Food Biochemistry and Food Processing (2 edition)

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

6 Part 1: Principles/Food Analysis

Glucose

Glucose 6-P

Fructose 6-P

Dihydroxyacetone-P

Fructose 1,6-Bis-P

Glyceraldehyde 3-P

1,3-Bis-phosphoglycerate

3-Phosphoglycerate

2-Phosphoglycerate

Phosphoenolpyruvate

Pyruvate If anaerobic Lactate

If aerobic

[TCA cycle]

Figure 1.1.Outline of glycolysis; 3-C reactions beginning with

glyceraldehyde 3-P occur twice per glucose.

Glycolytic processing of each glucose molecule results in a mod-

est gain of only two ATPs (the universal biochemical energy

currency). Although the gain of two ATPs is small, the creation

of pyruvate feeds another metabolic pathway, the tricarboxylic

acid (TCA) cycle, which yields two more ATPs. More impor-

tantly, glycolysis and the TCA cycle also generate the reduced

forms of nicotinamide adenine dinucleotide (NADH) and flavin

adenine dinucleotide (FADH 2 ), which drive subsequent oxida-

tive phosphorylation of ADP. An overview of the TCA cycle is

depicted in Figure 1.2. NADH and FADH 2 generated by gly-

colysis and the TCA cycle are subsequently part of oxidative

phosphorylation, where they transfer their electrons to O 2 in a

series of electron transfer reactions whose high energy potential

is used to drive phosphorylation. Overall, a net yield of 30 ATPs

is gained per glucose molecule as the result of glycolysis, the

TCA cycle and oxidative phosphorylation.

Pyruvate (from glycolysis)

Acetyl CoA

Oxaloacetate Citrate Isocitrate

Malate α-Ketoglutarate

Fumarate Succinyl CoA

Succinate

Figure 1.2.An overview of the TCA cycle.

Intermediates of carbohydrate metabolism play an important

role in many food products. The conversion products of glycogen

in fish and mammalian muscles are now known to utilise dif-

ferent pathways, but ultimately result in glucose-6-phosphate,

leading into glycolysis. Lactic acid formation is an important

phenomenon in rigor mortis, and souring and curdling of milk

as well as in manufacturing sauerkraut and other fermented veg-

etables. Ethanol is an important end product in the production of

alcoholic beverages, bread making and in some overripe fruits to

a lesser extent. The TCA cycle is also important in alcoholic fer-

mentation, cheese maturation and fruit ripening. In bread mak-

ing,α-amylase (added or present in the flour itself) partially

hydrolyses starch to release glucose units as an energy source

for yeast growth and development, which is important for the

dough to rise during fermentation.

During germination of cereal grains, glucose and glucose

phosphates or fructose phosphates are produced from starch.

Some of the relevant biochemical reactions are summarised in

Table 1.1. The sugar phosphates are then converted to pyruvate

via glycolysis, which is utilised in various biochemical reac-

tions. The glucose and sugar phosphates can also be used in the

building of various plant structures, e.g. cellulose is a glucose

polymer and is the major structural component of plants.

In addition to starch, plants also possess complex carbohy-

drates, e.g. cellulose,β-g1ucans and pectins. Both cellulose

andβ-glucans are composed of glucose units bound byβ-

g1ycosidic linkages that cannot be metabolised in the human

body. They are important carbohydrate reserves in plants that

can be metabolised into smaller molecules for utilisation during

seed germination. Pectic substances (pectins) act as the “glue”

among cells in plant tissue and also are not metabolised in the

human body. Together with cellulose andβ-g1ucans, pectins are

classified as dietary fibre.

Derivatives of cellulose can be made through chemical

modification under strongly basic conditions where side chains

such as methyl and propylene react and bind at sugar hydroxyl

groups. The resultant derivatives are ethers (oxygen bridges)

joining sugar residues and the side chain groups (Coffee et al.

1995). A major function of cellulose derivatives is to act as

a bulking agent in food products. Two examples of important

food-related derivatives of cellulose are carboxymethylcellulose

and methylcellulose.

Pectin substances include polymers composed mainly of

α-(1,4)-d-galacturonopyranosyl units and constitute the middle

lamella of plant cells. Pectins exist in the propectin form in

unripe (green) fruit, contributing to firm, hard structures. Upon

ripening, propectins are metabolised into smaller molecules,

giving ripe fruits a soft texture. Controlling enzymatic activity

against propectin is commercially important in fruits such

as tomatoes, apples and persimmons. Research and the de-

velopment of genetically modified tomatoes allowed uniform

ripening prior to processing and consumption. Fuji apples can

be kept in the refrigerator for a much longer time than other

varieties of apples before reaching the soft, grainy texture stage

due to a lower pectic enzyme activity. Persimmons are hard

in the unripe stage, but can be ripened to a very soft texture

as a result of pectic- and starch-degrading enzymes. Table 1.