Dense-phase carbon dioxide or supercritical CO2 denotes phases of CO2 that remain fluid, yet are
dense with respect to gaseous CO2. Moreover, in the supercritical state, CO2 has low viscosity
(3–7 × 10−5 Ns/m2) and zero surface tension (McHugh and Krukonis 1993), so it can quickly pen-
etrate complex structures and porous materials. Finally, CO2 is inexpensive and readily available,
which makes switching to CO2-based sterilization economically feasible (Zhang et al. 2006). When
100
Olive oil
7 h Ozonated olive oil 7 h Ozonated soybean oil Soybean oil 90 80 70 60 η/mPa · s 50 40 30 20 20 25 30 T/°C 35 40
FIGURe 9.7 Changes in the dynamic viscosity of ozonated and unozonated olive oil, and soybean oil as a
function of varying temperatures. (From Sadowska, J., Johansson, B., Johanssen, E., Friman, R., Broniary- Press, L., and Rosenholm, J.B., Chemistry and Physics of Lipids, 151, 85–91, 2008. With permission.)
Ozone and CO2 Processing: Rheological and Functional Properties of Food 135
gaseous or liquid CO2 is heated and compressed above the critical temperature (31°C) and pres-
sure (73 atm), it becomes a dense, highly compressible fluid that demonstrates properties of both liquid and gas. Hence, at relatively low pressures and temperatures carbon dioxide transitions to a supercritical state (Figure 9.8). The liquid state of CO2 conserves some of the solvent properties of
the supercritical state, with low viscosity and high diffusion coefficients, therefore the term dense- phase fluid refers to both the supercritical and liquid states. The properties of supercritical CO2 lend
themselves to deep penetration of substrates, which has led to uses in areas ranging from bioreme- diation to natural product extraction (van der Velde and de Haan 1992; Ge and Yan 2002).
9.3.1 Dense-pHase carbon DioxiDe (DpcD) treatment system
To date supercritical CO2 sterilization has not delivered on its promise as a potential preservative, due
in part to the inability of existing methodologies to achieve industrial levels of treatment (Spilimbergo and Bertucco 2003). Several systems for possible application of SCCO2 have been developed, includ-
ing batch, semicontinuous, and continuous. A typical batch-type system mainly comprises of a car- bon dioxide cylinder, a pressure regulator, a pressure vessel, a water bath or heater, and a CO2 release
valve (Figure 9.9). CO2 and the solution under study are stationary in a container during treatment
whereas the semicontinuous system allows flow of CO2 through the treatment chamber, unlike in
continuous systems where both CO2 and the liquid food flows through the system (Balaban and Sibel
2006). Figure 9.9c shows a schematic diagram of continuous DPCD treatment unit.
9.3.2 effectsof Dense-pHase carbon DioxiDe (DpcD) on fooD texture
The use of DPCD as a nonthermal means of food preservation has been reported by various research- ers, concentrating mainly on microbial and to some extent on enzymatic inactivation. Table 9.1 shows some of the food applications involving DPCD. There are a limited number of studies avail- able in the literature regarding the effect of DPCD or SCCO2 on product quality and its influence on
the rheological and functional properties of foods. Furthermore, most of the work is concentrated on liquid foods as shown in Table 9.1. Haas et al. (1989) treated whole fruits with DPCD to inhibit mould growth but observed tissue damage in some fruits even at low pressures, influencing textural properties of fruits. DPCD as a nonthermal pasteurization technique for beer was found to preserve aroma, flavor, foam capacity, and stability coupled with reducing beer haze and extending shelf life (Dagan and Balaban 2006). Parton et al. (2007) tested a continuous DPCD system for liquid foods such as grape must, orange juice, and tomato paste. No qualitative physical or chemical changes after high-pressure CO2 treatment were reported with respect to control samples. DPCD treatment of
Supercritical fluid –140 –120 –100 –80 –60 –40 –20 0 20 40 60 80 Temperature (°C) 10,000 1000 100 10 1 0.1 0.01 0.001 31.1°C 74 atm Pressure (atm) Solid Liquid Critical point Triple point Gas
orange juice improves some physical and nutritional quality attributes such as cloud formation and stability (Arreola et al. 1991). Moreover, color and cloudiness of DPCD-treated juice was preferred over untreated juice. Dagan and Balaban (2006) employed a continuous DPCD system for pasteuri- zation of beer and reported a reduction in haze by DPCD processing from 146 NTU to 95 NTU com- pared to fresh beer at processing conditions of 26.5 MPa, 21°C, 9.6% CO2, and 4.77 min residence
time. The aroma and flavor was found to be unaffected, which could be due to the pH-lowering effect of DPCD processing. Haze is an important rheological property of beer that can be defined as the formation of a colloidal suspension that scatters light and makes a beverage appear cloudy. However, they reported a slight decrease in foaming capacity and stability by DPCD. Changes reported for foam characteristics due to DPCD processing may have been caused by the extraction of yeast cell membrane or cell wall parts that may have changed the amount of hydrophobic compounds in the beer, therefore affecting foaming.
