Food Chemistry

1.2 Amino Acids 11

Aspartic Acidwas isolated from legumes byRitt-
hausenin 1868. It occurs in all animal proteins,
primarily in albumins at a concentration of 6–
10%. Alfalfa and corn proteins are rich in aspar-
tic acid (14.9% and 12.3%, respectively) while its
content in wheat is low (3.8%). Aspartic acid is
nonessential.

Cystine was isolated from bladder calculi by
Wolastonin 1810 and from horns byMoerner
in 1899. Its content is high in keratins (9%).
Cystine is very important since the peptide
chains of many proteins are connected by
two cysteine residues, i. e. by disulfide bonds.
A certain conformation may be fixed within
a single peptide chain by disulfide bonds. Most
proteins contain 1–2% cystine. Although it
is itself nonessential, cystine can partly re-
place methionine which is an essential amino
acid.

Glutamine was first isolated from sugar beet
juice by Schulze and Bosshard in 1883. Its
occurrence in protein (edestin) was confirmed
by Damodaranin 1932. Glutamine is readily
converted into pyrrolidone carboxylic acid,
which is stable between pH 2.2and4.0,
but is readily cleaved to glutamic acid at
other pH’s:

(1.1)

Glutamic Acid was first isolated from wheat
gluten by Ritthausen in 1866. It is abundant
in most proteins, but is particularly high
in milk proteins (21.7%), wheat (31.4%),
corn (18.4%) and soya (18.5%). Molasses
also contains relatively high amounts of glu-
tamic acid. Monosodium glutamate is used
in numerous food products as a flavor en-
hancer.

Glycineis found in high amounts in structural
protein. Collagen contains 25–30% glycine. It
was first isolated from gelatin by Braconnot
in 1820. Glycine is a nonessential amino acid
although it does act as a precursor of many
compounds formed by various biosynthetic
mechanisms.

Histidinewas first isolated in 1896 independently

byKosseland byHedinfrom protamines occur-

ring in fish. Most proteins contain 2–3% histidine.

Blood proteins contain about 6%. Histidine is es-

sential in infant nutrition.

5-Hydroxylysine was isolated by van Slyke

et al.(1921) andSchryver et al.(1925). It occurs

in collagen. The carbohydrate component of gly-

coproteins may be bound O-glycosidically to the

hydroxyl group of the amino acid (cf. 12.3.2.3.1).

4-Hydroxyprolinewas first obtained from gelatin

byFischerin 1902. Since it is abundant in col-

lagen (12.4%), the determination of hydroxypro-

line is used to detect the presence of connec-

tive tissue in comminuted meat products. Hydrox-

yproline is a nonessential amino acid.

Isoleucine was first isolated from fibrin by

Ehrlichin 1904. It is an essential amino acid.

Meat and ceral proteins contain 4–5% isoleucine;

egg and milk proteins, 6–7%.

Leucinewas isolated from wool and from mus-

cle tissue byBraconnotin 1820. It is an essen-

tial amino acid and its content in most proteins is

7–10%. Cereal proteins contain variable amounts

(corn 12.7%, wheat 6.9%). During alcoholic fer-

mentation, fusel oil is formed from leucine and

isoleucine.

Lysinewas isolated from casein by Drechsel

in 1889. It makes up 7–9% of meat, egg and

milk proteins. The content of this essential amino

acid is 2–4% lower in cereal proteins in which

prolamin is predominant. Crab and fish proteins

are the richest sources (10–11%). Along with

threonine and methionine, lysine is a limiting

factor in the biological value of many proteins,

mostly those of plant origin. The processing of

foods results in losses of lysine since itsε-amino

group is very reactive (cf.Maillardreaction).

Methioninewas first isolated from casein by

Muellerin 1922. Animal proteins contain 2–4%

and plant proteins contain 1–2% methionine.

Methionine is an essential amino acid and in

many biochemical processes its main role is as

a methyl-donor. It is very sensitive to oxygen

and heat treatment. Thus, losses occur in many

food processing operations such as drying,

kiln-drying, puffing, roasting or treatment with

oxidizing agents. In the bleaching of flour