Chapter 20: Dehydration and Concentration: Controlling Water Activity Flashcards
What is Water Activity (Aw)?
Water activity is an indicator of perishability
Reducing Water activity does what?
Reducing Water Activity can increase shelf life
Categories of Processed foods preserved by reduced water activity are?
- Dehydrated
- Concentrated
- Intermediate-moisture
What is Dehydration?
The oldest method of food preservation
- caused by lowering water content
In the Food Industry, What is Dehydration?
It is the artificial drying of food under controlled conditions
- functions to protect food spoilage
What are the Benefits of Dehydration?
- Lighter foods that take less space are less costly to package and ship
- A more-convenient form of food
Dehydrated foods are?
Left in a dehydrated state, like prunes and raisins
- Reconstituted by adding water, such as with instant mashed potatoes
What does Dehydration do?
Dehydration preserves food by lowering the water activity level, thus preventing spoilage
What is Surface Area and why is it important?
Surface area is an important factor; because the greater the exposed surface area is, the faster the food dries
ie) Thin uniform pieces speed the process
If food is not dried enough what can happen?
If food is not dried enough, bacteria or mold can grow
Dehydrators use what for drying?
Dehydrators use a heat source, a fan or a blower and a ventilated drying surface
- home food dehydrators operate on the same principle
For every 15C (27F) rise in temperature what happens?
For every 15C (27F) rise in temperature, air holds twice as much moisture in vapour form
What does faster drying mean?
Faster drying means less change in food quality
What is a Case Hardening?
If drying is too rapid, a dry skin forms and traps moisture inside called case hardening
Oxygen exposure causes what?
Oxygen exposure causes formation of tannic acid (browning) in foods high in polyphenols such as apples, grapes and tea
- increasing surface area increases exposure to oxygen
Can Dehydration denature enzymes?
Dehydration temperature are not high enough to denature enzymes so many foods are pretreated to inactive enzymes
What is Pasteurization?
Pasteurization is used with animal-based products like milk and eggs
What is Blanching?
Blanching in boiling water is used with vegetables
What is Sulfiting?
Sulfiting involves soaking food in a sodium bisulfite and water solution for 10 to 30 minutes
What does Sulfiting do?
it extends drying time, may cause breathing difficulties for people allergic to sodium bisulfite
What is Sulfuring?
it involves exposing fruits to fumes from burning sulfur for up to 4 hours
What is a disadvantage to Sulfuring?
A disadvantage is that some people are allergic to sulfur dioxide
What happens in Sulfuring?
Food is placed on stacked trays in a covered area where sulfur dioxide (SO2) can circulate
- may be dipped in sulfur dioxide
What are some advantages of Sulfuring
Shortened drying time, inhibited mold growth due to sulfur dioxide fumes, and the ability of sulfur dioxide odours to repel insects
What are the advantages of Sulfuring and Sulfiting?
Few or no effects on heat sensitive nutrients, food colour and texture
- BOTH methods destroy thiamin
What are the factors Food Processors consider when determining the best dehydration method?
- The type of food
- What quality is desired
- How much consumer are willing to pay
- Whether food is whole, divide, pureed or liquid
What is Tray Drying (Dehydration Method)?
Food is held on trays with holes in an enclosed cabinet for up to 20hours
- Air is blown over the food or rises up through the trays
- Moisture-filled air is vented out of the system
(This method is small-scale operations for fruits and vegetables)
What is Belt Drying (Dehydration Methods)?
This methods continuously feeds food into a tunnel dehydrator on moving belts
- Food can reach moisture levels of 5% to 7% in as little as 1 hour
- Using a belt Trough dryer causes food to tumble for even drying
(grains, peas, and beans work best in this dryer)
What is Drum Drying (Dehydration Method)?
Rotating heated drums pick up food and dry it as the drums rotate
- The drying time needed determines the size of the drum and the speed of rotation
- The method is best with heat-resistant foods that are brittle when dry like…
mashed potatoes, mashed sweet potatoes and tomato pastes
What is Spray Drying (Dehydration Method)?
Very small drops of the food are sprayed into the top of a heated chamber or tower and dehydrate as they fall
- The method is used with heat-sensitive product like milk, eggs, protein powders, flavourings and instant coffee
- FASTEST drying method (can dehydrate food in seconds)
What is Vacuum Drying (Dehydration Methods)?
Heated trays or shelves called “Platens” transfer heat in a vacuum chamber
- Lowering the atmospheric pressure lowers the boiling point, minimizing heat damage
- Produces HIGHEST quality (expensive)
- Is used for fruit juices
What is Freeze-drying (lypholization) or dehydrofreezing (Dehydration Method)?
Food is frozen, then placed in a vacuum chamber and heated
- Food temperatures and atmospheric pressure are lowered until water sublimates
- The process maintains better flavour and colour than other drying methods
What is Oven Drying (Home-Dried Foods)?
Food is pretreated and arranged in single layers on each oven rack and then oven door is propped open slightly to create airflow
- Oven temperature is maintained at 60C-66C (140F-150F)
- Microwave ovens may be used to dry fresh herbs
Foods that are about to undergo dehydration should be what?
Should be clean and free from blemishes
- Drying should take place right after harvest
- Trays need to be rotated every couple of hours
Where should Home-dried foods be stored?
Home-dried foods should be stored in reclosable plastic bags
- Food showing signs of mold growth should be discarded
What is Rehydration?
some foods need to be rehydrated before eating them
- Nonfat dry milk is dissolved in water before use
- Instant potatoes or soup mixtures are added to boiling water and cooked until tender
Why are some foods undergoing rehydration dusted with starch?
Foods such as raisins for breakfast cereal are dusted with starch to prevent moisture transfer
What is Food Concentration?
Food concentration is removing a portion of the water from a food product
- It may be the first step to dehydration
What are Concentrates?
concentrates are foods that are reduced in volume by having part of their water removed, such as
- fruit juice concentrate, maple syrup, condensed milk, and condensed soups
What are the benefits of Concentrates?
Lowering shipped costs because of less volume and weight, extended shelf life and better handling ease prior to dehydration
Some Problems with concentrates are?
Cooked flavours, colour changes, gritty textures, and denatured proteins
Low-acid foods require what?
Low-acid foods require additional preservation methods to destroy pathogens
What is the Open Kettle (Concentration Methods)?
Is the oldest way to remove water, requires frequent stirring, can result in flavour and colour changes with high heat and long cooking times
- Is the method used to make some jams, jellies and condensed soups
What is the Heat Evaporation Method (Concentration Method)?
food enters and leaves an evaporator in a continuous process
- is exposed to high temperatures for short periods
- has a fresher flavour than with the open kettle method because heat damage is limited
What is Vacuum Evaporation (Concentration 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?
Small particles pass through filters, isolating and concentrating the original food source
What are Intermediate-Moisture foods?
have a moisture content of 20% to 50% with enough dissolved solutes to prevent the growth of microbes
- Are nutrient and calorie dense because they are not concentrated
- May or may not need refrigeration
- May need preservatives or antioxidants added to prevent enzymatic activity
