Turmeric readiness for harvest is indicated by the drying of the plant and stem, approximately 7 to 10 months after planting, depending on cultivar, soil and growing conditions. The rhizome bunches are carefully dug out manually with a spade, or the soil is first loosen with a small digger, and clumps manually lifted. It is better to cut the leaves before lifting the rhizomes. Rhizomes are cleaned from adhering soil by soaking in water, and long roots are removed as well as leaf scales. Rhizomes are then further cured and processed, or stored for the next year’s planting.
Rhizomes for seed purposes must be stored in well-ventilated rooms to minimize rot, but covered with the plant dry leaves to prevent dehydration. They can also be stored in pits covered with sawdust, sand, or panal (Glycosmis pentaphylla) leaves that may act as insect repellent. The Indian Institute of Spice Research recommends the following fungicides as a pre-storage dip treatment for rhizome seeds: quinalphos at 0.075%, and mancozeb at 0.3%.
Studies indicate that bulbs (mother rhizomes) are preferred to fingers as a seed stock.
1.2 Post-harvest handling: curing, drying and polishing
Turmeric rhizomes are cured before drying. Curing involves boiling the rhizomes until soft. It is performed to gelatinize the starch for a more uniform drying, and to remove the fresh earthy odor. During this process, the coloring material is diffused uniformly through the rhizome. Recommendations as to the acidity or alkalinity of the boiling water vary by author.
The Indian Institute of Spice Research, Calicut, Kerala, and the Agricultural Technology Information Center simply recommend boiling in water for 45 min to one hour, until froth appears at the surface and the typical turmeric aroma is released. They report the color deteriorates as a results of over-cooking, but that the rhizome becomes brittle when undercooked.
Optimum cooking is attained when the rhizome yields to finger pressure and can be perforated by a blunt piece of wood.
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Boiling in alkaline water by adding 0.05% to 1% sodium carbonate, or lime, may improve the color. For the curing process, it is important to boil batches of equal size rhizomes since different size material would require different cooking times. Practically, fingers and bulbs are cured in separate batches, and bulbs are cut in halves. Cooking may vary from one to four or six hours, depending on the batch size. Curing is more uniform when done with small batches at a time. It is recommended to use perforated containers that allow smaller batches of 50 to 75 kg, which are immersed in the boiling water; by using this method, the same water may be used for cooking several batches. Curing should be done two or three days after harvest, and should not be delayed to avoid rhizome spoilage. The quality of cured rhizomes is negatively affected for material with higher initial moisture content.
Benefits of curing turmeric include reduction of the drying time, and a more attractive product (not wrinkled) that lends itself to easier polishing. However, it was reported that while the total volatile oil and color remained unchanged, curcuminoid extractability might be reduced. The curing by boiling process has the advantage of sterilizing the rhizomes before drying.
Slicing the rhizomes reduces drying time and yield turmeric with lower moisture content as well as better curcuminoid extractability. In rural Bolivia, slicing the boiled rhizomes is done by women. The “Fundación Poscosecha”, with the support of FAO has developed a slicing machine in order to ease the women’s work. The slicing machine has a simple design, is easy to use, and can be made at a low cost. It consists of a metallic structure (rack), a transmission system, and a metallic box containing a disc and two stainless steel circular blades. The transmission system is made of a pinion, an escape wheel, and a chain, and has a transmission report of 3:1. The slicing machine is all metallic, and when well maintained (lubrication of axles and bearings), it can be used up to eight years. The advantages of this machine are ease of use and installation, and ease of transport (it weighs about 40 kg). It has a high capacity (up to 120 kg/hr), and considerable reduces the traditional cutting by hand.
Cooked fingers or bulbs are dried to a moisture level of 5% to 10%. Sun drying may take 10 to 15 days, and the rhizomes should be spread in 5-7 cm thick layers to minimize direct sunlight that results in surface discoloration. Turmeric is one of the spices for which it is more advantageous to use mechanical driers because of the sensitivity to light. Those can be drums, trays, or continuous parallel or cross-flow hot air tunnels. Like with ginger rhizomes, the optimum drying temperature is 60 ºC.
