Drying and Food Preservation
The preservation of foods by drying is the time-honored and most common method used by humans and the food processing industry. Dehydration (or drying) is defined as ‘the application of heat under controlled conditions to remove the majority of the water normally present in a food by evaporation’ (or in the case of freeze drying by sublimation ). This definition excludes other unit operations which remove water from foods (for example mechanical separations and membrane concentration , evaporation and baking as these normally remove much less water than dehydration.
Drying methods
Drying methods
Classification of dryers according to heat mode transfer
Sun Drying
Sun drying tomatoes
Disadvantages:
-Contaminations from the environment
-Product losses and contaminations by insects and birds
-Floor space requirements
-Difficulty in controlling the process
-Inconsistent sensory quality
-Contaminations from the environment
-Product losses and contaminations by insects and birds
-Floor space requirements
-Difficulty in controlling the process
-Inconsistent sensory quality
Solar Drying
Direct solar dryer
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Indirect Solar Dryer
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-Direct solar dryer
Direct solar dryers expose the substance to be dehydrated to direct sunlight.
-Indirect solar dryer
Indirect driers are constructed so the sun shines upon a solar collector (a shallow box, the insides painted black, topped with a pane of glass) heating air which then moves upward through a stack of four to six trays loaded with produce.
Direct solar dryers expose the substance to be dehydrated to direct sunlight.
-Indirect solar dryer
Indirect driers are constructed so the sun shines upon a solar collector (a shallow box, the insides painted black, topped with a pane of glass) heating air which then moves upward through a stack of four to six trays loaded with produce.
Hot-air dryers
Spray dryer
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Tunnel dryer
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-Spray dryer
A fine dispersion of pre-concentrated food (40–60% moisture) is first ‘atomized’ to form fine droplets and then sprayed into a co- or counter-current flow of heated air at 150–300ºC in a large drying chamber.
-Tunnel dryer
Layers of food are dried on trays, which are stacked on trucks programmed to move semi continuously through an insulated tunnel. Able to dry large quantities of food in a relatively short time However, the method has now been largely superseded by conveyor drying and fluidized bed drying as a result of their higher energy efficiency, reduced labor costs and better product quality.
A fine dispersion of pre-concentrated food (40–60% moisture) is first ‘atomized’ to form fine droplets and then sprayed into a co- or counter-current flow of heated air at 150–300ºC in a large drying chamber.
-Tunnel dryer
Layers of food are dried on trays, which are stacked on trucks programmed to move semi continuously through an insulated tunnel. Able to dry large quantities of food in a relatively short time However, the method has now been largely superseded by conveyor drying and fluidized bed drying as a result of their higher energy efficiency, reduced labor costs and better product quality.
Bin dryer
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Rotary dryer
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Fluidised-bed dryer
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-Bin dryer
Bin dryers are large, cylindrical or rectangular containers fitted with a mesh base. Hot air passes up through a bed of food at relatively low velocities
-Rotary dryer
The food stuff is contained in a horizontal cylinder through which it travels. Heated either by air flow through the cylinder or by conduction of heat from the cylinder walls.
-Fluidised-bed dryer
Warm air is blown upwards directly underneath the food, causing it to flow and remain separated. This procedure is suitable for small items.
Bin dryers are large, cylindrical or rectangular containers fitted with a mesh base. Hot air passes up through a bed of food at relatively low velocities
-Rotary dryer
The food stuff is contained in a horizontal cylinder through which it travels. Heated either by air flow through the cylinder or by conduction of heat from the cylinder walls.
-Fluidised-bed dryer
Warm air is blown upwards directly underneath the food, causing it to flow and remain separated. This procedure is suitable for small items.
Cabinet dryer
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Conveyor drier
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Heated-surface(contact) dryer
Drum dryer
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Vacuum band dryer
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Heat is supplied to the food by conduction.
-Drum dryer(Roller dryer)
-Vacuum band and vacuum shelf dryers
Two main advantages over hot-air drying:
-It is not necessary to heat large volumes of air before drying commences and the thermal efficiency is therefore high.
-Drying may be carried out in the absence of oxygen to protect components of foods that are easily oxidized.
-Drum dryer(Roller dryer)
-Vacuum band and vacuum shelf dryers
Two main advantages over hot-air drying:
-It is not necessary to heat large volumes of air before drying commences and the thermal efficiency is therefore high.
-Drying may be carried out in the absence of oxygen to protect components of foods that are easily oxidized.
Freeze drying
Frozen material is subjected to a pressure below the triple point (at 0°C, pressure:610 Pa) and heated to cause ice sublimation to vapor.
This method is usually used for high-quality dried products, which contain heat-sensitive components such as vitamins, antibiotics, and microbial culture. The virtual absence of air and low temperature prevents deterioration due to oxidation or chemical modification of the product. It also gives very porous products, which results in high rehydration rates. However, freeze drying is a slow and expensive process.
