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1) Introduction to Food Science > Chapter 20: Dehydration and Concentration: Controlling Water Activity > Flashcards

Chapter 20: Dehydration and Concentration: Controlling Water Activity Flashcards

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1
Q

What is water activity an indicator of? Reducing it can do what?

A
  • Indicator of perishability

- Reducing water activity can increase shelf life

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2
Q

Name 3 categories of processed foods preserved by reduced water activity.

A

– dehydrated
– concentrated
– intermediate-moisture

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3
Q

What is the oldest method of food preservation? What is it caused by?

A
  • Dehydration

- Caused by lowering water content (water activity)

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4
Q

What is the artificial drying of food under controlled conditions?

A

Dehydration

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5
Q

Name 2 benefits of dehydration.

A

– lighter foods that take less space and are less
costly to package and ship
– a more-convenient form of food

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6
Q

Can dehydrated foods be reconstituted? Give an example.

A

reconstituted by adding water, such as with instant mashed potatoes

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7
Q

Name 4 factors affecting dried food quality.

A
  • Surface area (greater area, faster it dries)
  • Airflow (moving air with heat speeds drying)
  • Temperature (faster drying = less change in food)
  • Oxidation (browning)
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8
Q

What happens if food is not dried enough?

A

Bacteria or mold can grow

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9
Q

For every ____oC rise in temperature, air holds TWICE as much moisture in vapor form.

A

15oC

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10
Q

What happens if drying is too rapid?

A

a dry skin forms and traps moisture inside, called case hardening

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11
Q

Oxygen exposure causes the formation of what? In foods high in what?

A

formation of tannic acid (browning) in foods high in polyphenols such as apples, grapes, and tea

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12
Q

Are dehydration temperatures high enough to denature enzymes?

A

No, so many foods are pretreated to inactive enzymes

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13
Q

Name 2 ways to control enzymatic activity with heat. What products are they good for?

A
  • Pasteurization (animal based: eggs, milk)

- Blanching in boiling water (vegetables)

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14
Q

What is sulfiting?

A

involves soaking food in a sodium bisulfite and water solution for 10 to 30 minutes
- to control enzymatic activity

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15
Q

What are 2 downsides to sulfiting?

A

– extends drying time

– may cause breathing difficulties for people allergic to sodium bisulfite

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16
Q

What is sulfuring?

A

involves exposing fruits to fumes from burning sulfur for up to 4 hours
- to control enzymatic activity

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17
Q

What is a disadvantage of sulfuring?

A

A disadvantage is that some people are allergic to sulfur dioxide

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18
Q

What nutrient do both sulfiting and sulfuring destroy?

A

Thiamin (vitamin B1)

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19
Q

What are advantages of sulfuring?

A

shortened drying time, inhibited mold growth due to sulfur dioxide fumes, and the ability of sulfur dioxide odors to repel insects

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20
Q

What are advantages of sulfuring and sulfiting?

A

few or no effects on heat sensitive nutrients,

food color, and texture

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21
Q

Name the 6 dehydration methods.

A
  • Tray Drying
  • Belt Drying
  • Drum Drying
  • Spray Drying
  • Vacuum Drying
  • Freeze-Drying
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22
Q

What is tray drying? What foods use this method?

A
  • Food is held on trays with holes in an enclosed cabinet for up to 20 hours
    – Air is blown over the food or rises up through the trays
    – Moisture-filled air is vented out of the system
  • Small-scale operations (fruits and vegetables)
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23
Q

What is belt drying? What moisture levels can it reach in an hour? What is an advantage? What foods use this method?

A
  • This method continuously feeds food into a tunnel dehydrator on moving belts
  • 5 to 7% in an hour
  • Even drying
  • Grains, peas, beans
24
Q

What is drum drying? What determines the size of the drum and the speed of rotation? What foods use this method?

A
  • Rotating heated drums pick up food and dry it as the drums rotate
  • The drying time
  • Heat-resistant foods that are brittle when dry: mashed potatoes, tomato pastes
25
Q

What is spray drying? What foods use this method?

A

Very small drops of the food are sprayed into the top of a heated chamber or tower and dehydrate as they fall
- Heat-sensitive products (milk, eggs, protein powders, flavorings, instant coffee)

26
Q

Which drying methods is the fastest? How long?

