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
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.
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.
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:
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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