Biomedical Engineering Reference
In-Depth Information
13.2.3 High Pressure Carbon Dioxide (HPCD)
The inhibitory effect of carbon dioxide (CO 2 ) and its combination with pressure on the
growth of microorganisms has been known for more than 100years (Liao et al ., 2007 ).
However, the application of pressurized CO 2 , so-called high pressure carbon dioxide
(HPCD), is relatively new (Damar and Balaban, 2006). HPCD processing involves contacting
food with either subcritical or supercritical CO 2 in a batch (using a pressure vessel), semi-
batch or continuous system. Besides the environmentally benign nature of the HPCD process
(CO 2 is non-toxic and easily removed by simple depressurization and degassing), the CO 2
pressures applied for preservation purposes are much lower (generally 20 MPa) than the
pressures employed in HPP (García-González et al ., 2009 ).
Kincal and co-workers (2005) and García-González co-workers (2007) have hypothesized
that the elimination of oxygen in the system during HPCD reduces growth rate of aerobic
bacteria. Furthermore, when CO 2 is dissolved in water carbonic acid is produced. Presence
of carbonic acid in the medium increases cellular acidity and decreases intracellular pH,
affecting metabolic activities. Carbon dioxide can also rapidly penetrate into the cells and
may enhance its chemical activity on the internal metabolic processes. Finally, carbon
dioxide has an inhibitory effect on certain enzyme systems. Higher temperatures and
agitation increase the diffusivity of CO 2 and the fluidity of the cell membrane, making CO 2
penetration into the product easier (Hong et al ., 1997; Hong and Pyun, 1999). Treatments
conducted with supercritical CO 2 have proven to be more effective in inactivating microbial
cells than CO 2 under subcritical conditions due to liquid-like density and gas-like viscosity
and diffusivity of supercritical CO 2 , which enhances its penetration into cells resulting in
increased cell disruption (Tomasula, 2003; Gunes et al ., 2005). Vegetative cells in a low
water content system seem to be more resistant to HPCD processing, maybe due to the
direct result of a decreased CO 2 solubility and higher cellular concentrations, which seem to
exert a protective effect on microbial cells because of the decreased exposure of cells to the
effects of pressurized CO 2 (García-González et al ., 2009). Usually pressures between 4 and
50 MPa at room temperature or mild temperatures (35 -C) for 5-120 min are enough to
inactivate or cause at least five-log reduction of several spoilage and pathogen microorganisms
(García-González et al ., 2007). Yuk and co-workers (2010) reported inactivation of E. coli
K-12 in apple cider as a function of CO 2 concentration, process temperature and residence
time. A seven-log reduction was achieved at 42 °C and 8% CO 2 in the cider.
The enzyme inactivation exerted by pressurized CO 2 is believed to be due to pH lowering,
conformational changes of the enzyme, and inhibitory effects of molecular CO 2 on enzyme
activity (Damar and Balaban, 2006). As opposed to microorganisms, some enzymes, such
as peroxidase (POD) and polyphenoloxidase (PPO), are less sensitive to pressure changes
under supercritical conditions than subcritical CO 2 conditions. This may be due to the ability
of subcritical CO 2 to better interact with the molecules of the enzymes and thereby affect the
rate of denaturation (Liu et al ., 2008). It is believed that HPCD processing has a minimal
impact on some bioactive compounds found in fruit juices, such as ascorbic acid and
carotenoids, due to the exclusion of O 2 from the system and pH lowering (Boff et al ., 2003 ;
Zhou et al ., 2009). The pH decrease observed after HPCD processing is related to CO 2
dissolution, which further dissociates into bicarbonate and carbonate and formation of
hydrogen ions (Liu et al ., 2008 ). Color ( L * and a * values) of the product is slightly affected
by HPCD (Park et al ., 2002 ; Kincal et al ., 2006). No difference in sensory properties among
HPCD-treated and fresh samples has been detected after two weeks of storage at low
refrigeration temperatures (1.5°C). The CO 2 concentration threshold at which panelists
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