We’ve discussed refrigeration and canning as ways of preserving our food from spoiling.  We now know that refrigeration works by keeping food cold, thus slowing down the growth of bacteria and other microbes.  We also know that when we can food we heat it first, thus killing microbes and preventing new ones from entering the food.  However, we know there are foods that are not refrigerated, and not canned either, yet they still seem to remain safe to eat for long periods without spoiling.  Some examples are dried foods like fruits and jerky, cereals, and seasonings.  Although these foods may not taste as good after an extended time, they are still usually safe to eat.  Why don’t these foods spoil?

To understand why these foods never seem to develop bacterial or mold growth, we must ask what it is that these microorganisms need to grow.  If microorganisms (also called microbes) are present on our food, they will need four basic things to cause food spoilage:

1.       Food—the sugars and proteins in our food provide food for the microbes to multiply

2.       Warmth—refrigerator temperatures (20°F) are cold enough to slow bacteria down considerably

3.       Time—microbes require some time to grow, however under ideal conditions some can produce a new generation (that is, they can be multiplied by two) every 20 minutes!

4.       Water—most food has some water in it, which allows bacteria to digest the food

If any one of these things is not available, microbes will not grow, or at least will grow very slowly.  Anything that prevents the microbes from getting these basic needs will prevent them from growing, and will thus preserve our food. 

Water is extremely important to microorganisms, as it is to all life!  Water makes up an average of 70% of all living things, and it has many necessary roles.  Water makes up most of the interior fluid of the microorganisms, their food is dissolved in water, and all the processes that take place inside them to keep them alive must have water to happen.  Without water, all these processes would grind to a halt, and the organism would die. 

While canning works by removing the microbes themselves from the food and refrigeration works by removing warmth, other preservation methods work by removing water.  Some foods by themselves are dry enough to last in our pantry without needing to be canned or put in the refrigerator; dried pastas are a good example, as are some ingredients like flour and cornmeal (provided they stay dry).  However other foods need help—this is where dehydration comes in!

When it comes to dehydrating foods, there are essentially two methods:

1.       Air drying

2.       Freeze drying

Let’s start with air drying.  If you have a food dehydrator at home, you already have a device that will air dry food!  These machines are very readily available in department and kitchen stores, and they work by using a heat source and fan to blow warm air over whatever food is inside.  This air flow removes moisture, leaving a dry piece of food that will resist microbial growth.  This process may also be done in an oven, or by simply putting food out in the sun. 

Next, freeze drying.  If you have ever tried “astronaut food”, you’ve had freeze dried food!  Freeze drying works by taking food that is frozen at a very low temperature (around -40°F!) and placing it in a vacuum while maintaining its frozen state (for more information about vacuums, see our blog on the subject).  This causes the water molecules to go directly from their frozen state to the gas state, a process called sublimation.  What is left is a dry, airy food product that also resists microbial growth.  This works well on foods that have lots of liquid in them, like ice cream or fruit (astronaut ice cream is delicious, and can be purchased on astronauticecreamshop.com).

While removing water from food does prevent microbial growth, many dehydrated foods have a preservative added to them also to ensure that no bacteria can grow in any water that may remain, and sometimes to protect the food’s flavor.  Preservatives commonly added to dried foods include antioxidants (such as ascorbic acid and tocopherols) (Source: Bhat), various salts, and sugar, as well as some synthetic compounds. 

Antioxidants work by preventing loss of electrons to oxidizing agents, which can cause fruits to discolor and fats in meat to turn rancid (for more information, please see our lesson on antioxidants).  As an example, let’s look at what happens when the flesh of an apple starts to oxidize, and then what happens when we add an antioxidant—ascorbic acid from lemon juice:

Both salt and sugar work by pulling water out of the microbes.  This happens through a process called osmosis.  During osmosis, water flows from areas with fewer solutes (salts or sugar) to areas with more solute through the outer membrane surrounding the microbe, causing them to shrivel and die.  Traditional sugar plums are soaked in sugar water before drying for this reason, and it made the fruit sweeter! 

TRY PRESERVING SOME FRUIT THROUGH DRYING!

Here’s what you’ll need:

1.       Two cups of a fruit of your choice.  Small fruits like cherries should be cut in half and the pit removed.  Fruit larger than ½ inch in diameter should be sliced evenly into about ¼ inch slices. 

2.       One large baking sheet

3.       One large cooling rack

4.       An oven set to the lowest possible temperature, 130°F to 200°F

5.       One extra piece of fruit, the same kind as you dehydrate

6.       Two small, sealable containers

Here’s what to do:

1.       Place the cooling rack on top of the baking sheet.

2.       Spread the fruit on the cooling rack in a single layer.  Don’t let any of the pieces touch each other.

3.       Place the baking sheet with rack in the preheated oven

4.       Allow the fruit to dry for at least 6 hours, or until the fruit feels like soft leather

Store this fruit at room temperature for five days in an open container to allow any excess moisture to evaporate, stirring it every day.  Then cover the container and store for up to 10 months!

