The effects of sample storage and handling on differential scanning calorimetry of commercial mung bean starch
School of Applied Sciences, RMIT University, Melbourne VIC 3000, Australia
Mung bean is widely consumed in Asia in various forms, some with minimal processing, but it has considerable potential as a source of ingredients for incorporation into processed food products. One of these is starch and Differential Scanning Calorimetry (DSC) is widely used in the characterisation of starches. Based upon the approaches of a number of workers to DSC analysis, a slurry is generally prepared with a starch-water ratio of 1:2 (Donovan 1979), then sealed and stored overnight at room temperature (Blanshard 1987). The primary aim of this study has been to investigate the impact of various parameters on the application DSC to characterisation of starch gelatinisation.
Sample and instrument
The starch used in this study was commercial mung bean starch (Pine Brand Mung Bean Starch, Thailand). DSC analyses were performed using a Perkin Elmer DSC7 system to study starch gelatinisation, and 30μL aluminium DSC pans were used. The instrument was calibrated with Indium in accordance with the procedure described by the manufacturer.
Preparation of starch slurry
Mung bean starch and water were weighed in a small beaker at a ratio of 1:2, stirred thoroughly and sealed. The starch-water slurry was then set-aside for various time period at room temperature before running DSC. For each treatment, at least for duplicate determinations were carried out.
DSC analyses were performed at various intervals. The slurry was stirred prior to transfer (8-12mg) to DSC pans, then sealed hermetically. These were heated at a rate of 10°Cmin-1 from 25°C to 130°C.
The impact of premixing a slurry
For comparison with well mixed starch slurries, samples of starch powder were weighed directly into DSC pans using a spatula. Using a pipette, water was added, the weights recorded and finally each pan was sealed with a lid.
The influence of storage of prepared starch slurries
Following preparation, a starch slurry was sealed in a small beaker for 6 days. Sub-samples were taken and run on DSC at various times including immediately after mixing (day 0), and following storage for 1, 5 and 6 days. In another experiment, a slurry was analysed at varying times from 10 min up until 26 hr.
Placement of sample in the pan
Comparisons were made between samples for which the slurry was ‘centered’ or ‘non-centered’. For ‘centered’ the material was pipetted into centre of the pan and was not in contact with the side at any point. For ‘non-centered’ it had an irregular shape and was in contact with the side of the pan at some point.
A variety of specific approaches may be applied to the preparation and analysis of starch by DSC. The impact of each of these was investigated in relation to both the temperature and enthalpy of gelatinisation.
The impact of premixing a slurry
Handling of samples and addition of water presents challenges particularly when relatively small amounts are dispensed or weighed. The addition of water and starch separately to DSC pans was compared with premixing of a slurry and sub-sampling into the pan. Similar results were obtained for the two approaches. However samples without premixing showed lower precision for both gelatinisation peak and enthalpy values. This probably results from the relative ease of pipetting the slurry without spillage or loss.
The influence of prolonged storage
There was little change in the gelatinisation peak temperature throughout the 6 days. There appeared to be some increase in enthalpy if the starch slurry was left for more than two days.
Storage for up to 24 hours
DSC analyses were also performed at more frequent intervals (Figures 1 and 2). The values for both parameters show greater variability in the early stages of storage. Particularly for measurements of enthalpy, storage for at least five hours would be recommended. Almost identical readings were also found beyond 24 hours, but are not presented in the figures. Based upon the results obtained between 13 and 26 hours, mung bean starch, has a peak gelatinisation temperature of 73.4 ± 0.1°C and an enthalpy of 12.0 ± 0.2 Jg-1 starch.
Figure 1. The influence of time period from preparation of starch slurry upon gelatinisation temperature measured by DSC
Figure 2 The influence of time elapsed from preparation of starch slurry upon gelatinisation enthalpy (ΔH) measured by DSC
Positioning of slurry sample in the DSC pan
In the course of carrying out replicate determinations, it was observed that the placement of the starch slurry within the DSC pan appeared to influence the precision of the data. Although no difference in gelatinisation temperature was observed, the level of variability in gelatinisation enthalpy values was very much higher for the non-centered samples. The difference in contact area of starch slurry with the bottom of the pan might affect heat conduction from the instrument to the slurry. It is likely that the surface area for heat transfer fluctuated more for the non-centred samples, thereby resulting in the greater variation in DH values.
The results show that sample preparation plays an important role in minimising the variability of results. The longer the slurry is mixed the more stable are the values obtained. The results also clearly demonstrate the importance of placing the starch slurry centrally within the sample pans as a means of enhancing the reproducibility of the resultant data. It is concluded that the optimal sample preparation approach involved premixing the starch powder with water and storing at room temperature in a sealed container for at least 13 hours.
A special thanks to Dr Antonietta Genovese for help in setting up the DSC.
Donovan, J. (1979). Biopolymers 18, 263-275.
Blanshard, J. (1987) in Critical reports on applied chemistry volume 13: Starch: Properties and potential (Ed. T. Galliard) John Wiley & Sons, Chichester, U.K.