“…For purely convective packs, there is little or no lag period (i.e. the come-up time is very short); hence, PID and ID coincide, so that j h is equal to 1.0 (Jones, 1968). However, in actual practice, there is always some gap in time period before a product could reach the retort temperature, leading to differences in come-up time depending on the composition, size etc.…”