Food Biochemistry and Food Processing (2 edition)

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1 Introduction to Food Biochemistry 17

Table 1.13.Lipid Degradation in Seed Germination

Enzyme Reaction

Lipase (oil body) Triacylglycerol→Diacylglycerol+Fatty acid

Triacylglycerol→Monoacylglycerol+Fatty acids

Diacylglycerol→Monoacylglycerol+Fatty acid

Fatty acid+CoA→Acyl CoA

β-Oxidation (glyoxysome) Acyl CoA→Acetyl CoA

Glyoxylate cycle (glyoxysome) Acetyl CoA→Succinate

Mitochondrion Succinate→Phosphoenol pyruvate

Reverse glycolysis (cytosol) Phosphoenol pyruvate→Hexoses→Sucrose

Source: Bewley and Black 1994, Murphy 1999.

aggregate until a continuous network forms (partial coalescence)

yielding a ‘solid’ product. TG globules cement to one another at

globule interfaces due to interacting fat crystal networks (Goff

1997).

The physical and functional properties of TGs are highly vari-

able and are dependent on FA chain length, number of FA double

bonds and position/order of FAs on the glycerol backbone (e.g.

oil seeds tend to contain more double bonds in the middle posi-

tion, whereas animal TGs contain saturated FAs at this position).

Furthermore, reactions such as oxidation or lipolysis affect TG

behaviour.

Phospholipids

Another class of lipids are phospholipids (PLs), most of which

consist of a glycerol backbone with two FAs and the third back-

bone position containing various substituents such as serine,

choline and ethanolamine. These polar substituents render PLs

amphipathic (one end hydrophilic, the other FA end hydropho-

bic). An example of a PL used in food is lecithin, a natural

emulsifier and surfactant. In aqueous environments, PLs spon-

taneously form phospholipid bilayers in spherical-shaped struc-

tures called liposomes or lipid vesicles. Liposomes have been

used for the study of membrane properties and substance perme-

abilities, drug delivery, and can be used for microencapsulation

of food ingredients. Encapsulation of vitamin C significantly

improves shelf life by about 2 months when degradative compo-

nents like copper, Lys and ascorbate oxidase are present. Also,

ingredients can be sequestered within liposomes that have well-

defined melting temperatures, thus releasing the contents in a

controlled fashion (Gouin 2004).

Food Lipid Degradation

Hydrolysis and oxidation reactions are the principal ways in

which TGs are degraded in foods. Lipolysis refers to the hydrol-

ysis of the ester linkage between the glycerol backbone and FAs,

thereby releasing free FAs. Lipases are enzymes that release FAs

from the outer TG positions by hydrolysis. Free FA’s are more

susceptible to oxidation and they are more volatile compared to

FAs within TGs. Lipase activity is potentially problematic post-

slaughter, and controlling temperature minimises their activities.

By contrast, oil seeds ‘naturally’ undergo lipolysis pre-harvest

and, therefore, contain a significant amount of free FAs, result-

ing in acidity that should be neutralised in the extracted oil. In

terms of dairy products such as cheeses, yoghurts and bread, con-

trolled lipolysis is used as a means of producing desired odours

and flavours via microbial and endogenous lipases. However,

lipolysis is also responsible for development of rancid flavour in

milk, resulting from the release of short chain FAs, e.g. butyric

acid. Deep frying also produces undesirable lipolysis due to the

high heat and introduction of water from foods cooked in the oil

medium.

During seed germination, lipids are degraded enzymatically

to serve as an energy source for plant growth and develop-

ment. Because of the presence of a considerable amount of seed

lipids in oilseeds, they have attracted the most attention, and

various pathways in the conversion of FAs have been reported

(Table 1.13). The FAs hydrolysed from the oilseed glycerides are

further metabolised byβ-oxidation followed by the citric acid

cycle to produce energy. Seed germination is important in the

production of malted barley flour for bread making and brewing.

Autoxidation

The principal cause of lipid oxidation isautoxidation.Thispro-

cess takes place via the action of free radicals on the FA hydro-

carbon chain in a chain reaction. Initiation of the chain reaction

is the creation of free radicals by metal catalysis, light or per-

oxide decomposition. The initial free radicals then act on the

FA by abstracting a hydrogen atom from the hydrocarbon chain,

thereby making a new free radical, which then reacts with O 2 ,

resulting in hydroperoxy free-radical formation of the FA. This

FA-free radical then acts on other FAs abstracting a hydrogen

atom, creating a new free radical and the formation of a sta-

ble hydroperoxide. The chain reaction terminates when two free

radicals react together.

Initiation-- creation of R•

Propagation-- R•+O^2 →ROO•

ROO•+RH→ROOH+R•

(the new free radical can react with O 2 anew)

Termination-- R•+R•

R•+ROO•

ROO•+ROO•