It is substitutes for milk in coffee. See here: Coffee Whitener from GEA Niro
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From Daily Express | UK News | Drug police made me eat my milk powder says MP | Drug police made me eat my milk powder says MP
In the sentence below, what does "coffee whitener" mean?
MP Michael Fabricant yesterday told how he was threatened by armed Colombian soldiers who thought that his coffee whitener was cocaine.
Generally no. It is a mixture of glucose, emulsified fat and other substances. There are milk powders that can be used in coffee, but they are not technically "coffee whiteners".