ESTUDIANTIL UNIVERSITARIO 1
2. Metodología y datos
If repeated, it would be useful to ask participants to keep a basic diary to record fridge usage during the time when the data loggers are in the fridge. This wouldn’t entail tracking every time the fridge door is opened but would record if, for example, the door is open for a longer period of time than usual (e.g. to clean the fridge or to unpack the shopping).
4.2.4 Comparison with other studies
A review of survey work carried out to determine domestic fridge temperatures has been undertaken (Campden BRI, 2004. Evaluation of product shelf life for refrigerated foods, Guideline No. 46). This showed that five domestic fridge air temperature surveys had been carried out, all of them between 1990 and 1992.
Reducing food waste through the chill chain 40
A survey of 75 households recorded that mean operating temperatures were <5°C (Rose, S.A., Steadman, S.
and Brunskill, R. (1990). A temperature survey of domestic refrigerators. CCFRA Technical Memorandum No.
577. Campden BRI).
A MAFF survey of 252 households recorded mean operating temperatures of 6°C, with 22% of fridges at temperatures >8°C (James, S.J. and Evans, J. (1992). Consumer handling of refrigerated foods: temperature performance. International Journal of Refrigeration, 15 (5), 299-306).
A survey of 150 domestic fridges recorded average operating temperatures of 6.5°C. It was also noted that 26% of larder fridges and 29% of freezer-box fridges operated at average temperatures above 8°C (Flynn, O.M.J., Blair, I. and McDowell, D. (1992). The efficiency and consumer operation of domestic refrigerators.
International Journal of Refrigeration, 15 (5), 307-312).
Temperature distributions were measured in empty and loaded fridges by James and Evans (1992) 'The temperature performance of domestic refrigerators'. International Journal of refrigeration, 15 (5), 313-319.
When loaded with foods at 5°C, mean temperatures ranged from 0.3-8°C on the top shelf and 2-3.7°C in the fridge door. Average temperatures of two fridges loaded with food at 20°C for 4 hours showed that mean temperatures ranged from 10.8-12°C.
The in-home temperature survey completed as part of this research shows that the majority of domestic refrigerators operate at a mean temperature of around 7°C. It was apparent that a proportion of the fridges tested (14 fridges, 29% of the sample) were operating at mean fridge temperatures of 9°C or above. Only 14 of the 48 fridges (29% of the sample) were found to be at mean temperatures of 5°C or less. With 34 fridges (70%) operating below 8°C.
The average temperature reading across the whole survey population, as recorded by the interviewers, using the thermometers provided was 5.9oC.
As part of the FSA’s ‘Forward Evidence Plan’27 a larger scale survey of domestic refrigerator temperatures in the UK is being considered, which would build on the current research. This new survey will provide evidence to support the Agency’s food hygiene messaging, assess whether refrigerator temperatures have changed since the last survey, inform risk assessments for microbiological hazards in the domestic kitchen and provide information for manufacturers on consumer practices and conditions for storage of chilled foods.
27 http://www.food.gov.uk/multimedia/pdfs/evidenceplan10.pdf
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5.0 Experimental programme at Campden BRI
5.1 Controlled domestic fridge temperature study
For these tests, six fridges were used under laboratory controlled conditions (e.g. temperature and humidity measurements, fridge loading, door opening and environmental conditions) and the factors affecting temperature distributions were determined.
5.1.1 Method summary
The six fridges chosen for this test encompassed, as far as practicable, a range of makes, models and designs.
This included two identical larder fridges, two fridges within integrated fridge-freezers, an 'American-style' fridge and a fridge with integrated icebox. The objectives of these tests were to measure the typical temperature /humidity distributions within domestic fridges, when filled with a model food load and subjected to a series of controlled door opening/closing actions.
Table 4 indicates the fridges used for this test along with their main design features and technical specifications.
The volumes quoted represent the volume of the fridge and are taken from the rating plate found on the fridge.
None of the fridges had circulation fans - all relied on natural circulation of air in the fridge.
Table 4 Fridges used in the controlled laboratory tests.
Fridge no. Shelf Energy rating
Refrigerant Setting* Volume Fridge type***
1 Glass 100 W 55g
* The setting used on the fridge for these tests was set to provide a nominal fridge temperature of 5°C. The temperature was marked directly on the display for fridge 6.
** Two identical models of larder fridge were chosen to give an idea of not just inter-model variability, but also variability between identical appliances. Note the fact that to get a nominal fridge temperature of 5°C the two fridges required a different setting, suggesting variability in the operation of the thermostats.
*** The makes/models have been anonymised.
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5.1.2 Results – temperature distribution
Figure 23 shows the fridge air temperature distributions within Fridge 1 (fridge freezer). During the night-time operation (no door opening), the average temperatures ranged from 1.8°C (middle shelf, back) to 9.3°C (top shelf, front).
