Food Biochemistry and Food Processing (2 edition)

BLBS102-c28 BLBS102-Simpson March 21, 2012 13:54 Trim: 276mm X 219mm Printer Name: Yet to Come

28 Biochemistry of Fruit Processing 559

nonenzymatic Maillard-type browning reactions by reacting

with aldehyde groups of sugars so that they are no longer free to

combine with amino acids. Inhibition of browning is especially

important in dried fruits such as apples, apricots, and pears.

Sulfite also exhibits antimicrobial properties. The disadvantage

of sulfite is that some people are allergic to this chemical, which

makes its use restricted, for example, FDA prohibited the use

of sulfite in fresh produce and limited the residual in processed

products to 10 ppm, with appropriate label.

Sugar syrup has long been used to minimize oxidation before

the mechanisms of browning reactions were understood, and

still remains as a common practice today. Application of sugar

syrup provides a coating to the fruit and thus prevents the contact

of the cut surface and oxidizable components from contact with

atmospheric oxygen. Sugar syrup also increases the sensory at-

tributes of fruits, by reducing the loss of volatiles and improving

the taste.

Vacuum treatment is used in combination with the chemical

dips in order to improve their effects. While fruits are submerged

in the dip, vacuum is applied to draw air from the fruit tissue,

allowing better penetration.

In order to reduce the cost of handling and shipping, some

high-moisture fruits are pureed and concentrated to two or three

times their natural solids content. Many others are dried for

various purposes to different moisture levels. The majority of

fruit including apricot, apple, figs, pears, prunes, and raisins are

sun dried. Sulfite is commonly used to preserve the color when

fruits are dried under high temperature that does not inactivate

the oxidative enzymes.

Nonenzymatic Browning

Also called Maillard reaction, the nonenzymatic browning is of

great importance in fruit processing. During Maillard reaction,

the amino groups of amino acids, peptides, or proteins, react with

aldehyde groups of sugars resulting in the formation of brown

nitrogenous polymers called melanoidins (Ellis 1959, deMan

1999). The velocity and pattern of the reaction depend on the

nature of the reacting compounds, the pH of the medium, and

the temperature. Each kind of food shows a different browning

pattern. Lysine 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 involved in the Maillard reaction are:

1. An aldose or ketose reacts with a primary amino group of
   amino acid, peptide or protein, to produce anN-substituted
   glycosylamine;


2. The Amadori reaction, which rearranges the glycosy-
   lamine to yield ketosamine or aldoseamine;


3. Rearrangement of the ketosamine with a second molecule
   of aldose to yield diketosamine, whereas the rearrange-
   ment of the aldoseamine with a second molecule of amino
   acid leads to a diamino sugar;


4. Amino sugars are degraded by losing water to give rise to
   amino or nonamino compounds;


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

In Maillard reaction, the basic amino group is the reactive

component and therefore, the browning is dependent on the

initial pH, or the presence of a buffer system. Low pH results

in the protonation of the basic amino group, and therefore it

inhibits the reaction. The effect of pH is also heavily dependent

on the moisture content of the product. When the moisture is

high, the browning is caused by caramelization, whereas at low

water content and pH of about 6, the Maillard reaction prevails.

The flavors produced by the Maillard reaction in some cases are

reminiscent of caramelization.

The nonenzymic browning can be prevented by closely mon-

itoring the factors leading to its occurrence, including temper-

ature, pH, and moisture content. The use of inhibitors such as

sulfite is an effective way of controlling nonenzymic brown-

ing. It is believed that sulfite reacts with the degradation prod-

ucts of the amino sugars to prevent the condensation of these

products into melanoidins. However, sulfite can react with thi-

amine, which prohibits its use in foods containing this vitamin

(Figs. 28.1 and 28.2).

Fruit Juice Processing

The quality of the juice depends on the quality of the raw ma-

terial, regardless of the process. Often, the quality of the fruit

is dependent on the stage of maturity or the stage of ripening.

The major physicochemical parameters used in assessing fruit

ripening are sugar content, acidity, starch content, and firmness

(Somogyi 1996a, 1996b).

The main steps involved in the processing of most type of

juice include the extraction of the juice, clarification, juice de-

aeration, pasteurization, concentration, essence add-back, can-

ning or bottling, and freezing (less frequent). Juice extractors

for oranges and grapefruits, whose peel contain bitter oils, are

designed to cause the peel oil to run down the outside of the

fruit and not enter the juice stream. Since apples do not contain

bitter oil, the whole apples are pressed. Juice extraction should

be done as quickly as possible, in order to minimize oxidation of

phenolic compounds in fruit juice by naturally present enzymes.

Apple Juice Processing

Apple juice is processed and sold in many forms including

American apple cider, European apple cider, and shelf-stable

apple juice. American apple cider is the product of sound, ripe

fruit that has been pressed and bottled or packaged with no

form of preservatives added and stored under refrigeration. This

is a sweet type of cider, which has not been fermented. Be-

cause of an increased number of food-borne outbreaks ofE. coli

0157:H7 andCryptosporidium parvumin apple cider, many ju-

risdictions are working toward implementation of a kill step in

the processing of apple cider. Several pathogen outbreaks make

unpasteurized apple cider not suitable for general consumption,

and especially for children, elderly, and immunocompromised

people.

European type of apple cider is a naturally fermented ap-

ple juice, usually fermented to a specific gravity of one or less

(Anonymous 1980). Shelf-stable apple juice includes clarified