Starch-lipid interactions of tuber starches

Moorthy, S.N.1 and Eliasson, A.C.,2

  1. Senior Scientist, Central Tuber Crops Research Institute, Trivandrum 695 017, INDIA
  2. Professor, Food Technology department, Lund University, SWEDEN.

Lipids and surfactants can form strong complexes with starch and thereby modify the properties of the starches. Unlike cereal starches which contain considerable quantities of lipids present in them, the tuber starches are devoid of the lipids. A study was conducted to compare the types of starches using DSC. The starches were moistened with equal quantity of water in aluminum pans and the DSC run using a Seiko Calorimeter using indium as standard The heating-cooling cycle used was as follows: 15 to 1000CC at 50CC/min., cooling to 300C at 300CC/min., reheating from 30 to 1000CC at 50CC/min., and final cooling. The reheating is carried out to mask starch gelatinisation peak during the second cycle. Ten tuber starches were studied. It was found that the tuber starches do not exhibit starch-lipid complex melting peak during the heating in the first cycle and further confirmed during the second heating. This shows absence of starch-lipid complex in the native tuber starches. When lipids/ surfactants were added to the starches and the DSC run, the peaks corresponding to complex did appear further corroborating the results.

Starch-lipid interaction has been a topic of much study because it has important implications in deciding the starch properties but also provides an interesting system1-4. The changes that can be brought about have been exploited by the food industry for improving the texture and properties of various starch based foods. These include the improving the texture of potato mash by adding glyceryl monostearate5, reducing the staling of bread by adding some surfactants6, and imparting a firmer texture to pressure boiled rice flour7. In fact the importance of the resistant starch in nutrition has added further interest in the use of lipids to bring about resistance to starch digestion. However it has been observed that only cereal starches harbour lipids in the native state. Hence these starches possess characteristic lipid taste compared to bland taste of the root starches. The lipids associated with starch granules can be either on the surface of the granules as well as inside8. Those occurring on the surface include triglycerides, free fatty acids, glycolipids and phospholipids and are absorbed into the granules during starch isolation9. The internal lipids are monoacyl lipids lyso-phospholipids and free fatty acids. It is assumed that both the external and internal lipids are present in the free sate and bound to starch components either in the form of inclusion complexes or linked via ionic or hydrogen bonding to hydroxyl groups of the starches10.

The question whether the lipids form complexes only with amylose has been examined by various workers and lot of evidence with waxy starches has shown that the amylopectin molecules also can form complexes with the lipids by using the outer chains in them11. Our interest was to see whether there is any inherent restriction of the tuber starches to form complexes with lipids and surfactants.

Differential Scanning Calorimetry (DSC) is being increasingly used to study starch gelatinisation in view of the fact that the method quite fast, requires only very small quantity of the material and is reproducible 12-14. When starch is heated in excess water, it undergoes a series of changes which can be followed using the DSC. The peak corresponding to starch gelatinisation occurs at a range of temperature depending on the type of starch used. This process is endothermic and irreversible, There can be another peak corresponding to the melting of the starch-lipid complex which occurs at a much higher temperature (80-1300)15 , This process is also endothermic, but it is reversible. The latter appears as a peak during second heating cycle and hence a second heating cycle is followed for studying starch-lipid interaction.

It is well established that root and tuber starches contain only very small quantity of lipids in them. The DSC of some of these starches have been studied and they do not show the starch-lipid melting peak10,16-17. However it was desired to find out if this is true for all tuber starches and hence ten of these tropical starches were studied and DSC was run under identical conditions using the second heating cycle also. In order to also find out if there is some factor which inhibits the formation of starch-lipid complexes in tuber crop starches, two surfactants and lipids extracted from cereals were externally added and the DSC studied and the results are presented in the paper

Experimental

Starch was extracted from freshly harvested tubers of cassava, Colocasia esculenta, Xanthosoma saggittifolium, Amorphophallus paeonifolius, Dioscorea alata, Dioscorea esculenta, Dioscorea rotundata, Arrowroot, Pacchyrrhizus, and Canna edulis by standard method 18(Moorthy, 1991),. Accurately weighed quantity of the dry starch was transferred to the aluminium pans , water was added to get starch: water ratio of 1:2 and the pans were hermetically sealed. The pans were transferred to the heating chamber of the DSC equipment. The DSC was run on a Seiko 6000 DSC equipment using indium as standard and under nitrogen atmosphere. The heating cycle used for the study was as follows;

I heating 15-150 at 50/min
I cooling 150-30 at 300 /min
II heating 30-130 at 50/min
II cooling 130-30 at 300/min

The onset of gelatinisation To, end of gelatinisation Te and enthalpy of gelatinisation H were obtained from the graphs.
Starch lipids interaction was studied by using a starch-lipid mixture such that the starch:lipid ratio was and starch:water ratio was 1:2. Similarly starch-surfactant interaction was examined by using Sodium dodecyl sulphonate and cetyl trimethylammonium bromide solutions (5%) in distilled water.

