A taste for flavour characterisation

Flavour and aroma analysis is important to many industries, from the obvious – such as the food, drink and fragrance sectors – to the less evident, such as cosmetics and pharmaceuticals. The techniques and tools now available to flavour analysts are extremely sophisticated; sometimes however, you just can’t beat old fashioned human sensation Flavour is a familiar concept in everyday life, but before attempting a scientific analysis of flavour it is necessary to delineate what exactly is being investigated. A useful working definition is that flavour results from the combination of taste and aroma. Taste is the sensory perception of non-volatile compounds by receptors on the tongue, mouth and soft palate. There are generally acknowledged to be five tastes: sweet, sour, salty, bitter and umami. Aroma compounds, meanwhile, are volatile and detected in the nose; their presence leads to the perception of a particular food or other substance’s characteristic flavour: peppermint, strawberry or citrus, for example. There are a great many aroma compounds (in the thousands); they combine in a huge number of different ways to produce substances’ characteristic flavour profiles; and the human nose is very sensitive to them – more so than to taste compounds by many orders of magnitude. What’s more, dramatic variations in flavour can arise from the smallest of chemical differences. Even two different enantiomers (that is, molecules which are chemically identical, but are non-superimposable mirror images of each other) may yield entirely different sensory qualities. Carvone is just one example of this: S-carvone is perceived as caraway, R-carvone as spearmint. When we talk about flavour analysis, therefore, usually we are focused on aroma, as there is so much potential for variation amongst the many volatile compounds that produce characteristic flavour notes. A common approach to flavour analysis is the construction of a volatile profile by first extracting these compounds from a sample, then separating and identifying them using chromatographic methods combined with mass spectrometry. There are numerous factors that the analyst must take into account during this analysis, one of the most fundamental being the effect of the chosen sample preparation method on results. Different methods may lead to differences in the profile, and care must be taken to avoid sampling procedures which may alter the substance being studied – heating can affect a sample’s composition, for example. There is also always the risk of sample contamination and of losing key volatile compounds during a series of analytical steps. Extraction of volatiles may take the form of direct solvent extraction, solid-phase extraction (SPE), steam distillation (SDE), solvent assisted flavour evaporation (SAFE), or headspace extraction. Each technique has particular strengths and weaknesses, with SAFE and headspace extraction being generally recognised as particularly effective approaches. SAFE involves distilling the sample under vacuum conditions, with the distillation products collected as fractions in a flask cooled with liquid nitrogen. It is the low temperature and pressure conditions that make this technique a “gold standard” – the potential alteration of the sample already noted as a danger is avoided. SAFE allows the extraction of both volatile and non-volatile components of the sample, in contrast to headspace extraction which – by sampling the headspace above it – ensures only volatile components are captured. Once the analysis has been conducted and results obtained, another factor that must be taken into account is that although the complete profile may comprise a huge number of compounds, it is possible – even probable – that only a small number of these contribute significantly to the sample’s flavour. If these key aroma compounds can be identified then understanding of the flavour and how it can be altered or optimised is much simpler. An area of growing importance is flavour release over time. This is of particular note for food manufacturers, who may wish to market products as having long-lasting flavour and thus trial different methods of flavour delivery in order to produce the best customer experience. The challenge comes in how to analyse and quantify in the laboratory how long a particular flavour lasts. For this, various real-time mass spectrometry techniques are used, ion molecular reaction (IMRMS), selected ion flow tube (SIFTMS), proton-transfer reaction (PTRMS), to measure the concentration of specific flavour compounds in “nose-space” – exhalations during food consumption – samples taken over time. The most important part of flavour analysis is linking the chemical features of a substance to its sensory qualities, and therefore laboratory analysis alone is never sufficient to characterise a sample. People and their perceptions are fundamental to an understanding of flavour, and thus far we are nowhere near creating a means of analysis as sensitive to flavour as the human olfactory receptors. A useful technique is gas chromatography/olfactometry (GC/O). This involves combining GC analysis with human input; after the sample has passed through the GC column it is split into two detectors – one of which is a human assessor. Chromatographic peaks produced can then be correlated with the sensory assessment. Efforts have been made to replicate sensory analysis for use in certain scenarios, for example during food production as a quality check. Some progress has been made; recent research at the Universitat Autònoma de Barcelona produced an “electronic tongue” that could distinguish between certain types of beer through analysis of the ions present, whilst a pilot study at the University of Tokyo used NMR spectra to pinpoint particular sensory characteristics of coffee. Electronic Nose systems, either based on sensors or using mass spectrometry, are commercially available. Nevertheless, this type of ‘sensory’ analysis is extremely limited and provides much less information than traditional sensory panels. It should also be remembered that human experience of a product’s flavour is influenced by other sensory factors that on first glance might be thought unrelated. Texture is important; crunchiness, creaminess and other such qualities all affect perception of flavour. Research has also shown that such factors as colour and presentation of a product induce particular effects in the consumer that influence how flavour is experienced. There are many reasons why manufacturers invest in flavour analysis of their products. Identification and quantification of the key flavour compounds is the starting point for any optimisation of flavour, trials of new ingredients, or process variation; manufacturers will usually wish to compare the flavour profiles of products produced under different conditions or with different formulations. The link to sensory data here will also be of particular importance. Linking consumer preferences (as determined by sensory analysis) with the analytical data can support the development of successful products that appeal to consumer tastes. Flavour analysis is also used as part of quality control, and can inform quality specifications for a product. This may be particularly important when considering how a product’s flavour changes as it ages. Obtaining a characteristic flavour profile for a particular product (a flavour “fingerprint”) may also be of use in differentiating the product from competitors, in patent defence, and as substantiation in intellectual property claims to protect competitive advantage. The flavour team at Reading Scientific Services (RSSL) performs flavour analysis for clients for many purposes: finding alternative ingredients for formulations, comparing the flavour profiles of different products, investigating the effects of manufacturing process change and much more. The wide scope of flavour analysis as a field, and the infinite variation of flavour compounds that may require analysis, means that every project is different and there is always more to learn about what constitutes flavour. Author Kathy Ridgway, Flavour Technical Specialist, Reading Scientific Services Ltd