Freeze drying of hemp

Written by: Martina Vavrusova Hedegaard, Ph.D. / Technologist / SD Freeze Drying

Cannabis sativa L. is a non-drug variety known as hemp. It is one of the oldest cultivated crops, which has provided food, oil, fibre, and even medicine for many centuries.
Carl Linnaeus was assigned its name in 1753. The words ´´canvas´´ and ´´cannabis´´ derive from similar-sounding words in Greek, Latin, and Arabic for fabric. The second part of the name ´´sativa´´ comes from the Latin word sativus, which means sown or cultivated.
The botanical association with the drug cannabis is certainly not doing any favour to this complex plant. In brief, the production of tetrahydrocannabinol and other cannabinoids is under strict genetic control.

In fact, hemp contains very low concentrations of δ-9-tetrahydrocannabinol (THC), which is the main psychoactive compound in drug varieties of Cannabis. It is important to note, that the amount of THC in the mature hemp plant is less than 0.5% of the plant´s dry weight.

Hemp fibres have already been used by ancient Asian mariners to make good sturdy sail canvas and many historical documents have been written on paper made of hemp fibres.
Today, hemp fibre is used for the production of a variety of products including special types of paper such as cigarette papers, banknotes, tea bags, fibre for textiles, clothing, shoes, building insulation, construction materials, biofuels, and many other durable materials.
Although hemp is recognized as a valuable industrial crop for both food and fibre, hemp seed food products have not yet entered the mass market in the West. Luckily, this seems to be changing now. Hemp seed foods are not new and have been present for many centuries as traditional foods for example in the Baltic states, Eastern Europe, and China. Generally, hemp seeds have been used as food for humans and animals in Asia, India, Russia and Eastern Europe for many centuries.
As written by Callaway (2004), a long history and the variety of uses over a large geographic area and through so many different cultures are strong reminders of hemp seeds’ utility as a powerful source of nutrition.

Hemp seed oil production

Hemp seed oil which is used for human consumption is produced ideally from fresh seeds which have been air-dried at low temperatures (< 25 °C) for several days or weeks.

Fast-drying at elevated temperatures makes the control of the moisture difficult and results in a subsequent loss of the delicate flavours.

Hemp seed oil is pressed from the seeds of hemp, which are also a source of highly digestible protein.
The hemp seed moisture content is typically between 15-20 % at the time of harvest and the final moisture content for storage and pressing should be below 10%.
As noted by Callaway & Pate (2009), it must be ensured that the seed does not develop mould growth between the time of harvest and the time of drying. If no attention is paid to special care at the harvest, the final product quality will be affected, and the oil will have a relatively short shelf life.
Bulk oil is stored in glazed-metal, ceramic or glass containers after pressing. Subsequently, the oil is either filtered for immediate bottling into a glass or allowed to settle before it is bottled for further distribution.
The fine sediment from freshly pressed hemp seed oil has a high nutritive value and can be used directly as a nut-butter spread or in other food products.
The fresh, cold-pressed hemp seed oil has the natural dark colour described as a clear green to olive colour due to the presence of chlorophyll. Typical flavours of the high-quality oil are described as combinations of citrus, mint, and pepper and some people describe it as a nutty taste with a pleasant smell.
Generally, oxidation is the main problem of any polyunsaturated oil (discussed later). Therefore, it must be ensured that the seeds are kept in an inert atmosphere before it reaches the press head, and the inert atmosphere must be maintained throughout the processing until the oil is bottled.
The final bottled product should be protected from light and stored at relatively low temperatures.

The composition and the exceptional qualities of hemp seed oil

Hemp seed is interesting because of its composition, which contains highly unsaturated oil (⁓25 – 35 % ), easily digestible protein (⁓20 – 25 % ), carbohydrates (⁓20 – 30 %), and fibre (10 – 15 %), along with various phytosterols, important minerals and vitamins.
As a side note, hemp seeds do not contain gluten, which makes them a vegetable protein suitable for people suffering from celiac disease.
Hemp seed oil is valued for its high nutritional quality and health benefits.
As mentioned, hemp seed oil has a higher content of polyunsaturated fatty acids (PUFAs) than most other food oils and it is low in saturated fats.
The amount of PUFAs is ranging from 75-85% of the total oil content.
The hemp seed oil contains omega-6 polyunsaturated fatty acid – linoleic acid (LA) and omega-3 polyunsaturated fatty acid – α-linolenic acid (ALA).

