Characterization of the odor‐active compounds responsible for atypical aroma notes in cocoa (Theobroma cacao L.)

Abstract

Cocoa is the key raw material in chocolate manufacturing. The quality of cocoa is crucial for the pleasant aroma of the final confectionery products. Ideally, cocoa shows a rich aroma with floral, sour, malty, earthy, and fruity notes. However, chocolate manufacturing companies occasionally report the presence of off‐flavors in fermented cocoa at the level of the incoming goods inspection. Among them, moldy‐musty and coconut‐like notes are frequently observed. It is hard to establish the presence of off‐flavors on the base of sensory data only, as it has mainly been done so far, because this type of assessment is very subjective. Therefore, there is a strong need for an objective method based on analytical data to affirm whether a cocoa batch is suitable or not for further manufacturing. To clarify the molecular background of the moldy‐musty and the coconut‐like notes, the volatiles were isolated from fermented cocoa samples tainted with off‐flavors by solvent extraction and solvent‐assisted flavor evaporation (SAFE). The odor‐active compounds were determined by comparative aroma extract dilution analyses (cAEDA), using flawless cocoa samples as reference.The first part of the investigation was focused on cocoa samples with a moldy‐musty off‐flavor. Application of cAEDA revealed (‐)‐geosmin, 4‐methoxy‐2,5‐dimethylfuran‐3(2W)‐one, 1W‐indole, and 3‐methyl‐1W‐indole as potential off‐flavor compounds. This compound selection was based on their odor quality and higher flavor dilution factors in the off‐flavor cocoa than in the reference sample. Quantitation of the four compounds in nine off‐flavor cocoa samples and calculation of odor activity values (OAVs; ratios of the concentrations to the odor threshold values) suggested crucial roles of (‐)‐ geosmin and 3‐methyl‐1W‐indole for the off‐flavor. In the chocolate industry, their quantitation can be used to objectively assess the moldy‐musty off‐flavor at the level of the incoming goods inspection. Because both compounds are inhomogeneously distributed between testa and embryo, separate quantitation in the two parts of the seeds is required.The second part of the investigation was focused on cocoa samples with a coconut‐like note. Application of cAEDA revealed coconut‐like smelling compounds δ‐octalactone, δ‐2‐octenolactone, γ‐ nonalactone, γ‐decalactone, δ‐decalactone and δ‐2‐decenolactones as potential causative odorants. Quantitation of these six compounds and calculation of OAVs suggested δ‐2‐decenolactone as the crucial compound. Chiral analysis showed the presence of pure (R)‐δ‐2‐decenolactone, commonly referred to as massoia lactone. Its key role for the coconut note was finally demonstrated in a spiking experiment: the addition of (R)‐δ‐2‐decenolactone to the reference cocoa in an amount corresponding to the concentration difference between the two samples was able to provoke a coconut note in an intensity comparable to the one in the atypically smelling cocoa. To avoid an undesired coconut note caused by (R)‐δ‐2‐decenolactone in the final products, the chocolate industry may consider its odor threshold value, that is 100 μg/kg, as a potential limit for the acceptance of fermented cocoa in the incoming goods inspection.Caterina's thesis is publication‐based and includes the following three papers: