Extraction describes a wide variety of unit operations found in the food industry, including the manufacture of soluble coffee and tea, vegetable oils, fruit juices, and flavors and the removal of undesirable substances such as caffeine. Strictly speaking, some of these operations should be called leaching, washing, or expression, but it is common to call them extraction.
Operations may seem relatively simple, as when fruits are crushed and their juices removed by pressure. However, to enhance recovery, the remaining pulp may be washed with water, and sometimes the pulp is treated with enzymes to convert some of the insoluble material into juice solids. Thus, the apparently simple operation adds a biochemical reaction and a washing step. The recovered juice is more dilute than straight-run juice and is usually concentrated by evaporation.
Common Features of Extraction
This dilution illustrates a universal feature of extraction with solvents (which washing with water is): there is an inverse relationship between recovery and concentration—using more solvent removes more of the solute, but the resulting solution has a lower concentration. Usually, the solute is the desired component, so the excess solvent must be removed, and that usually involves some cost. This means that in extraction processes, there is almost always an optimum way to operate, determined by the costs of concentration and the value of the recovered material.
Water is an inexpensive solvent, but in other operations, the solvent may be an organic fluid, such as hexane, alcohol, or ether. There are both economic and safety motivations to recover as much solvent as possible. Another universal feature of extraction processes is that the solvent saturates the residue as well as appears in the extract solution. Thus, the residue usually is “dried” to drive off the retained solvent, and the extract solution may be evaporated for the same purpose.
Vegetable oil manufacture illustrates this practice. Olives and oilseeds such as soy, cotton, peanut, canola, and sunflower are usually crushed to express first-run oil. Some seeds may be cooked first or after crushing. Since pressing alone leaves some residual oil, the crushed meal is usually extracted with solvents, often hexane, which is flammable. Solvent-extracted oil may be considered inferior for some purposes but after refining may be difficult to distinguish from first-run oil. Olive is a case where the first-run (“virgin”) oil is considered superior in flavor. Because of the heat generated in pressing and cooking, extracted oil may contain free fatty acids, which are removed in refining. Gums and other impurities are often found as well; some, such as lecithin from soy, are recovered as valuable products in their own right.
The solvent-extracted meal is heated to remove solvent, which must be recovered by condensation to reduce cost and to prevent discharge to the atmosphere, where it is a contributor to air pollution as volatile organic compound (VOC). Obviously, there is some risk in heating a flammable material. Since the meal is often valued as animal feed, minimal heating is desired to maintain nutritive value.
Meanwhile, the solvent and oil solution is evaporated to remove solvent and concentrate the oil. Since there is usually a large difference in boiling point between the oil and solvent, the separation is relatively easy and mostly complete, but there can be some residual solvent in the oil. Few separations are 100% efficient, so there usually is some solvent loss, which contributes to cost.
Coffee and Tea Extraction
Coffee and tea solubles are removed industrially much as they are in the home, by contact with hot water. Again, there is an inverse relationship between recovery and concentration. At home, the second pot of tea using the same tea bag is noticeably weaker. Recovery is increased, but at a lower concentration. Avid coffee brewers know that overextraction does not result in better coffee; in fact, it is usually worse, because the more flavorful compounds are also most easily extracted and the later compounds are bitter or acidic.
In commercial operations to produce soluble coffee and tea, yield is important, so extraction is very thorough and complete. The joke in the industry is that there should be only air left in the vessels when the process is finished. It is not unusual to use high temperatures and pressures to increase yield by breaking down the otherwise insoluble material. One should not be surprised that commercial soluble coffee and tea do not taste like their freshly brewed counterparts.
Coffee and tea can also be decaffeinated using extraction. This illustrates a case of selective dissolution. Caffeine is soluble in some solvents, such as methylene chloride, while the carbohydrates, acids, and phenolics are not. An early process directly contacted coffee beans or tea leaves with solvent to remove caffeine. However, some solvents are carcinogenic and left residues in the beans, so an improvement was to contact a water extract with the solvent, which was mostly immiscible with water. This improvement comes at the cost of complexity: first, the beans are contacted with hot water, then the water is contacted with solvent, the solvent is removed from the water, the water is removed from the beans, the solvent is removed from the caffeine, and the coffee solubles are removed from the water stream and returned to the beans somehow.
