How Turmeric is Processed: from Plant to Powder

Turmeric has been used in Indian culture for nearly 4,000 years [1] and is now appreciated by cultures all over the world. However, many consumers of turmeric powder see this plant only in the form of a yellow powder. The process of turning the turmeric plant into a bright yellow spice requires specific conditions, skilled work and patience.

Turmeric powder is the product of a plant named Curcuma Longa, a member of the ginger family named Zingiberacaea [1]. This family is very varied, with 133 species of Curcuma being identified so far. More information on the many types of turmeric can be found in this article. It is native to South Asia, with India producing the greatest amount of this crop yearly, but it is now also cultivated in other countries including Ethiopia. Turmeric needs fairly warm temperatures between 20°C and 35°C to grow, with a large amount of rainfall per year [2]. It demands large amounts of nutrients while growing, particularly nitrogen, and can drain the nutrient content of the soil it grows in if not carefully managed.

This is partly due to its long growing period; up to nine months from planting to harvesting is typical [2]. The crop is harvested between July and March, once the leaves turn dry and the color of the plant is light brown and yellow. The most important part of the plant is the Rhizome, or rootstalk; this is the stem which grows underground and sends out roots [3]. The rhizomes are carefully lifted from the plowed land and sprayed to remove mud.

Image: Field Workers with Turmeric in the Background [6].

Before these rhizomes can be used to create turmeric powder, they must go through a boiling, drying and curing process. The first step in this process is to ‘sweat’ the turmeric; the long roots are trimmed off and the individual rhizomes, or ‘fingers’, are split from the main root bulb, also called the ‘mother’ rhizome [3]. They are spread onto the ground, covered with leaves and left alone for one day [2].

Next, the fingers and bulbs are boiled to remove their raw smell, soften them and create a more uniform color. This also gelatinizes the starch contained within the stems [1]. Shallow pans are used to do this, placed inside large vats containing slightly alkaline water. The boiling time ranges from 40 minutes to 6 hours, depending on the variety of turmeric plant and the country it is produced in. They are removed from the vats when white fumes appear from them, giving out a smell which is typical of this stage [2].

Once boiled, the rhizomes are immediately dried in the sun to prevent overcooking; this stage is called curing and is performed to reduce drying time and further improve the uniformity of the color. This must be done within 10 days of harvesting to create a high-quality product [2], so there is a significant amount of pressure on farmers at this stage to work quickly with large harvests. They are left in the sun until the moisture content is reduced to 8-10%; at this stage the rhizomes will produce a metallic sound when tapped [1].

The cured turmeric is then dried, to produce a root with a final moisture content of 5-10% which can be processed into a dry powder. At this stage, they are kept out of direct sunlight to prevent the bright yellow color from being lost. Some growers slice the roots to reduce their drying time; this improves the quality of the powder [2]. The warm climate of the growing country is often enough to dry rhizomes, but modern methods often use mechanical dryers which reach temperatures of 60°C to make the process more efficient. Even after using these dryers, the turmeric is spread outdoors to complete the process. This usually takes around 10-12 days, extending to as long as one month in Ethiopia depending on the weather conditions [3].

Once dry, the turmeric rhizomes are polished by being rubbed against the surface of the drying floor, stamped under the feet of the workers or placed into polishing drums [2]. This removes the rough outer surface and creates an attractive finish, enhancing their natural deep yellow color.

Various methods can be used to improve the appearance of the root at this stage; this is partly why it is important to select quality suppliers of turmeric powder who select their sources carefully. Some growers polish the rhizomes with turmeric powder mixed with water [3], others make a coating by grinding raw turmeric juice with lemon juice [2]. However many growers still use lead chromate despite this practice being strongly discouraged [1].

Lead poisoning cases reported in the US since 2010 have been attributed to turmeric contaminated with lead, leading researchers to conclude that producers were using this chemical to increase the weight, improve the color and hide imperfections on the root. Turmeric with high concentrations of lead have been found in India and Bangladesh, both major sources of turmeric powder in the US [4].

The final stage in this process is to grind the dried, polished rhizomes into the fine powder which has become so popular on the shelves of stores around the world. Traditionally, the rhizomes are ground into powder at ambient conditions, in temperatures of up to 43-95°C. Although this is still the main method of grinding turmeric, new methods such as cryogenic grinding are being explored as they may reduce the loss of essential oils and quality during the process and produce a finer powder [5].

Image: Turmeric Grinding Machine [7].

This powder keeps its intense yellow color indefinitely, but the flavor quality decreases over time [1]. This is yet another reason to buy fresh powder from a quality source, and use it up quickly while it still imparts its distinctive flavor. Fresh turmeric powder should have a musky, peppery smell, with an aromatic and slightly sweet taste [3]

[1] Prasad, S. and Aggarwal, B.B., 2011. Turmeric, the golden spice. Herbal Medicine: Biomolecular and Clinical Aspects. 2nd edition.

[2] Hailemichael, G. and Zakir, M., 2021. Pre-and post-harvest practices influencing yield and quality of turmeric (Curcuma longa L.) in Southwestern Ethiopia: A review. African Journal of Agricultural Research, 17(8), pp.1096-1105.

[3] Balasubramanian, S., Roselin, P., Singh, K.K., Zachariah, J. and Saxena, S.N., 2016. Postharvest processing and benefits of black pepper, coriander, cinnamon, fenugreek, and turmeric spices. Critical reviews in food science and nutrition, 56(10), pp.1585-1607.

[4] Cowell, W., Ireland, T., Vorhees, D. and Heiger-Bernays, W., 2017. Ground turmeric as a source of lead exposure in the United States. Public Health Reports, 132(3), pp.289-293.

[5] Barnwal, P., Mohite, A., Singh, K.K., Kumar, P., Zachariah, T.J. and Saxena, S.N., 2014. Effect of cryogenic and ambient grinding on grinding characteristics of cinnamon and turmeric. International Journal of Seed Spices, 4(2), pp.26-31.

[6] Field workers with turmeric leaves in background. Maharashtra, India Feb 2017. Photo by Bill Chioffi. https://sustainableherbsprogram.org/explore/plants-in-commerce/turmeric-2/