Transcript storage stability of dried foods

Protection of Foods by
Drying
Introduction:
 The preservation of foods by drying is based on the fact
that microorganisms and enzymes need water in order
to be active.
 Dried or low-moisture (LM) foods are those that
generally do not contain more than 25% moisture and
have a water activity (aw) between 0.00 and 0.60.
 These are the traditional dried foods.
 Freeze-dried foods are also in this category.
 Intermediate-moisture (IM) foods: Another category of
shelf-stable foods are those that contain between 15%
and 50% moisture and an aw between 0.60 and 0.85.
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PREPARATION AND DRYING OF LOW-MOISTURE FOODS
 The earliest uses of food desiccation consisted of
exposing fresh foods to sunlight until drying had been
achieved.
 Fruits such as grapes, prunes, figs, and apricots may be
dried by this method, which requires a large amount of
space for large quantities of the product.
 The drying methods of greatest commercial importance
consist of :
 Spray
 Drum
 Evaporation
 freeze-drying
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In the drying of fruits such as prunes, alkali dipping is
employed by immersing the fruits into hot lye solutions
of between 0.1% and 1.5%.
 This is especially true when sun drying is employed.
 Light-colored fruits and certain vegetables are treated
with SO2 so that levels of between 1,000 and 3,000
ppm may be absorbed.
 Treatment with SO2 treatment helps to maintain color,
conserve certain vitamins, prevent storage changes,
and reduce the microbial load.
 After drying, fruits are usually heat pasteurized at 150–
185◦F (65.6–85◦C) for 30–70 minutes.
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Blanching:
Similar to the freezing preparation of vegetable foods,
blanching or scalding is a vital step prior to dehydration.
This may be achieved by immersion from 1 to 8 minutes,
depending on the product.
The primary function of this step is to destroy enzymes that
may become active and bring about undesirable changes in
the finished product.
Leafy vegetables generally require less time than peas,
beans, or carrots.
For drying, temperatures of 140–145◦F (60–62.8◦C) have been
found to be safe for many vegetables.
The moisture content of vegetables should be reduced
below4%in order to have satisfactory storage life and quality.
Many vegetables may be made more stable if given a
treatment with SO2 or a sulfite.
Meat is usually cooked before being dehydrated.
 The final moisture content after drying should be
approximately 4% for beef and pork.
 Milk is dried as either whole milk or nonfat skim milk.
 The dehydration may be accomplished by either the
drum or spray method.
 The removal of about 60% water from whole milk
results in the production of evaporated milk, which has
about 11.5% lactose in solution.
 Sweetened condensed milk is produced by the addition
of sucrose or glucose before evaporation, so that the
total average content of all sugar is about 54%, or over
64% in solution.
 The stability of sweetened condensed milk is due in
part to the fact that the sugars tie up some of the
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Freeze drying :
 In freeze drying (lyophilization, cryophilization), actual
freezing is preceded by the blanching of vegetables and
the precooking of meats.
 The rate at which a food material freezes or thaws is
influenced by the following factors:
1. the temperature differential between the product
and the cooling or heating medium;
2. the means of transferring heat energy to, from, and
within the product (conduction, convection,
radiation);
3. the type, size, and shape of the package;
4. the size, shape, and thermal properties of the
product.
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After freezing, the water in the form of ice is removed
by sublimation.
 This process is achieved by various means of heating
plus vacuum.
 The water content of protein foods can be placed into
two groups: freezable and unfreezable.
 Unfreezable (bound) water has been defined as that
which remains unfrozen below −30◦C.
 The removal of freezable water takes place during the
first phases of drying, and this phase of drying may
account for the removal of anywhere from 40% to 95%
of the total moisture.
 The last water to be removed is generally bound water,
some of which may be removed throughout the drying
process.
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 In
studies on freeze-dried meats, it has
been shown that 40–80% of the enzyme
activity is not destroyed and may be
retained after 16 months of storage at
−20◦C.
 The final product moisture level in
freeze-dried foods may be about 2–8% or
have an aw of 0.10–0.25.
Freeze drying is generally preferred to hightemperature vacuum drying.
