Food Biochemistry and Food Processing (2 edition)

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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 3.2.1.108), 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.

Starch

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).