A

Spray drying

Food can dehydrate in seconds

27
Q

What is vacuum drying? What is this used for?

A

Heated trays or shelves called platens transfer heat in a vacuum chamber
- Used for fruit juices (expensive)

28
Q

How does vacuum drying minimize heat damage?

A

Lowering the atmospheric pressure lowers the boiling point, minimizing heat damage

29
Q

Which dehydration method produces the highest quality?

A

Vacuum drying

30
Q

What is freeze-drying also called?

A

Lypholization or dehydrofreezing

31
Q

What is freeze-drying?

A

– Food is frozen, then placed in a vacuum chamber and heated

– Food temperatures and atmospheric pressure are lowered until water sublimates

32
Q

Which dehydration method maintains better flavour and colour than other drying methods?

A

Freeze-Drying

33
Q

How do you oven dry foods at home?

A
  • Oven door is propped slightly open to create airflow

- Oven temp : 60-66oC

34
Q

What can be used to dry fresh herbs?

A

Microwave

35
Q

When should drying take place?

A

Right after harvest

36
Q

How should home-dried foods be stored?

A

In reclosable plastic bags

37
Q

Give examples of foods that need to be rehydrated before eating.

A
  • Nonfat dry milk

- Instant potatoes or soup mixes

38
Q

Raisins are dusted with what? For what?

A

Dusted with starch to prevent moisture transfer

39
Q

What is food concentration?

A

Removing a portion of the water from a food product

- First step in dehydration

40
Q

What are concentrates?

A

foods that are reduced in volume by having part of their water removed

41
Q

What are benefits of concentrates?

A

lower shipping costs because of less volume and weight, extended shelf life, and better handling ease prior to dehydration

42
Q

What are problems with concentrates?

A

cooked flavors, color changes, gritty textures,

and denatured proteins

43
Q

Low-acid foods require additional what? Why?

A

Additional preservation methods to destroy pathogens

44
Q

What is the oldest way to remove water? What does it require? What can it result in? It is the method to make what?

A

– is the oldest way to remove water
– requires frequent stirring
– can result in flavor and color changes with high heat and long cooking times
– is the method used to make some jams, jellies, and condensed soups

45
Q

Name 4 methods of concentration.

A
  • Open kettle
  • Heat evaporation
  • Vacuum evaporation
  • Filtration
46
Q

What is the heat evaporation method? What is it exposed to? Why does it have a fresher flavour than open kettle?

A

– enters and leaves an evaporator in a
continuous process
– is exposed to high temperatures for short periods
– has a fresher flavor than with the open kettle method because heat damage is limited

47
Q

What is the vacuum evaporation method?

A

– a vacuum is added to the evaporator and food moves through a series of chambers, each with lower atmospheric pressure

48
Q

What is the filtration method?

A

smaller particles pass through filters, isolating and concentrating the original food source

49
Q

What is the moisture content of intermediate-moisture foods? They have enough dissolved solutes to prevent the growth of what?

A
  • 20 to 50%

- Prevent the growth of microbes

50
Q

Intermediate-moisture foods are dense in what? Why? Do they need refrigeration, preservatives, or antioxidants? Why?

A

– are nutrient and calorie dense because they are concentrated
– may or may not need refrigeration
– may need preservatives or antioxidants added to prevent enzymatic activity

51
Q

Do you want to lower or raise the atmospheric pressure in a dehydrator? Why?

A

Lower, since it will enable water to escape from the food at low temperatures. It lowers the energy needed for a liquid-to-gas phase change.

52
Q

How do the platens in vacuum drying transfer heat to the food?

A

Through conduction

53
Q

Why is freeze-drying costly?

A

The pores will fill up with oxygen if it is removed from the vacuum. So, they seal and pack the food in a nitrogen-filled environment

54
Q

What is the moisture level of dehydrated foods? Of concentrated?

A

Dehydrated: below 15%
Concentrated: as much as 80%

55
Q

What causes low water activity in intermediate-moisture foods?

A
  • they have enough dissolved solutes (sugars, salts, minerals) to prevent the growth of microbes.
  • The lack of free water causes microbes in these foods to dehydrate and die.
  • ex: honey and molasses

1) Introduction to Food Science (20 decks)

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