Can anyone explain that what is the reason behind decrease in percent transmittance %T of starch samples when subjected to refrigerated storage for a period of 5 days. According to articles with increase in storage time there is a decrease in percent Transmittance of all samples tested with native having the lowest %T and Resistant Starches higher %T. This phenomenon is based on retro-gradation tendency of starches but is there a direct relation between Retrogradation and %T? Greater the retro-gradation gretater the %T or vice versa?
By retrogradations starch molecule start to reorganise and interact with other molecules to form bigger structure. The molecular solution of starch turn it in colloidal solution. A molecular solution will allow light to travel absorbing it only if it is coloured. Colloidal solution will disperse light. To see better the effect use a LASER pointer and send the light through a fresh solution and through a refrigerated solution. In fresh solution (or any molecular solution) you barely will see the light while in in older solution (or any other colloidal solution) you will see the light and the light will be dispersed. Starch is a very big molecule and the solution is a little bit turbid from the beginning.
Thankyou so much for your help. Therefore, referigerated storage leads to colloidal solution of starch which has greater dispersion but decreased light transmission right? But what can be the reason behind native starch samples showing decreased light transmittance as compared to the enzymatically produced resistant starches in all the storage periods .For example if such kind of values have been observed then How can i explain this trend ?
starch samples AvgT%120h
Native 0.019
RS(A) 0.069
RS(P) 2.513
RS(P+A ) 4.012
Water is a rather poor solvent for starch which means that aqueous starch systems are not stable. This may result in a number of phenomena, including turbidity and viscosity increase, precipitation, crystallization, gelation, syneresis, increased firmness of gels (and bread crumb). What actually happens depends on starch type, concentration, temperature, and dissolution state (suspension of swollen granules or molecular starch solution). This collection of phenomena is termed 'retrogradation'. The underlying process is the tendency of starch to revert to the crystalline state, generally the B-type crystallinity. The process is thermoreversible (< 100 degrees C) in waxy-type starches (no amylose) and partially irreversible in amylose-containing starches. So it is expected that the decrease in %T is lowest in waxy starches and highest in resistant starches, which generally have enhanced amylose-like character.
The reason that you have observed higher %T for resistant starch than for native starch could be caused by your experimental procedure. In a truly comparative test on different starches it is important that you start at the same dissolution state, for example a clear, true molecular solution. The latter is achieved by heating the suspension at 140 degrees C. A good alternative is to make a starch suspension in water, add an equal amount of 2M NaOH with stirring, neutralize with 1 M HCl and start the time test.
Thankyou so much . Yes my resistant starch is rarely soluble in water due to which true molecular solution is not obtained. This time i will try making the suspension as you suggested by heating the starch suspension in boiling water bath and then neutralization by equal amounts of NaOH and HCl. If suspension in water is 2.5ml then 2.5ml of NaOH and 2.5ml of HCl should be added for neutralization?
I am thankful for your help.
The dissolution in NaOH has to be done at room temperature because starch is oxidatively degraded by NaOH at high T. For example, suspend starch (1 g) in 25 ml water, then add 25 ml 2M NaOH with stirring until a clear solution is obtained. (It may be favourable to use a high impact stirrer to disrupt the swollen granules of native starch.) Then add 50 ml 1 M HCl with stirring to adjust pH of the solution to 6-7. (pH must not be higher than 7.) Starch concentration of the final solution is 1 %. (It is recommended to do a trial without starch first in order to establish the amount of 1M HCl.) After neutralisation, start directly taking aliquots of the sample as a function of time for the measurement of %T. If comparing different starches, make sure that final starch concentrations are equal because retrogradation is concentration dependent. Of course you can work at a smaller scale but mixing of the sample is required, e.g. on a tube stirrer.
Hi, Samiya Siddiqui
the decrease in transmittance might be related with short-term retrogradation, as linear amylose is much easier to retrograde, this might explain why native one has lowest T
Hi, Dr. Peter A. M. Steeneken ,
Different gelatinization temperature produce different retrogradation behavior? and how to explain waxy starch retrogradation is reversable while starch contain amylose is not?
To start with the second question of Xiaohong Lan: the crystallites in amylopectin (formed by double-helical organization of the short side chains) have a lower melting temperature (because of their restricted size and because of imperfections due to the vicinity of branching points) than crystallites in linear amylose where such restrictions do not play a role.
Regarding your first question: decrease in transmittance (T) is only a meaningful indicator of retrogradation if T at t=0 is about equal for all starches to be tested. Native starch after cooking at 100 C still contains swollen granules that contribute to turbidity. Resistant starch may not dissolve at all at 100 C. That is why in my previous answers I stress the importance of starting from the same state of dissolution.
So in order to determine any of the functional properties of Resistant starch samples for example iodine staining index the starch sample should be made completely soluble first and then treated for the evaluation of its properties?
It depends on what you want to measure. For studies of turbidity development and iodine binding it is essential to completely dissolve your samples, but of course not for studies of digestibility, crystallinity, etc.
citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.473.653&rep=rep1...pdf
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