It's A GasIt's A Gas

 It's A Gas
April 1997 -- New Technologies

By: Scott L. Hegenbart
Contributing Editor

  Extracting flavor essences, fats, oils and other food-ingredient substances constitutes a fundamental food-industry process, featuring time-proven methods. During the last 10 to 20 years, however, significant industry attention has focused on fluid gas extraction -- in particular, supercritical fluid extraction (SFE).  Several factors account for the interest. Many solvents used in traditional extraction are considered harsh, even hazardous. The risk potential of some is even being investigated by the U.S. Food and Drug Administration and the U.S. Department of Agriculture. Already, many are being replaced due to government mandate as well as consumer health and environmental concerns. Due to high flammability, safety concerns surround some alternatives commonly used for spice and seed oil extraction.  Extracting with fluid gases -- supercritical carbon dioxide in particular -- offers an attractive option because carbon dioxide is nontoxic, nonflammable and environmentally ubiquitous. It also is one of the least expensive solvents available.Altered states  Like all gases, carbon dioxide can exist in either solid, liquid or gaseous states, depending on the temperature and pressure to which it's exposed. Above a specific critical temperature of 31°C and critical pressure of 73 atm, however, carbon dioxide enters a "supercritical" state in which it behaves in some ways like a gas, in others like a liquid. One of these behaviors is the ability to act as a solvent. This property is pressure-dependent: the higher the pressure, the higher the dissolving power, according to Phasex Corporation, a Lawrence, MA-based designer of industrial systems that use supercritical fluid technology.  An actual food industry example of supercritical fluids acting as extraction solvents is the process for extracting flavors from hops. The major processing elements include:the extractor , which contains the hops from which flavors are to be extracted;the compressor, which recycles the carbon dioxide;the pressure-reduction valve , which reduces gas pressure; andthe separator, which collects the flavor extract and separates the carbon dioxide.  To perform extraction, carbon dioxide is compressed to a selected pressure and its temperature adjusted to the desired level to achieve a supercritical state. As the supercritical carbon dioxide passes through the extractor's charge of pelletized hops, it dissolves and extracts flavors and oils. The gaseous solution leaving the extractor passes through the pressure-reduction valve. Here, the pressure (and the dissolving power) of the carbon dioxide is reduced, causing the hops' flavor components to precipitate in the separator. This separates them from the carbon dioxide. The compressor then recycles the gas, and the cyclic process is continued until all flavor components are extracted and collected in the separator. After the extractor vents the carbon dioxide, the spent hops pellets are removed from the vessel. A new charge is then added and the process repeated.  By modifying the process to include multiple extraction vessels connected in series, and a small gas compressor connecting the extractor and carbon dioxide supply, the process can be given near-continuous efficiency.  At the end of one vessel's extraction period, the carbon dioxide flow can be routed to another vessel, which has previously been filled with hops to be extracted. While the extraction process is continuing in the other vessels, the vessel containing the spent hops can be unloaded and loaded with fresh hops. Instead of being vented to the atmosphere, the carbon dioxide present in the off-line extractor at the end of the extraction period can be sent to the supply vessel and reused.  Hops extraction for beer flavor isn't the only currently used application for supercritical fluid extraction. Coffee and tea are decaffeinated using carbon dioxide SFE. Spices and essential oils previously extracted with methylene dichloride or ethylene dichloride also use the technique. For each of these applications, the extraction is similar to the general description for extracting hops.Pluses and minuses  SFE offers many advantages over traditional extraction methods. First, it's efficient and highly specific. The process can be adjusted to leave behind undesirable components more readily than traditional extraction methods. When carbon dioxide returns to a gaseous state, it leaves behind no residue, so the finished extract is clean and solvent-free. Finally, supercritical temperatures are relatively low (31°C). This offers an advantage for processing heat-labile materials such as natural colors, fish oils and sensitive proteins.  Of course, no process is perfect. Possibly the biggest disadvantage is high capital costs due to equipment necessary for high-pressure processing. The food industry's lack of experience with high-pressure processing also may impede SFE-extraction acceptance. Some critics also indicate that open-ended SFE processes that vent gas are too costly to operate. With correct comparisons and identification of ideal applications, these concerns can be minimized, according to SFE advocates such as Phasex Corporation.  SFE certainly isn't just a laboratory curiosity, but it also isn't a magical solution to any and all extraction situations. Potential applications for supercritical fluid extraction must be carefully examined to determine if the benefits outweigh those of traditional extraction to a degree that will offset any equipment and engineering costs.  Many recent research projects have attempted to shoehorn SFE into inappropriate applications. For example, some researchers have tried defatting and decholesterolizing whole fresh meat, whole eggs and fluid milk. Fluids and large pieces of solids simply do not have the proper mass transfer properties for SFE any more than they would have for traditional liquid extraction methods. Supercritical fluid extraction resembles traditional extraction in this requirement. Successfully SFE-extracted materials -- flaked oil seeds, properly conditioned coffee beans and whole hops, and spices -- are a more appropriate starting material.  This doesn't mean that meat, eggs and milk are totally inappropriate SFE candidates. Researchers at the Department of Food Science and Technology at the University of Nebraska-Lincoln have successfully and significantly reduced the fat and cholesterol content of eggs and muscle foods. The trick is to extract dehydrated, rather than fresh, materials. Dry eggs are commonly used in food products as are powdered chicken and beef. Reducing the fat and cholesterol content helps product designers formulate reduced-fat foods.Below the critical stage  Supercritical fluid extraction isn't the only way gases can be used to reduce fat. CF Systems Corporation, Arvada, CO, has developed a proprietary liquefied gas-solvent extraction (LG-SX) process. As the name implies, liquefied gas-solvent extraction uses liquefied gases as the solvent to extract organic materials.  The process begins by screening or grinding the food material to a uniform size prior to charging the extraction vessel. These vessels combine the food and liquefied gas solvent with heat and agitation. After extraction, the agitators stop, and the fat-free food falls out of the oil/solvent mixture. The latter is moved to a recovery system where the food oil and solvent are separated and the solvent is cleaned and compressed for use again.  In bench and pilot-plant testing, LG-SX has successfully extracted oils and fats from grains, nuts, seeds and other foods, including corn, rice bran, wheat germ, soybeans, sunflower seeds, peanuts, jojoba and cocoa beans.  In addition to performing similarly, liquefied gas-solvent extraction also offers many of the same advantages as SFE. Like supercritical fluid extraction, the process is selective, performed at temperatures that minimize thermal degradation, and leaves no solvent residue because the gas is flashed off.  Unlike SFE, liquefied gas-solvent extraction doesn't require achieving supercritical temperatures and pressures, so less energy is required. On the downside, some ingredient processors might be reluctant to use propane as a solvent even though it has Generally Recognized as Safe status.  Supercritical fluid and liquid gas extraction are efficient, environmentally friendly methods for extracting food materials. As government regulations, consumer health concerns and environmental pressures continue limiting solvent selection, such methods will only increase in importance. The value of extracting fat, flavors, essences, caffeine and other food materials might not be that obvious. But without extraction, creating a food product would be about as easy as catching a gas vapor.Back to top