Vegetable oil created from a seed oil press can be usable without going through any process of refining, bleaching or deodorizing. This is especially true if those using the oil are comfortable with the natural smell, taste, color and appearance of the oil. However, in order to meet the standards required to be classified as commercial grade oil, vegetable oil produced by a seed oil press must be further processed using specific methods.
The need for refining, bleaching and deodorizing goes beyond simple, aesthetic considerations. Oil which is to be used by restaurants needs to have a relatively high smoke temperature. Safe smoke temperatures cannot be reached by oil made from a seed oil press without the use of additional refining and bleaching methods. In addition, many restaurants specifically ask for oils free of the natural tastes that many customers are unfamiliar with.
Restaurants are not the only market for commercial grade vegetable oil. More and more individuals and organizations are using biodiesel to fuel their vehicles. In this context, ensuring that oil is up to commercial grade standards is even more important, as issues of vehicle performance and safety come into play.
Refining involves two main steps: degumming and neutralization. In degumming, undesirable chemicals known as phospholids are taken out of the oil. It is especially important to ensure that non-hydratable phospholids are removed for those who are seeking to produce 100 percent biodiesel certifiable as such by the American Society of Testing and Materials.
The second step, neutralization, involves the removal of the free fatty acids. This is achieved by the use of sodium hydroxide in a water based solution. Following this, there are several possibilities. The oil can be filtered, put through a centrifuge or simply allowed to settle.
This step is intended primarily to remove unwanted color from the oil. Secondary benefits include the removal of other undesirable materials from the oil, such as sulfur compounds, trace metals and soap. These changes are achieved by combining the oil with specific kinds of clay that have bleaching properties.
As the name implies, the final step absorbs unwanted smells from the oil. Along with this, tastes and some remaining free fatty acids may also be removed. There are two main methods for achieving deodorization. The first involves the use of activated charcoal. The second occurs under conditions of low pressure and high temperature, and consists of steam heating and vacuum treatment.