An example of solar drier was developed in Bolivia, the “Secadora Solar”. It was designed to dry turmeric and ginger under the hot and humid conditions of tropical Bolivia, but it can also be used to dry other foods.
|Tumerc/ginger solar drier which was made in Bolivia|
The maximum temperature achieved by the drier depends on the outside climatic conditions.
The body of the machine is quite simple, it consists of a metallic rack supporting the rest of the components, two parallel inserted plastic trays where the products are put on to dry, and a plastic cover that should be designed to assure major protection from ultraviolet radiation.
Ideally, the plastic should be black, or contain a UV protector. Because the sun is serving as energy source, satisfying outputs cannot be achieved in regions where cloudiness and humidity is high. Approximately 4 KWH/m² of solar energy is needed to use these techniques successfully. The best outputs are obtained in regions with a humidity of 40% to 60% and average temperatures of 14º to 18ºC.
The advantages of this type of drier are simple construction with appropriate technology, ease of dismantling and transport, and versatility of use.
Dried fingers are polished to remove scales and rootlets from the rhizomes by using rotating drums lined with a metallic mesh that abrades the rhizome’s surface. Turmeric powder suspended in water is sprinkled over the rhizomes at the final stage of polishing to give an attractive color.
|Traditonal sun drying of tumeric|
1.3 Grading, packing and storage
Quality specifications are imposed by the importing country, and pertain to cleanliness specifications rather than quality of the spice. Proper care must be taken to meet minimum requirements, otherwise a lot may be rejected and need further cleaning and/or disinfection with ethylene oxide or irradiation.
Bulk rhizomes are graded into fingers, bulbs and splits. The Indian Standards for turmeric follow the Agmark specifications (Agricultural Directorate of Marketing), to insure quality and purity.
Rhizomes may be packed in jute sacks, wooden boxes or lined corrugated cardboard boxes for shipping. Storage of bulk rhizomes should always be in a cool and dry environment, to prevent moisture absorption and chemical degradation. Turmeric has traditionally been adulterated with related Curcuma species, specifically C. xanthorrhiza Roxburg, C. aromatica, and C. zedoaria. However, due to strong competition between spice processors, the quality of turmeric destined to the Western markets is usually guaranteed by the exporter in contracts negotiated between the buyer and the seller.
1.4 Grinding and milling
Grinding is a simple process involving cutting and crushing the rhizomes into small particles, then sifting through a series of several screens. Depending on the type of mill, and the speed of crushing, the spice may heat up and volatiles be lost. In the case of turmeric, heat and oxygen during the process may contribute to curcumin degradation. Cryogenic milling under liquid nitrogen prevents oxidation and volatile loss, but it is expensive and not widespread in the industry. Ground spices are size sorted through screens, and the larger particles can be further ground. Most quality control laboratories use the U.S. Standards (U.S.S.) screen size system. However, there are other systems that use a different numbering, and comparisons between specifications may be difficult. For instance, the U.S.S. screen numbering goes from 4 to 80 mesh screens (i.e. 4 to 80 openings per inch), while the Mill screen system goes from 4 to 55 mesh with different increments than the U.S.S. system.
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1.5 Extraction: oleoresin production
Since curcumin is the compound of interest in turmeric rhizome, it is important to know the solubility of curcumin in different solvents in order to choose the appropriate solvent. Curcumin is soluble in polar solvents (acetone, ethyl acetate, methanol, ethanol), and quite insoluble in non-polar solvents such as hexane, and insoluble in water. Dried powdered rhizomes are extracted by percolation with the polar solvent. The particle size, uniform packing in the extractor, temperature and percolation rate of the solvent are all important parameters for optimum extraction.26 If the oleoresin is the desired product, the solvent is completely evaporated by distillation at 45-55 °C.
If curcumin is the final product, the solvent is only partially removed, and the color material is separated from the solvent by freezing, then centrifugation or vacuum-filtration.55 At this step, curcumin is further purified with a wash with hexane. Hexane will extract all the gummy matter, oils, fats, and volatile essential oils that would otherwise impart a turmeric flavor. The yield of curcumin from dried turmeric root is about 5%.Oleoresin composition will vary greatly with the type of solvent, temperature and extraction methods, in addition to the effect due to quality of the raw material.15 The commercial methods of extraction will vary by manufacturer and are proprietary information.The yield of oleoresin from dried root is typically 10-12%.
