22
destroyed at 70–90 °C (Manheem et al. 2012). This method has some disadvantages. These are loss of vitamin, aroma, texture and colour, as well as loss of
water-soluble compounds when water or steam is used. It also has a lot of energy
requirements and waste problems.
Pre-cooking is another process that inactivates enzymes and inhibits melanosis.
Keeping for 2 min at the boiling temperature is reported to be sufficient for PPO
inactivation (Martínez-Alvarez et al. 2009).
3. Dehydration
The water content affects the enzyme activity as it forms a solvent in which the
reactants can dissolve, move, and react. Water activity in enzymatic reactions is
important. Reduced water activity also reduces enzyme activity.
4. Freezing
Dissolved salts, sugars and other carbohydrates are potent inhibitors in high concentrations. Increased solute concentration during freezing increases the enzyme
inhibition. As with high temperature inactivation, low temperature also has some
negative effects. Freezing can change texture and other properties. Rotllant et al.
(2002) reported that the rapid freezing of shrimps was a good way to prevent
melanosis and that melanosis was not observed in shrimps kept frozen for
3 months.
5. High pressure
High pressure is also recommended to prevent melanosis (Encarnacion et al.
2010). The effect of high pressure on enzymes is related to alternating and nonreversible changes in protein structure. Losses in catalytic activity may be different depending on the type of enzyme, the nature of the substrate, the temperature,
and the length of the process. A suitable combination of pressure temperature
should be applied to increase enzyme inactivation.
6. Use of inhibitors
The inhibitors used in the prevention of darkening are limited by their effects on
toxicity, health, taste aroma and texture. Inhibitors are categorized according to
activity. Inhibitors used to prevent melanosis as follows:
Reducing Agents
The main role of reducing agents in the prevention of darkening is to reduce the
orthoquinones to colourless diphenols or to react irreversibly to form colourless
products with orthoquinones. The use of reducing compounds is the most effective
method of preventing darkening (Kim et al. 2000; Gokoglu and Yerlikaya 2008).
Sulphides and derivatives (especially sodium met bisulphite) are polyphenoloxidase inhibitors, which are commonly used to prevent darkening in practice.
Treatment with sulphide-based bleach inhibitors may delay the darkening for a long
time depending on the concentration used (Nirmal and Benjakul 2009a). Bisulphites
show competitive inhibition by binding sulfhydryl groups of the active part of the
enzyme polyphenoloxidase. On the other hand, bisulfide inhibition depends on the
reaction of the sulphides with the quinones and results in the formation of irreversibly inhibited sulfokinone forms of polyphenoloxidase (Kim et al. 2000). Sulphide
2 Crustacean Shellfish
destroyed at 70–90 °C (Manheem et al. 2012). This method has some disadvantages. These are loss of vitamin, aroma, texture and colour, as well as loss of
water-soluble compounds when water or steam is used. It also has a lot of energy
requirements and waste problems.
Pre-cooking is another process that inactivates enzymes and inhibits melanosis.
Keeping for 2 min at the boiling temperature is reported to be sufficient for PPO
inactivation (Martínez-Alvarez et al. 2009).
3. Dehydration
The water content affects the enzyme activity as it forms a solvent in which the
reactants can dissolve, move, and react. Water activity in enzymatic reactions is
important. Reduced water activity also reduces enzyme activity.
4. Freezing
Dissolved salts, sugars and other carbohydrates are potent inhibitors in high concentrations. Increased solute concentration during freezing increases the enzyme
inhibition. As with high temperature inactivation, low temperature also has some
negative effects. Freezing can change texture and other properties. Rotllant et al.
(2002) reported that the rapid freezing of shrimps was a good way to prevent
melanosis and that melanosis was not observed in shrimps kept frozen for
3 months.
5. High pressure
High pressure is also recommended to prevent melanosis (Encarnacion et al.
2010). The effect of high pressure on enzymes is related to alternating and nonreversible changes in protein structure. Losses in catalytic activity may be different depending on the type of enzyme, the nature of the substrate, the temperature,
and the length of the process. A suitable combination of pressure temperature
should be applied to increase enzyme inactivation.
6. Use of inhibitors
The inhibitors used in the prevention of darkening are limited by their effects on
toxicity, health, taste aroma and texture. Inhibitors are categorized according to
activity. Inhibitors used to prevent melanosis as follows:
Reducing Agents
The main role of reducing agents in the prevention of darkening is to reduce the
orthoquinones to colourless diphenols or to react irreversibly to form colourless
products with orthoquinones. The use of reducing compounds is the most effective
method of preventing darkening (Kim et al. 2000; Gokoglu and Yerlikaya 2008).
Sulphides and derivatives (especially sodium met bisulphite) are polyphenoloxidase inhibitors, which are commonly used to prevent darkening in practice.
Treatment with sulphide-based bleach inhibitors may delay the darkening for a long
time depending on the concentration used (Nirmal and Benjakul 2009a). Bisulphites
show competitive inhibition by binding sulfhydryl groups of the active part of the
enzyme polyphenoloxidase. On the other hand, bisulfide inhibition depends on the
reaction of the sulphides with the quinones and results in the formation of irreversibly inhibited sulfokinone forms of polyphenoloxidase (Kim et al. 2000). Sulphide
2 Crustacean Shellfish