Quality in natural products

Key questions: What do we mean by quality in natural products? What factors affect quality of plant material?

Plant metabolism and the chemicals plants produce are dependent on a range of factors. Plant chemical profiles are influenced by geography, the type of soil, the weather, degree of light exposure, competition with other plants and attack by pathogens or herbivores.

Hugely important also is the age at harvesting or the season of harvesting. We can think of the set of metabolites or chemicals produced by a plant as its chemical profile or chemical fingerprint. Potential for variation in this chemical fingerprint results in variation in the levels of bioactive chemicals in plant material, and thereby the suitability of the material for specific purposes.

We can look at Quality from different points of view. From a consumer’s point of view, a product is a quality product when it meets the consumer’s expectations. For products, quality standards are necessary to ensure that products are what they are supposed to be and that they can reasonably be expected to do what they are supposed to.

Products must conform to specifications. For instance, a synthetic substance produced in a laboratory will need to undergo quality testing to ensure it is the desired substance at the expected level of purity. Quality can be more difficult to assess for natural products due to their complexity and the natural variation that occurs.

Honey – the importance of source

Honey is produced by bees from the nectar they collect. The taste, colour and viscosity of honey vary depending on the species of flowers from which the bees collect the nectar. Honeys have been used medicinally since ancient times and we now know that honeys have antibacterial and healing properties.

However, not all honeys are the same. The antibacterial action of Manuka honey, a viscous, dark honey with a strong flavour and aroma produced from the nectar of the manuka tree, is particularly powerful due to the presence of signature antibacterial compounds. The manuka tree is native to New Zealand and it is the nectar of the Manuka flowers that provides the signature chemicals.

The bioactivity of Manuka honey is dependent on the presence of these chemicals and so the quality of the product. In New Zealand, Manuka honey undergoes quality testing, including testing for the presence of these compounds so that it can be labelled as pure authentic New Zealand Manuka honey.

Toxins in plants can also accumulate in honey. Honey produced by bees that collect nectar from species of rhododendron can cause ‘Mad Honey’ intoxication. Mad honey is produced in the Black Sea region of Turkey and consumed to give a feeling of euphoria. It is also sometimes used in traditional medicine but can cause toxic symptoms such as vomiting, diarrhoea and seizures.

Peppermint – the importance of time of harvest

We are all familiar with the difference in appearance and taste of unripe fruit compared to ripe fruit. Think again of the apple. When you taste an apple you are tasting the apple’s chemical fingerprint. We know when it is time to harvest. There can also be an optimum age of harvest for other plant parts.

One example is peppermint. The leaves and stems of Mentha piperita L. are harvested and extracted to produce peppermint oil. The quality of the oil and the yield are important. Peppermint oil is used medicinally and a good quality oil requires a content of at least 45% menthol and 15-18% menthone. Menthol is at highest levels in older leaves whereas menthone is highest when the leaves are small and immature declining as they mature.

Total oil content increases until plants flower and then begins to decline. Harvesting must be carried out at the optimum time, particularly if the material is to be used to prepare peppermint oil of medicinal quality.

Lavender – the impact of climate

Lavender is an aromatic plant cultivated for its essential oil. Climate can significantly affect growth of the plant and the yield and chemical composition of the oil, influencing its quality and economic value. The plant needs direct sunlight daily during the growing season. Understanding the effects of climate helps growers to optimise cultivation for high yield of good quality lavender oil.

Tobacco – the impact of herbivory

The tobacco plant produces nicotine as a defence mechanism against herbivores. Nicotine makes the plant less palatable and also acts as a neurotoxin to insects, attacking their nervous systems. When attacked by herbivores, the tobacco plant responds by increasing production of nicotine, particularly in the leaves, which may or may not be desirable for growers.

Many plants have such defence responses and the ability to adapt to stresses, diverting resources to the production of protective molecules. This changes the overall chemical profile of the plant. Understanding natural defence responses can be useful. Plants can be bred with enhanced defence capabilities making them more resilient to environmental stresses and requiring less use of chemical pesticides.

Tomato – the importance of genetic diversity

Cultivation and selective breeding of plants to enhance specific traits leads to different varieties of the same plant species that can have vastly different chemical profiles. Genetic variations in tomatoes lead to differences in colour, taste, aroma and nutritional content. Cultivation tends to reduce genetic diversity, and wild plants will often have greater diversity in genetics compared to cultivated counterparts. Biosynthetic pathways are under genetic control and this is reflected in the chemical profiles of the plants.

Ensuring quality

Good Agricultural Practice (GAP) refers to a set of principles, standards and procedures that are followed to ensure safe and sustainable farming practices. Good Manufacturing Practice (GMP) ensures that that products are consistently produced and controlled according to quality standards. It is important to understand the potential for variability in plant material.

Natural material sourcing must be carefully considered as not all sources offer the same attributes. Quality attributes are particularly crucial for medicinal plant materials to ensure safety and efficacy of medicinal products for the public. How can quality be assured? Quality of plant materials is monitored through a combination of methods to ensure it complies with required specifications.

The methods include:

• Botanical identification: It is imperative to ensure that the plant species or subspecies is correctly identified. Plant material is visually and microscopically examined and may also undergo genetic testing such as DNA barcoding to ensure it is the desired plant.
• Physical attributes: Assessment of the colour, aroma and texture may be important. Moisture content is also assessed. The presence of too much moisture makes plant material subject to degradation and microbial growth.
• Chemical profiling: Signature compounds can be monitored by analytical techniques such as chromatography. Plant material extracts are prepared and can be tested for the presence or absence of one or more compounds. The amount of particular compounds in an extract, the percentage content, may also be of interest. In some cases, the analysis may be used as a fingerprint, a picture incorporating all the components of the plant extract.
• Contaminant testing: Contaminants such as pesticide residues and heavy metals should be determined as below defined limits in the quality assurance of plant materials.
• Traceability: Plant material should be traceable through the supply chain and detailed records should be kept of the source, batch number, method of processing and results of quality testing.