Thermal sterilization of canned food is widely used in the food preservation industry. This method ensures the microbiological safety of the products, helps to define their sensory properties, and preserves their nutrients. However, sterilization requires a significant amount of energy, which has an impact on the cost of the final product and the environment. In this paper, we present an experimental study into the energy balance in a vertical steam retort in a transient state during the venting stage with an initial product temperature of 4 °C and a container size of 63.5 × 30.16 mm. We carried out the experiments in accordance with the protocol for conducting temperature distribution studies developed by the Institute for Thermal Processing Specialists (IFTPS). Additionally, by means of a heuristic optimization method, we determined an optimal venting program for the process and its energy efficiency. The total energy consumption was 32.5 MJ; the thermal efficiency of the process, 43.88%; the losses through walls, drainage, venting, and purging, 27.82%; the energy required to heat the retort and the fittings, 22.91%; and others losses, 7.0%. Furthermore, we studied the phenomena associated with the sterilization process to propose strategies to reduce energy consumption.