A two-dimensional gas chromatography (GC×GC) study of 43 volatiles shows hexanal alone cannot gauge hazelnut aging.
The aroma of hazelnuts changes in complicated ways while they are stored by manufacturers, mostly because the fats in them slowly react with oxygen and go stale. The food industry usually checks hazelnut quality using standard tests for fat spoilage, or by measuring a single compound, hexanal, which builds up as fats break down. However, these tests do not capture the many ways hazelnuts can deteriorate.
In a recent study, researchers tracked 43 scent compounds from six different chemical groups in 428 hazelnut samples. The samples came from three harvests (2020 to 2022), two countries (Turkey and Italy), and two storage methods (vacuum-packed and stored in regular air). The researchers tested the samples at seven points over two years using headspace solid-phase microextraction comprehensive two-dimensional gas chromatography coupled with mass spectrometric and flame ionization detection (HS-SPME-GC × GC-MS/FID), which captured the scent compounds released by the nuts and separated them in two stages for a highly detailed analysis. A paper based on this research was published in the journal Food Research International.1
Hazelnuts can deteriorate in several ways that affect their quality, safety, and nutritional value. The main one is the breakdown of their fats, since hazelnuts are rich in unsaturated fats that react easily with oxygen. This sets off a chain reaction that first produces unstable compounds, which then break down further into other substances that give the nuts stale, rancid, or "off" flavors.2,3
Besides reacting with oxygen, hazelnuts also spoil because of natural enzymes still active inside the nut. One enzyme, lipase, breaks fats down into free fatty acids, which can give the nuts a sour, rancid taste, while another, lipoxygenase, speeds up the reaction of fats with oxygen.4When enzymes break down the fats in hazelnuts, they release free fatty acids, which react with oxygen and go stale up to ten times faster than fats that are still intact.5How hazelnuts are handled and dried after harvest has a big effect on how active these enzymes remain, with temperature and moisture levels being especially important in keeping the nuts fresh for longer.6
Even though it is well known that hazelnuts spoil in complex ways, most studies still rely on just a handful of scent compounds to judge freshness, most often hexanal and a few similar compounds produced when fats break down.7 “While these compounds are undoubtedly informative,’ write the authors of the paper,1 “they capture only one aspect of a broader and more complex process. Expanding the panel of markers beyond aldehydes is therefore essential to achieve a more comprehensive and mechanistically grounded description of hazelnut aging across diverse storage conditions and time scales.”
The researchers found that the year the hazelnuts were harvested turned out to be the biggest factor affecting their starting chemical makeup, showing that studies need to cover several years to find reliable signs of spoilage. How the nuts were stored mattered more over time, with vacuum packing offering clear protection after about 9 to 12 months. The data analysis consistently showed that hazelnuts do not go stale because of one compound or one chemical process. Instead, several groups of compounds change together, one after another. Hexanal, the compound commonly used to check freshness, behaved inconsistently and depended on how the nuts were stored, raising further doubts about relying on it alone to judge quality.1
“These findings,” write the authors of the paper,1 “demonstrate that an expanded multi-class volatile panel is necessary to capture the full complexity of hazelnut shelf-life deterioration, providing broader mechanistic coverage, earlier detection of degradation, and more reliable quality assessment in real industrial conditions.
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