Food Microbiology & Safety
Helen Billman-Jacobe
Sources of microorganisms in food-Milk
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Ray and Bhunia Chapter 3
Intended learning outcomes
Differentiate between normal flora in plant and animal derived foods, contaminants, spoilage organisms, pathogens and functional organisms
Relate the intrinsic and extrinsic factors of food substances to how microorganism can grow and survive
Food microbes and food safety
Many aspects of food microbiology are aimed at
• Understanding the sources of microorganisms in food
• Controlling access of some organisms to food
• Killing microorganisms in food or reducing their numbers
• Determining the microbiological quality of food
• Using microbiological standards and specifications for food safety
Collectively these measures manage product quality
This lecture is about where the microorganisms in food come from. We will focus on milk, meat, fish and plants
Whole , clean, fresh, lacteal secretion obtained by complete removal or milking of one or more healthy cows
A complex mixture of organic and inorganic food substances such as water, fat, carbohydrate, protein, minerals, salt, gases, vitamins, enzymes, hormones, antibiotics , white blood cells, somatic cells and bacteria
Intrinsic factors
pH and buffering Redox potential
Low -0.2mV
Antimicrobial factors Nutrient content
Gaseous atmosphere temperature
Extrinsic factors
Present but conc too low to effect keeping quality
Water activity
Moderate pH 6.4 – 6.6
Nutrient rich Lactose
Volume to headspace ration may influence amount of dissolved oxygen
Variable 4-37°C
2. % protein 4.1% lactose
3 – 5% fat
3.4 – 4.9% protein ~5% lactose
Milk from different animals differs slightly in
• Smallfatglobules
compositions. The season and diet will also have an
• Higher amount of medium chain triglycerides effect however the main constituents remain the
same. Fat, protein and lactose (carbohydrate)
Sources of microflora of milk
Sources of microflora of milk
Exterior of udder
• Environment
• Heavily contaminated udders may contribute 105 cfu/ml to milk
• Good hygiene can control this
Interior of udder
Low numbers of bacteria in a healthy cow udder (102 -103 cfu/ml)
Cow with mastitis may have up to 108 cfu/ml of bacteria in her milk
Mastitis is an infection of the mammary glands caused by bacteria including
• Pseudomonasaeruginosa
• Staphylococcusaureus
• Staphylococcusepidermidis • Escherichiacoli
• Klebsiella pneumoniae
Main health concerns associated with milk
Pasteurization is an effective control measure
Food Safety Australia and (FSANZ) set standards for the effective pasteurization of milk
All milk for consumption is Australia must be pasteurized
https://www.foodauthority.nsw.gov.au/sites/default/files/_Documents /foodsafetyandyou/raw-milk-advice.pdf
Primary Production and Processing Standard for Dairy FSANZ Standard 4.2.4
Processing of milk and dairy products
(1) Milk must be pasteurised by –
• (a) heating to a temperature of no less than 72°C and retaining at such temperature for no less than 15 seconds; or
• b) heating, using any other time and temperature combination of equivalent or greater lethal effect on any pathogenic micro-organisms in the milk; or
• (c) using any other process that provides an equivalent or greater lethal effect on any pathogenic micro- organisms; unless an applicable law of a State or Territory otherwise expressly provides.
Heat treatment of milk
French chemist and microbiologist , originally used heat treatment to stop beer and wine souring. The process, called Pasteurisation, was later used to kill pathogens from milk.
Name of treatment
Temp and time
Low temperature holding (LTH)
63°C for 30 min
High temperature short time (HTST)
72°C 15 sec
Ultra high temperature (UHT)
135°C for 1 sec
“sterilized”
> 100°C 20-40 min
Raw milk vs Pasteurised milk
Raw milk is milk that has not been heat treated
Pasteurised milk is not sterile
It should have
• < 3 x 104 cfu/ml bacteria
• < 1 coliform/ml (these are E. coli-like bacteria) • <105 cfu/ml after 5 days at 6°C
Thermoduric bacteria , probably Bacillus spp.
https://www.cdc.gov/features/rawmilk/index.html
Thermoduric bacteria
Curdled milk
pH of milk drops and milk protein precipitates + lipids breakdown=“curdles”
Bitty Cream syndrome
•Bacillus cereus (Gram positive, spore former)
• spores can survive pasteurisation and grow in milk
•Lecithinase digest the phospholipids in milk fat globules
•Remaining protein/fat globules float on surface of hot drinks and clings to glass
•Mainly happens when through improper refrigeration
Growth of bacteria in pasteurised milk
1.00E+06 1.00E+05 1.00E+04
0 2 4 6 8 10 12 14 16 18
Milk products
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