Based on their molecular structures and compositions,terpenes are high non-polar molecules,and have very low solubility in aqueous solution.In addition,they are in general highly volatile,and each terpene has its unique aroma.Some typical terpene compounds found in the hemp,their aroma characteristics and alternative sources are shown in Fig.4B.The typical boiling point of terpenes ranged from 119 °C to 198 °C.The terpenes are also not thermally stable and are susceptible to be oxidized into terpenoids.Conventionally,terpenes are usually used in cosmeceuticals,perfumes and aroma therapy.Recent studies have shown that terpenes in the hemp may play an important role in the ‘entourage effects’,that is ‘1 + 0 > 1’,of the medicinal properties and functionalities of the CBD.Such a phytocannabinoid-terpenoid synergy could enhance the treatment effectiveness of inflammation,pain,anxiety,and infections.Additionally,the therapeutic and pharmaceutical values,and functionality of terpenes themselves are usually underestimated.For example,myrcene is a potent analgesic,and has great antioxidant and anti-inflammatory activities; Limonene shows good antiviral,anti-bacteria and antidiabetic properties; Pinene has memory improvement,anti-inflammatory and antimicrobial functions ; Linalool shows sedative,anti-influenza,antimicrobial and neuroprotective effects; Caryophyllene is an excellent analgesic agent and antispasmic in muscles.It is worthy of mentioning that terpenes are GRAS additives approved by FDA,which means they can be used in food and beverages as flavor or nutrient supplements.Due to their abundance in the plant,hemp terpenes are promising sources of future functional food ingredients.Together with the booming business of CBD,terpenes are also drawing more and more interest in both food industry and academia.However,application of hemp terpenes is not easy due to their highly volatile and thermally unstable nature.
Particularly,they tend to evaporate and oxidize during the processing stages of the hemp plants,rolling benches canada such as drying,decarboxylation and extraction,and suffers significant loss.In fact,most of the terpene aroma in the currently available hemp products should offer additional value.More research needs to be done to collect the terpenes that are lost during the processing.Similar to CBD,how to improve the solubility,stability and bio-accessibility of terpenes in different food and beverage systems are also the key questions to be answered to determine the feasibility of using hemp terpenes as a future functional food ingredient.Furthermore,the interactions between each terpene species and CBD and the role of them in the ‘entourage effect’ are still not clear,and need to be better understood.Some cigarette and vape consumers claimed that they prefer the aroma of certain terpene species.Therefore,it is foreseen that exploration in adding certain types of high quality,food-safe hemp terpenes in foods accurately to reconstruct or create new terpene profiles in favor of consumers’ preference will be important and will deliver a more consumer friendly experience.At harvest,floral industrial hemp plants have high initial MCs.Chen et al.found the initial MCs of 3 California varieties were around 78% on wet basis.Before harvesting,plants usually have good defense mechanisms against microbial spoilage.However,after detached from the plant,the high moisture makes them susceptible to rapid quality deterioration and microbial spoilage.Therefore,successful additions of hemp CBD and terpenes in foods rely heavily on the preservation of these bio-active compounds and microbial safety of the hemp biomass and avoiding product loss via suitable processing.In addition,efficient and green extraction of these substances are critical.Likewise,the processing cost plays an important role in determining the pricing of CBD products.This section reviews and discusses the current status and recent advance in the drying,extraction and purification technologies for hemp in research and productions.The high MC of hemp at harvest makes them vulnerable to microbial spoilage during storage and is not desirable for downstream processing such as extraction and purification,thus the fresh floral hemp needs to be dried efficiently.At present,the harvested hemp is dried indoor in the industry by using two methods: hang-drying and tray drying.In the hang-drying scenario,the entire hemp plants are hung upside down on wires in a well-ventilated drying facility ,where the temperature is commonly 15‒21 °C,and relative humidity is 55%‒65%.In such a configuration,the stalks and branches of hemp plant droop are down and form a ‘closing umbrella’ structure,where the outer parts cover the center part of the plants.Since the hang-drying is normally conducted under natural convection conditions,the airflow is relatively weak,which leads to slow convective heat and moisture transfer rate and low penetration into the center.As a result,moisture in the center part cannot be effectively removed,and the hemp plant is not uniformly dried.
