Lesson 8 Flashcards
preservation by dehydration
2 steps in dehydration
- heat/energy transferred to the food promoting water removal from the food
- water transferred from the food into the dehydrating environment
Preservation principle of dehydration
- microorganisms cannot grow below 0.6 water activity (closer to 1 = more free water)
- water is physically removed to lower the water activity
- e.g. Staphylococcus aureus (a pathogen) grows at 0.85 and mould grows at ≥ 0.6
- enzymatic reactions require some water while chemical reactions (e.g. maillard browning) occurs at 0.3
Water activity (aᵥᵥ) in dehydrated foods
low water activity (0.2-0.6) but microorganisms are not killed and resume growth after food is rehydrated
Dehydration vs concentration
both use physical removal of free water from foods
* dehydration: removal of as much water from food as possible, providing long storage life
* concentration: partial removal of water, concentrating food constituents (e.g. fruit juice concentrates, jams, evaporated milk, condensed milk, maple syrup)
concentrated foods are not fully shelf-stable so it must be performed along with a more severe process (e.g. freezing, thermal processing, refrigeration, dehydration)
3 changes that occur during dehydration
- Shrinkage
- Case hardening
- Chemical changes
newer dehydration methods try to protect the product against these changes
Shrinkage
water soluble substances dissolved in food migrates from the interior to the surface, where it evaporates and is carried away by the dehydrating medium
suction effect leads to shrinkage!
Case hardening
- rapid drying causes compounds (e.g. sugars) to form a hard, fairly impermeable case around the food piece
- can cause the rate of dehydration to decrease (i.e. becomes harder to dehydrate)
common in high-sugar products (e.g. tropical and temperate fruit products)
Chemical changes during dehydration
- browning and flavor changes due to reactions (e.g. increased maillard browning with increased concentration of solutes)
- denaturation of proteins and aggregation of polysaccharides = loss of water-binding capacity
- loss of water-soluble components or concentration on food’s surface (leads to case hardening)
- loss of volatiles (e.g. flavor compounds)
why dehydrated foods have less flavor!
Grapes vs raisins
differences in appearance, texture, flavor
- raisins are darker (more concentrated pigment), sweeter (more concentrated sugar), and are more shelf stable at room temperature (lower water activity)
- grapes are more flavorful
4.5lb grapes = 1 lb raisins (water content)
sugars migrate to the surface of the grape during the dehydration process, where they are deposited as the water evaporates
5 factors affecting dehydration
- surface area (surface to volume ratio)
- temperature of the drying air
- air velocity
- humidity of the drying air
- atmospheric pressure and vacuum
Effect of surface area in dehydration
greater surface to volume ratio (smaller food piece) = faster moisture loss
Effect of temperature in dehydration
an increase in temperature will increase dehydration rate
Effect of air velocity in dehydration
maximize velocity of heated air moving around the food particles
shouldn’t be saturated with moisture
Effect of humidity of drying air in dehydration
- % RH of drying air determines the final moisture content of food
- the drier the air, the more moisture it can absorb
Effect of atmosopheric pressure and vacuum in dehydration
- water boils at 100°C (at a pressure of 1 atm = 760 mm)
- at lower air pressure, boiling temperature decreases (e.g. under vacuum, water will boil at 32°C)
vacuum is suitable for heat-sensitive food products, and it results in no quality problems and shrinkage
8 dehydration or drying methods
- sun drying
- spray drying
- tray and tunnel air drying
- drum drying
- freeze drying
- vacuum microwave drying
- deep fat frying
- vacuum extrusion drying
all have the same preservation principle!
Sun drying
first and oldest method
- inexpensive and slow drying method (takes several days) that requires dry and warm climates
- prone to invasion by insects, birds, rodents, and microorganisms
- prone to shrinkage and loss of volatiles
- poor rehydration
- inexpensive
used for fruits, vegetables, and fish
Tray (air) drying
- heated is air is blasted across trays or racks at a set velocity and with low % RH
- quick (faster than sun drying) and inexpensive
- prone to food shrinkage (similar quality as sun drying) and case hardening (typical with fast dehydration)
- poor/moderate rehydration properties
used for pasta, vegetables, fruit, and spices
Spray drying
- tiny droplets of food are sprayed into a steam of heated air (heated chamber), making a fine powder
- rapid method with good rehydration
used for liquid foods and purees (e.g. powder skim milk, instant coffee, tea, eggs)
Drum drying
- paste poured on top of rotating heated drum, creating films (thin layers) that is scraped off (flakes)
- rapid method with fairly good rehydration
used for semi-solid foods or thicker liquids like food paste and purees (e.g. mashed potatoes, baby cereal)
How does freeze drying (lyophilization) occur?
one of the most modern methods designed for pharmaceuticals
- food must be in frozen state (-80°C)
- then placed in a vacuum chamber with low heat provided from radiant heaters
- water will sublime (evaporate from solid solid state directly to vapor)
no case hardening (because no transition through liquid state)
not a rapid method! may take hours/days to complete depending on the size
Pros and cons of freeze drying
- excellent rehydration due to voids (pores) from ice crystals when water sublimes that act as channels
- shape of food retained (no shrinkage) because no translocation of water-soluble constituents to the surface
- flavor retained
- very expensive and slow (takes several days)
food closely resembles starting material (remarkable quality!)
used for instant soups, high-quality coffee, vegetables, military rations, and space food
Vacuum microwave drying
- food placed under vacuum (maintain low temp = lower boiling point) and heated using microwave energy
- rapid method (takes minutes)
- high-quality products (less nutrient loss and color change, flavor retention, complete rehydration) with no shrinkage and case hardening
but very expensive!
Deep fat frying
- submersion in hot oil (food “picks up” oil) > water evaporation
- at 140-150°C for 1-2 mins (vs 80°C for 30 mins in hot-air drying)
use for snack foods and bakery products (e.g. potato chips, instant noodles)
Extrusion drying
- dough (slurry of food) undergoes steam heating under pressure then puffs once pressure is released and water is expelled
- moist heat causes starch gelatinization (more flexible) and cooking of the product
used for breakfast cereals and snack foods
3 packaging requirements for dehydrated foods
- protect against moisture reabsorption (or transmission of water vapor) as DH foods are hygroscopic
- physical protection to prevent crushing (e.g. freeze dried foods have a porous structure and are easily crushed)
- protection from oxygen and light (e.g. photooxidation leads to degradation)
