Final answer:
Lactic acid is the compound in postmortem muscle that lowers pH and gives fermented sausages a tangy flavor. It is produced through lactic acid fermentation in muscles that have run out of oxygen.
Step-by-step explanation:
The compound in postmortem muscle that lowers pH and gives fermented sausages a tangy flavor is lactic acid. Lactic acid is produced through lactic acid fermentation, which occurs when muscles run out of oxygen during exertion. The production of lactic acid and carbon dioxide stimulates the release of oxygen from oxyhemoglobin in the blood passing through the muscle, ensuring an adequate oxygen supply to the working muscles.