Rural Industries Research & Development Corporation

The New Rural Industries
A handbook for Farmers and Investors

Spearmint

Introduction

The two main types of commercial spearmint oil are obtained from the leaves of the perennial herbs, Mentha spicata L., more commonly known as native spearmint, and Mentha cardiaca L., Scotch spearmint. These are members of the Labiatae family, which includes many well-known essential oil plants such as spearmint, basil, lavender, rosemary, sage, marjoram and thyme. The spearmint plant is a summer-growing perennial with upright square stems reaching a metre in height at maturity.

The oil is found on the undersides of the leaves, is extracted by steam distillation and is generally followed by rectification and fractionation before use. Its major end-uses are in toothpaste and mouthwashes, chewing gum and food flavourings.

Spearmint is produced mainly in the USA with some recent development in China and South America. The world market for spearmint oil is approximately 1500 t/year.

The environmental requirements for spearmint cultivation are not as narrow as those for peppermint. Nevertheless, as the general growing requirements, management practices and harvesting techniques are the same for both crops, spearmint is grown in the same production localities as peppermint.

Production in Australia has been very limited, with a maximum of 1500 kg produced in Tasmania in the early 1990s. This production has been based purely on native spearmint. Collaborative research is currently under way in Victoria and Tasmania examining the feasibility of Scotch spearmint production.

One of the main advantages of spearmint production is the ability to integrate the crop with other essential oil production. This system allows for considerably increased utilisation of plant and equipment, thereby offsetting the capital attributed to each enterprise.

Markets and marketing issues

While the production of spearmint oil world-wide is smaller than peppermint oil, it is substantial and increasing.

Many oral care products are now using a combination of spearmint and peppermint oils to `soften' the flavour and appeal to a wider market. In some cases blends may use equal proportions of the two oils.

The price of spearmint oil fluctuates year to year, with native spearmint at 20-40% lower than peppermint oil and Scotch spearmint 0-10% higher than peppermint oil. Maximum prices are achieved only if the major oil components, carvone and limonene, are in the correct balance with other flavour components.

Buyers are primarily concerned with the quality of the product as determined by the characteristics and consistency of the oil and its organoleptic (smell) properties. The need for consistency from year to year derives from the requirement for consistency of taste in consumer end-products.

The world-wide trade in spearmint oil is generally from growers to traders who may blend or rectify the oils from many growers or even regions to provide a consistent product to the flavour and fragrance houses which then supply pre-formulated product to the manufacturer.

Sales direct to flavour and fragrance houses and manufacturers does occur, but requires considerable long-term marketing commitment and well established production history.

The level of pesticide residues in the oil is now a key marketing issue. The top end of the market is very discerning and well equipped for residue detection.

Production requirements

Spearmint requires a relatively free-draining soil with a pH around 6.5. Areas that lie wet in winter will not perform vigorously and plants may die.

Spearmint has high water requirements during summer. Crops are currently grown under managed flood, high pressure and low pressure spray/sprinkler irrigation systems.

Pest and disease management are paramount and efficient broadacre spray equipment is a prerequisite.

Harvesting uses conventional forage harvesting technology but distillation of the oil requires specialist equipment and expertise and needs to be regionally based as large volumes of material must be processed. This impinges on transport costs and the need to process quickly to minimise product deterioration.

Spearmint differs from peppermint in that a healthy crop will regularly produce two harvests each season whereas peppermint usually produces only one harvest. In general, experience has shown that it is better to concentrate on maximising the oil potential for one harvest because of the high costs of harvest and costs and the additional costs of disease management with double harvest.

Varieties

Mentha spicata L. and Mentha cardiaca L. are sterile perennial herbs, and therefore must be propagated vegetatively.

Mentha spicata L. is characterised by a high carvone content which accounts for 60-70% of the total oil analysis, accompanied by a limonene content of 8-15%. The odour profile of native spearmint is a fresh and green with very high floral topnotes. The body of the oil is warm and herbaceous.

The carvone content of Mentha cardiaca L. is also 60 and 70%, but typically it has a higher limonene content of up to 20%. The oil also has a menthone content of up to 2%. This is the prime indicator of Scotch spearmint oil.

The odour of the oil is light, fresh, and diffusive, with an ethereal topnote and a woody, rapid body expansion. The body of the oil is typically very sweet.

Agronomy

Generally a nursery site of material is planted from lifted stolons in early winter. This is typically 1-2 ha in area. From a nursery site a plant-up ratio of
8-10:1 can usually be achieved the following year.

As spearmint is a perennial crop, pre-planting weed control is imperative to the long-term viability of the crop. A well-planned fallow and weed eradication program before planting is therefore strongly recommended.

The same specialised lifting equipment as is employed in Victoria and Tasmania to lift peppermint plants and remove soil can be used, provided the machinery is cleaned well to avoid cross contamination

Strong healthy planting material is essential for correct density of established crop. Stolons are typically the thickness of a pencil and white to pale green-brown. Black stolons should not be planted as this colour indicates disease or poor condition.

Fertiliser rates are generally high, as development of the maximum number of leaves and their retention through to harvest is the target. Frequent nitrogen applications are required through the growing season and careful maintenance of soil fertility is needed to ensure the crop remains productive. A commercial crop correctly maintained will yield well for at least 5 years.

