Food Biochemistry and Food Processing (2 edition)

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

1 Introduction to Food Biochemistry 5

metabolic intermediates). A glycosidic bond between a carbonyl
carbon and the nitrogen of an amine group (R–NH) is termed
anN-glycosidic bond. Similarly, reactions of carbonyl carbons
with thiols (R–SH) producethio-glycosides.

Food Disaccharides

The food carbohydrates sucrose, lactose and maltose (see struc-
tures below) are disaccharides (two monosaccharides joined by
anO-glycosydic bond) and are three of the principal disaccha-
rides used in the food industry. Used as a sweetening agent
and fermentation carbon source, sucrose exists naturally in high
amounts only in cane and beet, is composed of one glucose and
one fructose and is anon-reducing sugarsince it contains no free
aldehyde. The enzymes responsible for catalysing the hydrolysis
of sucrose to glucose and fructose are sucrases and invertases,
which catalyse the hydrolysis of the sucrose glycosidic bond.
Acid and heat also cause hydrolysis of sucrose, which is impor-
tant in the commercial production of invert sugar, where sucrose
is partially converted to glucose and fructose, thereby producing
increased sweetness and water-binding ability.
The disaccharide lactose is made up of galactose and glucose,
and is often referred to as milk sugar. Lactose is hydrolysed
by lactase (EC, an enzyme of theβ-galactosidase
enzyme class, produced by various mammals, bacteria and fungi.
By adulthood, some humans produce insufficient amounts of
lactase, thereby restricting the consumption of dairy product in
significant quantities. In deficient persons, a failure to hydrolyse
lactose in the upper intestine results in this simple sugar passing
into the large intestine, which in turn results in an influx of water
as well as fermentation by lower gut bacteria, leading to bloating,
cramping and diarrhoea. Lactose levels in dairy products can
be reduced by treatment with lactase or by lactic acid bacteria
fermentation. ‘Lactose-free’ milk products are widely sold and
marketed with most of the lactose hydrolysed by treatment with
lactase. Also, fermented dairy foods like yoghurt and cheese
contain less lactose compared to the starting materials where the
lactose is converted into lactic acid by bacteria, e.g. old cheddar
cheese contains virtually no remaining lactose.
Maltose is composed of two glucose units and is derived
from starch by treatment withβ-amylase, thereby increasing the
sweetness of the reaction mixture. The termmalt(beer making)
refers to the product whereβ-amylase, produced during the
germination, has acted on the starch of barley or other grains
when steeped in water.

Carbohydrate Browning Reactions

There are three general categories of browning reactions in
foods: oxidative/enzymatic browning,caramelisationand non-
oxidative/non-enzymatic/Maillard browning. Oxidative brown-
ing is discussed later in the section on proteins. The latter two
types of browning involve carbohydrate reactions. Caramelisa-
tion involves a complex group of reactions that are the result
of direct heating of carbohydrates, particularly sugars. Dehy-
dration reactions result in the formation of double bonds along
with the polymerisation of ring structures that absorb different
light wavelengths, hence the flavour development, darkening

and colour formation in such mixtures. Two important roles
of caramelisation in the food industry are caramel flavour and
colour production, a processes in which sucrose is heated in so-
lution with acid or acid ammonium salts to produce a variety of
products in food, candies and beverages (Ko et al. 2006).
The Maillard reaction is one of the most important reactions
encountered in food systems, and it is also called non-enzymatic
or non-oxidative browning. Reducing sugars and amino acids
or other nitrogen-containing compounds react to produceN-
glycosides displaying red-brown to very dark brown colours,
caramel-like aromas, and colloidal and insoluble melanoidins.
There are a complex array of possible reactions that can take
place via Maillard chemistry, and the aromas, flavours and
colours can be desirable or undesirable (BeMiller and Whistler
1996). Lysine is a nutritionally essential amino acid and its side
chain can react during the Maillard reaction, thereby lowering
the nutritional value of foods. Other amino acids that may be
lost due to the Maillard reaction include the basic amino acids
l-arginine andl-histidine.


Polysaccharides, or glycans, are made up of glycosyl units in a
linear or branched structure. The three major food-related gly-
cans are amylose, amylopectin and cellulose (cellulose is dis-
cussed below), which are all chains ofd-glucose, but are struc-
turally distinct based on the types of glycosidic linkages that join
the glucose units and the amount of branching in their respec-
tive structures. Both amylose and amylopectin are components
of starch, the energy storage molecules of plants, and cellulose
is the structural carbohydrate that provides structural rigidity to
plants. Starch is a critical nutritional component of many foods,
especially flour-based foods, tubers, cereal grains, corn and rice.
Starch can be both linear (amylose) and branched (amylopectin).
Amylose glucose units are joined only byα-1,4-linkages and
it usually contains 200–3000 units. Amylopectin also contains
α-1,4-linkages, but additionally it has branch points atα-1,
linkages that occur approximately every 20–30α-linkages. The
branched molecules of amylopectin produce bulkier structures
than amylose. Most starches contain approximately 25% amy-
lose, although amylose contents as high as 85% are possible.
Starches containing only amylopectin are termed waxy starches.
Starch exists ingranulesthat are deposited in organelles called
amyloplasts. Granule size and shape vary with plant source,
and they contain a cleft called the hilium, which serves as a
nucleation point around which the granule develops as part of
plant energy storage. Granules vary in size from 2 to 130μand
they have a crystalline structure such that the starch molecules
align radially within the crystals.

Metabolism of Carbohydrates

The characteristics of carbohydrates in both their natural states
and as processed food ingredients determine the properties of
many foods as well as their utilisation as nutrients.Glycolysis
is a fundamental pathway of metabolism consisting of a series
of reactions in the cytosol, where glucose is converted to pyru-
vate via nine enzymatically catalysed reactions (see Figure 1.1).
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