No more rancid hazelnuts: a research team at the URV has developed a method that can identify nuts that have gone bad due to oxidation. The technique uses infrared light to determine the chemical composition of hazelnuts without even removing them from their shells. The new system overcomes the limitations of traditional methods and makes it possible to identify the condition of all the hazelnuts in a packet in a single analysis, without the need to prepare or destroy the sample. The authors argue that the application of this technology would help to improve packaging techniques and distribution systems and significantly reduce losses in the nut trade, while offering new quality standards to the sector.
Catalonia is a land of nuts, especially in the southern regions of the country. Although the sector is dominated principally by almonds, the hazelnut is second in terms of annual production. In Catalonia, there are more than 90 cooperatives producing nuts with an overall worth of more than 75 million euros and the sector is strongly export-oriented. Most of the cooperatives that produce hazelnuts are in the Tarragona area, according to data from the Catalan Federation of Agricultural Cooperatives.
Good practices in the processing, packaging and distribution of this product are crucial in order to prevent losses and guarantee its long-term quality. In hazelnuts, the oxidation of the unsaturated fatty acids they contain causes them to turn rancid. Contact with oxygen and the action of light increase these reactions, "This means that the speed of oxidation increases when the nuts are not properly packed," says Jokin Ezenarro, a researcher at the URV's Department of Analytical and Organic Chemistry and lead author of the research.
Hyperspectral cameras
With this in mind, the research team has developed a system for monitoring the oxidation of hazelnuts, which would allow producers and traders to determine their quality before buying or selling them. The method developed by Ezenarro uses a hyperspectral camera, a device capable of determining the state of oxidation throughout the package: "It is a spectrophotometer; it applies a beam of light at each point and provides information on the composition of the sample depending on how it interacts".
The device uses infrared radiation, which has a longer wavelength than visible light and a lower frequency than green light, which makes it invisible to the human eye. "All organic molecules absorb infrared light; the frequencies at which they do so and how intensely they do so vary according to their composition," points out the URV researcher. This is what allows them to identify nuts with chemical compounds that have been caused by oxidation.
While spectrometers were traditionally designed to study a single point in a sample, hyperspectral cameras are changing this paradigm. In the same way that in a conventional camera, where many points of light - pixels - make up the image, these devices determine the infrared spectrum of an entire surface. In this case it is a competitive advantage that makes it possible to determine the oxidation state of a whole bag of hazelnuts, without even removing them from the bag. —
According to Ezenarro, in analytical chemistry there is a move away from destructive and laborious analysis methods: "These new techniques are greener; they do not need reagents and do not require sample preparation; in fact, with this method the measuring instrument does not even need to come into contact with the sample". The correct functioning of the technique will depend on variables such as the material or the thickness of the packaging, which can affect the infrared spectrum. In order to establish a relationship between the electromagnetic spectrum captured by the camera and the quality - and oxidation status - of the hazelnuts, the research team had to calibrate the device. To do that, the hazelnuts were incubated for 78 days in various conditions, some of which were more conducive than others to preservation. These included being vacuum packed, storage in a protective nitrogen atmosphere, exposure to the atmosphere and exposure to various degrees of light. With these data they constructed a mathematical model capable of comparing the analytical data of the sample with its conservation status.
The new method has allowed the researchers to confirm that the main causes of hazelnut oxidation are the atmosphere that they are in contact with and the light to which they are exposed, with storage time being the main driving force behind the oxidation process. "We were able to prove that the vacuum packaging process was the most effective and that exposure to light significantly affects the stability of the product," explains Ezenarro.
Despite having demonstrated that there are measurable chemical changes on the surface of hazelnuts due to oxidation processes, the team wanted to go a step further and determine whether these have an impact on the consumer's sensory experience. According to Ezenarro,the purpose here was more about "validating the methodology; that is, determining whether what we measure is also perceptible by humans". The results of the sensory tests showed that there is a relationship between the data observed with spectroscopy and the sensory experience of people: the samples stored in contact with the atmosphere and exposed to light were significantly more rancid.
The new trend towards developing methods to control the quality of products without destroying them offers a competitive advantage to companies in many different sectors. In fact, in the nut sector, it would help to improve packaging techniques, storage and distribution systems and to significantly reduce losses, while offering new quality standards. Although the technology is not yet available to everyone (the instruments needed can cost more than 50,000 euros) systems are beginning to appear that, with processes very similar to the one developed by Ezenarro's team, can distinguish between bitter and sweet almonds or between plastics in a recycling chain. In the words of the researcher himself, "the hyperspectral camera is here to stay".
Reference: Jokin Ezenarro, Ines Saouabi, Ángel García-Pizarro, Daniel Schorn-García, Montserrat Mestres, Jose Manuel Amigo, Olga Busto, Ricard Boqué, NIR-HSI for the non-destructive monitoring of in-bag hazelnut oxidation, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, Volume 333, 2025, 125906, ISSN 1386-1425, https://doi.org/10.1016/j.saa.2025.125906
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