52
evaporation rate on the muscle surface should be kept in balance with the speed of
water passage from the inner layers to the surface layer. If the water on the surface
of inner layer evaporates very quickly, this creates a layer that affects the drying
process and extends the drying time. The heat transfer during the drying process can
occur by convection, conduction, radiation, or a combination of these (Gavrila
et al. 2008).
Water activity is defined as the ratio of the vapour pressure of the water in the
caste and the vapour pressure of the pure water at the same temperature. At the
beginning of the drying process (constant phase of drying rate), the water activity
remains close to 1. However, the water activity decreases in the subsequent stages
of the drying process. Water activity is of great importance for food storage. Water
activity is the most important factor limiting chemical, biochemical and microbiological changes in foods. As water activity decreases, the durability of food
increases. Bacteria that cause food spoilage cannot multiply in foods with water
activity below 0.90. Some halophilic bacteria can operate up to 0.75 water activity.
Moulds easily multiply until water activity drops to 0.80. In addition, the decrease
in water activity also limits or prevents enzymatic changes. Enzymes such as peroxidases and phenoloxidases are inactive in water activity down to 0.85. However,
lipase enzymes remain active until 0.25–0.30 (Gokoglu 2002; Rahman and Labuza
2007; Belessiotis and Delyannis 2011). To prevent microbial growth, it has been
reported that the dried water activity should be kept below the critical value of 0.60
(Perera and Rahman 1997). The water activity of the dried shrimp at 27 °C was
found to be below 0.80, which is considered the low limit for the growth of most
food spoilage microorganisms. Moisture content in dried shrimps should be reduced
to 20–25% (Tapaneyasin et al. 2005) (Fig. 2.9).
Drying Methods
The methods of drying seafood are found in a wide range from traditional drying
methods in sun to high technology methods with computer control.
Sun Drying
Dried shrimp is one of the most important export products of Southeast Asia. Price
depends largely on quality characteristics such as dryness, colour and size.
Commonly traditional drying is carried out by boiling shrimps in salt water and then
drying them in the sun. In the traditional drying method, the shrimp is dried under
the sun for 3–5 h depending on the presence of sunlight. It is one of the most widely
used drying methods in many parts of the world. It is one of the most widely used
drying methods in many parts of the world. In this method, solar energy is used to
reduce the amount of water in the product, that is, to ensure the evaporation of
water. The pre- treatments before drying vary according to the region and the person
who will perform the operation. During sun drying, the heat is transferred to the
product by convection and radiation. The converted heat is partly transmitted to the
2 Crustacean Shellfish
evaporation rate on the muscle surface should be kept in balance with the speed of
water passage from the inner layers to the surface layer. If the water on the surface
of inner layer evaporates very quickly, this creates a layer that affects the drying
process and extends the drying time. The heat transfer during the drying process can
occur by convection, conduction, radiation, or a combination of these (Gavrila
et al. 2008).
Water activity is defined as the ratio of the vapour pressure of the water in the
caste and the vapour pressure of the pure water at the same temperature. At the
beginning of the drying process (constant phase of drying rate), the water activity
remains close to 1. However, the water activity decreases in the subsequent stages
of the drying process. Water activity is of great importance for food storage. Water
activity is the most important factor limiting chemical, biochemical and microbiological changes in foods. As water activity decreases, the durability of food
increases. Bacteria that cause food spoilage cannot multiply in foods with water
activity below 0.90. Some halophilic bacteria can operate up to 0.75 water activity.
Moulds easily multiply until water activity drops to 0.80. In addition, the decrease
in water activity also limits or prevents enzymatic changes. Enzymes such as peroxidases and phenoloxidases are inactive in water activity down to 0.85. However,
lipase enzymes remain active until 0.25–0.30 (Gokoglu 2002; Rahman and Labuza
2007; Belessiotis and Delyannis 2011). To prevent microbial growth, it has been
reported that the dried water activity should be kept below the critical value of 0.60
(Perera and Rahman 1997). The water activity of the dried shrimp at 27 °C was
found to be below 0.80, which is considered the low limit for the growth of most
food spoilage microorganisms. Moisture content in dried shrimps should be reduced
to 20–25% (Tapaneyasin et al. 2005) (Fig. 2.9).
Drying Methods
The methods of drying seafood are found in a wide range from traditional drying
methods in sun to high technology methods with computer control.
Sun Drying
Dried shrimp is one of the most important export products of Southeast Asia. Price
depends largely on quality characteristics such as dryness, colour and size.
Commonly traditional drying is carried out by boiling shrimps in salt water and then
drying them in the sun. In the traditional drying method, the shrimp is dried under
the sun for 3–5 h depending on the presence of sunlight. It is one of the most widely
used drying methods in many parts of the world. It is one of the most widely used
drying methods in many parts of the world. In this method, solar energy is used to
reduce the amount of water in the product, that is, to ensure the evaporation of
water. The pre- treatments before drying vary according to the region and the person
who will perform the operation. During sun drying, the heat is transferred to the
product by convection and radiation. The converted heat is partly transmitted to the
2 Crustacean Shellfish