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Detecting Organic Fruit and Vegetables Using Isotope Fingerprints

Team TFS
Team TFS

shutterstock_94720522The Problem

Organically grown products, such as fruit and vegetables, are more expensive because the farming practices used for growing them are more environmentally friendly, the yield of good quality product is lower and they are considered heathier and safer to eat. The higher cost to consumers is also related to the certification process that allows produce to be labelled and certified as organic. The certification process follows standards set out in EC Council Regulation No 834/2007 which, among other things, excludes the use of synthetic fertilizers.

The higher price that organic fruit and vegetables attract inevitably leads to economically motivated fraud through the mislabelling of produce as ‘organic’ when they have been grown using synthetic fertilizers on non-organically certified farms. Identification of mislabelled products is essential for protecting the brand reputation of organic food-producing companies and maintaining consumer confidence.

Of the range of organic produce available, tomatoes are one of the highest volume fruits (and yes, a tomato is definitely, scientifically speaking, a fruit!) sold globally, so the cost of fraud resulting from their mislabelling is potentially significant.

Differentiating between organic and non-organic produce presents a challenge as it requires an analytical technique that is capable of clearly and confidently distinguishing between products grown using organic fertilizers (such as such as peat, sewage sludge and animal manure) from those grown using synthetic fertilizers (such as potash and ammonium nitrate).

The Analytical Solution

So, how can this be done? There are various potential approaches available which vary in their specificity, complexity and cost. Examples include elemental profiling, metabolite and protein measurement and stable isotope ratio comparison.

Trace element profiling offers some differentiation capability as a result of different concentrations of these elements in fertilizers from organic and non-organic sources, but this is highly dependent on these concentrations being consistently different across different batches of fertilizers.

Studying differences in metabolite and protein composition in organic and non-organic products shows promise but requires more complex and costly instrumentation, as well as a higher degree of technical knowledge to interpret the results.

Measuring differences in stable isotope ratios between samples of different origin, in particular the 15N/14N ratio, offers the required specificity at a reasonable cost and suitable level of practical usability. Vegetables grown using organic fertilizers, tend to have nitrogen isotope fingerprints between +8‰ to +20‰, whereas products grown using synthetic fertilizers have nitrogen isotope fingerprints of +3‰ to +6‰. This is because of enrichment of 15N due to processes such as ammonia volatilization, denitrification and nitrification of the synthetic fertilizer that occur in the soil prior to plant uptake (the delta value is a means of expressing the ratio of a particular isotope pair in a sample relative to the same ratio in a certified isotope standard). The instrumentation used to make these measurements is isotope ratio mass spectrometry (IRMS), and one example of this technology is the Thermo Scientific™ Delta V™ Isotope Ratio Mass Spectrometer.

Applying Stable Isotope Analysis to Tomatoes

In order to measure the nitrogen isotope fingerprint in samples such as tomatoes using IRMS, the nitrogen needs to be extracted from the sample first. This is usually done by drying and homogenising the sample, combusting it in the presence of oxygen and finally reducing the nitrogen oxides so formed over hot copper to produce nitrogen gas. This nitrogen is then passed into the IRMS and the 15N/14N ratio measured.  This approach, referred to as elemental analysis isotope ratio mass spectrometry (EA-IRMS), has been successfully applied to analysis of tomatoes from different sources using the Thermo Scientific™ EA IsoLink™ IRMS System. The figure below shows just how clear the distinction between organic and non-organic tomatoes can be using EA-IRMS technology.

[caption id="attachment_18972" align="alignnone" width="737"]isotope-fingerprint-graph Click to enlarge[/caption]

If you’d like more details on this analysis, the full application note can be found here.

EA-IRMS isn’t limited to identifying whether fruits and vegetables are organically grown. This powerful technique can also be effectively applied for food authenticity testing, including detection of honey adulteration and proving the provenance of wine.

As this article has shown, the combination of organic elemental analysis and isotope ratio mass spectrometry provides an excellent solution for detecting and combating food and beverage fraud. If you’d like to learn more about what these technologies can do for your food testing applications, take a look at our Food Authenticity and Labeling web pages. To browse application notes, scientific posters and webinars visit our dedicated food integrity webpage.

Thermo Scientific offers a wide range of other analytical solutions to help you achieve your food safety, authenticity and QA/QC objectives. If you have any questions about methods, workflows or products for these application areas, from trace elemental analysis and chromatography to organic elemental analysis and high resolution mass spectrometry, just let us know via the comments box below.