To demonstrate the ability of this dried fruit to withstand spoilage:

1.       Take one piece of fruit you dried, and place it in one of your sealable containers. 

2.       Take one ¼ inch slice of the same kind of fruit that has not been dried, and seal it in the other container

3.       Allow these containers to sit at room temperature for one to two weeks, observing them daily for changes and microbial growth.  Make sure they are kept in the same place to ensure they both experience the same conditions.

What did you observe?  How long did it take for you to see changes in the fresh fruit?  Were there any changes in the dried fruit?  Be sure to write down all your observations!

References for further reading:

Bhat, Rajeev; Alias, Abd Karim; Paliyath, Gopinadham (2011). Progress in Food Preservation. Retrieved from http://www.eblib.com

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Last week we discussed how refrigerators work to keep your food cold (click here for a review).  Keeping food cold or frozen is very important to prevent it from spoiling—if you keep milk on the counter at room temperature it will spoil in only a day or two, but if you keep it in the refrigerator it will stay fresh for at least one week.  Keeping food frozen will prevent it from spoiling even longer—it should be safe to eat for years, although it may develop a rubbery texture!  But what about food in a can?  Canned foods can last for years, and never need refrigeration (as long as the can is sealed).  How can this be?

Before we discuss why canned foods can last so long without refrigeration, we need to understand why food spoils.  We’ll use milk as an example:  think about the last time you opened a carton of milk that had spoiled (gone sour).  It had a sour, rotten smell, and it may have looked chunky or curdled.  This spoilage is caused by bacteria that grow in the milk. 

Bacteria are very tiny, single-celled living things  (organisms); most are just a few micrometers long (one micrometer is 1/1,000,000 of one meter)—that’s less than half the width of a human hair!  These bacteria live in the milk, eating the sugars and other nutrients, and releasing the by-products of acid and other smelly compounds.  If left to themselves, these bacteria will continue to grow and multiply until the milk has so much acid in it that it curdles (acid makes the proteins in milk curdle—for more information see our lesson on adding acid to milk).  At this stage there are so many bacteria that the milk smells bad, too!  If left at a warm temperature these bacteria will grow quickly, causing the milk to spoil in less than one day.

The reason milk keeps fresh much longer in the refrigerator is because the bacteria grow much more slowly at cold temperatures.  Slowing down the growth of the bacteria prevents them from eating the sugar in the milk and reproducing, preventing spoilage.  Foods that are frozen will not spoil for a very long time because freezing foods usually stops the growth of bacteria completely!  This is true for other microorganisms that can cause food spoilage, such as yeasts and molds (if you’ve ever seen a piece of green, fuzzy bread...that’s mold!).

We know we can prevent food spoilage by slowing down or stopping the growth of microorganisms (like bacteria) in or on the food.  In general, there are two ways to do this:

1.       Slow down or stop the growth of microorganisms that are already there

2.       Kill all the microorganisms in the food, then prevent any new ones from getting in

Canning works using this second method. 

When foods are canned, they are sealed in metal cans or in jars and heated to 100°C (212°F) or higher for enough time to kill the bacteria and other microorganisms living in the food.  All microorganisms in the food are now dead, and so cannot feast on the sugars or other nutrients, multiply, and cause spoilage.  Because the can is sealed, no new living microorganisms can get in.  This is how canned foods can sit at room temperature for over a year and not spoil (Source: Shepard)!

TRY THIS!

Here’s what you’ll need:

1.       Two 1-pint canning jars, with lids

2.       Two packets of yeast

3.       Four tablespoons of sugar

4.       Two pints clean water

5.       One large pitcher

6.       One small saucepan

7.       A stove, cook top, or hot plate

8.       One large pot or canner

Here’s what to do:

1.       Clean the canning jars very thoroughly in very hot soapy water, or simply run them through a dishwasher on the sterilize cycle.  Wash the lids in very hot soapy water, rinse them well, and keep them soaking in hot water.

2.       In the pitcher, combine the two pints of water, two packets of yeast, and four tablespoons of sugar.  Mix well until all the sugar dissolves.

3.       Pour some of this mixture into one of the jars, and some into the small saucepan.  Only fill the jar to 1 inch from the top of the rim!  Be sure to wipe the rim of the jar clean with a paper towel before placing the lid on the jar. 

4.       Place the saucepan on the stove over high heat, and bring it to a boil.  Once the mixture boils, take it off the heat and pour it into the other jar.  Carefully place the lid on the jar, being sure to wipe the rim of the jar clean with a paper towel.  Make sure the lid is on the jar tightly!

5.       Place the jar with the boiled yeast mixture in the canner or pot, and fill with water until the water comes up just to the lid of the jar.

6.       Bring to a boil, and keep at a boil for 10 minutes.

7.       When the 10 minutes of boiling are up, carefully remove the jar from the hot water and place it at room temperature beside the jar with the un-boiled yeast mixture.  As the jar cools the lid should not bounce back when pressed—this means the jar has sealed.