 Among the disadvantages of the high-temperature
vacuum drying compared to the Freeze drying are the
following:
 1. pronounced shrinkage of solids
 2. migration of dissolved constituents to the surface
when drying solids
 3. extensive denaturation of proteins
 4. Case hardening: the formation of a relatively hard,
impervious layer at the surface of a solid, caused by
one or more of the first three changes, that slows the
rates of both dehydration and reconstitution.
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5. formation of hard, impervious solids
when drying liquid solution
 6. undesirable chemical reactions in
heat-sensitive materials
 7. excessive loss of desirable volatile
constituents
 8. difficulty of rehydration as a result of
one or more of the other changes
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EFFECT OF DRYING ON MICROORGANISMS
 Although some microorganisms are destroyed in the
process of drying, this process is not lethal per se to
microorganisms, and, indeed, many types may be
recovered from dried foods, especially if poor-quality
foods are used for drying and if proper practices are
not followed in the drying steps.
 Bacteria require relatively high levels of moisture for
their growth, with yeasts requiring less.
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Because most bacteria require aw values above 0.90 for
growth, they play no role in the spoilage of dried foods.
 With respect to the stability of dried foods, aw levels
related to the probability of spoilage in the following
manner.
 At aw values of between 0.80 and 0.85, spoilage occurs
readily by a variety of fungi in 1–2 weeks.
 At aw values of 0.75, spoilage is delayed, with fewer
types of organisms in those products that spoil.
 At an aw of 0.70, spoilage is greatly delayed and may
not occur during prolonged holding.
 At an aw of 0.65, very few organisms are known to
grow, and spoilage is most unlikely to occur for even up
to 2 years.
 Some investigators have suggested that dried foods to
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At aw levels of about 0.90, the organisms most likely to
grow are yeasts and molds.
 This value is near the minimum for most normal yeasts.
 Even though spoilage is all but prevented at an aw less
than 0.65, some molds are known to grow very slowly
at aw 0.60–0.62.
 Osmophilic yeasts such as Zygosaccharomyces rouxii
strains have been reported to grow at an aw of 0.65
under certain conditions.
 The most troublesome group of microorganisms in dried
foods is the mold, with the Aspergillus glaucus group
being the most notorious at low aw values.
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As a guide to the storage stability of dried foods, the
“alarm water” content has been suggested.
 The alarm water content is the water content that
should not be exceeded if mold growth is to be
avoided.
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Although these values may be used to advantage, they
should be followed with caution because a rise of only
1% may be disastrous in some instances.
 In freeze-dried foods, the rule of thumb has been to
reduce the moisture level to 2%.
 Although
drying destroys some microorganisms,
bacterial endospores survive, as do yeasts, molds, and
many Gram-negative and Gram-positive bacteria.
 Staphylococcus aureus added prior to freeze-drying
could survive under certain conditions.
 Some or all foodborne parasites, such as Trichinella
spiralis, have been reported to survive the drying
proces.
 The goal is to produce dried foods with a total count of
not more than 100,000/g.
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It is generally agreed that the coliform count of dried
foods should be zero or nearly so, and no foodpoisoning organisms should be allowed with the
possible exception of low numbers of Clostridium
perfringens.
 With the exception of those that may be destroyed by
blanching or precooking, relatively fewer organisms are
destroyed during the freeze-drying process.
 More are destroyed during freezing than during
dehydration. During freezing, between 5% and 10% of
water remains “bound” to other constituents of the
medium.
 This water is removed by drying.
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 Death
or injury from drying may result
from denaturation in the still-frozen,
undried portions, due to concentration
resulting from freezing, the act of
removing the “bound” water, and/or
recrystallization of salts or hydrates
formed from eutectic solutions.
 When death occurs during dehydration,
the rate is highest during the early stages
of drying.
 Young cultures have been reported to be
more sensitive to drying than old
The freeze-drying method is one of the best known
ways of preserving microorganisms.
 Once the process has been completed, the cells may
remain viable indefinitely.
 Upon examining the viability of 277 cultures of
bacteria, yeasts, and molds that had been lyophilized
for 21 years, only three failed to survive.
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STORAGE STABILITY OF DRIED FOODS
 In the absence of fungal growth, desiccated foods are
subject to certain chemical changes that may result in
the food’s becoming undesirable upon holding.
