Drying is a method of food preservation that works by removing water from food.
This reduction in water inhibits the growth of microorganisms and minimizes some
natural food decay. In ancient times the sun and wind would have naturally dried
foods. Evidence shows that Middle East and oriental cultures actively dried foods as
early as 12,000 B.C. in the hot sun. Later cultures left more evidence and each would
have methods and materials to reflect their food supplies—fish, wild game, or
domestic animal flesh. Vegetables and fruits were also dried from the earliest times.
The ancient Romans were particularly fond of any dried fruit they could make. In the
Middle Ages purposely built “still houses” were created to dry fruits, vegetables and
herbs in areas that did not have enough strong sunlight for drying. A fire was used to
create the heat needed to dry foods and in some cases smoking them as well.
2
The early traditional methods to dry foods included sun drying, wind drying, and
drying over a fire. Most likely the earliest dried foods were simply fruits that naturally
dried on the vine. For example, grapes will become natural raisins after drying in the
sun. Humans wanting to leave nothing to waste probably ate these and noticed that
they were quite good to eat. It is probably a similar eureka moment when a fishing
culture left their catch to hang on a line. The sun and wind dried the fish so that it
didn’t spoil. Dried fish certainly didn’t taste as good as raisins, but it was lighter and
equally nutritious. It is no doubt that some of the most ancient cultures understood
drying could preserve foods. They then would apply this knowledge to drying game
meats and drying simple grains or legumes.
3
Modern foods are dried because they are safe from pathogens, convenient and have
a long shelf life. Examples are milk powder and egg powder. The key to safety is to
have a moisture content below the level at which molds can grow. The crudest drying
techniques may leave 20‐30% moisture. Better processes leave only 10‐15%. It is
difficult to remove all moisture. The lower the residual moisture, the more difficult to
re‐dissolve the powder in liquids for use.
4
Modern foods are dried because they are safe from pathogens, convenient and have
a long shelf life. Some very common dried foods includes dried powdered milk and
dried powdered infant formula. Trail mix, soft chewy cookies, dried ethnic products
like coconut powder and dry dog foods are also foods preserved with a low water
activity. These latter products are not traditionally dried like milk, but are more
complex.
5
Utah has a history of storing foods. Natural dried foods will store best. An entire
industry has been built around this market. Here is just one company’s products
including dried whole egg powder, dehydrated carrots, dried cheese sauce, dried
textured vegetable protein (TVP), dried butter, and freeze dried blueberries.
6
Lets look at powdered milk. It has been around since the middle 1800’s. Today,
powdered milk is usually made by spray drying nonfat skim milk, whole milk,
buttermilk or whey. Pasteurized milk is first concentrated in an evaporator to
approximately 50% milk solids. The resulting concentrated milk is then sprayed into a
heated chamber where the water almost instantly evaporates, leaving fine particles
of powdered milk solids.
7
A older method of drying foods is drum drying. Compared to spray drying, drum
drying is a more intense heat treatment which results in more denatured proteins
and a possible cooked flavor. Denatured proteins also result in a less soluble powder.
Drum drying is still used for some products because it is very economical.
8
A fluidized bed dryer is a more modern drying system. Basically, wet materials are
deposited on a porous bed. Heated air from below the bed flows up through the
product. The heated air both dries the food and propels the food as a liquid through
the machinery. Think of how an air hockey puck moves along. This fluidized bed is
advantageous when product transitions from wet to dry. Picture trying to convey a
sticky non‐flowing food through ordinary equipment.
9
During World War II research was being conducted on how to dry penicillin and other
antibiotics. The scientists developed a method of removing moisture by freezing
items under a vacuum. This technology became freeze drying. After the war the
company working on this technology formed the Minute Maid Corporation to freeze
dry orange juice. Freeze drying works on the principle of sublimation of water.
Basically at very low pressure and a low temperature solid (frozen) water will
sublimate directly to gaseous (water vapor). On the chart water solid will sublimate
at any point where the red and green portions meet. The important factor here is
that the food is never heated much above freezing. Without heat this process
preserves vitamins, preserves enzymes and their activity, and will not denature any
proteins. Nearly all food volatile compounds will be retained, preserving the delicate
flavor profile of foods. Approximately 95% of water is removed from a freeze dried
food leaving the final water activity between 0.1 and 0.2.
