Viticulture and wine chemistry: In hot-climate vineyards, why do grapes typically lose acidity during ripening?

Introduction / Context:
Acid balance is a cornerstone of grape and wine quality because acidity shapes freshness, microbial stability, and sensory balance. Warm or hot growing conditions frequently yield fruit with lower total acidity and higher pH. Understanding which organic acid is lost, and why, is essential for vineyard decisions and cellar corrections (for example, acid additions).

Given Data / Assumptions:

• Major grape acids are tartaric acid and malic acid.
• Hot climates increase berry temperature and respiration.
• Observed outcome: lower acidity and higher pH at harvest compared with cooler sites.

Concept / Approach:
Malic acid is metabolically labile and is consumed by respiration in the berry when temperatures are elevated, especially at night. Tartaric acid, in contrast, is relatively stable metabolically and mainly decreases by dilution or precipitation (tartrate salts), not by respiration. Therefore, the primary driver of acidity loss in hot climates is degradation of malic acid.

Step-by-Step Solution:

Verification / Alternative check:
Comparative must analyses from cool versus hot sites show lower malate and higher pH in warm-climate fruit. Post-harvest cold soaks do not restore malate, supporting respiration-based loss, not merely dilution.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing dilution or potassium uptake effects with true metabolic respiration of malate; assuming tartaric degrades the same way as malic acid.

Final Answer:
Metabolic breakdown (respiration) of malic acid at higher temperatures.