Our freezer labels are based on our CheckPoint® I labels.  These are "B" formulations, and work well at deep frozen temperatures. 

All About Thawing and Partial Thaw

Everywhere food is stored and distributed at subfreezing temperatures, problems occur. These can arise from mechanical malfunction and employee neglect. Frozen foods may thus be exposed to above optimum temperature during transit or handling.  These "temperature abuse events" can range from mild to extreme.

Mild events are inconsequential departures from an ideal 0°F (subfreezing, or "partial thaw" event).  At the opposite extreme, temperature abuse can be a damaging, above-freezing-point-of-water event (normally called "thaw").  Any of these events can compromise and affect deep frozen commodities.

What happens during a thaw?  If the time is short, the interior of a food item will have more exterior exposure to above-ideal temperatures than the interior.  If the time is long enough, there is almost equivalent exposure of the interior, over time, as the exterior.

Information About Food Temperatures During Thaw

No simple information is available regarding the effects of thawing or partial thawing on the outside and inside of a food product item.  Although there has been much research about thaw effects, there is not much information on what actually happens to temperatures.  In order to calibrate the response of our CheckPoint® Freezer Labels, we have performed tests on sample products to determine these effects.  We measured temperature changes in several different simulated food items at the surface, and at various depths inside the food item.  Each item was subjected to partial thaw or complete thaw temperature changes.  

Example Data

In one test from our studies, a food item (density = 1.3 specific gravity, thickness = 3" [7.5 cm]) was kept initially at 1º F.  Then it was exposed immediately to 64º F for exactly 100 minutes, then the ambient temperature was returned to 1º F (the imposed temperature regime is shown by the red line).  

Clearly, the food item did not reflect ambient "outside" temperatures on the interior for quite some time.  The deepest layer (3 cm Deep) lagged behind the surface layer by approximately 1 hour, and the intermediate layer (1 cm Deep) lagged by about 30 minutes.  What is interesting about these curves is that the total thermal exposure in terms of degree minutes of exposure is about the same (ca. 600 - 650 degree minutes).  So, we cannot conclude that the interior of a food item as "not affected" or "insulated" from the effects of exterior temperature abuse.

Other tests showed the same warm-up and cool-down profiles, and confirmed that the total degree-minutes of exposure was relatively independent of depth in the food object:  the interior warms more slowly than the exterior, but also cools more slowly than the exterior. Of course if the warming event is a short lived "spike" of higher temperatures, then the interior will not reach elevated and damaging temperatures.

In another test (shown here on the left), the same item was exposed to a lower temperature simulated partial thaw event.  The item - initially at 0.5º F was exposed to an immediate rise to 10.7º F. 

Equilibration time is somewhat less than in the extreme example above (about 1.5 hours compared to 2 hours, above).  What is evident from this example is that there is really not a large difference in equilibration time from an extreme example, and this case of a subfreezing event.

The return profile (back down to 0.5º F, not shown here) was almost identical to the profile shown in the top figure, a "down arc".  In each case tested, the temperature exposure in degree-minutes on the interior was about the same as the exterior as long as the exposure time was above 1-2 hours.

The pattern which emerges in every case is a set of profiles that underscore the basic picture.  If thaw or partial thaw occurs for even a relatively short period of time, there are effects that go to the core of the product.

In the summary diagram at the left, the typical pattern is shown.  Exterior temperature rise rapidly to match the ambient air temperature and equilibrate rapidly.  When the event is over, the exterior temperature drops rapidly to reflect the change in ambient air.  Interior temperatures are much slower to react.  Most significant is that interior higher temperatures fall much more slowly.  The end result is that there is about the same cumulative exposure of interior and exterior of the food item.

Conclusions

	Temperature abuse as short as 1 hour in frozen food affects the interior about as much as the surface
	Total thermal exposure in degree-minutes is about the same in the interior as the surface
	Equilibration time is not substantially different for a mild temperature abuse event compared to an extreme temperature abuse event

So, it can be assumed that if there is any type of significant thaw or partial thaw event, that the whole food item has been affected.  This is important in consideration of monitoring frozen products.  

Freezer Label Responses Under Different Thaw Conditions

For examples of how TTI thaw monitoring works in actual practice, see example time-temperature thaw profiles for each of the CheckPoint® Freezer Label types:  

CheckPoint® B2-3H Labels  thaw profiles

CheckPoint® B2-12H Labels thaw profiles

CheckPoint® B2-36H Labels thaw profiles

These illustrate the effectiveness of the CheckPoint® approach to monitoring for partial thaw events. Details on the exact time-temperature characteristics of the B-series are available; click here to see a graph and table of these characteristics.

Special Thaw Indicator Application?

Intermediate values between B2-3H and B2-12H and B2-12H and B2-36H are available in the normal Vitsab® product series on request and demonstration of volume commitment. 

Contact us (Stephen Marlowe + 704-825-8150 | + 704-591-2327 (mobile) | s.marlowe@vitsab.com) to inquire about how we can solve your freezer monitoring needs.