Food Microbiology & Safety
Helen Billman-Jacobe
Normal microbiological quality of food
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Ray and Bhunia Chapter 4
Intended learning outcomes
Describe the microbial types and their levels that can be expected under normal conditions in different food groups
Raw and ready-to-eat meat products
Following slaughter, dressing, processing, the meat of animals and birds contain many types of bacteria from different sources
– normal flora:
– farm environmental contaminants: – abattoir:
– meat processing:
The microbial load of fresh meat varies greatly intact carcass ~10-1000 bacteria/cm2 minced meat 10,000 – 100,000 bacteria/g
skin, hair, feathers, gastrointestinal tract feed, water, soil and manure equipment, air, water and humans equipment, air, water and humans
Chilled meat
Chilled meat is stored at low temperature; -1 to 5°C
mesophiles will not multiply much at the low temperatures
Psychrotrophs are a major problem
predominant psychrotrophs lactobacilli and leuconostocs Brochthrix thermosphacta
Some coliforms
psychrothrophs can grow rapidly i.e. Pseudomonas
If products are kept under anaerobic conditions then the facultative anaerobes and anaerobes grow
i.e. Lactobacillus, Clostridium
The intrinsic properties of the meat and extrinsic properties of storage determine which organisms grow
Pseudomonas, Serratia, Proteus…..
If products are kept under aerobic conditions then
Low heat processed meat
Examples: frankfurters, ham, bacon
• Low heat processed meat has been subjected to
temperatures ~70°C.
• The treatment kills most microorganisms except for some thermodurics and spores of Bacillus and Clostridium
• This expected microbial level ~10-100 bacteria/g
• LHP meet is expected to have a storage life of ≥50 days
• Further processing such as slicing can introduce additional bacteria yeasts and moulds that affect shelflife
Raw and pasteurised milk
Raw milk contains, 103 bacteria per ml
• Refrigerated storage before pasteurisation: only psychrotrophs can grow
• Psychrotrophic pathogens such as Listeria monocytogenes can multiply in refrigerated for milk
Pasteurised milk contains microorganisms that
– survive the heat of pasteurisation = thermodurics i.e.
Pseudomonas and spores of Bacillus and Clostridium
– Enter the milk after heating but before packaging = post-
pasteurisation contaminants
– Psychrotrophs can multiply in refrigerated, pasteurised milk
– Heat stable enzymes can affect product quality i.e. Proteases, lipases
Fish and shellfish
• Scale fish, crustaceans (prawns, lobster, crabs) and molluscs (oysters, mussels, scallops)
• Microbial population varies with the pollution level and temperature of the water where the animals lived
• Saltwater fish and shellfish and generally have different bacteria than freshwater fish and shellfish
• animals harvested from polluted water spoil rapidly
• Psychrotrophs can multiply
• If fish is not consumed fresh then it must be salted, dried, cooked or otherwise processed to preserve it
Rawandfresh.com
Vegetables, fruits and nuts
Microbial levels and types depend on the conditions of farming and harvesting
Microbial load
vegetables ~ 103 – 105 bacteria/g fruit ~ 103 – 106 bacteria/g
Nuts ~103 -104 bacteria/g
Bacteria and moulds and yeast
Bacterial endospores ( Bacillus and Clostridium) and fungal spores
Can also have pathogenic protozoa and parasites
This vegetable products that are not consumed fresh can be processed to prolong the shelflife
Cereals, starches and gums
Cereals = grains, flower, for cereals, pasta, baked products, dry mixes and frozen in refrigerated products
Starches = Flour of cereals, tapioca, potatoes and other tubers
Gums = stabilisers and gelling agents i.e pectin, agar, xanthan
Pulses = peas and beans
Unprocessed products may contain high bacterial levels
May contain mycotoxins from fungal growth during storage
Many products are dried (low Aw)- may contain bacterial spores
Carbohydrate
Canned foods
Food in tin cans are treated with a high heat during canning process The amount of heat required depends on the pH
• pH≥4.6 are heated to obtain commercial sterility
• pH<4.6 heated to ~100°C
“commercial sterility”. Cans can still contain low numbers of thermophilic spoilage bacteria. These do not germinate if cans are stored below 30°C. If cans are stored ≥40°C spores germinate and products spoil.
Cans heated to ~100°C can contain spores of mesophilic bacteria which may be spoilage bacteria or pathogenic Bacillus cereus, C. perfringens or C. botulinum
Home bottled food
Home bottling is a method to preserve food however temperatures greater than 100°C cannot be achieved therefore home bottling relies on using food materials that are naturally acidic or are acidified by the addition of food acids.
Tomatoes and variety of fruit are popular for home bottling
pH neutral vegetables can be very risky due to the survival of C. botulinum spores
fowlersvacola.com
Sugars and confectionery
In general, sugar and confectionery have low water activity (Aw ≤0.84) and some have a low pH
The low-water activity and low pH prevents germination of bacterial spores
Preserving fruit by making jam involves the application of heat and the lowering of Aw through addition of sugar and boiling off excess water
Soft drinks, fruit and veg drinks, juices
fruit juices ≤pH4
vegetable juices ≥4.5
Fruit juices, spoilage by acid resistant and spore forming species
Vegetable juices, spoilage by mould, yeast and lactic acid bacteria
Juices may be heat-treated through pasteurisation or canning to prolong their shelf life.
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