Food Biochemistry and Food Processing (2 edition)

BLBS102-c14 BLBS102-Simpson March 21, 2012 13:17 Trim: 276mm X 219mm Printer Name: Yet to Come

14 Seafood Enzymes: Biochemical Properties and Their Impact on Quality 275

R

R O

O

R

PPO + O 2

OH OH

OH

PPO + O 2

Monophenol Diphenol

Reducing agent

o-Quinone

Protein – NH 2

Brown pigments

Figure 14.6.Enzymatic oxidation induced by PPO.

stability, and kinetic parameters can vary with species, body

part, and so forth. (Montero et al. 2001).

Temperature

PPO shows different optimal temperatures among various ani-

mals or species. Montero et al. (2001) reported that the optimum

temperature of PPO from imperial tiger prawn was between

40 ◦C and 60◦C. Thermal stability was considerably reduced

when the enzyme extract was subjected to temperatures up to

35 ◦C. Rolle et al. (1991) observed that the PPO from Taiwanese

tiger shrimp (Penaeus monodon) exhibited a temperature opti-

mumof45◦C and showed stability over the temperature range

of 30–35◦C. The optimum temperature for PPO from the impe-

rial tiger prawn carapace was between 40◦C and 60◦C; however,

thermal stability was greatest at temperatures below 35◦C (Mon-

tero et al. 2001). Zamorano et al. (2009) found that the maximal

activity of PPO of deepwater pink shrimp was observed in the

15–60◦C range. Benjakul et al. (2005) reported that the PO from

the kuruma prawn cephalothorax showed the maximum activ-

ity at 35◦C and the enzyme was unstable at temperatures above

50 ◦C.

The determination of the temperature/time profiles required

to activate the proenzyme and to prevent melanosis and improve

postprocessing quality and acceptability is a means to regulate

or control the quality of crustaceans deteriorated by melanosis

(Williams et al. 2003).

pH

The optimum pH depends largely on the physiological pH in

which the enzyme activity occurs in nature. For example, the

pH of the carapace of the cephalothorax was 7.16, while in the

abdominal cuticle, it was 8.76. Therefore, the enzymes would

probably present different optimum pH characteristics, depend-

ing on the locus of extraction. Montero et al. (2001) reported that

high PPO activity occurred at pH 5 and 8 inPenaeus japonicus.

PPO from deepwater pink shrimp had the highest activity at pH

4.5 and was most stable at pH 4.5 and 9.0. Benjakul et al. (2005)

found that PO from kuruma prawn cephalotorax showed the

maximal activity at pH 6.5 and it was stable in a wide pH range

of 3–10. The stability of the PO was very different depending

on a number of factors such as temperatures, pH, substrate used,

ionic strength, buffer system, and time of incubation (Kim et al.