Module 8 Flashcards
1
Q
Why dehydrate foods? [4]
A
- preservation of the food (dried milk, juices, fruit)
- retention of the size and shape of the food while imparting storage stability (freeze-dried steak, vegetable pieces)
- reducing weight and bulk of food for easier storage and transportation
- production of convenience items (instant coffee, instant mashed potatoes, vegetables that rehydrate in instant soup preparations).
2
Q
What is the basis of preservation of food by dehydration?
A
- Microbial growth, chemical and enzymatic reactions occur only if sufficient free water is present.
- When water activity is lowered there is a direct impact on microbial growth as well as chemical and enzymatic reactions.
It is important to remember that with dehydration, microorganisms are not readily killed. Once the food is rehydrated, microorganisms resume growth if favourable conditions exist.
3
Q
Discern between dehydration and concentration.
A
- Dehydration implies the removal of as much water from the food as possible in order to impart a long storage life.
- Concentration, on the other hand, implies that some of the water is removed from the food to concentrate the food constituents. Concentrated foods are not inherently shelf-stable and require the use of other forms of food preservation (e.g., refrigeration, freezing, dehydration, thermal processing) to extend storage life.
4
Q
Describe cell/tissue shrinkage during dehydration.
A
- As water is removed from food pieces during dehydration, the cells within the tissue shrink and lose their elasticity.
- Part of the reason for shrinkage in foods that have been dehydrated is that the water migrates from the interior of the food to the surface where it finally evaporates and is carried away by the dehydrating medium.
- As the water migrates to the surface of the food it carries with it the water-soluble substances dissolved in it.
- The loss of these substances from the interior of the food pieces contributes to the shrinkage observed in dehydrated foods and also contributes to the poor rehydration properties of such foods.
- Loss of the water-soluble components from the interior portions of the food pieces decreases the driving force for the attraction of water into the food pieces during rehydration.
5
Q
What is case hardening?
A
- Occurs when rapid drying causes compounds such as sugars to form a hard, fairly impermeable case around the food piece.
- This phenomenon can cause the rate of dehydration to decrease.
- Case hardening can occur in high-sugar products such as tropical fruit and many temperate fruit products.
- Dehydration procedures are designed to minimize the development of case hardening as much as possible.
6
Q
Describe chemical changes that may occur during dehydration.
A
- The Maillard browning reactions cause the development of brown colours and the formation of flavours not originally associated with the fresh product.
- The Maillard reactions proceed most rapidly when the water content of the food is in the range of 20% down to 15% because the reactants are in very close proximity, which increases the probability of reactions occurring. Thus, drying systems are designed to remove water through the 20-15% range of moisture content as rapidly as possible.
7
Q
In addition to the composition of the food, what other factors are important to consider that affect heat and mass transfer within food materials during dehydration. [5]
A
- Surface area - desirable to maximize surface-to-volume ratio to minimize resistance to heat and mass transfer
- Temperature - hotter air can hold more moisture before becoming saturated; an upper limit to drying air temperature is dictated by adverse chemical reactions that can take place in food at high temperatures
- Air velocity - faster velocity means faster moisture removal
- Humidity - drier air can absorb more moisture
- Atmospheric pressure and vacuum - some dehydrators are operated at pressures below atmospheric pressure to increase the rate of boiling and moisture removal
8
Q
Describe sun drying.
A
- Sun drying is mostly used in dry, warm climates. This is a very slow drying method (several days). It is mostly used for fruits, vegetables, and fish.
- An advantage of this method is the fact that it is quite inexpensive; however, disadvantages include long drying periods (up to several weeks) and the risk of invasion by insects, birds, rodents, and microorganisms.
The appearance is shrunken and has poor rehydration capacity.
9
Q
Describe spray drying.
A
- Spray driers are used to produce the greatest quantities of commercially dehydrated foods.
- Spray driers are restricted to use with liquid foods since the principle of the operation is the introduction of the food as a spray of small droplets into a high-velocity stream of warm air.
- Because droplet sizes are small, drying rates are very rapid and high-quality dehydrated food products can be produced.
- Foods most commonly dehydrated by spray driers include skim milk, coffee, tea and eggs.
