Abstract
Amidst rising demand for modified starch, hydroxypropylated derivatives from water yam, an underutilized tropical botanical source remains unexplored. The objective of this work therefore is to extract starch from water yam, modify same by hydroxypropylation and determine their physicochemical, functional and morphological characteristics. 
 
 Extraction of starch was carried out by blending peeled water yam previously soaked in 0.2%(w/v) NaHSO3 solution and filtering the resulting slurry. The filterate was next suspended in 0.2% NaOH solution, allowed to sediment and the supernatant oven-dried (400C) for 24 hrs. The dried product was subjected to hydroxypropylation (HP) (propylene oxide 4-12%/100g starch) and extent of molar substitution (MS) determined. The native and Hydroxypropylated starch (HPS) were evaluated for physicochemical, functional and morphological characteristics following standard methods. Pasting properties were analysed using Rapid Visco Analyzer (RVA) and elucidation of inherent functional groups was carried out by analyzing the FTIR Spectrum.
 
 Starch yield of 84.2% (dry wt. basis) and molar substitution (0.0024-0.05) of HPS were established. Bulk density increased (0.4988-0.6005g/cm3) with MS. There was significant (p=0.05) increase in the degree of Whiteness (W) (42.4-63.6%). Although granule size reduction was evident (33.88-33.43µm), hydroxypropylation did not affect their morphology. There was decline in concentrations of crude protein (0.18-0.01%), ash (1.35-0.34%) and amylose (44.19-37.48%) as MS rose. In contrast, there was significant (p=0.05) increase in water (1.76-2.66g/g) and oil absorption capacities (0.72-1.42g/g), swelling power (1.54-4.19g/g) and solubility (3.17-5.84g/g) at 500C. Freeze thaw cycles showed marked reduction in syneresis (10.3-1.09%) as MS increased. Peak Viscosity, pasting temperature and peak time of the HPS ranges were 297.83-583.6, 81.9-86.40C, and 4.5-7.0 mins respectively. FTIR band spectra indicated the presence of hydroxypropyl substituent groups in the modified starch.
 
 Hydroxypropylated starch (from water yam) at different molar substitution has been characterised and its properties established with strong potentials for wider applications in food systems.
Highlights
Traditional sources of starch are no longer sustainable because the current industrial demand for starch is being met only by limited number of crops essentially corn, potato, wheat and tapioca (Ellis et al, 1998)
The dried product was subjected to hydroxypropylation (HP) and extent of molar substitution (MS) determined
The yield is greater than values obtained by Verwimp et al,. (2004) for rye starch (42.2%) and Chavan et al (2010) for horse gram starch (22 to 31%) but less than values obtained by Nadiha et al (2010) for sago, potato and corn starches, which were 93.6%, 93.4% and 96.5% respectively
Summary
Traditional sources of starch are no longer sustainable because the current industrial demand for starch is being met only by limited number of crops essentially corn, potato, wheat and tapioca (Ellis et al, 1998). With increasing industrial demand for starches, there is need to explore new and alternative sources of starch. Tropical root and tuber crops such as water yam (Dioscorea alata) could offer this opportunity as they are rich in starch (Hoover, 2001, Wickramasinghe, 2009). Water yam is grown widely in tropical and subtropical regions of the world. They are plants yielding tubers with a starch concentration of between 70 and 80% of dry matter. The flesh of the tuber varies in color shades from white to purple (FAO, 1994).the texture of its flesh is not as firm as http://jfr.ccsenet.org
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