Lecture 5 Flashcards
What is heat transfer governed by?
Goverened by the first law of thermodynamics
Energy can not be created nor destroyed. It can only be converted into other forms of energy
What are the different heating mediums?
Water
Air
Steam
What is the difference between late and sensible heat?
Latent Heat? Change in Phase (liquid to Gas/Vapour)
Sensible Heat? Change in Temp
What is the conversion of water from liquid phase to gas phase ?
Need x amount of energy to convert 1kg of water to 1 kg of steam
How much energy is in 1kg of water and steam?
1 kg Water = 419.04 KJ
1 Kg Steam = 2676.1 KJ
how much energy must be put in to turn 1kg of water into 1kg of steam?
Need to put in to convert water to steam
Energy Difference = 2676.1 - 419.04 = 2257.06 KJ
How quickly does phase transition happen?
does not occur instanteously
-is gradual
What is steam quality?
is the term to describe the extent of the conversion
What is the difference between dry, steam, liquid water and wet steam?
Dry Steam = 100% quality steam (100% vapour)
-100% water converted to 100% steam
Liquid Water = 0% quality (no vapour)
Wet Steam = Any steam less than 100% quality
Water in food is classified according to its ??
Mobility
-Free water vs bound water which is more difficult to move
What is sun drying?
Economical
Slow
Not suitable for some high quality products
Moisture content ~ 15% (high for storage stability)
Contamination from dust, insects, rodents
Over the 60 years what has happened to the dehydration?
During the past 60 years drying has become faster and better controlled, and the product quality has improved
How can we increase shelf life of dehydrated fruit?
Shelf life of dried products increases by the use of packages which are resistant to:
Moisture
Oxygen
Odours
Light
What is the purpose of drying?
To lower moisture content and water activity >…Preservation
To inhibit growth of bacteria/yeast/mold
To decrease Enzymatic/Chemical Rxns.
To decrease weight/bulk Lower shipping cost
Convenience (instant coffee, instant mashed potatoes)
What is the difference between dehydration and concentration ?
Dehydration:
Almost complete removal of water from foods under controlled conditions
Final moisture 1-5% (dried milk, eggs, …)
Concentration:
Partial removal of water
Final moisture 20-40% (syrup, evaporated milk, condensed soup)
Is it possible to decrease water content of a food product to 0%?
No
-We have no food with 0% water it is impossible, we could destroy the food
What products are use the sundering method?
Hot air and low humidity
Small fruits or small pieces of fruits (spread on trays under sun)
SO2 can be used to prevent browning
Contamination from wind blown dust and dirt
Moisture content is reduced to 10-35%
What is the hot air drying method?
Products are placed on:
Metal mesh belts in a tunnel (Tunnel Drying)
Trays in a cabinet (Cabinet Drying)
Heated air is blown over the products
What does the time required for dehydration depends on for hot air drying?
Moisture content of product
Composition (Fat, Sugar, Salt)
-Higher the moisture, sakt sugar and fat will take longer for the product to dy
• Sugar and salt binds to water making it take longer
Shape (Irregular shapes, folded pockets)
Size
Temperature, Velocity and
Humidity of the air in the dryer
Why is high temps required at the beginning of the drying cycle?
Most of the energy is used to vaporize surface water
Food does not get very hot due to evaporation of surface water
Liquid > Vapour
What temps do we Dre vegetables at?
Initial Temperature = 82-93 C
Later Temperature = 54 – 71 C (b/c of Lower Surface Water)
-to not burn the food
What is fluidized-bed drying?
Porous Plate
Products are fed at one end to the plate, agitated and hot air is blown over them
Air ocomes from all over, is wuite fast
What is drum drying?
(one or two stainless steel drums rotating in opposite directions)
Liquid or Semi-Liquid: Milk, Juice, Eggs, Potato Paste
Product is flown onto the surface of drums and dried on the drums
Dried product is scraped off by stationary blade
does drum drying damage food?
Heat damage is High
-Dreied directly on the surface of the druum ther heat damge is. high
-Vacuum Drum Drying Decreases Heat Damage
What is spray drying?
Milk, Syrup, Coffee, Eggs or other Liquids/Semi-Liquids
The liquid material is sprayed into the chamber with Hot Air (74C) for a Short Time
-Because of low direct contact between product and heat
Final Moisture Content is about 1-5%
Heat Damage: Low
What is spray drying suitable for?
Suitable for heat sensitive products
What is the downfall of spray drying?
Fine Particles: Hard to Reconstitute
-They don’t go into solution very easily
Agglomeration:
Process of Re-hydration and Re-drying to produce porous particles that can easily be reconstituted
-Done after the drying process
What is freeze drying?
