Recent Developments in Non-thermal Processess - Food and Industrial Bioproducts and Bioprocessing - page 306

13.2 RECENT ADVANCES IN NON-THERMAL
TECHNOLOGIES
13.2.1 High Pressure Processing (HPP)
The potential of using pressure as a food preservation mechanism has been known for over
a century (Wilson et al ., 2008). High pressure processing (HPP) is mainly based on the
“isostatic” principle, which indicates that pressure is transmitted in a uniform and quasi
instantaneous manner throughout the whole food independent of its shape and size
(Heremans, 2002). The main components of the HPP equipment are: (i) vessel and yoke; (ii)
hydraulics (pressure generation system); (iii) temperature control system. HPP technology
involves a batch operation. The packaged product to be treated is placed in a vessel and a
pressurizing fluid (water or oil) is pumped into the vessel. Once the vessel is filled with the
liquid, pressure is built up by a hydraulic pump and is transmitted through the liquid to the
vessel until the desired pressure (100-1000MPa) is attained. The system is held at this
pressure for the duration of treatment time (1-20min) then rapidly decompressed.
Temperature is controlled by a thermostated mantle connected to a cryostat that surrounds
the vessel (due to adiabatic heating a temperature increase of 3 °C/100 MPa is expected)
(Rovere, 2002). The measurement of the temperature is usually carried out by thermocouples
placed inside the vessel in contact with the pressure medium.
The mechanism of microbial inactivation by HPP is a combination of different
reactions, such as non-covalent bonds breakdown and increase in cell membrane
permeability (Rastogi et al ., 2007). A review of the latest literature on microbial inactiva-
tion by HPP in different food matrices clearly indicates that pressures between 200 and
600 MPa at room or mild temperatures (35-50 °C) are usually sufficient to inactivate the
majority of vegetative cells, including molds and yeasts, in several minutes (Wilson et al .,
2008 ; Rastogi et al ., 2007). Based on this knowledge, industrial HPP treatment is
conducted at a pressure of 600 MPa for a holding time of 1-5 min (Torres and Velazquez,
2005 ). Listeria monocytogenes is used as a HPP target microorganism, especially in RTE
products, due to its high resistance to pressure and presence in low acid refrigerated foods.
However, in acidic environments, E. coli O157:H7 is commonly used as a microorganism
of concern.
The primary goal of any non-thermal processing is food preservation while maintaining
the quality of fresh product. The inactivation of quality deteriorative enzymes is also
extremely important. Covalent bonds are not affected by HPP treatment, thus the primary
structure of the enzymes will not be damaged. The hydrogen bonds are also relatively
baroresistant, hence secondary enzyme structure will not be affected up 700MPa.
However, HPP treatment affects electrostatic and hydrophobic interactions that maintain
the stability of tertiary and quaternary protein structures. HPP treatment also produces
structural damage to the active site interfering in the enzyme-substrate interaction
(Ludikhuyze et al ., 2002). In vegetable-based products, enzyme baroresistance is generally
higher than that for the indigenous microorganisms. For that reason, preservation treatment
of such products is based on the inactivation of the enzymes responsible for the quality
deterioration. Low water activity seems to have a detrimental effect on HPP effectiveness
due to the need of a fluid for pressure transmission (Oxen and Knorr, 1993). As commented
earlier, HPP processing does not affect covalent bonds and for that reason, most of the
bioactive compounds present in food (water-soluble vitamins such as ascorbic acid,