Starch consists mainly of linear amylose and highly branched amylopectin and is stored as semicrystalline granules [1
]. Starch not only is used as food to provide nutrition for humans and animals, but also is widely used as an important ingredient in the food, textile, medicine, papermaking, casting, metallurgy, petroleum, and chemical industries [2
]. The diversified applicability of starch demands specific structural and functional properties. Starches from different plant sources are varied in their morphologies, structures, and properties, which determine their utilization [3
]. Some studies have reported the structural and functional properties of starches from commercial important sources, such as cereals, tubers, and legumes [5
]. In recent years, in order to save food, broaden the source of starches, and develop new functional starches, substantial efforts have been made to find starches from non-conventional sources and study their structural and functional properties [2
Yam is a generic name for the plants of twining climbers that form tubers or rhizomes in the genus Dioscorea
of the monocot family Dioscoreaceae. World production of yams reached over 65.9 million metric tons in 2016, and the top three producers are Nigeria, Côte d’Ivoire, and Ghana with production quantities over 31.5, 4.9, and 4.5 million metric tons, respectively, every year from 1997 to 2016 [12
]. Chinese yam (Dioscorea opposite
Thunb.) is a rhizome crop mainly cultivated in China, Japan, and Korea [13
]. The rhizome of Chinese yam is an important edible and pharmaceutical food in China, and has nutritional and economic significance [14
]. Starch is the main component of rhizome. Its structural and functional properties have been studied by some previous reports [15
]. The bulbil, also known as Shanyaodou (in Chinese), is an aerial tuber of Chinese yam, which is generated from leaf axil [2
]. The bulbil can be released from the parent plant and grow independently into a new plant, making it an important propagation organ [13
]. The bulbil of Chinese yam has been used as an important food in people’s everyday life and as an important ingredient in livestock feed. It has also been used as one of the important ingredients for invigorating the spleen and stomach, promoting the production of body fluids, benefiting the lung, and invigorating the kidney [2
]. The yield of bulbils of Chinese yam is from 3000 to 4500 kg dry weight per hectare. Starch is the main component in bulbil and can account for about 60% in dry bulbil [2
]. However, the structural and functional properties of starch from bulbil have not been studied but there has been one previous study investigating its physicochemical properties [2
]. It is of great importance to understand the structural and functional properties of rhizome and bulbil starches from the same Chinese yam cultivar.
In this study, starches were isolated from rhizomes and bulbils of Chinese yam cultivar Shuishanyao. Their morphology, granule size, amylose content, crystallinity, short-range ordered structure, lamellar structure, swelling power, water solubility, thermal and pasting properties, thermal stability, and hydrolysis and digestion properties were investigated using many physicochemical measuring methods and spectral analysis machines. Our objective was to compare the structural and functional properties of rhizome and bulbil starches and provide some information as to how these starches can be used.
3. Materials and Methods
3.1. Plant Materials
The fresh rhizome and bulbil of Chinese yam (Dioscorea opposita Thunb. cultivar Shuishanyao) were obtained from a local natural food market (Yangzhou City, China) in December 2016.
3.2. Measurements of Soluble Sugar and Starch Contents in Rhizome and Bulbil
The rhizomes and bulbils were sliced into small pieces and dried in an oven at 110 °C for 2 h and 80 °C for 2 days, and then ground extensively. The flour was passed through a 100-mesh sieve, and its starch and soluble sugar contents were determined using the colorimetric method of anthrone-H2
as previously described [42
3.3. Isolation of Starch
Rhizomes were peeled and sliced into small pieces, and bulbils were cleanly washed. The samples were homogenized with deionized water and filtered with 100- and 200-mesh sieves. The starch suspension was settled for 6 h. The precipitated white starch was treated three times with 0.2% NaOH to remove surface protein and mucopolysaccharide. The treated starch was washed three times with deionized water and two times with anhydrous ethanol, dried at 40 °C for 2 days, and ground and passed through a 100-mesh sieve.
