3. Natural Flavouring Materials

Plants are a universal of foods, which are both nutritious and flavoured. With careful choice, plants can provide a quite adequate balance of proteins, carbohydrates, fats, vitamins and mineral acids required for a healthy diet. However, many plants are more valuable for their aromatic properties, the spectrum of which is immense. Some must be regarded as fragrances whereas others are primarily of value as flavourings, but in the creation of imitation flavourings and fragrance compounds the demarcating is very imprecise.

Plant materials contain both volatile and non-volatile constituents who affect their odour and/or flavour profiles as well as their sensory impact. The volatile constituents are what gives to the plant its distinctive odour whereas the non-volatile constituents are either inert (e.g., cellulose) or influence some gustatory reaction (e.g., bitterness, pungency, astringency, etc.) sometimes coupled with a physiological effect (e.g., coffee, cocoa leaves, etc.). The intensity and quality of these effects show very wide variations between plant families and species. Although, qualitatively, the specific aromatic profiles remain recognisable and within acceptable limits, a quantitative variation is often observed between different crops and batches of the same plant material and also between the several parts of the plant which may be used. Because of this variation it is difficult to be precise in describing the aromatic effects of plant materials and assessment is generally a matter of judgement of acceptability for a particular use.

The plant materials can be classified in order to introduce the idea of impact without attempting to produce a precise grouping of similar attributes:

Natural ingredients can be divided into three types:

• Natural aromatic raw materials: vegetable or animal product used for its flavouring properties, either as such or processed and suitable for human consumption in the form in which it is used. Examples are fruits, fruit juices, spices, herbs, cheese and vinegar.

• Natural flavour concentrate: concentrate preparations obtained exclusively by physical methods from a natural aromatic raw material. Examples are essential oils, oleoresins, extracts and distillates.

• Natural flavouring substance: defined chemical component isolated from a natural aromatic raw material by exclusively physical methods. Examples are citral, geraniol, and menthol.

The ingredients with major industrial use are in the form of essential oils and extracts.

There are several processes to extract essential oils: solvent extraction, hydrodistillation, and extraction with dense CO₂.

Most essential oils are used directly as starting materials in the production of flavour and fragrance compositions. However, some essential oils are fractionated or concentrated by distillation, partitioning, or adsorption. Substances that are important for the desired characteristic odour and taste are thus concentrated, and other components, which possess either an unpleasant or very faint odour or are unsuitable for the application in question, are removed.

Individual compounds can be isolated from essential oils containing one or only a few major components by distillation or crystallisation. Examples are eugenol from clove oil, menthol from cornmint oil, citronnellal from Eucalyptus citriodora oil, and citral from Litsea cubeba oil. These compounds are used as such or serve as starting materials for the synthesis of derivatives, which are also used as fragrance and flavour substances. However, the importance of some of these oils has decreased substantially because of the development of selective synthetic processes for their components.

The chemicals present in essential oils may be classified as follows:

• Hydrocarbons of the general formula (C₅H₈)_n - the terpenes.
• Oxygenated derivatives of these hydrocarbons.
• Aromatic compounds having a benzenoid structure.
• Compounds containing nitrogen or sulphur.

These constituents are synthesised by the plant during its normal development and the chemical composition of the oil is generally characteristic of any given plant species. The proportion of the chemicals present in any given essential oil remains within predictable limits so that the quality of oil may often be expressed in terms of one or more specific components. The constituent ranges over the whole field of organic chemistry and their clear relationship have long been recognised in their biogenesis, chemical character and sensory attributes. However, research is still required to establish the precursors and the precise biochemical pathways involved in their formation in the growing plant.

Extracts of fragrance and flavour substances obtained from plants are termed pomades, concretes, absolutes, resinoids, or tinctures according to their method of preparation.