Lecture 25: The Microbiology of Food and Water

Perishable foods

Easily support microbe growth. Ex. Green salad items, fresh meta

Semi-perishable foods

Don’t spoil as quick. Ex. nuts, potatoes

Non-perishable foods

Remain edible for long periods. Ex. flour, sugar, dried beans

Intrinsic factors

Food properties. Ex. osmolarity, pH and buffering capacity, water availability, etc

Extrinsic factors

Storage environment. Ex. temperature, humidity, concentration of gases.

Multiple methods can be used to prevent food spoilage

• Reduction of water activity (aw) of food 

• Control of temperature 

• Increase in acidity of food 

• Addition of chemical preservatives 

• Irradiation of food 

• Use of modified-atmosphere packaging (MAP)

• Hurdle technology

Reduction of water activity

Achieved by drying food out or by adding solutes (sugar/salts)

Pure water a_w

1.0

Seawater a_w

0.98

Honey a_w

0.6

For growth most bacteria requires

a_w > 0.9

Bacterial pathogens cannot grow at…

a_w < 0.86

Yeasts and molds cannot grow at…

a_w < 0.65

Pasteurization

Cooling or heating can be used. Usually cheap and highly effective

Canning

May be combined with pressure to eliminate endospores. May alter taste or texture of food

Sodium benzoate

Artificial preservatives. Lowers pH - in ketchup, salad dressing, soft drinks

Nitrites

Artificial preservatives. Often used in curing meats interferes with cellular respiration

Bacteriocins (nisin)

Natural antimicrobial preservatives. Small proteins produced by some bacteria that inhibit closely related bacterial species

Irradiation

Prevents spoilage of food using radiation. Strength of microbe elimination depends on type of radiation

Non-ionizing radiation (UV)

Surface level only, not strong. Causes formation of damaging thymine dimers in DNA

Ionizing radiation (gamma/X-rays)

Stronger, more penetrating. Produces oxidative damage and toxic free radical generation damaging DNA

Modified-atmosphere packaging (MAP)

Vacuum packing takes oxygen out of the package. Many microbes need oxygen to survive and multiple. Sometimes packaging is flooded with CO2 to achieve the same effect.

Hurdle technology

Using multiple levels of antimicrobial control in food. Employs multiple constrains (hurdles), each of which the microorganism in question must overcome in order to proliferate

Starter cultures

Preparations of microorganisms that are added to food to aid in the production of fermented products

Lactic acid bacteria (LAB)

Capable of fermenting the milk sugar lactose, a disaccharide of glucose and galactose, and are used in producing fermented dairy products such as cheese, buttermilk, koumiss, kefir, sour cream, yogurt

Koji

A starter culture of various molds used for aerobic fermentation of grains to break down proteins and polysaccharides

Maromi

Secondary anaerobic fermentation where bacteria and yeast further ferment the products from Koji

Trickle/Quick method

Uses acetic acid bacteria growing on inert support material for fermentation

Foodborne intoxication

Due to microbial toxins in food

Foodborne infection

Due to actual microbes in food

A wastewater treatment plant

Is designed to achieve several goals before the outflow (effluent) of cleaned wastewater exists the plant and is discharged into the receiving water, such as a lake or stream

Goals for treating wastewater

• Reducing total organic carbon (TOC)

• Removal or inactivation of harmful microbes in wastewater

• Reduction of inorganic compounds (especially ammonium, nitrogen, and phosphorus levels) to prevent eutrophication of natural waters

Persistent organic pollutant (POP) levels

These compounds (synthetic pesticides and pharmaceutical drugs excreted in urine), even in low levels, may affect fish and mammal reproductive and development

WW Pre-treatment

Physical removal of large objects

WW Primary treatment

Physical removal of sediments and grease that form primary sludge

WW Secondary treatment

Uses trickling filter/activated sludge unit to form complex biofilms that break down organic compounds over time

WW Tertiary treatment

Not always used; filtration method

WW Disinfection

Chlorination, UV light exposure, or ozonation

Trickling filter system

• Wastewater is sprayed evenly over a filtration bed of rock or over plastic matrices 

• The nutrients in the percolating wastewater support a microbial biofilm on the bed matrix 

• Biofilm formed by microbes that have colonized the extensive inner surface area of a plastic matrix forming the bed of a treatment system

• Microbial metabolism removes pollutants from the effluent