Food Biochemistry and Food Processing (2 edition)

(Steven Felgate) #1

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


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•
Free download pdf