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For organic production, synthetic solvents are not allowed. Solvents derived from petrochemicals such as hexane, pentane, di- and tri-chloroethanes, acetone, cannot be used in organic production. The International Federation of Organic Movement (IFOAM) specifies that only ethanol, water, edible oils or carbon dioxide are allowed. Therefore, possible solvents for curcumin extraction would be ethanol in the first step, and wash with vegetable oils and water for purification. There is no published study by using these restricted solvents, and it would be worth pursuing experimentally.
2. Proposed improvements
In India, the region of production determines the name of the type of turmeric. Although the average productivity of cultivated turmeric remains at an average low, breeding for crop improvement is difficult due to plant sterility. The Indian Institute of Spice Research in Calicut, Kerala, has however proceeded to crop improvement by clonal selection. This
Institute currently maintains a collection of 500-600 accessions of turmeric, all named for the distinct locations from which each was collected. Other institutions are also proceeding towards a similar effort, observing production yields and qualities of turmeric collected from the Tarai area of Northern India transplanted under plain conditions. The following Indian cultivars were released by the Indian Institute of Spice Research.
2.2 Post harvest handling
It is recommended that washed rhizomes be dried as soon as possible to minimize contamination, mold growth and fermentation. Boiling the turmeric rhizome in the curing process significantly reduces the microbial load on the rhizomes. If the rhizome is additionally dried in a mechanical drier, the potential for dust contamination is lessened.
After drying, specific equipment is suggested for optimum cleaning of the dried rhizomes: a plain sifter and an air screen separator will help remove small materials such as dead insects, excreta and extraneous matter, while a rotary knife cutter, a screen separator and a de-stoner will help remove residual insects and other extraneous matter.
In spite of the curing and drying process, turmeric still carries a heavy bacterial load. Specifically, toxigenic molds are reported on turmeric and other spices. Among the post-processing disinfection treatments available for spices, ethylene oxide is currently approved for use on spices in the United States with a maximum residue of 50 ppm after treatment.
However, ethylene oxide is dangerous to apply, and highly carcinogenic by inhalation. It has to be applied in specially-built facilities. The use of ethylene oxide is banned in the European Union and in Japan. The U.S. Environmental Protection Agency (EPA) has a maximum tolerance residue for ethylene oxide of 50 ppm on spices (40 CFR, 180.151). Propylene oxide is an alternative to ethylene oxide, but is not as efficient. If ethylene oxide is withdrawn form the market, it is likely that propylene oxide will follow the same route.
Irradiation is becoming an increasingly accepted technique to sterilize spices and other food products, mostly meat products and fruits. It too requires specially built and secure facilities. The common dose applied to spices is 3 to 10 kGy. By law, a spice irradiated once cannot be irradiated a second time; therefore, bulk irradiated spices must be well labelled to avoid a second irradiation if it enters as an ingredient in a meat or other product that will be irradiated.53 Irradiation of turmeric rhizomes with 10 kGy at a dose rate of 19 Gy·min-1 was reported not to modify the composition of volatile oils extracted after one week of storage at 5 ºC. The antioxidant activity as measured by the protective ability of linoleic acid oxidation (thiobarbituric acid value and peroxide value) was also reported to be unaffected by irradiation. Laws allowing irradiation on foods vary greatly by country within the European
Union. Several test methods are available to detect whether a spice has been irradiated. One method is based on the observation that irradiated spices exhibit thermo-luminescence. However, it appears that inorganic dust present in spice powders have the highest thermo-luminescence capacity from irradiation. Therefore, one study suggested that salt (NaCl) could be added before irradiation and serve as an indicator for irradiation. This may not be very practical for all spices. Other reported methods include electron spin resonance spectroscopy.
Turmeric pigment is highly unstable as compared to the yellow synthetic colorant, tartrazine. However, if protected from light and humidity, the curcuminoid pigments in turmeric powder and oleoresin are stable. Therefore, turmeric rhizomes and powder should be stored away from light and in a very dry environment. Additionally, all water or ethanol solvent should be removed from the oleoresin to assure pigment stability.
Organisation: Food and Agriculture Organization of the United Nations (FAO), AGST
Prepared by Anne Plotto.
Edited by François Mazaud, Alexandra Röttger, Katja Steffel
Last reviewed: 22/04/2004
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