I know that phenol is something to avoid.  
What is the origin (how does it develop) ? 
How can it be avoided or kept at a minimum (maybe there are some
regulations indicating maximum permitted) ?
It is in particular phenol related to coffee which has my interest,
but phenol in other products can also be mentioned.

The above is apparently from this or a related website:
http://www.atsdr.cdc.gov/tfacts115.html

http://en.wikipedia.org/wiki/Phenol

1.Metabolic activation of phenol by human myeloperoxidase and
horseradish peroxidase
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3023815&dopt=Abstract
The oxidation of phenol catalyzed by human myeloperoxidase and
horseradish peroxidase resulted in extensive binding of phenol-derived
metabolites to boiled rat liver protein. This binding paralleled
closely the removal of phenol from the incubations and was inhibited
from 83 to 99% by the addition of the antioxidants, ascorbate and
glutathione, suggesting that metabolism and binding were occurring via
a one-electron oxidation pathway. Metabolic studies employing both
human myeloperoxidase and horseradish peroxidase resulted in the
identification of 4,4'-biphenol and diphenoquinone as the principal
identifiable metabolites. The addition of reduced glutathione to
incubations containing horseradish peroxidase resulted in the
formation of two conjugate species. These conjugate species were
identified by fast atom bombardment mass spectrometry to be
glutathione conjugates of diphenoquinone. The major gluthathione
conjugate was identified as 3-(glutathion-S-yl)-4,4'-biphenol by NMR
spectroscopy. These results suggest that the formation of highly
reactive species through the peroxidase-mediated metabolism of phenol
and other phenolic compounds could play an important role in the
hematopoietic toxicity observed during chronic benzene exposure.

2.Scientists Warns 'No-Phenol' Digestive Enzymes 'Dangerous' Autism
Treatment Enzymes, amino acids and phenol in autism
http://www.autismtoday.com/articles/nophenoldangerous.html

3.Metabolic pathways of the diseased potato
http://www.scri.sari.ac.uk/TiPP/pps/Chart.pdf