A come-up time correction factor of 42% has been traditionally accepted for thermal processing, and was found to be applicable for model foods heated in cans in a Steritort and a still retort when the retorts were heated to processing temperature with a constant rate of temperature rise. Correction factors were larger when both retorts were heated with an initial rate of temperature rise faster than linear heating. A 77% correction factor was measured during Orbitort processing, and was more conservative for predicting processing time than assuming 42%. Product initial temperatures of 70 and 160°F resulted in correction factors of 70 and 45% when the still retort was heated linearly, and reached 83 and 69% with normal venting. The experimental procedure for predicting the come-up factor does not require instantaneous retort heating as a baseline for comparison.