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Issue 17

RIRDC Research Report: Food Safety of three species of Mint

Extracts

This report was undertaken by Dr Anne Fulton, Northern Melbourne Institute of TAFE for RIRDC IN 2000

In recent times some members of the Prostanthera (the native mint bushes) genus have been used to a limited extent by bush food specialists in syrups, cordials, salsas, chutneys etc. One attractive feature of the mints is their attractive, clean, strongly minty eucalyptal fragrance. The oils of two species have been reported to display antimicrobial activity. Mints of this genus should be investigated for commercial use. A mint which has good flavour and the prospect of acting as a preservative makes an excellent food ingredient if shown to be safe for human consumption. This report develops methods for easy identification of the selected three species of Prostanthera and studies their characteristic texture, smell, taste and storage and cooking stability. It identifies the major chemical constituents of the oils, investigating the possible side effects after eating by studying published reports involving each constituent. It also looks into the antimicrobial activity of the oils from Prostanthera.

The Prostanthera as a genus are native plants which are fairly common and readily grown. The members of this family possess a distinctive minty/eucalyptal fragrance. This, together with their dainty mauve coloured flowers render many Prostanthera species to be popular with many gardeners. In recent years there has been increasing interest taken in the role

of indigenous plants as raw materials. Native plants with various distinctive properties have been introduced into a number of diverse fields such as cosmetics, pharmaceuticals, toiletry, etc, including the important arena of cuisine. Due to their characteristic fragrance, a few Prostanthera species have been used by isolated chefs as an ingredient in the production of cordials, syrups, sauces and chutneys (personal communications from S. Hess Buschmann, `Gold Coast Bush Foods' and from J. Robyn, 'Robyns Bush Food'.) Given that as a group they are fairly robust and not too particular about growth conditions the Prostanthera would appear to be able, potentially, to support a large market once this has been developed. However, even before the development of a market, the safety of these plants for human consumption must first be established. Then the stability of their most prized characteristic, their aroma/ fragrance, must be investigated in order to determine likely shelf life. Finally commercial users should have some means whereby the particular species that are targeted as raw materials can be readily distinguished from other species.

Antimicrobial activity had been reported in the essential oils derived from a number of species of Prostanthera, leading Y to the speculation that the inclusion of the dried powdered leaves in foodstuffs could act as a complete or partial substitute for preservatives in the finished product. For this reason microbiological studies were included in this work. Obviously the allure of Prostanthera is increased if by its inclusion, the addition of 'unnatural chemical preservatives' with the attendant spectres of allergies, systemic poisoning, and chemical contamination is rendered unnecessary or to a lowered dosage. The three species selected for this study were Prostanthera rotundifolia, prostanthera lasianthus, and Prostanthera incisa.  The first two are found throughout the southern cooler temperate regions of Australia where they are commonly grown as windbreaks. The Prostanthera incisa (Ballinyah clone) studied came from the Lismore region of northern NSW.

These three species were selected for investigation because they are already grown in significant amounts as crop production and so ample quantities were available for experimental use.

Moreover, Prostanthera rotundifolia and Prostanthera incisa have already undergone a measure of commercial exposure. They are at present included as a minor constituent of certain products by manufacturers  of bushfoods. It is hoped that from this study the results that emanate will be of value to the native food/culinary industry and thereby increasingly realise the market potential of Prostanthera.

Executive Summary

The purpose of this project was to enhance the market potential for three prostanthera species by screening each species for food safety and antimicrobial activities in the herb and in the manufactured product, thus gathering data to form a basis for food standards. This broad aim was fulfilled by a number of studies which were meant to

provide means and some reference criteria for the rapid identification of the three Prostanthera species as raw materials;

identify the main chemicals present in each species and establish the toxicity status of each;

clarify the toxicity to humans after ingestion of each specie;

establish the stability of their primary marketable trait their fragrance,

investigate their antimicrobial activity and the potential to serve as food preservative. 

The first of these aims has been completed by the microscopic and macroscopic study coupled with the TLC results. GCMS data identified most of the major lipophilic chemicals present at two time points,  yielding thereby a lead to the possible toxicity of the herb oil and its stability to storage, to microwaving, and to baking. Three prominent components in the fresh oil of Prostanthera rotundifolia were detected but could not be identified.

From an extensive literature search centred on the chemicals defined by GCMS the conclusion was drawn that while ingestion of moderate quantities of the leaf was unlikely to produce untoward effects, conservatively a safe adult dose consists of about four and a half teaspoons of the ground dried leaf, or about 0.2 ml of the pure oil, assuming a safety limit that is parallel to that of Eucalypt oil. Sensory tests pointed to a loss of fragrance with storage, the loss being only partial in the case of Prostanthera incisa and Prostanthera rotundaolia but quite complete in the case of Prostanthera lasianthus. The varying degree of loss was supported by GCMS data, which also indicated that the oil of Prostanthera incisa was more resistant to storage decay than that of Prostanthera rotundifolia. The fragrance survived microwaveing to a degree but was completely destroyed by baking. The intense bitterness of all three species of leaf was their most unattractive triat. This decreased slightly with storage but was even then still off putting. However the oils were less bitter than expected. This lead to the speculation that the bitter taste may be due to other components as well as the terpenoids that were present in the oils. The fresh oils from Prostanthera incisa and Prostanthera rotundifolia demonstrated activity against Gram positive bacteria, which activity was still evident in oils from herbs that had been stored for five months. That a number of terpenoid chemicals displayed antimicrobial behaviour had been reported by various groups. This and our data supported the conclusion that the antibacterial activity originated from the lipophilic components of the leaf. In relation to the inactivity of Prostanthera lasianthus, it is interesting to note that the levels of cineole present were found to be lower at both time intervals than those of the other two species. When added to water, sugar solution, and oily solutions, the ground freshly dried leaves of all three species, with a single exception, showed temporary (one week duration) bacteriostatic action.

This property should be further looked into if decisions are to be formed about its usefulness as a preservative.

In closing, the following suggestions are put forward for consideration. The composition of the oils of Prostanthera rotundifolia and of Prostanthera incisa in this study were slightly different from that found by others, hence giving rise to the suggestion that oil composition may be affected by environmental conditions, ie soil, climate, harvest period, period of sunlight, etc. The correlation between composition and other factors should be investigated since products with wider markets could be developed, for example specimens whose leaves retain the original fragrance but are devoid of bitterness, or whose oils show greater antimicrobial prowess. The as yet unidentified chemicals in Prostanthera rotundifolia oil, as well as the flavonoid compounds in all three species, should be identified and included in an expanded safety investigation. Bacteriostatic activity, which has been shown in this work to be present in Prostanthera incisa and Prostanthera rotundifolia merits deeper and more rigorous experimental work for commercial applications. The intense bitterness of the Prostanthera leaves weighs against their inclusion in foods which are not highly sweetened. Developmental work before marketing should include ways of masking or removing altogether this taste while leaving the fragrance and antimicrobial property.

The full report can be found at their website:

www.rirdc.gov.au/reports/Index.htm