Food Biochemistry and Food Processing

78 Part I: Principles

O 2 (Soliva-Fortuny et al. 2001) seem to be adequate
approaches to improving the shelf life of foods by
inhibition of PPO.
Within the emergent tools for the control of PPO,
the application of genetic techniques should be men-
tioned. Transgenic fruits carrying an antisense PPO
gene show a reduction in the amount and activity of
PPO, and the browning potential of transgenic lines
is reduced compared with the nontransgenic ones
(Murata et al. 2000, 2001). These procedures may
be used to prevent enzymatic browning in a wide
variety of food crops without the application of var-
ious food additives (Coetzer et al. 2001).

NONENZYMATIC BROWNING

THEMAILLARDREACTION

Nonenzymatic browning is the most complex reac-
tion in food chemistry due to the large number of
food components able to participate in the reaction

through different pathways, giving rise to a complex

mixture of products (Olano and Martínez-Castro

1996). It is referred to as the Maillard reaction when

it takes place between free amino groups from amino

acids, peptides, or proteins and the carbonyl group

of a reducing sugar.

The Maillard reaction is one of the main reactions

causing deterioration of proteins during processing

and storage of foods. This reaction can promote

nutritional changes such as loss of nutritional qual-

ity (attributed to the destruction of essential amino

acids) or reduction of protein digestibility and amino

acid availability (Malec et al. 2002).

The Maillard reaction covers a whole range of

complex transformations (Fig. 4.4) that lead to the

formation of numerous volatile and nonvolatile

compounds. It can be divided into three major phas-

es, the early, intermediate, and advanced stages. The

early stage (Fig. 4.5) consists of the condensation of

primary amino groups of amino acids, peptides, or

Figure 4.4.Scheme of different stages of Maillard reaction (Hodge 1953, Ames 1990).