History of fermentation science: Who first identified the chemical that causes the souring of milk (lactic acid)?

Introduction / Context:
Lactic acid is the key organic acid responsible for the sour taste of fermented milk products. Distinguishing who discovered the acid itself versus who explained the biological mechanism of lactic fermentation is a classic history-of-science question.

Given Data / Assumptions:

• The question asks about the chemical (lactic acid), not the microbial mechanism.
• Multiple historical figures contributed to fermentation science in different ways.

Concept / Approach:
Carl Wilhelm Scheele (often cited simply as Scheele) isolated and described lactic acid in the 18th century from sour milk. Decades later, Louis Pasteur elucidated that lactic acid fermentation is a biological process carried out by microorganisms, establishing the role of lactic acid bacteria. The distinction is between chemical discovery and microbiological explanation.

Step-by-Step Solution:

Verification / Alternative check:
Chemistry histories record Scheele’s many acid discoveries (e.g., oxalic, lactic), predating Pasteur’s microbiology era.

Why Other Options Are Wrong:

• Pasteur: established microbial causation of fermentation, not the first to discover lactic acid.
• Ronald Ross: known for demonstrating malaria parasite transmission by mosquitoes.
• Watson: associated with DNA structure (with Crick), unrelated to lactic acid discovery.

Common Pitfalls:
Conflating discovery of a compound with later elucidation of its biogenesis.

Final Answer:
Scheele