Variation in the Chemical Composition of Five Varieties of Curcuma longa Rhizome Essential Oils Cultivated in North Alabama

Turmeric (Curcuma longa L.) is an important spice, particularly is Asian cuisine, and is also used in traditional herbal medicine. Curcuminoids are the main bioactive agents in turmeric, but turmeric essential oils also contain health benefits. Turmeric is a tropical crop and is cultivated in warm humid environments worldwide. The southeastern United States also possesses a warm humid climate with a growing demand for locally sourced herbs and spices. In this study, five different varieties of C. longa were cultivated in north Alabama, the rhizome essential oils obtained by hydrodistillation, and the essential oils were analyzed by gas chromatographic techniques. The major components in the essential oils were α-phellandrene (3.7–11.8%), 1,8-cineole (2.6–11.7%), α-zingiberene (0.8–12.5%), β-sesquiphellandrene (0.7–8.0%), ar-turmerone (6.8–32.5%), α-turmerone (13.6–31.5%), and β-turmerone (4.8–18.4%). The essential oil yields and chemical profiles of several of the varieties are comparable with those from tropical regions, suggesting that these should be considered for cultivation and commercialization in the southeastern United States.


Introduction
Turmeric (Curcuma longa L.), belonging to Zingiberaceae, is a rhizomatous plant native to Southeast Asia, but is extensively cultivated worldwide, particularly in tropical countries (e.g., India, Pakistan, Bangladesh, China, Taiwan, Thailand, Sri Lanka, East Indies, Burma, Indonesia, northern Australia, Costa Rica, Haiti, Jamaica, Peru, and Brazil) [1][2][3][4]. Turmeric is well known for its use as a culinary ingredient and a traditional herbal medicine [5]. It is extensively used in Asian cuisine and is one of the key ingredients in curry powders [6]. Turmeric, either fresh or in dried form, has a long history of medicinal use, dating back 4000 years [7]. Due to its bright yellow/orange color, turmeric is often referred to as the "Indian saffron" or "golden spice". Curcuminoids are considered the main bioactive components of turmeric. Turmeric's medical properties are credited mainly to the curcuminoids, which are abundant in turmeric rhizome [8]. The total curcumin (sum of all curcuminoids) was proven to have significant health benefits along with the potential to prevent various diseases, including Alzheimer's, coronary heart disease, and cancer [9]. Due to a plethora of scientific articles on the health benefits of turmeric, the demand for turmeric is steadily increasing in the US and now represents an estimated US$36 million per annum. The US imports 90% of its requirements from various countries [10]. Due to recent inconsistencies associated with quality and production methods of raw materials imported from Asian countries, many US manufacturers of herbal products are seeking domestically-produced materials that meet their standards and requirements. Turmeric (Curcuma longa) is one such crop. The rhizome from which the curcumins are derived is tuberous, with a rough and segmented skin. The primary rhizome is called the "mother rhizome" or bulb, which is pear-shaped in the center (Figure 1). The branches of mother rhizomes are the secondary rhizomes, called lateral or "finger rhizomes" [11]. The mother rhizomes are more matured than finger rhizomes, therefore containing higher curcuminoid concentrations and perhaps higher essential contents than finger rhizomes. However, the curcumin yield from finger rhizomes is higher than that from mother rhizomes [12]. Turmeric (Curcuma longa) is one such crop. The rhizome from which the curcumins are derived is tuberous, with a rough and segmented skin. The primary rhizome is called the "mother rhizome" or bulb, which is pear-shaped in the center ( Figure 1). The branches of mother rhizomes are the secondary rhizomes, called lateral or "finger rhizomes" [11]. The mother rhizomes are more matured than finger rhizomes, therefore containing higher curcuminoid concentrations and perhaps higher essential contents than finger rhizomes. However, the curcumin yield from finger rhizomes is higher than that from mother rhizomes [12]. One of the important components of turmeric is its volatile oil. The role of turmeric oil in the treatment of a wide variety of diseases in animals and humans were reviewed in detail [4,7]. Thus, curcuma oil appears to be a promising agent for the treatment of simple dermatitis, cerebral stroke, and other disorders associated with oxidative stress [13]. The essential oils of turmeric are relatively complex, with hundreds of components. The major components, however, are α-turmerone (12.6-44.5%), β-turmerone (9.1-37.8%), ar-turmerone (12.2-36.6%), β-sesquiphellandrene (5.0-14.6%), α-zingiberene (5.0-12.8%), germacrone (10.3-11.1%), terpinolene (10.0-10.2%), ar-curcumene (5.5-9.8%), and α-phellandrene (5.0-6.7%) [14]. One of the important components of turmeric is its volatile oil. The role of turmeric oil in the treatment of a wide variety of diseases in animals and humans were reviewed in detail [4,7]. Thus, curcuma oil appears to be a promising agent for the treatment of simple dermatitis, cerebral stroke, and other disorders associated with oxidative stress [13]. The essential oils of turmeric are relatively complex, with hundreds of components. The major components, however, are α-turmerone (12.6-44.5%), β-turmerone (9.1-37.8%), ar-turmerone (12.2-36.6%), β-sesquiphellandrene (5.0-14.6%), α-zingiberene (5.0-12.8%), germacrone (10.3-11.1%), terpinolene (10.0-10.2%), ar-curcumene (5.5-9.8%), and α-phellandrene (5.0-6.7%) [14].
Turmeric is considered a viable cash crop with a ready market in Alabama and in the US. Similar to any essential oil crop, turmeric's essential oil also varies with variety, soil type, and environmental conditions [15,16]. Hence, evaluating different varieties for essential content and composition is an important consideration for determining a variety for cultivation in a particular location. Turmeric is a tropical crop and grows well in warm and humid environments with mean air temperatures between 20 and 30 • C. It can be planted in all soil types, but it does best in well-drained clay loam or sandy loam soils rich in humus or organic matter with a soil pH of about 5.5 to 6.5. It grows in a wide range of climatic conditions but requires about 100 to 200 cm of rainfall a year. Thus, Alabama's hot, humid, and rainy summer season is suited for turmeric production in the southeastern US. Furthermore, turmeric is potentially suited for catering to the herbal products industry, which prefers locally sourced materials. The purpose of the present study was to determine the variation in the essential oil chemistry of five C. longa genotypes that could potentially be cultivated for commercial purposes in north Alabama and to note any differences between the mother rhizome and the lateral rhizomes of each cultivar.

