1) High-pressure sterilization of foods can maximize benefits by taking into account adiabatic heating during pressurization. Maintaining an adiabatic (no heat loss) process allows lower temperatures and pressures to achieve sterilization. 2) Adiabatic heating causes the food temperature to rise 3-9°C for every 100 MPa of pressure increase. Most research has not controlled for heat loss, resulting in lower than expected temperatures. 3) A single, high-pressure pulse sterilization method could be made economically feasible by using vessel materials limiting heat transfer and rapidly pressurizing food to over 700 MPa while maintaining an adiabatic process.

1. FOOD
TECHNOLOGY Feature
High-Pressure Sterilization:
Maximizing the Benefits
of Adiabatic Heating
Taking into account the importance of temperature and temperature control
could make high-pressure sterilization economically more feasible.
Wouter B.C. de Heij, Ludo J.M.M. van
Schepdael, Roy Moezelaar, Hans
Hoogland, Ariette M. Matser,
and Robert W. van den Berg
W hen a product is pressurized, an adiabatic temperature
increase always occurs as a result of compression heating.
However, most researchers working to develop high-
pressure sterilization processes have not taken full advantage of that
phenomenon.
This article describes an approach that maximizes the benefits of adia-
batic heating and thereby increases enormously the feasibility of high-
pressure sterilization of foods. It reports on the importance of tempera-
ture control during high-pressure sterilization and describes the develop-
ment of a new high-pressure sterilization process utilizing a single pres-
sure pulse.
High-Pressure Sterilization
High-pressure processing has emerged as an attractive processing tech-
nology for mild preservation of foods. At ambient temperature, pressures
in the range of 200–600 MPa reduce the number of microorganisms and
inactivate enzymes involved in product spoilage, while the product retains
its fresh or just-prepared appearance, organoleptic characteristics, and nu-
tritional quality.
Since the introduction of high-pressure processed products in 1990,
the range of products has gradually expanded and now includes jams, jel-
Authors De Heij, Moezelaar, and Matser are, respectively, lies, sauces, fruit juices, avocado puree, and ham. The effect that these
Senior Scientists in Process Engineering, Microbiology, and pressure treatments have on microorganisms is comparable to that of pas-
Food Technology, and author Van den Berg is Head of the Dept. teurization, and the products therefore have an extended shelf life under
of Preservation Technology & Food Safety, Agrotechnological refrigerated conditions.
Research Institute (ATO b.v.), P.O. Box 17, 6700 AA To achieve a commercially sterile product that can be stored at ambient
temperatures, bacterial endospores also need to be inactivated. Two mech-
Wageningen, The Netherlands. Author Van Schepdael is
anisms have been shown to result in high-pressure inactivation of bacteri-
President, Solico, Everdenberg 97, 4902 TT Oosterhout, The
al spores. At lower pressures and temperatures, pressure induces spores to
Netherlands. Author Hoogland is Senior Technologist, germinate, and the germinated spores are subsequently inactivated by the
Mechanical Engineering, Unilever Research Vlaardingen, P.O. pressure treatment (Gould and Sale, 1970; Wuytack et al., 1998). At higher
Box 114, 3130 AC Vlaardingen, The Netherlands. Send reprint temperatures, a direct spore-inactivation mechanism that bypasses germi-
requests to author Moezelaar. nation is more likely (Maggi et al., 1996; Mills et al., 1998; Ananta et al.,
VOL. 57, NO. 3 • MARCH 2003 FOODTECHNOLOGY 37

2. High-Pressure Sterilization
2001). The inactivation curves that are observed under such near the vessel wall cools down and does not reach the same
conditions usually do not follow first-order kinetics and are temperature as the product fraction in the center of the vessel
therefore difficult to interpret (Okazaki et al., 2000; Ananta et (De Heij et al., 2002; Ting et al., 2002).
al., 2001). Many experimental designs for studying the kinetics of in-
In the patent literature, three different processes have been activation by high-pressure lack measures to control the
described for the production of commercially sterile food achieved temperature during treatment (FDA, 2000); this may
products: be the cause of tailing inactivation curves often observed (Ting
• Application of a single pressure pulse of at least 700 MPa et al., 2002).
