Prepared by Assoc Prof Darryl Small (v4 July 2012) questions/feedback to <darryl.small@rmit.edu.au>

Molecule of the week MSG, glutamic acid and glutamate ions

The glutamates are an interesting group of molecules

which are important as components of proteins as well

as through their influence on the taste of foods.

Glutamates occur in quite a number of forms and the

first important distinction is between free and bound

forms. Free glutamates are those that are not linked

covalently to any other molecule and these do occur

naturally to varying extents in foods. Bound glutamates

are those linked to one or more other molecules and

most of the bound form is present in the many different

proteins found in our foods [1, 2].

Glutamate occurs naturally in foods Free

forms occur in many common foods including

tomatoes, meats and some cheeses [3]. These cannot be

distinguished chemically from molecules that have

been incorporated as an additive.

Bound glutamate occurs in all of the many different

protein molecules in foods (and living cells). Here it

can be a large proportion of the amino acids. For

example in wheat flour it is approx 33% of the total

weight of the proteins [4]. Less well known is that all

active forms of folate also contain at least one

glutamate unit as part of their structure [1, 2].

Use as an additive Glutamate is approved as an

additive around the world and can be incorporated in

one of six forms [5]: MSG (E621) (monosodium

glutamate), monopotassium (E622), calcium (E623),

monoammonium (E624) or magnesium (E625) as well

as glutamic acid (E620). The function of added

glutamate is as a flavour enhancer – the taste provided

is known as umami [1-3]. Under the ANZFSC these

additives can only be added to particular foods and the

addition must be declared on the label [5] so that

consumers can make informed choices when selecting

foods to eat.

Functions and safety As well as being a

component of all proteins., glutamate functions in the

transmission of nerve impulses particularly in our

brains. Although some people may have adverse

reactions to glutamate, for most it is very safe [1-3].

Structure The two

diagrams represent

glutamic acid and are

different depictions of

the same molecule.

The shading in the

upper structure shows

the part of the

molecule which is

common to all amino

acids. This becomes

involved in the peptide

bonds when proteins

are biosynthesised.

The lower depiction

attempts to show the

actual 3D arrangement

H2N CH C

CH2

OH

O

CH2

C

OH

O

NH2

O

OH

O

HO

of bonds around the carbon atoms, while omitting the

hydrogens that are present.

Forms of glutamate in food In reality, in most

foods glutamate does not occur in the acid form shown.

Regardless of the source or form of the molecule added,

it is the acidity (pH) of the food which determines the

extent of ionisation. This is shown in the equilibrium

reactions below. The pKa values indicate the pH at

which half of the molecules in the equilibrium are in

each of the two forms shown.

As an example of how this works, in very acidic

conditions the equlibria will shift strongly towards the

left. From this we can calculate that glutamate has no

overall charge at pH 3.32 (its isoelectric point, pI).

Therefore, as with many amino acids glutamate often

occurs as the double ion which as referred to as a

zwitterion.

References and further reading

[1] Fennema and [2] Coultate both have sections on

folates, amino acids and MSG as a flavour enhancer.

[3] Food Reviews International 18(1):23-38.

[4] NUTTAB has amino acid data.

[5] ANZFSC standards 1.2.4 and 1.3.1.

H3+N CH C

CH2

OH

O

CH2

C

OH

O

pKa 2.23

H3+N CH C

CH2

O-

O

CH2

C

OH

O

pKa 4.42

H3+N CH C

CH2

O-

O

CH2

C

O-

O

pKa 9.95

H2N CH C

CH2

O-

O

CH2

C

O-

O

+ H

+

+ 2 H

+

+ 3 H

+