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Thyme - Thymus vulgaris

Malcolm Douglas : DouglasM@crop.cri.nz
Redbank Research Station
A printed copy with photos is available. Updated: June 1993

Synonyms: wild thyme, common thyme, garden thyme.

Thyme, a small, upright shrub growing 20-40 cm in height, belongs to the Labiatae family. Pink to lilac, and rarely white, flowers are borne in clusters near tips of shoots in early summer. Leaves are small and grey-green.

Other species of regional importance for both culinary herb and essential oil production include T. serpyllum, T. zygis and T. mastichina. Lemon-scented cultivars of thyme are grown on a small scale for fresh and dried culinary herb production.

Thymus vulgaris is native to the western Mediterranean region, extending to south eastern Italy. Thymus serpyllum has a wider natural range in much of western Europe, while T. zygis and T. mastichina are restricted to the Iberian Peninsula.

Uses

Fresh and dried thyme is used as a traditional culinary seasoning. The essential oil is also extracted from the herb and used in both perfumery and flavouring applications. The essential oil has useful antioxidant activity. Thyme is also regarded as a medicinal herb with antispasmodic, expectorant and flatulence-reducing action. Thymol, a major constituent of the oil, has antibacterial, antifungal and anthelmintic activity.

The dried herb is used to give flavour to meat and meat products, condiments, relishes, soups and gravies. Thyme is an ingredient in mixed herbs. Lemon thyme is often used to flavour fish dishes.

The essential oil of thyme is mainly used in flavouring applications in the processed food industry. It is also used to fragrance soaps and detergents where its characteristic fresh, antiseptic aroma is desired.

Thyme produces a distinctive honey that is beginning to find niche markets in Europe and Asia.

Growing environment

Thyme is unique among herbs in New Zealand. It became well established over several thousand hectares of dry, sunny country centred along the Clutha and Kawarau river valleys in Central Otago. It was probably introduced by gold miners in the 1860s and 70s. Cockayne reported its naturalisation and spread on gold tailings near Clyde in 1928, and since then its spread has been rapid. As rainfall increases towards the coast or the mountains other species become more dominant and thyme disappears from the flora.

Although thyme grows without irrigation in very dry conditions, its productivity is low and increases substantially if irrigation is applied to the crop.

Soils must be very free draining and pH should be increased to at least 6.0 by applying lime, if necessary.

Agronomy

The crop can be established from seed or by cuttings. Some variability in plant growth habit, flowering time and productivity occurs in a crop established from seed. Thyme seed is very small but can be sown directly in the field at a rate of 5 kg/ha. Best results at Redbank Research Station were obtained by spring sowing in very light, stony soil. Variable crop establishment is a risk in a direct-seeded crop. Alternatively, seedlings can be established in a nursery bed or cell trays, and transplanted into the field.

Thyme grows easily from 5-10 cm cuttings taken in spring. Root-promoting hormone may be beneficial. Plants with superior characteristics can be selected and propagated from cuttings to reduce variability in the crop.

Transplants (cuttings or seedlings) can be established in beds approximately 1.5 m wide. Ideally, beds should be designed so that machinery straddles the crop and plants are not squashed, which thyme will not tolerate. A spacing of 25-30 cm between plants in beds is recommended.

Thyme tolerates a range of herbicides, although questions of residues on herb material, particularly for the export market, need to be addressed. No herbicides are registered for use on thyme, and only limited research has been carried out. Herbicides showing potential for selective weed control in Crop & Food Research trials include Sinbar, Stomp, Foresite, Linuron and Versatill. Although non-chemical weed control may ultimately be desirable, it may not be possible to grow thyme on a large scale in New Zealand without some herbicide assistance.

Crops of thyme have been established successfully without herbicides by transplanting the crop into an area covered with weed-suppressing matting. Organic mulches can be used in a similar way.

Harvest management of a thyme crop is critical. If the crop is left too long between harvests or is cut too hard, plant death can occur, particularly in wetter environments. Disease infection may contribute to plant deaths in crops weakened by heavy cutting. Harvesting should, therefore, be frequent and cutting height adjusted to leave some green herbage on the plants after cutting.

In Central Otago, a cultivated crop of thyme was harvested four times in a season for dried herb production. Harvest was carried out when 10% of the crop was beginning to flower. Thyme tends to produce new flowers soon after it has been harvested, so inclusion of some flowers in the final product is unavoidable.

Thyme suffers from few pests and diseases in Central Otago. However, in wetter environments with imperfect soil drainage, root-rotting fungal diseases have been implicated in plant death.

Essential oil yields peak in hot summer conditions. Yields of 1.0% (10 ml oil/kg fresh thyme) can be expected from wild thyme growing in Central Otago in mid-summer. Yields decrease to 0.10% in winter. Yields from cultivated material range from 0.05 to 0.50% depending on variety. However, herbage yields under cultivation far exceed production in the wild, so more oil would be produced per hectare in cultivated crops.

Soil tests should be carried out to determine fertiliser requirements. A basal fertiliser application containing nitrogen, phosphorus, potassium and sulphur should be applied annually. Thyme responds well to additional applications of nitrogen during the growing season, usually applied after each harvest to promote new shoot growth.

Processing and quality

Traditionally, large amounts of thyme were sun-dried, but final product quality was poor. Artificial drying methods allow better control of product quality. A forced air-flow drier would be suitable. Thyme should be dried at temperatures lower than 40°C to reduce loss of flavour through volatilisation of essential oil, and to maintain a good green colour.

