DD represents a good source of valuable minor compounds such as sterols, tocopherols and squalene, which can be recovered and further used as food additives, in the pharmaceutical industry and cosmetics. Furthermore, the FFAs, one of the major compounds present in DD, are mostly used as additives for animal food, fluidizing agents, for lecithin or as medium-grade soaps. Such fatty acids can also be used as precursors in a wide variety of molecular synthesis schemes such as the production of dibasic acids of different chain lengths (Gangopadhyay et al., 2007). Alternatively, DD has non-food applications, such as their mixing with fuel oil (5-10%) to fire steam boilers (Svensson, 1976). A great interest was shown in DD for its possible application in the production of high-quality (biodiesel) or low-quality (biofuel) methyl esters.

Rough estimates of the quantity of DD are available. The amount mainly depends on the content of FFA, gums and impurities present in the oil and on the efficiency of refining. Using Mielke (2009) figures for the worldwide vegetable oils production in 2009 and assuming that (1) palm oil is entirely physically refined (100% RBD), (2) soybean oil is mainly chemically refined (100% NBD) and (3) rapeseed and sunflower oils are mainly physically refined (75% RBD/25% NBD), one can estimate the DD production. Considering that by physical refining a multiple of 1.2 times the FFA content is removed from crude oil as DD (Vries, 1984), the DD production can be estimated (Table 22.2). The FFA content before deodorization step for the chemically refined oil (NBD) is difficult to estimate. However, it is generally considered that the vegetable oil contains ca. 0.10% FFA before deodorization and 0.05% FFA after deodorization (final oil) in order to make it suitable for human consumption.

The palm oil production increased significantly in 2009 (46.5 mil. t/year) compared with 2007 (36.8 mil. t/year) that accordingly determined an increase in the DD (RBD).

Considering Mielke (2009) figures for the vegetable oil production in 2009, approximately 3.1 mil. t/year of DD should have been produced, where 3.02 mil. t/ year comes from physical refining (DD-RBD) and 0.03 mil. t/year comes from chemical refining (DD-NBD).

Owing to the high content of FFA present in the DD from physical refining (RBD), this side-product is suitable for the biodiesel production and the DD from chemical refining (NBD) is mainly valorized for the recuperation of minor compounds. However, there is no clear distinction in the literature of the origin of the feedstocks.