Hello there, I have a new drawback about my project of cassava starch nanoparticles, specially about solubility: I heated the water until 73 °C to improve this property, and I notice the starch is not solubilising at all. I mean, even though I increse the temperature and adding the same mass of last time, I do not understand why my solution still has lumps in it.
One hypotesis I have raised is ambient humidity is affecting it because I live in a tropical country (colombia) and its humidity is high, also I prepared the starch 2 months ago and I left with any use for weeks. It does not smell bad and have not change the color.
I hope your answers resolve this trouble
Thaks for your help
It is a usual observation that starch granules do not dissolve into a molecular state when heated in excess water at temperatures below 100 °C. At the so-called initial gelatinization temperature T0, the granules start to lose their crystallinity (gelatinization). This enables them to adsorb increasing quantities of water (swelling). For cassava starch this process spans a temperature range between about 65-75 °C. At the latter temperature Tc (c stands for completion), % crystallinity has dropped to zero and a suspension of swollen starch granules has resulted. At further heating, the swelling may continue to increase, but some of the starch goes progressively into molecular solution, preferentially amylose. Complete molecular solubilisation will be generally occur only at a temperature not lower than 140 °C. This can be achieved in a pressure cooker and is accompanied by some molecular degradation. Typical swelling power of cassava starch in excess water is 16 or 21 ml/g at 70 or 92 °C, respectively. Solubilty amounts to 15 or 28 % at these temperatures (own results).
The gelatinization and dissolution process can be followed by microscopy, preferably by taking samples during the heating process, and cool and dilute these prior to microscopic observation. Preferred optics are bright-field, polarization optics (to observe whether crystallinity is still present: crystalline structures light up brightly against a dark background), and phase contrast for highly swollen granules where refractive index differences have become very slight. Also iodine staining in combination with bright-field optics is a helpful technique.
With regard to possible spoilage of your sample due to high temperature and water content, starch is stable at a water content of 15 % corresponding to a water activity (aw) of ca. 0.65 at 25 °C, provided it is stored in a closed container. In case of doubt, determination of a germ count is advised.
The presence of lumps (in the dry state) may be caused by the drying method during isolation of the starch. Lumps in the dry powder or in the unheated suspension can be removed by sieving. The powder can also be washed with ethanol or acetone and dried. Other options are to use high-speed stirring of the unheated suspension and /or suspending the powder into a small amount of ethanol and add water only after a lump-free suspension has been obtained.
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