Figure 1. Examples of enzymatic browning in apple, banana and potato (A. Fresh cut, B. Browning).

Figure 2. Peroxidase and polyphenoloxidase activities in two species of tobacco plants infected with tobacco mosaic virus and a cross between the two. (Adapted from Goy et al., 1992)

Figure 3. Schematic of the activation of polyphenol oxidase by microorganisms.

Figure 4. Different developmental stages in the formation of Florida Spiny Lobster Cuticle. A. No new cuticle, only epidermis from old cuticle; B. Beginning signs of a newly forming cuticle; C. Advanced stages of a newly forming cuticle; and D. Newly formed cuticle almost completely formed as in late premoult.

Figure 5. Polyphenol oxidase activity and trypsin activated polyphenol oxidase activity at various stages of lobster moulting.

Figure 6. Melanosis progression scale of shrimp. (Courtesy of Dr. W.S. Otwell, University of Florida)

Figure 7. Monophenol oxidase pathway producing the diphenol.

Figure 8. Polyphenol oxidase activity for the substrates aromatic amines and o-aminophenols. (From Toussaint and Lerch, 1987)

Figure 9. Diphenol oxidase pathway producing the quinones.

Figure 10. Formation of melanin from tyrosine. (From Lerner, 1953)

Figure 11. Simplified mechanism for the hydroxylation and oxidation of diphenol by phenoloxidase.

Figure 12. Comparison of reactions catalysed by catecholase (o-DPO) and laccase (p-DPO). (From Walker, 1995)

Figure 13. Structures of common phenolic compounds.

Figure 14. Thermal inactivation of the thermostable fraction of potato lipolytic acyl hydrolase, lipoxygenase, polyphenol oxidase, and peroxidase. (From Svensson, 1977)

Figure 15. Effect of high hydrostatic pressure on relative activity of polyphenoloxidase in Tris buffer (pH 7) at 45 ^oC. (From Seyderhelm et al., 1996)

Figure 16. The distribution of species of sulfurous acid at various pH values. (From Ough, 1984)

Figure 17. The primary role of reducing agents such as sulphiting agents in the inhibition of enzymatic browning is to reduce the pigment precursors (quinones) to colourless, less-reactive diphenols. (Adapted from Walker, 1977)

Figure 18. Mechanism of prevention of colour formation by ascorbic acid.

Figure 19. Effect of cysteine and cyteinyl addition compounds with o-quinones on the enzymatic oxidation of o-diphenols. (From Richard-Forget et al., 1992)

Figure 20. Inhibition of potato browning by N-acetyl-L-cysteine (5% potato in water; inhibitor 0.8 mM; 5 h; room temperature).(From Friedman et al., 1992)

Figure 21. Major antioxidants used in food.

Figure 22. Schematic and chemical structure of a β-cyclodextrin molecule.

Figure 23. The inhibitory effect of 4-hexylresorcinol on PPO. (Adapted from Kahn and Andrawis, 1985)

Figure 24. Blackspot(%) in raw head-off brown shrimp treated with 4-HR and 1.25% sulfite, and stored at 2 ^oC. (From Lambrecht, 1995)

Figure 25. 4-HR treatment (0.02% for 2 min dip) of star fruit. A. Treated B. Non-treated.

Figure 26. Postulated mechanism for the inhibition of PPO induced enzymatic browning by thiols. (From Friedman and Molnar-Perl, 1990)

Figure 27. Simplified schematic showing how antisense RNA can be used to control gene expression at the translational level (P represents the promoter). (From Martinez and Whitaker, 1995)