9.3.3 effectof Dense-pHase carbon DioxiDe (DpcD) on
rHeological propertiesof Dairy proDucts
DPCD can inactivate pathogenic and spoilage microorganisms and enzymes pertinent to milk and milk products (Hong and Pyun 2001). A major disadvantage for potential applications of DPCD in dairy products is its negative influence on the rheological properties of milk due to changes in
Filter Filter Filter Valve Valve Valve Valve Pump Pump Pump Pump Pump Heating jacket Sample
Carbon dioxide cylinder
Sample
Mixing pump
Multibatch reactor system Temp indicator
Sample
Pressure vessel
(a) Batch system
(b) Semicontinuous system (c) Continuous system Holding and heating system Expansion valve Treated product Vaccum Chiller CO2 CO2 CO2
Ozone and CO2 Processing: Rheological and Functional Properties of Food 137
milk protein (casein). As CO2 dissolves in the aqueous portion of food it undergoes reaction of
CO2 with water to form carbonic acid, thus lowering pH (Damar and Balaban 2006). This further
dissociates to yield bicarbonates, carbonates, and H+ ions, lowering extracellular pH and aiding inactivation on microorganisms. This lowering of pH in milk causes precipitation of casein, which has an isoelectric point of pH 4.6. This pH-lowering effect of DPCD can be employed in casein production (Tomasula 1997; Hofland 1999).
CO2 + H2O↔H2CO3
tABLe 9.1
effect of DPCD on Food Preservation and quality
Food Product treatment system quality Attributes Reference
Apple cider Continuous Organoleptic quality (√) Gunes et al., 2006
Apple Juice Batch Liao et al., 2007
Carrot juice Batch Color (×), Cloud (×) Park et al, 2002
Orange juice Continuous Kincal et al., 2005
Orange juice Batch pH (√), oBrix (√), Color (↑), acidity
(↓), AA (↓), organoleptic quality (~)
Arreola et al., 1991 Orange juice Continuous pH (~), oBrix (~), acidity (↑), Color
(↓), Cloud (↑), organoleptic quality (~)
Kincal et al., 2006
Orange juice Continuous pH (~), oBrix (~), acidity (~),
Vitamin C (~) and folic acid (~), and aroma profile (~).
Ho, 2003
Orange Juice Batch Cloud (↑), pH (~), oBrix (~),
Color (↑), organoleptic quality (~)
Balaban, 2005
Orange juice Color (↓), pH (↓) Wei et al., 1991
Grape juice Continuous pH (~), organoleptic quality (~) Gunes et al., 2005
Milk Continuous Werner and Hotchkiss,
2006 Mandarin juice Continuous Cloud (↑), pH (~), oBrix (~), acidity
(~), organoleptic quality (~), color (×)
Yagiz et al., 2005
Coconut water Continuous organoleptic quality (√), shelf life (↑) Damar and Balaban, 2005
Beer Continuous Haze (↓), aroma (~) and flavor (~)
Foam capacity and stability (~)
Dagan and Balaban, 2006
solid or semi solid
Kimchi Batch pH (↑), TA (↓), organoleptic quality
(~), acceptance (↑)
Hong et al., 1997 Hong et al, 1999 Meat (porcine muscle) Batch Muscle pH(√), cooking loss (√),
protein solubility (√), tenderness (√), water holding capacity (√), protein denaturation (×)
Choi et al., 2008
Chicken Color (×), cooking quality (×),
expulsion of liquid Water Holding Capacity (×)
Wei et al., 1991
Shrimp Color (×), cooking quality (×),
expulsion of liquid (Water Holding Capacity ) (×)
H2CO3↔H+ + HCO3–
HCO3–↔H+ + CO3–2
Precipitation of casein has a negative influence on consumer sensory perception. Dairy products prepared from DPCD-processed milk such as cheese have resulted in improved rheological param- eters of cheese making such as reduced clotting time, augmentation of curd hardness, and whey losses along with a slight increase of cheese yield (Ruas-Madiedo et al. 2002). DPCD treatment of milk is reported to reduce particle size distribution (PSD) compared to controls. The change in the PSD indicates a modification to the fat when milk was treated with CO2 and pressure (Tisi 2004).
