Dehydration as a Food Preservation Method Flashcards
Give some reasons to dehydrate a food.
- 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; and
- production of convenience items (instant coffee, instant mashed potatoes, vegetables that rehydrate in instant soup preparations).
Food preservation by dehydration is based on what principle?
Food preservation by dehydration is based on the principle that microbial growth, chemical and enzymatic reactions occur only if sufficient free water is present.
When the water activity of foods is lowered there is a direct impact on microbial growth as well as chemical & enzymatic reactions.
What is the water activity of dehydrated foods?
Microbial activity, enzymatic activity and chemical reactions can occur only in the free water phase of foods
Water activity of foods ranges from 0 to 1.0
Water activity of dehydrated foods is in the range of 0.2 to 0.6
Microorganisms cannot grow at water activity below 0.6
Does dehydration kill microorganisms?
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.
Distinguish between food dehydration and concentration, both of which involve the removal of water from foods.
Dehydration implies 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 in order 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.
Similar to the other preservation methods we have reviewed, dehydration will cause changes in the food that need to be controlled in order to maintain the highest quality possible. Give examples of some of these changes.
- Cell/tissue shrinkage
- Case hardening
- Chemical changes
Describe cell/tissue shrinkage due to dehydration.
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 of 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 attraction of water into the food pieces during rehydration.
Describe case hardening due to dehydration.
Case hardening 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 possible.
Describe chemical changes due to dehydration.
The Maillard browning reactions cause the development of brown colours and the formation of flavours not originally associated with the fresh product.
Loss of volatile substances that contribute to the flavour of foods occurs during dehydration.
Generally the higher the drying temperature, the larger the loss of volatiles, with the result that the dehydrated food is less flavourful than the initial product.
Why are drying systems designed to remove water through the 20-15% range of moisture content as rapidly as possible?
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.
This will minimize the negative effects the Maillard reaction has on the flavour of dehydrated food products.
The flavour of rehydrated skim milk powder is due largely to the products of the Maillard reaction during dehydration of the milk.
Prior to dehydration of egg whites, they are treated with an enzyme, glucose oxidase, which “de-sugars” the egg whites and minimizes the colour and flavour changes that could be caused by the Maillard reactions involving glucose.
Why are dehydrated fruit pieces much sweeter than the fresh fruit?
The reason for this phenomenon is that the sugars are concentrated on the outside of the fruit.
In addition to the composition of food, what factors can also affect heat and mass transfer within food materials undergoing dehydration?
These factors are important to consider in order to control undesirable changes.
Surface area. It is desirable to maximize the surface-to-volume ratio of the food to be dehydrated to minimize the resistance to heat and mass transfer. Generally, the smaller the food piece, the more rapid the rate of moisture loss.
Temperature. The hotter the air (there is an upper limit though), the more moisture it will hold before becoming saturated. Drying systems are designed to maximize temperature differences between the product and the drying air to increase the rate of dehydration.
Air velocity. The faster the air velocity within a dehydrator, the more rapid the rate of moisture removal.
Humidity of the drying air. The drier the air, the more moisture it can absorb before it becomes saturated.
Atmospheric pressure and vacuum. Water boils at 100°C when it is at a pressure of 1 atm (760 mm Hg). As the pressure lowers, the boiling temperature will decrease. If the temperature is maintained constant, a decrease in pressure will increase the rate of boiling. Some concentrators and dehydrators are operated at pressures below atmospheric pressure in order to increase the rate of boiling and moisture removal. This is especially important in the case of heat-sensitive food products.
Why do crackers experience a loss of crispness in an opened package?
Crackers have a low equilibrium relative humidity and they tend to adsorb water from the air.
Describe sun drying.
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.
Describe spray drying.
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.
Describe tray (air) drying.
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 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, spices.
Describe drum drying.
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.
Describe freeze drying.
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 is called sublimation.
During freeze drying, because the food remains rigid during dehydration, the subliming water leaves voids where the ice crystals were located.
There is no translocation of water-soluble constituents because there is no movement of liquid, allowing freeze dried foods to retain their shape.
Also, freeze dried foods rehydrate almost completely because the voids left by the subliming water provide channels through which water can enter the food, and the water-soluble components of the food in their original locations provide the driving force for rehydration.
Freeze dried foods do not usually exhibit the shrinkage and chemical changes noted earlier to occur in other dehydrated foods.
Describe extrusion.
Slurry of food is passed though a tube, under pressure, that is heated by steam.
The moist heat causes starch gelatinization and cooking of the product.
Product is forced though 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.
Describe packaging considerations for dehydrated food products.
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.
