L3B Milk Processing (products)

Question 1

How is yoghurt made?

Answer 1

Converting lactose to lactic acid by the addition of starter culture under the right temp. and moisture conditions.

Question 2

What is starter culture?

Answer 2

Starter Culture

Contains typically a mixture of two symbiotic bacteria (mixed culture)

Streptococcus thermophilus
Lactobacillus delbrueckii ssp. Bulgaricus

Question 3

What is the difference between set type yoghurt and swiss type yoghurt?

Answer 3

The two styles of yogurt commonly found in the grocery store are set type yogurt and swiss style yogurt. Set type yogurt is when the yogurt is packaged with the fruit on the bottom of the cup and the yogurt on top. Swiss style yogurt is when the fruit is blended into the yogurt prior to packaging.

Question 4

How much SNF must yoghurt contain?

Answer 4

at least 8.25%

Question 5

What is the difference between staphylococcus thermophilous and lactobacillus delbrueckii ssp. bulgaricus?

Answer 5

Streptococcus thermophilus

Heterofermentor – bacteria which produce >1 compound e.g. lactic acid or ethanol

Non-proteolytic – i.e. does not secrete a protease to digest proteins to get amino acids

L. delbrueckii ssp. bulgaricus

Homofermentor – bacteria which produce only 1 compound e.g. lactic acid

Proteolytic – secretes a protease to digest proteins to get amino acids

Question 6

Explain the process of yoghurt formation.

Answer 6

Step 1:
ST grows exponentially while LD does not as ST uses the amino acids and peptides that are readily available in milk and has fewer nutritional requirements hence growing faster than LD. Amino acids eventually become a limiting factor as ST does not secrete protease.ST also produces formic acid which is later used by LD.

Step 2:
ST growth slows down and LD grows exponentially.ST produced the metabolites necessary for LD. (Formic acid which is required to synthesise purines in DNA. LD lacks pyruvate-formate lyase necessary for formate synthesis whereas ST produces large amounts.

Step 3:
The second exponential growth of ST. LD expresses a protease PrtB anchored on the cell wall. PrtB breaks down casein proteins into peptides, uptake of peptides and release of amino acids. LD also produces lactic acid which lowers the pH eventually causing milk to coagulate.(pH=5.5). The lactic acid and amino acids are what stimulate the growth of ST.ST also secretes an exopolysaccharide that forms a matrix with milk proteins to give a gel like structure.

Step 4:
ST produces more formic acid till pH drops to 5.5 and LD resume its growth causing the pH to drop further to 4.3-4.5.Fermentation process is then slowed down by cooling yoghurt to 4 degrees celsius and the final pH drops to 4.2

Question 7

What is the general yoghurt making process flow?

Answer 7

Adjust Milk Composition & Blend Ingredients

may be adjusted to achieve the desired fat and solid content.often dry milk is added to increase the amount of whey protein for texture and ingredients such as stabilizers are added.

Pasteurize Milk

Pasteurized at 85 degrees Celsius for 30min or 95 degrees for 10min. High heat treatment is used to denature whey proteins allowing them to form a more stable gel which prevents separation of water during storage. High heat treatment also further reduces the spoilage microorganisms numbers in the milk to provide a better environment for starter culture to grow. Yoghurt is pasteurized before starter cultures are added to ensure they remain active in yoghurt after fermentation as probiotics. If pasteurisation happens after fermentation culture will be inactivated.

Homogenize
Done to improve yoghurt consistency

Cool Milk (42 degrees Celcius)
bring the yoghurt to the ideal temperature for starter culture

Inoculate with Starter Cultures

Hold

Milk is held at 42 degrees Celcius until pH 4.5 is reached. Allows fermentation to progress to form a soft gel and the characteristic flavours of yoghurt. Can take several hours.

Cool

Cooled to 7 degrees celcius to stop the fermentation process

Add Flavors & Fruit
Package

Question 8

What is ice cream?

Answer 8

Ice cream is a complex colloidal system of dispersed and clustered fat droplets in a concentrated mixture of sugars and proteins, with the additional presence of dispersed ice crystals, and air bubbles.

Question 9

What is the purpose of fat, proteins and emulsifiers in ice cream?

Answer 9

Fats are important for the creaminess of the ice cream.

Proteins from milk form a membrane around fat droplets making them hard to come into contact with one another.

Emulsifiers replace some milk proteins on the surface of the fat droplets.

As ice cream is made some of the fat in the droplets solidify, and the fat “needles” that form help droplets to cluster. These clusters along with milk proteins, help stablise air bubbles in the ice cream.( Membrane consists of emulsifiers and milk proteins that enclose liquid fat which is mixed in with some solid fat crystals)

Question 10

How do the emulsifiers affect fat droplets?

Answer 10

Replacing some of the proteins with emulsifiers leads to a thinner membrane meaning they are more likely to coalesce and cluster during whipping. This makes them more likely for fat droplets to mix with each other instead of creating an emulsion.

Question 11

Why do fats need to be de-emulsified in ice cream?