You will note that the equipment is designed to maximize the drying rate, produce dry particles of uniform size and prevent the sticking of partially dry food particles to the walls of the spray driers. You should also note that spray driers operate continuously, that is, the product is sprayed into the drying chamber and the dried product and moist air are separated and removed from the driers.
10
Q
Describe tray drying.
A
- Food placed on trays or racks is exposed to heated air at a set velocity.
- This type of drying can be quite fast and requires heated air with a relative humidity (%RH) lower than that of the product to be dried.
- Water-soluble components migrate to the surface of food and are deposited as water evaporates.
- The dried food has relatively poor re-hydration properties, is shrunken in appearance and is very dense.
- Products dried this way are: pasta, vegetables, fruit, and spices. The diagram below shows the sequence of events that occur during the dehydration of fruit tissue.
11
Q
Describe drum drying.
A
- During the process of drum drying, food paste is applied to a heated drum in a thin layer to promote rapid drying.
- As the drum rotates, it picks up a thin film of food material that dries rapidly.
- The dried food is scraped off the drum near the end of a full rotation of the drum. Dehydrated mashed potatoes and some ready-to-eat breakfast cereals are dried this way. Some popular low-fat snack foods and potato chip like products contain drum-dried potato flakes as the primary ingredient.
12
Q
Describe freeze drying.
A
- Freeze drying is restricted to high-value foods because of the high costs associated with this dehydration method.
- Water is removed from food in the frozen state without transition through the liquid state. This phenomenon, called sublimation.
- Sublimation of water is accelerated under vacuum conditions.
- During freeze-drying, food is frozen and then placed in the freeze dryer, the dryer is sealed and a vacuum is created and maintained.
- The application of heat from radiant heaters within the shelves of the freeze drier provides the energy required for sublimation to occur.
- During freeze-drying the amount of heat applied to the food is carefully controlled to maximize the rate of drying without causing the transition of water from the solid to the liquid phase.
13
Q
Describe vacuum microwave drying.
A
- The technology consists of a combination of vacuum (to keep the temperature low) and microwaves (for ultra-rapid energy transfer), producing high-quality products with less nutrient loss, better flavour retention, and less colour change.
- Vacuum microwave dried (VMD) products retain a more natural appearance and have the advantage of complete re-hydration (reconstitution).
- VMD is a quick drying method when compared to freeze drying and air drying methods (VMD can take only a few minutes, compared to hours in freeze-drying).
Vacuum microwave or radiant energy vacuum (REV) technology is being developed for the dehydration of food, nutraceutical and pharmaceutical products by Dr. Tim Durance in the Food Science program at the University of British Columbia.
14
Q
Describe deep fat frying.
A
- During deep fat frying, the high temperature of hot oil causes the water in the food to evaporate rapidly, often accompanied by substantial pick up of oil by the food being dried.
- Dried foods such as potato chips have a low water content (as well as a low water activity) but a high oil content. Many snack foods and bakery products, such as donuts, are produced using deep fat frying.
Do you know how instant noodles are made? After cooking and shaping, the noodles are dried either by deep-fat frying or non-frying (hot-air) drying. Frying is usually done at 140-150°C for 1-2 minutes, while hot-air drying uses a temperature of about 80°C for 30 min. The two drying methods yield products that differ greatly in fat content.
15
Q
Describe extrusion drying.
A
- A slurry of food is passed through a tube, under pressure, that is heated by steam.
- The moist heat causes starch gelatinization and cooking of the product.
- The product is forced through a narrow opening (a die which can produce a product with a variety of shapes) at the end of the tube and escaping steam causes the dehydrating product to puff.
- Many ready-to-eat breakfast cereals and snack foods are produced this way.
16
Q
Describe packaging requirements for dehydrated foods.
A
- Dehydrated foods, because of their low water activity, are hygroscopic (readily pick up moisture).
- The primary requirement that must be met by packaging material used with dehydrated foods is that there must be no transmission of water vapour from the surrounding environment into the food.
- Packaging should also protect against oxygen and light.
- Many dried foods also require that the packaging material provide physical protection to prevent the food from becoming crushed during distribution and handling.
