What causes milk
to spoil?
Let’s make some cheese!
What is milk made of?


Most of it is water (about 85 %).
Other components of milk are
 milk
fat (3-5 %)
 proteins (2-4 %)
 lactose (4-6 %)
 minerals (0.2-0.8 %)
(Of course, the exact composition depends on which
animal you are talking about and the nutritional status of
the animal)
Milk Proteins play a Key Role in
Making Cheese!

Casein is the major protein from milk. As
you know, proteins are very valuable for
nutrition. Milk also has small quantities of
proteins such as albumin and globulin,
which are essential for disease resistance
in the young.
Milk Sugars are also Very
Important!

Lactose is the main sugar present in the
milk. People in many countries are known
to be lactose intolerant, which means they
cannot digest lactose. This is the main
component that is transformed when other
products are made from milk.
What do the Minerals do?

Milk contains important minerals such as
calcium and phosphorous which are
necessary for the growth and repair of
teeth and bones.
Why does Milk Spoil?
Well, it is the very high moisture content,
along with the abundant supply of nutrients
(so that bacteria can eat and grow!).
 The pH value of milk is 6.7; this means that it
is slightly acidic, but very close to being
neutral.

How does the Milk Spoil?
Because of the bacteria or microorganisms in milk
 The bacteria belong to three categories

 those
that spoil the milk
If the milk is spoiled, we do not consume it. So, we do not have to worry about
these bacteria causing infections in us
 those
that cause infections and diseases in
humans
Before 1930s, milk and milk products were the most important vehicles of
diseases such as typhoid, tuberculosis and diphtheria. As you all know, we can
protect ourselves against this problem by pasteurization of milk.
 those
that bring about natural fermentation
(for example, those involved in converting
milk to curd).
So how do we get the bacteria to
make Cheese?


The action of bacteria on the milk is very
important for us to get curd.
These bacteria convert lactose to lactic acid.
 The
sour taste comes because of the production of
acid
 This changes the pH of the curd (becomes acidic)


Because bacteria cannot thrive as they can in milk, the curd
does not spoil so easily
If somebody has problems digesting lactose, curd is easier
for them to digest
How do we get bacteria to start
their action?

We boil the milk to eliminate any of the bacteria which
might spoil the milk
(our milk is already pasteurized so we don’t have to boil it!)

Then, we cool it so that it becomes lukewarm (Why
lukewarm?)
(This is where we will start the process)

We put a spoonful of curd producing bacteria into the
milk
(Our starter culture will be buttermilk)

The bacteria from the curd will find lots of lactose in the
milk. They will consume it, produce lactic acid and make
the milk sour
Milk is a freely flowing fluid, but curd (stuff that is turned into
cheese) is a thicker pasty kind of stuff…WHY?

It has a lot to do with the protein Casein!
 These
proteins normally exist as negatively
charged groups that repel each other, and are
thus distributed evenly throughout the milk.
 When the acidity increases in the milk, the
groups of casein proteins lose their negative
charges.

They also lose their ability to repel each other.
They then bond with each other, causing
coagulation, or curdling of the milk
Then what?

Once the curd is
formed from the
coagulation of
protein, a separation
of the curds from the
whey begins to take
place
What should I expect to happen?
What happens to the whey?

The whey looks like cloudy, yellowish water. It
tastes exactly like milk, and is full of nutrients.
 Most
cheese makers recycle the whey from their
cheese making process.

If the cheese maker owns pigs, for example, they may feed
them the residual whey.



Parma hams, for example, are made from pigs fed on the whey
from Parmigiano Reggiano.
Other cheese makers convert the whey into ricotta, or
cottage cheese, fully extracting the last milk solids from the
liquid.
But more often than not, the whey ends up being distributed
over the farmer’s pasture, fertilizing the land the dairy herd
will end up grazing on once more.