10
What affect do you think freeze drying will have on microorganisms? Is this process
lethal to them? What will happen to the microbes when the food is rehydrated?
11
Drying is bacteriostatic and bactericidal. Cell death is based on heat intensity.
Drying has little sporocidal activity against yeast, mold, or bacterial spores.
Most spores dry in a viable state. If drying was via high heat some death may occur in
yeast and molds. However, dry heat is far less lethal than wet heat. This is
mostly due to the ability of water to transfer heat.
12
No microorganisms, even xerophiles, cannot grow at a moisture content below 15%.
At this level, all spoilage will be chemical including oxidative rancidity, non‐enzymatic
browning, and degradation of vitamins such as vitamin C. To minimize chemical
spoilage there are several things food product developers can do. It includes
reducing the moisture level below 15%, minimizing reducing sugars, blanching foods
to destroy enzymes, using SO2 if possible, and packaging foods to minimize moisture
pick up.
13
The extrinsic properties of water activity and moisture were discussed previously.
Moisture is a good predictor of microbial growth only at its lowest end of the scale.
What happens if there is more than 15% moisture? Will that automatically mean that
microbes can grow? Honey has around 20% moisture, yet no microorganisms can
grow. The reason is all of the sugars in honey lowers the water activity to 0.6. Water
activity has been known and understood since the 1950’s.
14
This is an interactive chart that can be used in dried food product development. Note
that once we get to a water activity of approximately 0.65 we have precluded the
possibility of microbial growth. Further reductions can be used to preserve product
quality. For a product high in fats a water activity of 0.2 to 0.5 would be optimum.
For a product high in reducing sugars, non‐enzymatic browning can be reduced by
reducing Aw from 0.6 to 0.2.
15
An expression of the amount of water present in a food or ingredient that is available
to support microbial growth. As Aw is reduced, the rate of growth of microorganisms
declines.
A key food preservation principle is based on the reduction of water activity by
removing water or by adding solutes such as sugar or salt.
16
Water activity is based on the ability of the water molecule to participate in chemical
reactions or to be transport
ed across a cell membrane. In the diagram free water
molecules can pass through the cells membrane and participate in chemical reactions
for the cell. Chemically bound water cannot pass through the cell’s membrane. If
bound water cannot get into the cell, it cannot be used.
17
Water activity is measured in an Aw meter. Basically, the meter is reading the relative
humidity around a sample to determine the Aw. Aw meters are expensive and can
cost from 2000‐4000 dollars. The Aw values obtained go from 0 meaning no
moisture present to 1.0 which is equal to pure water. To manipulate Aw food product
developers would need to either reduce mositure or add more humectants such as
sugar, salt, or proteins.
18
What water activity is needed to prevent all bacteria from growing? Click on the
appropriate row.
19
Dried foods are classified into three categories. The first is high moisture with an Aw
of 0.85 or greater. At that water activity Staphylococcus aureus can start to grow. At
Aw’s just a little higher we see other bacteria growing. The opposite end is low
moisture foods. These have a water activity below 0.6. At that level, no
microorganisms can grow unless the food picks up moisture from the environment. In
the middle is the intermediate moisture foods with an Aw from 0.6 to 0.84.
Pathogenic bacteria cannot grow, but yeasts and molds can. Some intermediate
moisture foods are quite interesting.
20
Here are some examples of intermediate moisture foods. Dried fruits have an Aw
from 0.6‐0.7. Cakes and pastries have an Aw from 0.6‐0.9. Sugar syrups have an Aw
from 0.6‐0.75. jams and jellies have an Aw from 0.8‐0.9. Some fermented and dried
sausages can have an Aw from 0.83‐0.87.
21
To formulate an intermediate moisture food we have to first ensure that the water
activity is at or below 0.85. Then we must design a process that eliminates yeast and
mold or prevents their growth.
22
Now lets look a little closer. What water activity is safe for a cheese spread that is
pasteurized in the jar? Why?
23
Let’s say we have a soft chew cookie at Aw 0.78. How many potential organisms can
survive? How must that cookie be packaged?
24
Yeast and molds can be killed using pasteurization heat or propylene oxide gas. They
can also be inhibited from growth using antifungals such as sorbates, benzoates, or
propionates. It is important to know that some osmophiles are resistant to these
antifungals. Modified atmosphere packaging can be used. Removing oxygen will
prevent mold growth. Adding carbon dioxide as a MAP gas will inhibit both yeast and
mold growth.
