Understanding food flavour: the interaction of flavour chemistry and sensory science

D. Zabaras, D.C. Frank, P. O’Riordan

and P. Varelis

Centre for Food Quality and Stability

O

SH

2-methyl-3-furanthiol

SCHO

methional

Food Flavour: Definition

• Food flavour consists of three sensations:

• Odour (orthonasal perception)• Aroma (retronasal perception)• Taste

• Volatile compounds are responsible for the odour and aroma of a food

Precursors of Volatile Compounds

• Foods contain amino acids, carbohydrates and lipids, carotenoids

• Reactions involving amino acids, sugars and lipids are responsible for the formation of volatile compounds:

• Maillard reaction• Lipid decomposition

Maillard Reaction

− H2Orearrangement

NHROH

OOHH

OH

OH

Amadori intermediate

carbonyl compounds

− H2O− RNH2

O

OH

O

O

O CHOHO

O

OOH

O OH

OO CHO

O OH

O

ASPARAGINE

H2Saldehydes

H

O

OH OH

OHOH

OH

R NH2+

reducing sugar aminocompound

Amino acids

Lipid oxidation

(e.g PUFA) aldehydes

Acrylamide

Strecker reaction

Melanoidins

Flavour compounds

+ Lipid oxidation

Maillard reaction: Aromas generated from various sugars / amino acids

ChocolateOvercooked cabbage

Boiled meatCharred meatBeef broth

Sucrose

SweetHarsh horseradish

Stale potatoBaked hamBeef broth

Maltose

Dirty dog Bean soupFried potatoes

ChickenBeef broth

Fructose

CaramelCabbageBurnt fried potatoes

Chicken trayBurnt candy

Glucose

PhenylalanineMethionineLysineGlutamic AcidGlycine

(obtained from various literature sources)

Food volatiles

• Analytical investigations employed mostly GC and GC/MS.

• An estimated 8000 food volatiles had been reported until 1997.

• Initially it was assumed that all volatiles contribute to food aroma.

• The determination of odour activity values (OAV) showed that not all volatiles were important as flavour compounds

OAV = Odorant concentration/Odour threshold value

Odour analysis

• Results in the qualitative and quantitative determination of the important odorants in a food product:

• identity of important odorants• relative concentration in food of interest

• This information is useful for the prediction, control and re-creation of the flavour of a food product.

Odour analysis: Steps

1. Extraction of the volatiles from the food matrix

• dynamic headspace or purge-and-trap (DHS)• static headspace (SHS)• liquid or solvent extraction • solvent-assisted flavour evaporation (SAFE)• simultaneous distillation-extraction (SDE)

high purity N2

Sample

Tenax-TA® trap

Rapid sampling: 30 minutes

Up to six samples simultaneously

Extraction of highly volatile components

No heat-related artefacts

Odour analysis: purge-and-trap

Odour analysis: Steps

2. Separation and tentative identification of the volatiles in theextract by GC/MS and localisation of the odorants by olfactometry (GC/MS-O)

• Commonly employed GC/MS-O approaches:

• OSME (perception measurements of a non-diluted extract by trained panelists averaged)

• AEDA (extract serially diluted and measured until no perception)

Real time GC/MS trace

voice recorder

sample introduction

GC/MS

perceptualtrace

time intensityrecorder

GC-MS-olfactometer and time intensity recorder

instrumental trace

Time-intensity recorder interface

mouse-controlled sliding intensity scale

0 100

Information obtained from GC/MS-O

Damian Lindt

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1 205 409 613 817 1021 1225 1429 1633 1837 2041 2245 2449 2653

5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00

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Time-->

Abundance

TIC: LINXC.DGC/MS trace (from detector)

Aromagram (from human nose)

The human nose still remains the most sensitive detector of odour-active volatiles!!

Det

ecto

r res

pons

eP

anel

ist r

espo

nse

Time (sec)

Odour analysis: Steps

3. Quantification of important odourants using GC/MS

• Potentially most reliable quantification approach:

• SIDA (stable isotope dilution analysis)

30 40 50 60 70 80 90 100 110 120 130 1400

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Abundance

Scan 4498 (25.391 min): SL_100.D (-4484) (-4517) (-)126

7945

64

111

9633 54 13338

30 40 50 60 70 80 90 100 110 120 1300

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Abundance

Scan 4496 (25.376 min): SL_101.D (-4480) (-4511) (-)128

8046

64

112

9633 54 5938

00 20.00 25.00 30.00

on 126.00 (125.70 to 126.70): SL

00 20.00 25.00 30.00

on 128.00 (127.70 to 128.70): SL

Example: 13C2-dimethyl trisulfide

H313C

SS

S13CH3

H3CS

SS

CH3

Odour analysis: Steps

4. Preparation of ‘synthetic’ blends of odourants in ‘blank’ matrices using the quantitative data (including omission experiments)

5. Sensory evaluation of the prepared blends by trained panels

Whole process successful when:

Aroma of synthetic blend (usually containing 3-4 odourants) = aroma of original food

Example: Chocolate aroma

• Chocolate: a matrix of solid particles (cocoa, sugar, milk) set in a continuous fat medium

