Page last updated Jul 11, 2011 @ 08:39pm

Freezer Failure

 

Freezer Test After Loss of Power

 

OBJECTIVES

This test was performed to determine the temperature characteristics of a freezer after normal power is lost. Specific objectives were:

  1. Measure temperature rise profile to determine how long before food thaws and spoils.
  2. Compare results between a partially filled freezer with one that is completely full.

 

TEST SETUP

The freezer contents was simulated using jugs of water. Initial monitoring was performed using a digital multimeter with a temperature probe connected to a computer for frequent automatic data logging. During later time periods when the temperature was changing very slowly, data was recorded manually approximately twice daily. The probe was positioned in free air in a central location in the freezer and the jugs were spaced to allow free air circulation throughout the interior. See Figure 1 showing the setup for the first test using 12 gallons (100 pounds) of ice. The freezer was located in a basement with a relatively cool ambient of 68 F.  The freezer itself was a commercial type old model (early 70s vintage).  More recent models probably have better thermal insulation.  Thus, results herein probably show a faster warm-up rate than newer models.

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Figure 1
Test 1 with 12 Gallons of Ice

The second test was conducted similarly except 26 gallons (210 pounds) of ice was used. See Figure 2 below.

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Figure 2
Test 2 with 26 Gallons of Ice

RESULTS

Test results in terms of the two measured temperature profiles are shown in Figure 3 below. As expected, the freezer that was full remained cold much longer. The background colors and descriptions are based on USDA information.

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Figure 3
Measured Temperatures

CONCLUSIONS

  1. The prolonged flat temperature profile around 32 F shows that ice is very effective in maintaining a relatively constant temperature near its freezing point.
  2. For this freezer, thawing did not begin until about 8 hours after the loss of power for the partially filled test and about 20 hours for the filled test. Here, thawing is assumed to begin when the probe temperature reaches 32 F. When food is thawed and in the range of 32 F to 40 F slow growth of some spoilage bacteria can occur. These durations will vary with the effectiveness of the freezer's insulation, the ambient temperature outside the freezer, the temperature setting of the freezer and how many times the door is opened.
  3. The probe temperature did not reach 40 F until 110 hours (4+ days) after the loss of power for the partially filled Test 1. For Test 2 it took 190 hours (7+ days) to reach 40 F. At this temperature, most of the ice had melted and food would have completely thawed. This temperature is significant because above this point some growth of food poisoning bacteria occurs.

 

ADDITIONAL GUIDANCE

These results and conclusions apply only to freezers, not to refrigerators.   Due to the lack of frozen products in a refrigerator, the temperature will rise much faster. The USDA states that you can normally expect your refrigerator food to last at least 4 to 6 hours, depending on how warm your kitchen is.

USDA also states that ordinarily, a half-full freezer can maintain freezing power for roughly 1 day after losing power. A fully stocked freezer will keep food frozen for 2 days. Meat or poultry that still contains ice crystals may safely be refrozen. For meat and poultry products that have been kept in a refrigerator section, though, or have only managed to stay "cool-feeling," cooking is a better option. After you cook these items, you can refreeze them. Throw out any product that has even a slightly unusual color or odor.

 

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