Microbial spoilage 2

Question 1

What are the + and - for bacteria in milk?

Answer 1

good for bacteria because high water content, near-neutral pH, available nutrients
bad because low amounts of available AA

Question 2

what process is used to eliminate pathogens in milk?

Answer 2

pasteurization eliminates pathogens and increases shelf life

Question 3

how do milk spoilage bacteria get energy?

Answer 3

rely on proteolysis or lipolysis

• milk lipolysis occurrs faster at 4C than 7C because during cold storage psychotropic bacteria have a faster growth rate and produce more lipases
• ptoteolysis occurs more quickly at 7C than at 4C

Question 4

What is the carbon availability in milk? what is the nitrogen availability?

Answer 4

Carbon - use glucose as a carbon source, but lactose is the more common carbon source for bacterial spoilage in milk. Only a select bacteria can degrade lactose.
-Few can use fat as a carbon source (and only if the globule membrane is physically damaged)
Nitrogen: casein proteins, but also urea, peptides, and AA

Question 5

natural inhibitors of bac in milk

Answer 5

lactoferrin and lactoperoxidase
lactoferrin: binds iron (makes it a limiting nutrient)
lactoperoxidase: survives pasteurization. Catalyzes oxidation of thiocyanate and creates hypothiocyanite, a highly reactive oxidant (antimicrobial)
lysozyme

Question 6

what is the role of H2O2 with lactoperoxidases?

Answer 6

• h2O2 is the limiting substrate. - adding H2O2 increases effectiveness of lactoperoxidase
• LABs and coliforms are limited if there is enough H2O2 present

Question 7

which psychrotrophic bacteria spoil milk?

Answer 7

• spoil both raw and pasteurized milk: Gram neg rods
• Pseudomonas commonest
• obligate aerobes are most popular in refrigerated milk.
• Enerobacteriacea increase in numbers during milk storage, but eventually they will be overgrown by Pseudomadaceae

Question 8

are the spoilage bacteria of milk able to ferment lactose?

Answer 8

The bacteria most associated with milk spoilage Pseudomonas sp. are unable to ferment lactose

Question 9

preoteolysis and lipolysis in psychrotrophic spoilage milk

Answer 9

Psychrotrophic milk spoilage usually relies on proteolysis to spoil the milk, and to a lesser extent lipolysis

Question 10

spoilage of LAB in milk

Answer 10

• occurs when the milk is temperature abused

- milk can be spoiled by “wild” LAB that produce unwanted gas or off flavours