Small scale production - CBD oil extraction

The CBD oil extraction from cannabis flowers gains more and more interest globally to meet medical requirements. Typical extraction methods are via ethanol or supercritical CO2. In both cases, the sample material must be ground in order to increase the sample density in the extraction unit. Usually, sample sizes around 2-5 mm or around 1-2 mm are most suitable.

The Retsch Cutting Mill SM 300 316L meets the requirements for the sample processing prior to the extraction process, without any warming effects, so no loss of volatile ingredients and no sticking effects. Sample throughput higher than 40 kg/h is possible, and via the selection of the aperture sizes of the bottom sieves, the final fineness can be adapted to the user´s requirements. Thanks to the sophisticated design, the SM 300 316L is disassembled within seconds and can be cleaned very easily. 

The video on the right shows how easily dried cannabis flowers are ground as sample preparation for CBD oil extraction with the Retsch SM 300 is done. The mill achieves that with a throughput of over 40 kg / h and a fineness of 1-2 mm, other finenesses are also possible by selecting the bottom sieve) and without heating at 700 rpm. All parts in contact with the product are made of 316L stainless steel and offer quick and easy cleaning, thanks to particularly smooth surfaces.

Extraction Method THC / Cannabinoids Improved method, derived from Shimadzu protocol

Solution for Potency Testing

For potency testing, the Mixer Mill MM 400 with the falcon tube adapter can be used to simultaneously homogenize up to 8 samples. For this purpose, the dried flowers are deep-frozen at -20°C and then 4 g are filled in each tube. After adding 2 x 15 mm steel grinding balls, homogenization can take place at 30 Hz for 3 minutes. A grind size of 1-2 mm can be achieved in this way. This method leads to highly reproducible analysis values with minimized sample loss for CBD, CBDA, THC and THCA and also saves time due to high sample throughput, short grinding times and disposable tubes which don’t require cleaning. After grinding, a portion of 500 mg can be used for the subsequent steps like extartion. This method is also suitable for homogenizing samples for pesticide analysis.

Heavy Metal Analysis

During their growth, cannabis plants can absorb toxic heavy metals such as lead, cadmium, arsenic and mercury from the soil. If samples are analyzed for heavy metals, the Knife Mill GM 200 is also suitable in addition to ball mills and the ZM 300 rotor mill. It is particularly easy to use and homogenizes samples up to 200 ml in one go. A particle size of <1 mm is sufficient to achieve very good reproducibility. The standard deviation was below 5 % for all analyzed elements. If a shorter grinding time is selected, which results in larger (2 mm) particles, standard deviations of up to 12 % can be expected. Appropriate care in the homogenization process therefore pays off by minimizing such particle size effects.

^{Grain size effects on the reproducibility of heavy metal analysis in cannabis flowers: After 10 s grinding (interval at 4000 rpm) in the Knife Mill GM 200, fibers remain, the corresponding standard deviations in heavy metal analysis are higher than with samples ground for 20 s with a final fineness < 1 mm (10 s interval, 4000 rpm + 10 s 10000 rpm).}

In the case of heavy metal analysis, special attention must be paid to choosing the right grinding tools. Since mechanical particle size reduction is always associated with abrasion of the grinding tool, the use of steel tools inevitably leads to a falsification (increase) of the heavy metal values in the sample. This can be avoided by choosing, for example, zirconium oxide tools in the case of ball mills or titanium tools in the case of GM 200 or ZM 300. Steel tools increase the measured concentration of heavy metals and lead to incorrect results.

^{Effects of the grinding tool material on analysis results. Abrasion from steel tools leads to increased values of heavy metals and thus falsifies the analysis. This falsification can be avoided by using zirconium oxide or titanium tools.}

There are different methods for determining trace metals in plant material such as cannabis and hemp or edibles. All require mineral acid digestion to destroy the organic matrix and dissolve trace metals to obtain a liquid sample. For microwave systems, 0.5 g ground sample are sufficient.

Suitable mills for quality control of cannabis samples

Hemp is a versatile plant material that has gained increasing attention in recent years due to its numerous applications and industrial importance. Hemp is a variety of the cannabis sativa plant species and is known for its high levels of cannabidiol and low levels of tetrahydrocannabinol. Hemp has been used for centuries for a variety of purposes including fiber, food, and medicine. However, in the last three years, there has been a significant development in the use of hemp for industrial purposes such as building materials, biofuels, and textiles.

The RETSCH Cutting Mill SM 300 was used for grinding pre-cut stem parts of the plants down to 1 mm particles without too long fiber parts > 10 mm. The 6-disc rotor and a 0.75 mm bottom sieve were employed, and the mill was operated at 3000 rpm. Depending on the individual sample, a throughput of 1 kg in 5 min was possible, resulting in an average throughput of ~7.5 kg / h. It is strongly recommended to use the cyclone unit to improve the sample discharge from the grinding chamber and prevent clogging of the sieve with fine particles. Glass bottles of various sizes or receptacles of 5 l or 30 l can be attached to the cyclone for sample collection.

Pre-cut stems of hemp plants before (left) and after grinding in the Cutting Mill SM 300 (right)

Cutting Mill SM 300 with cyclone

To obtain finer particles, a two-step grinding process using the Cutting Mill SM 300 for-precutting and the Rotor Beater Mill SR 300 for fine grinding is recommended. While the SM 300 grinds the sample much quicker, the SR 300 produces much finer particles. Therefore, combining the two milling systems greatly improves the process. This is demonstrated in the following with two samples, hemp hurd and hemp fibers.

The hemp hurd sample was slowly poured into the funnel of the mill whereas the hemp fibers were fed as bundles to the machine. Both samples were pre-cut in the SM 300 using the V-rotor and a 1 mm bottom sieve. To facilitate sample discharge, the cyclone was used as described above. The sample was ground in both cases to fibers sized approximately 1-10 mm.

The fine-grinding step was done for both samples in the SR 300 at 10.000 rpm using a 360° ring sieve with aperture size 0.08 mm. Again, the cyclone was employed to improve sample discharge. The hemp hurd was pulverized to 86 % < 100 μm, the hemp fibers to 76 % < 100 μm. The required grinding times for 1 kg of sample are shown in table 1.

Hemp hurd (above) and hemp fibers (below) after pre-cutting in the SM 300 and subsequent pulverization in the SR 300.

The Retsch Cutting Mill SM 300 and Rotor Beater Mill SR 300 are versatile laboratory-scale mills for grinding hemp and similar materials to a fine powder or flour. Both mills can handle a wide range of fibrous samples and produce consistent particle sizes. While these mills are designed for laboratory-scale applications, they are also suitable for grinding at least 2-3 kg/h in pilot-scale applications. Safe and user-friendly operation and a wide range of accessories of different materials make both mills easy to use in many application fields.