with Bevan Skaalen
ver the course of the last 10 years or so, wineries in the U.S.
have been dealing with a difficult problem: 2,4,6-
trichloroanisole (2,4,6-TCA). The cost of this chemical compound
to the wine industry has been tremendous — in wine
quality, in negative publicity, and financially.
Generally termed a haloanisole, 2,4,6-TCA is known to
cause a musty, mold taint at very low concentrations (parts
per trillion [ppt]). It has likely always been a major cause of
wine taint but has been recognized as a serious problem only
in the past 10 years because scientific instrument technology
has advanced to be able to identify 2,4,6-TCA as one of the
sources of the musty, moldy taint found in wines.
Another haloanisole, 2,4,6-tribromoanisole (2,4,6-TBA) has
recently been identified as a similar contributor to wine taint.
2,4,6-TBA was first identified by Pascal Chatonnet and
collaborators in 2004 in French wines.1 Like 2,4,6-TCA,
2,4,6-TBA causes a musty, mold taint in wine at very low concentrations,
but it has the potential to be an even more serious
problem to the U.S. wine industry because its precursor (2,4,6-
tribromophenol [2,4,6-TBP]) can be found in so many sources
commonly used in wineries.
This column provides an overview of the emerging
2,4,6-TBA problem and suggests what preventive actions
wineries can implement to protect their wines.
What is the taint resulting from haloanisole contamination
of food and beverages? Taint is a taste or odor that typically
arises from an external source, as opposed to off-odors or off flavors,
which are attributed to internal changes to a product
(such as microbial spoilage).
The taste or odor threshold for a taint is defined as the lowest
concentration of the chemical compound detectable by a
defined population group. Generally, a food or beverage taint
can be tasted or has an odor at extremely low concentrations.
Chemically, 2,4,6-TBA is a derivative of the anisole (or
methoxybenzene) family of compounds, which contain at
least one atom of a halogen (fluorine, chlorine, bromine, or
iodine) and thus are termed haloanisoles. Haloanisoles are
formed from halophenol compounds by microbes, such as
filamentous fungi, via a process called biomethylation.
Specifically, 2,4,6-TBA is formed from the biomethylation
of its precursor 2,4,6-TBP. Figure I shows the chemical structure
of 2,4,6-TBA (three bromine atoms attached to the 2, 4,
and 6 positions on a benzene ring).
Origin of the 2,4,6-TBA precursor 2,4,6-TBP
Chlorophenols (like 2,4,6-trichlorophenol [2,4,6-TCP],
which is precursor to 2,4,6-TCA) are primarily anthropogenic
in origin; that is, they are produced by human activity.
Bromophenols (like 2,4,6-TBP) can be produced both
naturally in the environment and anthropogenically. In
nature, 2,4,6-TBP can be formed in marine environments by
brown algae as a way to remove
excess bromine from their
surroundings,2 and anthropogenically, it can be produced in
wastewater that has been treated with chlorine in the
presence of bromine ions and low levels of organic phenols.3
2,4,6-TBP and its derivatives have been used as 1) fire-retardant
agents in epoxy resins, polyurethanes, plastics, paper, textiles,
and fire extinguishing media; 2) wood preservatives; 3)
general fungicides for the leather, textiles, paint, plastics, paper,
and pulp industries; and 4) antiseptic agents. 5) They have also
been found in detergents containing bromine.
The winery environment has several possible sources of
2,4,6-TBP, such as painted surfaces in the cellar, sealants,
barrels, oak adjuncts, wood ladders, wooden catwalks, wood
pallets, plywood, wooden rafters, wood beams, water, water
hoses, wine hoses, plastic tank liners, plastics, insulation,
filter pads, fining agents, packaging materials (cardboard,
adhesives, paper bags), cleansers, and sanitizers.
Formation of 2,4,6-TBA
The only scientifically proven molecular mechanism responsible
for formation of 2,4,6-TBA (and all other anisoles found in
wine — 2,4,6–TCA, 2,3,4,6-tetrachloroanisole [2,3,4,6-TeCA], and
pentachloroanisole [PCA]) is the biomethylation (O-methylation)
of its precursor 2,4,6-TBP. (See biochemical reaction in
Figure I.) In the conversion to 2,4,6-TBA, 2,4,6-TBP is catalyzed
by the enzyme chlorophenol O-methyltransferase (CPOMT),
and the oxygen group on 2,4,6-TBP is methylated. This enzyme
has been shown to methylate several different halophenols,
including chlorophenols, bromophenols, and iodophenols.4
Figure I. Biochemical reaction for the conversion of 2,4,6-TBP to
2,4,6-TBA by O-methylation
This biomethylation reaction is performed primarily by
filamentous fungi (Trichoderma longibrachiatum, Penicillium
spp., Fusarium spp., Cladosporium spp., and Paecilomyces
variotii)5,6,7 and has been shown to be catalyzed by the winerydwelling
Biomethylation is a biochemical defense mechanism for
microbes. It allows them to detoxify their immediate
environment by converting the highly toxic halophenols to
non-toxic haloanisoles. In the absence of this defense mechanism,
the filamentous fungi and Streptomyces could die or
suffer significant physiological damage. It is worth noting
that filamentous fungi also have a second defensive strategy
to detoxify halophenols: secretion of oxidative enzymes
(laccase) that attack and degrade halophenols.