The common techniques for oil extraction are mechanical pressing, the usage solvents, and supercritical fluid extraction. Each of these different methods pre­sents its own advantages and disadvantages. The first oil extraction method can be divided into expression and ultrasonic-assisted extraction and the efficiency nor­mally ranges from 70 to 75 % (Rengel 2008). The main drawback of this method is that it generally requires drying the algae beforehand, which is an energy intensive step.

Using solvents such as n-hexane, benzene, ethanol, chloroform, and diethyl ether can efficiently extract the fatty acids from algae cells. However, the use of chemi­cals in the process could present environmental, safety, and health issues. In many cases, manufacturers of algae oil use a combination of mechanical pressing and chemical solvents in extracting oil to improve efficiency (around 95 %).

Supercritical extraction requires high-pressure equipment that is both expensive and energy intensive. In this process, carbon dioxide is heated and compressed until it reaches a liquid-gas state. Then, it is applied to the harvested algae and acts like a solvent (Mendes et al. 1995; Ferreira et al. 2013).

Apart from these, there are some other more expensive and less known and uti­lized methods which are enzymatic extraction that uses enzymes to degrade the cell walls with water acting as the solvent; and osmotic shock is a sudden reduc­tion in osmotic pressure that can cause cells in a solution to rupture.

Once the oil is extracted through these methods, it is referred to as “green crude.” However, it is not ready to be used as biofuel until it undergoes a process called transesterification. This step is a chemical reaction in which triglycerides of the oil react with methanol or ethanol to produce (m)ethyl esters and glycerol (Rengel 2008). This reaction creates a mix of biodiesel and glycerol that is further processed to be separated and leaves ready to use biodiesel.

Direct conversions from a non-dry state are being studied and some pos­sibilities that may play an important role in offsetting the costs and improve oil extraction efficiency are arising. Among these, it is important to highlight in situ transesterification and hydrothermal liquefaction (Chen et al. 2009; Patil et al. 2008) Nevertheless, due to limited-level information in these processes for algae, more research in these areas is still needed.

Meanwhile, a lot of work is being made to reduce energy input and costs of extraction processes. Many industries claim they have come up with cost-effective methods in this area; however, until large-scale facilities are deployed, it is hard to tell which one will work in a large-scale basis.

The whole algae, bio oil, or the residues from oil extraction are excellent feed­stock for making other fuels and products via different processes. Some of these products will be presented in the next chapter.