The nature of the feedstock is influencing a number of parameters such as the ester content, CN, CP/CFPP, viscosity and the oxidative stability. However, there is little influence on the heat of combustion, flash point, lubricity and emission. The ester content of the biodiesel is dependent on the feedstock due to the presence of FFAs which are not converted into esters, the amount of unsaponifiable fraction (1-2%) which is not removed during reaction and impurities such as dimers and polymers of TAG which are transformed into dimeric and polymeric alkyl esters.

The physical properties of biodiesel are mainly influenced by the fatty acid composition of the feedstock (see Chapter 4 for more details).

The melting point (MP) of FAME is mainly dependent upon the fatty acid alkyl chain:

• The longer the alkyl chain length the higher MP

• Presence of unsaturation determine lower MP

• Trans configuration and conjugation of FAME with identical C-atoms and unsaturation are leading to higher melting points

• Branching of alkyl chain is decreasing the MP

• The alcohol chain is influencing the MP: methyl>ethyl>iso-propyl.

In this way, the MP of the FAAE is highly influencing the physical properties (CP, CFPP and viscosity) of biodiesel. The CP is higher for longer alkyl chains,
unsaturation is leading to lower CP and trans and conjugated esters have higher CP. Similarly, the kinematic viscosity of biodiesel is also influenced. In addition, substituents in the FAAE are leading to substantial higher viscosity. Dimeric fatty acids also produce biodiesel with a higher viscosity.

Physical properties of individual FAAE and of biodiesel from various feedstocks are given in Table 5.5 and Table 5.6. The oxidative stability is mainly influenced by the degree of unsaturation, with allylic and crs-allylic position more easily oxidized.

Other factors are the presence of natural anti-oxidants. Purification of biodiesel by distillation is decreasing the oxidative stability as a part of tocopherols remain in the distillation residue. The presence of hydroperoxides, metals (FE and Cu) and pro-oxidants (e. g. chlorophylls) cause a lower oxidative stability. In relation to physical properties of biodiesel, there is a conflict between saturation and unsaturation. Biodiesel produced from more saturated feedstocks has a higher CN and a better stability. However, the cold properties are negatively influenced by a high degree of saturation. It has been observed that sedimentation of insoluble

contaminants in biodiesel prepared from soy and palm oil might occur well above CP (Van Hoed, 2010).