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Microbes and Food Preservation. 1. Why study how certain microbes cause disease in animals and humans? Because the safety of your food and products used to make it affects you every day!.

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Microbes and Food Preservation


Why study how certain microbes cause disease in animals and humans? Because the safety of your food and products used to make it affects you every day!

March 2013: Food Safety & Inspection Services finds E. Coli bacteria in many frozen foods (e.g. pizzas, quesadillas) made by Rich Products. The company recalls 10.5 million pounds of its foods after 24 people in 15 states are sickened…

May 2013: U.S. Food and Drug Administration (FDA) investigates an outbreak of hepatitis A virus linked to a frozen organic berry mix. 30 illnesses so far reported in CO, NM, NV, AZ and CA. The disease is contagious, and usually spread by poor hygiene.

October 2012: Major outbreak of fungal meningitis linked to injectable steroids made by New England Compounding Center (MA) used to treat arthritis/pain. At least 620 cases of patients with evidence of infection, with 39 people dead.

May 2012: Dimond pet food company expands recall of pet food possibly contaminated with Salmonella bacteria. Deaths of pets have been noted in many states, including CT!!

September 2011: Centers of Disease Control (CDC) confirm outbreak of Listeria food poisoning in cantaloup spreading to 18 states & leading to at least 30 dead, hundreds sick, mostly young and elderly.




Food can become contaminated by microbes, a general term for very small organisms that are found nearly everywhere (air, soil, water). Microbes include…

  • Bacteria: consist of only one cell (single-celled), have a cell wall, have no nucleus or mitochondria. A few types make their own food, but most compete for food with humans.

Salmonella bacteria (causes food poisoning)

  • Protists: very diverse group. Have a nucleus, can be either single or multi-celled. Some have cell walls, others have cell membranes. Some make their own food, but some don’t.

Plasmodium (causes malaria)

  • Fungi, mold & yeast: Have a nucleus. Either single or multi-celled. Don’t make own food, must land on their food source or starve.

Mold that caused Potato Famine (1845) in Ireland


You Need Us!


Microbes are a vital part of any ecosystem!

  • Decay or decomposition: bacteria & fungi break down complex organic matter into simple materials to be reused (mostly by plants).
  • Bacteria can be symbiotic - they live with and often help other organisms. Bacteria live in the small intestine of humans and help digest food.
  • Bacteria can be very helpful to humans:

Used to make food (e.g. cheese, yogurt, beer…)

Used to decompose sewage, break down toxins in soil, even oil floating in water

Bacteria Streptomycin have been engineered to produce antibiotics such as Bacitracin and Erythromycin.



But You Don’t Need Us!

Still, when people think of bacteria, they think harmful.

  • Lots of microbes are responsible for disease:

Tetanus, typhoid fever, pneumonia, syphilis, cholera, and tuberculosis are all caused by bacteria.

Bacteria and molds release toxins (poisonous substances) as they grow which is actually what cause the illness.

Don’t mess with me!

E. Coli is normally found in stomachs of animals. So, the likely source of E. Coli poisoning is feces from animals, possibly as manure used as fertilizer.

Bacteria cause other “everyday” problems such as body odor and bad breath!


Methods of Food Preservation - remove moisture


A variety of methods to prevent microbes from contaminating food have been developed, and depend on giving microbes a poor environment to grow and live in…

  • Remove moisture: all organisms need water. Two methods to remove water:

Dehydration: removing water under vacuum or by heating it (sun drying). Good for many foods, including vegetables.

Salting: adding salt not only sucks water out of food and microbes, but most organisms cannot live in a high salt environment. Good for fish & meats, changes the taste of the food.

Sea salt being added to ham to remove water

Sun-dried tomatoes



Methods of Food Preservation - remove oxygen

  • Remove oxygen: most organisms need oxygen. Works well without changing taste of food or removing nutrients. Two methods to remove oxygen:

Vacuum packing: get rid of oxygen by sucking it out under a vacuum. Good for many kinds of foods, and lasts as long as the vacuum is good.

Inert gas: storing food under an inert gas like nitrogen prevents many microbes from growing.

Vegetables stored in vacuum sealed bags

Inert: something that has no ability to react


Methods of Food Preservation - cooling


  • Change temperature: most organisms cannot live outside a certain temperature range. Heating and/or cooling are good ways to kill microbes.

Refrigeration or freezing: very common & easy, lasts as long as food stays frozen. Some microbes can survive freezer temperatures, so freezing should be used in combination with another method.

Fruit & veggies stored in a freezer

Supermarket frozen food section



Methods of Food Preservation - heating

Cooking & boiling: one of most effect methods. Often changes taste of food, destroys or removes nutrients. Must be done long enough to ensure microbes are killed, but once done, you don’t have to keep heating.

Pasteurisation is the process of heating foods hot enough to kill bacteria, but not so long that you ruin the taste (e.g. milk & juice)

Milk being pasteurised at a factory

Pork needs to be cooked at a high enough temperature and long enough


Methods of Food Preservation - pickling


  • Pickling: not very common. Most microbes cannot live outside a certain pH range. Storing food in an acidic solution (contains salt & vinegar) kills bacteria, but can still be eaten. Pickling works by lowering the pH in the microbe’s environment.
  • What is pH? pH is a measure of how much hydrogen ion (H+) is in a solution. We measure pH on a scale of 0-14.

A pH of 7 is called neutral. Pure water has pH of 7.

A pH lower than 7 is said to be acidic (there’s lots of H+ around)

A pH above 7 is said to be basic (there’s not a lot of H+ around).

  • Good for only a few kinds of foods (some meats & veggies). Dramatically changes the taste!


Methods of Food Preservation - irradiation

  • Irradiation: food is exposed to radiation (e.g. X-rays) which kills microbes and slows ripening of fruit. Will not make food safe if microbes have already left toxins on the food.
  • Irradiation is not used as much because of the perception (idea) that exposing food to radiation is somehow harmful to the food, and possibly people. However, irradiation is believed to be safe.

Irradiated foods such as meat, grains, fruits & vegetables with the international symbol for irradiated food

Left: factory where peanuts are irradiated. Right: protest to stop irradiated food from being used in school lunches


Organisms pass their resistance to chemicals on to offspring through DNA and genes.

Other Methods - chemicals


  • Disinfection: use of chemicals (bleach) to kill microbes to control infectious diseases on surfaces where food is stored or prepared.
  • Applying insecticides: spraying chemicals onto crops has long been used to control microbes and insects. Chemicals are very effective, easy to apply, but can be harmful to the environment (plants, animals, humans).
  • Bacteria and some insects have learned to survive certain chemicals, including antibiotics, through DNA mutations. The organisms are now resistant to the chemical, so it won’t work any more. Resistant bacteria then multiply…


In the end…

The best techniques to keep food safe are the easiest…

  • Wash food with at least water. Chemicals, like vinegar, can also help.
  • Cover or wrap food in containers, even in paper, especially after cooking.
  • Storing leftovers in refrigeration and covered helps decrease contamination.
  • Thoroughly cook food! Undercooked food can result in foodborne illness.