During daytime operation (door opened for two minutes each hour for six hours), average temperatures for these periods ranged from 2.3°C (middle shelf, back) to 10.6°C (top shelf, front).
Figure 23a Temperature distributions within Fridge 1 (fridge freezer) (night).
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Figure 23b Temperature distributions within Fridge 1 (fridge freezer) (day).
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Reducing food waste through the chill chain 43 Figure 24 shows the fridge air temperature distributions within Fridge 2 (fridge freezer). During the night time operation (no door opening), average values of the temperature ranged from 5°C (middle shelf, back) to 8.7°C (top shelf, front).
During daytime operation (door opened for two minutes each hour for six hours), average temperatures for these periods ranged from 6.7°C (middle shelf, back) to 9.7°C (top shelf, front).
Figure 24a Temperature distributions within Fridge 2 (fridge freezer) (night).
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Figure 24b Temperature distributions within Fridge 2 (fridge freezer) (day).
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Reducing food waste through the chill chain 44 Figure 25 shows the fridge air temperature distributions within Fridge 3 (larder fridge). During the night time operation (no door opening), average values of the temperature ranged from 1.4°C (middle shelf, back) to 9.1°C (top of door).
During daytime operation (door opened for two minutes each hour for six hours), average temperatures for these periods ranged from 1.8°C (middle shelf, back) to 5.5°C (top of door).
Figure 25a Temperature distributions within Fridge 3 (larder fridge) (night).
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Figure 25b Temperature distributions within Fridge 3 (larder fridge) (day).
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Reducing food waste through the chill chain 45 Figure 26 shows the fridge air temperature distributions within Fridge 4 (larder fridge). During the night time operation (no door opening), average values of the temperature ranged from 4.8°C (middle shelf, back) to 8.4°C (base of door).
During daytime operation (door opened for two minutes each hour for six hours), average temperatures for these periods ranged from 4.9°C (middle shelf, back) to 9.8°C (base of door).
Figure 26a Temperature distributions within Fridge 4 (larder fridge) (night).
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Figure 26b Temperature distributions within Fridge 4 (larder fridge) (day).
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Reducing food waste through the chill chain 46 Figure 27 shows the fridge air temperature distributions within Fridge 5 (icebox fridge). During the night time operation (no door opening), average values of the temperature ranged from 5.2°C (top shelf, centre) to 10.5°C (salad crisper).
During daytime operation (door opened for two minutes each hour for six hours), average temperatures for these periods ranged from 6.3°C (top shelf, back) to 11°C (top shelf, front).
Figure 27a Temperature distributions within Fridge 5 (icebox fridge) (night).
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Figure 27b Temperature distributions within Fridge 5 (icebox fridge) (day).
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Reducing food waste through the chill chain 47 Figure 28 shows the fridge air temperature distributions within Fridge 6 (‘American’ style fridge). During the night time operation (no door opening), average values of the temperature ranged from 4.6°C (middle shelf) to 6°C (top shelf).
During daytime operation (door opened for two minutes each hour for six hours), average temperatures for these periods ranged from 4.6°C (middle shelf) to 6.5°C (top shelf).
Figure 28a Temperature distributions within Fridge 6 (‘American’ style fridge) (night).
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Figure 28b Temperature distributions within Fridge 6 (‘American’ style fridge) (day).
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Reducing food waste through the chill chain 48 The increased air temperature in fridges during the daytime is a result of both the higher ambient air temperature during the day and the increased frequency with which the door is opened. This highlights the importance of locating the fridge in a suitable place within the kitchen - away from radiators or other sources of heat e.g.
ovens, hobs, grills or the ventilation outlet of microwave ovens. The fridge should also have access to an appropriate airflow to assist in dissipation of the heat removed from the food by the fridge; the rear of the fridge often has coils mounted externally to assist with airflow. Fridges which are built into kitchen units should be installed so as to allow adequate airflow around the fridge.
The booklet produced by MAFF (Ministry of Agriculture, Fisheries and Food) in 1984 'Keeping food cool and safe' (extract shown in Figure 29) highlights a number of useful pointers in relation to keeping food cool. This includes advice on where the coldest part of the fridge might be, which foods should be kept in which locations within the fridge and keeping the coldest part of the fridge cold enough for safe food storage. The results of this research were consistent with the results of the earlier work.
The conclusions that can be derived from this work are:
Consumers should check their fridge temperatures with a fridge thermometer and not assume the fridge temperature dial correlates to fridge temperature. Temperature dials traditionally carry numbers relating to the degree of cooling, but not actual temperature, and there is often confusion whether increasing the number increases or decreases the fridge temperature.
