Method for milk and cheese
Gas chromatography based determination of the fatty acid content of milk and cheese samples.
For this, the fat from the milk or cheese samples is first extracted by means of an organic solvent and then converted into more volatile substances. The fatty acid solutions (fatty acid methyl ester) thus obtained are measured in a gas chromatograph using flame ionization (GC-FID). For this the solution is evaporated and the vapour passed through a thin glass capillary. As soon as a particular component (in this case fatty acid) passes the detector it draws a peak. On the basis of the length of time taken to pass through the tube, these can be identified and, the components can be quantified by means of the area under the peak.
Image: Chromatograph of a fatty acid standard: the x axis shows the time in minutes, the Y axis the intensity of the components, which enables quantification of the fatty acids.
Method for meat
Determining the mineral content by means of inductive coupled plasma (ICP).
The minerals are identified using the ICP method. The meat is minced and then macerated in concentrated acid so that all the minerals dissolve. Those minerals which occur in higher concentrations (e.g. sodium and potassium) are analysed using an optical emissions spectrometer and inductive coupled plasma (ICP-OES) and the trace elements (e.g. selenium and cobalt) are analysed by means of inductive coupled plasma (ICP-MS).
The plasma is obtained by heating the solution to over 5000 °C in a noble gas atmosphere, causing a mixture of partially or fully charged particles ( ions and electrons) to form. These charged particles are accelerated through an electrical field. A detector enables the individual minerals and trace elements to be identified.
Method for apples
Determining the consistency of the flesh:
The consistency of the flesh is determined by means of a texture analyser. For this a stamp is applied to the surface of the apple and the maximum force required in order for the stamp to pierce the flesh of the apple is measured. The consistency of the flesh is an important indicator of the quality of the fruit at the time of harvest and after storage.
Determining the total sugar content:
Apple juice is obtained from the apples and measured in a refractometer. For this the characteristics of light are exploited, whereby this is refracted differently when passed through the juice (or another liquid being measured) and a prism, depending upon the composition of the juice. The total sugar content (expressed in °Brix) is calculated from the refractive index obtained in this way. Refractometers are used in viticulture among other things in order to measure the total sugar content of the grapes and thus to determine the ideal time to harvest them.
Determining the acidity:
The acidity is determined by means of titration. For this the juice obtained from the apples is mixed with dilute sodium hydroxide solution in order to reach a defined pH tipping point. That is the point at which the amount of solution added has neutralised the acid in the juice. From the amount of solution added, the amount of total acid present in the apple juice used can then be calculated.
Measuring the pH value of the apples:
The pH value is determined by means of a pH meter. For this the juice obtained from the apples is measured using an electrode, whereby the voltage between a known reference solution and that in the apple juice is determined. From this voltage the hydrogen ion concentration can be calculated, which indicates the pH value.
Determining the monosaccharides through ion chromatography:
The monosaccharides are analysed through an ion chromatography procedure. First the juice is obtained from the apples and then diluted with water. The resultant solution is analysed using an ion chromatograph. For this the electrolytic current flow in the solution is measured, this being directly proportional to the concentration of the relevant matter (monosaccharide). A chromatograph with peaks is produced, with each peak representing a particular monosaccharide. It is thus possible to obtain a profile of the monosaccharides in the apples being examined.
In the laboratory for flavour and metabolites, apples are analysed with state-of-the-art methods of analytical chemistry. Foto: I. Corrà.
| Measuring equipment / method | To measure | Used for |
|---|---|---|
| GC-FID | fatty acid profile | milk and cheese |
| inductive coupled plasma (ICP) | minerals | meat |
| refractometer | total sugar content | apples |
| titration | acidity | apples |
| texture analyser | consistency of flesh | apples |
| pH meter | pH value | apples |
| ion chromatography | monosaccharides | apples |