Native starch cannot be used in a vast range of starch applications due to the fact it cannot exhibit the desired properties. The desired properties can be obtained by the modification of native starch. Most commonly used method is the chemical modification of starch molecules. More so, by using specific moisture and temperature conditions, there can be alterations in the physicochemical properties of starch since a lot of physical modifications involve the use of water and heat [Senanayake et al., 2013]
Cross- linked maize and rice starch had the highest pore sizes that trap the highest amount of water resulting to the highest moisture content. At 40% formaldehyde concentration, the moisture content of maize when treated with 40% formaldehyde is the highest as compared to wheat and rice. This shows that treatment with formaldehyde has a direct proportional effect on maize starch i.e., the percentage moisture contents varied but generally increased with increase in level of cross-linker [Oladunmoye et al.,2014; Belibi et al.,2014]. However, the variation in the concentration of formaldehyde treatment on rice showed a constant effect on moisture content. In all the starches, the batches cross-linked with the 40% formaldehyde had the highest moisture content.2.5% sucrose gave a decrease in the moisture content of maize and rice, while 10, 20 and 40% sucrose gave a reduced effect in wheat starch. This shows that a raise in the concentration of sucrose gave an inversely proportionate effect on the moisture content of wheat starch. High moisture content may lead to enzyme activation and microbial proliferation. Low moisture content usually shows a high level of stability during storage, protecting starches from growth of moulds, and giving a high yield of dry weight [Jubril, 2012]. Percentage moisture contents close to 12% and above will provide enough moisture for drug degradation and microbial activities [Odeku et al., 2005].
The flow properties of powders are important in assessing the adequacy of a material as a direct compression excipient. Hausner index and Carr’s percent compressibility are regarded as indirect ways of measuring the flow property of powder. The Hausner index reflects inter-particle friction, while the Carr’s index shows the ability of a material to reduce in volume. Hausner ratio which is higher than 2.5% signifies poor flow, Carr’s index less than 16% signifies good flowability while values more than 35% signifies cohesiveness. As the value of these indexes increase, there is a reduction in the flow of the powder and this increases the likelihood of producing tablets with more weights variation [Okunola & Odeku, 2011]. All starches from all the sources had an Hausner’s ratio less than 2 and Carr’s index greater than 16% (except for 10%, 20% and 40% formaldehyde treated starches). Wheat cross-linked with 5% sucrose, had a good flow ability with Carr’s index of 17.4 which indicates low flowability and chances of producing tablets with weight variation [Jubril, 2012].Bulk, tapped and true densities are the usually measured density values which are used to analyses the major properties of powders. Bulk density gives details on the volume occupied by the inter-granular spaces, inner and external pores of the solids. Bulk density indicates the overall degree of packing in a specific volume. Tapped density is referred to as the density after tapping or vibration. The bulk densities of maize reduced with a rise in the concentration of sucrose and increased with an increase in concentration of formaldehyde. Wheat and rice showed a slight irregularity at 10% and 40% sucrose, and decreased with an increase in the concentration of formaldehyde. The tapped densities showed an irregular/wavy effect at different concentrations of sucrose and formaldehyde. This shows that the rise in concentration of the cross-linking agents enhances the bulk and tapped density of the starch powder.
Swelling is widely accepted as an assessment of tablet disintegration ability. From the results obtained for swelling profile of different cross-linked starches in Table 3. The percentage swelling of the native starches of maize, wheat and rice are higher than the cross-linked derivatives, and could be attributed to the fact that cross linked starches experienced granule modification that decreased the hydration capacity. Increase in concentration of cross linking agent led to an increase in the amount of cross-links and this confers a greater stability on the starch granule. Therefore, the water absorption reduction was more pronounced at higher concentrations of the cross-linkers. Presumably, cross linking has an effect on the easy access of water to the starch. This in turn causes a reduction in the swelling properties of the cross linked polymer [Yu et al., 2016]. Porosity determines the swelling ability of starch. The higher the porosity, the more the inter-particulate spaces where water could be absorbed [Carmona-Garcia et al., 2009]. The increase in the ionic strength of the cross-linked starch decreased the osmotic pressure inside the charged paste and a reduction in its swelling. Cross-linking caused a high elastic contraction of polymer network which counteracted the swelling process. Hydration which leads to swelling is dependent on the type and number of hydrophilic groups in the polymer structure. The highest swelling for the cross-linked starches was observed in both sucrose and formaldehyde cross-linked rice starch, while the least swelling was observed in 5% and 40% sucrose cross-linked maize and wheat starches. Swelling power is a parameter that is analysed in theory of disintegration, which must be preceded by water penetration, therefore the high percentage swelling of sucrose and formaldehyde cross-linked rice starches to compare with the native starch will enhance disintegrating properties of tablets formulated with, more than others in this studies.
