Chapter 3 Materials and Methods
4.8 DEA experiments
The capacitance measurement of freeze dried fructose at 500 Hz at 10-80 °C is shown in Figure 4.95. It was stable before 40 °C then increased with temperature.
Figure 4.95The capacitance measurement of freeze dried fructose at 500 Hz.
The derivative of capacitance over temperature range of this measurement before
smoothing is shown in Figure 4.96. The derivative of capacitance over temperature range of this measurement after smoothing gave a peak of derivative as shown in Figure 4.97.
-1.0E-08 3.0E-08 7.0E-08 1.1E-07 1.5E-07
10 30 50 70
temperature (°C)
dC/dT
500 Hz
Figure 4.96Derivatives of capacitance at 500 Hz against temperature for freeze dried amorphous solid fructose without grease or data smoothing.
Figure 4.97Derivatives of capacitance at 500 Hz against temperature for freeze dried amorphous solid fructose with data smoothing.
The moisture content in the samples was found to change during the DEA tests. Dow Corning high vacuum grease was then applied to seal at all connecting parts of the DEA cell. There was no variation of moisture content of samples during DEA tests after application of the grease. The capacitance measurements were smoothed using a moving average. Heating and cooling measurements gave similar results. The results presented here were from heating measurement only. The same sample of freeze dried amorphous fructose showed good trends after smoothing (Figure 4.98). The trial with many materials confirmed the change in peak derivative with the chemical component of materials.
Figure 4.98Derivatives of capacitance against temperature of freeze dried amorphous fructose when Dow Corning high vacuum grease and data smoothing was used. The peak of derivative at 500Hz matched Tgfrom DSC.
The melted fructose, glucose and sucrose were analysed to determine Tg. DEA at 200 Hz gave a peak derivative at a temperature very close to the DSC Tgfor glucose and sucrose.
The DEA results of melted fructose are shown in Figure 4.99. Melted fructose was the most difficult substance to get good curves from DEA. The 200 and 500 Hz curves did not show the peak derivative that matched the DSC Tg.The peak derivative at 500 Hz was around 35 °C.
0.0E+00 5.0E-12 1.0E-11 1.5E-11 2.0E-11
18 28 38 48 58 68 78
temperature (°C)
dc/dT
1000
10000
100000
1 MHz 500
Figure 4.99Derivatives of capacitance against temperature for melted fructose. The peak of derivative at 500Hz showed a much higher temperature when compare to DSC Tgof melted fructose and freeze dried fructose. The derivative at 1000 Hz showed unusual jump of the capacitance before went back to normal range.
The DEA of melted glucose is in Figure 4.100. The peak derivative of glucose at 200 Hz was found at around 50 °C which matched Tgof this material from the visual experiment.
All other peak derivatives at all frequencies including at 200 Hz were at around 37 °C which is very close to onset DSC Tgof glucose from cooling scan (35 °C). This point was also very close to Tgvalue of glucose at 38 °C reported by Finegold et al. (1989). The work of Ollett & Parker (1990) indicated Tgof glucose in a range of 20-35 °C.
0 5E-13 1E-12 1.5E-12 2E-12 2.5E-12 3E-12
20 25 30 35 40 45 50 55 60 65
temperature (°C)
dc/dT
200 1000
1 MHz 100000 10000
Figure 4.100Derivatives of capacitance against temperature for melted glucose.
Figure 4.101 shows the derivatives of capacitance with respect to temperature for melted sucrose. The peak derivative of sucrose at 200 Hz found at around 60 °C which matched the Tgof this material from the visual experiment. The DSC onset Tgvalues from both cooling and heating scans found the transition at around 54 °C. This value was similarly to Tgof dry melted sucrose reported at 56.6 °C by Roos & Karel (1990).
0 1E-12 2E-12 3E-12 4E-12 5E-12
25 35 45 55 65 75 85
temperature (°C)
dc/dT
200 1000
10000
1 MHz
100000
Figure 4.101Derivatives of capacitance against temperature for melted sucrose.
The DEA trial of a mixture of sucrose: glucose: fructose at a 1: 1: 1 ratio found the peak derivative at 1000 Hz at 50 °C. This value matched the Tgof this mixture from the visual experiment (Figure 4.102). The DSC onset Tgfrom cooling scan of the same ratio mixture had found at much lower than the values from this measurement (32 °C).
-3E-12 -1E-12 1E-12 3E-12 5E-12 7E-12
20 30 40 50 60 70 80
temperature (°C)
dc/dT
200
1000
10000
100000 1 MHZ
Figure 4.102Derivatives of capacitance against temperature of melted mixture of sucrose: glucose:
fructose at ratio of 1: 1: 1.
The fraction of acid in freeze dried fructose + 50% carrot fibre was changed and it was found that the peak derivative varied with the acid fraction. Figure 4.103 shows the peak derivative (200 Hz) of freeze dried fructose + 50% carrot fibre + 1% citric acid at 70 °C.
The peak derivative at the same frequency appeared at around 60 °C when the citric acid in the sample was 3% (dry weight) (Figure 4.104). It was also found that the Tgdecreased when adding more citric acid. There was no experiment from DSC of fructose +50% fibre + citric acid to compare to these results.
0.00E+00 1.00E-08 2.00E-08 3.00E-08 4.00E-08 5.00E-08
50 60 70 80
temperature (°C)
dc/dT
200 Hz
Figure 4.103Derivatives of capacitance against temperature of freeze dried fructose + 50% carrot fibre + 1% citric acid.
0.00E+00 1.00E-08 2.00E-08 3.00E-08 4.00E-08 5.00E-08
40 50 60 70 80
temperature(°C)
dc/dT
200 Hz
Figure 4.104Derivatives of capacitance against temperature of freeze dried fructose + 50% carrot fibre + 3% citric acid.
The trial of DEA with fructose + 3% sodium chloride at the temperatures from 10 to 80
°C showed that the first peak derivative might occurred below 10 °C (Figure 4.105). It was shown that DEA at low frequencies had a possibility of measuring the glass transition of a single substance or a few mixed food components.
0.00E+00 1.00E-07 2.00E-07
10 20 30 40 50 60 70
temperature (°C)
dc/dT
200 Hz
500 Hz
1000 2000 3000
Figure 4.105Derivatives of capacitance against temperature of freeze dried fructose + 3% sodium chloride.
DSC and DEA of freeze dried fructose + carrot fibre
The results from DSC and DEA of freeze dried mixtures of fructose + carrot fibre are in Table 4.24. It was difficult to match the DSC Tgresults and the peak derivative from DEA. The DEA results showed the frequency independence of each sample. The
measurement frequency range was set by guessing the possible performance of each range until it was successfully matched the DSC Tgto the peak derivative of DEA.
Table 4.24Tgfrom DSC and corresponding DEA frequencies for carrot fibre in freeze dried amorphous fructose
Carrot fibre: fructose ratio (dry basis)
Moisture content (% wb)
DSC Tg
(°C)
DEA frequency for same Tg(Hz)
0:100 0.05 18 500
14:86 5.0 59 1,300
21:79 5.0 69.5 20,000
28:72 5.0 82 6,000
21:79 0.01 98.2 10,000
21:79 2.0 90.5 7,000
21:79 5.0 69.5 20,000
In conclusion the frequency used in DEA was found to vary with the food components.
This showed a possibility of DEA to measure the Tgvalues of a single and double mixed food components. However, when working with DEA, repeating at different range of frequencies until it is matched the results from typical equipments used to measure Tgis required.