Chewing the fat: Aroma volatiles from animal fats extracted by two complementary techniques

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  • Published: Apr 23, 2012
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: Chewing the fat: Aroma volatiles from animal fats extracted by two complementary techniques

Fats and flavour

SPME and SDE have been compared for the extraction of volatile compounds emitted from hot beef and sheep fats for subsequent GC/MS analysis. The differences found in the profiles suggest that that the two techniques should be regarded as complementary.

Compare the flavour and aroma of raw and cooked meat and you will appreciate the effects of cooking. Raw meat has little flavour compared with cooked meat, the improvements being caused by thermal reactions which produce a whole host of compounds.

The so-called Maillard reaction takes place between sugars and amino acids to create many compounds that are responsible for the meaty flavour and darken the meat, hence the alternative name of browning reaction. These products also decompose to produce yet more flavour compounds. In addition, some of the lipids present in meat fat are thermally degraded during cooking to produce the volatile compounds that are responsible for distinguishing between different meat species.

A team of scientists in Australia has been studying the flavour of sheep and beef to identify the volatiles responsible. Sheep are vital to the national economy. The country is one of the leading producers of lamb and mutton, the largest exporter of mutton and live sheep, and second largest exporter of lamb. What is perhaps less well-known is the fact that Australia is also the world's second largest exporter of beef.

Peter Watkins and colleagues from the Co-operative Research Centre for Sheep Industry Innovation at the University of New England, Armidale, the Department of Primary Industries, Werribee, Murdoch University, and the University of Melbourne, wanted to examine the volatiles produced by heating beef fat and sheep fat, which produce much of the flavour.

Distillation versus fibre

The volatiles in both types of fat were analysed by GC/MS but the researchers wanted to test SPME as a means of extraction and compare its performance to that of simultaneous distillation and extraction (SDE), which has been more popular for meat volatiles.

As a one-step procedure, SDE uses steam distillation to extract the volatiles but it is plagued by long extraction times. SPME is also one-step and has the potential to be much faster but its suitability for animal fat extraction must be proven.

For SPME, molten fat was placed in a pre-heated vial and extracted for 60 minutes using a commercial fibre coated with divinylbenzene-carboxen-polydimethylsiloxane. The fibre was then inserted directly into the GC injector.

SDE was conducted with a mixture of dichloromethane and saturated brine, which was added to molten fat samples and distilled for 60 minutes. The organic extract was cooled for analysis. In both cases, the volatile compounds were identified from their retention indices and by comparing their mass spectra with those in a commercial library.

Sheep and beef fat volatiles

The beef and sheep fats yielded a total of 100 and 97 compounds, respectively, using both techniques, with only 44 compounds common to both techniques for both fats. There were many more compounds which could not be identified using the current methodology.

Most of the compounds belonged to the four main classes of hydrocarbons, aldehydes, acids and ketones/lactones, although the proportion of each class was not totally consistent between SPME and SDE. In all cases, hydrocarbons were most abundant, but the order of the three remaining classes did vary between technique and the type of fat.

For some compounds that were common to SPME and SDE, different proportions were extracted by one technique compared with the other but no overall trends were observed between compound class and extraction technique.

The compounds from both fats originated either from lipid oxidation or from a pasture diet. The aroma of cooked lamb or mutton is commonly associated with "mutton" or "pastoral" notes. The mutton note is attributed mainly to a range of branched fatty acids that are produced in greater amounts as the meat is cooked. Pastoral notes originate from sheep fed on pasture rather than on grain and are attributed principally to 3-methylindole and p-cresol.

In this study, branched fatty acids were not detected by GC/MS after SPME or SDE and other fatty acids were in low abundance. This could be due to their poor extraction or to the known difficulty in processing fatty acids by GC/MS, which generally requires derivatisation to esters for successful analysis.

There were some compatibility issues between SPME and SDE which affect their comparison. For instance, SPME extracted higher amounts of volatile compounds compared to semi-volatiles, due to their increased presence in the headspace where the fibre was positioned. Nonetheless, it is clear that neither technique is superior to the other, and they should be regarded as complementary for the extraction of aroma compounds from beef fat and sheep fat.

Related Links

Meat Science 2012, 91, 99-107: "A comparison of solid-phase microextraction (SPME) with simultaneous distillation-extraction (SDE) for the analysis of volatile compounds in heated beef and sheep fats"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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