Industrial hemp is a cultivar of the Cannabis sativa plant grown as an agricultural commodity, distinct from psychoactive cultivars. Once harvested, the raw plant material undergoes specialized processing to convert it into usable products like fiber, food, and cannabinoid extracts. Processing directs the material to one of three paths: the stalk for fiber, the seeds for food-grade oil, or the flowers and leaves for chemical extraction.
Post-Harvest Preparation and Component Separation
The first step after the hemp harvest is to reduce the plant’s moisture content, which is a process known as drying or curing. This is an important step to prevent microbial contamination, such as mold and mildew, that can quickly ruin the crop. Hemp biomass intended for cannabinoid extraction requires careful curing in a controlled environment, typically between 60 to 70 degrees Fahrenheit with 45 to 55% relative humidity, to preserve delicate terpenes and cannabinoids.
The moisture content must be brought down to a specific range, often between 10 and 15% for flower material. Once dried, the whole plant is mechanically separated into its primary components using specialized harvesting and threshing equipment. The tough outer stalks are collected for fiber processing, the grain-like seeds are separated for food and oil production, and the cannabinoid-rich flowers and leaves, collectively called biomass, are collected for chemical extraction.
Processing Hemp for Industrial Fiber
Processing hemp stalks for industrial fiber involves a series of steps designed to separate the long, strong outer fibers, known as bast fiber, from the inner woody core, called the hurd. This separation process begins with retting, which is a controlled biological or chemical action that breaks down the pectin “glue” holding the components together.
Retting methods include dew retting, where stalks are left in the field for several weeks, allowing natural moisture and microbial action to degrade the pectin. While cost-effective, dew retting can result in inconsistent fiber quality due to unpredictable weather. Alternatively, water retting involves submerging the stalks in tanks, providing a more uniform breakdown, but requiring careful management of wastewater and higher capital investment.
Once the retting is complete, the stalks are dried and moved to a decortication machine. Decortication is the purely mechanical process where the retted stalks are crushed by rollers and then beaten or hammered to physically strip the bast fiber from the fragmented hurd.
This mechanical action yields two distinct products: the long, durable bast fibers used for textiles, ropes, and biocomposites, and the absorbent, cellulose-rich hurd. The hurd is commonly utilized in non-textile applications like animal bedding, lightweight concrete alternatives such as hempcrete, and composite building materials.
Processing Hemp for Seeds and Edible Oil
Hemp seeds, or hemp nuts, are processed primarily for the edible oil and nutritional components within the kernel. After mechanical separation from the chaff and debris, the seeds must be thoroughly cleaned, sorted, and dried down to a moisture content below 10% to ensure safe storage and prevent rancidity.
Dehulling removes the hard outer shell to produce the soft inner kernel, known as hemp hearts. Specialized impact hullers are used to gently crack the shell and separate the lighter shell fragments from the heavier kernels. Hemp hearts are sold directly as a food product or further processed into protein powder.
To extract the oil, the seeds are subjected to cold pressing, a mechanical method that uses an expeller press without chemical solvents or external heat. The term “cold pressed” signifies that the processing temperature remains below 120°F (49°C). Maintaining this low temperature preserves the integrity of the oil’s delicate polyunsaturated fatty acids, such as Omega-3 and Omega-6 compounds, and antioxidants like Vitamin E. The resulting unrefined oil retains a green hue and a distinct nutty flavor, indicating its high nutritional quality.
Processing Hemp for Cannabinoid Extracts
Extracting cannabinoids, such as CBD and CBG, from the dried hemp flower and leaf biomass requires sophisticated chemical and physical methods. The two primary industrial techniques are supercritical carbon dioxide (\(\text{scCO}_2\)) extraction and cold ethanol extraction.
\(\text{scCO}_2\) extraction utilizes carbon dioxide pressurized and heated above its critical point of 31.1°C and 73.8 bar, where it exhibits properties of both a liquid and a gas. In this supercritical state, \(\text{CO}_2\) acts as a tunable solvent, allowing operators to adjust temperature and pressure, typically in the range of 45-60°C and 3,000-5,000 psi, to selectively extract desired compounds. This method produces clean, solvent-free extracts, as the \(\text{CO}_2\) can be easily separated from the crude oil by simply lowering the pressure, allowing the gas to return to its natural state.
Alternatively, ethanol extraction is often preferred for high-volume processing due to its scalability and solvent efficiency. Ethanol is a polar solvent, but when chilled to cryogenic temperatures, usually between \(-40^\circ\text{C}\) and \(-50^\circ\text{C}\), its selectivity increases, minimizing the co-extraction of undesirable plant components like chlorophyll and waxes. This cold method results in a cleaner crude oil, reducing the need for extensive post-extraction refinement.
The resulting crude oil from either method must then undergo purification steps to achieve high-purity concentrates. The first step is winterization, where the crude oil is dissolved in ethanol and chilled to sub-zero temperatures. This causes fats, waxes, and lipids to solidify and precipitate out of the solution, which are then removed through filtration.
Following winterization, the extract is further refined using distillation, often a technique called fractional or short-path distillation. This process operates under a high vacuum to lower the boiling points of the compounds, preventing thermal degradation. The oil is heated gradually, and compounds are separated based on their unique boiling points, allowing the isolation of individual cannabinoids, such as pure CBD or CBG, often reaching purities exceeding 95%.