• Characteristic flavour is a result of several manufacturing steps

• Flavour depends on many parameters including cocoa variety used, bean fermentation, conching process, added ingredients (e.g. vanillin)

Example: Chocolate aroma

• Total volatile and aroma profiles of two milk chocolates compared

• Both chocolates contained a similar amount of cocoa and milk solids (approx. Cocoa Solids 26%, Milk Solids 27%, Fat 30%)

GC/MS profiles

5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00

-600000

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Time-->

Abundance

TIC: LINCH.D (*)TIC: CADMILK.D (*)

acetic acid

3-methylbutanal

vanillin

• Traces very similar (> than 90 volatiles detected)

Chocolate 1

Chocolate 2

Aromagrams

• Traces different (> than 70 odour-active volatiles detected)

0102030405060708090

100

1 205 409 613 817 1021 1225 1429 1633 1837 2041 2245 2449 2653

Chocolate 1

0102030405060708090

100

1 205 409 613 817 1021 1225 1429 1633 1837 2041 2245 2449 2653

Chocolate 2

Odo

ur in

tens

ity (0

-100

)

Time (sec)

Instrumental and perceptual data combined Chocolate 1

0102030405060708090

100floral,mandarin (linalool)honey/rotten,cheesy (phenylacetaldehyde/butanoic

acid)green, floral (unknown)sweet, floral (unknown)honey, sweet (5-methyl-2-furanmethanol)fruity (ethyl-2-methylpropanoate)herbal,fresh (unknown)herbal,pine (a-pinene)

herbal (unknown)fruity, bubblegum (ethyl-2-methylbutanoate)

pineapple (unknown)crushed leaves (hexanal)

herbal (b-pinene)herbal,dirty (a pyrazine)fruity, sweet (unknown)sweet,fresh (2-heptanone/limonene)fruity (unknown)sweet (unknown)melon,green (octanal)sweet,peach (unknown)baked potato (methional)

mushroom (1-octen-3-one)mushroom, fungus (unknown)

garlicky (unknown)rotten egg (dimethytrisulfide)

curry, spicy (abhexon)bacon (unknown)

cooked spaghetti (unknown)bacon, smoked ham (unknown)

spicy, spaghetti (sotolon)fatty (1-hexanol)fatty, green (E-2-octenal)fatty,cheesy (unknown)fatty, oxidised (Z-2-nonenal)fatty, oxidised (E-2-nonenal)sweet, fatty (unknown)

cheese (unknown)cheesy, smelly socks (2-methylpropanoic acid)rotten, parmesan (2/3-methylbutanoic acid)popcorn,cheesy (butyrolactone)fatty, burnt (unknown)stewed apple (phenylethylacetate)wine, fermented (2-phenylethanol)

vinegar (acetic acid)vitamin B (2-furanmethanol)

medicinal (guaiacol)medicinal (unknown)

sweet, phenolic (unknown)metalic (unknown)

phenolic (unknown)sweet (2,3-pentanedione)

sweet,caramel (unknown)sweet,caramel (unknown)

chocolate, malty (3-methylbutanal)caramel (2,3-butanedione)

sweet (unknown)caramel (d-pentolactone)coconut (d-octenolactone)

sweet, vanilla (vanillin)sweet, vanilla (unknown)

sweet, vanilla (unknown)malty (unknown)

sweet (benzyl alcohol)sweet (unknown)

chocolate (unknown)coffee, chocolate (2-methyl-3-(methyldithio)furan)peanut, green (2-methoxy-3-isopropyl pyrazine)dirt (2-ethyl-3,5-dimethylpyrazine)nutty, herbal (trimethylpyrazine)roasted nut (tetramethyl pyrazine)

Aroma profile comparison

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Vinegar/Wine

Chocolaty/Nutty

Sweet/Caramel

Mushroom/Spicy

Fatty/Cheesy

Floral/Herbal

Chocolate 1 Chocolate 2

Chocolaty/Nutty

Floral/Herbal

Mushroom

/Spicy

Fatty/Cheesy

Sweet/Caramel

Vinegar/Wine

Medicinal

Volatiles contributing most to milk chocolate aroma

• 3-methylbutanal (malty)• 2-ethyl-3,5-dimethylpyrazine (potato, green)• Z-2-nonenal (green, fatty)• acetic acid (vinegar)• methional (baked potato)• 2-methyl-3-(methyldithio)furan (coffee, chocolate)• hexanal (crushed leaves)• phenylacetaldehyde/butanoic acid (honey/cheesy, rotten)

• These volatiles could be now used (by going through the remaining steps of the process) to re-create chocolate aroma.

This has recently been achieved at FSA with kiwifruit flavour.

Bibliography

Belitz, H.D, Grosch, W. Food Chemistry, 2nd edition, Springer-Verlag, 1999.

Grosch, W. Evaluation of the key odorants of foods by dilution experiments, aroma models and omission. Chem. Senses 2001,26, 533-545.

Sides A, Robards K, Helliwell S. Developments in extraction techniques and their application to analysis of volatiles in foods. TiAC 2000,19, 322-329.

Pillonel L, Bosset J.O, Tabacchi R. Rapid pre-concentration and enrichment techniques for the analysis of food volatile: A review. Lebensm.-Wiss. u-Technol. 2002, 35, 1-14.

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