Hexanes - Article by Karl Loren

Edible Oil Extraction: Severe Limitations of Hexane (Solvent Extraction)

For practical and economic reasons, hexane (C6H14) is the dominate extraction solvent for oil seeds, used throughout the world. It is cheap (7 cents per pound) and abundant: every petroleum refinery on earth produces it, as it is an essential high-vapor pressure easily-ignited component of gasoline. As total world petroleum is about 3,500 million metric tons annually, a simple 5% early stripping of this "natural gasoline" would allow 175 million tons (385 billion pounds) of availability. Since world oil-seed extraction using hexane solvent probably accounts for around 60 million metric tons of edible oil, which represents about 250 million tons of feedstock, at a moderate loss rate of 0.15% (3 pounds per ton of feedstock), total annual hexane losses to the environment might be 750 million pounds (130 million gallons), or about 0.2% of what is potentially available from refineries. This is an extremely minor area for the petroleum industry. Thus, the combination of extremely large availability, very low cost, and simple effectiveness have created 50 years of inertia which has tended to obscure six very menacing features. These are as follows:

1. Even with modern DST units (desolventizer/toaster), some residual hexane remains in the final meal (animal feed). In many facilities, this can run as high as 0.5%. The known mild toxicity of this trace contaminate in animal feed precludes the use of solvent-extracted meal in a number of markets (it kills baby piglets). There are deeper issues involving the near-irreversible adduct that the hexane molecule forms with the interior, highly hydrophobic regions of the folded proteins. Presumably, upon digestion in an animal's gut, this toxic hexane is released. It is then free to adduct to other hydrophobic acids, etc. This is supported in the literature going back 30 years.

2. Hexane is "too efficient" - it extracts virtually every oil soluble fraction, including up to 5% of unwanted (but nutritionally very valuable) materials, usually referred to as "unsaps." This means that the meal (now contaminated with hexane) no longer contains these oil-soluble, critical nutrients (phytosterols, vitamin E family tocols, and phosphatidyl choline and other natural lecithins). Additional chemicals particularly caustic soda, and severe thermal processing, etc., are now required, and this creates quantities of waste sludges and greatly degraded unsaps. This is both a cost problem (chemicals) and a waste problem (sludges).

3. Even where the recovery of sterols, vitamin E, etc., from the unsaps is attempted, the final yield is only a fraction of the content in the original feedstock. Furthermore, it has been degraded and requires many subsequent stops of purification and modification to become edible again.

4. The inevitable daily loss of hexane (by inherent design) into the environment is probably the most severe problem. Older plants lose up to 0.5% per ton of feed, and the current state of the art guaranteed on new facilities have a maximum loss of 0.15%, with practical operations at 0.1%. Even at this guaranteed loss of 0.15%, a moderate-sized 100,000 bushel per day soybean facility would lose 6,000 pounds of hexane per day to the environment (atmospheric leaks from distillation, decanting, open vessels, and the meal). This translates to an entire 40,000 pound (tank wagon) every week lost to the environment.

5. The EPA now categorizes hexane as a HAP (haxardous air pollutant). It is included on the list of 189 toxic chemicals. It is controlled under the TRI (toxic release inventory) of the U.S. EPA. Refer to Inform, Vol. 9, No.7, July 1998:p 708.

6. Since hexane is very volatile, flammable and explosive, it is also a simple physical hazard. Numerous plants have exploded and burned over the past 50 years, often with the loss of multiple lives. Usually, one or two "go up" annually, throughout the world. After all, this is high-vapor-pressure gasoline, which is being endlessly redistilled in a closed loop, 24 hours per day, 7 days per week.

In light of the above, it is absolutely impossible to site a new hexane extraction plant in a number of locations in the United States. This trend is continuing.

Overton SV and JJ Manura (1997) using new technology found higher than expected levels of pentane, hexane, heptane, octane and benzene derivatives in all 6 hexane extracted cooking oil samples tested. This means that humans and livestock may be ingesting greater amounts of petroleum derivatives than previously thought.

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Posted : 03/31/2010

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