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1- Oxidative rancidity:
 In dried foods that contain fats and oxygen
2- Maillard reaction (nonenzymic browning):
 In foods that contain reducing sugars
 This process is brought about when the carbonyl
groups of reducing sugars react with amino
groups of proteins and amino acids, followed by
a series of other more complicated reactions.
 Maillard-type browning is quite undesirable in
fruits and vegetables (because of the unnatural
color and bitter taste).
Freeze-dried foods also undergo browning if the
moisture content is about 2%, thus the moisture
content should be held below 2%.
 Other chemical changes that take place in dried foods
include
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3- loss of vitamin C: in vegetables
4- General discolorations
5- Structural changes:
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At least four methods of minimizing chemical changes
in dried foods have been offered:
1. Keep the moisture content as low as possible.
2. Reduce the level of reducing sugars as low as
possible. These compounds are directly involved in
nonenzymic browning, and their reduction has been
shown to increase storage stability.
3. When blanching, use water in which the level of
leached soluble solids is kept low.
4. Use sulfur dioxide. The treatment of vegetables
prior to dehydration with this gas protects vitamin C
and retards the browning reaction.
One of the most important considerations in preventing
fungal spoilage of dried foods is the RH of the storage
environment.
 If improperly packed and stored under conditions of
high RH, dried foods will pick up moisture from the
atmosphere until some degree of equilibrium has been
established.
 Because the first part of the dried product to gain
moisture is the surface, spoilage is inevitable; surface
growth tends to be characteristic of molds due to their
oxygen requirements.
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INTERMEDIATE-MOISTURE FOODS
 Intermediate-moisture foods (IMF) are characterized by
a moisture content of around 15–50% and an aw
between 0.60 and 0.85.
 The developed IMFs are characterized not only by aw
values of 0.60–0.85 but also by the use of additives
such as glycerol, glycols, sorbitol, sucrose, and so
forth, as humectants, and by their content of fungistats
such as sorbate and benzoate.
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Preparation of IMF
 Because S. aureus is the only bacterium of public
health importance that can grow at aw values near
0.86, an IMF can be prepared by formulating the
product so that its moisture content is between 15%
and 50%, adjusting the aw to a value below 0.86 by use
of humectants, and adding an antifungal agent to
inhibit the rather large number of yeasts and molds
that are known to be capable of growth at aw values
above 0.70.
 Additional storage stability is achieved by reducing the
pH.
 For determination of the aw of a food system, One can
also use Raoult’s law of mole fractions where the
number of moles of water in a solution is divided by the
total number of moles in the solution .
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In preparing IMF, water may be removed either by
adsorption or desorption.
 By adsorption, food is first dried (often freeze dried)
and then subjected to controlled rehumidification until
the desired composition is achieved.
 By desorption, the food is placed in a solution of higher
osmotic pressure so that at equilibrium, the desired aw
is reached.
 Although identical aw values may be achieved by these
two methods, IMF produced by adsorption is more
inhibitory to microorganisms than that produced by
desorption
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The following general techniques are employed to
change thewater activity in producing an IMF:
 1. Moist infusion. Solid food pieces are soaked and/or
cooked in an appropriate solution to give the final
product the desired water level (desorption).
 2. Dry infusion. Solid food pieces are first dehydrated
and then infused by soaking in a solution containing the
desired osmotic agents (adsorption).
 3. Component blending. All IMF components are
weighed, blended, cooked, and extruded or otherwise
combined to give the finished product the desired aw.
 4. Osmotic drying. Foods are dehydrated by immersion
in liquids with a water activity lower than that of the
food. When salts and sugars are used, two simultaneous
countercurrent flows develop:
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The composition of a model IMF product called
Hennican is given in Table 18–6.
 This is an adaptation of pemmican, an Indian trail and
winter storage food made of buffalo meat and berries.
 Hennican is the name given to the chicken-based IMF.
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Storage Stability of IMF
 The undesirable chemical changes that occur in dried
foods occur also in IMF.
 Lipid oxidation and Maillard browning are at their
optima in the general IMF ranges of aw and percentage
moisture.
 However, there are indications that the maximum rate
for Maillard browning occurs in the 0.4aw–0.5aw range,
especially when glycerol is used as the humectant.
 A food in moist air exchanges water until the
equilibrium partial pressure at that temperature is
equal to the partial pressure of water in the moist air.
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