As air temperature is not consistent across the whole cabinet, a single point temperature measurement in the fridge may not provide adequate information for the consumer. It is recommended that two or three temperature points are used - top, middle and bottom of fridge and then the consumer can ensure
temperature sensitive products (such as uncooked meats and pre-prepared foods) are placed in the coldest part of the fridge.
The central areas of the fridges tested were generally the coldest. Current advice to consumers to load the fridge with uncooked meats, fish and ready meals in these areas of the fridge should be re-emphasised.
Quantifying actual temperature ranges within the fridge and mapping this (along the lines of the 'Keeping food cool and safe' booklet) would help consumers minimise food safety risks, improve food quality and reduce waste of refrigerated foods. A simple illustration could be produced graphically within the fridge or included as part of the fridge manufacturers hand book.
Modern fridges are increasingly designed to be more energy efficient, as a response to environmental legislation and consumer demands. This increase in energy efficiency is largely gained through the use of better insulation, which should have the additional benefit of maintaining a more stable temperature.
Reducing food waste through the chill chain 49 Figure 29 Extract from 1984 'Keeping food cool and safe' booklet showing where the coldest part of a domestic fridge is likely to be.
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5.1.3 Results – fridge shelf construction
The effects of different fridge shelf constructions were also investigated, using the two fridge-freezers (Fridges 1 and 2). Figure 30 shows the temperature fluctuations at the centre of two fridge freezers, one having a wire shelf and one a solid glass shelf. With the wire shelf, temperature within the fridge rose from 5°C to over 12°C on opening the door, taking some 35 minutes after door closure to recover a fridge temperature of less than 5°C.
With the solid glass shelf, temperature within the fridge rose less, by just a couple of degrees, but also took around 40 minutes to recover to a fridge temperature of less than 5°C.
Figure 30a Temperature fluctuations with different fridge shelves (wire shelves).
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Mi ddl e Shel f Left Front Mi ddl e Shel f Center
Figure 30b Temperature fluctuations with different fridge shelves (glass shelves).
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Mi ddl e Shel f Left Front Mi ddl e Shel f Center
The large gap between the two dataloggers in the glass shelf test (Figure 30b) showed that the glass (solid) shelf had an effect that 'stratified' the temperature throughout the fridge (i.e. could create different temperatures at different levels). Whereas the wire shelves (open) enabled air to flow freely between the top and bottom of the fridge, the solid shelves sometimes created layers of different temperatures. It is expected that this could have advantages and disadvantages - an open shelf would allow temperature to equalise more uniformly throughout the entire fridge but any changes e.g. door opening or placing a warmer food in the fridge etc. would help raise the temperature of the entire fridge. Conversely, a solid shelf would perhaps create very different temperature layers in different parts of the fridge. However, for any changes due to door opening, warm food etc. glass shelves would isolate the effect from some areas within the fridge.
Reducing food waste through the chill chain 51 These results suggest that glass shelves may help maintain a more consistent temperature over time within the fridge and can be used to minimise rapid temperature rises. This aspect of work will be considered more fully under the ‘fridge design’ part of this report (Section 6). Data28 would suggest that the majority of UK fridges have glass shelves (approximately 90%).
5.1.4 Results – humidity
Humidity was also measured using the two fridge-freezers (Fridges 1 and 2).
Figure 31 shows the humidity profiles measured in these two fridge freezers. Both profiles show a cyclic pattern.
The cycling of humidity (peaks and troughs) relate to the frequency of the thermostatic temperature control used in the fridge (the temperature results also showed that one of the test fridges operated within a narrower temperature range than the other; Figures 4 and 5).
During night time operation, the average mean dew point temperature29 in both fridges dropped (Figure 31a).
Fridge 1 dropped to around -7°C and Fridge 2 to around -1°C. Both of these levels represent quite a dry environment.
During day time operation (Figure 31b), when the door was opened each hour, humidity levels were higher as warm, moist air was let into the fridge.
28 Beijing Inforce Consulting Ltd, July 2009
29 The Dew Point Temperature is the temperature at which condensation will start to occur.
Reducing food waste through the chill chain 52 Figure 31a Humidity profiles within two models of Fridge freezer (night)
Figure 31b Humidity profiles within two models of Fridge freezer (day)
NB. It is not clear why the spike occurred (Figure 12b - between 100 and 150 minutes). From the results of humidity measurement (and also temperature measurement) it appears that spikes in temperature or humidity do occur from time to time and for no apparent reason – it is suspected that sometimes the thermostat doesn’t kick in to regulate temperature (which would have the effect of producing the humidity spike). It is possible that the thermostat used to control the fridge temperature is located in a place where it doesn't pick up the fridge temperature as quickly as the temperature dataloggers used in these experiments.
