Microstructural, physicochemical, thermal, and rheological properties of starches from commonly consumed yam varieties in West Africa

TL;DR

This study examines the properties of starches from ten yam varieties in West Africa to determine their potential for food and industrial uses.

Contribution

The study provides detailed microstructural, thermal, and rheological data for yam starches, supporting their application in food and non-food industries.

Findings

Yam starches showed significant variation in functional, thermal, and rheological properties.

Sample KNE-C had the highest amylose content at 42.7%, while sample KPO-C had higher relative crystallinity than sample OBI-N.

Starches displayed solid-like behavior and viscoelastic properties, with gel hardness ranging from 5.74 N to 10.74 N.

Abstract

This study assessed the starch content of ten yam varieties commonly consumed in West Africa to clarify their structural, thermal, physicochemical, and rheological properties and to support their potential food and industrial applications.

The yam starches differed significantly in functional, thermal, and rheological properties. The different samples were given different codes. The sample coded KNE‐C exhibited the highest amylose content at 42.7%. The starch granules were round and elliptical, with sizes ranging from 14.8 μm (sample SD3‐G) to 24.1 μm (sample BET‐C). X‐ray diffraction analysis indicated peaks at 5.7°, 15°, 17°, and 23° 2θ, which correspond to a type C crystallinity pattern. The starch relative crystallinity was significantly lower (P ≤ 0.05) in sample OBI‐N (32.49%) than sample KPO‐C (40.76%). Gelatinization varied significantly (P ≤ 0.05) among the yam starch varieties, with a temperature range (ΔT) of 14.9 to 19.4 °C. A rapid visco analyzer (RVA) was used to show that peak time ranged between 5.0–5.5 min, pasting temperature ranged between 77.6–84 °C, and viscosity ranged between 4.114 Pa.s and 5.820.5 Pa.scP. The starches displayed viscoelastic properties and solid‐like behavior (G′ > G″). Gel hardness varied from 5.74 N in sample KPO‐C to 10.74 N in sample SD4‐G. Principal component analysis showed that components PC1 and PC2 together accounted for 55% of the total variance, and PC3 increased the cumulative variance to 71%.

The starches differed in microstructure, physicochemical, thermal, and rheological properties. This study provides a valuable basis for the selection of yam starch with specific properties suited for specific food applications in both food and non‐food industries. © 2025 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Figures

Scanning electron micrographs (SEM) of yam starch varieties.

X‐ray diffraction spectra of yam starch varieties.

Fourier transform infrared (FTIR) spectra of yam starch varieties.

Differential scanning calorimetry (DSC) curves of yam starch varieties.

Pasting properties of yam starch varieties.

Functional and textural properties of yam starches. (A) The swelling powers and water solubilities, (B) Gel strength and syneresis rate of yam starch varieties.

Oscillatory amplitude sweeps of gels of yam varieties. The storage modulus (A), loss modulus (B), phase angle (C), and complex modulus (D).

Principal component analysis of yam starch varieties.

Tables

Table 1. Variety, country and sample code of yam starch varieties

Variety	Country	Sample code
Katala	Benin	KAL‐B
Laboko	Benin	LBO‐B
SDr1403003	Ghana	SD3‐G
SDr1403004	Ghana	SD4‐G
Amadouo	Côte d'Ivoire	AMD‐C
Krenglè	Côte d'Ivoire	KNE‐C
Kponan	Côte d'Ivoire	KPO‐C
Bêtê‐bêtê	Côte d'Ivoire	BET‐C
Obiaturugo	Nigeria	OBI‐N
TDr8902665	Nigeria	TDR‐N

Table 2. Moisture content, total starch, granular size, apparent amylose content of yam starch varieties

