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Why does your fridge
start to smell,

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and where does that icky
black liquid come from?

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Wouldn't life just be
easier if things didn't rot,

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if things lasted forever?

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Well, maybe, but probably not.

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The average American household
wastes about 25% of its food,

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and a lot of restaurants
are even worse.

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So if we didn't
have decomposition,

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what would happen to all
the food we throw away?

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Food in landfills usually
gets dealt with by bacteria,

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protease, and fungi, who
allow nutrients in the food

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to return to the soil and
eventually other living things.

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These decompositions
also break down

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other dead stuff like trees.

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In fact, they're pretty much the
only thing that can eat wood.

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So even if in a
world without rot,

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you don't have to worry
about your house getting

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eaten by termites who rely on
protease in their stomachs.

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Pretty soon forests
and landfills

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will be flooded with
a lot of dead stuff.

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How do we avoid this problem?

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Well, basically in your
fridge, on a forest floor,

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in a dumpster, almost
anywhere, there

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are fungi, bacteria,
and protease that

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live entirely by eating stuff.

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These dead things can
be more or less divided

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into three categories--
carbohydrates, sugar

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and starches; lipids, think
fats; and proteins like meat.

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All of these are
chemically different,

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so they each get broken
down by different enzymes

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in different ways before
being absorbed by decomposers.

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For example, proteases
break down proteins

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into amino acids, the
cell's building blocks.

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Lipids rely on lipases
and carbohydrates

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on things like amylase
and cellulases.

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So how does a perfectly
nice broccoli floret

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start giving off this
foul black liquid?

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Fruits and vegetables are
almost entirely made of water.

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So on the most basic
level, you could

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say the cells are like extremely
complex water balloons.

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The exterior, or
cell wall, is made

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of cellulose, a complex
carbohydrate that

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gets broken down by enzymes
into small sugars the cell can

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get energy from.

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The bacteria or fungi
uses cellulase to eat

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the exterior of
a cell, it's like

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if I were to pop the balloon.

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That's the muck you
see in your fridge.

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What about that stink?

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For fruits and vegetables,
a lot of time the smell

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happens after the icky
black liquid forms.

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Other bacteria that
weren't involved

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in the initial
colonization move in

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and start to stink
everything up.

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Meat get's smelly when lipases
break down fat in the meat

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into glycerol and fatty
acids to energy sources,

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and fatty acids
are kind of gross.

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And so while food rotting may
smell absolutely terrible,

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because of it, the
environment is able to recycle

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crucial nutrients it needs.

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And, well, that's
why we have all this.

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