challenges of flavour analysis

8/9/2019 Challenges of Flavour Analysis

1/4

White Paper

The Challenges of

Flavour AnalysisComparison and Choices of

Extraction Techniques

Kathy Ridgway

Technical S ecial is t


2/4

Page 2

Challenges of Flavour AnalysisComparison and Choices of Extraction Techniques

The term flavour analysis, will mean different things

to different people and a number of approaches can be

employed This may include a volatile profile of a

product: a comparison of products or ingredients for

volatiles or off-flavours; determination of specificflavour related components; or perhaps a full study

using aroma extract dilution analysis (AEDA) with gas

chromatography-olfactory (GC-O) to fully characterise

the flavour active compounds in a product or

ingredient. In some instances where the taste as well

as the aroma is important, this may also involve liquid

chromatography techniques, such as liquid

chromatography-mass spectrometry (LC-MS). The

perception of flavour can also be linked to physical

parameters such as texture, and consumer preference

can be strongly linked to the flavour releasemechanisms, which can also be studied.

In the most part, flavour analysis concentrates on

volatile components and this means that gas

chromatography (GC) is the instrumental method of

choice. However, the method of sample extraction can

be key in the information obtained from the

instrumental analysis and it is important to recognise

the strengths and weaknesses of each approach in

order to select the most appropriate technique for each

application.

Why do Flavour Analysis?

Flavour is major driver of liking in most food and

consumer products and in particular the aroma of a

product correlates well with consumer preference and

perceptions of quality. Sensory analysis can help

differentiate products and indicate differences, but only

by chemical analysis can you understand why one

sample is different from another. Sensory and

analytical results can be correlated and linked to

changes in process. A full understanding of how a

flavour behaves or is produced in a production process

can be critical in optimisation and can aid product

development and drive meaningful quality

specifications.

Once the preferred profile of a product or ingredient is

defined then key compounds can be used for ongoing

quality and specification checks. This can help ensure

that a products flavour is consistent with regard to

process variation, aging, change of production lines or

equipment, or to check equivalent production indifferent manufacturing sites. Depending on the

required information, many approaches can be used to

assess the flavour of a product.

Sampling

As with any food analysis, in order to obtain

meaningful results, it is critical that a representative

sample is taken. For some ingredients or products

(such as liquids or powders), this can be relativelystraight forward. However, for multicomponent

products this may require a large sample size to be

taken initially, with subsequent homogenisation and

subsampling steps.

In order to understand the flavour of a product, it is

also important to understand the natural variance in

the ingredient or product and this needs to be

considered when determining the samples to be

analysed. This may be natural variation due to season,

origin, or supplier and all may be important forcharacterisation or defining specifications for quality

control purposes. When producing a flavour map for

instance, it is key to include samples from the

extremes of the spectra as described by sensory

analysis.

Choice of Extraction Method

Assuming the samples required have been determined

and are sufficiently homogeneous, the next

consideration is the choice of sample preparation or

extraction that will be employed. This will depend on

the purpose of the analysis, as well as equipment and

expertise available. For flavour characterisation

studies, it can be important to ensure the product is

not changed during sample extraction and excessive

heat should be avoided. For other applications or

comparative studies, this can be less critical.

Equilibrium techniques may provide an indication of

relative levels of flavour, whereas more exhaustive

extraction can be used to obtain more accurate

quantitation. Assuming the final analysis achieved

using GC, generally with a mass spectrometry

detector, a number of approaches can be taken and

these are outlined below and choices summarised in

Table 1.

Direct Liquid Extraction (LLE)

Direct liquid extraction by dilution or liquid-liquid

extraction are most commonly used for liquid samples,

although can be employed for solid samples following

an initial extraction. Selectivity is through choice of

solvent, and potentially adjustment of pH. For simpleflavour solutions this approach is ideal for quality

specifications or identification of components. In

complex matrices it may be necessary to isolate


3/4

Page 3

compounds of interest from the matrix further using

selective clean up approaches such as solid phase

extraction (SPE), this can also be used for solvent

exchange and to concentrate the sample extracts.

Without further clean up, the complexity of the extract

can result in key flavour compounds not being

detectable above matrix components, therefore this

approach is more often used for targeted analysis,

where selective clean up protocols can be developed.For particularly volatile compounds, these additional

steps can lead to losses during extraction.

