Food Biochemistry and Food Processing

522 Part V: Fruits, Vegetables, and Cereals

the pH of the medium, and the temperature. Each
kind of food shows a different browning pattern. Ly-
sine is the most reactive amino acid because it con-
tains a free amino group. The destruction of lysine
reduces the nutritional value of a food since it is a
limiting essential amino acid. The major steps in-
volved in the Maillard reaction are as follows:

• An aldose or ketose reacts with a primary amino
  group of an amino acid, peptide, or protein to
  produce an N-substituted glycosylamine;


• The Amadori reaction, which rearranges the gly-
  cosylamine to yield ketoseamine or aldoseamine;


• Rearrangement of the ketoseamine with a second
  molecule of aldose to yield diketoseamine, or the
  rearrangement of the aldoseamine with a second
  molecule of amino acid to yield a diamino sugar;


• Amino sugars are degraded by losing water to
  give rise to amino or nonamino compounds; and


• The condensation of the obtained products with
  amino acid or with each other.

In the Maillard reaction, the basic amino group is
the reactive component, and therefore the browning
is dependant on initial pH, or the presence of a buf-

fer system. Low pH results in the protonation of the

basic amino group; it therefore inhibits the reaction.

The effect of pH is also heavily dependent on the

moisture content of the product. When the moisture is

high, browning is caused by caramelization, whereas

at low water content and pH of about 6, the Maillard

reaction prevails. The flavors produced by the Mail-

lard reaction in some cases are reminiscent of car-

amelization.

The nonenzymic browning can be prevented by

closely monitoring the factors leading to its occur-

rence, including temperature, pH, and moisture con-

tent. The use of inhibitors such as sulphite is an ef-

fective way of controlling nonenzymic browning. It

is believed that sulphite reacts with the degradation

products of the amino sugars to prevent the conden-

sation of these products into melanoidins. However,

sulphite can react with thiamine, which prohibits its

use in foods containing this vitamin.

FRUITJUICEPROCESSING

The quality of the juice depends on the quality of the

raw material, regardless of the process. Often the

Figure 22.1.Mechanism of browning reaction.