COMPARING INFRARED AND PROBE THERMOMETERS TO MEASURE THE HOT
HOLDING TEMPERATURE OF FOOD IN A RETAIL SETTING
Rohit C., Moos M., Meldrum R., Young I.
School Of Occupational and Public Health, Ryerson University
• The fundamental technical difference between probe and
infrared thermometer is that infrared thermometer
provide the surface temperature of the food item whereas
the probe thermometer offers the internal temperature.
• Food safety regulations in Ontario for the food service
and retail sector requires that potentially hazardous food
that is kept hot must be held at an internal temperature of
≥60°C. This limits the use of an infrared thermometer for
food safety compliance.
• This research examines the relationship between the
internal temperature of the hot holding food measured
via probe thermometer and the surface temperature
measured via infrared thermometer in a retail setting.
BACKGROUND
PURPOSE
• Seven different stores (Store A to Store G) from one
major retail chain in Greater Toronto Area were visited
six times each during February and March 2018.
• Seven different hot held food items (whole chicken,
chicken strips, chicken pieces, breaded chicken wings,
honey garlic chicken wings, potato wedges, ribs) were
sampled twice for internal temperature via probe
thermometer and twice for the corresponding surface
temperature via infrared thermometer during each visit.
• Both types of measurements were taken using a ParTech
TMD (ParTech, Inc. Toronto).
• The sampling time and store cooking time record were
used to calculate a variable ‘elapsed time’ – amount of
time a food product setting in the hot hold unit. Other
variables included in the research were total quantity of
food items in the hot holding unit (number of food
boxes), temperature of the hot-holding unit, and outside
weather temperature (via the Weather Network mobile
app).
• The data were analyzed using Microsoft Excel and SPSS
software.
Figure 1 Figure 2
Figure1: This graph shows probe measurement values for given infrared values at each sample point. At
the lower infrared temperatures (<60°C), the internal temperature is usually higher, and at the higher
temperature level, the effect is inversed. This can be explained by the surface of the food cooling down
faster in comparison to the center of the food.
Figure 2: This graph shows the strong positive correlation between the probe temperature and the
infrared temperature.
MATERIALS AND METHODS
• The Pearson Correlation test shows a significant
positive correlation (r = 0.706, n = 212) between the
probe and infrared measurements.
• The correlation between the probe and infrared
measurements differed among the food items, with
breaded chicken wings having the highest correlation (r
= 0.867, n = 35) and potato wedges having the lowest
correlation (r = 0.592, n = 39)
• The correlation between the two thermometer types
also differed by store.
• The elapsed time variable (preparation time – sampling
time) had a negative correlation with both the probe (r
= -0.261, n = 182) and infrared (r = -379, n = 182)
measurements.
RESULTS
DISCUSSION
• The research showed a strong positive correlation
between the probe and infrared thermometers and that
indicate a possibility of developing a predictive model
to estimate the internal temperature of food via
infrared thermometers.
• Such a model will make the use of infrared
thermometers more reliable for food safety
compliance by the food service industry.
• The elapsed time had a negative correlation with both
the probe and infrared temperatures and the result
suggests that longer the food stays in the hot holding
unit the more likely that it will have lower
temperature.
• The research had some limitations, including use of
only one type of thermometer, one retail chain, and
limited types of food products.
DESCRIPTIVE ANALYSIS OF STUDY
VARIABLES FOR ALL FOOD ITEMS
Mean SD N
Hot Holding Unit
Temp(°C) 74.64 10.69 33
Hot Holding Unit Food
Quantity21.53 9.75 43
Time Elapsed(h:mm) 1:59 1:32 182
Probe Temp(°C) 66.05 6.38 212
Infrared Temp(°C) 64.95 9.19 212
Difference b/w Probe &
Infrared(°C) 5.38 1.10 212
Outside Temp(°C) 1.76 3.30 294
PEARSON CORRELATION BETWEEN EXPLANATORY VARIABLES AND TEMPERATURE
MEASUREMENTS
VariableProbe Infrared
r p N r p N
Hot Holding Unit Temp -0.109 0.171 159 -0.293 0.01 159
Hot Holding Unit Food Quantity 0.021 0.766 212 -0.131 0.58 212
Time Elapsed -0.261 0.01 182 -0.379 0.01 182
Outside Temp 0.053 0.439 212 0.059 0.396 212
CORRELATION BETWEEN THE PROBE AND
INFRARED TEMERATURES, STRATIFIED BY
FOOD TYPE AND STORE LOCATION
Variable r p N
Food Type
Whole Chicken 0.741 0.01 42
Chicken Stripes 0.837 0.01 32
Chicken Pieces 0.756 0.01 36
Breaded Chicken Wings 0.867 0.01 35
Honey Garlic Chicken Wings 0.677 0.01 27
Potato Wedges 0.592 0.01 39
Ribs - - 1
Store Type
Store A 0.569 0.01 35
Store B 0.744 0.01 26
Store C 0.866 0.01 35
Store D 0.531 0.01 28
Store E 0.603 0.01 26
Store F 0.907 0.01 29
Store G 0.719 0.01 31
All data 0.706 0.01 212