Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: July 3, 2004
Publication Date: September 26, 2004
Citation: Tomasula, P.M., Kozempel, M.F. 2004. Acid precipitation using co2 - ready for commercialization.(Abstract). 11th International Symposium & Exhibit-Supercritical Fluid Chromatography - Extaction and Processing Meeting. Paper No. 92. Technical Abstract: High pressure or supercritical carbon dioxide (CO2) precipitation may be used as a green alternative to processes that use acid precipitation for isolation of proteins from aqueous solution. The process has been used on the batch scale to isolate casein protein from milk, soy protein from an aqueous solution of soy flour and corn proteins from a side stream in a corn-milling process, and to facilitate fractionation of whey proteins from a whey protein concentrate. CO2, when dissolved in the aqueous solution, hydrolyzes to form carbonic acid according to the equation:CO2+H2O=H++ HCO3-. The pH is reduced so that the protein precipitates near its isoelectric point. This process has been demonstrated at temperatures in the range from 40 to 70 'C and pressures up to 14 MPa. In early experiments, casein was isolated from milk using CO2 in a pilot-scale batch reactor to determine the effects of pressure and temperature on yield of casein and casein quality. Initial pH of the milk was 6.6. The pH decreases linearly with addition of CO2 to a value of 5.4. Then, pH decreases at a lower rate with further addition of CO2 to pH of 4.8. Upon depressurization of the vessel, casein pH was 4.8 and pH of the associated whey was only 6.0. The higher pH for the whey indicates that much of the CO2 was evolved upon release of pressure while casein was converted to its acid form. The major advantage of using CO2 as a substitute for mineral acid precipitation is elimination of the precipitant from the product upon release of pressure. Washing steps are not needed to remove the precipitant from the product and neutralization of the acid product stream is not required. Less handling of the product in subsequent washing stages means that the potential for particle breakage and loss is minimized and as a result, there is less waste water generation. For potential commercial applications, a continuous process for protein precipitation from aqueous solution using CO2 also was developed. The continuous process is comprised of steps for contacting high pressure or supercritical CO2 with a pressurized aqueous stream, a stage for reaction and precipitation of the protein and CO2, and a stage for removing the protein product. Two separate reactor/precipitators were designed and tested. In one of the reactors tested, milk was sprayed into a column filled with pressurized CO2; in the other, five streams of liquid CO2 were pumped into a stream of pressurized milk, mixed in a static mixer and then pumped through a heated double-pipe tubular reactor. Hot water flowed in the outer pipe of the reactor to control temperature. The diagram of the process using the spray reactor is similar with the spray reactor replacing the tubular reactor. The process is novel in that the product may be removed from the reactor without depressurizing the system as required in batch operation. Depressurization occurs stepwise, reducing pressure only about 0.48 MPa/stage, so that particle size is maintained throughout processing. Depressurization through a valve or nozzle directly from the reactor/precipitation step was shown to limit particle size to 45 microns. In this study, a 12 stage progressing cavity pump, operated in reverse and fed through its outlet port was used to depressurize the reactor/precipitator, although the method is not limited to use of a pump to accomplish depressurization stagewise. The continuous reactors yield casein with slightly larger particle sizes than batchwise operation but the stepwise depressurization step using a pump sheared the particles. Casein precipitated at higher pressure (lower pH) was firmer and therefore more susceptible to shear while that precipitated at lower pressure (higher pH) was softer and less susceptible to shear while passing through the pump. Differences in particle sizes between batch and continuous tubular operation may be attributed to the different methods used for addition of CO2. Sparging of CO2 into the batch reactor is a slower process with gradual pressure elevation from the gas to liquid phase and pH depression until system pressure is reached compared to the rapid mixing of the CO2 and milk streams already at system pressure that may impact precipitation rates and particle formation. This technology has been licensed for an application other than casein precipitation and should be available shortly. This presentation will focus on some of the issues that are being addressed to bring this technology and the protein products that result from it to commercialization.