Kynurenic Acid: A New Potential Sensory Marker in Sweet Chestnut ( <i>Castanea sativa</i> ) Honey

Kynurenic acid (KYNA) is a tryptophan metabolite with demonstrated pharmaco-active properties determined for the first time in several Spanish monofloral honeys collected in five regions with protected quality designation. Among monofloral honeys, sweet chestnut ( Castanea sativa ) and bell heather ( Erica cinerea ), both bittertasting, are often confused. Although chestnut honey meets all of the parameters established for this kind of monofloral honey, it is commonly recognized sensorially as heather honey by mistake, making their commercialization as chestnut honey unfeasible. In the present study, KYNA has been defined as a new sensory marker as a corrective action. A total of 246 honey samples were analyzed by direct injection UHPLC/QTOF-MS/MS, equipped with selected reaction monitoring (SRM) as static mass filters with high mass-resolution capacity. Samples with C. sativa pollen levels (>75%) showed KYNA higher contents but varied significantly between areas: northwest (68–1893 mg/kg), south (19–919 mg/kg), and west (51–539 mg/kg). In forest and mountain honeys, KYNA content depends mainly on the presence of honeydew honey. In rosemary, bell heather, and avocado honeys, only traces are detected (average <2.5 mg/kg). The bitterness threshold determined by KYNA is 78 mg/kg, which facilitates the sensory resolution between chestnut and heather honeys. Below that threshold, KYNA becomes decisive to characterize chestnut honey. Thus, as characteristic of chestnut honey, a KYNA content of at least 50 mg/kg was found in 97.5% of the samples and a taste activity value (TAV > 1) in 88% of the samples.