Triglicerides and fatty acids

Phospholipids and cell membranes

Sterols: cholesterol and testosterone


Extra topics:

Fuel made from fat: Biodiesel

Eating fat without gaining weight:Olestra

Trans fats




Lipids (or fats)

Fuel made from fat:


Biodiesel is a fuel made of animal or plant fat, which is produced to power diesel engines.
However, biodiesel is not the same as fat. Although diesel engines can  run on vegetable fat (the first diesel engine run on peanut oil), this is not a convenient fuel because it causes some damage to modern diesel engines. A simple chemical transformation (transesterification) can transform fat into a fuel which in many respects is superior even to petro diesel ( a discussion of its advantages and disadvantages is presented below).
The formation of alkyl esters (biodiesel) from fat consists basically of adding an alcohol to the fat. The transesterification process is accelerated by means of a catalyst, normally a base like sodium hydroxide (caustic soda).
The alkyl esters formed are a much better fuel than simply fat, for two main reasons: decreased viscosity and increased cetane number.
Fats are too viscous. This fact is inconvenient because the fuel injector in a diesel engine must produce an spray where the fuel droplets are as small as possible. The smaller the better, because this way the oxygen can access more of the fuel and the combustion can be (almost) complete. The high viscosity of fat causes the formation of relatively large drops, which causes a loss of power and also the production of deposits of unburnt oxidised material inside the combustion chamber. In biodiesel this problem is lifted; it has approximately the same viscosity as petro diesel.
The cetane number is a measure of the self-ignition quality of the fuel. Biodiesel has a higher cetane number than fat. Higher cetane numbers means a better engine performance and also better cold starting.

Advantages and disadvantages

Use of land
One of the main arguments against biodiesel is with respect to the farming area occupied and the possibility of deforestation for planting. That raises 2 issues:

  1. The carbon emissions resulting from deforestation / Large amounts of land required. It has been calculated that , using soya, the entire arable land of the US would have to be occupied in order to fulfil the diesel fuel needs of the country.
  2. The availability of food for human consumption in the third world and the need to use the land to produce edible food.

However, there are satisfying answers  to these issues.
Firstly, it is possible to choose a feedstock that doesn’t require large amounts of good agricultural land.  Algae are a good alternative. They don’t require land currently being used for food production and they can be grown on waste materials like sewage or in non-arable lands like deserts or marine areas. In addition, their potential oil yield is higher than from plants, so that less area would be needed to fulfil the diesel fuel needs of the country.
Another alternative is the use of   halophytes, which are plants that grown on saltwater, where conventional food crops cannot grow. Their oil yield is similar to yields of soybeans.
Finally, there various other species of non-edible plants that can grown in poor soil, which is unsuitable for food crops, and that have a significant oil yield.
Recycling dirt oil
Oil that has been used up, especially in restaurants that have extensive frying, can be collected and  transformed in biodiesel. That is an almost free resource. It also avoids the dumping of dirt oil in landfills.
Biodiesel degrades about 5 times faster than petro diesel. This is normally seen as a disadvantage, although some people claims it is a positive aspect with respect to environmental issues, because it means biodegradability.
Biodiesel is hygroscopic, e.g., it attracts moisture. Water maybe a problem because it may cause corrosion to fuel systems and also promote the growth of microorganisms.
Biodiesel is non-toxic. It has been claimed that it can be eaten and it causes less damage than salt .
Biodiesel emissions are lower in CO2 , NOx ,  sulphur and particulate matter. That means that it contributes to a reduction in greenhouse gases like CO2 . It also reduces the production of NOx and also SO2 (sulphur dioxide) , which are gases that cause acid rain among other problems.
Particulate matter are a serious health hazard. Submicron particles can penetrate deep into the lungs, into regions where they cannot be expelled. In addition, they contain metals that act as catalysts of undesirable chemical reactions in the atmosphere, for instance the formation of strong acids like sulphuric and nitric acids.
Energy security
Biodiesel offers countries that lack oil reserves the opportunity of becoming energy independent. Diesel is very important because is the main fuel involved in the transportation of people and goods, especially in the third world where the rail infrastructure is precarious.
Power density
The calorific value of biodiesel is about 37.27 MJ/kg This is 9% lower than regular petrodiesel. Some people (
U.S. Department of Commerc) claim that this is not an issue because it is due to the fuel density difference.

Biodiesel cannot operate at low temperatures. Gelling is the temperature at which pure biodiesel (B100) starts to gel and as a result cannot work efficiently as a fuel. This temperature is dependent on the feedstock used and can roughly vary from about -10 to +16 degrees Celsius.

Biodiesel is a better lubricant than petro diesel and it increases the life of fuel injection systems. Methyl esters (biodiesel) are also sold as lubricants for diesel engines.

The cost of biodiesel is usually higher than petro diesel.


Biodiesel is usually mixed with petro diesel and mixtures have ranged
from 2/98% (B2), 20/80% (B20) up to 100%(B100).
The use of mixtures with up to 20% biodiesel (B20) doesn’t require any modification of the engine and doesn’t affect manufacturers’ warranty.
If the proportion of biodiesel used is higher than this value, minor modifications must be made to the engine and some manufacturers will void the warranty.





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