Answer 11

Some of the fat needs to be de emulsified because it plays an important role in trapping air.

Question 12

What is the structure of ice cream and how is it formed?

Answer 12

The structure of ice cream is 30% ice crystals,15% liquid syrup,5% fat globules and 50% air bubbles.

During freezing, most water is frozen into ice. Small ice crystals are needed for smooth ice cream. Beating and aeration occur at the same time as freezing to form small air bubbles, stabilised by the de emulsified fat. Air makes up 30-50% of the ice creams final volume. Sugar sweetens the ice cream and lowers the freezing point of water, reducing the amount of ice. Soft ice cream contains less ice.

Question 13

How are the fat globules formed in ice cream?

Answer 13

Fat is forced through a small valve under high pressure to break into small droplets, milk proteins stick to the surface of these droplets creating a membrane. This membrane of proteins helps prevent fat from coalescing back into bigger droplets as the proteins coating individual fat droplets repel each other when they come into close contact.

Question 14

Why is the melting temperature of fat important in ice cream?

Answer 14

Melting temperature of fat used in ice cream is important as fats that melt at too high temperatures give a waxy feel in the mouth whilst its difficult to make stable ice cream with those that melt at too low temperature.

Dairy falls into the right range, pal oil and coconut oil can also be used as they have similar melting points.

Question 15

What substances help incorporate air into products?

Answer 15

Fats and proteins. eg sobert do not have dairy products.

Question 16

How does air content affect ice cream quality?

Answer 16

Better ice creams tend to have lower air content and hence a higher density while higher air density ice cream tends to melt more quickly.

Question 17

What is the freezing process in ice cream?

Answer 17

Freezing adds the ice itself to the ice cream, modern facilities use liquid ammonia to produce low temperatures before that, mixtures of salt and water were used. Adding salt and water can lower temperatures to -21.1 degrees Celsius whereas liquid ammonia is used at around -30 degrees celsius. The quicker the refrigerant used the quicker the ice cream is made. Ice cream is made in a barrel with rotating scraper blades, when the ice cream touches the sides of the barrel, it freezes but is immediately scraped off by the blades. The very small ice crystals will then be dispersed throughout the mixture. We want thecrystals to be as small as possible as the smaller the crystals the smoother the ice cream.

Question 18

How are stabilisers used in ice cream?

Answer 18

Stabilisers are added in small amounts about 0.2% into ice cream. They are often extracted from plants (sodium alginate) the sodium salt of alginic acid is extracted from brown seaweeds.

Stabilisers reduce the rate at which ice cream melts, add smoothness and increase the viscosity of liquid phase of ice cream. Using multiple stablisers can produce synergistic effects.

Another stabiliser is carrageenan (less frequently used due to its cost)

Question 19

How are flavours and colourings incoporated into ice cream?

Answer 19

Natural ice cream flavours contain a number of flavouring contributing compounds. Flavouring can usually be achieved artificially, artificial flavouring often called vanillin other can be more complex. Other compounds like skatole also found in feces which has a flavoural and odour at low concentrations. Colours can be added artificially, anthocyanins from plants among other colouring agents are used.

Question 20

What are the general percentage of ingredients used for ice cream?

Answer 20

Milkfat: >10% - 16%
Milk solids-not-fat (snf): 9% - 12%
Sucrose: 10% - 14%
Corn syrup solids: 4% - 5%
Stabilizers: 0% - 0.4%
Emulsifiers: 0% - 0.25%
Water: 55% - 64%

Note minimum milk fat in ice cream is 10%

Question 21

What is the difference between soft-serve ice cream and hard frozen ice cream?

Answer 21

Soft serve ice cream has less milk fat and more air than harder ice cream. Hard ice cream is usually served at -12 degrees Celsius. Soft serve ice cream is served at -6 degrees Celsius. A difference of only 6 degrees makes soft serve ice cream much softer and creamier than its harder cousin.

Question 22

What is fat destabilization?

Answer 22

Fat destabilization or fat agglomeration refers to the process of clustering and clumping (partial coalescence) of the fat globules which leads to the development of a continuous internal fat network or matrix structure in the product.

Question 23

What is the purpose of milkfat or butterfat in ice cream?

Answer 23

Milkfat (or “Butterfat”) / Fat

• increases the richness of flavour in ice cream
• produces a characteristic smooth texture by lubricating the palate
• helps to give body to the ice cream, due to its role in fat destabilization
• aids in good melting properties, also due to its role in fat destabilization
• aids in lubricating the freezer barrel during manufacturing

Question 24

What is the purpose of SNF in ice cream?

Answer 24

The serum solids or milk solids-not-fat (MSNF) - lactose, caseins, whey proteins, and minerals (ash content)

• improve the texture of ice cream, due to the protein functionality
• help to give body and chew resistance to the finished product
  are capable of allowing a higher overrun without the characteristic snowy or flaky textures associated with high overrun, due also to the protein functionality

Question 25

What is overrun and overrun %?