3.4. Morphology Observation and Size Analysis of Starch
The morphology of starch under normal light and polarized light was observed using a BX53 polarizing light microscope (Olympus, Tokyo, Japan) equipped with a charge-coupled device (CCD) camera (DP72, Olympus, Tokyo, Japan) as previously described [22
]. The granule size distribution of starch was analyzed using a laser diffraction particle size analyzer (Mastersizer 2000, Malvern, UK) as previously described [43
3.5. Measurement of Amylose Content in Starch
The amylose content was determined using the iodine colorimetric method as previously described [44
3.6. Analysis of Crystalline Structure of Starch
The crystalline structure of starch was analyzed using an X-ray powder diffractometer (D8, Bruker, Karlsruhe, Germany) at 200 mA and 40 kV as previously described [43
]. The scanning region of diffraction angle was from 3 to 40° 2θ with a step size of 0.02° and a count time of 0.8 s.
3.7. Analysis of Short-Range Ordered Structure of Starch
The short-range ordered structure of starch was analyzed using a Varian 7000 FTIR spectrometer with a DTGS detector equipped with an attenuated total reflectance single-reflectance cell containing a germanium crystal (45° incidence angle) (PIKE Technologies, Madison, WI, USA) as previously described [43
3.8. Determination of Swelling Power and Water Solubility of Starch
The swelling power and water solubility of starch were determined by heating starch-water slurries at temperatures from 50 to 95 °C in 5 °C intervals as previously described [43
3.9. Analysis of Thermal Properties of Starch
The thermal properties of starch were measured by a DSC (200-F3, NETZSCH, Selb, Germany) as previously described [44
3.10. Analysis of Pasting Properties of Starch
The pasting properties of starch (8% solids) were analyzed using an RVA (3D, Newport Scientific, Warriewood, NSW, Australia) following the temperature program: holding at 50 °C for 1 min, heating to 95 °C at 12 °C/min, maintaining at 95 °C for 2.5 min, cooling to 50 °C at 12 °C/min, and holding at 50 °C for 1.4 min.
3.11. Thermogravimetric Analysis of Starch
Ten milligrams of starch were placed on a platinum pan and heated from room temperature to 800 °C using a Pyris 1 TGA system (PerkinElmer, Waltham, MA, USA) at a heating rate of 10 °C/min. Nitrogen was used as a purge gas at a flow rate of 20 mL/min. Change in sample weight against temperature was measured.
3.12. Measurement of PPA Hydrolysis of Starch
The starch was hydrolyzed by PPA (A3176, Sigma Aldrich, St Louis, MO, USA) for different times as previously described [42
]. After hydrolysis, starch slurries were quickly centrifuged at 5000 g
for 5 min. The supernatant was used for the measurement of the solubilized carbohydrates to quantify the degree of hydrolysis using the anthrone-H2
3.13. Analysis of In Vitro Digestion of Starch
The in vitro digestion of starch was determined using both PPA (Sigma A3176) and AAG (E-AMGDF, Megazyme, Bray, Ireland) as previously described [44
]. The released glucose quantity was determined using a glucose assay kit (K-GLIC, Megazyme, Bray, Ireland).
3.14. Statistical Analysis
The mean value and significant difference were analyzed using SPSS software (IBM Company, Chicago, IL, USA).
The bulbil and the rhizome of Chinese yam had 66.9% and 84.6% starch content, respectively. Their starches were oval in shape and had eccentric hila. The volume- and surface-weighted mean diameters were 16.4 and 8.5 μm for the bulbil starch, respectively, and 17.7 and 9.4 μm for the rhizome starch, respectively. The bulbil starch and the rhizome starch had 38.3% and 35.2% amylose content, respectively. Both rhizome and bulbil starches had CA-type crystallinity and showed a similar short-range ordered structure and lamellar structure. The rhizome starch had lower swelling power, water solubility, and setback viscosity and higher onset and peak gelatinization temperatures and breakdown viscosity than the bulbil starch. The bulbil starch had higher thermal stability, resistance to hydrolysis, and in vitro digestion than the rhizome starch.