Chemistry and Physics of Vanillin

Chaim Frenkel, ProfessorDepartment of Plant Biology and Pathology Rutgers-the State University of New JerseyNew Brunswick, NJ 08901-8520

(732) 932-9711 x 365(732) 932-9441 fax

<frenkel@aesop.rutgers.edu>

Cross section of vanilla bean showing a dark central cavity(seeds) surrounded by vanillin forming cells.

C H

OH

OCH3

O

C H

OH

OCH3

O

Hydrophobicity Hydrogen bonds (intra and intermolecular) of the aromatic ring

C H

OH

OCH3

O

Reactivity of a carbonyl group

(Maillard Reaction)

Molecular features of vanillin, including hydrophobicity, efficacy for forming hydrogen bonds and reactive carbonyl group, which could influence the fate of vanillin during various handling stages.

π- π stacking of aromatic rings. This phenomenon, stemming from the hydrophobicity of aromatic compounds contributes to the tendency of vanillin to form aggregates.

C

O

OO

C

O

H

OH

O

CH3

CH3

C

O

H

OH

O

CH3

H

H

* Adapted from: Aihara A (1973) A study of hydrogen bonds in Vanillin I.

Denki Tsaushin Daigaku 24:71-75

Inter and Intra Hydrogen Bonds in Crystalline Vanillin*

0 10 20 30 40 50

80O C

50O C

60O C

40O C

20O C100

80

60

40

20

0

Perc

ent V

anilli

n

Days

Rate of disappearance of dry vanillin held at atmospheric pressure and at different temperatures.

Time (days)

Perc

ent V

anilli

n

40O C

60O C

0 2 4 6 8 10 12 14

60O C

0O C, 20O C

40O C

50O C

80O C

100

80

60

40

20

0

Rate of disappearance of dry vanillin held at 0.1 atmospheric pressure and at different temperatures.

Perc

ent V

anilli

n

Temperature (OC)

100

80

60

40

20

020 30 40 50 60 70 80

0.1 atmospheric pressure

1.0 atmospheric pressure

Disappearance of vanillin held at 1.0 or 0.1 atmospheric pressure.

Vanillin was held for 18 and 4 days at 1.0 or 0.1 atmospheric pressure,respectively. The data shows that vanillin disappearance is greatly accelerated above 40o C.

0 10 20 30 40 50 600

2

4

6

8

10vanillin

methanol

Temperature (OC)

µ M v

anilli

n

ml M

etha

nol a

fter 2

4 ho

urs

290

280

270

260

250

Vanillin content solubilized in 100% methanol and held for 24 hours at different temperatures. The data shows that while methanol is volatilizedvanillin remains in solution.

0 10 20 30 40 50 60 70 80

0

2

4

6

8

10310

270

230

190

150

Temperature (OC)

Vani

llin C

onte

nt (µ

M)

vanillin

water

ml W

ater

Vanillin content solubilized in water and held for 48 hours at different temperatures. The data showsThat disappearance of vanillin is correlated to the rate of water disappearance.

0 10 20 30 40 50 60 700

2

4

6

8

10vanillin

methanol (40%)

µ M V

anilli

n

Rem

aini

ng V

olum

e of

Liq

uid

(40%

met

hano

l)

Temperature (OC)

340

320

310

300

Vanillin content solubilized in 40% methanol-water solution and held for 2 days at different temperatures.The data shows that presence of water caused some disappearance of vanillin.

C

O

OH

O

CH3

H

C

O

OO

C

O

H

OH

OCH3

CH3

H

H

WATER

C

O

OH

O

CH3

H

HEAT

C

O

OH

O

CH3

H

A hypothetical scheme indicating that hydration of vanillin,due to vanillin-water hydrogen bonding, leads to the volatilization of vanillin-water complex.

0 1000 2000 3000 4000 5000 60003

4

5

6

7

8

9

10

11

pH

µ mole KOH or NaOH

KOHNaOH

Titration curve of vanillin in water. The data shows that native vanillin is acidic and, moreover, exhibits a substantial buffering capacity.

0 10 20 30 40 50 60 70 80

0

2

4

6

8

10

watervanillin pH 8.0vanillin pH 6.5

3.0

2.6

2.2

1.8

1.4

Van

illin

Con

tent

(µ m

ole)

Temperature (OC)

ml W

ater

Vanillin content in water, adjusted to pH 6.5 or 8.0 and held for 48 hours at different temperatures. Disappearance of vanillin is function of solubility (greater at pH 8.0 that 6.5, above 40o C).

0 10 20 30 40 50 60 700

2

4

6

8

10

Vani

llin C

onte

nt (µ

mol

e)

Temperature (OC)

3.4

3.0

2.6

2.2 watervanillin pH 2.0vanillin pH 3.0

ml

Wat

er

Vanillin content in water, adjusted to pH 3.0 or 2.0 and held for 48 hours at different temperatures. Disappearance of vanillin is function of solubility (greater at pH 3.0 that 2.0, above 30o C).Lower levels of vanillin at pH 3.0, below 30o C, reflect precipitation of the compound in the aggregate state.

A schematic view of water structure showing a tight inter-molecular arrangement created by hydrogen bonding of neighboring water molecules.

Solute cavity in water showing disengagement of waterstructure to accommodate a guest molecule (middle) *

* Martínez JM, Pappalardo RR., Marcos ES, Mennucci B, Tomasi J (2002) Analysis of the Opposite Solvent Effects Caused by Different Solute Cavities on the Metal-Water Distance of Monoatomic Cation Hydrates. J Phys Chem B 106:1118-1123

Maillard Reaction

Inhibitors Promoters

F¯ Cl¯ Br¯ I¯ ClO4¯ SCN¯

Li+ Na+ K+ Rb+ Cs+ NH4+

Water Structure Water Structure Breakers Makers (Chaotropes)(Kosmotropes)

Some ions, which enhance or diminish the structure of water. Water structure-breaking ions are presumed to enhance the solubility and, thereby, the chemical reactivity of vanillin.

C=O + H2NR

HO C N R

Carbinolamine Schiff's base

C=O + H2NR

H

C OH

C H

H

HO

C OH

C H

H

HO

HO C N RH

C OH

C H

H

HO

C N R + HOH H

H

H

C N R + HOH

Carbinolamine Schiff's base Aldose

Schiff's base formation

C OH

C H

H

HO

C N R H

C OH

C HHO

H C N R

H

C O

C HHO

H C N R

HH

Schiff's base 1-amino-2-keto sugar

Amadori Rearrangement

C N R + HOH + HC O RNH3

Sciff's base Hydrolysis

In acid medium Schiff's base will undergo hydrolysis

+

H

A generalized scheme of Maillard Reaction showing Schiff base formation followed by Amadori Rearrangement, leading to further degradation including formation of brown color.

0 20 40 60 80 1000.1

0.2

0.3

0.4

0.5

OD

420

nm

Concentration of CsCl or LiCl (mM)

CsCl

LiCl

Rate of Maillard reaction occurring between vanillin and lysine in presence of varying concentrations of water structure-breaking ions (CsCl) or water structure-maker ion (LiCl). Solutions containing 5 mM vanillin and lysine adjusted to pH 6.5, held at 65o C for 2 days. Brown pigment formation estimated by spectroscopy.

Visualization of Maillard reaction for vanillin and lysine occurring in absence or presence of varying concentrations of water structure-breaking ions (CsCl) or water structure-maker ion (LiCl).

Summary

Conditions, which increase the water solubility of vanillin (alkalinity or chaotropic agents) enhance water co-volatility and chemical reactivity of the compound.