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HDSO Food Science > Lecture 5 > Flashcards

Lecture 5 Flashcards

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1
Q

What is heat transfer governed by?

A

Goverened by the first law of thermodynamics

Energy can not be created nor destroyed. It can only be converted into other forms of energy

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2
Q

What are the different heating mediums?

A

Water
Air
Steam

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3
Q

What is the difference between late and sensible heat?

A

Latent Heat? Change in Phase (liquid to Gas/Vapour)

Sensible Heat? Change in Temp

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4
Q

What is the conversion of water from liquid phase to gas phase ?

A

Need x amount of energy to convert 1kg of water to 1 kg of steam

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5
Q

How much energy is in 1kg of water and steam?

A

1 kg Water = 419.04 KJ

1 Kg Steam = 2676.1 KJ

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6
Q

how much energy must be put in to turn 1kg of water into 1kg of steam?

A

Need to put in to convert water to steam

Energy Difference = 2676.1 - 419.04 = 2257.06 KJ

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7
Q

How quickly does phase transition happen?

A

does not occur instanteously

-is gradual

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8
Q

What is steam quality?

A

is the term to describe the extent of the conversion

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9
Q

What is the difference between dry, steam, liquid water and wet steam?

A

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

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10
Q

Water in food is classified according to its ??

A

Mobility

-Free water vs bound water which is more difficult to move

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11
Q

What is sun drying?

A

Economical

Slow

Not suitable for some high quality products

Moisture content ~ 15% (high for storage stability)

Contamination from dust, insects, rodents

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12
Q

Over the 60 years what has happened to the dehydration?

A

During the past 60 years drying has become faster and better controlled, and the product quality has improved

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13
Q

How can we increase shelf life of dehydrated fruit?

A

Shelf life of dried products increases by the use of packages which are resistant to:

Moisture
Oxygen
Odours
Light

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14
Q

What is the purpose of drying?

A

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)

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15
Q

What is the difference between dehydration and concentration ?

A

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)

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16
Q

Is it possible to decrease water content of a food product to 0%?

A

No

-We have no food with 0% water it is impossible, we could destroy the food

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17
Q

What products are use the sundering method?

A

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%

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18
Q

What is the hot air drying method?

A

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

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19
Q

What does the time required for dehydration depends on for hot air drying?

A

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

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20
Q

Why is high temps required at the beginning of the drying cycle?

A

Most of the energy is used to vaporize surface water

Food does not get very hot due to evaporation of surface water
Liquid > Vapour

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21
Q

What temps do we Dre vegetables at?

A

Initial Temperature = 82-93 C
Later Temperature = 54 – 71 C (b/c of Lower Surface Water)
-to not burn the food

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22
Q

What is fluidized-bed drying?

A

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

23
Q

What is drum drying?

A

(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

24
Q

does drum drying damage food?

A

Heat damage is High
-Dreied directly on the surface of the druum ther heat damge is. high

-Vacuum Drum Drying Decreases Heat Damage

25
Q

What is spray drying?

A

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

26
Q

What is spray drying suitable for?

A

Suitable for heat sensitive products

27
Q

What is the downfall of spray drying?

A

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

28
Q

What is freeze drying?

A

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

29
Q

What is puff drying?

A

Product is heated (oven) and suddenly subject to a high vacuum

The final product is porous with a puffed structure

30
Q

What is microwave drying?

A

**Use Radient Energy and convert it to Kinetic Energy to increase the temp.

Not Much used in Food Industry for
Drying purposes

31
Q

What is the rate of water removal?

A

Rate of water removal is not constant:

-High at the beginning and then it slows down

32
Q

What are the reasons why water removal is high at the beginning and then it slows?

A

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

33
Q

What are the factors affecting drying rate?

A

1) Surface Area
2) Temperature
3) Air Velocity
4) Humidity
5) Time/Temperature
6) Atmospheric Pressure & Vacuum
7) Evaporation & Temperature

34
Q

How does surface area affect drying rate?

A

↑ S.A. > ↑ Heat Transfer

Recomnende to slice things into sections to increase surface area and the rate of heat transfer will be faster

35
Q

How does temp affect drying rate?

A

↑ 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

36
Q

How does air velocity affect drying rate?

A

High velocity air sweeps moisture away from the drying food surface

37
Q

How does humidity affect drying rate?

A

The Drier the Air > ↑ Rate of Drying

Equilibrium Relative Humidity?

Humidity at which the food neither loses nor gains moisture
-When drying stops

38
Q

What is the ratio for drying?

A

Lower temps need to be drier (have low RH)

39
Q

How does time and temp affect drying rate?

A 
HTST: high temp short time
LTLT: low temp long time 
Vegetable  
-Oven Dry (4h)
-Sun Dry (2 days)
40
Q

How is better HTST LTLT?

A

High temp short time- will always give you the best results for preservcing sensory products o fthe food

41
Q

How does atmospheric pressure and vacuum affect drying rate?

A

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

42
Q

What does vacuum mean?

A

Vaccuum means decrease pressure

43
Q

How does evaporation and temp affect drying rate?

A

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 ↑

44
Q

What are the effects of food properties on dehydration?

A

Increase solute concentration

Binding of water

Cellular structure

Porosity

45
Q

how does an increase solute concentration effect food properties when dehydrated?

A

↑ B.P. ↓ Drying Rate

Salt and sugars

High Na and sugar, thy will bind water and make it harder to remove this water

46
Q

how does binding of water effect food properties when dehydrated?

A

Free Water
Bound Water

Much easier to remove

Harder to remove

47
Q

how does cellular structure effect food properties when dehydrated?

A

Cook Food> Cells become more Permeable to Water

And then easier to remove water fomr the food high

48
Q

how does porosity effect food properties when dehydrated?

A

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)

49
Q

Why is porous food drying rate higher?

A

Higher because of higher surface area

50
Q

What are the advantages of porous food?

A

1) Quick Solubility

2) Greater Volume Appearance

51
Q

What are the disadvantages of porosity?

A

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

52
Q

What is the isothermal curve?

A

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

53
Q

What was the most popular dried fruit?

A

Rasins (69.9%)
Prunes
Dates
Apples

HDSO Food Science (17 decks)

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