Milk & Cheese

Question 1

What are the constituents of milk in solution?

Answer 1

• Water soluble vitamins (good source of Riboflavin, thiamin/niacin/ascorbic acid in small amounts)
• Minerals (⅓ of Ca, K, Mg, Na)
• Lactose (sugar) which participates in Maillard browning reaction
• Salts

Question 2

What are the constituents of milk in colloidal dispersion?

Answer 2

• ⅔ of Ca, Phosphorous

- Proteins: whey proteins + casein proteins

Question 3

What are the constituents of milk in emulsions? What kind of emulsion is milk and how does it keep in that emulsion?

Answer 3

• Milk fat
• Fat-soluble vitamins (A and D)
• Milk is fat-in-water emulsion (fat is dispersed, water is continuous)
• Fat in milk exists as globules kept in emulsion by an emulsifying layer around fat globules
• Emulsifying layer is composed mainly of lipoproteins

Question 4

What are the 4 stabilizing factors in milk?

Answer 4

• Brownian movement
• Like charges repelling
• Water of hydration
• Kappa in casein

Question 5

What are the kinds of milk proteins?

Answer 5

• 80% casein proteins (alpha, beta, kappa) which exist with calcium and phosphate as calcium phosphocaseinate/casein micelle
• 20% whey proteins (lactalbumin and lactoglobulin)

Question 6

What is homogenization?

Answer 6

• Milk is pumped under extreme pressure through very small holes
• Prevents fat globules from forming larger clusters which rise to surface of milk
• Reduces size of fat globules
• Fat globules permanently distributed as a very fine emulsion throughout milk (continuous phase)
• Better ratio of emulsifying layer to fat globules
• Homogenized milk is whiter, more opaque, easier to digest when fat particles are smaller

Question 7

What is pasteurization?

Answer 7

• In Canada, all commercial fluid milk and cream must be pasteurized
• Destroys pathogenic microorganisms by heating
• Especially Tuberculosis brucellosis
• 15 seconds at 72 C (high temperature, short time or HTST)
• 2 seconds at 138 C (ultra-pasteurizing)
• 99.4% of bacteria are destroyed, the remaining 0.6% are good bacteria

Question 8

What is the difference between regular commercial milk and PurFiltre milk?

Answer 8

• Milk is passed through extremely fine filter to remove bacteria, then pasteurized
• Separate fat from whole, 2%, or 1% milk
• Filter milk
• Add fat back
• Pasteurize milk
• Increased level of purity of milk (99.9% bacteria gone)
• Longer storage time and shorter pasteurization time
• Some say there is better flavour

Question 9

What are cultured dairy products?

Answer 9

• Inoculated milk with non-pathogenic bacterial culture (Lactobacillus bulgaris or Streptococcus thermophilus)
• Ferments lactose into lactic acid
• pH decreases from 6.6 to 4.6
• IEP of casein proteins
• Casein proteins denature and coagulate

Question 10

How do you denature whey proteins? What happens and how do you prevent it?

Answer 10

• Denatured by heat only, stabilizing factors no longer present
• Denatured whey proteins precipitate out of colloidal dispersion
• Settle to bottom of container
• If you continue to overheat, causes the whey proteins to scorch the bottom of the pot
• To prevent overheating (direct heat), use double boiler (<100 C) to heat milk

Question 11

What can denature casein?

Answer 11

• Acid
• Polyphenolic compounds
• Enzyme rennin

Question 12

How can acid denature casein?

Answer 12

• Casein micelles are colloidally dispersed at pH of milk (6.7)
• Negative charges around casein micelles repel each other and keep micelles stable
• Adding acid lowers pH by adding H+
• Neutralizing negative charges around micelles
• Overall charge on micelles becomes 0
• Brings casein micelles to their IEP (4.6), unstable, micelles are said to be denatured
• Adhere together forming larger, less stable molecules
• Precipitate out of colloidal dispersion
• Heat accelerates reaction

Question 13

How can polyphenolic compounds denature casein?