9.3.4 effectof Dense-pHase carbon DioxiDe (DpcD)-
assisteD extractionon rHeological properties
Supercritical CO2 has been proposed as an effective way of removing cholesterol from egg yolk
(Froning et al. 1990; Paraskevopoulou et al. 1997). Such low-fat, low-cholesterol egg products are used in various food formulations. Paraskevopoulou et al. (1999) studied the effect on the viscoelas- tic properties of mayonnaise prepared from reduced-cholesterol yolk after extracting with 20:80 ethanol:water or SCCO2. They found that the method of lipid extraction influenced both the emul-
sion droplet size and the viscoelastic parameters. The yolk extracted with an ethanol:water mixture, containing 1.5% polysorbate 80, resulted in emulsions that exhibited higher viscoelastic moduli values compared to those prepared with supercritical CO2-extracted yolk. Figure 9.10 shows the
changes in complex viscosity as a function of generated strain for mayonnaise emulsions prepared from reduced cholesterol yolk after extraction.
9.3.5 Dense-pHase carbon DioxiDe (DpcD)-assisteD extrusion
Extrusion is a well-known commercially adopted cooking process to produce a large variety of expanded food products. The final texture and quality is closely related to the morphology and cell structure of the extrudate (Cho and Rizvi 2008). Supercritical fluid extrusion (SCFX) is an innovative process for continuous generation of microcellular structures in various matrices
2.0 1.5 Log (η*/Pa · s) 0.5 0.1 1 10 Strain (%) 100 1.0
FIGURe 9.10 Changes in complex viscosity as a function of strain for mayonnaise emulsions (25°C; 1 Hz;
emulsions aged for 24 h). Key: , spray-dried yolk (Dv = 10.1 µm); , yolk extracted with 20:80 (v/v) ethanol/
water containing 1.5% (w/v) polysorbate 80 (Dv = 13.2 µm); ◊, yolk extracted with supercritical CO2 (Dv = 11.7
µm). (From Paraskevopoulou, A., Kiosseoglou, V., Alevisopoulos, S., Kasapis, S., Colloids and Surfaces B: Biointerfaces, 12, 107–111, 1999. With permission.)
Ozone and CO2 Processing: Rheological and Functional Properties of Food 139
under low shear and low process temperatures (<100°C) (Rizvi et al. 1995; Mulvaney and Rizvi 1993; Rizvi and Mulvaney 1992). This process offers an attractive extension of the conventional extrusion cooking process to create a new generation of expanded products. It is well estab- lished that macro/microstructure formation in extrusion processes is the consequence of several overlapping events including biopolymer structural transformations (starch gelatinization and/or protein denaturation), nucleation, die-swell, cell growth, and cell collapse (Moraru and Kokini 2003).
In an extrusion process when the melt passes through the extruder die, it undergoes a sudden pressure drop resulting in water vapor nuclei generation. These cells grow in size as additional water vapor diffuses into the nuclei. Furthermore, thermal expansion of water vapor causes further expansion. At the maximum level of extrudate expansion, it starts to experience some collapse due to elastic forces in the cell wall (Arhaliass et al. 2003). Conventional steam-based extrusion usu- ally requires low moisture (18–28 wt%), high temperature (120–180°C), and high shear conditions for good expansion. These harsh operating conditions often lead to excessive dextrin formation and limit the application of heat- and shear-sensitive ingredients such as proteins, vitamins, and certain flavors (Cho and Rizvi 2008). Although limited control of the extrudate expansion and the microstructure in conventional steam-based extrusion can be obtained by manipulating parameters such as moisture content, die geometry, die temperature, screw rotation speed, and feed rate, steam- expanded products usually show nonuniform cellular structure and cell sizes in the range of 1–3 mm (Barrett and Peleg 1992). Conversely, use of DPCD as a blowing agent instead of steam uncouples the dual role of water as a blowing agent and a plasticizer during SCFX. In this process, expansion of the melt is achieved by first solubilizing DPCD in the melt and then inducing nucleation due to pressure drop in the die, which is followed by cell growth caused by diffusion of CO2 into the
nucleated cells (Rizvi et al. 1995). A higher moisture content (30–45 wt%) in the extruder barrel is utilized to keep the product temperature low via reduction of viscous dissipation of energy and to maximize SCCO2 solubilization in the melt. Since SCFX extrusion is conducted at temperatures
lower than 100°C as well as at lower shear, it enables the use of temperature- and shear-sensitive ingredients in product formulations. Due to the pressure-dependent solubility of DPCD in melts, it can also be used to adjust the pH by formation of carbonic acid to modify the in-barrel rheological properties of the extrudate. The dual role of water, which acts both as a plasticizer and a blowing agent for expansion in conventional extrusion cooking, is decoupled since the expanded structure formation is via puffing (Alavi et al. 2003).