Results and discussion

The DSC graphs of the ten native tuber starches are presented in Figures. The thermograms clearly show the absence of peaks corresponding to melting of the starch-lipid complex in any of the starches whether during the first heating or second heating whereas the expected peaks for starch gelatinisation were present during the I heating. When a suspension of starch in water is heated, the starch undergoes a series of changes in their morphology and physicochemical and rheological properties. The changes in the thermal properties are clearly visible in the DSC thermograms. When a specific temperature is attained, a peak corresponding to starch gelatinisation is observed. This takes place over a range of temperature and is known as gelatinisation temperature. The granules absorb a large quantity of water and swell which leads to increase in viscosity. There will be first absorption of water by the amorphous regions which is followed by the disorganisation of the crystalline regions . These occur together under excess water whereas they can get resolved into two distinct peaks at lower levels of water, The temperature depends on the starch source and also on the presence of other ingredients which can modify the gelatinisation temperatures considerably. Salts, sugars, lipids are known to affect the gelatinisation temperatures.

If lipids are present in the starch, they can form complexes with the starch and the melting of the starch-lipid complex occurs at a much higher temperatures. The temperature range can be 80-1350C compared to the gelatinisation temperatures of 55 to 900C. Whereas starch gelatinisation is an irreversible process, the starch-lipid melting is a reversible process and so on cooling, the complex is reformed and during second heating the starch gelatinisation peak is absent, but the peak corresponding to the melting of the starch-lipid complex appears. Hence studies on the starch lipid interaction are always carried out during the second heating. It has also been observed that the enthalpy of starch-lipid melting is higher during second heating and this is attributed to the release of amylose when the starch has gelatinised and this extra amylose complexes with the starch. In the present studies, we did not observe any peak corresponding to the starch-lipid melting even during the second heating confirming the absence of starch-lipid complex in any of the tuber starches studied. In our studies we have tried all types of tuber starches with varying properties and none of them gave any peak corresponding to starch-lipid melting. We were particularly interested in Pacchyrrhizus starch since the crop has leguminous characteristics in producing pods and hence whether the starch has some lipid similar to pulse starches. Even for this starch, there was no peak corresponding to melting of starch-lipid peak showing absence of the leguminous character in the starch. Similarly we also did not find any difference between the starches having different XRD patters. Whereas cassava, aroids, arrowroot and Pacchyrrhizus starches have A pattern, the yam starches and canna starches possess B pattern. However all the starches did not exhibit the starch-lipid melting peak. Hence the arrangement of the starch molecules in the granules do not have any bearing on the starch-lipid complex formation. It is well documented that cereal starches show the peak whereas potato starch does not exhibit the starch-lipid complex melting. So all the tuber starches are free from lipids in their native state similar to potato starch

In order to find out whether the starches have some inhibition in forming complexes with lipids or surfactants, these starches were complexed with externally added lipids and surfactants and the DSC of the complexes examined. On addition of these, the peak corresponding to melting of the starch-lipid or starch surfactant complex was observed and that too for all the starches examined. Both the surfactants used gave complex formation showing there is no inhibition in complex formation. Various workers have compared the complexing ability of different surfactants and found the optimal length for complex formation is eighteen to eighteen carbon surfactants19. The stability is very low beyond this limit. Though there was not much difference between the SLS and CTAB, the latter seems to form slightly stronger complex because of the slightly higher enthalpy obtained for them. Similarly there was not much difference among the polar and nonpolar lipids in forming complex with starch. Polar lipids were expected to form stronger complexes , but such effect was absent Thus the results clearly indicate that there is no inherent problem for the tuber starches to form complexes with lipids It is possible that t during synthesis of starch, the tuber starches do not come into contact with lipids and hence unlike the cereal starches, they do not have inherent lipids in them It also shows that the starch properties of the tuber starches can be modified by adding lipids or surfactants. Such practice is already there in cereal starches and potato starch but not for the tuber starches. Some of the tuber starches have some special characteristics and by incorporation of lipids or surfactants , the properties may be further improved. This is also important for producing resistant starch using the tuber starches and lipids or surfactants.

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