Biofuels in Perspective

Production of Biodiesel from Waste Lipids 157
In this section the physical and chemical reactions will be described which will be
harmful for biodiesel production. In the next section the various technological approaches
for the biodiesel production from waste fats and oils will be studied followed by a number
of case studies.
9.3.1 Physical and Chemical Reactions in Lipids
Crude vegetable oils and animal fats are containing a number of artifacts which are changing
the physical and chemical properties in comparison to refined oils. The presence of these
unwanted components is due to a series of reactions: oxidation, hydrolysis, dimerization
and polymerization. Similar reactions are occurring when refined oils and fats are used for
food production, especially when their production is performed at higher temperatures, in
oxidative conditions and in the presence of water. In addition, food product ingredients can
migrate into the oil.
Physical changes in processed oils and fats are an increase in viscosity due to polymerization,
an increase in the specific heat, changes in the surface tension and especially the
higher tendency for foaming and browning and/or darkening of the color (Cvengros and
Cvengrosova, 2004; Nawar, 1984).
9.3.1.1 Oxidation Reactions of Fatty Acids
Crude vegetable oils and fats and especially frying and cooking oils are oxidized due to
the presence of unsaturated fatty acids. Oxidation can take place via three mechanisms.
Auto-oxidation, where oxygen is attacking the methylene function next to the double
bond, involves an initiation, propagation and eventually a termination step. Auto-oxidation
can only be stopped via the addition of anti-oxidants.
The second mechanism is photo-oxidation due to the presence of sensitizers (chlorophyll,
haemoglobin, colorants, metals Fe, Cu) which are leading to radical formation followed by
auto-oxidation or by the formation of singlet oxygen which is reacting 1200 times faster
than triplet oxygen with unsaturated fatty acids.
An enzymatic oxidation involves the presence of lipoxygenase which is only active in
the presence of a 1,4-pentadiene (e.g. linoleic, linolenic acid).
The rate of oxidation is dependent on the number of double bonds. The relative rate for
oxidation in comparison to C18:1, is respectively 10 and 100 for C18:2 and C18:3.
The three oxidation pathways are giving rise to similar oxidation products. The primary
reaction products are hydroperoxides and peroxides. The compounds undergo cleavage of
the O-O bond with the formation of alkoxy radicals which are converted into secondary
oxidation products such as aldehydes, ketones, alcohols and acids. Especially the formation
of conjugated mono- and di-unsaturated aldehydes which are rapidly reacting with
nucleophiles via a Michael addition can form unwanted components.
Considering the use of crude and waste lipids for the production of biodiesel it is
important to consider the oxidative stability of the fatty acids. Therefore, used cooking and
frying palm oil seem to be the best choice due to the higher degree of saturation followed
by high oleic sunflower oil.