3.1. Waste edible oil (WEO): availability, economic value and
properties
The amount of WEO generated from every country worldwide
is huge and varies accordingly to the amount of edible oil
consumed. Annually, a total of more than 15 million tons of WEO
is generated from selected countries in the world as shown in
Table 5. It should be noted that the worldwide WEO is way much
larger than that value. Therefore, WEO is readily available
feedstock that can be used for biodiesel production. The cost for
WEO is generally lower than fresh edible oil as a major fraction
contributing to the cost lies in the collection and purifying
processes. For comparison purposes, the costs of soybean and
WEO are given. Currently, WEO from soybean is sold as yellow
grease that has a market value of approximately $1.09/US gallon
and is expected to rise to $1.21/US gallon by the year 2013. On the
other hand, the price of soybean in the market is about $2.22/US
gallon in year 2004 and is expected to rise to $2.47/US gallon in
2013 [45].
Generally, the physical and chemical properties of WEO are
almost similar to fresh edible oil and are different from source to
source. For example, the chemical and physical properties of palm
WEO might be different from rapeseed WEO due to the different
oil composition. Apart from that, the water content and FFA
content in WEO are relatively higher than fresh edible oil as a
result of frying process. During the frying process, edible oil is
heated in the presence of air and light at temperatures of
160–2001C for a relatively long period of time. Some common
physical changes observed in edible oil after frying are: increase in
viscosity and specific heat, change in surface tension, color and
higher tendency of fat formation. All these changes are due to
three common reactions during the frying process: thermolytic,
oxidative and hydrolytic [47].