Time-Dependent Degradation of Naphthoquinones and Phenolic Compounds in Walnut Husks

SIMPLE SUMMARY: Some studies have examined the degradation of individual phenolic compounds. However, the degradation of naphthoquinones has been poorly investigated, with contradicting reports. This study investigated how individual phenolic compounds and phenolic groups (e.g., naphthoquinones, flavanols, flavonols, hydroxycinnamic acids) are oxidized over time in walnut husk gratings, to better explain, or confirm, the process of the juglone release pathway. The study was designed to initially determine whether the proposed juglone synthesis pathway of hydrojuglone glucoside → α-hydrojuglone → juglone is indeed correct, or whether an alternative pathway is seen. The study also provides new data about the degradation of individual phenolics and phenolic groups over time when damaged tissue is exposed to the air. As phenolic compounds are considered highly beneficial to human health, increases upon processing indicate the need for further investigations into healthier food preparation processes. ABSTRACT: The aim of the present study was to investigate how individual phenolic compounds and phenolic groups in walnut husk gratings (e.g., naphthoquinones, flavanols, flavonols, hydroxycinnamic acids) are oxidized over time, with a particular focus on the juglone synthase pathway. Walnut husk gratings were prepared and left under ‘degradation’ conditions (exposure to the air, room temperature) at increasing times. Following methanol extraction of these husk gratings, the HPLC profile of methanolic extract of husk gratings exhibited twenty-six compounds over time, then hydrojuglone glucoside, α-hydrojuglone, and juglone were detailed by HPLC-mass spectrometry. Initially (0–20 min), the content of hydrojuglone glucoside in the husk gratings decreased by 40.4%, while the content of α-hydrojuglone increased by 20.0%, and then decreased. After an initial delay (0–20 min), juglone increased by 47.9% from 20 to 40 min, and then decreased. This initially confirmed that hydrojuglone glucoside and α-hydrojuglone could be considered as precursors of juglone. Different phenolic groups showed different degradation processes, although they all reached their highest content after 40 min. This might arise from degradation of the phenols, increased free phenols, or activation of the plant defense mechanism due to damage to the tissue, similar to the effects of stress or a pathogen attack. Although it has been reported that the phenolic compounds decrease when food is processed or damaged, they showed increases, which were not indefinite, but time dependent. As phenolic compounds are considered highly beneficial to human health, increases upon processing indicate the need for further investigations into healthier food preparation processes. This is the first study on the degradation pathways of juglone, using a mass spectrometer, in which we suggest that hydrojuglone glucoside and α-hydrojuglone are indeed the precursors of juglone. However, it is possible that there are other degradation pathways of hydrojuglone glucoside, since less juglone is synthesized than expected.