digital.csic.es/bitstream/10261/2453/1/Elaboration.pdf#page=7
> 4.2.3. Colour fixation
The black surface colour obtained is not stable
and fades progressively after oxidation and during
the shelf life of the packed product. To prevent this
deterioration, only the use of ferrous gluconate and
ferrous lactate is legally permitted for ripe olive
processing (García et al., 1986a). Normally, ferrous
salts were added at a concentration of 100 ppm
(parts per million) of iron in the liquid. Iron diffusion
into the flesh is complete in about 10 hours, but
normally, this phase is prolonged to about 24 hours
(García et al., 2001).
