Corylus avellana L. Aroma Blueprint: Potent Odorants Signatures in the Volatilome of High Quality Hazelnuts

Abstract

The volatilome of hazelnuts (Corylus avellana L.) encrypts information about phenotype expression as a function of cultivar/origin, post-harvest practices, and their impact on primary metabolome, storage conditions and shelf-life, spoilage, and quality deterioration. Moreover, within the bulk of detectable volatiles, just a few of them play a key role in defining distinctive aroma (i.e., aroma blueprint) and conferring characteristic hedonic profile. In particular, in raw hazelnuts, key-odorants as defined by sensomics are: 2,3-diethyl-5-methylpyrazine (musty and nutty); 2-acetyl-1,4,5,6-tetrahydropyridine (caramel); 2-acetyl-1-pyrroline (popcorn-like); 2-acetyl-3,4,5,6-tetrahydropyridine (roasted, caramel); 3-(methylthio)-propanal (cooked potato); 3-(methylthio)propionaldehyde (musty, earthy); 3,7-dimethylocta-1,6-dien-3-ol/linalool (citrus, floral); 3-methyl-4-heptanone (fruity, nutty); and 5-methyl-(E)-2-hepten-4-one (nutty, fruity). Dry-roasting on hazelnut kernels triggers the formation of additional potent odorants, likely contributing to the pleasant aroma of roasted nuts. Whiting the newly formed aromas, 2,3-pentanedione (buttery); 2-propionyl-1-pyrroline (popcorn-like); 3-methylbutanal; (malty); 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel); dimethyl trisulfide (sulfurous, cabbage) are worthy to be mentioned. The review focuses on high-quality hazelnuts adopted as premium primary material by the confectionery industry. Information on primary and secondary/specialized metabolites distribution introduces more specialized sections focused on volatilome chemical dimensions and their correlation to cultivar/origin, post-harvest practices and storage, and spoilage phenomena. Sensory-driven studies, based on sensomic principles, provide insights on the aroma blueprint of raw and roasted hazelnuts while robust correlations between non-volatile precursors and key-aroma compounds pose solid foundations to the conceptualization of aroma potential.

Keywords: Corylus avellana L.; chromatographic fingerprinting; hazelnuts volatilome; key-aroma compounds; metabolomics; primary metabolites; volatile organic compounds (V.O.C.).

Figure 1

In-shell hazelnut production during the years 2017-2019 according to the FAO.

Figure 2

Histograms illustrating the % response ratios for analytes with larger variation between spoiled hazelnuts class-image pairs. OK samples are adopted as reference classes for comparisons. Error bars correspond to ± SD over analytical replicates.

Figure 3

Qualitative sensory analysis results illustrated as spider graphs and corresponding to four hazelnuts cultivars/blends as raw (A) and roasted (B) [23 min at 160°C] kernels. Figure from Kiefl and Schieberle (2013).

Figure 4

Linear regression functions (confidence interval 95%) between primary metabolites and/or precursors with corresponding key-odorants in roasted kernels. (A) 3-Methylbutanal with Leucine (R² 0.9577), (B) 2-Methylbutanal and Isoleucine (R² 0.9284), (C) 2,3-Butanedione and (D) 2,3-Pentanedione and the sum of fructose (syn- and anti- forms) and glucose (glucopyranose and glucose) (R² 0.8543 and 0.8860), (E) 2,5-Dimethylpyrazine and Alanine (R² 0.8822), and (F) pyrroles and the sum of Ornithine and Alanine (R² 0.8604) (from Cialiè Rosso et al., 2020).