Grape Sunburn Phenomenon

Grape berries undergo sunburn phenomena similar to human skin. This phenomenon introduces burnt-like flavors into wine, particularly affecting the quality of white wines where delicate aromas are crucial. Sunburned berries must be sorted and removed during harvest.

Progressive Stages of Sunburn

Grape sunburn progresses through distinct stages:

1. Initial stage: Berries turn pink in color
2. Intermediate stage: Color changes to red
3. Final stage: Berries become dark and desiccated

Completely desiccated berries yield no juice upon extraction. Even mildly sunburned berries pose risks of introducing negative flavor components into the juice during pressing. Regardless of the degree of sunburn, affected berries require removal during harvest.

True Cause of Sunburn

While direct sunlight (ultraviolet radiation) was traditionally considered the cause, recent research has identified high temperature as the actual causative factor. The effect of direct sunlight operates through an indirect mechanism whereby solar radiation increases berry temperature, subsequently triggering sunburn. This phenomenon represents a chemical change more closely resembling "carbonization" than "sunburn."

Relationship Between Sun Exposure and Quality

Necessity of Sun Exposure for Red Wine Grape Varieties

Anthocyanins, the coloring compounds in red wine grape varieties, are produced as a defensive response of berries to ultraviolet radiation. Therefore, appropriate sun exposure contributes to enhanced pigment concentration. Regional differences in solar radiation constitute a primary factor creating varietal color variations.

The Dilemma of Sun Exposure Management

The presence of anthocyanins does not improve sunburn resistance. Even in red wine grape varieties, sunburn occurs under excessive high-temperature conditions, leading to quality deterioration. Excessive leaf removal aimed at increasing pigment concentration may elevate quality risks, particularly during high-temperature years.

Considerations for Viticultural Management

During abnormally hot years, leaf removal operations require careful judgment, considering that sunburn risks may outweigh the benefits of enhanced pigment concentration.