3316
M. KITSUNAI et al.
hand, even if the solution is re-cooled after exceeding
the upper-limit temperature (that is, the phase transition
temperature for the amide used), visually detectable
biphasic states remain, and the solutions can be used as
an indicator that records the thermal history. Moreover,
the solution reforms a single phase when vigorously
agitated after being adequately cooled. Therefore,
depending on the shape of the container sealed with
the amide solution, it can be applied to a thermal history
indicator that can be used repeatedly. In addition, the
amide compounds were stable in aqueous solution even
when they were allowed to stand at room temperature
for 6 months.
2
5°C
40°C·1 min 40°C·10 min
Fig. 3. Phase Transition of the Mixture of N-Isopropylpropionamide
[
9] and Water.
The amide phase contained azulene (8 mM) as a color marker.
0
.9/1.1–1.0/1.0 (v/v, water/amide). Also, it was found
In conclusion, aqueous solutions of varied low-
molecular-weight amides showed marked temperature-
dependent phase transitions. The temperatures could be
controlled by the ratios of amide and water or by the
content of sodium chloride in the solution. The hydro-
philic property of the amide bond should generally
become higher with hydration of the corresponding
carbonyl bond on cooling, as observed in the poly(N-
isopropylacrylamide). In the case of the present low-
molecular-weight amides, the hydrophilic and hydro-
phobic balance of the molecules should affect the phase
transition temperature of the corresponding aqueous
solutions. Since those aqueous solutions gave clearer
visual changes with the phase separation in the presence
of pigments, they can perhaps be applied as thermal
history indicators, by sealing in small-sized capillaries
or chips, for example.
that the addition of sodium chloride resulted in a
downward shift of the phase transition temperature for
the same volume ratio. In other words, the phase
separation temperature could be freely controlled.
The addition of sodium chloride should decrease the
water solubility of those amide compounds, lowering the
phase transition temperatures of the aqueous solutions.
Amide [9] was mixed with water at a 1:1 volume ratio,
and a portion (1.0 ml) of the solution was sealed in a
sample container. This remained in a single phase at
1
0)
ꢁ
ꢁ
2
0 C. Even when the temperature was raised to 35 C
and held at that temperature for longer than 20 min, the
solution did not change. When the temperature was
ꢁ
raised to 36 C, after 10 min the solution became cloudy.
ꢁ
When a temperature of 36 C was maintained, the
solution gradually separated into two phases, and it
completely separated after more than 20 min. Further-
ꢁ
more, when the temperature was raised from 20 C to
ꢁ
Acknowledgment
4
0 C in one step, the solution quickly became cloudy
and separated into two phases in about 10 min, as shown
in Fig. 3. This property was generally observed for the
amides in Scheme 1.
This work is partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan.
When the same amide [9] was mixed with 0.2 mol/l
sodium chloride solution at a 1:1 volume ratio, a single
phase was maintained at 15 C. A portion (1.0 ml) of the
References
ꢁ
1
2
)
)
Shinguu, K., Technical considerations and future direc-
tion of frozen foods. Reito, 77, 69–71 (2002).
solution was sealed in a sample container and the
temperature was gradually raised until the solution
became cloudy, at which point the time was noted.
Jagannath, A., Ramesh, A., Ramesh, M. N.,
Chandrashekar, A., and Varadaraj, M. C., Predictive
model for the behavior of Listeria monocytogenes
Scott A in Shrikhand, prepared with a biopreservative,
pediocin K7. Food Microbiol., 18, 335–343 (2001).
ꢁ
Although there was no change up to 17 C, the solution
ꢁ
became cloudy at 18 C after 10 min, and it completely
separated into two phases after more than 20 min. A
clear-water solution that changed to cloudy was instantly
observable.
3) Vaz-Velho, M., Duarte, G., McLauchlin, J., and Gibbs,
P., Characterization of Listeria monocytogenes isolated
from production lines of fresh and cold-smoked fish.
J. Appl. Microbiol., 91, 556–562 (2001).
Water solutions of the synthesized amide compounds
when heated above the set phase transition temperatures
became cloudy after about 10 min, and gradually the
phase separated. After phase separation, even when the
solutions were re-cooled, the phases did not recombine
naturally at the lower temperature. Thus a history of the
warming event could be recorded by the visual change.
In other words, if no visual change occurs in these water
solutions, this indicates that the accompanying products
that require storage at low temperatures have been
maintained below the control temperature. On the other
4
)
Kogure, H., Kawasaki, S., Nakajima, K., Sakai, N.,
Futase, K., Inatsu, Y., Bari, M. L., Isshiki, K., and
Kawamoto, S., Development of a novel microbial sensor
with baker’s yeast cells for monitoring temperature
control during cold food chain. J. Food Prot., 68, 182–
1
86 (2005).
5
)
Zhang, X., and Kirsch, L. E., The physical stability
of thermally-stressed phospholipid-based emulsions
containing methyl, propyl and heptyl parabens as model
drugs. Int. J. Pharmac., 265, 133–140 (2003).
Kitsunai, Makoto