The dried product must then be processed to remove leaf material from stems, and sieved to remove dust and produce a uniform product.

An International Standard detailing quality requirements for dried thyme is available. An important factor contributing to the flavour intensity and, therefore, quality of thyme is the essential oil content of the dried herb. Whole thyme leaves must contain 0.5% essential oil (5 ml/kg dried herb), and ground thyme must contain 0.2% essential oil to meet International Standard requirements.

Essential oil of thyme can be extracted by steam distillation of the fresh herb. The essential oil is located in small glands on the leaves. Yield and quality of essential oil varies according to the genetic make-up of plant material, crop maturity at harvest, environment and distillation practice.

Chemical composition of thyme oil is very variable with seven distinct races (chemotypes) identified in Europe. Two chemotypes occur naturally in Central Otago, but the thymol chemotype is dominant. Preliminary assessment by European flavour and fragrance companies suggested that Central Otago wild thyme oil is not favoured because it is too unlike their traditional sources of material.

Yields

The wild thyme resource in Central Otago formed the base of a local dried herb processing industry in the past. More recently, one or two operators have harvested thyme mechanically from accessible areas for Cerebos-Greggs. Yields of around 1000 kg/ha of unprocessed, dried herb could be expected from wild populations every three years.

Production of thyme at different locations in New Zealand is given in the table. Good horticultural management, irrigation, fertiliser and weed control were used in this study. Recovery of dried leaf after processing depends on the stalkiness of the material. Cultivated material has a higher proportion of leaf to stalk than wild material. The percentage of dried rubbed herb obtained from whole dried herb was around 60-70%.
Indicative yields of thyme grown at six sites in New Zealand.
Production of unprocessed dried herb (kg/ha)
Site Year of establishment Established crop
Hamilton 6260 Crop failure
Hastings 1580 13'400
Blenheim 2920 7860
Clyde 3740 7850
Mosgiel 3950 Crop failure
Invercargill 1660 Crop failure

Crop failure occurred at Hamilton, Mosgiel and Invercargill through high plant mortality after a hard crop harvest. Quality of dried herb material produced in these trials is currently being assessed.

Seed sources

Anon, 1992: The New Zealand nursery register 1992/93. Reference Publishing Co. Auckland, New Zealand.

Philip, C. (ed. Lord, T) 1991: The plant finder 1991/92 Edition. Headmain Ltd. UK.

Isaacson, R.T. 1989: Andersen Horticultural Library's Source List of Plants and Seeds. Andersen Horticultural Library, University of Minnesota, USA.

Markets

New Zealand imports all its dried thyme mainly from Spain and Morocco, the major world producers. Wild thyme from Central Otago is no longer being collected commercially because imported material is cheaper. Most bulk dried herbs are produced in countries with low labour costs, so the challenge for New Zealand growers is to produce crops of superior quality at a competitive price.

A certain amount of import substitution could be expected, but New Zealand production would be largely destined for, the export market. Europe is a major consumer of thyme together with the US. Trade statistics indicate that the US imports around 1000 t of dried thyme annually. Spot market prices in the US ranged from NZ$3.70 to 5.40/kg in January 1993 depending on quality and source: Spanish thyme was offered for higher prices than Moroccan.

Ninety percent of the thyme oil of world trade is produced in Spain. The estimated world production of thyme oil in 1984 was around 25 t. Spain also produced 2 t of wild thyme oil (T. serpyllum) in 1984. The spot price for thyme oil on the US market was NZ$ 108/kg in January 1993.

Future prospects

Crop & Food Research is continuing research on thyme production in New Zealand. The crop is being grown at several sites nationwide to assess the effect of environment on dried herb production and quality. Drying and processing methods are also being evaluated.

The potential of new accessions and chemotypes of thyme for both culinary herb and essential oil production is being assessed.

Further reading

BS 7807:Part 2:1989: Herbs and spices for food use. Part 2. Specification for dried thyme (whole and ground). British Standards Institution, UK.

Guenther, E. 1949: Oil of thyme. In: The essential oils. Vol. 3. Individual essential oils of the plant families Rutaceae and Labiatae. Kreiger, Florida, USAI. pp- 744-763.

Heath, H.B. 1981: Source book of flavours. AVI, Connecticut, USA

Leung, A.Y. 1980: Encyclopedia of common natural ingredients in food, drugs and cosmetics. Wiley, New York, USA.

Ministry of Agriculture, Fisheries and Food 1980: Culinary and medicinal herbs. Reference Book 325. Her Majesty's Stationary Office, London, UK.

Morgan, R.K. 1989: Chemotypic characteristics of Thymus vulgaris L. in Central Otago, New Zealand. Journal of Biogeography 16: 483-491.

Morgan, R.K. 1990: Thyme in the Central Otago landscape. In: Kearsley, G.; Fitzharris, B. ed., Southern landscapes, Department of Geography, University of Otago, NZ. pp. 213-232.

Wilkinson, E.L.; Dann, G.M.; Smith, G.J.S. 1979: Thyme in Central Otago. Tussock Grasslands and Mountainlands Institute, Lincoln College, NZ. Special Publication No. 14.


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While every care has been taken when preparing this document, no liability will be accepted by the New Zealand Institute for Crop & Food Research Limited for any loss or damage suffered as a result of applying the information contained in this document. Copyright © 1999 The New Zealand Institute for Crop & Food Research Limited, Private Bag 4704, Christchurch, New Zealand.