Lecture 23 Outline
Why does my refrigerator smell? (food spoilage), What my grandmother knew about canning and smoking (food preservation), Preservatives in foods. Good or bad for me?
1856 soup can - Still good?
Veal can circa 1830, cast iron and tin lined
Salmonella
the typhi species causes typhoid fever (like typhus, but not caused by Rickettsias)
the enteritidis species causes food poisoning
very survivable outside of the body, withstands freezing, resistant to chemical disinfectants, and motile
generally obtained through the fecal-hand-food-oral route
found in raw meats, eggs, salads, poultry, cream desserts and milk products
thrive at 40 to 140 F in foods
disease severity depends on the dose ingested
the bacteria grow in the intestines causing illness
typical symptoms occur 12-36 h after ingestion: headache, diarrhea, cramps, vomiting, fever
bacteria can survive freezing and refrigerator storage
when the food is warmed again, the bacteria grow again
high heat (boiling) kills the bacteria
Campylobacter jejuni
a spiral, motile bacilli that prefers low oxygen concentrations
commonly found in surface waters, raw and undercooked meats, raw milk, and raw shellfish
needs 85 F to grow, but is killed by high heat
after ingestion, the bacteria grows in the intestines
2 to 5 days after ingestion illness ensues with diarrhea, cramps, headache, and fever
in most cases the illness is self-limiting in 2 to 7 days
but it can lead to meningitis and even death
initially thought to be a minor ailment, but now recognized as perhaps the most common food poisoning
Listeriosis
Listeria monocytogenes has only recently been recognized as a major food poisoning threat
it is common in the environment, salt tolerant, drying tolerant, pH tolerant
thus, pickling and acidic pHs, or drying of foods does not kill the bacteria
it is killed by high heat and cooking
Listeriosis is really a problem for newborns, pregnant women, and the immunocomprimised
healthy adults will get a mild illness of vomiting and fever (fun!)
it can be deadly to fetuses and if not stillborn, have a 20-35% mortality rate
the incubation period can be as long as three weeks
it is commonly contracted from cole slaw, raw milk, and cheese made from raw milk
Staphylococcal bacteria
these are very common in the environment and on the human body
people with colds or blisters are particularly infective
it is also found in raw milk, untreated water, and sewage
the bacteria grows in warm foods and produces toxins that are released into the food
the toxin is heat resistant and is not destroyed by cooking
the bacteria is killed at high temperatures and inhibited by cold temperatures
once ingested, the toxin produces illness that includes cramps, diarrhea, and vomiting 1 to 8 after eating
commonly found in protein foods (meats), milk products, and any food that are frequently handled
since "Staph" is found on skin, the more raw foods are handled, the more likely the bacteria is to contaminate the food
such things are salad bars are prime suspects
also note that the severity of the disease is dependent on the dose of toxin ingested
the toxin will not reproduce in the gut like Salmonella
also not how quickly the illness ensues, just a few hours
most of the toxin induced food poisonings have a very rapid onset
Clostridium botulinum
this was in your reading
lives on decaying matter in soil, fruits, and vegetables
often the result of poor home canning, especially of low acid foods
the bacterium forms spores that are very resistant to environmental agents and disinfectants
the bacterium also grows in the anaerobic environment found in canned foods
cans that are bulged out may be infected as the anaerobic growth of the bacterium produces gases that cause the can to swell
the bacterium produces very deadly toxins
the toxins cause muscle paralysis by blocking nerve transmissions
Clostridium perfringens
closely related to botulinum but not as deadly
again it forms spores that are not killed by hours of boiling
cooking actually activates the spores while killing bacterial competitors
then given a warm, moist, anaerobic environment (like a can), the perfringens grows
8 to 24 after ingestion, fever and diarrhea ensue
Prevention
cook foods thoroughly, quickly cool leftovers
avoid contamination with surface waters
wash hands often, especially after defecating and before preparing foods
Food preservation
heat: cooking causes proteins to denature, membranes to melt and bacteria to be killed
cooling: low temperatures can inhibit the growth of bacteria or kill them
freezing: freezing kills by forming water ice crystals within the bacteria and rupturing membranes.
some bacteria produce cryoprotectant compounds like glycerol that prevent ice crystal formation
the bacteria actually vitrify into a glass at low temperatures
drying: microbes need water to live and drying can rupture membranes
salting: kills bacteria that are not salt tolerant, causes dehydration effects
acidifying: again bacteria not tolerant to low pH are killed or inhibited
irradiation: in your readings
canning: essentially the process is heat foods within a closed container
bacteria are killed by the heating and other bacteria cannot enter the closed container
very effective. if the sterile environment inside the can is not breached, then the foods in the cans can remain safe to eat indefinitely
preservatives: things like BHA and sulfites are not antimicrobial
they simply prevent the degradation (rancidity) of foods due to oxidation reactions
Western blots
back to some genetic engineering techniques
remember, we can use gel electrophoresis to separate proteins of different sizes
now we want to see if our "protein of interest" is one of the protein bands on the gel
first we transfer the protein in our gel into a special piece of paper (a nitrocellulose membrane) using electrophoresis, moving the proteins (as two-dimensional array) from the gel and onto the paper
the proteins are now stuck to the paper in exactly the band pattern of the gel
then we use antibodies that are specific for only our protein of interest
these bind to only our protein band
then we add a second antibody that is specific for the first antibody
this second antibody has a "reporter" on it, something radioactive, or an enzyme activity that makes a colored product
this we can see and know if our protein of interest is present
this technique is called a Western blot
