For more than 100 years scientists have endeavoured to develop methods to quantify the free fatty acids in milk because of their impact on flavour and now scientists at Aarhus University have succeeded in doing so.

Under the management of associate professor Trine Kastrup Dalsgaard from the Department of Food Science, PhD-student Bashar Amer and lab technician Caroline Nebel have succeeded in developing a new method of deriving the free fatty acid concentrations using gas chromatography and mass spectrometry (GC-MS) measurements. For measuring with GC-MS the free fatty acids most be apolare and that is obtained by derivatizing. The  little trick, which makes this method excellent involved in the derivatization of the free fatty acids directly in the milk without no previously extraction. The in-solution-derivatization ensures the quantification of all the individual fatty acids, including the short-chain ones that are causing milk rancidity. The short-chain fatty acids are water-soluble in contrast to the fat-soluble long-chain fatty acids that are much less likely to taint the milk. 

The literature lists several methods of quantifying free fatty acids and the majority of them use the step of extracting the fat from the other milk components using an organic solvent.

- This is not really a good solution because the short-chain fatty acids are water-soluble and therefore difficult to extract. The accepted method currently used for quantifying free fatty acids in milk is the BDI method, though this has problems too with measuring all free fatty acids. The BDI method and many other methods also have the drawback that they cannot distinguish between individual free fatty acids and therefore are not able to tell whether the concentration of one is higher than the other. Since there is a large difference in the impact on off-flavour of the different fatty acids, it is actually quite important to be able to distinguish between them, explains Trine Kastrup Dalsgaard.

The GC-MS method developed is able to quantify all individual fatty acids in milk and this has been achieved by attaching an ethyl group to the fatty acids directly in the milk before the extraction. This makes the short-chain water-soluble fatty acids apolar and therefore also soluble in organic solvents.

Better milk for the consumer

In this way you are able to quantify all the free fatty acids in milk (see Table 1 below). The levels of butyric acid are 4.56, 4.71, 1.99 and 1.91 µg/mL in, respectively, untreated milk (from Holstein cows), commercial full-cream milk, semi-skimmed milk, and skimmed milk. The method can also be used in milk samples with elevated level of butyric acids and samples up to 200 µg/mL butyric acid has been measured

- The method is both fast and cheap and it meets all the requirements for a good reference method, and speedy spectroscopy methods such as FT-IR and Raman can benefit from it.

The development of good and rapid methods that can be implemented as quality control in the industry will give a better milk quality on the breakfast table, says Trine Kastrup Dalsgaard.

Further information: Associate professor Trine Kastrup Dalsgaard, Department of Food Science, telephone: +45 8715 7998, e-mail:trine.dalsgaard@agrsci.dk   

Table 1. Free fatty acids in untreated milk and three commercial milk types: full-cream, semi-skimmed and skimmed. The concentrations are the averages of five independent milk samples. (-): below the detection limit.