Pest, disease and weed control

The most significant disease problem encountered with spearmint is a rust fungus which, if left unchecked, will totally defoliate the plants. The current method of control relies on use of the fungicide `Tilt' at strategic times in the life cycle of the fungus. Other important factors in control are efficient spray application, and the removal of rouge plants and areas that are difficult to spray.

Scotch spearmint appears to be more sensitive to rust attack than the native spearmint, but both are prone to significant oil loss if the rust is left unchecked.

Pest problems encountered in Australia include cut worms, twospotted mites and the wingless grasshopper.

Weed control programs must be strictly maintained to reduce plant competition but more importantly to eliminate oil contamination. Very few herbicides are registered for use in spearmint crops. Growers should seek specialist advice before applying any pesticides.

Varieties

Mentha piperita L. is a sterile, perennial herb and therefore must be propagated vegetatively.

Mentha spicata L. is characterised by a high carvone content which accounts for 60-70% of the total oil analysis, accompanied by a limonene content of 8-15%. The odour profile of native spearmint is a fresh and green with very high floral topnotes. The body of the oil is warm and herbaceous.

The carvone content of Mentha cardiaca L. is also 60 and 70%, but typically it has a higher limonene content of up to 20%. The oil also has a menthone content of up to 2%. This is the prime indicator of Scotch spearmint oil.

The odour of the oil is light, fresh, and diffusive, with an ethereal topnote and a woody, rapid body expansion. The body of the oil is typically very sweet.

Agronomy

Generally a nursery site of material is planted from lifted stolons in early winter. This is typically 1-2 ha in area. From a nursery site a plant-up ratio of
8-10:1 can usually be achieved the following year.

As spearmint is a perennial crop, pre-planting weed control is imperative to the long-term viability of the crop. A well-planned fallow and weed eradication program before planting is therefore strongly recommended.

The same specialised lifting equipment as is employed in Victoria and Tasmania to lift peppermint plants and remove soil can be used, provided the machinery is cleaned well to avoid cross contamination.

Strong healthy planting material is essential for correct density of established crop. Stolons are typically the thickness of a pencil and white to pale green-brown. Black stolons should not be planted as this colour indicates disease or poor condition.

Fertiliser rates are generally high, as development of the maximum number of leaves and their retention through to harvest is the target. Frequent nitrogen applications are required through the growing season and careful maintenance of soil fertility is needed to ensure the crop remains productive. A commercial crop correctly maintained will yield well for at least 5 years.

Pest, disease and weed control

The most significant disease problem encountered with spearmint is a rust fungus which, if left unchecked, will totally defoliate the plants. The current method of control relies on use of the fungicide `Tilt' at strategic times in the life cycle of the fungus. Other important factors in control are efficient spray application, and the removal of rouge plants and areas that are difficult to spray.

Scotch spearmint appears to be more sensitive to rust attack than the native spearmint, but both are prone to significant oil loss if the rust is left unchecked.

Pest problems encountered in Australia include cut worms, twospotted mites and the wingless grasshopper.

Weed control programs must be strictly maintained to reduce plant competition but more importantly to eliminate oil contamination. Very few herbicides are registered for use in spearmint crops. Growers should seek specialist advice before applying any pesticides.

Harvesting, transport and distillation

The timing of harvest is not as critical to the quality of the oil as it is with peppermint and maximisation of oil yield is generally more important. Nevertheless, in Tasmania and Victoria an extensive pre-harvest sampling program is essential to ensure optimum quality oil.

Spearmint crops are mown using conventional hay mowers or windrowers. It is very important not to bruise any of the leaves at any time during harvest as this will result in oil losses.

Once the cut herb is wilted it is chopped directly into a distillation vessel, usually referred to as a tub, using a forage harvester. The herb must have the correct moisture content for complete and economic oil extraction.

The tubs are then transported to the distillation facility where either wet or super-heated steam is passed through the herb and the resulting steam and oil vapour are condensed and separated.

Condensing and separating equipment should be made from stainless steel, and general processing hygiene followed to ensure no contaminants are present.

In general, most economic units distil five or more tonnes of herb at a time. The time taken for oil extraction varies depending on the type of steam source, the herb weight and the moisture content.

Most distillation units are diesel fired but some wood fired units are used in Tasmania with success.

Once the oil is separated the product is relatively stable for many months provided it is stored out of direct sunlight and away from heat. Epoxy-lined and galvanised drums are the most commonly used storage and transportation units.

Economics

The costs of establishing a spearmint crop are considerable because propagation is vegetative, as described above. In general, a minimum area of 5 ha is needed within an existing essential oil distillation region of radius 30 km.

Capital outlay is considerable for dedicated equipment such as boilers, condensers, separators, tubs, planting equipment. In general, even with second-hand equipment set-up costs have been in the order of $150,000 to $250,000 for a regional facility. Consequently, spearmint production is at present viable only if integrated with existing essential oil production regions.

The table on the following page (not available in this html version) is a gross margin analysis for a typical Tasmanian or Victorian spearmint crop of 5 ha or more.

It should be noted that this gross margin is highly sensitive to yield and price changes. Production costs tend not to vary greatly.

Key contacts

Key references

Guenther (1948) The Essential Oils - Krieger

Hay and Waterman (1993) Volatile Oil Crops - Longman Scientific and Technical

Aretander (1960) Perfume and Flavor Materials of Natural Origin - Aretander

R&D Plan for the Essential Oils and Plant Extracts Program
1996-2001, RIRDC October 1996

Proceedings of the Essential Oils Planning Workshop, RIRDC, June 1996

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