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I'm Scott Campbell and I work here at Decagon Devices we are the makers of
the Aqualab line of what activity meters and as a water activities meter
maker a lot of people often ask us what is water activity
to explain that a little bit better let's pretend
you're out in the marketplace and your or wanting to buy a car or a vacuum
cleaner or even uh... projector for your home theater system
well each of these products has specs
some of the specs on a car might be the the horsepower or on a vacuum cleaner could be
the motor amps
or even on the projector it could be lumens to represent brightness
now these are all fine but what you really want to know about these things
is for example how clean will a vacuum cleaner get my carpet
or for a car is how well it pushes you back in the seat when you hit the
accelerator
now when it comes to measuring water in lots of different products there's a
similar dynamic
moisture content can tell you how much water senior product but what you really
want to know is
how long till this product that i have goes moldy on the shelf or how long
till this product starts to taste rancid if you're working with the pharmaceutical
when i put a powder inside of a gel cap, will it split the gel cap
or even in a plastic if I am injection molding a helmet will it be unsightly
streaks in the helmet uh... and will it impact the structural integrity of that
product
so some people have a difficult time understanding what what activity is
but everybody can understand what water activity does for example if you know
the water activity
you will know whether your product is going to mold was sitting on the shelf
another thing you'll know if you know the white activity of
two components within a product like say a cheese and cracker is whether that
cheese is going to transfer its moisture to the cracker and turn the crackers soggy
another thing you know is it your product has vitamin c on it
you'll know how long you can sit on the shelf and still have an acceptable level
of vitamin c
another thing you'll know is let's say you have a lotion
and that lotion has a high water activity
if the water activity is too high
you'll know whether that product is going to mold while sitting on the shelf
moisture content can't tell you any of these things
so what makes what activity so powerful
water activity is a measurement of potential energy
so as such these uh... this measurement of potential energy
obeys the laws of thermodynamics
now because of that we know that differences in water activity
uh... or differences in potential energy will cause moisture to moves say from
our cracker or sorry for our cheese
into are cracker
now once we know that these water activity differences obey the laws of
thermodynamics
that allows us
to use them in the world of physics in a bunch of different models and equations
important thing here is that once we know the what activity numbers and
can plug them into these models and equations they make predictions about
what's gonna happen for a product
so how long is it gonna be before
a product on the shelf
goes rancid because of an oxidation reaction
or how long before it loses a critical level of vitamin c in the product
the power is that
these equations allow us to make predictions about what's going to happen
to the product
so let's look at a real-life example about how this works so this is an
animation that we put together that shows a cookie and a jar full of
water now the water that's in the cookie has a lower water activity
or in other words the lower capacity to do work
because the water that's in there has already expended energy
energy to bind with the salts and sugars that are in the cookie
now the water that's in the jar the pure water hasn't expended any of this energy
because
it's pure water there is nothing else in the jar except water
because the cookie has a water activity of .55 water activity units
uh... the cookie cannot support the growth of mold or bacteria well why not
it's because that uh... the water activity is so low
that if there is mold or bacteria in this cookie it doesn't have enough
energy to take up water and of those microorganisms become dormant
obviously measuring water activity is pretty important
but how is it that we can actually measure
the potential energy water inside a solid sample
well in reality it's pretty easy that's because by the laws of thermodynamics
makes all systems tend to move towards equilibrium
so if we take a sample and we put it inside a closed chamber feel comfy
equilibrium with that air this inside that chamber
now once that vapor equilibrium has been reached
all we do is we use typically a chilled mirror sensor
to measure the relative humidity of the air in that head space
and that relative humidity number
gives us our water activity
so that covers the basics
water activity is an important part of reactions that can damaged food and other
products
and water activity typically measured by putting a sample inside a sealed
chamber waiting for to come to equilibrium and measuring the relative
humidity of the air above the sample
many people need more information about their specific product in order to use
water activity knowledge
and we have a lot of this information available at AquaLab.com
now if you don't find information you're looking for their
please feel free to give us a call and we would be happy to discuss your
application with you