Chapter 20: Dehydration and Concentration: Controlling Water Activity

• Indicator of perishability

- Reducing water activity can increase shelf life

– dehydrated
– concentrated
– intermediate-moisture

• Dehydration

- Caused by lowering water content (water activity)

Dehydration

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

reconstituted by adding water, such as with instant mashed potatoes

• Surface area (greater area, faster it dries)

• Airflow (moving air with heat speeds drying)

• Temperature (faster drying = less change in food)

• Oxidation (browning)

Bacteria or mold can grow

15oC

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

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

No, so many foods are pretreated to inactive enzymes

• Pasteurization (animal based: eggs, milk)

- Blanching in boiling water (vegetables)

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

– extends drying time

– may cause breathing difficulties for people allergic to sodium bisulfite

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

A disadvantage is that some people are allergic to sulfur dioxide

Thiamin (vitamin B1)

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

few or no effects on heat sensitive nutrients,

food color, and texture

• Tray Drying

• Belt Drying

• Drum Drying

• Spray Drying

• Vacuum Drying

• Freeze-Drying

• 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)

• This method continuously feeds food into a tunnel dehydrator on moving belts

• 5 to 7% in an hour

• Even drying

• Grains, peas, beans

• 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

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)

Spray drying

| Food can dehydrate in seconds

Heated trays or shelves called platens transfer heat in a vacuum chamber

• Used for fruit juices (expensive)

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

Vacuum drying

Lypholization or dehydrofreezing

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

| – Food temperatures and atmospheric pressure are lowered until water sublimates

Freeze-Drying

• Oven door is propped slightly open to create airflow

| - Oven temp : 60-66oC

Microwave

Right after harvest

In reclosable plastic bags

• Nonfat dry milk

| - Instant potatoes or soup mixes

Dusted with starch to prevent moisture transfer

Removing a portion of the water from a food product

| - First step in dehydration

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

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

cooked flavors, color changes, gritty textures,

| and denatured proteins

Additional preservation methods to destroy pathogens

– 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

• Open kettle

• Heat evaporation

• Vacuum evaporation

• Filtration

– 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

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

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

• 20 to 50%

| - Prevent the growth of microbes

– 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

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.

Through conduction

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

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

• 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