Cleaner Environment with Carbon Dioxide (Supercritical Carbon Dioxide in Cocoa Butter Equivalent Extraction)

Supercritical Fluid Extraction (SFE) Systems uses supercritical carbon dioxide (SC-CO₂) instead of an organic solvent. Yes, carbon dioxide. However, the word ‘supercritical’ is an important keyword in this system. Supercritical fluid is a state that happens when a fluid is above its critical Temperature (T_C) and critical pressure (P_C), when it is between the typical gas and liquid state. For CO₂, the T_C is 32.1 ^oC and the P_C is 73.8 bar. Around the supercritical region, CO₂ can dissolve triglycerides at concentrations up to 1% mass.

Figure 1. Phase diagram of carbon dioxide

Why industry gone through the hassle of providing the environment that makes CO₂ liquid when other traditional organic solvents are readily liquid at STP for extraction? This is because carbon dioxide is inexpensive, readily available, non-toxic and harmless to the environment.

So how does it work? We need to know about the equipment first.

Figure 2. Parts of the equipment involved in SFE

Let’s start with the CO₂ cylinder, it stores CO₂ (obviously). Then there is cooler which involves in CO₂ liquefaction. It is connected to a high-pressure syringe pump which is capable to build pressure up to 69 MPa (690 bar). The parts in the dotted square in Figure 2 are classed as SC-CO₂. They are equipped with a heated capillary restrictor with maximum operating temperature of 150 ^oC. The pressure and temperature are controlled by software.

“The density of the SC-CO₂ at around 200 bar is close to that of hexane; and the solvation characteristics are also similar to hexane”

To extract the desired compound, the sample need to be placed in the extraction cell and pressurized with CO₂ to dissolve the sample. The SC-CO₂ together with the dissolved compound are transferred to a fraction or oil collector. Then the contents are depressurized, and SC-CO₂ will lose its solvating ability which will precipitate the desired material in the trap section. The condensed CO₂ can be recycled back or released to atmosphere.

Now that we know the concept of SFE, it’s time for us to learn the sciency stuffs that optimize the process. To optimize the extraction process, we need to control pressure, temperature and incubation time as they are the factors that can affect the extraction yield. In this article, we will focus on optimization of enzyme interesterification of camel hump fat to maximize cocoa butter extraction by using SC-CO₂ as a solvent to ease the production.

Higher pressure can indirectly cause low yield of cocoa butter. How? Physical properties of SC-CO₂ are density dependant and the density of SC-CO₂ depends on pressure. The power of SC-CO₂ will increase as the density increase. This will cause the SC-CO₂ to react with the enzyme and produces carbamates and frees the amine group at the surface of the enzyme. Hence, the enzyme will be inactivated thus lowering the cocoa butter yield.

Temperature also plays a role in maximizing the cocoa butter production as increase in temperature will increase the enzyme activity in SC-CO₂ thus increases the cocoa butter yield. However, once the temperature goes above 50 ^oC, enzyme can be destroyed as they are sensitive to high temperature. If the enzyme used can withstand high temperature, the production of cocoa butter will still decreases as the density of SC-CO₂ lowers down and loses its solvation power.

The optimum incubation time to achieve maximum yield of cocoa butter is 3 hours. Lower than that it will not have enough time for the desired reaction to occur and more than that, acyl migration (Oleic acid from sn-2 position of triglycerides which about 90% of it comprise the TAG of cocoa butter) will increase. Both will lower down the yield of cocoa butter.

            Cleaner technology is relatively a new thing and method of approach to control pollution. This method, which promotes reduction, recycling and even elimination of source of waste by preventing and recycling the wastes generated, is clearly more environmental friendly and acceptable. Since the concept is still new, there is no specific regulation yet regarding the usage of SC-CO₂ in cocoa butter equivalent extraction.

            In most countries, regulation regarding waste handling is focused on implementation of waste treatment and disposal scheme with the aim to meet discharge or emission standards. Although this contributed significantly to protection of environment, it is still not the best approach to manage environmental pollution. In some cases, this method is problematical and resulted in transfer of pollutants, especially the hazardous one from one medium to the other.

            In conclusion, SFE is an alternative to liquid extraction using traditional organic solvent such as hexane or dichloromethane. There will always remain some residual solvent in the extract and matrix, so there will always be some level of environmental contamination from the usage of traditional organic solvent. This is the complete opposite when SC-CO₂ is used. CO₂ is easy to be removed (simply reduce the pressure), it leaves almost no trace, and also non-destructive to environment. Soil Association approves the usage of CO_­2 in SFE for organic products.




References:

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