Drying And Dehydration Flashcards
Higher drier/dry bulb temperature
Faster drying rate
Most expensive and unique drying method
Freeze drying
Why do high fat foods have slower drying rate?
Contains less water. Fat, due to its hydrophobic nature, may create a barrier that slows down water migration from interior to surface.
Smaller cubes or those with higher surface-to-volume ratio
Higher drying rate
Heat is supplied by convection
Drying using heated air
Process of cabinet drying
Fresh air is utilized -> moves in the heater making it hot -> fan sweeps away hot air going to the tray -> hot air moves in above & below the trays -> hot air then sweeps moisture becoming humid which contains moisture -> moves out in the exhaust
Classification of drying processes
Thermal drying
Osmotic drying
Mechanical dewatering
True or false. Cubes are smaller and with concave surfaces.
True.
In drying using heated air, what are the various forms of product chamber?
Cabinet dryer - dried mangoes
Tunnel dryer - industrial
Belt dryer
Spray dryer - industrial, powdered milk
How does drying affect solubility?
Increase in product temperature —> increase in protein denaturation —> decrease in solubility
Basic drying methods
Drying using heated air
Drying by direct contact with a heated surface
Freeze drying
Higher surface temperatures
Faster drying rate
Which is more porous? Air-dried or freeze-dried? Why? How does porosity affect lipid oxidation?
Freeze-dried. Lack of ice in the matrix —> more porous —> more oxidation
Typical solutes used for osmotic drying
Sugar or salt
Factors that influence quality during drying include
Chemical
- browning reactions
- lipid oxidation
- color loss
Physical
- rehydration
- aroma loss
- texture
- solubility
- shrinkage
Nutritional
- vitamin loss
Not a major nutritional loss during drying
Protein loss
True or false. You can achieve 100% rehydration.
False
Advantages of dehydration
Generally superior in terms of quality
Sanitary conditions are easily controlled
More expensive but superior quality commands a higher market value
Shorter drying period
Higher yield
Can be done under any weather conditions
Follows osmosis, commonly used in fruits
Osmotic drying
Maillard browning depends on
Water activity and temperature
Sensitive to high temperatures at high moisture content
Vitamin C
Associated chemical changes with textural changes
Crystallization of cellulose
Degradation of pectins
Starch gelatinization
Why dehydrate food?
Preservation (reduce water activity)
Obtain specific attributes (flavor, texture, etc.)
Reduce bulk and transport costs
Enable formulation of specific products
Color changes during browning reactions are affected by
Amount of ingredients
Control of heated air
Why is bound water more difficult to remove during drying? What does it require?
Attached to food components and interacts with the molecular structure of food. Requires higher temp or longer time to break bonds
Carotenoid and chlorophyll leads to
Higher drying temperature
The degree to which a dehydrated sample will rehydrate depends on the
Structural and chemical changes
Two types of browning
Enzymatic browning
Non-enzymatic browning
- caramelization
- Maillard reaction
Lipid oxidation causes what?
Rancidity
Off flavors
Loss of fat soluble vitamins and pigments
Dehydration
Removal of moisture under controlled conditions.
What is the effect of drying on pigments?
Longer drying times cause higher pigment loss.
Enzymatic browning is prevented by
Blanching or treatment with sulfur dioxide or ascorbic acid
Heat is used to dry the product
Thermal drying
Shrinkage reduces
Wettability
Product texture
Product absorbency
More bound water
Slower drying rate
Faster moisture loss
Faster drying rate
Effect of oxygen on lipid oxidation is closely related to
product porosity
Oxidation during storage contributes to increase in ____________ losses.
Organic compounds for aroma and flavor
Best method of preparation for vegetables in terms of drying. Why?
Blanching. Opens up spores for moisture to easily escape.
More free water
Faster drying rate
Mechanism of drying
Simultaneous application of heat and removal of moisture —> dry product
Heat supplied mainly by conduction
Drying by direct contact with a heated surface
Drying rates of meat depend on
Fat and lean content
Position of drying air
Arrangement of fat and lean mass
Moisture is frozen and then sublimed to vapor by the application of heat at very low pressure
Freeze drying
Drying
Generally applies to all methods of moisture removal; = natural method or sun drying
Stress cracks are induced by
Uneven volumetric changes resulting in uneven moisture and temperature distribution
Higher fat content
Slower drying rate
Uses a mechanical presser to squeeze out the water from the food
Mechanical dewatering
Describe the mechanism of drying.
Heat penetrates the food’s surface.
Moisture travels up through diffusion or osmosis and reaches the surface.
Dry air sweeps away moisture.
The process continues to reach equilibrium.
Factors affecting the drying rate.
Properties of the drier
Relative humidity
Air velocity
Dry bulb temperature
Properties of the food
Moisture content
Method of preparation
Rate of moisture loss
Surface-to- volume ratio
Surface temperature
Fat content
Osmotic drying makes use of _________ solutions, wherein….
Hypertonic, solutes are higher than solvent, water inside the fruit will exit to reach equilibrium
Temperature of driers
60-70 C
Osmotic drying
Water:
Osmotic substances:
Natural solubles:
Exits
Enters
Exits
Maillard reaction
Reducing sugars form a complex with amino acids
Higher relative humidity
Slower drying rate
Often lost during dehydration; responsible for flavor and aroma
Organic compounds
Lower SVR has slower drying rate. Why?
Moisture from interior takes longer to reach the surface.
Higher air velocity
Faster drying rate
Rehydration is maximized when
cellular and structural disruption are minimized
Disadvantage of mechanical dewatering
Liquid/soluble compounds are still present
Browning reactions cause
Changes in color
Decreased nutritional value and solubility
Off flavors
Textural changes
