The Oldest Thing in Your Fridge

Chapter 13. The Oldest Thing in Your Fridge

Biology Connections with Assessment
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You must read each slide, and complete the question on the slide, before proceeding to the next one.
What's the oldest thing in your fridge?

Read the article below. Then answer the questions that follow.

Is that safe to eat that?

How long do you leave food in your refrigerator? A couple of days? A week? Until it turns blue and fuzzy? Or, until it walks out of the fridge by itself? Knowing about osmosis will help you understand why old Chinese takeout should be tossed, wheras the jelly left by your roommate last year might actually be edible.

A cold refrigerator will help slow the growth of microbes on our food. But even if you keep your leftovers cold, sooner or later fungi and bacteria will start growing, and the food will spoil. Yet foods that have a lot of sugar and salt last longer than others. This is because the process of osmosis works in their favor.

Osmosis helps out your fridge’s old timers

Think about some of the things that have been in your fridge for a long time. Typical long-term residents in a fridge include pickles, mustard, ketchup, salad dressing, and jelly. Let’s see how osmotic pressure works in your refrigerator, starting with a jar of jelly. When did you first open it? One month ago, or was it six? Is it still safe to eat? Yes. Jelly has far more sugar in it than bacteria. This means it’s a hypertonic environment.

As bacteria have semi-permeable membranes, water moves from the less concentrated solution inside the cell towards the higher sugar concentration in the jelly. This process of osmosis kills the bacteria, which are often 90 percent water.

Osmotic pressure forms when concentrations outside the cell are different than the solution inside the cell membrane. Hypothetically speaking, this pressure would go away if the water flowing out of the bacteria diluted the sugar—once the concentrations equalized, osmosis would stop. Of course there is far too much sugar in the jelly jar, and not enough water inside the bacteria, for equilibrium to ever be reached.

While the sugar in jelly sucks the life out of most bacteria, you might find a few bacteria and fungi that manage to survive in hypertonic environments. However, even these hardy microbes can’t grow quickly. Osmotic pressure makes jelly a bad environment for bacteria.

Can you think of other foods that last a long time due to high osmotic pressure–even outside of the refrigerator?

Salt causes osmotic pressure too

Leftover dessert should stay fresh a bit longer than leftover steak.

Ever notice that many restaurants leave ketchup on the table? Take a look at the label on a typical bottle: 17 grams (1 tablespoon) of ketchup contains 4 grams of sugar and 190 milligrams of sodium, or salt.

Salt depends on osmosis to preserve food—it removes water from the bacterial cells just like the sugar in jelly. When rubbed on beef and fish, salt can also remove water from the meat by osmosis. Microbes don’t grow well in such a dry environment, which is why you don’t need to worry about salty jerky going bad for a very long time.

Most desserts are made with a lot of sugar. This increases the osmotic pressure, and the food stays safe to eat for a little longer than other leftovers. This is why desserts can be kept in the fridge for five days, but other foods are usually tossed after three days.

When in doubt, throw it out

But without sugars or salt to create a hypertonic environment, bacteria can colonize foods very quickly. This is one reason raw meat spoils easily. Even when kept in the refrigerator, raw meats should be eaten within a couple of days. Cooked meats (with denatured proteins) should be safe for about three days in the fridge.

If you are ever unsure about whether food has been in the fridge too long, it’s a good idea to toss it. Bacteria-induced food poisoning can be an extremely painful experience, as anyone who has gone through it can tell you. Measured up against the possibility of enduring a bout of food poisoning, those suspect short ribs are something you can get by without.

© 2015 WH Freeman and Company.

Answer the following questions to demonstrate your understanding of the article.

1.

Refrigerating perishable foods the rate at which bacteria can divide.

Right. Cold temperatures slow down the rate at which bacteria divide. Even with refrigeration, bacteria numbers will eventually increase to dangerous levels and cause food to decompose. Anyone who eats the food can become very ill.
Incorrect. Cold temperatures slow down the rate at which bacteria divide. Even with refrigeration, bacteria numbers will eventually increase to dangerous levels and cause food to decompose. Anyone who eats the food can become very ill.

2.

Fill in the missing words to complete this paragraph:

The process of , where water moves across a cell’s semi-permeable from an area of solute concentration to an area of solute concentration, works in favor of certain foods such as jelly, salad dressing, and mustard, which can last for a long time without spoiling.

2
As the article notes, osmotic pressure forms when concentrations outside a cell are different than the solution inside a cell membrane. Osmosis benefits foods like jelly, which can go months without spoiling.
Try again. You have one attempt remaining.
As the article notes, osmotic pressure forms when concentrations outside a cell are different than the solution inside a cell membrane. Osmosis benefits foods like jelly, which can go months without spoiling.

3.

Foods like ketchup and pickles have a(n) environment that kills most bacteria.

