Food Biochemistry and Food Processing (2 edition)

BLBS102-c01 BLBS102-Simpson March 21, 2012 11:8 Trim: 276mm X 219mm Printer Name: Yet to Come

1 Introduction to Food Biochemistry 9

FOOD PROTEIN BIOCHEMISTRY

Properties of Amino Acids

Proteins are polymers ofamino acidsjoined bypeptide bonds.

Twenty amino acids commonly exist and their possible combi-

nations result in the potential for an incredibly large number of

sequence and 3D structural protein variants. Amino acids con-

sist of a carbon atom (Cα) that is covalently bonded to an amino

group and a carboxylic acid group. Thus, they have an N–Cα–C

‘backbone’. In addition, the Cαis bound to a hydrogen atom and

one of 20 ‘R groups’ or ‘side chains’, hence the general formula:

+NH 3 −CHR−COO−

The above general formula describes 19 of the 20 amino

acids except proline, whose side chain is irregular, in that it

is covalently bound to both theα-carbon and the backbone

nitrogen. The covalent bonds among different amino acids in

a protein are called peptide bonds. The 20 amino acids can

be divided into three categories based on R-group differences:

non-polar, polar and charged polar. The functional properties of

food proteins are directly attributable to the amino acid R-group

properties: structural (size, shape and flexibility), ionic

(charge and acid–base character) and polarity (hydrophobicity/

hydrophilicity).

At neutral pH, most free amino acids are zwitterionic, i.e. they

are dipolar ions, carrying both a positive and negative charge, as

shown in the general formula above. Since the primary amino

and carboxyl groups of amino acids are involved in peptide

bonds within a protein, it is only the R groups (and the ends

of the peptide chains) that contribute to charge; the charge of a

protein being determined by the charge states of the ionisable

amino acid R groups that make up the polypeptide, namely

aspartic acid (Asp), glutamic acid (Glu), histidine (His), lysine

(Lys), arginine (Arg), cysteine (Cys) and tyrosine (Tyr). The

acidic amino acids are Tyr, Cys, Asp and Glu (note: Tyr and Cys

require pH above physiologic pH to act as acids, and therefore,

are less important charge contributors in living systems). Lys,

Arg and His are basic amino acids.

The amino acid sequence and properties determine overall

protein structure. Some examples are as follows: Two residues

of opposite charges can form a salt bridge. For example, Lys and

Asp typically have opposite charges under the same conditions,

and if the side chains are proximate, then the negatively charged

carboxylate of Asp can salt-bridge to the positively charged

ammonium of Lys. Another important inter-residue interaction

is covalent bonding between Cys side chains. Under oxidising

conditions, the sulfhydryl groups of Cys side chains (–S–H) can

form a thiol covalent bond (–S–S–) also known as adisulphide

bond. Lastly, hydrophobic (non-polar) amino acids are generally

sequestered away from the solvent in aqueous solutions (which

is not always the case for food products), since interaction with

polar molecules is not energetically stable.

Protein Nutritional Considerations

In terms of survival and good health, the contributions of protein

in the diet are to provide adequate levels of what are referred to

as ‘essential amino acids’ (Lys, methionine, phenylalanine, thre-

onine, tryptophan, valine, leucine and isoleucine), amino acids

that are either not produced in sufficient quantities or not at all

by the body to support building/repairing and maintaining tis-

sues as well as protein synthesis. Single source plant proteins are

referred to as incomplete proteins since they do not have suffi-

cient quantities of the essential amino acids in contrast to animal

proteins, which are complete. For example, cereals are deficient

in Lys, while oilseeds and nuts are deficient in Lys as well as

methionine. In order for plant proteins to become ‘complete’,

complementary sources of proteins must be consumed, i.e. the

deficiency of one source is complemented by an excess from

another source, thus making the combined protein ‘complete’.

Although some amino acids aregluconeogenic, meaning that

they can be converted to glucose, proteins are not a critical

source of energy. Dietary protein breakdown begins with cook-

ing (heat energy) and chewing (mechanical energy) followed by

acid treatment in the stomach (chemical energy) as well as the

mechanical actions of the upper GI. The 3D structures of pro-

teins are partially lost due to such forces and are said to ‘unfold’

or denature.

In addition to protein denaturation, the stomach and upper

intestine produce two types ofproteases(enzymes that hydrol-

yse peptide bonds) that act on dietary proteins.Endopeptidases

are proteases that cleave interior peptide bonds of polypeptide

chains, whileexopeptidasesare proteases that cleave at the ends

of proteins exclusively. Pepsin, an acid protease that functions

optimally at extremely low pH of the stomach, releases pep-

tides from muscle and collagen proteins. In the upper intestine,

serine proteases trypsin and chymotrypsin further digest pep-

tides, yielding free amino acids for absorption into the blood

(Champe et al. 2005). An important consideration regarding the

nutritional quality of proteins is the effect of processing. Heat

and chemical treatments can serve to unfold proteins, thereby

aiding to increase enzymatic hydrolysis, i.e. unfolded proteins

have a larger surface area for enzymes to act. This may increase

the amino acid bioavailability, but it can also lead to degrada-

tive/transformative reactions of amino acids, e.g. deamidation of

asparagine and glutamine, reducing these amino acids as nutrient

sources.

Animal Protein Structure and Proteolysis in

Food Systems

Animal tissues have similar structures despite minor differ-

ences between land and aquatic (fish and shellfish) animal

tissues. Post-mortem, meat structure breaks down slowly, result-

ing in desirable tenderisation and eventual undesirable degrada-

tion/spoilage. Understanding meat structure is critical to un-

derstanding these processes, and Table 1.4 lists the location

and major functions of myofibrillar proteins associated with

the contractile apparatus and cytoskeletal framework of animal

tissues. Individual muscle fibres are composed of myofibrils,

which are the basic units of muscular contraction. The skeletal

muscle of fish differs from that of mammals, in that the fi-

bres arranged between the sheets of connective tissue are much

shorter. The connective tissue appears as short, transverse sheets