Physical Chemistry of Foods

2. The formation of more or less ordered macroscopic structures,
   especially gels. This is governed by the tendency of many proteins to
   aggregate or to form intermolecular cross-links on heating, change of pH,
   etc. Gels are discussed in Section 17.2.
   In the present chapter, protein solubility, conformation, and
   conformational stability are the main subjects. See Chapter 8 for water
   relations.

7.1 DESCRIPTION

The chemistry of proteins is covered in most texts on food chemistry and, of
course, on biochemistry. For the convenience of the reader, a brief review is
given in this section.

7.1.1 Amino Acids

Proteins are linear polymers ofa
L amino acids linked by the formation of
peptide bonds:

NH 222 CHR 22 COOHþNH 222 CHR* 22 COOH?

NH 222 CHR 22 C0 22 NH 22 CHR* 22 COOHþH 2 O

The degree of polymerizationnranges from 50 to several 100. R denotes a
side group; basically 20 different ones exist in nature and are given in Table
7.1, together with some properties. According to the side group, they can be
categorized as

Aliphatic: Ala, Val, Leu, Ile

Aromatic: Phe, Tyr, Trp, His

Charged: Asp
;Glu
;Lysþ;Argþ;Hisþ;ðCys
Þ

Somewhat polar, uncharged: Asn, Gln, Ser, Thr, (Tyr)

Sulfur containing: Cys, Met

This leaves Gly, which has a mere 22 H as a side group, and proline, which is
not a primary amino acid.

Note Proline is commonly called an imino acid, although it does

not contain an imino group, C 55 NH, but a secondary amino group,

C 22 NH 22 C.

Note that some belong to more than one category.