2
Correct. Ketchup and pickles have a hypertonic environment, so water moves from the less concentrated solution inside bacteria cells into the surrounding environment, killing the bacteria.
Try again. You have one attempt remaining.
Incorrect. Ketchup and pickles have a hypertonic environment, so water moves from the less concentrated solution inside bacteria cells into the surrounding environment, killing the bacteria.

4.

Fill in the Blank:

Commonly used as a preservative, also enhances the taste and desirability of many foods, including jams and jellies.

2
Correct. Sugar not only helps preserve foods by creating a hypertonic environment, which prevents bacteria growth, but also enhances the desirability of foods by making them sweet.
Try again. You have one attempt remaining.
Incorrect. Sugar not only helps preserve foods by creating a hypertonic environment, which prevents bacteria growth, but also enhances the desirability of foods by making them sweet.

5.

True/False:

Canning fresh vegetables in a salt brine solution would keep them fresh and edible for a long period of time.

True. Salt brine solution is used as a preservative in many canned foods because it creates a hypertonic environment that prevents bacterial growth. While this is good for keeping food fresh, it is a disadvantage for individuals on low-sodium diets.
No, this statement is true. Salt brine solution is used as a preservative in many canned foods because it creates a hypertonic environment that prevents bacterial growth. While this is good for keeping food fresh, it is a disadvantage for individuals on low-sodium diets.

6.

Bacteria are almost 90 percent water. Therefore, we can surmise that bacteria cannot survive in a solution that is less than _____ percent water.

A.
B.
C.
D.
E.
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Good job! As the article points out, bacteria are about 90 percent water. So if bacteria are in a solution that is less than 90 percent water/10 percent solute, that means they are in a hypertonic environment. Osmotic pressure will cause water inside the bacteria to flow out of the cell into the area of higher solute concentration, killing the bacteria.
Try again. You have one attempt remaining.
Incorrect. As the article points out, bacteria are about 90 percent water. So if bacteria are in a solution that is less than 90 percent water/10 percent solute, that means they are in a hypertonic environment. Osmotic pressure will cause water inside the bacteria to flow out of the cell into the area of higher solute concentration, killing the bacteria.

7.

Why is it not possible to keep fruit fresh for long periods of time without the use of refrigeration or preservatives?

A.
B.
C.
D.
E.
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Bacteria thrive in this isotonic (or nearly-isotonic) environment, feeding on the starches and sugars in fruit. Refrigeration slows this process. Likewise, canning was developed to store fruits (and vegetables) past their usual “sell-by” dates.
Try again. You have one attempt remaining.
Incorrect. Bacteria will thrive in this isotonic (or nearly-isotonic) environment, feeding on the starches and sugars in fruit. Refrigeration slows this process. Likewise, canning was developed to store fruits (and vegetables) past their usual “sell-by” dates.

8.

Why should you toss out cooked meat after three days (even if it is refrigerated)?

A.
B.
C.
D.
E.
2
There is a good chance that bacteria found in meats can cause illness if ingested, especially when bacteria multiply.
Try again. You have one attempt remaining.
Incorrect. There is a good chance that bacteria found in meats can cause illness if ingested, especially when bacteria multiply.

9.

The graph above shows the rate of bacterial growth in four different foods, all kept at room temperature for 17 days. Based on the results, under which conditions—at room temperature, in the refrigerator, or in the freezer—would you need to store each food if you planned on keeping them fresh for at least 2 weeks? (Click the graph to expand for a closer view.)

A. Food A

B. Food B

C. Food C

D. Food D

2
Both “Food B” and “Food D” could be stored at room temperature, considering the bacterial growth rate was quite slow, even after 17 days at room temperature. “Food C” most likely would need to be kept in the refrigerator because, after 1 week at room temperature, a significant amount of bacteria had started to grow. Refrigeration would likely slow down the bacterial growth and keep the food safe for consumption for a few days. Finally, “Food A” had significant bacteria growth at room temperature after just 3-4 days. Simple refrigeration probably would not be enough to keep this food fresh; this food would need to be stored in the freezer.
Try again. You have one attempt remaining.
Not quite. Both “Food B” and “Food D” could be stored at room temperature, considering the bacterial growth rate was quite slow, even after 17 days at room temperature. “Food C” most likely would need to be kept in the refrigerator because, after 1 week at room temperature, a significant amount of bacteria had started to grow. Refrigeration would likely slow down the bacterial growth and keep the food safe for consumption for a few days. Finally, “Food A” had significant bacteria growth at room temperature after just 3-4 days. Simple refrigeration probably would not be enough to keep this food fresh; this food would need to be stored in the freezer.

10.

You are cleaning out your fridge. Your roommate is convinced that the four items pictured below all need to be thrown away. But it's ultimately up to you to decide which ones are still safe to eat. Based on the number of days (in red) that each has been in the fridge, which item should be thrown away immediately? Click the correct figure below to record your answer.
Feedback
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