**6. Extraction of lipids**

Microalgae oils can be extracted similarly to other oilseed biomasses, which usually use physical extraction by adding a chemical solvent to improve the extraction process; solvents such as hexane and methanol are widely used [24].

The extraction of oil with solvent is a process of transferring soluble constituents (oil) from an inert material (biomass) to a solvent, in a purely physical process, without any chemical reaction. Solvent extraction is currently the most economical method and the use of hexane and chloroform has made it the fastest and most efficient method for lipid extraction from the most diverse biomasses. However, the presence of these chemical solvents can affect the lipid composition of the extracted oil [39].

Mechanical presses such as screw and piston, among others, can rupture the cells and release the oil, but these operation lead to low efficiency. On the other hand, when combined with the use of organic solvents, oil extraction efficiency can be reached in the order of 95% [10].

High concentrations of salts can cause a sudden change in the osmotic pressure, which can lead to the rupture of cells and the release of oil from inside; a method known as "osmotic shock" [39].

A method that has received notable attention is supercritical extraction, which uses fluids above the critical point of temperature and pressure, providing properties such as low viscosity and high diffusivity, allowing to achieve greater efficiency in the lipid extraction step. In this context, the use of CO2 as a supercritical fluid is highly attractive due to its high selectivity, non-toxicity and short processing period. The only drawback is the high cost associated with installing adequate equipment to operate under safe conditions at high temperature and pressure [10].

In the search for environmentally friendly processes, enzymatic extraction is highly recommended, since it uses enzymes such as pectinase and cellulase, to degrade the cell wall, providing highly efficient extractions without affecting the microalgal lipid composition, unlike the method that uses organic chemical solvents [39]. On the other hand, these methods are still used only on a laboratory scale, due to the high costs involved with the acquisition of enzymes [24].

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*Microalgae Cultivation in Photobioreactors Aiming at Biodiesel Production*

have lipids can, in theory, be useful in the production of biodiesel [6].

not compete with the food chain can also be used to produce biodiesel [6].

water. Both reactions occur in the presence of alcohols and catalyst [41].

Biodiesel is the natural and renewable substitute fuel for petroleum diesel. It is produced from vegetable oils such as soy, palm, sunflower and jatropha and/or animal fat, such as beef tallow and chicken fat. In other words, any sources that

First generation raw materials such as soy, palm and sunflower are not considered socially sustainable because they compete with the demand for food. In contrast, second generation raw materials such as beef tallow, residual frying oils and chicken fat, stand out as alternatives to produce biodiesel. Physic nut, which grows in semiarid regions, where other sources of oilseeds are not able to grow and, therefore, do

Microalgae, as well as cyanobacteria, belong to the group of third generation raw materials. These are a promising alternative, as they are able to accumulate high levels of lipids combined with high growth rates and biomass productivity compared to conventional oilseeds, and they do not compete with the food chain [40].

Biodiesel can be defined as a mixture of alkyl esters of fatty acids obtained by transesterification of triacylglycerides from vegetable oils or animal fat, or by the esterification reaction of free fatty acids resulting in alkyl esters of fatty acids and

Alcohols are considered to be transesterification agents, and may be methyl alcohol (methanol), ethyl alcohol (ethanol), propyl, butyl or amyl. Methanol is the most widely used alcohol due to its low cost and its physical–chemical properties (polarity and lower carbon chain), while ethanol has stood out for its potential for

Transesterification is a multi-stage reaction, including three reversible stages in series, so that triglycerides are converted to diglycerides, then diglycerides are converted to monoglycerides and finally monoglycerides are converted to esters (biodiesel) and glycerol (co-product). Stoichiometrically the transesterification reaction requires 3 moles of alcohol for every mole of triacylglyceride, with an excess of alcohol being used to shift the balance towards the formation of the products, as it is a reversible reaction. The esterification reaction requires 1 mole of alcohol to 1 mole of fatty acid and requires excess alcohol to favor the direction of

The biodiesel synthesis reactions can be catalyzed homogeneously or heterogeneously. Homogeneous catalysts can be acidic (sulfuric, sulfonic and hydrochloric acid) or alkaline (sodium or potassium hydroxides), while heterogeneous catalysts can be enzymes or metallic compounds. In the catalyst determination, the acidity index is the main characteristic of the oil to be observed since high amounts of fatty acids do not allow alkaline catalysis (soap formation in the reaction medium). In

On enzymatic catalysis, enzymes such as lipases, present in several organisms including animals, plants, fungi and bacteria have the biological function of accelerating the hydrolysis of fats and vegetable oils, releasing fatty acids, monoglycer-

Enzymes have some advantages when compared to the chemical catalytic process, such as high selectivity, reaction temperature in the range of 30–40°C and pH between 4 and 9. On the other hand, it requires long reaction periods to achieve

Leung, Wu and Leung [44] explained that in general there will be advantages and disadvantages associated with the choice of the type of catalyst. The main

*DOI: http://dx.doi.org/10.5772/intechopen.93547*

low toxicity and easy availability [37].

this case, the use of acid catalysts is suggested [18].

ides, diglycerides and glycerol [43].

high conversions and, therefore, costly [43].

product formation [42].

**7. Synthesis of biodiesel**