Freeze the Food > Subject to High Vacuum
Sublimation:
-Removal of moisture without going through liquid phase
Rapid Drying w/out changing the shape of the food
What is puff drying?
Product is heated (oven) and suddenly subject to a high vacuum
The final product is porous with a puffed structure
What is microwave drying?
**Use Radient Energy and convert it to Kinetic Energy to increase the temp.
Not Much used in Food Industry for
Drying purposes
What is the rate of water removal?
Rate of water removal is not constant:
-High at the beginning and then it slows down
What are the reasons why water removal is high at the beginning and then it slows?
1) Water is lost from the surface
- (Constant Rate of Drying)
- A lot of free and surface water at the begining
2) Thick layer develops on the surface
- (Insulation Barrier)
- Barrier prevents moisture loss
3) Dry food picks up moisture from air as fast as it loses it (Equilibrium Relative Humidity)
-As soon as it loses the moisture it picks it back up and at that point it stops drying
Drying Stops
What are the factors affecting drying rate?
1) Surface Area
2) Temperature
3) Air Velocity
4) Humidity
5) Time/Temperature
6) Atmospheric Pressure & Vacuum
7) Evaporation & Temperature
How does surface area affect drying rate?
↑ S.A. > ↑ Heat Transfer
Recomnende to slice things into sections to increase surface area and the rate of heat transfer will be faster
How does temp affect drying rate?
↑ Temperature difference between the heating medium and the food > ↑ Rate of Heat Transfer
Higher the temp difference the higher the rate of heat transfer
Moisture Must be Removed by Applying Air Flow
How does air velocity affect drying rate?
High velocity air sweeps moisture away from the drying food surface
How does humidity affect drying rate?
The Drier the Air > ↑ Rate of Drying
Equilibrium Relative Humidity?
Humidity at which the food neither loses nor gains moisture
-When drying stops
What is the ratio for drying?
Lower temps need to be drier (have low RH)
How does time and temp affect drying rate?
HTST: high temp short time LTLT: low temp long time Vegetable -Oven Dry (4h) -Sun Dry (2 days)
How is better HTST LTLT?
High temp short time- will always give you the best results for preservcing sensory products o fthe food
How does atmospheric pressure and vacuum affect drying rate?
At Constant Temp:
↓Pressure > ↑ Rate of Boiling
Food in a heated vacuum chamber will lose moisture faster at lower temp.
(Important for Heat Sensitive Foods)
Drum drying under vaccume agt lower pressure so you don’t have to increase the temp and not damage food, preserve food quality
What does vacuum mean?
Vaccuum means decrease pressure
How does evaporation and temp affect drying rate?
Hot Food> Cool Down b/c some of the heat is used to convert water to vapour
*Air In (200C)> Food(70C) > Air Out(120)
However, as Moisture Content of Food ↓ > Evaporation ↓ > Food Temperature ↑
What are the effects of food properties on dehydration?
Increase solute concentration
Binding of water
Cellular structure
Porosity
how does an increase solute concentration effect food properties when dehydrated?
↑ B.P. ↓ Drying Rate
Salt and sugars
High Na and sugar, thy will bind water and make it harder to remove this water
how does binding of water effect food properties when dehydrated?
Free Water
Bound Water
Much easier to remove
Harder to remove
how does cellular structure effect food properties when dehydrated?
Cook Food> Cells become more Permeable to Water
And then easier to remove water fomr the food high
how does porosity effect food properties when dehydrated?
1) By whipping or foaming > Dry the Foam
2) By Vacuum Drier (Rapid escape of water)
3) By Creating Steam Pressure Within the Product (escaping steam tends to puff the product)
Why is porous food drying rate higher?
Higher because of higher surface area
What are the advantages of porous food?
1) Quick Solubility
2) Greater Volume Appearance
What are the disadvantages of porosity?
1) ↑ Bulk >↑ Shipping Cost
2) Lower Storage Stability b/c of ↑ Surface Exposure to Air and Light
Hiugher chanes for oxidative damage and therefore damage to sensory properties
What is the isothermal curve?
ZONE I : Monolayer H20/ Bound H20
-not available for any activity
Why we can never have 0% water in foods
ZONE II : Intermediate H20
- avilable for some physical, bio and chem activity
ZONE III: Free H20
-Free water: 0.8-1 aviailable for all physical, biological and chemcials activity
• East moulds etc sue this to grow
• When freezing foodm we are freezing hthis water, same with dehydrating
What was the most popular dried fruit?
Rasins (69.9%)
Prunes
Dates
Apples
