BIOLOGY 2EE3 Lecture Notes - Lecture 13: Red Meat, Sauerkraut, Total Organic Carbon
23 Jun 2018
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Microbiology of Food and Water
Food Spoilage
Likelihood of spoilage is related to the intrinsic (food) and extrinsic (storage environment) factors
oIntrinsic: Water availability, osmolarity, nutrient content, pH + buffering capacity, antibacterial
constituents, biological structures such as rinds or shells
oExtrinsic: Temperature, humidity, presence + concentrations of gas
Microbial load – number of microorganisms contaminating an object or organism
Perishable food – Easily support microbial growth (Green salad items, fresh meat)
Semi-perishable food – don’t spoil as quickly (nuts, potatoes)
Non-perishable food – edible for long periods (dry beans, sugar, flour)
Some food spoilage is desirable, example: changes to milk to form cheese, yogurt and butter
oMilk: Refrigerated, pasteurized milk begins with a reduced microbial growth from heat
processing and storage at cold temperatures increases generation time
oSalted butter: low aw, a solid structure to reduce microbial mobility and low pH
oCheese: low aw and solid structure reduces microbial mobility
oYogurt: pH<4.4 which is inhibitory to many contaminating microbes
Food Preservation
1. Reduction of water activity of food
To reduce: Dry or add solutes (sugar/salt)
Most bacteria require aw > 0.91: Bacterial pathogens can’t grow at aw < 0.86 and yeasts and molds
can’t grow at aw < 0.65
Exception - Staphlococcus aureus – a type of exotoxin, it produces enterotoxins that act on the
intestine leading to foodborne illnesses, often induces vomiting and is most commonly found in
improperly cooked or insufficiently refrigerated food
2. Control of temperature
Cooling or heating (pasteurization)
Cheap and highly effective
Combined with pressure to eliminate endospores (canning)
May alter taste or texture of food
3. Increase in acidity of the food
Many microbes die and can’t grow in a low pH environment
Fungi tend to tolerate low pH
Pickling in vinegar or fermentation – naturally drops pH below 4.6
4. Addition of food preservatives
Natural: Bacteriocins, Lactic acid and vinegar
-Bacte riocins ; nicin – unus u al broad spe ctrum bacteriocin ag ains t Gram + orga nis m s
produced during ferme n t a tion by Lactococc u s lactis
Artificial: Sodiu m benzoa t e, Propriona t e , Sorba t e s , Sulfur dioxide and Nitrites
5. Irradiation of food:
Does not cause food to beco m e radioac tive
Degree of microb e elimina tion dep e n d s on the type of radiation – only causes da m a g e to
DNA. Non- ionizing radiation – surface level only, not strong. Ionizing radiation – stron g er a
mor e penetr ating
6. Use of modified atmo s p h e r e packaging
oVacuu m pack agin g take s O2 out of the package as many microbes need O2 to grow
oPacking with CO2 to achiev e same effect as O2
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Document Summary
Microbial load number of microorganisms contaminating an object or organism. Perishable food easily support microbial growth (green salad items, fresh meat) Semi-perishable food don"t spoil as quickly (nuts, potatoes) Non-perishable food edible for long periods (dry beans, sugar, flour) Food preservation: reduction of water activity of food. To reduce: dry or add solutes (sugar/salt) Most bacteria require aw > 0. 91: bacterial pathogens can"t grow at aw < 0. 86 and yeasts and molds can"t grow at aw < 0. 65. Combined with pressure to eliminate endospores (canning) May alter taste or texture of food. Many microbes die and can"t grow in a low ph environment. Pickling in vinegar or fermentation naturally drops ph below 4. 6: addition of food preservatives. Artificial: sodiu m benz o a t e , proprion a t e , sorb a t e s , sulfur dioxide and nitrites. Does not cau s e food to beco m e radioa c tiv e.
