Why are many zinfandel wines good in alcohol? Several factors are at work, but one of the leading ones is this: zinfandel has a genetic proclivity to ripen unevenly. On the same cluster, some grapes will be perfectly ripe, some will still be under-ripe, and others will be so ripe that they’re virtually raisins. If a winemaker picks grape clusters like this, there’s a high chance that the wine will taste discombobulated—both unripe and overripe at the same time. To avoid this, most winemakers let zinfandel hang on the vine until the last of the grapes have become perfectly ripe (at which point some of them are overly ripe). Then, during fermentation, the yeasts convert the abundant sweetness of the overripe grapes into alcohol, resulting in zinfandels with a naturally high alcohol content.