Turmeric Post Harvest Operations

As a dried rhizome of a herbaceous plant, turmeric is closely related to ginger. The spice is also sometimes called “Indian saffron”. The underground rhizome imparts a distinctive flavour to food but it is also used to provide food with a deep, indelible orange colour. In the form of this fine, dried, yellow powder, turmeric is mostly sold to customers in developed countries. Turmeric is used in a wide variety of foods of the cuisines of Southern Asia but locally it also applies as an antiseptic for skin abrasions and cuts.

Global Production and Trade

While there is speculation that turmeric may have originated from South or South-East Asia, its centre of domestication is certainly the Indian subcontinent. Currently, India is the major producer of turmeric, and it is also the major user of its own production.

Post Harvest Management Aspects

Harvest: Turmeric's 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 percent, and mancozeb at 0.3 percent. Studies indicate that bulbs (mother rhizomes) are preferred to fingers as a seed stock.

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 odour. During this process, the colouring material is diffused uniformly through the rhizome. 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. Optimum cooking is attained when the rhizome yields to finger pressure and can be perforated by a blunt piece of wood.

Boiling in alkaline water by adding 0.05 to 1 sodium carbonate, or lime, may improve the colour. 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 with better curcuminoid extractability.

Grading, Packing and Storage

Quality specifications are imposed by the importing country, and pertain to cleanliness 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.

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 may 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.

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.

Post Harvest Handling

It is recommended that washed rhizomes be dried as soon as possible to minimize contamination, mould 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. Irradiation is becoming an increasingly accepted technique to sterilize spices and other food products, mostly meat products and fruits.

Several test methods are available to detect whether a spice has been irradiated. One method is based on the observation that irradiated spices exhibit thermoluminescence. However, it appears that inorganic dust present in spice powders has the highest thermoluminescence 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.

Storage

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.


Extracts from Report released from Food and Agriculture Organisation (FAO) of the United Nations. Copyrights reserved with FAO. Read more at www.fao.org