RESEARCH UPDATE
E. coli-free tea

New research looks at various methods of brewing compost teas and their ability to support E. coli production

By Cara Hungerford

January 7, 2005: The debate surrounding compost tea used to linger solely around its effectiveness in controlling plant fungal diseases and other pathogenic systems -- that was until reports of compost teas contaminated with E. coli starting emerging. Focus quickly shifted from efficacy to safety. Concerns that tea usage could lead to bacterial contamination of food crops caused such an outcry within the organic community that the National Organic Standards Board (NOSB) created a Compost Tea Task Force and a slew of research experiments were set up to investigate the situation.

In April of 2004, the Compost Tea Task Force released its recommendations—which included using potable water, sanitizing equipment and brewing areas and following additional guidelines when using additives such as molasses—without much available science to go on. Now newly published research is emerging and at least initial studies are backing the committee’s common sense approach.

One such study, conducted at the Woods End Research Laboratory in Mt. Vernon, Maine, and titled, “Compost Teas: Microbial Hygiene and Quality in Relation to Method of Preparation,” was published in the summer 2004 edition of Biodynamics. Dr. William Brinton and team set out to determine the potential health risks of compost tea by recording the bacterial levels of several preparation methods. The methods studied were commercial compost tea kits, commercial kits in which farm-made compost was substituted for purchased materials and a European method in which homemade compost is steeped in water and lightly stirred for several days. The results showed that after 24 hours of aeration and 36 additional hours of steeping none of the preparation methods tested positive for E. coli provided the ingredients were not contaminated prior to mixing. The researchers found that small amounts of E. coli in the tea will actually decrease to the point of extinction in 72-120 hours.

The results of the study lead researchers to conclude that hygiene is the greatest threat facing compost teas. When teas were produced with bacteria-free water and compost in a laboratory setting, E. coli was virtually undetectable. On the other hand, when E. coli was present in the compost or water at the time of brewing, the process served as an incubator and bacteria levels grew rapidly. In order to reduce the risk of bacterial contamination researchers stressed the importance of clean handling techniques including washing tools and storing brews in areas protected from outside contamination and using composts that have been tested for E. coli.

Although supplemental tea materials were not the focus of the study, researchers also found support for the cautious approach taken by the task force with regard to the use of additives, especially molasses, in compost teas. Based on the findings that showed the European-style teas to actually inhibit the growth of E. coli over time, researchers hypothesized that the lack of available carbon (provided by the molasses) and the absence of active aeration may have created an inhospitable environment for E. coli and hindered replication.

Researchers recommended avoiding teas made with a brew time of 24 hours or less and those using molasses additives, determining that these popular brews may in fact be among the least safe and—according to previous research*—least effective methods of producing compost teas.

In October, the NOSB voted to accept the recommendations of the Compost Tea Task Force and is now working to merge the April report with its previously established guidelines on Compost Tea.


*Weitzien, H.C. 1963. Untersuchengen über die Uraschen der keimhemmung von Pilzsporen im Boden. Zbl. Bakt. Parasit. Infekt. Hyg. 116:131-170.


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