Mold and mildew may then grow in the center,and the quality of the dried hemp is not uniform.The hang-drying process usually takes 7‒10 days to finish and has high labor cost.Since CBD and terpenes reside mainly in the ‘cola’ of the hemp,drying the stalk and stems causes energy waste and is not necessary.Tray drying is an ‘upgrade’ method to the hang-drying.As shown in Fig.5C,colas are detached from the plant and placed in thin layers on drying trays and dried indoor under similar conditions as the hang-drying.Challa et al.found that tray drying is faster than hang-drying,but the drying rate is still relatively low,requiring 3‒5 days to dry the moisture down to a safe level for storage.Due to the high production yield and increasing cultivation,a larger amount of harvested hemp will need to be dried in a short season,and the currently applied low efficient drying methods may not fulfill the needs.Novel efficient mechanical drying technologies are thus needed to improve the processing efficiency,moisture uniformity,quality and safety of the products.Hot air drying is one of the most commonly used technology for foods and agricultural products due to their advantages of easy implementation,low operating cost and availability of equipment in the industry.The heat and moisture transfer rates during the drying process are significantly improved with the forced convection under HA drying compared to the natural air drying.However,it should be noted that thermally drying the hemp may result in vaporization of terpenes due to their volatile nature and degradation of terpenes and cannabinoids.Chen et al.studied the drying characteristics of hemp inflorescence and leaves under HA drying at different temperatures and found the drying time was significantly reduced compared with ambient air drying.Meanwhile,it was found that drying temperature did not have significant influence on the CBD contents in the hemp biomass,but increasing the temperature caused significant loss of terpenes.It was also found that increasing the drying temperature facilitated the conversion of CBDA to CBD.Similarly,Challa found increasing the HA temperature above 40 °C did not significantly affect the CBD contents in hemp,and non-isothermal HA drying led to the 90% drying time reduction compared to the control condition.Chasiotis et al.found that increasing the drying temperature from 40 °C to 60 °C and applying a nonisothermal drying regime increased the CBD and Δ9 -THC levels in cannabis inflorescence and leaves.Metin Ozguven et al.applied HA drying with a stepwise change temperature for peppermints and found using a lower temperature HA in the first 4 h and then raised to 55 °C to finish the drying could reduce the drying time while retaining the desired product quality.Zotti-Sperotto et al.applied intermittent HA drying to Lippia origanoides HBK leaves and Schinus terebinthifolius Raddi fruits,and found intermittent drying led to increased essential oil and thymol yield of L.origanoides without significantly affecting the color and quality attributes.
In addition,previous studies have shown that non-isothermal drying can improve the energy efficiency,achieve effective disinfection,disinfestation and enzymatic deactivation of foods and agricultural products.More research is needed in studying the efficacy of non-isothermal HA drying on the efficacy of disinfection and enzymatic deactivation of hemp biomass.Infrared drying is another promising drying technology that has been used for foods and plants.Due to the intensive transfer by thermal radiation,IR drying is particularly efficient for drying of thin materials or surface pre-drying of thick materials.Far-IR can penetrate 2‒5 mm into the surface of foods and generates strong thermal effects via vibrations of water molecules.IR drying has been successfully used for different herbs that contain valuable bio-active compounds.Nadee et al.found that IR drying was faster than HA drying and more energy efficient,while maintaining similar product qualities of Pandanus tea leaf.Xie et al.suggested that IR heating is suitable for the efficient drying and quality preservation of Chinese and Indian medicinal herbs.However,there were also studies that reported that IR drying led to non-uniform drying and uneven moisture distribution in herbs,as well as significant loss of color,aroma and bio-active compounds.Freeze drying is considered to have minor effects on the volatile and bioactive compounds in plants,which should be attributed to the very low temperature and low oxygen availability during the freeze drying conditions.Chen et al.found that freeze drying led to much less terpene loss than the HA drying.However,freeze drying is usually time consuming and energy intensive.Chimsook studied the drying characteristics,antioxidant activity