M. P. Cano

High-Pressure-Assisted Heating as a Method for Sterilizing Foods

In 1899, Hite (1899) subjected milk to high hydrostatic pressure instead of high temperature to keep it from turning sour. His attempt was inspired by the sensory shortcomings of heatsterilizedmilk, which had a ‘‘cooked’’ taste, and by his knowledge of recent discoveries in the field of marine ecology, in which researchers had demonstrated that microorganisms are affected by pressure. Hite achieved a 4-log reduction in microbial count in milk with a 10min treatment at approximately 700 MPa at room temperature. A 30-min treatment at 400 MPa successfully preserved grape juice, cider, peaches, and pears without destroying the fresh flavor. Below 200 MPa, the lethal effect of pressure was found to be significantly reduced, which was in agreement with earlier findings of Chlopin and Tammann (1903). These authors already reported the resistance of bacterial spores to hydrostatic pressure, which was later proven by Larson et al. (1918). These authors found that a pressure of even 1200 MPa was not sufficient to kill Bacillus subtilis spores. Shortly afterward, Bigelow published his data (Bigelow et al., 1920, 1921), and for the first time, a scientifically based method and calculation procedure for heat sterilization of food was introduced. In these publications, the effectiveness of heat treatment was proven by quantifying the death of the bacterial spores responsible for product deterioration or foodborne illnesses. Clear processing rules were developed that accounted for the transient nature of heat transfer. It is evident that the lack of success in spore inactivation impaired the quality-retaining benefits of high hydrostatic pressure as an alternative preservation treatment. The production of shelf-stable food was (and still is) the goal of the canning industry. From the viewpoint of a process engineer, it is understandable that the optimization of thermal preservation appeared to be more promising. In fact, the development of rotary retort systems, high-temperature–short-time treatment, aseptic processing, and novel packaging systems improved the quality characteristics of thermally processed food substantially. However, due to physical limitations, a certain class of products can be identified that warrants further consideration on the effectiveness of heat treatments. In this chapter, it will be demonstrated that high pressure can be used beneficially in those situations where conventional thermal sterilization processes fail to obtain high-quality products.