Most seed oils and vegetable oils, such as soybean oil, rapeseed oil, corn oil, sunflower oil, palm oil, or linseed oil, contain a variety of saturated and unsaturated fatty acids. The fatty acid profiles of oils commonly vary by source, but typically include a variety of saturated fatty acids, such as palmitic acid (C16:0) and stearic acid (C18:0); some monounsaturated fatty acids such as oleic acid (C18:1) and erucic acid (C22:1); and polyunsaturated fatty acids including linoleic acid (C18:2) and linolenic acid (C18:3). (The Cx:y designation refers to fatty acids wherein x is the number of carbon atoms and y is the number of double bonds.)
Polyunsaturated fatty acids, particularly linolenic acid (C18:3), have been found to lead to unacceptable rancid flavors in oil during baking, frying, etc. High contents of linolenic acid can also render edible oils unstable and easily oxidized during cooking and storage, which compromises the sensory characteristics of foods cooked in such oils. Many food oils are hydrogenated to increase stability by reducing the amount of linolenic acid and increasing saturated and monounsaturated fatty acids. For example, the maximum desirable linolenic acid content for many commercial frying oils is about two weight percent of the total fatty acid content of the oil.
Hydrogenating (mono)unsaturated fatty acids increases the saturated fatty acid content. Unduly high saturated fatty acid content in edible hydrogenated fat products, e.g., food oils, can adversely impact cardiovascular health by raising serum cholesterol levels. As a byproduct of hydrogenation, unsaturated fatty acids can be converted from their natural cis configuration to their trans isomer form. Recent studies have indicated that trans-fatty acids can impact cardiovascular health more negatively than saturated fatty acids do. In part due to this recent research, consumers are focusing more on trans-fatty acid content of edible hydrogenated fat products, with lower trans-fatty acid content being preferred by most consumers.
A variety of hydrogenation catalysts is known in the art. Each catalyst has shortcomings, though. Some particularly active catalysts, such as platinum or palladium, are able to hydrogenate food oils at relatively low temperatures, e.g., room temperature, but they generally are not very selective. As a consequence of unselective hydrogenation, a hardened fat that is solid at room temperature often will be achieved before C18:3 content of rapeseed oil or soybean oil, for example, is reduced to two percent. Consequently, such catalysts are unsuitable for producing a commercial pourable frying oil that is semi-liquid at room temperature.
Nickel-based catalysts are more selective, tending to hydrogenate trienic fatty acids at a higher rate than dienic or monoenic fatty acids. Most commercially available nickel-based hydrogenation catalysts comprise nickel on a relatively inert carrier, such as silica or alumina. The properties of nickel-based catalysts may be adjusted by additions of minor amounts of other metals, such as copper or even minor amounts of platinum or palladium. Commercially available nickel-based catalysts used in hydrogenating food oils, for example, are typically sold with the nickel-based catalysts distributed in a fully hardened fat matrix.
Nickel-based catalysts on inert carriers can be more selective than platinum and palladium, for example, but typically require either high temperatures or electrolysis to drive the hydrogenation reaction. In the absence of electrolysis, nickel-based catalysts typically require temperatures of 100° C. or more to conduct hydrogenation at an acceptable rate, with commercial hydrogenation processes being conducted at 120° C. or higher. Conducting hydrogenation at such high temperatures, however, tends to increase the formation of trans-fatty acids. For example, commercially available hydrogenated semi-liquid frying oils based on rapeseed or soybean oil and having C18:3 levels of 2% or less will typically have at least 15% of the fatty acids in the trans form, with trans-fatty acid contents of 20% or higher being commonplace.
Electrolytic hydrogenation using nickel catalysts can be conducted at relatively low temperatures, e.g., room temperature, but have limited use for commercial production of hydrogenated food oils. In electrolytic applications, the nickel catalyst is typically a monolithic block of nickel that can be electrically connected to a power source to serve as a cathode. An anode is spaced from the nickel cathode. Food oils tend to be fairly good dielectrics and significantly limit electrical conduction between the cathode and the anode. To render the food oils conductive, they typically must be emulsified in a conductive medium (e.g., saline solution or formic acid) or mixed with a solvent (e.g., alcohol or ketones). Adding and subsequently removing such conductive agents drives up the cost of the hydrogenation process and may render the resultant product less desirable as a food oil.