8.       Observe the jars carefully for up to 4 hours.  Watch for signs of yeast growth, including bubbles and froth forming at the surface of the water. 

9.       After 4 hours, remove the lids of the jars and observe the liquid carefully.  Take note of the way the liquid looks, as well as how it smells.

What did you observe in the two jars?  How is the boiled liquid now different from the un-boiled liquid?  Did you hear or smell anything different when you opened each jar?  Be sure to write down all your observations!

For an additional challenge, you could prepare these two jars again, but this time prepare a third jar without boiling, and place it in the refrigerator.  How do you think these three jars would be different from each other after 4 hours?

References for further reading:

Shepard, Sue. "Pickled, Potted, and Canned: How the Art of Food Preserving Changed the World." "Canning." Simon and Schuster. New York, 2000.

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Your teeth are extremely important!  Not only do they help you chew your food, but they are essential for your overall health.  People with an unhealthy mouth (like those who have gum disease) are 40% more likely to have other chronic (long-lasting) health conditions, like diabetes, heart disease, lung disease and even obesity (being very overweight).    When you neglect your teeth—that is, when you don’t brush and floss every day—you are setting yourself up for a lot of problems, from bad breath and cavities to tooth loss!  Not only is this uncomfortable and unattractive, it’s expensive:  dentures can cost over $2,500! 

For more information about how an unhealthy mouth can hurt you, please visit WebMD.com(1).

Why is neglecting your teeth so dangerous?  What causes all this trouble?  It’s because of the bacteria (very tiny organisms) that live in your mouth.  These bacteria are there all the time, and usually they are good because they help prevent other kinds of bacteria that could make you very sick from growing there(2).

However when you don’t brush your teeth, these bacteria grow out of control forming a sticky film on your teeth, called dental plaque.  If you don’t brush your teeth often, you can sometimes feel this plaque as it forms on your teeth; it feels rough, almost like your teeth are fuzzy!

The bacteria that are living in this plaque digest some of the sugar you eat, and as they do they create acid (click here for more information about acids).  This acid then begins to destroy the enamel of your teeth, causing cavities (holes in your teeth).

So how can you prevent all this from happening?  Simple—brush your teeth twice a day and floss them once a day, every day!  Be sure to brush with a soft-bristle toothbrush to prevent damage to your gums, and to brush gently for two minutes using toothpaste.  Carefully brush all the surfaces of your teeth, especially around your gums where the bacteria live. 

LET’S SEE HOW GOOD YOU ARE AT BRUSHING YOUR TEETH!

Here’s what you’ll need:

1.       A soft-bristle toothbrush (it doesn’t have to be new, as long as it’s yours!)

2.       Toothpaste

3.       Floss, one piece at least 12 inches long

4.       Plaque disclosing tablets, such as 2Tone or GUM Red Cote.  Almost all dentists’ offices carry these, and will give them to you for free if you ask.  You can also purchase them online at butlerGUMexpress.com or DentaKit.com. 

Here’s what to do:

1.       Using your soft-bristle toothbrush and toothpaste, brush and floss your teeth as you normally would. 

2.       Remove one disclosing tablet from the package, and chew it.  Swish it around your mouth for 30 seconds. 

3.       After 30 seconds spit it out, DON’T SWALLOW IT! 

4.       Look in the mirror at your teeth.  Do you see any colorful spots?  These are the spots where plaque is hiding! 

5.       Brush your teeth again, making sure to remove all the stained spots.

Do you notice anything different about how your mouth felt the second time?  Are your teeth smoother?  Feel them with your tongue, and pay attention to how they feel, because this is how they should feel after you brush them.  If they don’t you may need to brush a little more carefully, or you may need to replace your toothbrush (remember to replace it every 3 months)!

Now that you’re finished learning how best to brush your teeth, you probably have some disclosing tablets left.  What else can you test?  Here are a few ideas to try:

·         Does one brand of toothpaste remove plaque better than another brand?  Use one brand in the evening, and rinse with a disclosing tablet.  Take a picture of your teeth (or have a parent do it for you).  Use the competing brand the following evening, and rinse with a disclosing tablet.  Take another picture of your teeth.  Which time did your teeth appear cleaner?  Remember:  use the same toothbrush and brush at the same time of day for the same length of time, so the only thing that is different is the toothpaste!

·         Does one brand of toothbrush work better to remove dental plaque than another brand?  Follow the instructions above, but use a different toothbrush each evening.  For this test, you should make sure both toothbrushes are new, so that you can be sure they have both been used the same number of times. 

Whatever you try, be sure to write down everything you did, and everything you saw in your notebook! 

References for further research:

1)      Oral Health: The Mouth-Body Connection. WebMD.  2012, Jan 4.  Retrieved 4-24-14.  www.WebMD.com.

2)      Frei, Rosemary.  The Pros and Cons of Oral Bacteria.  DrBicuspid.  Retrieved 4-24-14.  www.drbicuspid.com.

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