THERMAL PROPERTIES OF BUTTER
E.L. Watson
Member CSAE
Department of Agricultural Engineering
University of British Columbia
Vancouver, B.C.
INTRODUCTION
A knowledge of the quantity of
thermal energy and its rate of transfer
during cooling or heating of foodstuffs is
of considerable importance in the design
of storage and handling systems for
foods. The quantity of heat will influence
the capacity of the cooling or heating
A finite difference numerical analysis
was developed to include the variation
with temperature of conductivity and
specific heat in the transient temperature
predicitions. The temperatures so calcu
lated were used to construct a theoretical
"warming" curve between —25 and 12°C.
This calculated warming curve is within
1°C of the measured temperature history.
equipment, and the rate of heat transfer
will, of course, govern the time required
to complete the process.
When a change of state occurs during
the heating and cooling process as, for
example, during the freezing of water or
the solidification of fats, the transfer of
heat becomes very complex because the
thermal properties are very temperaturedependent when changes of state are
taking place.
Fifty-five-pound cubical blocks of but
Further studies will be necessary to
fully evaluate the effects of water, salt
and composition of the fat upon the
thermal properties. Also, the influence of
temperature history and subcooling on
liquid-solid fractions of fat, solution and
water will require additional investiga
14, 15). Reported values of the thermal
conductivity of butter range from a low
value of 0.163 W/(m.K) to a high value of
0.329 W/(m.K). Reported values of spe
cific heats vary from 3,000 to 15,000
J/(kg.K). Many reports omit the composi
tion of the sample, and frequently fail to
specify the test conditions such as tem
perature, temperature difference or test
apparatus used. Tschubik and Maslow
(13) have reported the most comprehen
sive data for a wide variety of foodstuffs.
They have summarized the thermal con
ductivities and specific heats of salted and
unsalted butters at eight different tem
peratures between 17 and —35°C.
Specific and latent heats are reported
as enthalpies by Riedel (7) or as "mean"
specific heat by Staph and Woolrich (11).
tion.
LITERATURE REVIEW
Although the use of enthalpy is more
The thermal properties of butter have
been reported by several authors (4, 13,
logical, the concept of combining specific
and latent heats into a mean specific heat
ter are commonly stored at —20to —30°C.
Before the butter can be packed for the
retail trade, it must be warmed to about 1
0.301—
or 2°C. This process is known as "temper
ing" and is usually carried out at a
controlled temperature of 2 to 3 C. High
ambient temperatures cannot be used
because the butter would melt and lose
quality as well as becoming very difficult
u
to handle in the packaging machinery.
Three changes of state may be
observed during the warming of butter.
At -21°C, a NaCl-H20 eutectic melts
(10). If water is present in excess of the
eutectic concentration (^23%NaCl), ice
melts until the freezing point of the
solution is reached. The melting of the
fats commences at about 10°C, and most
butterfats are completely liquid at 40°C
(14). Some fats may melt below 10°C,
g).25
z
o
u
<
but the quantity is small and any such
melting is masked by the melting of the
ice and eutectic.
It was found that both the thermal
conductivity and the specific heat of
N
/
butter varied with temperature.
0.20
-30
-20
-10
TEMPERATURE
RECEIVED FOR PUBLICATION NOVEMBER
29, 1974
68
10
0
20
30
(°C )
Figure 1. Thermal conductivity of butter at various temperatures. Dotted lines represent values
used in the finite difference analysis (1 Btu/h°F ft = 1.731 W/m.K).
CANADIAN AGRICULTURAL ENGINEERING, VOL. 17 NO. 2, DECEMBER 1975
is required to carry out conventional
k
transient heat flow calculations when a
change of phase is taking place.
Q
thermal conductivity W/(m.K)
power consumed by probe heater
(W/m);
=
temperature in °C of probe thermo
Ti =
couple at time f i;
temperature in C of probe thermo
Ti
A number of methods have been used
to measure thermal conductivity (1, 2, 4,
5, 8). A very rapid method using a
thermal conductivity probe was develop
ed by Hooper and Lepper (5), and adapt
ed for small samples by Sweat and Haugh
couple at time t2;
tx and
h
'o
(12).
time since probe was energized (sec);
a time correction factor (sec).