These polyunsaturated fatty acids represent the most desirable content of hemp oil, especially due to the ratio in which they are present. The 3:1 ratio of LA to ALA is found to be optimal for nutrition.

In contrast, an unbalanced intake of omega-3 and omega-6 fatty acids is associated with many diseases like as example cardiovascular diseases, diabetes, and cancer.

Additionally, the presence of gamma-linolenic acid (GLA) in hemp seed oil makes this oil superior to other seed oils.
The nutritional value of hemp seed oil is attributed to the fatty acid composition, but it is important to understand that there are other natural compounds that contribute to its beneficial properties as well.
Natural compounds such as tocopherols, tocotrienols, phytosterols, phospholipids, terpenes, and minerals complement the nutritional value of hemp seed oil and increase its potential as a functional food. Just to mention the benefits of a few of them:
- Hemp contains significantly higher levels of γ-tocopherol than many other plants compared to the levels of α-tocopherol. They both have important antioxidant and anticancer properties which are exhibited in their respective physiological systems.
- β-Sitosterol is a plant sterol that has demonstrated its potential in affecting plasma cholesterol levels. Furthermore, it also has antiviral, antifungal, and anti-inflammatory properties.
- Terpenes such as β-caryophyllene and myrcene exhibit anti-inflammatory and antioxidant properties, respectively.
- Methyl salicylate can be hydrolysed to salicylic acid (a common active ingredient of aspirin) and thus have pharmacological effects like those of aspirin.
- Cannabidiol (CBD) is present in hemp seed oil in trace amounts as a result of contamination from other parts of the hemp plant during oil production. The presence of CBD is, however, small but interesting because of many health benefits as it has been documented to have anti-convulsive, anti-epileptic, antimicrobial, and skincare properties.
The potential and the value of hemp seed oil should give the oil a stable place in the market as it is a nutritionally highly valuable food product that also exhibits several pharmacological properties.

Applications of hemp seed oil

The nutritional and pharmacological benefits of hemp seed oil are clearly obvious.

However, cooking with hemp seed oil is generally not recommended due to its high level of unsaturation, which makes this oil on one hand very nutritionally interesting but on the other hand very unstable when exposed to high temperatures.

Therefore, hemp seed oil is a great addition to salad dressings, pesto sauce, smoothies, raw bars, and other beverages.

Due to the high content of PUFAs in hemp seed oil and their possible polymerisation when exposed to oxygen, hemp seed oil can be used as drying oil to produce paints, varnishes, durable floor coverings etc.
The hemp seed oil has also found used in body care products like soaps and shampoos. Adequate precautions should be taken when using hemp seed oil in such products because, as mentioned earlier, the high content of unsaturated fatty acids may cause the hemp seed oil to oxidize.
However, the presence of these essential fatty acids in body care products has demonstrated good skin benefits including healing properties. Therefore, stabilisation in emulsion and further drying techniques may provide certain protection of the essential fatty acids and make them more stable for further applications.
Hemp seed oil is also used in the production of biofuels especially biodiesel which has a lower freezing point (due to the high degree of unsaturation) than biodiesels produced from saturated fats.

Examples of microencapsulation when using the freeze-drying technique

As already mentioned, food matrices containing unsaturated fatty acids will oxidise and produce off flavours, rancid odours, and in some cases toxic compounds, which affect the overall quality of the product.
Different strategies have been developed to ensure the oxidative stability of polyunsaturated acid-enriched emulsions. One of the promising and interesting strategies can be the incorporation of phenolic extracts from natural sources such as berries into emulsions to effectively retard the oxidation process.

A study by Raikos et al. (2015) has shown that the addition of raspberry powder extracts significantly improved the oxidative stability of a hemp-based emulsion. Furthermore, the extract was even more effective compared to a synthetic antioxidant when samples were subjected to heat treatment.

