork Products. Journal of the American Dietetic Association, Vol. 69, No. 1, July 1976.

  10. Exler J, Weirauch JL:  Comprehensive evaluation of fatty acids in foods - VIII. Finfish. Journal of the American Dietetic Association, Vol. 69, No. 3, September 1976.

  11. Fristrom GA, Weirauch JL:  Comprehensive evaluation of fatty acids in foods - IX. Fowl. Journal of the American Dietetic Association, Vol. 69, No. 5, November 1976.

  12. Anderson BA, Fristrom GA, Weirauch JL:  Comprehensive evaluation of fatty acids in foods - X. Lamb and veal. Journal of the American Dietetic Association, Vol. 70, January 1977.

  13. Exler J, Avena RM, Weirauch JL:  Comprehensive evaluation of fatty acids in foods - XI. Leguminous seeds. Journal of the American Dietetic Association, Vol. 71, No. 4, October 1977.

  14. Exler J & Weirauch JL:  Comprehensive evaluation of fatty acids in foods - XII. Shellfish. Journal of the American Dietetic Association, Vol. 71, No. 5, November 1977.

  15. Anderson BA:  Comprehensive evaluation of fatty acids in foods - XIII. Sausages and luncheon meats. Journal of the American Dietetic Association, Vol. 69, 1976.

  16. Exler J:  Lipids and Fatty Acids of Important Finfish: New Data for Nutrient Tables. Journal of the American Oil Chemists' Society, Vol. 52, May 1975.
    [DOI: 10.1007/BF02557948]

  17. Sheppard AJ: Lipid Manual  -  Methodology for Appropriate Fatty Acid - Cholesterol Analysis. U.S. Food and Drug Administration, 1992. Published and Distributed by Wm. C. Brown Publishers, 2460 Kerper Boulevard, Dubuque, Iowa 52001, USA.

  18. Hyvönen and Koivistoinen - Fatty Acid Analysis, TAG Equivalents as Net Fat Value, and Nutritional Attributes of Fish and Fish Products. Journal of Food Composition and Analysis, Vol. 7, Issues 1-2, pp. 44-58, March 1994.
    [DOI: http://dx.doi.org/jfca.1994.1005]

  19. Hyvönen et al - Fatty Acid Analysis, TAG Equivalents as Net Fat Value, and Nutritional Attributes of Commercial Fats and Oils. Journal of Food Composition and Analysis, Vol. 6, Issue 1, pp. 24-40, March 1993.
    [DOI: http://dx.doi.org/jfca.1993.1004]

  20. Møller A: Fatty Acid Molecular Weights and Conversion Factors. Danish Food Information, 2011.
    http://toolbox.foodcomp.info/References/Anders Møller  -  Fatty Acid Molecular Weights and Conversion Factors

  21. Sofia Trattner, Wulf Becker, Sören Wretling, Veronica Öhrvik, Irene Mattisson, 2015:
    Fatty acid composition of Swedish bakery products, with emphasis on trans-fatty acids.
    Food Chemistry, May 2015.
    [DOI: http://dx.doi.org/10.1016/j.foodchem.2014.11.145] https://www.sciencedirect.com/science/article/pii/S0308814614018834/pdfft?md5=4101a31398979c40726f6cea5a902025&pid=1-s2.0-S0308814614018834-main.pdf


 



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