Milking & storage of milk
31 important questions on Milking & storage of milk
What is the anatomy of the mammary gland?
What is the weight of an udder?
How is the mammary gland bilt up?
- Alveoli (lobules) > ducts >gland cistern > teat cistern
- 4 separate glands
- Milk cistern stores just 20% of milk produced between milkings
- Main storage: alveoli
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What do the secretory cells lining the alveoli do?
- Use nutrients in blood to form milk components
- Synthesize lactose, protein, and fat (C4-16)
- Transport vitamin, minerals, and fat (C16+)
Which of these statements is wrong?
- Between milking, milk is mainly stored in alveoli
- The weight of the udder just before milking is more due to the milk than the tissue
- All milk components are synthesised in the udder
- From the alveolus, the milk travels through ducts, cistern and teat before it leaves the udder
they are not all synthesised over there!
When starts the ejection of milk? End when does it stop?
- Milk mainly accumulates in the alveoli
- Milk ejection is stimulated by oxytocin, due to stimuli (e.g. Sucking) - contraction of myoepithelial cells
- After a start-up phase, the ejection reaches a maximum, and than slowly decreases
- Usually, 80-90% of the milk is removed
With what kind a machine is the milking done?
- Vacuum system consists of a vacuum pump & lines
- The vacuum is needed for:
- the milking action in the milking unit
- the transport of the milk
How is the milk stored?
What are the bacterial growth phases?
- Lag
- exponential
- stationary
- death
- long-term stationary
- During the lag phase the bacteria adapt themselves to growing conditions and synthesise RNA, enzymes as well as other molecules
- The log phase is when the bacteria grow very rapidly
- The stationary phase occurs when a nutrient is depleted in the environment so death and growth is equal
- The death phase is when the bacteria die due to lack of nutrients
Why is the lag phase, of bacterial growth, so important
- Bacteria need time to adapt to their new environment
- new nutrients
- new temperature
Raw milk should stay in the lag phase!!!
What are the factors for the exponential growth of bacteria?
- Bacterial strain
- Time & temperature
Other factors
- pH
- Oxygen pressure
- Promoters &nutrients
- Inhibitors
What happens in the stationary phase, of bacterial growth?
- Not enough nutrients
- Production of inhibitors (e.g. Lactic acid)
- Approximate CFU is 10^9 (but spoiled milk: 10^7)
Growth & dying-off are in balance
What happens in the dying-off phase, of bacterial growth?
This phase is especially important in fermented dairy products
What are the difference places where bacteria come from? (sources)
- The inside of the udder
- The outside of the udder
- The milking equipment
The source from inside of the utter =
- Mastitis pathogens may occur in high numbers
- Milk from the infected udder is often very different (e.g. Clotting)
- Mastitis may also be present without clinical signs
The source form outside of the udder =
cleaning the udder is thus important
- however, if the number of bacteria in the environment is very high, cleaning is not sufficient!
The source from milking equipment =
- bacteria growing in the milking equipment can very rapidly grow in raw milk
Temperature control
- temperature in the whole chain should be below 4*C
Which of these statements is wrong?
- Spoiled milk is generally not a safety risk
- Having clean udders is important towards prevention of environmental bacteria entering the milk
- A cow with an infected udder is not a big risk, as this milk will be diluted by the milk of other cows
- Keeping milk in the lag phase of bacterial growth can prevent spoilage of raw milk
What are the 5 main groups of spoilage bacteria?
- Lactic acid bacteria
- Psychrotrophs
- Spore-forming bacteria
- Heat resistant bacteria
- Coli-forms
The spoilage bacteria - Lactic acid bacteria (LAB)
- LAB include Lactobacillus, Lactococcus, and Streptococcus species
- LAB mainly grow in uncooled milk
- Most LAB don't survive pasteurisation
- LAB are no longer very common due to hygiene
The spoilage bacteria - Psychrotrophs
- Pseudomonas is the main psychotroph
- They come from environment and milking equipment
- Main spoilage bacteria at refrigeration temperature
- Can produce heat-stable lipase and proteinases
The spoilage bacteria - Spore-forming bacteria
- Bacillus and Clostridium are the main spore-formers
- Vegetative cells are killed during pasteurisation, spores can survive
The spoilage bacteria - B cereus
- B. Cereus can come from soil, dust, manure, and feed
- B. Cereus may grow at refrigeration temperatures
- B. Cereus is the main limiting factor in the shelf life of pasteurized milk
The spoilage bacteria - Clostridia
- Clostridia mainly lives in manure and soil
- Contamination of milk mainly takes place via manure
- Spores of Clostridia don't germinate in milk. They can cause "late-blowing" of cheese
The spoilage bacteria - Heat-resistant bacteria
- Some non-spore forming bacteria survive low-pasteurisation
- These bacteria mainly originate from milking equipment
- These bacteria only cause deterioration if milk is stored at too high temperatures
The spoilage bacteria - Coli-forms
- They are wide-spread
- They enter the milk through contaminated milking equipment
- They grow rapidly in uncooled milk
- They form gasses and off-flavours
- Low-pasteurisation kills these bacteria
- Coli-forms are thus used as indicator for proper heating
Which of these statements is wrong?
- C. Tyrobutyricum can be prevented by not feeding silage to cows
- B. Cereus is the main safety risk associated to milk
- E.coli is a indicator of both hygiene and correct pasteurisation
- Lactic acid bacteria will never cause problems as long as milk is kept cold throughout the production chain
B. Cereus is more a spoilage bacteria .....
Growth inhibition
What are natural growth inhibitors?
What are contamination growth inhibitors?
- Cooling raw milk to <4*C is the most important step
- Presence of inhibitors (natural or contamination)
Natural
- immunoglobulins - specific inhibition
- Usually low levels (except agglutinins)
- Lactoferrin - inhibition by iron removal
- May also help against mastitis
- Lysozyme - induces bacterial cell lysis
- Peroxidase system - non-specific inhibitor
Contamination
- Antibiotics
- Disinfectants
- antibiotics and disinfectants may be harmful for both processing (fermentation) and human health
Enzymes can also effect the storage, why
- Synthesizer in the udder
- From the blood
- From bacteria (secreted or released after cell lysis)
- Main enzymes are lipase and proteases
Lipase - has effect on the storage - what are lipase
- Only short chain (e.g. Butyric) FFA cause off-flavours
- The fat globule membrane protects the fat
- Pasteurization destroys natural milk lipase
- Milk should be pasteurised before homogenisation, or immediately after homogenisation
Proteases - has effect on the storage - what are proteases
- The main protease is plasmin
- Plasmin converts beta-casein to gamma-casein and protease-pepton (even at refrigeration temperatures)
- Plasmin is very heat-stable and can therefore cause proteolysis in improperly heated UHT-milk
- possible bitter off-flavour
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