Drying of Fish

Surendraraj

Drying-Introduction

Drying-Concentrated Food– less costly to produce, – processing equipment is limited, – storage area required is less and – distribution costs are minimum

Drying Vs Immobilization of waterPrinciple:Water removal and reduction in water activity

Water activity Vs Preservation

Theory of Drying– Evaporation– Movement of water

Food Material– adsorption layer-Bound water – diffusion layer –Less tightly– free layer –Free layer

Two phases of drying– Constant rate of drying– Falling rate of drying

Theory of DryingConstant Rate of Drying-Evaporation

Air velocityRelative humidityTemperatureSurface areaProduct Thinkness

Falling Rate of Drying-Diffusion-capillary flow of water-viscous flow of water vapour-thermal diffusion-surface diffusion in a porous medium.-gaseous diffusion-molecular diffusion in a soild medium

Factors:vapor pressure, moisture content, chemical potential Fat, Thickness, Water content

Water Activity• Water activity is the measure of water available in a food to

support microbial growth and chemical reactions• Moisture content is measure of total moisture always higher

as bound water, salt, sugar moisture not available for microbe• expressed as a fraction in the range 0-1 or usual 0-100%

percentage range used for relative humidity• Aw = P/Psat• Where P is the vapour pressure of water vapor in air that is in

equilibrium with the food sample, and Psat is the saturation vapor pressure of water at the same temperature.

Method of DryingNatural or drying

• Drying on the ground• Rack Drying• Scoffolding• Solar Tent Drying• Solar Collector Drying• Solar Cabinet Dryer

Artificial or dehydration

Heat by hot gas/Hot air dryer• Kilns• Cabinet dryer• Tunnel dryer• Multi deck• Fluidized bed dryer

Heat by solid surface/Contact Dryer

• Drum dryer• Vacuum dryer

Drying on ground

Drying on ground

Drying on Raised Platform

Drying on paddy straw

Drying on mat

Drying on mat

Drying on Mat

Drying on mat

Drying on Mat

Drying on tray

Drying on rack

Drying by hanging

Drying by hanging

Bombay Duck Drying/Scoffolding

Bombay duck drying

Bombay Duck Drying

Bombay Duck Drying

Drying on Rack

Solar Tent Dryer

Solar Tent Dryer

Solar Collector Dryer

Solar Cabinet Dryer

Method of DryingNatural or drying

• Drying on the ground• Rack Drying• Scoffolding• Solar Tent Drying• Solar Collector Drying• Solar Cabinet Dryer

Artificial or dehydration

Heat by hot gas/Hot air dryer• Kilns• Cabinet dryer• Tunnel dryer• Multi deck• Fluidized bed dryer

Heat by solid surface/Contact Dryer

• Drum dryer• Vacuum dryer

Cabinet Dryer

Tunnel Dryer• Long tunnels, 10-15 m long• Trolleys loaded with fish are moved• parallel flow drying tunnel-air towards the

outlet side cool and humid-product not dried• Counter flow drying tunnel-prolonged drying

schedules-lead to spoilage• Provide umidification and humidity control

Fluidized bed dryer

• The product is passed on a perforated conveyor bed

• Air is passed below, • Product is partially lifted and three

dimensional drying

Vacuum Dryer

• Hollow shelves, through which the heating medium is circulated in a chamber

• Vacuum will be drawn and drying will proceed under vacuum

• Expensive• Suitable for fatty fishes

Rotary Dryers

• Mainly for fish meal• Hot air from steam jacket supplied to drum• Types: Rotary drum dryer, direct rotary dryer,

indirect rotary dryer and Steam dryer

Drum Dryers

• Fluid Materials• Drum heated• Dried material scrapped off• Types: single drum dryer, double drum drier,

and twin drum dryer