Osmotic Dehydration
This dehydration process generally does not produce a product of low moisture content that can be considered shelf stable. Need further processing (generally by air, freeze-, or vacuum-drying methods) to obtain a shelf-stable product, or the dehydration process could be used as a pretreatment for canning, freezing, and minimal processing. Osmotic dehydration is the process of water removal by immersion of water-containing cellular solid in a concentrated aqueous solution.
Water and solutes transfer in osmotic process
The driving force for water removal is the concentration gradient between the solution and the intracellular fluid. The removal of water during osmotic process is mainly by diffusion and capillary flow, whereas solute uptake or leaching is only by diffusion.
The osmotic dehydration process:
- dynamic period (mass transfer rates increased or decreased until equilibrium is reached).
- equilibrium period (equilibrium is the end point of osmotic process, i.e., net rate of mass transport is zero).
The osmotic dehydration process:
- dynamic period (mass transfer rates increased or decreased until equilibrium is reached).
- equilibrium period (equilibrium is the end point of osmotic process, i.e., net rate of mass transport is zero).
Problems in Applying the Osmotic Dehydration Process in the Food Industry
Product Sensory Quality
•Salty products when uses salt.
•Sweet products when uses sugar.
•Loss of acidity.
•Controlling solute diffusion and optimised process may be useful to overcome this problem
Syrup Management for long application and reuse of syrup.
- syrup recycling
- solute addition
- syrup composition and concentration
- reuse of syrup
- syrup disposal
Product Sensory Quality
•Salty products when uses salt.
•Sweet products when uses sugar.
•Loss of acidity.
•Controlling solute diffusion and optimised process may be useful to overcome this problem
Syrup Management for long application and reuse of syrup.
- syrup recycling
- solute addition
- syrup composition and concentration
- reuse of syrup
- syrup disposal
Smoking
One of the most ancient food preservation processes, and in some communities one of the most important. The use of wood smoke to preserve foods is nearly as old as open-air drying. The heat associated with the generation of smoke also causes a drying effect. Smoking has been mainly used with meat and fish.
The main purposes of smoking are:
-It imparts desirable flavours and colours to the foods.
-Some of the compounds formed during smoking have a preservative effect (bactericidal and antioxidant) due to the presence of a number of compounds.
In many cases, smoking is considered as a pre-treatment rather than a drying process.
-Smoke is effective in preventing lipid oxidation in meat and fish products.
-Smoke contains phenolic compounds, acids, and carbonyls, and the smoky flavour is primarily due to the volatile phenolic compounds.
-Wood smoke is extremely complex and more than 400 volatiles have been identified.
-Wood smoke contains nitrogen oxides which are responsible for the characteristic colour of smoked foods, whereas polycyclic aromatic hydrocarbon components and phenolic compounds contribute to its unique taste. These three chemicals are also most controversial from a health perspective.
The main purposes of smoking are:
-It imparts desirable flavours and colours to the foods.
-Some of the compounds formed during smoking have a preservative effect (bactericidal and antioxidant) due to the presence of a number of compounds.
In many cases, smoking is considered as a pre-treatment rather than a drying process.
-Smoke is effective in preventing lipid oxidation in meat and fish products.
-Smoke contains phenolic compounds, acids, and carbonyls, and the smoky flavour is primarily due to the volatile phenolic compounds.
-Wood smoke is extremely complex and more than 400 volatiles have been identified.
-Wood smoke contains nitrogen oxides which are responsible for the characteristic colour of smoked foods, whereas polycyclic aromatic hydrocarbon components and phenolic compounds contribute to its unique taste. These three chemicals are also most controversial from a health perspective.
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Hot and Cold Smoke
Hot smoking: cooks the product - Heat and smoke is applied. - Internal temperature <62.8oC for at least 30 min. - At least 3.5% salt in water-phase of muscle. Cold smoking: does not cook the product - Smoke is applied. - Internal temperature <35oC. - At least 3.5% salt in water-phase of muscle. |
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Drying Pretreatments
To prevent increasing of microbial loads
To accelerate drying process
To improve quality of the food
-Blanching
-Sulfur Dioxide Treatment
-Salting or Curing
-Other Dipping Pretreatments
-Freezing Pretreatment
-Cooking
To prevent increasing of microbial loads
To accelerate drying process
To improve quality of the food
-Blanching
-Sulfur Dioxide Treatment
-Salting or Curing
-Other Dipping Pretreatments
-Freezing Pretreatment
-Cooking
Quality change during drying
Additional reading materials:
| Drying and food preservation |
| Dehydration |
| Dehydration |
| Dehydration |
