10 High-Pressure-Assisted Heating as a Method for Sterilizing Foods Volker Heinz and Dietrich Knorr Technical University Berlin, Berlin, Germany I. INTRODUCTION 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 heat- sterilized milk, 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 10- min 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 pack- aging systems improved the quality characteristics of thermally processed food substan- tially. 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. 207 Copyright © 2005 by Marcel Dekker. 208 Heinz and Knorr II. PRESSURE AND TEMPERATURE Thermodynamic properties and phase equilibria in any system, as well as transport properties such as viscosity, thermal conductivity, or diffusivity, must be considered in their functional relationship with temperature and pressure. The equilibrium concentrations of chemical reactions also depend on these quantities. From experiments, it is known that the formation of ions from neutral molecules, like the self-ionization of water, is promoted by increasing pressure. Applying the ‘‘Le Chatelier’’ principle to the resulting drop in pH, it can be assumed that the volume of the ions is less than the volume of the neutral molecules. In general, the impact of pressure ( p) and temperature (T) can be derived quantitatively from Gibbs’s definition of free energy G [Eq. (1)]: GuH À TS ð1Þ where S and H are the entropy and enthalpy, respectively. Using HuU+pV (U: inner energy; V: volume), the total derivative of Eq. (1) yields: dG ¼ dU þ pdV þ Vdp À TdS À SdT ð2Þ This converts to Eq. (3) when the First and Second Laws of Thermodynamics (dUudqÀpdV and dquTdS, where dq denotes the transferred heat) are applied and when only changes in free energy (DG) are considered: dðDGÞ¼DVdp À DSdT ð3Þ From Eq. (3), it follows that transitions or reactions within physicochemical systems that show a difference in free energy between educts and products are determined by simultaneously occurring changes in volume and entropy. Hence, reactions such as phase transitions or molecular reorientation depend on both temperature and pressure and cannot be treated separately. Integration of Eq. (3) using a Taylor series expansion up to second- order terms yields Eq. (4): 2 2 DG ¼ DG0 þ DV0ð p À p0ÞDS0ðT À T0ÞþðDb=2Þðp À p0Þ ðDcp=2T0ÞðT À T0Þ þDað p À p0ÞðT À T0Þð4Þ with a u (dV/dT)p: thermal expansion coefficient; b u (dV/dp)T: compressibility; and cp u (dq/dT)p: heat capacity. It is evident that this approximation yields a quadratic dependence of the change in free energy on temperature and pressure. In Fig. 1, it is demonstrated that this quadratic two-variable function converts to an ellipsoidal phase transition line in the p–T plane for constant DG. In the diagram on the right, a particular situation where DG=0 is considered for the relevant pressure and temperature range ( p z 0andT z 0). Usually, states A and B denote molecular or physicochemical states, which are reversibly or irreversibly connected by reactive pathways that finally bring about the transition to the energetically favored configuration (DG<0). In a more general approach, this concept may also apply to the inactivation of spores. In spite of the limited knowledge on the mechanistic background of the lethal action, it is clear that biochemical reactions sensitive to changes in pressure and temperature are involved. Hence, it is still a matter of speculation whether only one or multiple target sites of the sporulated organism should be regarded as the weakest link when exposed to external stress; from the point of view of preservation technology, these considerations are of less importance. Therefore, it is justified at this point to reduce the Copyright © 2005 by Marcel Dekker. High-Pressure Heating for Sterilizing Foods 209 Figure 1 Changes in free energy in response to pressure and temperature. highly complex survival strategy of bacterial spores (described later in this chapter) to two distinct states: recoverable and not recoverable. In analogy to states A and B in Fig. 1, it can be assumed that the inactivation can be brought about at various pressure–temperature conditions along the equilibrium line DG=0. From the relations given above, the rate at which the spore alters from the recoverable to the nonrecoverable state cannot be deduced. This is because the thermodynamic formulations are based on the assumption that during transition, all components involved are equilibrated, which takes an infinitely long time theoretically. In fact, the kinetics of reactive pathways must be treated in a different way. For process engineering, it is most important to know which treatment time is required to obtain the expected results, and how the process is influenced by changes in pressure and temperature. The velocity of chemical reactions can be accelerated as well as delayed by variations in pressure and/or temperature. This effect is highly dependent on the molecules involved and the reaction mechanism under consideration. In order to find a quantitative formulation of this observation, the reaction rate (k) must be related to these parameters. Whereas a reliable relation on the temperature dependence of k was first published in 1889 by Arrhenius (1889) [Eq. (5)], it took much longer until a similar expression for pressure dependence [Eq. (6)] was derived by Eyring (1935a,b) in 1935. BlnðkÞ E ¼ a ð5Þ BT RT 2 p BlnðkÞ DV Ã B ¼ ð6Þ p T RT with R: universal gas constant (R = 8.314 J/K mol); Ea: activation energy; and DV*: activation volume. Observing the right-hand side of Eq. (6), it is evident that a negative DV produces an increased reaction rate k, whereas a positive DV * indicates a delayed reaction. Examples of accelerated reactions can be found among polymerizations, cyclo-additions, and solvolytic reactions. Unfortunately, the general application of Eq. (6) is hindered by the empirical finding that DV * is not always constant, but may also vary with pressure. Another limitation is that the order of reaction might be different at different pressure levels [which is not considered in Eq. (6)]. Copyright © 2005 by Marcel Dekker. 210 Heinz and Knorr From Eqs. (5) and (6), it is obvious that experimental designs must include both pressure and temperature in the set of independent variables. Hence, systematic inves- tigations of any pressure–temperature response of a system are based on kinetic experiments at constant pressure and temperature to identify the reaction order or, possibly, the underlying mechanism of action of a more complex response. An obvious way of showing the results of such experiments is to use a contour plot with lines of constant reaction rate k in a p–T diagram similar to that of Fig. 1. The importance of these results is even more relevant when the thermodynamic relation between pressure and temperature must be taken into account. If the boundary of a system under consideration is assumed adiabatic, and if phase transitions are excluded, an increase in pressure will produce a rise in temperature. The effect of compression on aqueous systems is presented in Fig. 2. This observation can be expressed by the thermodynamically derived Eq. (7), where q denotes the density of the system. In most cases, unfortunately, the thermal expansion coefficient and the specific heat capacity are not constant within the temperature and pressure range, which is of interest, and little experimental data on food materials are available. BT aT B ¼ ð7Þ P adiabat pcp It is important to note that the temperature increase DT under adiabatic conditions depends on the maximum pressure reached, as well as on the temperature of the system before compression. For example, a pressurization of water from ambient pressure up to 800 MPa with an initial temperature of 5jC and 100jC results in a level of 23jC and 140jC, respectively.