Organisation: Food and Agriculture
Organization of the United Nations (FAO), AGST
Prepared by Anne Plotto.
Edited by François Mazaud, Alexandra Röttger, Katja Steffel, Larissa
D'Aquilio (HTML transfer)
2. Post-Production Operations
2.1 Harvest
2.2 Washing, "killing", drying
2.3 Grading and packaging
2.4 Storage
2.5 Distillation
2.6 Extraction: oleoresin production
The time of harvest after planting depends on the end-use. For fresh products and preserves, one should harvest rhizomes while they are still tender, low in pungency and fiber content, therefore before they are fully mature.38 Harvest for dried spices and oil is best at full maturity, when the leaves turn yellow; leaving the rhizomes in the ground past that stage may reduce pungency and oil content, and increase the fiber content.38,42 Maximum oil and oleoresin contents are between 150 and 170 days after planting under Kerala's conditions.3 In Hawaii, (6)-gingerol, the pungent constituent of ginger, increased over time in rhizomes of 'Hawaiian' cultivar as measured on a fresh-weight basis, while it reached a peak 16 weeks after planting on a dry-weight basis, decreased and increased again to reach a second peak at 24 weeks.15 Likewise, the maximum oleoresin content was reached after 28 weeks on a fresh-weight basis. Time from planting to maturity may be highly affected by the type of soil in which ginger is grown.42
Harvest for planting material is further delayed until the leaves are completely dried out.21 Rhizomes may be left in situ, with the leaves cut serving as a mulch, and dug when needed.38 Alternatively, they may be dug out, treated with fungicide and insecticide, dried in the shade, and stored in pits covered with sand.10,21
Harvest is by manually lifting the rhizomes from the soil, that may have been loosen at first.21,42 In some countries such as Australia, harvest may be fully mechanized using special equipment; the crop must be planted in such way that interspacing between rows is adapted to equipment.42 Care should always be taken to assure integrity of the rhizomes during harvest and postharvest handling.
Fresh rhizomes should be washed, and cleaned from debris, shoots and roots. When available, pressure washing is preferred as it is more efficient and tends to reduce the microbial load33. Traditionally, rhizomes are killed by a 10 min. immersion in boiling water, which also inactivates enzymatic processes, then sun-dried.38, 42 Another method is to scrape, peel, or slice rhizomes prior to drying. Peeling or scraping is advised for reducing drying time, thus minimizing mold growth and fermentation.33 However, while this process decreases the fiber content by removing the outside corky skin, it also tends to remove some of the oils constituents, as they are more concentrated in the peel, and therefore reduces some of the pungency.38,42 The peeled rhizomes may be bleached to improve appearance.
After peeling and washing, rhizomes are first soaked in water for 2 to 3 hours, then steeped in a solution of 1.5 to 2.0% lime (calcium oxide) for 6 hours, then drained and sun-dried33. This procedure is used when a light bright color is desired. The Indian Spice Board recommends the following sequence for preparing dry spices: soaking in water overnight to loosen the soil, peeling/scraping with pointed-end bamboo splinters, washing off the residual peel, sun drying for one week, soaking in 2% lime for 6 hours, and final drying (www.indianspices.com/html/s1926pac.htm).1 Drying should be done to 8-10% moisture, and should not exceed 12%.21 Expected weight loss during drying is 60-70%.42
Cleaning and drying procedures should be done as fast as possible after harvest to ensure minimum loss from microbial contamination, mold growth and fermentation. Mechanical washers, slicers, and solar or hot air driers may help minimize contamination from dust during post harvest handling operations.42 Sun-drying peeled ginger takes 7 to 9 days to reach a moisture content of 7.8% to 8.8%.33 If the ginger is sliced, it takes only 5 to 6 hours by using a cross-flow drier, while it takes 16 to 18 hours to dry scraped whole ginger using the same equipment and conditions. Mechanical drying will ensure a more homogenous and cleaner product. When drying with hot air, care should be taken to adjust air flow and temperature. Drying should not exceed the critical temperature of 60 ºC to avoid flesh darkening and discoloration.33
Specific equipment is suggested for optimum cleaning of the dried rhizomes.33 An air screen separator will help remove dead insects, excreta and extraneous matter, while a rotary knife cutter with a screen separator will help remove residual insects and other extraneous matter.
Quality specifications are imposed by the importing country, and pertain to cleanliness specifications rather than quality of the spice (see cleanliness specifications in 1.5.1below). 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 may be packed in jute sacks, wooden boxes or lined corrugated cardboard boxes for shipping42. The following terms are used to describe the various forms of dried rhizome.
Dry slices or powder are packaged in Kraft multi-wall laminated bags.21,22 Some laminates may be better than others due to film permeability. Whichever film is used, storage in a cool and dry environment is crucial for dry spices.
Dried spices
Dried rhizomes, slices, or splits should be stored in a cool environment (10-15°C). When stored at room temperature (23-26 ºC), losses of up to 20% oleoresin (dry weight) were observed on dry ginger after 3 months, and the content of (6)-gingerol decreased 29 7. It is therefore recommended to extract or distill dried ginger rapidly, if cold storage is not available, when oil or oleoresin is the final product. The importance of a dry storage for dried ginger destined for distillation can only be emphasized because additionally, mycotoxins from mold may be co-distilled with the essential oil.
Mold and bacteria developing on dried rhizomes may be efficiently controlled with 60Co gamma-irradiation at doses of 5 to 10 kGy, with minor changes in the quality of ginger oil 20,30. Ethylene oxide is also used as a fumigation treatment on spices.21,22 The U.S. Environmental Protection Agency (EPA) has a maximum tolerance residue for ethylene oxide of 50 ppm on spices 8. Both disinfection by irradiation and ethylene oxide treatments require specially built and highly secured facilities.
