The application of differential scanning calorimetry to a study of gelatinisation of commercial mung bean starch
School of Applied Sciences, RMIT University, Melbourne VIC 3000, Australia
Mung bean flour and the starch extracted from it are used widely in food processing particularly for Asian food products. In the utilisation of starches in food manufacture, it is important to understand the changes occurring during heating and the impact of process parameters. Gelatinisation is a phase transition of starch granules heated in the presence of sufficient water. The process can be observed by the loss of crystallinity of the granule, an uptake of heat, hydration of the starch with swelling of the granule and a decrease in the relaxation time of water molecules (Donovan, 1979). This is caused by the breaking of hydrogen bonds between α–1,4 linked D-glucan molecules (Blanshard, 1987).
It has been reported that gelatinisation requires the presence of at least 60% moisture (Colonna et al. 1987). In the study of these changes, Differential Scanning Calorimetry (DSC) is widely used and based upon the approaches of a number of workers a slurry is generally prepared with a starch-water ratio of 1:2 (Stevens and Elton, 1971), then sealed and stored overnight at room temperature (Wootton and Mahdar, 1993). The aims of this study have been to compare the gelatinisation of starch in the presence of different moisture levels and to investigate the changes in mung bean starch during heating.
Sample and instrument
The starch used in this study was a commercial mung bean starch (Pine Brand Mung Bean Starch, Thailand). Starch gelatinisation was studied by DSC using a Perkin-Elmer DSC 7, and 30μL aluminium DSC pans were used throughout. The instrument was calibrated with Indium in accordance with the procedure described by the manufacturer.
Preparation of starch mixtures
Mung bean starch and water were weighed in a small beaker at various ratios (1:0, 1:0.5, 1:0.75, 1:1, 1:1.5, 1:2, 1:3, 1:4, and 1:5). The mixtures were stirred thoroughly and sealed. The starch-water slurry was then set-aside for 18 hours at room temperature before running DSC. Reliable results were obtained when prepared samples were stored under these conditions. For each treatment, at least duplicate determinations were carried out.
For slurries, a pipette was used to transfer sample to DSC pans. Drier samples were mixed with a mortar and pestle, then transferred to pans by spatula. These were weighed and sealed immediately. All sample weights were in the range of 8 to 12mg. Gelatinisation was studied with DSC using a heating rate of 10°Cmin-1 from 25°C to 130°C. A graph was generated for each sample; peak (°C), onset (°C), offset (°C) and enthalpy values (Jg-1) were calculated by Pyris software supplied by Perkin Elmer.
In order to investigate the impact of varying moisture levels on DSC data, a series of slurries were prepared and compared with samples of unwetted starch. Typical DSC curves for mung bean starch slurry at a ratio of 1:2 and 1:0 (starch:water) are shown in Figures 1 and 2 respectively. Comparison of gelatinisation enthalpy (ΔH) values of starch slurries prepared at different moisture contents is presented in Figure 3.
Figure 1. DSC thermogram of mung bean starch slurry (starch:water = 1:2) analysed by Perkin-Elmer Pyris software
Figure 2 DSC thermogram of mung bean starch slurry (starch:water = 1:0.5) analysed by Perkin-Elmer Pyris software
It was found that at lower moisture contents, no gelatinisation peak could be identified in the thermograms. However one clear, sharp peak was consistently found at 73°C when the moisture content was 50% or above. Small, additional peaks were observed at higher temperatures in some cases.
The enthalpy values for mung bean starch increased as the water content was increased, but remained constant once a certain amount of water was added to the starch slurry. Thus samples with 66.7% to 83.3% moisture gave consistent enthalpy values, but sample handling was facilitated towards the higher end of this range.
Figure 3 A comparison of gelatinisation enthalpy (ΔH) values measured using starch slurries prepared at different moisture contents. Values are presented as mean ± standard deviation.
It is concluded that the optimal approach involved preparation of a mixture at a moisture exceeding 66.7% and typical enthalpy values obtained were 12.6J per gram of starch.
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.
Colonna, P. Buleon, A. Mercier, C. (1987) in Critical reports on applied chemistry volume 13: Starch: Properties and potential (Ed. T. Galliard ) John Wiley & Sons, Chichester, U.K.,
Stevens, D. and Elton, G. (1971). Die Stärke, 23, 8-11.
Wootton, M. and Mahdar, D. (1993) Starch/Stärke 45, 295-299.