Chapter 20: Dehydration and Concentration: Controlling Water Activity Flashcards
What is water activity an indicator of? Reducing it can do what?
- Indicator of perishability
- Reducing water activity can increase shelf life
Name 3 categories of processed foods preserved by reduced water activity.
– dehydrated
– concentrated
– intermediate-moisture
What is the oldest method of food preservation? What is it caused by?
- Dehydration
- Caused by lowering water content (water activity)
What is the artificial drying of food under controlled conditions?
Dehydration
Name 2 benefits of dehydration.
– lighter foods that take less space and are less
costly to package and ship
– a more-convenient form of food
Can dehydrated foods be reconstituted? Give an example.
reconstituted by adding water, such as with instant mashed potatoes
Name 4 factors affecting dried food quality.
- Surface area (greater area, faster it dries)
- Airflow (moving air with heat speeds drying)
- Temperature (faster drying = less change in food)
- Oxidation (browning)
What happens if food is not dried enough?
Bacteria or mold can grow
For every ____oC rise in temperature, air holds TWICE as much moisture in vapor form.
15oC
What happens if drying is too rapid?
a dry skin forms and traps moisture inside, called case hardening
Oxygen exposure causes the formation of what? In foods high in what?
formation of tannic acid (browning) in foods high in polyphenols such as apples, grapes, and tea
Are dehydration temperatures high enough to denature enzymes?
No, so many foods are pretreated to inactive enzymes
Name 2 ways to control enzymatic activity with heat. What products are they good for?
- Pasteurization (animal based: eggs, milk)
- Blanching in boiling water (vegetables)
What is sulfiting?
involves soaking food in a sodium bisulfite and water solution for 10 to 30 minutes
- to control enzymatic activity
What are 2 downsides to sulfiting?
– extends drying time
– may cause breathing difficulties for people allergic to sodium bisulfite
What is sulfuring?
involves exposing fruits to fumes from burning sulfur for up to 4 hours
- to control enzymatic activity
What is a disadvantage of sulfuring?
A disadvantage is that some people are allergic to sulfur dioxide
What nutrient do both sulfiting and sulfuring destroy?
Thiamin (vitamin B1)
What are advantages of sulfuring?
shortened drying time, inhibited mold growth due to sulfur dioxide fumes, and the ability of sulfur dioxide odors to repel insects
What are advantages of sulfuring and sulfiting?
few or no effects on heat sensitive nutrients,
food color, and texture
Name the 6 dehydration methods.
- Tray Drying
- Belt Drying
- Drum Drying
- Spray Drying
- Vacuum Drying
- Freeze-Drying
What is tray drying? What foods use this method?
- 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)
What is belt drying? What moisture levels can it reach in an hour? What is an advantage? What foods use this method?
- This method continuously feeds food into a tunnel dehydrator on moving belts
- 5 to 7% in an hour
- Even drying
- Grains, peas, beans
What is drum drying? What determines the size of the drum and the speed of rotation? What foods use this method?
- 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
What is spray drying? What foods use this method?
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)
Which drying methods is the fastest? How long?
Spray drying
Food can dehydrate in seconds
What is vacuum drying? What is this used for?
Heated trays or shelves called platens transfer heat in a vacuum chamber
- Used for fruit juices (expensive)
How does vacuum drying minimize heat damage?
Lowering the atmospheric pressure lowers the boiling point, minimizing heat damage
Which dehydration method produces the highest quality?
Vacuum drying
What is freeze-drying also called?
Lypholization or dehydrofreezing
What is freeze-drying?
– Food is frozen, then placed in a vacuum chamber and heated
– Food temperatures and atmospheric pressure are lowered until water sublimates
Which dehydration method maintains better flavour and colour than other drying methods?
Freeze-Drying
How do you oven dry foods at home?
- Oven door is propped slightly open to create airflow
- Oven temp : 60-66oC
What can be used to dry fresh herbs?
Microwave
When should drying take place?
Right after harvest
How should home-dried foods be stored?
In reclosable plastic bags
Give examples of foods that need to be rehydrated before eating.
- Nonfat dry milk
- Instant potatoes or soup mixes
Raisins are dusted with what? For what?
Dusted with starch to prevent moisture transfer
What is food concentration?
Removing a portion of the water from a food product
- First step in dehydration
What are concentrates?
foods that are reduced in volume by having part of their water removed
What are benefits of concentrates?
lower shipping costs because of less volume and weight, extended shelf life, and better handling ease prior to dehydration
What are problems with concentrates?
cooked flavors, color changes, gritty textures,
and denatured proteins
Low-acid foods require additional what? Why?
Additional preservation methods to destroy pathogens
What is the oldest way to remove water? What does it require? What can it result in? It is the method to make what?
– 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
Name 4 methods of concentration.
- Open kettle
- Heat evaporation
- Vacuum evaporation
- Filtration
What is the heat evaporation method? What is it exposed to? Why does it have a fresher flavour than open kettle?
– 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
What is the vacuum evaporation method?
– a vacuum is added to the evaporator and food moves through a series of chambers, each with lower atmospheric pressure
What is the filtration method?
smaller particles pass through filters, isolating and concentrating the original food source
What is the moisture content of intermediate-moisture foods? They have enough dissolved solutes to prevent the growth of what?
- 20 to 50%
- Prevent the growth of microbes
Intermediate-moisture foods are dense in what? Why? Do they need refrigeration, preservatives, or antioxidants? Why?
– 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
Do you want to lower or raise the atmospheric pressure in a dehydrator? Why?
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.
How do the platens in vacuum drying transfer heat to the food?
Through conduction
Why is freeze-drying costly?
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
What is the moisture level of dehydrated foods? Of concentrated?
Dehydrated: below 15%
Concentrated: as much as 80%
What causes low water activity in intermediate-moisture foods?
- 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
