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What is causing this vinegar smell?

The Question: Whenever I walk by this certain house, there’s a noticeable smell, like vinegar, but more chemical. Very hard to pinpoint the source. Now at the same time, they had a giant white oak cut down, and some of the 5-foot diameter rounds from the trunk are still in the yard, freshly cut. Is it possible that the wood is the source of the smell?

Submitted by: Tom, Washington, DC, USA

Cut logsThe Short Answer: I suspect that you are smelling acetic acid being given off by the cut tree. Acetic acid, is, of course, the component of vinegar that gives it both its acidity and its characteristic smell. Healthy trees, when first cut, can release acetic acid, which evaporates and contributes to the fresh cut wood smell. Some reports suggest that oaks (especially red oaks, more on that later) in particular have an acetic acid smell when cut. There is also the possibility that the tree had a condition called bacterial wetwood. Some refer to this condition as slime flux, but the slime is really a symptom of the bacterial infection. The anaerobic bacteria that cause wetwood can create acetic acid as a byproduct of metabolism. It’s unlikely that wetwood would weaken such a large tree enough to cause it to be cut down. On the other hand, it seems that most trees, if they live long enough, are eventually infected, and if this tree were taken down for some other reason, it might still have had bacterial wetwood, which might be contributing to the acetic acid smell.

Slime Flux More Information: Acetic acid is a very common chemical in nature, found in metabolic pathways and produced by both aerobic and anaerobic bacteria (oxygen-loving and oxygen-hating, respectively). It’s been known to humanity for at least as long as we’ve been making beer and wine, because acetobacteria will convert alcohol to acetic acid in the presence of oxygen, souring beer and turning even the finest wines to vinegar. The various species of bacteria that cause wetwood are anaerobic, however, so they can thrive in the interior wood of trees, where there is little oxygen.

Ted Leininger, PhD., of the USDA Forest Service, points out that the slime flux/wetwood bacteria don’t actually destroy the wood. However, the bacteria release enzymes that degrade the walls of the wood cells. This degradation causes the cells to leak and the wood to take on a water-soaked appearance. Hence the name, wetwood. The typical sign that a tree has bacterial wetwood is when all that released water begins oozing out of any wounds the tree may have. This oozing has been given the descriptive name slime flux. It’s possible that a buildup of gaseous pressure caused by the byproducts of the bacteria can actually crack the tree and create the wounds from which the “slime” oozes, but in most cases, the slime exits through existing wounds. Dr. Leininger explains why wetwood can cause a tree to smell: “Wetwood bacterial infections can produce fatty acids such as acetic acid, butyric acid, valeric acid, caproic acid, or propionic acid, which impart the odor and cause bleaching or staining to the bark.  The odor has been described as rancid, fetid, fermented, or like vomit.”

Because AskaNaturalist reader Tom reported the smell as vinegary and not particularly fetid, Dr. Leininger thinks the smell may be simply cut oak smell. “It’s possible that healthy oak wood will have a distinct, perhaps acidic, odor because of tannins.” After looking at the photo of the cut sections of the tree, Dr. Leininger doesn’t believe the tree was suffering from an acute wetwood infection, as he says it would probably be visible in the cross section as darker, water-soaked areas.

At least one source also reports that red oaks are particularly high in natural acetic acid. Tom reported the tree as a white oak, but oak species are notoriously hard to identify, in part because they are highly variable and often hybridize. From the bark shown in the photo, Dr. Leininger is pretty sure this tree was a red oak, not a white oak. White oak, he says, tends to have scaly or shaggy bark. So the best guess seems to me to be that this is a red oak, and the remaining cut rounds are giving off a natural acetic acid odor, and imparting that salad dressing smell to the neighborhood.

There is yet another connection between wood and acetic acid. In an early case of making lemonade when life gives you lemons (though lemons have citric acid, not acetic), people quickly figured out that spoiled wine had its own benefits, both as food (vinegar) and as an acidic chemical agent. So they soon developed ways to deliberately manufacture it. One way is to simply encourage those acetobacteria that ruined the wine in the first place. This is done by inoculating wine with acetobacteria culture and then aerating the wine to make sure the bacteria have plenty of oxygen. Another method is to “cook” wood and distill off the acetic acid, both the acid naturally in the wood, and the acetic acid formed by the action of high temperature on the many biological chemicals present in wood. The resulting amber-colored liquid in its raw form is called pyroligneous acid, but its main acidic component is none other than our familiar acetic acid. Cooking wood remained the primary method of preparing acetic acid until methods of direct synthesis from methanol were invented in the 20th century.

Acetic Acid Trivia: Acetic acid as vinegar and in biological systems is dissolved in water. But if you hang around chemistry labs, you’ve probably heard of “glacial acetic acid.” This is simply pure acetic acid, with absolutely no water. Pure acetic acid freezes at 16.5 °C (62 °F), just a little below room temperature, forming acetic acid “ice,” hence the “glacial.”

Sources:

Wilson, A. 2008. Incidence of bacterial wetwood in southern bottomland hardwood logs and lumber. Phytopathology 98 (6): S172-S172.

MURDOCH, C., and R. CAMPANA. 1983. BACTERIAL SPECIES ASSOCIATED WITH WETWOOD OF ELM. Phytopathology 73 (9): 1270-1273.

Xu, Z., et al. 2001. Physical, mechanical, and drying properties associated with bacterial wetwood in red oaks. Forest Products Journal 51 (3): 79-84.

Granstrom, K., and B. Mansson. 2008. Volatile organic compounds emitted from hardwood drying as a function of processing parameters. International Journal of Environmental Science and Technology 5 (2): 141-148.

http://chestofbooks.com/reference/American-Cyclopaedia-V1/Acetic-Acid.html

http://toxics.usgs.gov/definitions/pyroligneous_acid.html

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