Hi Michael, good question. Zinfandel has a genetic proclivity to ripen unevenly. That means that on the same cluster, there will be perfectly ripe grapes, under-ripe grapes, and some grapes that are so ripe they’re virtually raisins. If a winemaker picks grape clusters like this, there’s a good chance that the wine will taste weird–both unripe and overripe at the same time. To avoid this, most winemakers let zinfandel hang on the vine until the unripe grapes have become perfectly ripe; at which point, many of the other grapes have turned overly ripe. Then, during fermentation, the yeasts convert the abundant sweetness of the overripe grapes into considerable alcohol. The result is that many zinfandels can’t help being a bit high in alcohol.