The modern decaffeination process uses supercritical carbon dioxide, which leaves no residue and is selective for caffeine.
Supercritical Fluid Extraction
Gases can be compressed at high pressure to form a phase that has both liquid-and gas-like properties. The compressed gas, called a supercritical fluid because it is above the substance’s critical pressure, can have high solvent power but low viscosity. Release of pressure dramatically reduces the solvency, precipitating the solute quickly. Carbon dioxide is popular for such use because it is inexpensive, nonexplosive, and nontoxic.
Val Krukonis, President of Phasex Corp., Lawrence, Mass. (phone 978-794-8686, www.phasex4scf.com) is a pioneer in the field of supercritical extraction, but is the first to say that it is not a panacea. In addition to decaffeination of coffee and tea, he says, it is in use to remove nicotine from tobacco, to impregnate wood, to replace harmful solvents in dry cleaning, and to prepare fine particles of drugs for pharmaceuticals. He says that 140 million lb of coffee is treated with supercritical fluids to remove caffeine each year. Phasex performs consulting and process development on supercritical fluid extraction and operates a small plant for custom manufacture.
Thar Technologies, Pittsburgh, Pa., has applied supercritical fluid extraction to a wide variety of products and uses, including spices, flavors, foods, nutraceuticals, solvent removal, coatings, impregnation, vegetable oil refining, and others. Supercritical fluid technology offers advantages such as the absence of any organic solvent residue and selective extraction and fractionation of different compounds, according to Tony De Prado, Sales Manager (phone 412-826-3939x214, www.TharTech.com). The company offers both pilot- and production-scale systems.
Another major application of supercritical fluids is extraction of hops to yield an easy-to-use flavor concentrate for beer.
Research on supercritical fluid extraction is also conducted at the U.S. Dept. of Agriculture’s National Center for Agricultural Utilization Research in Peoria, Ill. According to Jerry King (phone 309-685-4011), a major focus is on extraction of oil from various seeds and on subsequent refining of the oils. The Center has also applied supercritical fluid extraction to analysis of toxicants in meats, grains, and commercial food products. The supercritical fluids replace organic solvents in the laboratory.
Other Extraction Equipment
Crown Iron Works Co., Minneapolis, Minn. (phone 651-639-8900, www.crowniron.com) makes specialized extraction equipment primarily applied in the oilseed industry. The company’s extractor uses a chain to drag oilseed flakes through solvent-filled chambers in a countercurrent path. This equipment illustrates two more common features of extraction: the importance of solid size and shape and the concept of countercurrent flow.
Solids containing a solute of value or interest typically are composed of inert material, such as carbohydrate or protein, that entraps the solute. To remove the solute, solvent must penetrate the inert matrix and then diffuse out. This diffusion process is often the rate-limiting step. To reduce the time required, it is common to grind the particles to a fine powder or to make thin flakes, if possible. Powders may be difficult to handle and may plug equipment. Flakes are often easier to move. Sometimes, of course, one cannot change the shape of the substrate.
Countercurrent flow refers to the practice of contacting exhausted substrate with fresh solvent while fresh substrate sees the exiting solvent. This is easy to arrange when contacting one liquid with another. One typically relies on density differences to separate the fluids. With solids, it may be more complex to simulate countercurrent flow. One way is with complex piping and valves in which the relative position of vessels containing the solids is changed periodically. Another is by physically moving the solids, as is done in the Crown contactor.
Littleford Day, Florence, Ky. (phone 800-365-8555, www.littleford.com) applies its Ploughshare® technology to special batch extractions, using choppers and plows in a vessel to promote agitation and contact. Material is contacted in the vessel with solvent, the solution is removed through filters, the residual solvent is evaporated by heating the vessel, then the extracted solids are discharged.
Designing and operating an extraction process is a challenging task, even for experienced engineers. However, it is useful for anyone in the food industry to know a little of the potential applications and to understand some of the basic principles.
by J. PETER CLARK
Consultant to the Process Industries
Oak Park, Ill.