=
=
At
Sherbon and Dolby (10) have used
differential scanning calorimetry (DSC)
to study the influence of thermal history
on the physcial properties of butter. They
least
three
measurements
were
taken at each temperature. The mean
conductivities are shown in Figure 1.
transitions or phase changes within the
test sample. When techniques to measure
these changes directly in energy units are
available, the measurement is referred to
as
differential scanning calorimetry
(DSC). For additional information, the
reader is referred to the book by
MacKenzie (6).
The Dupont 900 Differential Scanning
Calorimeter (DSC) was used to obtain
thermograms. Ten-to 15-mg samples of
butter were cooled in the DSC cell to
—80° C at 8°C/min, using liquid nitrogen.
The cooling curves were not recorded.
reported that unsalted butter has one
melting transition associated with the
Differential thermal analysis (DTA) is
a technique for recording the difference
in temperature, At, between a test sub
aqueous phase at —0.5°C, whereas salted
stance and an inert reference material as
20°C/min. Variation in holding times did
butters have two such transitions: one at
samples of the two are warmed or cooled,
-21°C corresponding to the melting of the
at a constant rate. If the test substance is
not appear to
thermograms.
NaCl-H20 eutectic mixture, and the
other varying with salt concentration,
probably corresponding to melting of ice
in a NaCl solution. Also, these authors
have shown that the DSC is useful in
The samples were held at —80° C for
varying times (less than 1 h). The thermo
gram was recorded while heating at
materially
affect
the
thermally active, then the curve obtained
by plotting At against temperature shows
irregularities or peaks. These peaks
The specific heat data were obtained
as follows: Two empty pans were first
indicate the occurrence and measure the
warmed at 20°C/min on the DSC from
extent
-80° to 20° C to obtain a blank thermo-
of
energy-involving
reactions,
following the effects of cream treatment
upon the melting characteristics of the
resulting butter.
0.5
MATERIALS AND METHODS
The thermal conductivity of a number
of butter samples was measured using the
thermal probe developed by Sweat and
Haugh (12) with a plexiglass cell 7/8-inch
in diam and 2 inches long. The sample
cell and a separate reference cell were
0 -
u
filled with butter at room temperature,
taking care to disturb the sample as little
w_ q 5 _
as possible. The cells were cooled in
about 1/2 h to measuring temperature in
a refrigerated bath. A few samples were
held in -24°C storage for several days
before measuring the conductivity.
1.0
Each measurement was taken as fol
lows: when the temperature of the bath
was stable at the desired temperature, a
current of 0.2 A was applied to the
probe. The resulting temperature rise was
I—
<
^
^ - 1.5
recorded on a Riken Denshi millivolt
^
recorder. The temperature record thus
»-
obtained was used to provide the instan
taneous temperatures occurring at 1-sec
intervals between 6 and 19 sec ofheating.
_20
The values of logarithm of time and
" '
temperature
were
subjected
to
linear
regression on the IBM 360 computer. The
slope of the line thus obtained was used
to calculate the thermal conductivity
using the following equation:
.Qln[(t2- t0)/(t1 - t0)]
*nT2-Tx)
-60
-40
-20
TEMPERATURE
where
0
20
(°C)
Figure 2. Warming thermograms of butter and of emptypan.
CANADIAN AGRICULTURAL ENGINEERING, VOL. 17 NO. 2, DECEMBER 1975
69
gram. Then a sample was placed in one of
the pans and sample and reference pan
containing 1.4% salt melted at -5bC (10).
were again warmed from -80°C to 20°C
The magnitude of these low temperature
peaks is related to the quantity of water
at 20°C/min. The resulting thermograms
present in
The conductivity and mean specific
heat of butter containing 16% water and
2% NaCl have been measured at tempera
butter. The transition
occurring above 10°C is likely caused by
are shown in Figure 2.