Curcuma longa Varieties
The five varieties used in this study were selected out of fourteen varieties according to three criteria: high yield but low curcuminoid content (varieties, CL5, CL3), low yield but high curcumin content (CL10), and high yield and high curcumin content, thus, high curcumin yield (CL9, CL11), based on studies at Auburn University and Alabama A&M University. Thus, CL3 and CL5 may be considered for the fresh rhizome market, CL 10 for the high curcumin dry rhizome herbal products market, and CL9 and CL11, which have attractive, orange-colored rhizomes, could serve both fresh and dry herbal produce markets. The two varieties CL3 and CL9 were consistent in their performance over three years in both south and north Alabama. A knowledge of their relative oil yield and composition could help in value-addition for either fresh rhizomes or dry herbal markets.

Cultivation of Curcuma longa
The rhizomes of five turmeric varieties belonging to C. longa (CL3, CL5, CL9, CL10, and CL11) obtained from various sources (Table 1)  The plants were grown using organic production methods and irrigation was provided as and when needed by the drip method. Prior to making the raised beds, the soil was plowed with a rototiller, and a mixture of composted cow manure, poultry litter, and vermicompost was incorporated into the soil at a rate equivalent to 45.5 kg of N/ha. Once the crop was established, a fish emulsion-based organic soluble fertilizer, Neptune's Harvest™ (Seven Springs Organic Farming and Gardening Supplies, Check, VA, USA), was applied through the irrigation system at 3-week intervals. Three plants from the middle of each row were harvested in mid-February 2020 by digging the plants, separating the rhizomes from the shoot, and washing clean of soil and debris with forced water jets. The mother and lateral rhizomes ( Figure 1) were separated and placed in mesh trays and dried of excess water using fans at room temperature. The rhizomes were then placed in a cooler box with ice and carried to the chemistry department at the University of Alabama in Huntsville for oil extraction and profiling.

Essential Oils
The fresh rhizomes, both mother and lateral rhizomes (Figure 1), were chopped and hydrodistilled for 4 h using a Likens-Nickerson apparatus with continuous extraction of the distillate with dichloromethane. Evaporation of the dichloromethane gave pale yellow to yellow rhizome essential oils (Table 1), which were stored at −20 • C until analysis.

Results and Discussion
The fresh mother and lateral rhizomes ( Figure 1) were chopped and hydrodistilled to give pale yellow to yellow essential oils in yields ranging from 0.204% to 0.695% (Table 1). Varieties CL5, CL9, and CL11 gave better essential oil yields (0.443-0.659%) than CL3 or CL10 (<0.3%). The total oil content of CL5, CL9, and CL11 were similar to those reported for Indian chemotypes of C. longa [21,22]. In CL5, CL9, and CL10, the mother rhizomes had higher oil yields than the lateral rhizomes. A similar trend was reported for curcumin concentration of turmeric varieties grown in south-central AL [12].
Xu and coworkers examined the extracts of 160 samples of C. longa from China [26]. Gas chromatographic analysis of the volatiles from the extracts revealed three volatile profile types, while high-performance liquid chromatographic (HPLC) analysis showed three types based on curcuminoid content. Unfortunately, Xu et al. identified only 10 volatile components, whereas 79 components were identified in our essential oil work. Furthermore, percent compositions were not reported and only "representative" chromatograms were presented. Nevertheless, although comparison is tenuous, based on the chromatograms, the volatile profile types identified seem to be analogous to essential oil types in our work. Importantly, volatile profile types "a" and "b" correspond to highcurcuminoid type "B" [26]. Therefore, we conclude that high turmerone concentrations are desirable qualities in turmeric essential oil.

Conclusions
The chemical profiles of varieties CL5, CL9, and CL11 tested in this study in north Alabama were comparable to those growing in tropical regions of the world, suggesting that these varieties are suitable for commercialization in this region. However, CL3 and CL10 gave relatively poor essential oil yields and essential oils with lower concentrations of the turmerones. There is a growing market for Curcuma longa essential oils, with several varieties showing promise for development in the southeastern United States.