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(Question 5: Bread Staling)
There are a variety of roles for water in different food systems. One
important aspect of water in food is referred to as water activity, which
can affect both nutritional quality, functional degradation of nutrients
and food constituents, as well as regulating microbial growth. During the
middle and end of the 19th century, scientists began to discover that a
direct relationship existed between the presence of water in the food and
its tendency to spoil.
It is because of the fact that lowering water activity will enhance
shelf life , that the process of food dehydration (e.g. drying) remains
an active and likely the oldest form of food processing. On an industrial
scale in hot climates, dehydration is often the preferred method of preservation
because dried products pose fewer problems during storage and distribution
than alternative methods of food preservation. The next time you are in
a supermarket take a quick glimpse at the comprehensive range of dried
foods which we use regularly in our diet (e.g. dried drinks, baby food,
dried cereals and cereal products and dried fruits).
Let's have a look at what we dried in our own
labs.
| before
drying |
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| after drying |
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^_^ Do you like it?
By the way, could you think about what happened during drying?
Go back to principles.
Some of the modern dehydration methodsused today include: solar drying,
hot air drying, conduction drying, radiative drying, dielectric and microwave
drying, freezing drying, osmotic drying, acoustic drying, explosion puffing
and so on. No matter what drying method is used, scientists are concerned
about both the inside chemical changes taking place in the food as well
as how much energy can be saved during a certain drying process. We hope
that not only will the stability and safety of the food we eat be insured
but also the nutritional quality be maintaned.
|
Air Dried
|
Freeze Dried
|
Vacuum Microwave Dried
|
|
 |
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^_^ Can you think
about why we preceive the different sensory resppponses above from foods
that have undergone different processes?
Go back to principles.
Food scientists are usually concerned more about the chemical effects
of drying even during storage. Many food reactions can be categorized in
first-order kenetics, in spite of the complex reactions. We can indicate
nutritional loses as : 
Of course in some cases, nutritional losses follow a zero-order reaction.
Milk Powder
Loss of available lysine during storage of dry milk at 40C has been
studied as a function of water activity. When Aw=0.68, milk powder goes
brown, but when Aw=0.4,no browning occurs.
^_^ Do you know
why? Answer