145 | Lesson 5

Drying and Dehydration

What are the different factors affecting drying rate?

1. nature of the material

2. shape

3. size and arrangement of the material in the dryer

4. temperature

5. humidity

6. velocity of air used for drying

How does the shape of a food material affect drying rate?

• round = slower

• rectangle = faster (more SA exposed)

These trays are used for drying to allow air circulation.

perforated trays

What is case hardening?

Formation of a hard, dry outer layer while the interior remains moist

Temperature used in exercise 1 for drying.

55ºC

How does humidity affect drying rate?

low RH = faster moisture removal

What are the different effects of drying in food?

1. reduces weight to be carried per unit food value

2. brings un/desirable changes in food

3. oxidation

4. loss of heat-sensitive vitamins

5. concentration of fiber and mineral

T/F: High-sugar foods generally dry faster because sugar increases the availability of water for evaporation.

False

T/F: Low-humidity air generally promotes faster drying than high-humidity air.

True

T/F: An increase in mineral concentration after drying necessarily means that the food gained minerals.

False

What are the changes in food caused by drying?

• size and shape

• color

• texture

• flavor

What are the factors affecting the quality of dried foods?

1. mc

2. storage conditions

3. packaging

4. other treatments (sodium metabisulfite/benzoate)

What is the typical moisture content of dried fruits?

15-25%

What is the typical moisture content of dried vegetables?

5-10%

Why can dried fruits have a higher moisture content than dried vegetables?

• high sugar content - lowers Aw

• high acid content - inhibits mcgs

T/F: Dried foods should be stored in cool and dry conditions to prevent Maillard reaction.

True

What is the main purpose of packaging dried foods?

to protect them from moisture uptake, oxygen uptake, and light exposre

It prevents browning and helps maintain color during drying and storage.

sodium metabisulfite

It acts as an antimicrobial/antimycotic preservative by forming benzoic acid.

sodium benzoate

What are the three major areas used to compare sun-drying and mechanical dehydration?

• cooking quality

• sanitary issues/process requirements

• cost efficiency

T/F: The cooking quality of dehydrated foods is generally superior to sun-dried foods.

True

T/F: Mechanical dehydration allows better control of sanitary conditions.

True

T.F: Mechanical dehydration generally requires a shorter drying time.

True

T/F: The superior quality and higher yield of dehydrated products can allow them to command higher market value.

True

Give differences between sun-drying and mechanical dehydration.

Sun drying

• low equipment cost

• weather dependent

• longer drying time

• less sanitary control

• lower product quality

• lower market value

Mechanical dehydration

• higher equipment cost

• controlled conditions

• faster drying

• better sanitation

• better cooking quality and yield

• higher market value

What are the main pre-drying operations in dehydration?

• receiving

• sorting/selection

• washing

• pre-conditioning

• peeling

• cutting and trimming

• blanching/cooking

What are considered during sorting/selection?

maturity and appearance

What is the purpose of pre-conditioning?

to facilitate peeling

How is pre-conditioning performed in the drying process?

heating in water

What are the different types of peeling methods used in pre-drying operations?

• mechanical

• chemical (lye)

• thermal shock

Rapid application of heat, such as high-pressure steam, causing the skin to expand and separate from the tissue.

thermal shock peeling

What are the major purposes of blanching before dehydration?

• preserve color

• inactivate enzymes (PPO)

• reduce surface microbial load

• expel oxygen from tissue

Blanching that results in better rehydration.

steam

What type of blanching is preferred for fruits?

syrup

What treatments can fruits and vegetables undergo before/during drying to prevent browning?

dipping in sulfites, ascorbic acid, or fruit juice

What are antimycotic agents used in drying operations?

sodium benzoate and sorbate

What are the other additives used in drying operations?

• bleaching agent

• firming agent

• humectant

• anti-caking agent

This firming agent retains the firmness of dried fruit overtime.

CaCl

Give examples of anti-caking agents.

• silicon dioxide (SiO₂)

• calcium silicate

What are the different post-drying operations?

1. sweating

2. resulfuring

3. packaging

4. pasteurization

5. storage

A holding period that allows moisture to equilibrate among individual pieces of dried food.

sweating

What is the purpose of sweating in dried foods?

• equilibration of moisture among individual pieces

• prevention of moisture condensation

How long does sweating typically take?

7-10 days

This post-drying operation is done to prevent non-enzymatic browning during storage, insect activities, and for nutrient preservation.

re-sulfuring

Re-sulfuring is done to prevent _ during storage and insect activities.

NEB

T/F: In re-sulfuring of dried products, fresh sulfur dioxide must be used.

True

What is the role of packaging in post-drying operations?

For protection against moisture, air contamination with microorganisms, and infestation with insects

What is the purpose of pasteurization of dried foods?

To kill pathogens and destroy spoilage microorganisms

What are the two pasteurization methods?

• freezer

• oven

What are the conditions for the freezer method of pasteurization?

0ºC for at least 48 hours

What are the conditions for the oven method of pasteurization?

70-80ºC for 15 minutes

What are the desired characteristics of dehydrated products?

• Minimal browning 

• Comparable taste, odor, and appearance to other processed products

• Minimal nutrient loss and rancidity

What are the undesired characteristics of dehydrated products?

• Pronounced and uneven shrinkage

• Case hardening

• Darkening