The rate of warming of 55-lb cubical
blocks of butter was measured by placing
several blocks at -20°C in a room control
led at
the
CONCLUSIONS
12.5°C. Thermocouples were
inserted at several locations in the blocks
and temperature readings were
daily.
taken
the melting of the fats.
tures between —40 and +30°C. The values
The mean specific heats at various
temperatures as shown in Figure 3 were
calculated from the thermograms in
Figure 2. These mean specific heats
vary greatly with temperature. Further
studies will be required to establish the
influence of water and salt content,
delayed nucleation and/or recrystalliza
tion upon the thermal properties of but
include both latent and sensible heat as
ter.
defined by Staph and Woolrich (11). The
large contribution of the latent heat of
Theoretical temperature histories of
cubes of butter were predicted by finite
difference analyses using the measured
values of the thermal properties. The
fusion of the eutectic and of the ice is
RESULTS AND DISCUSSION
very evident at -20°C and -10°C. The
latent heat of fusion of fats is released
The butter used in these tests contain
predicted temperatures were within 1°C
over a temperature range of 5° to 40°C.
ed 80% butterfat, 16% moisture and 2%
The specific heat of the frozen butter at
NaCl.
-40°C is slightly more than
of the measured temperatures.
diction equation will be very
evaluating the effect of air
ambient temperature and size
upon the time required for
1,465
J/(kg.K) (0.35 Btu/lb°F) whereas the spe
The thermal conductivities are shown
in Figure 1. Each point is the average of
three measurements. Consecutive values
cific heat of melted butter at 40° C is ap
proximately 2,300 J/(kg.K) (0.55
Btu/lb°F).
reach any designated temperature.
for any one subsample agreed within 5%.
However, considerable variation (12%)
Using the techniques described by
Dusinberre (3), the Fourier equation can
was noted between replications.
be written in finite difference form for an
The conductivity of butter frozen at
-40°C for 1 h is about 0.26 W/(m.K)
infinite plate with both conductivity and
specific heat varying with temperature.
Temperature profiles for an infinite plate
(0.15 Btu/h°F ft). However, when the
temperature remained well below freezing
for several days, the conductivity was
found to increase, sometimes reaching
0.29 W/(m.k), (0.17 Btu/h°F ft). This in
crease may be caused by delayed nucleation or recrystallization of the compon
ents of the butter. As the temperature of
freezing was increased the conductivity
tended to decrease, approaching a value
This pre
useful in
velocity,
of block
butter to
SUMMARY
The thermal properties of salted but
ters were measured at a series of tempera
tures between -40 and +20 degrees Celsius
were calculated assuming symmetry on
either side of the center line of the plate.
using
the
thermal
probe
and
the
These profiles were used to calculate
unaccomplished temperature ratios at the
center of the plate. The third power of
the ratio for a plate should represent the
changes of state were observed. At -21
degrees Celsius the NaCl-H20 eutectic
differential scanning calorimeter. Three
melts.
second
of ice.
trolled
ratio for a cube (9). Center temperatures
calculated in this way are plotted in
Figure 4.
Just below 0 degrees Celsius a
transition represents the melting
This melting temperature is con
by the salt concentration. A third
transition
occurs between
10 and
40
of 0.2 W/(m.K) (0.11 Btu/h°F ft) at 0°C.
This is expected since the conductivity of
water is considerably less than the con
ductivity of ice. Quite unexpectedly the
conductivity of the melting butter in
creased as the temperature increased.
5
\
V
V-
\
\\
The variability in the values of the
conductivities
found
at
Y
temperatures
below 0°C are presumed to be related to
the complexity of the system, particular
ly to the possibility of recrystallization or
delayed nucleation (and subcooling) both
of fats and of water. Also, of course, the
crystal structure of the fats is influenced
by the rate of cooling. Further work will
be required to clarify the relationship of
Y
^4
Y
^
%
\\
\
V
\\
O)
<
\\
\
\
.—
h- 3
<
UJ
J
X
/
these various factors and establish their
u
relationship to the thermal properties.
u_
J
l
^^^^
u2
UJ
The thermograms measured on the
Dupont DSC are shown in Figure 2.
These thermograms are very similar to
those reported by Sherbon and Dolby
(10). At about -21°C the NaCl-H20
-^^^
Q.
co
—-—'
Z
<
LU
*1
70
_L_
i
-20
-10
Figure 3.
10
0
TEMPERATURE
indicates that the ice is all melted at
about -10°C. This is expected for butter
containing 2% salt. The ice in butter
I
-30
eutectic begins to melt. The second peak
20
30
40
(°C )
Mean specific heat of butter at various temperatures. Dotted lines represent values used
in the finite difference analysis (1 Btu/lb°F = 4186.8 J/(kg.K).