for 15 min or longer to a product which is first preheated to a In the experiments that formed the basis for the patented
temperature above 90°C (Wilson and Baker, 2000, 2001). high-pressure sterilization processes at elevated initial temper-
• Application of at least two pressure pulses of 700–1,000 atures (Meyer, 2000, 2001; Wilson and Baker, 2000, 2001), no
MPa to a product which is first preheated to a temperature of measures were taken to prevent heat losses during pressuriza-
70–90°C (Meyer, 2000, 2001). tion. Assuming that the lowest temperature that a product ex-
• Application of a single pressure pulse of at least 70 MPa periences during pressurization determines whether that prod-
for more than 12 hr (Hirsch, 2000). uct is indeed sterilized or not, a similar degree of microbial in-
Lifetime and maintenance costs of high-pressure equipment activation may be obtained with lower final temperatures when
are related to the number of pulses. Therefore, the processing heat losses to the vessel wall are minimized. Lower final tem-
cost per cycle increases with the number of pulses applied. Pro- peratures would allow the process to be operated at a lower
cessing cost also increases with the total cycle time. Besides re- initial temperature or a lower working pressure, and would
ducing the processing costs, a short processing cycle minimizes therefore make high-pressure sterilization economically more
the effects on the product quality. For a commercial produc- feasible.
tion process, a short production cycle involving a single pres- With an axi-symmetric one-dimensional finite element
sure pulse would be preferred. model for heat conduction, it can thus be predicted that in an
Since 1997, our research consortium consisting of Unilever adiabatic high-pressure sterilization process involving applica-
Research Vlaardingen, Stork Food & Dairy Systems, and the tion of a pressure pulse of 700 MPa, the initial product temper-
Agrotechnological Research Institute ATO has been studying ature may be lowered from 90°C to 80°C (Table 2) or the pres-
several aspects of high-pressure processing, including design of sure may be reduced to 500 MPa.
equipment (Bartels, 1998; Van den Berg et al., 1999); effects on
microorganisms and product quality (Krebbers et al., 2002a, Technical Solutions
b); and packaging and consumer acceptance, with the goal of To achieve a cost-effective and commercially feasible, (qua-
developing safe, cost-effective high-pressure processes that ren- si-) adiabatic high-pressure sterilization process, the following
der high-quality products. robust, technical solutions may be implemented (Schepdael et
al., 2002):
Adiabatic Temperature Increase • Apply a high pressurization rate (>5 MPa/sec). For exam-
In addition to elevated initial product temperatures, many ple, fast pressurization can be obtained by using a system with
studies reported in the scientific and patent literature that eval- an internal intensifier (Van den Berg et al., 1999).
uate high-pressure inactivation of bacterial endospores also ex- • Apply a vessel material with low heat-transfer properties
ploit the abiabatic temperature increase of the product that is (less than 1 W/m/°K) and/or with an adiabatic temperature
the result of compression heating. Depending on the nature of rise that is equal to or greater than the adiabatic temperature
the product, the initial product temperature, and the applied rise of the food product (4–8°K/100 MPa). Among the vessel
pressure, the adiabatic temperature increase may vary from 3 materials suitable for this application are polyoxymethylene
to 9°C/100 MPa (Table 1). (POM), polyetheretherketone (PEEK), or (ultra-high-molecu-
However, while the temperature of a product may thus rise
20–40°C during high-pressure treatment, the metal pressure
vessel that surrounds the product is not subjected to significant
compression heating (Fig. 1). As a result, the product fraction
Table 1—Temperature changes of selected substances
due to compression heating.
Initial temperature Temperature change
Substance (°C) (°C/100 MPa)
Water 20 2.8
60 3.8
80 4.4
Fig. 1—Temperature profile during high-pressure treatment in a typical
Steel 20 ~0
metal pressure vessel (left) and cross-section of the vessel (right). T0 is the initial
Chicken 20 2.9 product temperature, T1, adiabatic is the theoretical final product temperature that is
Cheese (Gouda type) 20 3.4 achieved as a result of compression heating, and T1,c and T1,w are the tempera-
tures of the product fraction in the center and near the vessel wall, respectively,
Milkfat 20 8.5 during pressure treatment.
