What effect does excessive heat have on fermentation?

Excessive heat during fermentation can significantly impact the winemaking process, primarily because yeast is highly sensitive to temperature. When the temperature rises too much, it can lead to the production of undesirable flavors, such as cooked or stewed fruit notes, and can inhibit the yeast’s ability to ferment sugars effectively. In extreme cases, very high temperatures can kill the yeast altogether, halting the fermentation process. This combination of producing off-flavors and potentially killing the yeast underscores the importance of monitoring fermentation temperatures closely to ensure a healthy and complete fermentation while preserving the wine's desired characteristics. Other options suggest benefits or neutral effects of heat, which aren't accurate in the context of excessive temperatures during fermentation.