Can I prime with fresh wort instead of corn sugar?
Yes, this is called krausening. You need to use enough (approximately 1 quart of 1.042 specific gravity wort) to raise the gravity of the finished beer by 3 or 4 points (from 1.040 to 1.043 for example). Stir in your fresh wort until the gravity rises by this amount. Keep in mind the wort must be freshly made up and sterile, as wort that has been sitting since the start of the brew (even if kept in the refrigerator sealed) will have some bacterial contamination. This is due to the fact that boiling for 1 hour does not kill heat resistant bacteria spores, which grow after a few days in freshly boiled wort. This is not a problem in freshly boiled wort, as the boiled bacteria spores will be dormant for long enough to let the yeast start and gain dominance before the bacteria can get going. The only way to truly sterilize wort is to pressure cook it at 250° F. for 20 minutes. This higher temperature and pressure is required to kill bacteria spores.
