FNH 200 Lecture Notes - Lecture 8: Maillard Reaction, Food Preservation, Instant Mashed Potatoes
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Dehydration of foodstuffs involves the removal of water to increase the storage stability of perishable food items.
You will learn about processing parameters that affect the ultimate quality of dehydrated plant and animal
tissues and fluids that are used as food. You will learn about the principles of spray drying and freeze drying of
foods that are amenable to these dehydration methods. The advantages and disadvantages of various
dehydration technologies will be discussed. You will learn packaging requirements for maintenance of the quality
of dehydrated foods.
Upon completion of this lesson, you will be able to:
describe the underlying concepts of various methods of food dehydration
scientific principles for extension of storage life of foods by dehydration
methods for dehydrating different foods, and the consequences in terms of food quality
explain factors affecting the rate of dehydration
describe the packaging requirements for foods dehydrated by various dehydration methods
Some reasons for dehydrating foods are:
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
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 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:
Recapping from Lesson 2:
Water activity (aw) defines the proportion of water in a food that is in the free, unbound form
Microbial activity, enzymatic activity and chemical reactions can occur only in the free water phase of
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 aw below 0.6
Chemical reactions (e.g. Maillard browning) can begin to occur at aw of 0.3
Please visit this website from the Cole Palmer Instrument Company for an overview on water activity, including examples of water
activity values for several food products
It is important to remember that with dehydration, microorganisms are not readily killed. Once the food
is rehydrated (reconstituted), microorganisms resume growth if favourable conditions exist.
It is also important to 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
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.
Changes in Food during Dehydration
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. Some of these changes are:
Cell/tissue Shrinkage. As water is removed from food pieces during dehydration, the cells within the
tissue shrink and lose their elasticity. If you have purchased dehydrated vegetables such as carrots, onion
slices, or dehydrated fruits such as apple cubes or slices you will have no doubt observed the shrinkage
that has occurred. 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.
Sequence of events that occur during dehydration. (click on the diagram to get a better
Case hardening. 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
Chemical changes. A number of chemical changes occur in foods during dehydration in systems
employing warm air as the drying agent. The Maillard browning reactions (Lesson 2) 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 in order to 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, egg
whites are treated with an enzyme, glucose oxidase, which "desugars" the egg whites and minimizes the
colour and flavour changes that could be caused by the Maillard reactions involving glucose.
Loss of ease of rehydration can occur because of the loss of the ability of some hydrophilic food
constituents to absorb water. Heat denaturation of proteins, starches and gums can decrease the water-
holding capacity of dehydrated foods. The salts and sugars concentrated on the outside of the food pieces
will dissolve in the water added to the food to rehydrate it. Since those water soluble components are not
inside the food pieces, there is less attraction for water to enter the food product. As result, rehydration is
less complete. You may have noticed that dehydrated fruit pieces are much sweeter than the fresh fruit.
The reason for this phenomenon is that the sugars are concentrated on the outside of the fruit.
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.
Dehydrated foods may show varying extents of shrinkage or chemical changes, depending on the method and
conditions used to dry the food.
Factors Affecting Dehydration
Dehydration of food requires that water (mass) be transferred from the food into the dehydrating environment,
and that heat (the driving force that encourages water removal) be transferred to the food to promote water
removal from the food. The objectives of food dehydration operations are to dry the food as fast as possible, at
the least cost, while creating the fewest changes in food quality.
The composition of the food itself can have an effect of the rate at which dehydration occurs. For example, if
water is bound to solutes in the food it will have a lower vapour pressure and therefore will be more difficult to
remove. The porosity of the food is also important. Efforts are made to enhance the porosity of foods to be
dehydrated in order to facilitate mass transfer and speed of drying rate, thus maximizing the efficiency of
dehydration. Porous (sponge-like) structures are formed by creating steam pressure within the product during
the drying process. The steam will "puff" the product. Another way of creating porosity is by making a stable
foam from a liquid food prior to drying.
In addition to the composition of food, the following factors can also affect heat and mass transfer within food
materials undergoing dehydration, and therefore are important to consider in order to control some of the
undesirable changes described previously:
Surface area. It is desirable to maximize the surface-to-volume ratios of the food to be dehydrated so as 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, 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. An upper limit to drying air temperature is dictated by adverse chemical reactions that
can take place in a food at high temperatures. The upper temperature limit is also dictated by the chemical
and physical nature of the food.
Air velocity. The faster the air velocity within a dehydrator, the more rapid the rate of moisture removal.
Food dehydrators are designed to maximize the velocity of heated air moving around the food particles to
Humidity of the drying air. The drier the air, the more moisture it can absorb before it becomes saturated.
The relative humidity of the drying air determines the final moisture content of the food being dried.
Knowledge of the equilibrium relative humidity of food is important for the proper design of dehydrators and
for the design of packaging systems that will prevent moisture adsorption by the dehydrated food during