Aroma analysis of vegemite 1. Sample preparation and aroma samplingPosted: June 11, 2013
Vegemite: that salty, yeasty, vitamin B-rich, Australian sandwich spread. Its taste and odour are distinctive, iconic and entrenched in the Australian cultural identity. But what are the chemical compounds that make up the unique smell of vegemite? This is a topic close to my heart (or nose), as I drive past the Kraft factory where vegemite is manufactured on my way to work, so the aroma often fills my nostrils and makes me crave a thick slice of sourdough toast slathered with butter and vegemite. I was shocked to find that this area of research into sensory analysis of vegemite is sorely lacking in the scientific literature, so I decided to do my own experiment* which I will take you through in a series of three blog posts.
VEGEMITE. IS GOOD.
The aromas we smell from foods, fragrances or anything with an odour, comes from chemicals that are released from the object into the surrounding atmosphere and then into our noses. In order to escape from the object into the air (and noses), these aroma chemicals must be ‘volatile’, where volatile means the compounds do not require much energy to enter the gas phase and may do so spontaneously, or with a little heat or pressure.
In order to do the analysis of the aroma of vegemite I will be using two popular analytical chemistry techniques: solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS). These will be discussed later on.
A sample of vegemite was taken directly from a refrigerated** 600 gram jar purchased from a supermarket and manufactured in Melbourne, Victoria. The sample was transferred by means of an ordinary household knife to a 20 mL glass headspace sampling vial with rubber septum. The weight of the vegemite sample transferred to the sampling vial was approximately 2 g. At the same time, a sample of vegemite from the same jar was applied to buttered toast in order to ensure the sample was of the acceptable quality.***
Headspace sampling vials will generally be of a much larger volume than the sample. The sample only takes up a small part of the vial, with the remaining volume available for the formation of an atmosphere concentrated with volatile compounds emitted from the sample. The vial lid has a rubber septum which permits a sampling needle to pierce the top while the vial remains sealed, with no gases escaping.
Headspace sampling vial showing rubber-lined cap and tool to crimp cap and seal the atmosphere inside.
Aroma sampling by solid phase microextraction (SPME)
The SPME sampler looks like this…
… and here it is taken apart…
The parts of a SPME sampler (left to right): body, needle/fibre assembly, retaining screw, depth gauge.
The needle-like part of the SPME sampler houses what we call a ‘fibre’. The fibre is made from fused silica or stainless steel and coated with a thin layer of polymer. The fibres can be made with different polymer coatings which can help you analyse different types of volatile compounds. The type of polymer coating is indicated by the colour of the needle hub (in this case red is polydimethylsiloxane). When the SPME sampler is assembled, pressure applied to the spring allows the delicate fibre to be either exposed or protected by the needle.
To encourage maximum release of volatile chemicals from the vegemite sample, the vial was placed in a laboratory oven set to 65 °C for about half an hour and allowed to come to thermal equilibrium. In order to sample the aroma, the SPME sampler needle housing the fibre is used to puncture the vial septum. Once the needle is pushed through into the vial, the delicate fibre can be exposed. When the fibre is exposed to an atmosphere of volatile chemicals, they become temporarily trapped on the polymer coating. The extraction of volatile compounds from the vial onto the fibre only needs a few minutes, in this case I did the extraction of the vegemite aroma for 15 minutes.
The SPME sampler inserted into the vial containing vegemite in a laboratory oven.
Close up of SPME sampler inserted into the vegemite-containing vial. You can just see the thin fibre circled in red.
Once the extraction of the aroma chemicals is complete, the next stage of the experiment can begin. This will be the gas chromatography-mass spectrometry and will be discussed in the next post.
*Although all of the work and analysis was done by me, full credit for the idea to do this goes to fellow practitioner of Teh Scients Dr Chris Slape (@is_chris).
**I am of the opinion (and so is Kraft) that it is entirely unnecessary to keep one’s vegemite in the fridge. However, storing vegemite in the fridge provides me with the benefits of accessibility and domestic harmony, so there it lives.