Answer 25

Overrun % increase in volume of ice cream greater than the amount of mix used to produce that ice cream. i.e. start off with 1 litre of mix and you make 1.5 litres of ice cream - the overrun is 50%). Overrun caused by air being whipped into ice-cream.

Question 26

What are some problems with using SNF in ice cream?

Answer 26

But excess of lactose which can lead sandiness as the lactose crystallizes out of solution.
Excess lactose in the serum phase may also lower the freezing point of the finished product to an unacceptable level.

Question 27

What are some sources of non-fat solids?

Answer 27

Milk, cream, condensed milk, evaporated milk, dry milk, whey.

Question 28

What are some sweeteners and their roles in ice cream?

Answer 28

Sweeteners

Sucrose, corn syrup

Improve flavour
depress freezing point so that the ice cream has some unfrozen water associated with even though below 0 ° C i.e., -15° to -18° C. makes the ice cream easier to scoop.

Question 29

What are some emulsifiers and their purpose in ice cream?

Answer 29

Emulsifiers

Egg yolk, polysorbate 80 (a sorbitan ester consisting of a glucose alcohol (sorbitol) molecule bound to a fatty acid, oleic acid, with oxyethylene groups added for further water solubility)

aid in developing the appropriate fat structure and air distribution necessary for the smooth eating and good meltdown characteristics desired in ice cream.

Question 30

What is usually used as stabilisers in ice cram and give some examples.

Answer 30

Stabilisers

usually polysaccharide food gums, that are responsible for adding viscosity to the mix and the unfrozen phase of the ice cream.

e.g. guar gum, carboxymethylcellulose, sodium alginate, carrageenan, xanthan gum, locust bean gum, gelatin
Stabilisers

Question 31

Why are stabilisers needed in ice cream?

Answer 31

Without the stabilizers, the ice cream would become coarse and icy very quickly due to the migration of free water and the growth of existing ice crystals.

To stabilize the emulsion to prevent creaming of fat (fat separates from water to form a cream layer)

To stabilize the air bubbles and to hold the flavourings, e.g., ripple sauces, in dispersion

• To prevent lactose crystal growth and retard or reduce ice crystal growth during storage also to prevent shrinkage from collapse of the air bubbles

to prevent moisture migration into the package (in the case of paperboard) and sublimation from the surface
In the ice cream at the time of consumption

• To provide some body and mouthfeel without being gummy, and to promote good flavour release

Question 32

Why does moisture want to migrate into the package in ice cream?

Answer 32

Moisture migration occurs due to the difference in water activity usually caused by temperature differences. Usually, moisture migrates from hot to cold from the outside into the package as higher temperatures tend to have higher water activity.

Question 33

What is the purpose of ageing the ice cream mix?

Answer 33

Improves whipping qualities of mix and body and texture of ice cream by

• providing time for fat crystallization, so the fat can partially coalesce;
• allowing time for full protein and stabilizer hydration and a resulting slight viscosity increase;
• allowing time for membrane rearrangement and protein/emulsifier interaction, as emulsifiers displace proteins from the fat globule surface, which allows for a reduction in the stabilization of the fat globules and enhanced partial coalescence.

Question 34

How do you age the ice cream mix?

Answer 34

Place the liquid ingredients (milk, cream or coffee whitener) in the upper section of the double boiler. Beat in the eggs and the skim-milk powder. Mix the gelatin with the sugar and add to the liquid with constant mixing. While stirring, heat to about 70C. Place the container in cold water and cool as rapidly as possible to below 18oC.

The ice cream mix is best if it is aged (stored in the refrigerator at 5 degrees) overnight. This improves the whipping qualities of the mix and the body and texture of the ice cream. If time does not permit overnight aging, let the mix stand in the refrigerator for at least four hours. After the aging process is completed, remove the mix from the refrigerator and stir in the flavouring.

Aging the mix cools it down before freezing, allows the milk fat to partially crystallize This improves the whipping properties of the mix.

Question 35

How is milk fermented into yoghurt?

Answer 35

Converting lactose to lactic acid by the addition of starter culture under right temp. and moisture conditions.

4 cups (1 quart) milk
3 tablespoons plain yogurt (purchased or homemade)* or powdered yoghurt starter
Flavouring

Place milk in a large saucepan over medium-high heat; cook until it reaches 180 degrees, stirring occasionally to prevent scorching, 5 to 7 minutes. Let cool to 115 degrees. (denature proteins and cool to temp that will not kill starter culture)
Whisk together 1 cup milk and the yogurt. Stir into remaining milk. Transfer to a 1-quart mason jar. Wrap jar (without lid) in 2 clean kitchen towels, completely covering sides and top. Let stand undisturbed in a warm place until yogurt has the consistency of custard, 4 to 5 hours.

Question 36

What causes yoghurt to be a semisolid?

Answer 36

Bacteria ferment the milk turning lactose in the milk to lactic acid. The lactic acid is what causes the milk as it ferments to ferment and taste tart. (sour or acidic)