38 FOODTECHNOLOGY MARCH 2003 • VOL. 57, NO. 3

3. lar-weight polyethylene (UHMWPE). These materials can be
applied to the interior of the high-pressure vessel as a liner, or
the product container can be constructed from these materials.
• Avoid heat losses from the product to the colder pressure
fluid entering the pressure vessel. This might be obtained by
(a) using an internal intensifier system to generate the pressure,
(b) heating the high-pressure pipes and/or the high-pressure
pump (external intensifier system), or (c) avoiding thermal
contact between the pressure liquid entering the vessel and the
product to be treated, e.g., by inserting the product into a
product container.
• Avoid heat losses from the product to the pressurization
fluid during pressure treatment by choosing a pressurization
fluid with an adiabatic temperature rise that is equal to or
greater than that of the product.
Fig. 2—Laboratory-scale prototype of a high-pressure sterilization system
Whether only one or a combination of these solutions is
which maximizes the benefits of the adiabatic increase in product temperature
implemented depends on the improvement in relation to the resulting from compression heating.
costs of implementation. In a small-scale laboratory device, ap-
plication of a product container constructed from, for exam- temperature and to a relatively small extent to pressure.
ple, polyethylene or POM with an appropriate wall thickness For example, an isothermal (T1 = 100°C) increase from 300
(>5 mm) will sufficiently control the temperature during treat- MPa to 800 MPa yields an inactivation rate similar to that of an
ment. isobaric (p = 300 MPa) increase from 100°C to 109°C. A similar
pattern was observed by Ananta et al. (2001), who evaluated
Lab-Scale System Tested both a first- and a 2.5-order kinetic reaction to calculate the k-
A high-pressure system in which all of the above solutions values. The importance of the final product temperature for the
have been implemented has been developed by our consortium degree of spore inactivation confirms the need for measures for
for laboratory-scale experiments (Fig. 2) and used to study the controlling the temperature during high-pressure sterilization.
kinetics of inactivation of Bacillus stearothermophilus spores. It has been suggested that high-pressure sterilization requires
This organism was chosen as a target organism because its at least two pressure pulses because a single pressure pulse would
spores are among the most heat resistant known. only sublethally injure cells (Meyer, 2000, 2001). This suggestion
For production of spores, B. stearothermophilus strain was based on the observation that no surviving spores were
ATCC 7953 was cultivated at 55°C on Spo8 medium (Faille et present immediately after treatment whereas growth had oc-
al., 1999). After 7 days of incubation, spores were harvested, curred in similarly treated samples after one week of storage.
washed in demineralized water, and stored at 7°C. Spores pro- We evaluated this phenomenon using spores of Bacillus subti-
duced according to this procedure were characterized by a lis. In contrast to B. stearothermophilus, this species is able to
thermal decimal reduction time D120 of 8.8–9.9 min and a z- grow both aerobically and anaerobically at ambient tempera-
value of 6.5–6.7°C. tures. Serially 10-fold-diluted spore suspensions of B. subtilis
For high-pressure treatments, spores were suspended in strain 168 were prepared in growth medium consisting of pep-
tryptone soy broth to a final density of 106–107 spores/mL. Ali- tone and sodium chloride and transferred to polyethylene
quots of this suspension were transferred to polyethylene pouches. Some of the pouches were used immediately after
pouches and stored on ice until processing. Prior to high-pres- treatment to determine the viable count by plate counting in
sure treatment, the spore suspensions were preheated for 1 min
at the desired initial product temperature. Spores in the un-
treated spore suspensions and surviving spores in the treated
pouches were enumerated by pour plating in tryptone soy agar.
From the observed reductions in viable count, the reaction
rate k was calculated as function of final pressure and final
temperature. A total of 29 k-values corresponding to various
combinations of temperature (84–122°C) and pressure (300–
800 MPa) were obtained and fitted with the modified Eyring-
Arrhenius equation:
where k and kref are in sec-1, T and Tref are temperature in oK,
and p and pref are pressure in MPa. The data fit this first-order
kinetic equation with an explained part of 99%.
