Pork: not to everyone's taste
The term halal is an important part of Islamic law. Translated as lawful or permitted, it is used to specify what is regarded as permissible in different walks of life. One of the key areas is halal food where several specific rules are applied.
Meat must be slaughtered in a particular way and no food products can contain any pork meat at all. Carnivorous animals are also banned along with alcoholic drinks. In addition, a number of foodstuffs such as gelatine, emulsifiers and flavours are regarded as questionable due to a lack of clarity about their origins.
Knowing that these items are banned is one thing, recognising that they are present in a foodstuff is quite another. Visual inspection and documentation may not be sufficiently reliable, especially for mixtures of meat such as sausages, or for ingredients like gelatine that are added to other products.
Over the years, a number of techniques have been devised to detect the presence of pork in foods, such as HPLC and DNA methods based on PCR, which have both been applied to pork and lard. FTIR spectroscopy has also been used to detect lard in mixtures of animal fats.
One further method is based on the detection of the volatile components emitted from food samples. The electronic nose, using GC to separate the volatiles for detection by a surface acoustic wave sensor, has been applied to detect lard in other foods. Now the same team of scientists, based at the Halal Products Research Institute at the Universiti Putra Malaysia, have adopted the method for halal authentication in a number of meat products.
Electronic nose sniffs out pork
The director of the Institute, Yaakoob Che Man presented the results of his studies with co-researchers Mokhtar Nurjuliana, Dzulkifly Mat Hashim and Abdul Karim Sabo Mohamed at the 9th Pangborn Sensory Science Symposium held in Kyoto during September 2011.
In the first instance, the aroma profiles of pork, beef, mutton and chicken were measured following heating for a short while at 60°C in sealed vials. The vapour was sampled with a needle and pumped into the electronic nose for preconcentration in a trap, before rapid heating and expulsion to the head of the GC column.
The compounds were separated by temperature programming and detected by a surface wave acoustic (SAW) detector comprising a quartz crystal. The retention time on the column and the changes in the crystal vibration frequency of the detector allow the various compounds and their relative amounts to be determined. The identities of the compounds were confirmed by headspace GC/MS analysis with electron ionisation.
For pure pork, a total of 43 volatiles compounds were identified by GC/MS and most were lipid oxidation products such as aldehydes and ketones. Some of these were identified by GC-SAW, such as diacetyl and 3-hydroxy-2-butanone (buttery odour), 2-methylpropanal (pungent), heptanal (soapy), trimethylpyrazine (roasted) and nonanal and decanal (soapy).
The retention times and concentration data were processed by the inbuilt software to provide circular two-dimensional representations known as VaporPrintsTM. In this format, the volatile profiles for the four meats were clearly distinguishable from each other. The analysis of sausages produced from pork, beef and chicken, as opposed to pure meat, also produced different profiles.
Although the unique 2D displays produced by the GC-SAW system were capable of distinguishing the presence of pork rather than other types of meat, they still required visual interpretation. As an alternative that did not need human judgment, the researchers turned to principal components analysis.
The first principal component alone gave good separation of the meat and sausage types describing 67% of the total variance and closer inspection revealed that the major discriminating volatile was heptanal. Three additional undefined principal components could also be introduced for further refinement of the model.
So, the electronic nose was able to discriminate between pork and other types of meat. The sample size is small, at less than 5 g, and the total analysis time is less than one minute, providing a rapid, accurate and low cost system for halal verification.
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