Week 3

Question 1

Factors affecting milk composition + milk quality

Answer 1

• Breed
• Diet
• Stage of lactation - Parity
• Season
• Health

Free from off flavors and odors
- Free form detectable drug residue, added water or adulterants
- Low total bacterial count (total plate count)
- Cultivation of milk on solid agar (48h at 32) and counting bacteria forming units
- Low somatic cell count (SCC)
- Leukocytes (‘white cells’ >90%): lymphocytes, macrophages - Epithelial cells (<5%): secretory cells from alveolar tissue

Question 2

SCC and mastitis

Answer 2

• High SCC is an indicator of mastitis (inflammation of mammary gland) - Individual quarter of SCC:
• Uninfected quarters <150000 cell/mL
• Possible infected quarters 150-250000 cell/mL
• Infected >250000 cells/mL

Question 3

Effects of high SCC

Answer 3

• Increase of Heat-stable lipases and proteases that cause: - In milk
• Rancin off flavors (lipolysis)
• Bitter and astringent off flavors (proteolysis) - In cheese:
• Reduced firmness
• Reduced yields
• Increased fat and protein loss in whey - Compromised sensory quality

Question 4

Environmental mastitis pathogens (streptococci)

Answer 4

• Uberis and bovis
• Normally shed in feces
• May be found on the skin of the teats and udder
• Teat injuries and poor udder preparation before milking predispose to infection - Most infection occur early in lactation or during the dry period

Question 5

Environmental mastitis pathogens (coliforms)

Answer 5

• Originate from fecal contamination on the udder
• Lipopolysaccharide incites a rapid often dramatic inflammatory response - Signs range from milk to sever
• The typical secretion is serum like
• Most infections are cleared spontaneously

Question 6

Factors Influencing Environmental mastitis pathogens

Answer 6

Stage of lactations (highest in early lactation)
- Environments (highest with wet weather, muddy paddocks) - Milking routine:
- Clean teat
- Contagious (milking infected cows last or as a separate herd) - Over milking (poor machine maintenance, machine striping)
- Negative stimulation

Question 7

Pasteurisation

Answer 7

HTST milk (high temp short time pasteurization)
- 72C for 15sec
- 63 C for 30 min (older method) - Destroys harmful bacteria
- No effect on flavor
- Ultra high temp pasteurization (UHT or UP) - At Least 138C for 2sec

Question 8

Homogenisation

Answer 8

Ensures consistent fat dispersal to reduce or prevent creaming
- Milk is subject to pressure (fat globules are broken in small particles - Not a food safety requirement as only affects taste not quality

Question 9

Psychrotrophic bacteria

Answer 9

Ability to grow at low temperatures (15-20C) - Main genus:
Pseudomonas spp
- Bacillus spp
- Microbacter spp

Question 10

Pseudomonas spp

Answer 10

Gram negative, non-spore forming, psychrotrophic
- Produce heat-stable proteases and lipases
- Not present in milk harvested under good hygienic protocols
- Destroyed by pasteurization
- Post pasteurization contamination can decrease shelf life
- Incidence reduced through better milk handling and packaging conditions

Question 11

Spore-forming bacteria

Answer 11

Bacillus spp; microbacter spp
- Gram positive, spore forming, psychotropic
- Bacteria and spores can be present in raw milk
- Vegetative cells destroyed by HTST but spores can survive
- Bacterial growth from spores and spoil milk and reduce shelf life - Microfiltration can be used

Question 12

Milk shelf life

Answer 12

Pasteurisation combines with bactofugation (centrifugal separation of bacteria and spores) 14 days
- Pasteurization combines with MF
- 1.4 um membrane: 2-4 weeks
- >4 weeks:
- 0.8 um membrane
- UHT pasteurization