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

14 Part 1: Principles/Food Analysis

Table 1.9.Secondary Amine Production in Seafood

Enzyme Reaction

Histidine decarboxyalse (EC 4.1.1.22) l-Histidine→Histamine+CO 2

Lysine decarboxylase (EC 4.1.1.18) l-Lysine→Cadaverine+CO 2

Ornithine decarboxylase (EC 4.1.1.17) l-Ornithine→Putrescine+CO 2

Source: Gopakumar 2000, IUBMB-NC website (www.iubmb.org).

Fruit Ripening

Ethylene, a compound produced as a result of fruit ripening, acts

as an initiator and accelerator of fruit ripening. Its concentration

is low in green fruits, but can accumulate inside the fruit and

subsequently activate its own production (positive feedback).

Table 1.10 lists enzymes in the production of ethylene starting

from methionine. Ethylene is commonly used to intentionally

ripen fruit. During shipping of green bananas, ethylene is re-

moved through absorption by potassium permanganate to render

a longer shelf life.

Analytical Protein Biochemistry

The ability to isolate food proteins from complex materi-

als/matrices is critical to their biochemical characterisation. Dif-

fering biochemical characteristics such as charge, isoelectric

point, mass, molecular shape and size, hydrophobicity, ligand

affinity and enzymatic activity all offer opportunities for sepa-

ration. Charge and size/mass are commonly exploited parame-

ters used to separate different proteins using ion exchange and

size exclusion chromatography, respectively. In ion exchange

chromatography, proteins within a sample are bound to a sta-

tionary phase having an opposite charge (e.g. anionic proteins

are bound to a cationic column matrix). A gradient of compet-

ing ions is then passed through the column matrix such that

weakly bound proteins will elute at low gradient concentration

and thus are separated from strongly bound proteins. In the case

of size-exclusion chromatography, protein mixtures are passed

through an inert stationary phase containing beads having pores

of known size. Thus, larger molecules will take a more direct

(and hence faster) path relative to smaller molecules, which can

fit into the beads, resulting in a more ‘meandering’, longer path,

and therefore, elute slower (in comparison to larger molecules).

Perhaps the most basic biochemical analysis is the determi-

nation of molecular mass. Polyacrylamide gel electrophoresis

(PAGE) in the presence of a detergent and a reducing agent is

typically used for this purpose (mass spectrometry is used for

precise measurement where possible). Detergent (sodium dode-

cyl sulphate; SDS) is incorporated to negate charge effects and

to ensure all proteins are completely unfolded, thereby leaving

only mass as the sole determinant for rate of travel through the

gel and hence the term SDS-PAGE. Proteins denatured with

SDS have a negative charge and, therefore, will migrate through

the gel in an electric field (see Figure 1.4). A standard curve

for proteins with known molecular masses consisting of rela-

tive mobility versus log [molecular mass] can be generated to

calculate masses of unknowns run on the same gel.

Food Allergenicity

The primary role of the immune system is to distinguish be-

tween self and foreign biomolecules in order to defend the host

against invading organisms. Antibody proteins that are produced

in response to the foreign compounds are specifically referred to

as immunoglobulins (Ig), where five classes exist, which share

common structural motifs: IgG, IgA, IgM, IgD and IgE. IgE, the

least abundant class of antibodies, are the immunogenic proteins

important to protection against parasites as well as the causative

proteins in allergic reactions (Berg 2002).

Allergic diseases, particularly in industrialised countries,

have significantly increased in the last two decades (Mine and

Yang 2007). In the United States, food-induced allergies oc-

cur in an estimated 6% of young children and 3–4% of adults

(Sicherer and Sampson 2006). The most common causes of

food allergic reactions for the young are cow’s milk and egg,

whereas adults are more likely to develop sensitivity to shellfish

Table 1.10.Ethylene Biosynthesis

Enzyme Reaction

Methionine adenosyltransferase (EC 2.5.1.6) l-Methionine+ATP+H 2 O→S-adenosyl-γ-methionine+

Diphosphate+Pi

Aminocyclopropane carboxylate synthetase

(EC 4.4.1.14)

S-adenosyl-γ-methionine→1-Aminocyclopropane-1-carboxylate+

5 ′-Methylthio-adenosine

Aminocyclopropane carboxylate oxidase

(EC 4.14.17.4)

1-Aminocyclopropane-1-carboxylate+Ascorbate+^12 O 2 →Ethylene

+Dedroascorbate+CO 2 +HCN+H 2 O

Source: Eskin 1990, Bryce and Hill 1999, Crozier et al. 2000, Dangl et al. 2000, IUBMB-NC website (www.iubmb.org).