The SCFX process allows control of not only the average cell size of extrudates, which may range from about 50 to 250 µm with a high polydispersity index of ~0.95, but also the volumetric expansion of final products by manipulating process parameters such as die dimension, DPCD con- centration, and residence time (Alavi and Rizvi 2005; Winoto 2005). SCFX extrudates show non- porous surfaces and predominantly closed cell structures. High moisture content in the melt reduces the glass transition temperature of the product and the SCFX extrudates are still in a rubbery state upon exiting the die and have a sufficiently low viscosity to cause cell growth or shrinkage. As a result, SCFX extrudates tend to expand further until their structure is set during postextrusion pro- cesses such as drying and frying (Alavi et al. 1999; Chen et al. 2002).
Cho and Rizvi (2008) investigated the effects of DPCD injection rate and die dimension on the time-dependent expansion behavior of SCFX extrudates using a visualization technique. Scanning electron microscope images of SCFX extrudates were produced using the formulation 49.5 wt% pregelatinized corn starch, 24 wt% pregelatinized potato starch, 24 wt% sugar, 1 wt% salt, and 1.5 wt% distilled monoglyceride as a dough conditioner with 7 wt% whey protein concentrate to improve the structural stability of the extrudates (Winoto 2005) with barrel moisture content of 43.8 wt%. They observed that, as the die diameter was decreased from 5.9 to 2.9 mm, the cross-sectional expansion and the number of cells increased, whereas the average cell size decreased (Figure 9.11). When the ratio of SCCO2 to the feed was increased from 0.5 to 0.75 wt% SCCO2, the expansion and
and using 5.9 mm die showed significant structure collapse, implying that the prevention of gas loss would be essential to increase the final volumetric expansion.
9.3.6 effectof Dense-pHase carbon DioxiDe (DpcD) on fooD microstructure
Alavi et al. (1999) observed that SCFX extrudates exhibited the unique characteristic of a nonpo- rous skin surrounding the internal cellular morphology (Figure 9.12). This skin was comprised of unexpanded starch and very small cells. Rapid diffusion of CO2 out of the sample creates a
depletion layer near the edges in which the gas concentration is too low to contribute significantly to cell growth. Moreover, rapid drying and gelation of the proteins near the edges sets the mate- rial quickly and inhibits growth of any nucleated sites. A combination of these factors caused the formation of the nonporous skin (Alavi et al. 1999). Conversely, the “skin effect” does not occur for extrudates dried at 100°C because of rupture of any skin that might have developed initially. The presence of skin reduces water penetration and delays onset of sogginess, which is a desirable characteristic for ready-to-eat breakfast cereal. Moreover, the skin provides a composite structure FIGURe 9.11 Effects of SCCO2 and die size on product morphology (21 × magnification). Left: 5.9 mm die;
right: 2.9 mm die. Top: 0.5 wt% SCCO2; bottom: 0.75 wt% SCCO2. (Adapted from Winoto, C. W., Process
parameters and their influence on supercritical fluid extrudate properties. Master thesis, Cornell University, Ithaca, NY, 2005. With permission and Cho, K. Y., and Rizvi, S. S. H., Food Research International, 41, 31–42, 2008. With permission.)
Ozone and CO2 Processing: Rheological and Functional Properties of Food 141
to the extrudates and variation of its thickness can provide another means for manipulating extru- date mechanical properties and texture (Alavi et al. 1999). The polydispersity index of cell size distribution of SCFX extrudates is reported to be greater compared to steam extrudates (Barrett and Peleg 1992; Alavi et al. 1999). This indicates that distribution of the cell size of SCFX extru- dates is uniform. The cell density of the SCFX extrudates can be explained on the basis of the classical nucleation theory. Hence it can be said that extrusion of products with SCFX results in a more uniform microcellular structure.
9.4 ConCLUsIons AnD FUtURe tRenDs
Within the food industry, there is an increasing emphasis and trend toward natural food preserva- tion technologies in response to growing consumer demand for “greener” additives. During the last two decades natural nonthermal technologies have been investigated for practical applications. These technologies have been shown to inactivate microorganisms and enzymes without signifi- cant adverse effects on organoleptic or nutritional properties. Reported studies have demonstrated that natural nonthermal food preservation techniques described in this chapter such as ozone and DPCD may offer unique advantages for food processing with minimal or desired effects on the rheological properties of food. More complex considerations arise for combinations of technologies, particularly with respect to optimization of practical application. Intelligent selection of appropriate systems based on detailed, sequential studies is necessary.
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