In a fridge, humidity levels are generally lower than the surrounding environment as the moisture in the air condenses on the cooled surfaces within the fridge and then is drained away. From the results, although differences in humidity were found between the two fridges, they are not thought to be significant. It is likely, however, that a greater number of fridges would show a wider range of humidity conditions.
During the course of the experiments, Campden BRI determined that the humidity levels within the fridge will indeed have an effect on the likely quality of the refrigerated food stored in the fridge. This will be more pronounced with foods that are unwrapped. Generally, fridges with low humidity levels (drier environments) will
Reducing food waste through the chill chain 53 tend to dry out the surface layers of some foods, particularly high moisture food (e.g. leftover meal components) and also foods higher in fats (e.g. cheese, hams, meats). The result is often that the food looks spoiled. In all circumstances, it is preferable to wrap the food - either in its original packaging (re-closing the packaging) or in an air-tight container. This would also have the effect of helping to mitigate any changes in temperature that might occur when the fridge door is opened.
In contrast to this, it is well understood that high humidity can encourage mould growth so wrapping some foods can cause them to spoil prematurely. Foods which are respiring and giving off water, such as fruit and
vegetables, need to be stored carefully as sealed wrappings can lead to moisture building up around the foods.
Such items need to be stored in packaging with vent holes; the original packaging will be designed to meet this need.
5.1.5 Limitations of the study
The experiments were not designed to align with ISO standards, which set out test methods for household refrigerating appliances, but to simulate a real life environment. Hence, the controlled domestic fridge study tests air temperature rather than product temperature as this is what the consumer will measure using most domestic thermometers.
It is important to note that depending on the temperature of food going into the fridge, its residency time, its density and its packaging the fridge air temperature will not necessarily directly correlate to product temperature.
Nor will product temperature fluctuate at the same speed or to the same extent as air temperature. This study was not designed to quantify product temperature changes and cannot suggest the extent to which product temperatures are affected by air temperatures and cycles. However it is obvious that at equilibrium the temperature of the food cannot be lower than the air temperature.
This study has provided a snapshot of domestic fridge air temperatures and humidity under controlled conditions of measurement. Although six disparate fridges have been used for these tests, it is likely that a wider study, encompassing more models and types of fridge, would be useful to further reinforce the conclusions from this work.
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5.2 Domestic fridge thermometer evaluation
A separate study undertaken for WRAP by Campden BRI evaluated a wide range of domestic fridge thermometers available to the UK consumer. The results of this work are presented in a separate report30 with an overview of the results given below and summary table repeated in full (Table 5).
A wide range of domestic fridge thermometers are available. The majority are liquid-in-glass, electronic, liquid crystal or bimetallic thermometers. These are widely available in most hardware stores, online and in larger supermarkets. The retail price of thermometers ranges from under £2 for the simple liquid crystal or liquid-in-glass types to over £25 for some electronic types. Given the low price options, fridge thermometers are a very affordable tool for consumers to ensure their fridge is operating at the correct temperature and to map temperature ranges so temperature sensitive foods can be stored in the right place.
From a desk-based review of literature and web-based sources, a total of 18 different domestic fridge
thermometers were purchased for this study, representing the range currently available to UK consumers. They were purchased from a variety of sources, including mail order catalogues, internet suppliers, magazine
advertisements and in-store. These included eight versions of liquid-in-glass fridge thermometers, three versions of bimetallic fridge thermometers, five versions of electronic fridge thermometers, one version of a liquid crystal fridge thermometer and one version of an infrared fridge thermometer.
The accuracy of the fridge thermometers tested was generally good, with the majority of devices measuring fridge air temperatures to within ±0.5°C of the actual fridge temperature31. Some of the fridge thermometers tested were not designed to directly measure fridge air temperature. One type had a standard liquid-in-glass thermometer embedded into a clear liquid gel; this was designed to slow down the rate of temperature change of the thermometer and be more representative of the temperature change actually experienced by the food products within the fridge. Another type used an infrared sensor to directly measure the food product/packaging temperature within the fridge and responded much more quickly than the other thermometers.
All of the thermometers had scales that spanned a much wider range than was necessary for domestic refrigerator use, this made the proportion of the scale that was of interest to the consumer (typically 0-10°C) small as a proportion of the entire range, which consequently made the thermometers more difficult to read and less precise than the ideal. This is something for consideration by fridge thermometer manufacturers.
The readability and usability of the thermometers varied considerably. While the electronic thermometers could be read easily and accurately (and in most cases they could be read without opening the fridge door) the liquid in glass thermometers were generally considered to be the most difficult to read as the design meant they often had to be looked at closely to determine the reading. This may lead to inaccuracies as a consumer may have to remove the thermometer from the fridge to read it, which in turn may affect the displayed reading.
The instructions provided to consumers alongside the fridge thermometers were limited and quite variable. Many
The instructions provided to consumers alongside the fridge thermometers were limited and quite variable. Many