DNA Analysis Addresses the Authenticity Challenge
As the molecule of life, DNA is what makes every individual unique. It is the tiny differences between one person's DNA and another's that defines every person, and makes him distinct from any other human being.
Other animals also share this individuality. So whilst every cow must have the DNA that makes it a cow, as opposed to a sheep or pig, there are differences at the species level, to make the cow a Friesian rather than an Angus. Then there are differences at the individual level, to make it cow 'a' rather than cow 'b'. Similarly, many plants share the same DNA in order to be plants, but a carrot that grows long and thin is different from a carrot that grows short and round, due to differences in their DNA, although environmental factors may also have an effect.
It is this combination of sameness, similarity, and difference that enables scientists to use DNA in authenticity studies of relevance to the food industry.
It is now common practice to use the analysis of DNA as a means of verifying the authenticity or purity of ingredients. The Molecular Biology Laboratory at Reading Scientific Services Ltd (RSSL), for example, can differentiate between >20 species of meat and >50 species of fish by using DNA techniques. These methods are used to verify the meat content of comminuted products, for example, or to identify the animal of origin for any body parts found in products as foreign bodies. RSSL can also use DNA to distinguish between nut species, and to detect allergens such as peanuts in finished goods.
Most recently, RSSL has developed a reliable DNA screening method for identifying adulteration of Basmati rice. The method has been validated on all of the commercially available varieties of Basmati rice and has been designed to give suppliers and consumers confidence in the accuracy of product labelling. The method can also differentiate between true Basmati and Basmati-like varieties such as Texmati and Jasmati.
The test can give two types of information about a rice sample. Firstly, results will identify the presence of any other non-Basmati rice present in the product. Secondly, for samples that are labelled as comprising of a single Basmati rice variety, it is possible to verify whether only that Basmati rice variety is present, or if other Basmati varieties have been used.
These tests have been designed to give organisations involved in handling rice a quick and reliable means of authenticating their supplies. Although the test has been developed to identify and distinguish Basmati rice varieties, in principle the method could be adapted to varietal testing of other premium rice varieties.
All authenticity studies, including that of Basmati rice, are characterised by three essential steps;
- DNA extraction and purification
- PCR amplification of microsatellite loci
- Analysis of PCR reaction products
Each stage must be conducted in physically separated laboratories using dedicated equipment and reagents, to avoid the risk of cross-contamination. The analytical process is tightly controlled and the procedure incorporates rigorous controls on the quality of results obtained.
Clearly, there will be differences in the analytical procedures according to the products/ingredients under test. The most obvious difference is in the selection of the microsatellite loci that are amplified in step 2. These are the regions of DNA that are common to one species or variety, but differ between species and varieties. In the case of Basmati rice, the PCR (DNA amplification) stage amplifies 9 rice-specific microsatellite loci, which have been validated for use in rice varietal identification. Fluorescent markers are incorporated into the PCR products during the amplification process to permit their detection and identification in the final stage of the test.
In some authenticity studies it is possible not just to identify the species present, but also to quantify them. This may be of value in determining the meat content of a mixed meat product for example, or in determining the extent of contamination of a non-GM crop with a GM crop.
With Basmati rice, at present, it is not possible to reach a reliable conclusion about quantities. It is only possible to confirm the presence or absence of a non-Basmati species, and to verify or refute the purity of a product being sold as 100 per cent pure. However, this represents a huge improvement on the previous situation, which left the premium-priced Basmati rice vulnerable to adulteration with lower-priced varieties. Until recently, determination of rice varieties was limited to a highly subjective visual inspection combined with fragrance assessment or simple grain size measurement. Use of DNA removes all of this subjectivity.
There are many other premium-priced ingredients that are vulnerable to adulteration, and to which DNA techniques might be applied. The challenge is in identifying the microsatellite loci that can be reliably used as definitive markers for a particular species or variety. However, once that challenge has been overcome, it is relatively straightforward to create a routine screening technique that can be applied in all cases, to give reputable suppliers the assurance they seek that their ingredients are entirely authentic.
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