All cross-linked starches showed an overall slight reduction in their pH as compared with the native starch. The pH of almost all the concentrations of sucrose and formaldehyde cross-linked maize, wheat and rice starches with cross-linked maize and rice had pH above 5. While the exception of 10%, 20% and 40% formaldehyde cross-linked maize and rice starch had pH bellow 5 respectively.
Swelling and Viscosity of cross linked are the very important and useful features of assessing the level of cross-linking. Table 3 also illustrates the dependence of viscosity of starch on the concentration and type of cross-linkers. It is observed that the viscosity of the cross-linked wheat and rice decreased as the concentration of the cross-linker increases for the two cross-linkers except for maize which showed a slight increase at 40% sucrose and formaldehyde. Also the viscosities of the cross-linked starches were generally different from those of the non-cross-linked (Native) ones. In general, the viscosities of the native starches were more than those of the cross linked starches. This is in conjunction with Shah et al., [2016], that the degree of peak viscosities of cross-linked starches is inversely proportional to the concentration of crosslinking agent. Starch with a greater crosslinking level will show a lesser peak viscosity as compared with starch with lesser crosslinking levels.
The thermal properties of the defatted and undefatted wheat, maize and rice starches were also analysed as shown in Table 5. For maize starch, both the onset and end set temperatures of the glass (Tg) as well as the energy change (∆H), recorded were higher for the undefatted than the defatted. This shows that the process of defatting probably lowered the intermolecular forces within the starch sample leading to the requirement of less energy for the Tg process. Except for the onset temperature, similar trend was observed for the rice starch. There were no significant differences between the melting peaks recorded for the undefatted and defatted starches.
Considering the thermal parameters of the defatted and formaldehyde – modified wheat starches, it was shown that the Tg took place at lower temperatures for the samples treated with 2.5–20% formaldehyde. However, with increased formaldehyde concentrations up to 25–40%, the Tg occurred at much higher temperatures. Similar trend was observed for the ∆Hs involved in the transitions. The melting peaks generally occurred at slightly elevated temperatures for the formaldehyde-modified wheat starch samples. These observations indicate that the chemical modification resulted to wheat starch of a more ordered (crystalline) molecular conformation than the natural moiety. In comparison with the sucrose-modified wheat starch, the end set temperature of the Tg as well as the ∆H were significantly higher than for the formaldehyde – modified sample. However, the melting endotherm was higher for the formaldehyde – modified wheat starch than for the sucrose-modified sample.
For the formaldehyde-modified maize- starch, the onset temperatures of the Tg were obviously greater than that of the defatted maize starch for the various range of concentrations tested. The end set temperatures were however lower except at 20% formaldehyde concentration. Similarly, the ∆H for the Tg of the defatted maize starch was higher than those of the formaldehyde – modified except at 20% formaldehyde concentration. The melting peaks of the formaldehyde-modified maize starch were greater than for the untreated defatted sample. In comparison with the sucrose-modified maize starch, the onset temperatures were also higher than for the untreated and defatted samples. The onset of transition occurred at approximately similar temperature except at 10% sucrose concentration where it was significantly much higher. The results showed that the ∆Hs for the Tg were generally higher for the formaldehyde than the sucrose – modified maize starches except at 20% chemical agent concentration where the latter had significantly higher value. The melting peaks generally occurred at higher temperatures for the formaldehyde-modified maize starch than the sucrose – modified except at 2.5% chemical agent concentration where the reverse was the case.
Both the onset and end set temperatures of the Tg for the formaldehyde-modified rice starch were greater than for the untreated and defatted sample. However, the ∆Hs for the Tg of the crosslinked rice starch samples were higher than for the untreated sample. On the other hand, the melting peaks of the formaldehyde-treated rice starch samples occurred at really lesser temperatures compared to the untreated sample.
For the sucrose-modified rice starch, the onset and end set Tg occurred at higher temperatures, at 2.5% sucrose concentration than for the untreated but defatted sample. However, the onset and end set Tg occurred at lower temperatures when the sucrose concentration was increased to 5%. The ∆Hs for the Tg were also higher for the sucrose-modified rice starch than for the untreated sample. However, the melting peak of the untreated but defatted rice starch was significantly higher than for the sucrose-modified samples. In comparison with the formaldehyde-modified rice starch, the sucrose-modified had higher onset and end set temperatures for the Tgs. Varied results were obtained for the ∆Hs and the melting peak temperatures. The overall results obtained for the various modified starches showed that the chemical (cross-linking) agents used had effects on their original molecular conformations of the native samples though the amorphous and crystalline structures were still present as indicated by the glass and melting transitions. The changes were due to the cross-linking of the starch moieties by the functional groups in the chemical agents used. The extent of changes might partly be attributed to the level of amorphosity and crystallinity of the original starch molecules. The resultant effect is that new starch motifs were produced with perhaps improved or new functionalities as pharmaceutical excipients.