Fig. 3 shows the dependency of k and D as a function of p Fig. 3—Reaction rate k in sec–1 (expressed as its natural logarithm) and
for several final product temperatures. From this graph, it can decimal reduction rate D (expressed in min) of high-pressure inactivation of
be concluded that over the range of 300–800 MPa, the degree spores of B. stearothermophilus ATCC 7953 as a function of the applied
of spore inactivation is largely determined by the final product pressure at various final product temperatures.
VOL. 57, NO. 3 • MARCH 2003 FOODTECHNOLOGY 39

4. High-Pressure Sterilization
Fig. 4—Simulation of high-pressure inactivation of spores of B. stearothermophilus ATCC 7953 in a system without (left) and with (right) a product container. Upper
panel shows the pressure profile; middle panel shows the temperature profiles in the center (red line) and near the wall (blue line) of the vessel; and lower panel
shows viable spore count.
tryptone soy agar. The remaining pouches were stored at 37°C a useful tool for optimizing high-pressure sterilization processes.
for 30 days, then were visually evaluated for growth. In combination with a database of kinetic inactivation parame-
From the pattern of occurrence and absence of growth in the ters of relevant target microorganisms such as Clostridium botu-
pouches (2–3 replicates per spore concentration), the number of linum, the model may also assist in the validation of high-pres-
surviving spores was estimated by the most probable number sure sterilization processes (Sizer et al., 2002).
(MPN) method, a statistical approach for the enumeration of
microorganisms in serial dilutions, according to Food and Drug Increased Use Foreseen
Administration guidelines. The MPN of surviving spores after As discussed above and shown in Fig. 5, compared to a con-
30 days of storage did not differ significantly from the plate ventional retort process, high-pressure sterilization involves a
counts immediately after treatment, so there was no indication smaller time–temperature integral as a result of the instanta-
of recovery of spores during storage. neous adiabatic temperature rise, and a lower maximum tem-
Prediction Model Developed
Based on the insights presented above, we have developed a
simulation model that predicts the degree of high-pressure spore
inactivation as a function of process parameters, equipment ma-
terial and dimensions, product characteristics, and target micro-
organism in a two-step process:
1. Calculate the temperature distribution inside the vessel
during processing, using an axi-symmetric one-dimensional fi-
nite element model based on heat conduction.
2. Then calculate spore inactivation as a function of time and
the realized product temperature and pressure, using the Eyring-
Arrhenius equation presented above.
Fig. 4 presents two examples in which the effect of the appli-
cation of an insulating product container in a standard steel ves-
sel on spore inactivation is demonstrated. The high-pressure
treatment consisted of a single high pulse of 700 MPa with a
hold time of 3 min applied to a product preheated to an initial
temperature of 90°C and containing 1010 spores of B. stearother-
mophilus/mL. In a standard steel vessel, such a process cycle Fig. 5—Time–temperature integrals of a conventional retort process and an
adiabatic high-pressure sterilization process consisting of a preheating phase
would result in a 6-log spore inactivation, whereas use of the (1), a pressurization phase (2), and a cooling phase (3). Compared to the retort
product container would result in a spore inactivation of more process, the high-pressure process has a smaller time–temperature integral and
than 10 log units. can use a lower maximum temperature because of the additional pressure
This predictive model for high-pressure spore inactivation is effect.
40 FOODTECHNOLOGY MARCH 2003 • VOL. 57, NO. 3

5. perature as a result of the additional pressure. Therefore, high- age-stability of high-pressure preserved green beans. J. Food Eng. 54: 27-33.
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fects of combined high pressure-temperature treatments on Clostridium sporogenes
ucts while minimizing the adverse effects of conventional heat- spores in liquid media. Industria Conserve. 71: 8-14.
ing processes on the sensory and nutritional quality. Use of the Meyer, R.S. 2000. Ultra high pressure, high temperature food preservation process. U.S.
proposed technical solutions to maximize the benefits of adia- patent 6,017,572.
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sterilization in the future. Clostridium sporogenes spores. Lett. Appl. Microbiol. 26: 227-230.
Okazaki, T., Kakugawa, K., Yoneda, T., and Suzuki, K. 2000. Inactivation behaviour of
heat-resistant bacterial spores by thermal treatments combined with high hydrostatic
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