Synthetics
2-mercaptopropionic acid smells like a flavoring ingredient.
Mosciano, Gerard P&F 17, No. 5, 127, (1992): Sulfury, meaty savory, brown with roasted brown nuances
Luebke, William tgsc, (1989): Meaty sulfurous roasted
R C Treatt & Co Ltd: Meaty, sulphurous, brothy, brown notes