For most parts of this exercise we will work in groups of two. Each of you should write up your own lab report based on the results your group got. Briefly describe what you did, the results you got, and your interpretation of those results. Did anything surprise you? Evaluate the experiment. Do you think this would work in the classroom? If so at what level? If not, why not? Would you make any changes in the exercise?

Several million people in the U.S. are poisoned each year by the food they eat. They can either become sick because of a toxin produced by the organism before they eat it or because the organism reproduces inside them and then makes them sick.

Other foods are unappetizing to us because their color, texture or taste has been altered by microorganisms such as fungi or bacteria. There is not necessarily a relationship between foods that are unappetizing and those that are unhealthy to eat. Some foods smell terrible or look disgusting because of bacterial and fungal growth but would be very nutritious if you could stand to eat them. Others may be quite tasty but contain disease causing organisms.

In 1787 the Emperor Napoleon of France offered a large reward for the person who could suggest a way to preserve food for the armies of France. Nicholas Appert put food in bottles, stoppered them, and heated the contents, killing the bacteria and preventing new ones from contaminating the food. He won the award in 1809. He was not actually aware that bacteria existed and were the cause of the spoilage. In 1819, Peter Dunrand did the same thing in metal containers he called canisters, from which we get the name can.

World-wide, about 40% of the food grown for human consumption is lost to pests and microorganisms.

Microorganisms can't grow well in the absence of moisture. Therefore, taking much of the water out of food will prevent microbes from growing on it. Pasta, grains, dried fruit and flour won't spoil if they are stored in a dry area. Evaporating the liquid from things like nectar and tree sap increases their concentration of sugar (See below) and thus preserves them as honey and maple syrup.

Many microbes are unable to grow when there is high sugar or salt content. Fruits and jams last longer when sugar is added. Pickles and sauerkraut are preserved by the addition of salt. Adding sugar or salt can also cause foods to lose their water and thus dry them (See above). Salting fish, bacon and ham keeps it from going bad.

The pH is a measure of the acidity of something. Examples of things showing a wide range of pH are listed on the next page.

pH

1. ACIDIC Stomach gastric juice
2. Vinegar, Peaches, Coke
3. Tomato juice
4. Black coffee
5.  
6. NEUTRAL Distilled water, Human saliva
7. Egg white, Sea water
8. Baking soda
9.  
10. Household ammonia
11.  
12. Bleach
13. Oven cleaner
14. BASIC

Most microorganisms grow best at a pH of around 7 and can't grow in an acidic environment. Adding an acid like vinegar or vitamin C (citric acid) will increase the acidity and limit bacterial and fungal growth. In buttermilk, yogurt and many cheeses, the bacteria that get started first lower the pH and produce something that tastes different than milk but is still tasty to some people. In sauerkraut, salt prevents most bacteria from growing (See above). Those that can survive produce large amounts of lactic acid that lowers the pH and prevents other bacteria from growing.

Refrigerating food slows the growth of microorganisms contaminating it. If foods were going to spoil at room temperature, they will eventually spoil in the refrigerator; it will just take longer. Freezing food stops growth of microorganisms contaminating it so the food will last a very long time. Changes in texture while in the freezer are caused by the freezing and thawing of the cells, not by bacterial or fungal growth. Cooling food will increase the shelf life of food but as soon as the food is warmed up, the bacteria and fungi take up where they left off. In other words, a frozen piece of slimy lunch meat will still be slimy when it is thawed out.

Increasing the temperature of milk to 63º C (145º F) for 30 minutes (pasteurization), will kill most pathogenic bacteria. Other bacteria that don't cause disease will survive; the milk will still spoil if not refrigerated. Heating to 100º C (212º F), the boiling temperature of water, will kill most bacteria but some can still survive, particularly those that have formed resistant spores. To kill them, you must use a higher temperature. This requires using a pressurized container since water can't be heated above boiling. A generation ago, cooks who didn't have pressure cookers would boil foods in jars to kill most of the bacteria. Then they cooled the jars for a few hours to allow the remaining spores to germinate. Then they boiled the foods again, killing the now vulnerable bacteria.

If the container of food is sealed as soon as the microbes are killed (canning), the food will last indefinitely. If someone is cooking in an unsealed container like a saucepan, new microorganisms will quickly replace the killed ones.

