Optical in situ analysis of starch granules under high pressure with a high pressure cell
Introduction
Starch is a major storage product in plants and one of the most important carbohydrate sources for human nutrition. It is composed of a mixture of the two glucanes amylose and amylopectin and occurs in form of granules (Belitz, Grosch, & Schieberle, 2001). Starch is semicrystalline and birefringent as can be observed under polarised light. Upon heating, it gelatinises accompanied by loss of birefringence. Gelatinisation is regarded as the hydration and irreversible swelling of the granule, the destruction of molecular order within the starch granule and the melting of starch crystals (Zobel, 1984). Svensson and Eliasson (1995) examining the thermal gelatinisation of potato and wheat starch in limited water found evidence for a two-phase gelatinisation process. First a hydration of amorphous regions of the granule accompanied by a minor reduction in crystallinity occurred, and secondly melting of starch crystals up to a total loss of crystallinity took place.
The crystalline order could also be destroyed by mechanical means, e.g. by a high hydrostatic pressure treatment (French, 1984). Muhr and Blanshard (1982) found a gelatinisation of wheat starch in excess water at ambient temperature and 450 MPa. The pressure range in which gelatinisation took place was typical for each starch (Stute, Klingler, Boguslawski, Eshtiaghi, & Knorr, 1996), e.g. starches having a B-type X-ray pattern were more resistant to pressure than starches with an A- and C-type crystalline structure Ezaki & Hayashi, 1992, Muhr & Blanshard, 1982, Rubens et al., 1999, Stute et al., 1996. Rubens et al. (1999) proposed a two-step mechanism for pressure-induced starch gelatinisation similar to the thermal gelatinisation process. In the first step, the amorphous regions were hydrated causing a swelling of the granules and a distortion of crystalline regions, and in the second step the crystalline regions became more accessible to water. The authors stated that under pressure a hydration of the starch occurred before changes in crystallinity proceeded during gelatinisation.
In other respects, the pressure-induced gelatinisation and the thermal gelatinisation differed. Typical for pressure-gelatinised starches was the limited swelling of the melted granule (up to twice in diameter) and the maintaining of the granular character (Stute et al., 1996). Furthermore, according to Douzals, Perrier Cornet, Gervais, and Coquille (1998) there was only little amylose release and according to Stute et al. (1996) sometimes even no amylose release. Pressurised starch suspensions were more condensed, with a different water binding capacity (Douzals et al., 1998). X-ray diffraction patterns of untreated and pressurised A-type starches in the presence of water showed a transformation from the A- to the B-type X-ray pattern Hibi et al., 1993, Katopo et al., 2002. Moreover, water played an important role in the high pressure-induced gelatinisation of starches. Suspended in alcohol even at very high pressures (up to 3 GPa) starch granules did not swell (Snauwaert & Heremans, 1999). Katopo et al. (2002) suggested that ethanol had a space filling effect stabilising the crystallinity of starches. According to Stute et al. (1996), a high moisture content was required for ultra high pressure gelatinisation.
Until recently, the microscopic analysis in high pressure research was mainly limited to ex situ observations whereas the samples were investigated with a microscope before and after the pressurisation step Begg et al., 1983, Sato & Kobori, 1995, Shimada et al., 1993. Snauwaert and Heremans (1999) and Rubens et al. (1999) observed pressure-induced starch gelatinisation in situ in a diamond anvil cell, Douzals, Maréchal, Coquille, and Gervais (1996) in a high pressure microscope. The authors discovered swelling of starch granules during pressurisation. Douzals et al. (1996) observed further swelling of the granules, i.e. an increase in granule volume after pressure release. The authors also detected a decrease in volume of the starch suspension during pressurisation, which partly remained after pressure release, and assumed that starch molecules linked with water occupy less volume than suspended in pure water and therefore the granule hydration would be preferential under pressure according to Le Chateliers Principle. Douzals et al. (1996) also viewed pressurisation of iodine stained starch granules and observed a decolouration of the swelling kernels which was regarded as an indication of starch melting.
The Lehrstuhl für Maschinen- und Apparatekunde at the Technical University München (Freising-Weihenstephan, Germany) in cooperation with the mechanical engineering company Record Maschinenbau (Königsee, Germany) developed the so-called Hartmann, Pfeifer, Dornheim, Sommer-High Pressure Cell (HPDS High Pressure Cell) enabling microscopic in situ analyses under pressures of up to 300 MPa (Hartmann, Pfeifer, Dornheim, & Sommer, 2003). Aim of this work was to validate the efficiency of this newly developed HPDS High Pressure Cell and furthermore to gain insight into the process of pressure-induced starch gelatinisation.
Section snippets
Chemicals
Potato starch, wheat starch (both from Overlack, Mönchengladbach, Germany) and tapioca starch (Thai World Import and Export, Bangkok, Thailand) with a water content of 19.1%, 12.7%, and 13.2%, respectively, were suspended in abounding distilled water (0.8% w/w).
The iodine solution consisted of 0.2 g iodine (Sigma, Deisenhofen, Germany) and 2 g potassium iodide (Merck, Darmstadt, Germany) in 100 ml distilled water.
Starch suspensions under the high pressure microscope
The high pressure microscope is described in detail elsewhere (Hartmann et al.,
Results and discussion
With the development of the HPDS high pressure cell in combination with an inverse microscope, it was possible to obtain in situ images with both high optical resolution and high quality up to pressures of 300 MPa. The loading of the cell was easy and uncomplicated.
The starches investigated were starches with an A-type X-ray pattern (wheat starch; Fig. 1a), with a B-type pattern (potato starch) and with a C-type pattern (tapioca starch) according to Stute et al. (1996). Examining wheat starch
Conclusions
With the development of the HPDS high pressure cell in combination with an inverse microscope, it was possible to obtain in situ images with both high optical resolution and high quality up to pressures of 300 MPa. The loading of the cell was easy and uncomplicated.
The maximum pressure of 300 MPa was not sufficient enough to observe swelling of potato and tapioca starch granules whereas wheat starch granules showed a thorough swelling of most granules during pressurisation. Further work is
Acknowledgements
This work was supported by the German Ministry for Education and Research (BMBF; Grant No. 0330099 and 0330089).
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