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Pressurized N-Propane Shows Promise as Extraction Method for Hemp Seed Oil

By Leo Bear-McGuinness
Published: Sep 27, 2018   
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Pressurized n-propane is a superior solvent to supercritical carbon dioxide for extracting hemp seed oil, according to recent research published in The Journal of Supercritical Fluids.

While an efficient and preferred practice, using supercritical carbon dioxide as a solvent often has high operating pressures and costs. In comparison, the researchers found that using pressurized n-propane could be a cheaper alternative that requires lower extraction pressures. And if that weren’t enough incentive, the end-products of the pressurized n-propane tests had higher concentrations of antioxidants and a lower acidity than those from both the carbon dioxide and conventional solvent tests – a strong sign of a higher quality food product.

With the hemp seed market expected to grow alongside the increasing investment in the cannabis industry, any information on how to reduce costs will be highly valued. The researchers’ results could, therefore, have a big impact on the future of industrial hemp seed oil extraction.

The research was a joint venture between the universities of Estadual de Maringá and Center Octávio Bastosde of Brazil and University la República of Uruguay.

The seed of production

Like marijuana, hemp varieties are cultivars of Cannabis sativa L. Unlike recreational cannabis, the hemp plant contains less than 1% of Δ9 -THC and its seeds don’t contain any of the psychoactive substance at all. Instead, the seeds constitute a balance of easily digested proteins, fibers, essential fats, antioxidants, amino acids, iron and vitamins.

The seeds’ healthy balance of fatty acids alone is quite remarkable. When converted into oil, many other properties are removed, leaving 80% of the oil as polyunsaturated fatty acids. One study into the consumption of the fatty acids omega-6 (Ω6) and omega-3 (Ω3) showed that the ideal ratio should be around 2.5:1 omega-6:omega-3. Hemp seeds have high levels of linoleic (Ω6) and α-linolenic (Ω3) fatty acids and one study of two varieties of hemp gave the ratio between the two to be between 2.5 and 2.7.

The seeds are thus heavily coveted for the ‘health food’ market, in addition to being used as main ingredients in soaps, shampoos, detergents, lubricants, paints, inks, fuel, and plastics.

What are the benefits of using n-propane as a solvent?

To begin the experiment, researchers milled a supply of seeds and placed the dehulled products into different extraction apparatus with three different solvents: n-propane, supercritical carbon dioxide and the conventional mixture of hexane and isopropanol at atmospheric conditions. They then used mathematical modeling to determine which solvent had produced the most nutritionally rich product at the lowest cost.

Looking at the statistical test results, the group found no significant variation in the composition of fatty acids between the different forms of extraction. On average, the fatty acid content of the oils was 8.8% saturated, 13.3% mono-unsaturated and 76.0% polyunsaturated. The Ω6/Ω3 ratio values were similar for both solvents under all operating conditions used, approximately 3:1 – a near enough match to the ideal 2.5:1 ratio.

Researchers also found that, at a constant pressure, the temperature didn’t have a significant influence on the extraction yield.

But, while these factors of fatty acid nutritional index and temperature were similar for both solvents, differences did arise when it came to the variables of pressure, the amount of solvent required and the antioxidant content of the oil.

Maintaining the high pressures needed for extraction can be a large part of the financial burden of hemp seed oil production. Unfortunately for manufacturers who use carbon dioxide as a solvent, the dense gas can exacerbate this financial encumberment. With its lower critical pressure values of 4.25 MPa compared to supercritical carbon dioxide’s 7.38 MPa, n-propane requires less power and thus can be used on an industrial scale at a lower cost.

In addition, the researchers’ experiment needed less n-propane to obtain similar percentage mass yield values to when using supercritical carbon dioxide. Plus, the extracts obtained with pressurized n-propane had higher total levels of β-carotene and tocopherols, valuable antioxidants that act as precursors for vitamins A and E, respectively.

Should hemp seed oil manufacturers switch to n-propane?

Of course, the use of propane as an extraction solvent does have its downsides. As a highly flammable substance, safety precautions such as spark-less rooms with powerful ventilation are a must. If n-propane were to be implemented by the hemp industry, the necessary retraining and restructuring of staff and facilities would also leave an initial spike in financial costs.

Even when it comes to achieving a desirable product outcome, n-propane doesn’t deliver on everything. During their experiment, the researchers found that the percentage mass yield obtained with pressurized fluids (supercritical carbon dioxide and n-propane) was lower than the percentage yields obtained when using a more conventional mixture of hexane and isopropanol at atmospheric conditions.

Ultimately though, the researchers’ results show that n-propane is a promising (if not perfect) alternative to obtaining hemp seed oil at mild conditions of temperature and pressure without losing the oil’s nutritional quality.


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