Using natural antioxidants or in general, antioxidants to prevent oxidation of polyunsaturated fatty acids in combination with other techniques such as microencapsulation of oil by using freeze-drying could be an alternative to prevent oxidation of PUFAs.
The freeze-drying technique shows great potential with respect to the oxidative stability of the final product.
Freeze-drying offers to dry at low temperatures in an inert atmosphere, and therefore offers advantages compared to other methods applying high temperatures which together with the presence of oxygen may lead to increased oxidation of PUFAs.
Besides the obvious advantages of using the freeze-drying technique in the manufacturing of dried microencapsulated oil with a high content of nutritionally beneficial PUFAs, the freeze-drying technique also makes it easier to introduce such a product into other food matrices and increase its consumption in our diet.
In the study by Heinzelmann et al. (2000), fish oil was microencapsulated using the freeze-drying technique.

Fish oil contains omega-3 long-chain polyunsaturated fatty acids. Therefore, it can be used as an example that can be applied to other oils containing PUFAs like hemp seed oil.

The microencapsulation process involves the following steps:

Preparation of the emulsion, which contains suitable emulsifiers together with stabilisers, water, and oil to make a stable emulsion. Mixing of these components requires low temperatures to avoid thermal damage to the oil.
Homogenisation.
Freezing of the emulsion where a freezing rate should be considered.
Actual freeze-drying of the frozen emulsion.

In this case, during the drying process, the product temperature increased from the initial value of -30 C and -80 °C, respectively, to a final temperature of 18 °C. The drying was performed at a pressure of 20 Pa and the time needed for drying was between 36 and 72 h.

The results of the study by Heinzelmann et al. (2000) indicate that the final freeze-dried product has a very porous structure and therefore it is very important to flush the freeze-drying chamber with nitrogen after finishing the drying process. This ensures that the micropores of the dried product are filled with nitrogen and therefore the contact with oxygen is reduced which improves oxidative stability.
The study by Heinzelmann et al. (2000) clearly demonstrates the possibility to produce dried microencapsulated oil containing omega-3 PUFAs by using the freeze-drying technique, which offers an opportunity to achieve a product with good oxidation stability.

In another study by Heinzelmann & Franke (1999), the results confirmed that freeze-drying is a suitable technique to achieve dried microencapsulated fish oil with good oxidation stability.

In conclusion

Hemp is a sustainable and eco-friendly agricultural plant that is globally recognised for its vast applications. Hemp seed oil is a nutritionally accomplished product with many other health benefits and a broad spectrum of uses.
Generally, food matrices containing unsaturated fatty acids are prone to oxidation which adversely affects the product quality.
However, it is important to remember that a balanced intake of PUFAs is associated with many health benefits and that these acids are essential in our nutrition.
Therefore, a good understanding of suitable techniques such as freeze-drying gives an opportunity to prevent hemp seed oil products from oxidising and makes their further applications in food or skincare products easier.

Several factors including the formulation of emulsion (choice of emulsifiers and stabilisers), the addition of antioxidants, homogenization temperature, freezing temperature and freezing rates, final texture of the powder, and storage conditions must be carefully considered in order to obtain a final product with good oxidation stability and with adequate shelf life.

Literature

Leizer, C. et al. (2000). The composition of hemp seed oil and its potential as an important source of nutrition. Journal of Nutraceuticals, Functional & Medical Foods, 2, (4), 35 – 53.
Callaway, J.C. & Pate, D.W. (2009). Hempseed oil. Chapter 5, pp. 185 – 213 In: Gourmet and Health-Promoting Specialty Oils, Robert A. Moreau and Afaf Kamal-Eldin (Eds.), American Oil Chemists Society Press, Urbana I1, ISBN 978-1-893997-97-4.
Callaway, J.C. (2004). Hempseed as a nutritional resource: An overview. Euphytica, 140, 65 – 72.
Mikulcova, V. et al. (2017). Formulation, characterization and properties of hemp seed oil and its emulsions. Molecules, 22, 700, 1 – 13.
Raikos, V. at al. (2015). Processing and storage effects on the oxidative stability of hemp (Cannabis sativa L.) oil-in-water emulsions. International Journal of Food Science and Technology, 1 – 7.
Heinzelmann, K., Franke, K. (1999). Using freezing and drying techniques of fish oil to improve oxidation stability. Colloids and Surfaces B: Biointerfaces, 12, 223 – 229.
Heinzelmann, K. et al. (2000). Protection of fish oil from oxidation by microencapsulation using freeze-drying techniques. European Journal of Lipid Science and Technology, 114 – 121.