What causes milk to spoil?

1. What causes milk to spoil? Let’s make some cheese!

2. 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)

3. 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.

4. Milk Sugars are also Very Important! • Lactoseis 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.

5. 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.

6. 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.

7. How does the Milk Spoil? • Because of the bacteria or micro-organisms 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).

8. 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

9. 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

10. 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

11. Then what? • Once the curd is formed from the coagulation of protein, a separation of the curds from the whey begins to take place

12. What should I expect to happen?

13. 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.