What are taste & odor causing compounds?
While there are several organic compounds classified as taste-and-odor-causing compounds, the most prominently encountered are trans-1,10-dimethyl-trans-9-decalol (geosmin) and 1,2,7,7-tetramethyl-bicyclo-heptan-2-ol (MIB) which can result in consumer complaints at levels as low as 5 – 10 ng/L. Other taste-and-odor-causing compounds include 2-isopropyl-3-methoxy pyrazine (IPMP), 2-isobutyl-3-methoxy pyrazine (IBMP), and 2,3,6-trichloranisole (TCA).
Typically, research focuses on MIB and Geosmin because these two compounds are more prevalent in nature and are more difficult to remove than IBMP, TCA, and IPMP.
Where do MIB and Geosmin come from?
MIB and Geosmin are secondary metabolites caused by certain species of phytoplankton and bacteria and are identified by their characteristic earthy-musty tastes and odors. Organisms most frequently linked to these taste and odor problems have been actinomycetes and several genera of blue-green algae (cyanobacteria). The most common documented taste-and-odor-producing blue-green algae include Oscillatoria spp., Aphanizomenon spp., Anabaena spp., and Microcystis spp. Some protozoans have also been linked to producing tastes and odors.
How do these compounds cause unpleasant tastes and odors?
Living algae produce numerous volatile and non-volatile organic substances that are either simple products of photosynthesis or more complex metabolites. Those compounds that are not used immediately, or stored within the algae, are excreted into the environment. These compounds then become food for organisms such as bacteria (e.g., cyanobacteria and actinomycetes). Some of these compounds, or their decomposed products, cause tastes and odors themselves. Additionally, when algae decompose, they can cause taste and odor problems in two potential ways. First, when algae die they, lyse (i.e., break apart) releasing odorous compounds. Second, the decaying biomass can produce adequate nitrogen to support bacterial growth, including that of actinomycetes.
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