Spices such as cinnamon and cloves have been used for centuries as preservatives. Other spices had little effect on microbes but were used to cover up the taste and smell of foods that no longer tasted and smelled fresh. Nitrites are added to bacon, ham and other meats to prevent the growth of Clostridium botulimum, the bacterium that causes a deadly form of food poisoning. Propionate, sorbate and benzoates are added to many commercially prepared foods to increase their shelf life.

Smoking dries out meat or fish at the surface and that stops some microbial growth. The smoke also contains a variety of compounds that limit microorganism's growth.

Ionizing radiation kills microorganisms and insect eggs. It also delays sprouting and ripening. If the food is in a container which prevents new organisms from entering, the irradiated food can be stored for long periods of time without spoiling. Imagine storing a fresh steak, sealed in plastic, on a shelf for a couple of months and then cooking and eating it.

The food should not be radioactive any more than you are radioactive after visiting your dentist. There could be risks to the workers who process the food. Studies need to be done in this area to make sure that it is safe. Few foods are currently available with this preservation method but the number is likely to increase in the near future.

Native Americans dried buffalo meat in the sun. It became known as jerky, after the Spanish word for this food, charqui.

1. Trim the fat from a flank steak.
2. Cut with the grain of the meat to make at least four strips that are 1 inch wide and 1/4 inch thick.
3. Lay the strips on a cutting board and pound them with a heavy bottle or a mallet until they are very thin.
4. Sprinkle a lot of salt on half of the strips and pound the salt in.
5. Turn these strips over, sprinkle with more salt and again pound the salt in.
6. Use a piece of tape to make a flag at the dull end of a long wooden skewer and label it.
7. Weigh the skewer. Thread a strip of meat like a ribbon on the skewer and weigh it again.
8. Repeat this for each strip of meat.
9. Lay the skewers on a tray that suspends the meat in the air and catches the juices that drip from the meat.
10. Place the tray in a warm dry area.
11. Weigh each of the pieces the next day and every day afterward until there is no longer a change in the weight.
12. Calculate the percentage of weight loss as follows.

(Fresh meat + skewer) - skewer = fresh weight.

(Dried meat + skewer) - skewer = dried weight

 

Percentage                        fresh weight - dried weight
of weight lost     =              fresh weight X 100

13. Compare the salted sample with the unsalted one. Which lost the most water? Which one lost it the quickest? Which seems the most flexible? Were there other differences?

How much moisture do plants and mushrooms contain?

1. Take several kinds of fruits and vegetables and a mushroom.
2. Slice them into pieces that are about a 1/4 of an inch thick.
3. Weigh a piece of weighing paper, place two slices of a food on it and weigh it again.
4. Do the same for each of the other foods you selected.
5. Place the foods on a tray in a warm dry area.
6. Weigh each of the foods the next day and every day afterward
7. until there is no longer a change in the weight.
8. Calculate the percentage of weight loss as follows.

   (Fresh food + paper) - paper = fresh weight.

   (Dried food + paper) - paper = dried weight

    

   Percentage                     fresh weight - dried weight
   of weight lost     =          fresh weight X 100

9. Make a bar graph comparing the percentage of weight each of your foods lost.

How much water will a dried mushroom reabsorb?

1. Weigh a commercially dried mushroom.
2. Put it in a glass and fill the glass with water.
3. After an hour, drain the mushroom and pat it dry.
4. Weigh it again.
5. Calculate the percentage of water absorbed as follows.

   Percentage of                          reconstituted weight - dried weight
   water absorbed     =                reconstituted weight X 100

6. How did the percentage of water reabsorbed in the last experiment compare with the percentage of water lost in the previous experiment with the mushroom?

Materials for 12 pairs of students

• 2 1-pound flank steaks
• 6 sharp knives
• 6 cutting boards
• 6 heavy bottles or mallets
• 1 bag coarse Kosher salt
• 6 scales
• 1 bag wooden skewers
• 6 small aluminum trays to hold skewers
• 3 rolls colored tape
• 2 pounds large fresh mushrooms
• Several apples, carrots, onions, oranges, fresh peas, potatoes, etc.
• weighing papers
• 12 small aluminum trays to hold foods
• 12 dehydrated mushrooms
• 12 glasses
• paper towels

  

  Saint Xavier University,  Dale Fast, Lab for Teachers

  Last updated 25 January 2000   fast@sxu.edu