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

18 Part 1: Principles/Food Analysis

Table 1.14.Enzymatic Lipid Oxidation in Food Systems

Enzyme Reaction

Arachidonate-5-lipoxygenase (5-lipoxygenase,

EC 1.13.11.34)

Arachidonate+O 2 →(6E,8Z,11Z,14Z)-(5S)-5-

hydroperoxyicosa-6–8-11,14-tetraenoate

Arachidonate-8-lipoxygenase (8-lipoxygenase,

EC 1.13.11.40)

Arachidonate+O 2 →(5Z,9E,11Z,14Z)-(8R)-8-

hydroperoxyicosa-5,9,11,14-tetraenoate

Arachidonate 12-lipoxygenase (lipoxygenase,

EC 1.13.11.31)

Arachidonate+O 2 →(5Z,8Z,10E,14Z)-(12S)-12-

hydroperoxyicosa-(12–5,8,10,14-tetraenoate)

Arachidonate 15-lipoxygenase (15-lipoxygenase,

EC 1.13.11.33)

Arachidonate+O 2 →(5Z,8Z,11Z,13E)-(15S)-15-

hydroperoxyicosa-5,8,11,13-tetraenoate

Lipoxygenase (EC 1.13.11.12) Linoleate+O 2 →(9Z, 11 E)-( 13 S)-

13-hydroperoxyoctadeca-9,11-dienoate

Source: Lopez-Amaya and Marangoni 2000a, 2000b, Pan and Kuo 2000, Kolakowska 2003, IUBMB-NC website (www.iubmb.org).

Another mode of lipid oxidation is enzyme-catalysed lipid

oxidation by a group of enzymes termed lipoxygenases (LOXs),

whose activities are important in legumes and cereals. In LOX

activity, a free radical of a FA initially is formed, followed by its

reaction with O 2 , yielding a hydroperoxide product (Klinman

2007). Linoleic acid and arachidonic acid are important FAs

from the health perspective and are quite common in many

food systems (Table 1.12). Because of the number of double

bonds in arachidonic acid, enzymatic oxidation can occur at

various sites, and the responsible LOXs are labelled according

to these sites (Table 1.14). In addition to rancid off-flavours,

LOX can also cause deleterious effects on vitamins and colour

compounds. However, LOX activity also causes the pleasant

odours associated with cut tomatoes and cucumbers.

The main enzymes involved in the generation of the aroma

in fresh fish are also LOXs, specifically the 12- and 15-LOXs

(Table 1.14) and hydroperoxide lyase. The 12-LOX acts on

specific polyunsaturated FAs and producesn-9-hydroperoxides.

Hydrolysis of the 9-hydroperoxide of eicosapentenoic acid by

specific hydroperoxide lyases leads to the formation of aldehy-

des that undergo reduction to their corresponding alcohols, a

significant step in the general decline of the aroma intensity due

to alcohols having higher odour detection thresholds than the

aldehydes (Johnson and Linsay 1986, German et al. 1992).

Milk contains a considerable amount of lipids and these milk

lipids are subjected to enzymatic oxidation during cheese ripen-

ing. Under proper cheese maturation conditions, these enzymatic

reactions starting from milk lipids create the desirable flavour

compounds for these cheeses. These reactions are numerous and

not completely understood, thus only general reactions are pro-

vided (Table 1.15). Readers should refer to Chapters 19, 20 and

26 in this book for a detailed discussion.

Elected Phytochemical Flavour and

Colour Compounds

Many fruits and vegetables produce flavours that are significant

in their acceptance and handling. There are a few well-known

examples (Table 1.16). Garlic is well known for its pungent

odour due to the enzymatic breakdown of its alliin to the thio-

sulfonate allicin, with the characteristic garlic odour. Straw-

berries have a very recognisable and pleasant odour when they

ripen. Biochemical production of the key compound responsible

for strawberry flavour, the furan 2,5-dimethyl-4-hydroxy-2H-

furan-3-one (DMHF), is also known as furaneol, which results

from hydrolysis of terminal, non-reducing∼-d-glucose residues

from DMHF-glucoside with release of∼-d-glucose and DMHF.

Lemon and orange seeds contain limonin, a furanolactone that is

bitter and is hydrolysable to limonate, which is less bitter. Many

cruciferous vegetables such as cabbage and broccoli have a sul-

furous odour due to the production of a thiol compound (R–SH)

after enzymatic hydrolysis of its glucoside. Brewed tea darkens

Table 1.15.Changes in Lipids in Cheese Manufacturing

Enzyme Reaction

Lipolysis

Lipases, esterases Triglycerides→β-Keto acids, acetoacetate, fatty acids

Acetoacetate decarboxylase (EC 4.1.1.4) Acetoacetate+H+→Acetone+CO 2

Acetoacetate-CoA ligase (EC 6.2.1.16) Acetoacetate+ATP+CoA→Acetyl CoA+AMP+Diphosphate

Esterases Fatty acids→Esters

Conversion of fatty acids

β-Oxidation and decarboxylation β-Keto acids→Methyl ketones

Source: Schormuller 1968, Kilara and Shahani 1978, IUBMB-NC website (www.iubmb.org).