Solvent Assisted Flavour Evaporation (SAFE)

Volatile components of a sample can be isolated from

matrix components using distillation. Steam distillation

extraction (SDE), using apparatus first reported by

Likens and Nickerson, can be employed for the analysis

of taints or off flavours, where the comparison between

a control and suspect sample is possible. As theoriginal apparatus set up involves heating the sample

for some time however, there is the possibility of

artefact formation. Therefore although useful for

comparison of samples, and extraction of taints

originating from an external source, it is rarely used for

flavour analysis. A related technique, which is

considered to be the gold standard for flavour

extraction is solvent assisted flavour evaporation

(SAFE). This uses a vacuum to distill at reduced

pressures and temperatures and liquid nitrogen cooling

of the distillate to ensure the sample is not altered

during extraction. In this approach, fractions of both

the volatile and non-volatile components are obtained.

When followed by GC-O analysis, which enables

detection also using the human nose, this approach

enables aroma compounds to be assessed by

correlating sensory perception with chromatographic

peaks as they elute from the instrument.

Headspace Extraction

Analysis of the headspace above a sample is another

way to minimise interferences from non-volatile matrix

components. The sample is heated and agitated to

enable an equilibrium to be reached and the

concentration in the headspace is determined and is

proportional to that in the sample. One of the

advantages of headspace analysis is that it is suitable

for the majority of matrix types, although a

reproducible partition is required, so addition of

water/salt and sample preparation to reduce particle

size need to be considered. It can be used for a

screening approach or optimised for more targeted

quantitative analysis. However, as this technique does

not provide exhaustive extraction, conditions should be

optimised and maintained for accurate quantitation.

Ideally internal standards (preferably isotopically

labelled) should be used, or the method of standard

additions can be employed. The disadvantage of direct

headspace analysis is that it can lack sensitivity and

this can be improved by using a further selection or

enrichment technique, such as dynamic headspace,

needle trap devices or solid phase microextraction

(SPME). Direct thermal desorption may also be

suitable for some applications.

Solid Phase Microextraction (SPME)

The use of SPME, in particular in headspace sampling is

extensively used in flavour analysis. It enables

sampling of only the volatile components, but with

increased sensitivity compared to direct headspace.

Use of different extraction phases on the fibre can also

provide some selectivity when required. Due to the

partitions involved, the optimum temperatures are

often lower than direct headspace, particularly for the

highly volatile compounds, which can be lost back offthe fibre at higher temperatures. As with headspace,

SPME is an equilibrium technique and thus the same

considerations apply for accurate quantitation. For

some matrices, effects can also be observed due to

displacement/competition between compounds for

active sites on the fibre. Careful selection of fibres and

conditions, or sample dilution, can overcome these

effects. SPME is extremely useful in comparing relative

levels of volatiles or in profiling of samples and mixed

fibres enable extraction of the majority of flavour

active compounds.

Stir Bar Sorptive Extraction

Stir bar sorptive extraction has also been employed for

volatile/flavour profiling. Like SPME it is a sorptive

extraction technique, which uses no solvent and can

provide high enrichment capacities. It uses a larger

volume of extraction phase than SPME and is

particularly suitable for direct extraction from liquid

samples. Due to the non-polar nature of the PDMS

coating most commonly used, it has particular

applications for profiling of alcoholic beverages, as

interference/fibre poisoning observed with SPME from

the alcohol matrix does not occur. The more recent

EG-silicone (PDMS/ethylene glycol copolymer) phase

enables better extraction for more polar compounds.

Quantitative methods have been reported, but it is

more commonly used for comparative studies or

qualitative profiling.

Conclusions

When deciding which sample extraction approach to

use for flavour analysis, it is important to understand

that each will result in a different profile. The

technique should be chosen to enable the required


4/4

Page 4

sensitivity and selectivity to be achieved. The nature

of the product or ingredient matrix should always be

considered and the link back to sensory data is

required to enable a full understanding of the impact of

analytical results.

Table 1

Choice of Extraction Techniques for Flavour Analysis

LLE SAFE #DHS SPME SBSE

Volatile Profiling

Identification of differences/comparison ofprofiles

Identification of known/target compounds Quantification of target compounds ? ? ?

Flavour Profiling

Identification of flavour relevant compounds + GC-O

Quantification of flavour relevant compunds ? ? ?

Preferred method

Can be used for some applications (# may lack sensitivity)

? May be suitable optimisation required (use of internal standards)

Kathy Ridgway Technical Specialist

Dr Kathy Ridgway is a Technical Specialist in the Investigative Analysis department ofReading Scientific Services Ltd. A graduate of the University of Surrey, she has over

20 years laboratory experience and has worked on determination of a variety ofchemical contaminants in foods

She has published papers including an extensive review of sample preparationtechniques for determination of trace contaminants in foods and a review of theanalysis and origins of taints and off flavours. She has contributed to several bookchapters and presented her work at international conferences

For further information, please contact Customer Services [email protected]

challenges of flavour analysis

Documents