Alternatives to irradiation or synthetic chemicals were investigated by the Indian Institute of Spices Research. They found that leaf powder of Glycosmis pentaphylla and Azadirachta indica added to dried ginger rhizomes in sealed polyethylene bags were effective at preventing damage from the cigarette beetle (Lasioderma serricorne) (http://www.iisr.org/department/cropprod/hhigh.htm).2
Fresh ginger
Fresh ginger should be stored in a cold and humid environment. However, cold storage may not always be available in the producing areas. A "zero energy" cool chamber was experimentally designed at the Peruvannamuzhi IISR farm to store fresh ginger, maintaining the temperature 6 to 7 ºC below the outside temperature (http://www.iisr.org/cropprod/postharvest.htm).2
Fresh ginger rhizome shelf life may be extended by storage at 10-12ºC and high humidity. In a study on Hawaiian ginger, quality was stable during 28 weeks when stored at 12.5ºC and 90% relative humidity (RH) as determined by dry weight, fiber content, oil content, sugars and phenols.31 In comparison, storage at 22 ºC and 70% RH shortened rhizome commercialization to 20 weeks due to excessive water loss and fiber contents.31
Irradiation at 0.05-0.06 kGy may be used to inhibit sprouting and extend shelf life of fresh ginger 27,43. However, irradiation at these low levels decreased volatile content of fresh ginger, which was perceived by sensory analysis after 5 months in storage.43
A combination of biocontrol with Trichoderma sp. and storage in polyethylene bags at 25-30ºC controlled storage rot due to the fungus Sclerotium rolfsii and prevented weight loss from dehydration. 27 The Indian Institute of Spices Research recommends storage of fresh ginger in polyethylene bags with 2% ventilation prevents both dehydration and mould development (http://www.iisr.org/department/cropprod/hhigh.htm). 2
Ginger oil may be produced from fresh or dried rhizomes. Oil from dried rhizomes will have less of the low boiling point volatile compounds since they tend to evaporate during the drying process41. The difference between oils produced from fresh and dried rhizomes can be seen in the citral content, usually lower in the oil from dried plant material.25 Additionally, unpeeled or coated rhizomes are preferably used for oil or oleoresin extraction to improve yield.38,42
For steam distillation, dried rhizomes are ground to a coarse powder and loaded into a still.42 Live steam is passed through the powder, thus entraining the volatile components, which are then condensed with cold water. Upon cooling, the oil separates from the water. Cohobation, or re-distillation, is practiced in India to increase oil yield.42 Oil yield from dried rhizomes is generally from 1.5% to 3.0%42. Indian (Cochin) ginger yields 1.5% to 2.2 % of an oil rich in citral.13 The rhizome powder stripped from its oil (marc) is made of about 50% starch and may be used as livestock feed42. It may also be further dried and powdered to produce an inferior spice.42
Major components in ginger essential oil are zingeberene (20-37%), ar-curcumene (5-20%), - and -farnesene, -bisabolene and -sesquiphellandrene.25 The low boiling point monoterpenes a-pinene, cineole, borneol, geraniol, geranial and neral are less abundant and present in various proportions, and they impart aromas characteristic to the products. For instance, citral with its two isomers geranial and neral, is especially high in the Brazilian-grown cultivars 'Capira' (6.6-7.0% citral) and 'Gigante' (14.3-20.7% citral), while it is only 1.9-4.3% in some Chinese oils25. Australian oils also have a high citral content, up to 27%, averaging 19%, imparting a lemony aroma to the final product.18, 41
Gingerols (6-, 8-, and 10-gingerol) are the compounds responsible for ginger pungency16,19; however, because they are readily decomposed to the less pungent shoagols and zingerones upon heating, oleoresins obtained by solvent extraction are preferred when pungency is desired. Commercial solvents include ethanol, acetone, trichloroethane or dichloroethane42, although the latter two are known carcinogenic and ethyl acetate or hexane are preferred.40
Dried powdered rhizomes are extracted by percolation, and the extract is then cold-distilled at 45-55 °C to remove all the solvent, while assuring integrity of gingerols by not overheating. Hydrophilic solvents such as ethanol, and acetone also extract water-soluble gums, which may need to be further separated by centrifugation. However, water-soluble solvents may be preferred to prepare extractive to be used by the beverage industry to assure water solubility. Supercritical fluid extraction uses carbon dioxide (CO2) under high pressure and cold temperature. This extraction technique is preferred for higher quality extracts because there is no thermal degradation, and the aromatic profile is therefore closer to the profile in the plant.42
Zingerone, shogaol and gingerol were present in cold pressed oil and supercritical
extract of Chinese ginger, and were absent from the steam distilled oil.44 Geranial
and citral were 10.9 and 2.0% in supercritical extract, as compared to 0.63
and 1.31% in steam distilled oil, respectively.44
The use of steam or CO2 is environmentally preferred over hydrocarbon or halohydrocarbon
solvents since they generate little or no hazardous wastes. Kim et al. (1992)
reported extracts yield of 6.9% with CO2.
For certifiable organic production, synthetic solvents are not allowed 9.
Therefore, 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 (see IFOAM standards, Appendix 4).
7 A similar range of decrease in oleoresin
content was reported earlier by Richardson (1967).
8 (CFR 40, Part 180.151)
9 (Federal Register, 7 CFR Part 205)