25
The last method to formulate intermediate moisture foods is to further lower the
water activity. As the Aw gets closer to 0.6 fewer osmophiles or xerophiles exist. For
example at Aw 0.74 only two spoilage organisms can potentially spoil our product.
For example the soft chewy cookie that had a water activity of 0.78 in our example
earlier could be reformulated with glycerol and propylene glycol until the Aw gets to
0.74. Using good sanitation after baking, these cookies are placed into wrappers in a
manner that prevents any possible environmental cross contamination by
Zygosaccharomyces or Monascus.
26
Proper drying or water activity levels are required to prevent growth of pathogenic
bacteria. As long as the entre food product maintains an Aw of 0.85 or less, there is
no concern. However, it is possible to raise the water activity of complex foods. For
example if a cake has an Aw of 0.84. It is frosted with a high moisture frosting. If
water were to migrate from the frosting to the cake, the Aw could be raised above
0.85. moisture could also be picked up from the environment. That same cake
stored open in a Louisiana summer could pick up quite a bit of moisture. The second
area of concern for pathogens in dried foods is the ability of Salmonella, E. coli and
Cronobacter to survive the drying process. Salmonella has caused numerous
outbreaks in low Aw chocolate and Cronobacter has caused illnesses in dried milk
formulas.
27
Cronobacter sakazakii is a Gram‐negative, rod‐shaped, pathogenic
Enterobacteriaceae. It is related to both Salmonella and E. coli. It is more of an
opportunistic pathogen affecting the immunocompromised including neonatals.
Some neonatal Cronobacter sakazakii infections have been associated with the use of
powdered infant formula with some strains able to survive in a desiccated state for
more than two years. In infants it can cause bacteremia, meningitis and necrotising
enterocolitis. Infant illness has a 20‐50% mortality. The natural habitat of
Cronobacter sakazakii is not well understood. The bacterium can be detected in the
gut of healthy humans, most probably as an intermittent guest. It can also be found in
the gut of animals as well as in the environment.
28
The slide depicts four different strains’ ability to survive desiccation. Cells were air
dried in either infant formula (solid circles) or tryptic soy broth (open circles). Note
how in most cases the infant formula dried samples had higher survival rates.
29
Over the last several decades, a number of outbreaks of salmonellosis have been
associated with the consumption of ready‐to‐eat low‐moisture products, including
chocolate, powdered
infant formula, raw almonds, toasted oats breakfast cereal, dry seasonings, paprika‐
seasoned potato chips, dried coconut, infant cereals and, more recently, peanut
butter and children’s
snacks made of puffed rice and corn with a vegetable seasoning.
30
There is a common misconception that low numbers of Salmonella are not a problem
in low moisture foods because these products do not support Salmonella growth.
However, low numbers of Salmonella in foods can cause illness, and the presence of
the organism in low moisture ready‐to‐eat foods must be prevented.
31
Here is a quick graph done by my lab of the survival of Salmonella in a dried meat
paste. The meat paste is primary preserved with low moisture and high salt. The
graph shows that we could get a 5.5 log reduction after 28 days and a 7 log reduction
after 5 weeks, but we were never able to completely eliminate all Salmonella. Not
shown on this graph was another dried meat paste product. There was no log
reduction at all during the 120 days it was monitored.
32
.awasam-promo3 {
background-color: #F5F9FF;
color: #000000;
text-align: center;
padding: 20px;
border-radius: 10px;
}
.button {
background-color: #4CAF50;
border: none;
color: white;
padding: 10px 20px;
text-align: center;
text-decoration: none;
display: inline-block;
font-size: 16px;
margin: 4px 2px;
cursor: pointer;
border-radius: 5px;
}
.button-whatsapp {
background-color: #41D07D;
border: none;
color: white;
padding: 10px 20px;
text-align: center;
text-decoration: none;
display: inline-block;
font-size: 16px;
margin: 4px 2px;
cursor: pointer;
border-radius: 5px;
}
.awasam-alert {
color: red;
}
Needs help with similar assignment?
We are available 24×7 to deliver the best services and assignment ready within 6-8 hours? Order a custom-written, plagiarism-free paper
Get Answer Over WhatsApp
Order Paper Now