Answer 13

• Found in light coloured, low acid fruits, and vegetables (potatoes, apples, asparagus, mushrooms, green peas, strong tea and coffee)
• Remove layer of water of hydration around casein micelles
• Micelles denature and adhere together and form larger, less stable molecules
• Precipitate out of colloidal dispersion (curdling)
• Heat accelerates reaction

Question 14

How can the enzyme rennin denature casein?

Answer 14

• Enzyme extracted from stomach of calves
• Not present in milk
• Synthetically produced in a form called chymosin
• Used mainly in cheese-making industry
• Rennin removes Kappa casein (which is an extra 4th stabilizing factor) from casein micelle
• Alpha and beta casein readily denature, but calcium does not split off from casein micelle
• Weak, unstable Calcium Caseinate GEL forms

Question 15

What happens during the cheese making process?

Answer 15

• By the end of cheese making process, proteins (whey and casein) are denatured and coagulated
• Water-in-fat emulsion has formed

Question 16

What happens when you age/ripen cheese?

Answer 16

Changes in texture (changes in protein)

• Firm, tough & rubbery to softer & crumbly
• Cheese blends better

Changes in flavour (changes in fat)
- Mild & bland to mellow & tangier

Question 17

How do you overheat cheese and what occurs?

Answer 17

• Too high a temperature or moderate temperature for an extended period of time (eg. leaving cheese mixture in double boiler for too long)
• Cheese proteins become over-coagulated
• Tough and stringy cheese due to excessive H bonds are happening along polypeptide chain
• Overcoagulated proteins shrink due to bonds forming and squeeze out water = water on top of cheese
• Fat emulsion breaks = oily layer on top of cheese

Question 18

What properties define cheeses?

Answer 18

Cheeses which melt are:

• Medium/high in moisture
• Medium/high in fat

Cheeses which blend must:

• Melt well
• Be aged/ripened (changes to proteins during ripening allow cheese to blend)

Question 19

How do you make a good whipped cream foam?

Answer 19

1. Use cream with ≥ 32% fat (lipoproteins in emulsifying layer around fat globules)
2. Use cold temperatures (≤ 4 C)
   - Cream, beaters, and bowl at refrigerator temperature (4C or lower)
   - Fat globules become more solid
   - Cream is thicker, making it easier to incorporate air bubbles (higher volume)
   - Solid fat globules clump together more easily on surface of air bubbles (more stable foam)

Question 20

What occurs when you whip/beat whipped cream?

Answer 20

• Incorporates air bubbles
• Disrupts emulsifying layer around fat globules causing them to clump together
• Fat globules collect on surface of air bubbles
• Results in stable foam (longer time before bubbles burst and foam collapses)
• Denatures lipoproteins in emulsifying layer which collect on surface of air bubbles
• Results in an even more stable foam

Question 21

How do you prevent denaturation of casein?

Answer 21

1. Minimize heating
   - Use double boiler (<100 C)
   - Heat only to serving temperature (do not boil)
2. Minimize time acid ingredient and casein in milk are together
   - Adding milk just before serving (last step then “serve immediately”)
3. Thicken water phase around casein micelles
   - Physically prevents denatured casein micelles from adhering together and precipitating out of colloidal dispersion
   - Starch if product involves heat (gelatinized)
   - Gelatin if conditions are cold

Question 22

What occurs when you add heat to whipped cream foam?

Answer 22

If you heat whipping cream, it will melt

You cannot coagulate whipping cream, therefore you cannot make a permanent foam like meringue

Question 23

What occurs when you add sugar to whipped cream foam?

Answer 23

• Decrease foam volume if added too soon and increase time to create foam (add after partial foam is created)
• Decrease foam stiffness (softer peaks)
• Decrease chance of over-beating which would result in butter
• Increase foam stability (longer time before foam collapses)