Factors affecting milk composition + milk quality
- 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
SCC and mastitis
- 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
Effects of high SCC
- 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
Environmental mastitis pathogens (streptococci)
- 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
Environmental mastitis pathogens (coliforms)
- 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
Factors Influencing Environmental mastitis pathogens
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
Pasteurisation
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
Homogenisation
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
Psychrotrophic bacteria
Ability to grow at low temperatures (15-20C) - Main genus:
Pseudomonas spp
- Bacillus spp
- Microbacter spp
Pseudomonas spp
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
Spore-forming bacteria
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
Milk shelf life
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