CANADIAN AGRICULTURAL ENGINEERING, VOL. 17 NO. 2, DECEMBER 1975
degrees Celsius representing the melting
of fats.
The mean specific heat of salted butter
varies from 1,300 joules per kilocalorie to
more than 5,000 joules par kilocalorie.
The high values are caused by the latent
heat of fusion associated with the various
changes of phase.
The thermal conductivity at —40
degrees Celsius is approximately 0.25
watts per calorie. When the butter is
warmed to 0 degrees Celsius the conduc
tivity tends to decrease to about 0.2
watts per calorie. At temperatures above
0 degrees Celsius the conductivity in
creases rapidly to a value near 0.3 watts
per calorie at 25 degrees Celsius.
The conductivity increases with time
of storage at constant temperatures below
0 degrees Celsius. It is suggested that such
increases may be caused by delayed
nucleation or recrystallization of the
components in the butter.
A number of transient temperature
predicitions were made using finite dif
ference numerical analysis. This method
of analysis permits the variation of
specific heat and conductivity with
temperature to be included in the predic
tion equation. The temperatures so calcu
lated were graphed as a theoretical
warming curve. This calculated warming
curve varies only 1 degree Celsius from a
measured temperature history.
ACKNOWLEDGMENTS
These studies were made possible by
funds supplied by the National Research
Council. Dairyland Division of the Fraser
Valley Milk Producers Association sup
plied the butter samples. The assistance
of Jeffery Chow in carrying out the
measurements and calculations is greatly
appreciated.
Figure 4. Measured and calculated center temperatures in a 55-lb cube of butter warming in still
air at 12-1/2°C: line A, measured center temperature; line B,calculated center tempera
ture using properties shown in Figures 1 and 3 and a surface film conductance of 0.5
Btu/h F ft ; line C, calculated center temperature assuming constant thermal properties
(k =0.135 Btu/h°F ft; Cp =0.8 Btu/lb°F; h =0.5 Btu/h°F ft2).
5.
Hooper, F.C. and F.R. Lepper. 1950.
Transient heat flow apparatus for
11.
determination of thermal conductivities.
Staph, Horace E. and W.R. Woolrich.
1951. Specific and latent heat of foods in
the freezing zone. Refrig Eng. 59:
Trans. Amer. Soc. Heat Vent. Eng. 56:
1086-1089.
309.
6.
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1.
7.
4.
Sweat, V.E. and C.G. Haugh. 1972. A
thermal conductivity probe for small food
samples. Paper 72-376 presented to 1972
Ann. Meet. Amer. Soc. Agric. Eng., Hot
Springs. Ark.
Dickerson,
Roger
W.
13.
Tschubik, LA. and A.M. Maslow. 1973.
Warmphysikalische
Konstanten
von
Riedel, L. 1950. Der Kaltebedarf beim
Gefrieren
Transfer. 94: 133-140.
3.
12.
Cooper, T.E. and G.J. Trezek. 1972. A
probe technique for determining the
thermal conductivity of tissue. J. Heat
2.
MacKenzie, R.C. (Ed.). 1970. Differential
thermal analysis Vol. 1, Academic Press,
New York, N.Y.
von
Obst
und
Gemuse,
Kaltetechnik. 8: 195-203.
Jr.
1965.
An
8.
Reidy, G.A. and A.L. Rippen. 1971.
Methods
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Dusinberre, G.H. 1961. Heat transfer
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Textb. Co. Scranton, Perm.
9.
Hooper, F.C. and S.C. Chang. 1953.
Development of the thermal conductivity
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59: 463-472.
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t r a n sf e r.
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Company, Cambridge, Mass.
Publishing
Sherbon, J.W. and R.M. Dolby.
1972.
Application of differential scanning
calorimetry to butter. J. Dairy Res. 39:
319-324.
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Webb, Byron, H. and Arnold H. Johnson.
1956. Fundamentals of dairy chemistry.
Avi Publishing Company Inc., Westport,
Schneider, P.J. 1955. Conduction heat
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Woodams, Edward, E. and Joseph E.
Nowrey.
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71