What Is Really in Imported Tung Oil? Understanding Extraction Processes

You might want to grab a cup of coffee before reading through this article.  It is going to get pretty technical, but the information is important for tung oil buyers who really want to know what they are buying. 

So, starting off let’s be clear, not every product labeled “pure tung oil” is produced the same way.  For buyers of Tallahassee Tung Oil, you know what you are getting.  We only use a mechanical screw press to extract the oil from tung fruit harvested in our orchards.  Our labelling tells you when the tung fruit was harvested, when the tung oil was processed, and a batch number that corresponds to a certificate of analysis from an independent laboratory.  We also sign each container of tung oil guaranteeing its purity. 

Unfortunately, this is not the case for imported tung oil.  Imported tung oil may be processed by mechanically pressing tung seeds, however, large processing facilities that export to the U.S. may also extract tung oil using solvents such as hexane or similar light-hydrocarbon mixtures.  Hexane, by the way, is a known neurotoxin (CDC Hexane Safety).  Buyers comparing tung oil products should understand how these processes differ—and why the extraction method matters.

The seeds of tung fruit contain a very high percentage of oil.  In mechanical processing, the kernels are crushed and pressed so that the oil is physically separated from the seed material.  Mechanical pressing does not remove every drop of oil.  Some remains in the press cake after the first extraction.  To increase total recovery, a processor may expose the seed material and/or press cake to a solvent that dissolves the remaining oil.

Hexane has historically been used for this purpose because it dissolves oil efficiently.  Industry literature describes solvent extraction as capable of recovering substantially more tung oil than pressing alone.  Mechanical pressing yields around 70% of available oil, while solvent extraction yields upwards of 95%.  That additional yield helps explain the attraction of solvent processing for large tung oil producers.

But does solvent remain in the finished oil?  A properly operated solvent-extraction plant is designed to evaporate, strip, recover and reuse most of its solvent.  Solvent extraction therefore does not automatically mean that finished tung oil contains a hazardous concentration of hexane.  Nevertheless, 100% solvent removal should not simply be assumed.  Residual hexane has been measured in other commercially processed vegetable oils (ScienceDirect). 

China, which produces approximately 80% of all tung oil imported by the U.S., and other large tung oil producing countries operate different types of facilities.  Some use mechanical presses; some may use a combination of pressing and solvent recovery.  Without accurate processing disclosures, buyers are unable to distinguish mechanically pressed oil from oil produced partly or entirely through solvent extraction.

A container purchased here in the U.S. may identify the product as “pure tung oil” without clearly stating:

• Where the tung oil was produced;

• If it was mechanically or solvent extracted;

• Whether the mechanically pressed and solvent-extracted streams were blended.

The term “pure” can describe composition while saying little about processing.  A product could potentially contain tung oil without added mineral spirits, dryers or resins and still have been produced through solvent extraction.  Conversely, mechanically pressed tung oil could later be blended with thinning solvents.  Tung oil that has been blended with other chemicals will usually be labelled as a tung oil “finish”.

There are also other factors to consider when buying imported tung oil in addition to how the tung oil was processed.  Imported tung oil often passes through several stages before reaching the store shelf.  At the foreign facility where it is processed the tung oil is stored after processing.  It is then exported through a second company that stores it at a shipping port until it is loaded onto a large freighter.  The journey continues across an ocean where temperatures and humidity levels greatly fluctuate.  The containers are then stored by U.S. wholesale buyer.  The companies that repackage the tung oil you buy in the store, order their supply from one of the U.S. wholesalers and they then fill and label containers for retail.  By the time you order online or buy at a local store, the “pure” tung oil you receive could be several years old.

The companies named on the retail container, particularly the popular brands sold in the U.S., are not the company that crushed the seed or extracted the oil.  A distributor may know the country of origin but have limited information about the original processing plant.  The supply-chain distance makes documentation particularly difficult but also very important.  Responsible suppliers should be prepared to identify the production method and support their claims with records rather than assumptions. 

When those facts are disclosed, customers - you - can make an informed decision based on performance, processing method, environmental priorities, and personal preference.  When they are not disclosed, the words on the label tell only part of the story.  As consumers become more aware of how oil seed products are processed, we’ll hopefully see more emphasis on accurate labelling.