and total phenolic contents of Cordyceps militaris for cordycepin production by freeze drying and HA drying.It was found that freeze drying led to higher retention of phenolics and antioxidant activity,but HA drying had much lower operating costs.Dielectric drying,including microwave and radio frequency drying,can penetrate the objects and generate volumetric heating,which is beneficial for improving the drying efficiency,reduces energy consumption and preserves of sensitive volatile and bio-active compounds.Rehkopf developed a RF drying technology to rapidly dry cannabis,prevent the microbial spoilage and retain the product quality.MW and RF have also been applied to assist other drying technologies to improve the drying efficiency and preserve bio-active compounds.Durance et al.developed a patent on a rapid drying technology for marijuana using MW-assisted vacuum drying,which has been commercialized by EnWave Technologies.Other combined drying technologies have also been studied for efficient drying different types of foods and agricultural products,such as MW-assisted HA drying,ultrasound-assisted HA drying,steam-assisted HA impingement drying,IR-assisted vacuum drying,and sequential IR and HA drying.The feasibility of these technologies for hemp drying and preservation of CBD and terpenes remains to be investigated.Currently,flood table research on developing efficient drying technologies for hemp,and on the influence of different drying methods on the retention of CBD and terpenes is scarce in the literature.The suitable drying methods and drying conditions may be tailored to the requirements of the final products and consumers’ needs.
The feasibility of different drying technologies for commercial scale hemp production will need to be evaluated by conducting techno-economic analysis.It will be important to study the potential measures to recover and collect the terpenes that are lost by vaporization during the processing.Currently,there are two major extraction methods that are widely used in the hemp industry: solvent extraction and super-critical fluid extraction.The purpose of extraction from plant materials is to obtain as much of the desired substances as possible,meanwhile,to minimize the co-extraction of undesired substances.It is noticed that besides cannabinoids and terpenes,hemp biomass is also rich in fatty acids and other minor substances such as chlorophyll,phenolics and flavonoids.Due to the lipophilic nature of both CBD and terpenes,they are usually co-extracted along with the lipids.Therefore,selection of suitable extraction methods with high yield and selectivity is very important.Additionally,food safety is one of the most important factors to be considered in the solvent selection for food applications.Solvent extraction is one of the most commonly used methods for plant extraction industry.This is the process involving the solid-liquid maceration with different solvents.In general,there are two major types of solvents that are mostly used in the industry: ethanol and hydrocarbons.Ethanol is a polar solvent,which usually has a higher throughput and extraction yield compared to other methods and is able to obtain full-spectrum distillate easily.Since ethanol is a GRAS solvent according to FDA,the food safety of ethanol extracts is not a concern.Meanwhile,due to the hydrophilic nature,ethanol extraction usually leads to less co-extraction of lipids and wax from the hemp biomass,and thus winterization and dewaxing could be avoided or even eliminated.However,its selectivity to cannabinoids and terpenes is usually low.Disadvantages of this method include harder post-separation and purification,low selectivity,large solvent loss,difficult solvent recovery and large solvent cost.Moreover,ethanol is very flammable and has fire safety risk.Hydrocarbon extraction is usually performed with butane,hexane,heptane,etc.,which are typical non-polar solvents.The extraction efficiency with hydrocarbon is usually higher than ethanol extraction,and the solvent recovery is much easier than ethanol,and thus the solvent cost with hydrocarbon should be cheaper.Due to their highly non-polar nature,hydrocarbon extraction can retain more terpenes than ethanol,but also co-extract the lipids and wax from the plants,thus winterization is necessary.In recent years,the use of hydrocarbons,particularly hexane,has rising concerns,as they may potentially cause concerns to human health.The United States Environmental Protection Agency indicates that ‘acute inhalation exposure of humans to high levels of hexane causes mild central nervous system effects’.Similar to ethanol,hydrocarbons are also flammable and have the fire safety risk during operations.In addition,emission of hydrocarbons contributes to the global warming and environmental impacts.