Deodorization construction materials — Heavy metals, particularly those possessing
two or more valency states, generally increase the rate of oxidation. Of
all metal, copper is the most potent catalyst. A concentration high enough to
produce a noticeable oxidative effect lies in the proximity of analytical detection
limits, probably 0.005 ppm. The corresponding content for iron is 0.03 ppm. Other
metals have exhibited varying catalytic powers; for example, the contents of metal
ions required to decrease the lard-keeping time by 50% at 208°F (98°C) are 0.06
ppm manganese, 1.2 ppm chromium, 2.2 ppm nickel, 3.0 ppm vanadium, 19.6
ppm zinc, and 50.0 ppm aluminum.100 This comparison stresses the importance
of avoiding the use of copper, iron, or some other alloy if the highest possible
flavor stability is required; therefore, deodorizers are fabricated from type 304
stainless steel at points contacted by the oil.