Lecture 5
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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