Gently whisk the lime juice and zest into the base.
Pour the mixture into the container of an ice cream machine and churn according to the manufacturer's instructions. Just after churning quickly stir in the graham cracker crumbs.
Transfer to an airtight container and freeze for at least 2 hours before serving.
NOTE
To crush graham crackers, place about 5 graham crackers in a large zip-top bag.
Place on a work surface and roll over them with a rolling pin, heavy jar, or wine bottle. Alternatively, break them up a bit by hand, then process in a food processor into crumbs.
