Food Chemistry

70 1 Amino Acids, Peptides, Proteins

1.4.4.8 Tryptophan Residue

N-Bromosuccinimide oxidizes the tryptophan
side chain and also tyrosine, histidine and
cysteine:

(1.139)

The reaction is used for the selective cleavage of
peptide chains and the spectrophotometric deter-
mination of tryptophan.

1.4.4.9 TyrosineResidue........................................

Selective acylation of tyrosine can occur with
acetylimidazole as a reagent:

(1.140)

Diazotized arsanilic acid reacts with tyrosine and
with histidine, lysine, tryptophan and arginine:

(1.141)

Tetranitromethane introduces a nitro group into
the ortho position:

(1.142)

1.4.4.10 BifunctionalReagents

Bifunctional reagents enable intra- and inter-

molecular cross-linking of proteins. Examples are

bifunctional imidoester, fluoronitrobenzene, iso-

cyanate derivatives and maleic acid imides:

(1.143)

(1.144)

(1.145)

(1.146)

1.4.4.11 ReactionsInvolvedinFoodProcessing

The nature and extent of the chemical changes in-

duced in proteins by food processing depend on

a number of parameters, for example, composi-

tion of the food and processing conditions, such

as temperature, pH or the presence of oxygen. As

a consequence of these reactions, the biological

value of proteins may be decreased:

• Destruction of essential amino acids


• Conversion of essential amino acids into
  derivatives which are not metabolizable


• Decrease in the digestibility of protein as a re-
  sult of intra- or interchain cross-linking.
  Formation of toxic degradation products is also
  possible. The nutritional/physiological and toxi-
  cological assessment of changes induced by pro-
  cessing of food is a subject of some controversy
  and opposing opinions.
  TheMaillardreaction of theε-amino group of
  lysine prevails in the presence of reducing sugars,