Samples	Moisture (%)	Total starch (%)	Size (μm)	AAM (%)
OBI‐N	10.0 ± 0.6c	71.3 ± 1.4b	17.2 ± 0.1e	37.3 ± 2.5d
SD3‐G	9.4 ± 0.7d	81.0 ± 3.8a	14.8 ± 0.0f	31.3 ± 1.4e
KPO‐C	12.4 ± 0.2a	65.1 ± 0.8c	18.8 ± 0.2d	39.7 ± 1.2b
AMD‐C	11.0 ± 0.5b	64.2 ± 1.4d	20.4 ± 0.1d	27.5 ± 1.8g
TDR‐N	8.4 ± 0.2f	62.0 ± 0.1f	21.9 ± 0.1b	38.3 ± 3.8c
LBO‐B	8.7 ± 0.2f	64.2 ± 0.5d	20.7 ± 0.0c	21.4 ± 3.1h
KAL‐B	9.1 ± 0.2e	61.4 ± 0.2g	18.1 ± 0.1d	31.6 ± 1.0e
SD4‐G	9.1 ± 0.6e	85.3 ± 0.2 a	18.7 ± 0.2d	30.3 ± 2.4f
BET‐C	12.7 ± 1.0a	63.0 ± 0.2e	24.1 ± 0.4a	38.7 ± 3.7c
KNE‐C	8.4 ± 0.2f	71.0 ± 0.4b	21.1 ± 0.1b	42.7 ± 3.1a

Table 3. Infrared ratio (IR) ratios, relative crystallinity, and thermal properties of yam starch varieties

Samples	IR1	IR2	RC (%)	Thermal parameters
	1045/1022 (cm−1)	1022/995 (cm−1)		To (°C)	Tp (°C)	Te (°C)	∆T (°C)	∆H (J g−1)
OBI‐N	1.20a	0.795a	32.49d	72.6 ± 1.1cd	84.2 ± 1.5c	89.2 ± 1.1e	16.6 ± 0.0ab	13.5 ± 0.6b
SD3‐G	1.16a	0.845a	36.39c	71.0 ± 1.1d	82.7 ± 1.7c	86.3 ± 2.0f	15.3 ± 1.0d	12.1 ± 0.7d
KPO‐C	1.22a	0.768a	40.76a	78.4 ± 2.0a	88.7 ± 0.8a	94.0 ± 2.0c	15.6 ± 0.0bc	15.3 ± 0.7a
BET‐C	1.19a	0.805a	33.91e	73.9 ± 1.7c	85.3 ± 2.5ab	88.8 ± 2.0e	14.9 ± 0.7e	14.4 ± 0.6b
TDR‐N	1.20a	0.807a	36.96b	74.0 ± 1.8c	80.1 ± 2.0d	90.5 ± 1.2d	16.5 ± 0.6ab	13.0 ± 0.2c
LBO‐B	1.16a	0.854a	35.88c	77.4 ± 1.7ab	86.6 ± 1.7a	96.2 ± 2.3b	18.8 ± 0.6a	14.6 ± 0.1ab
KAL‐B	1.22a	0.781a	38.64b	79.7 ± 0.8a	85.1 ± 0.6ab	99.1 ± 0.6a	19.4 ± 0.2a	15.5 ± 0.2a
SD4‐G	1.27a	0.724b	33.01e	75.8 ± 0.8b	86.9 ± 0.2a	93.5 ± 0.8C	17.7 ± 0.6b	10.7 ± 0.2e
AMD‐C	1.24a	0.734b	38.09b	72.1 ± 3.3d	85.1 ± 1.4ab	90.9 ± 1.7d	18.8 ± 0.5a	15.6 ± 0.8a
KNE‐C	1.17a	0.824a	36.78b	78.9 ± 0.6a	84.6 ± 0.7ab	97.5 ± 1.3b	18.6 ± 0.4a	15.7 ± 0.6a

Table 4. Pasting properties of yam starch

Sample	Peak time (min)	Pasting temp (°C)	Viscosity (cP)
			Peak	Trough	Final	Breakdown	Setback
OBI‐N	5.2 ± 0.1a	81.2 ± 0.6b	5121.0 ± 186.7d	4610.5 ± 140.7a	7864.0 ± 38.2b	510.5 ± 46.0j	3253.5 ± 102.5c
SD3‐G	5.0 ± 0.1a	83.0 ± 0.6b	5820.5 ± 323.1a	4535.0 ± 192.3b	6624.0 ± 104.7d	1285.5 ± 31.0a	2089.0 ± 97.0g
KPO‐C	5.3 ± 0.1a	77.6 ± 0.1c	5354.5 ± 211.4c	4275.5 ± 19.0c	5370.0 ± 70.7i	1079.0 ± 52.3d	1094.5 ± 55.0i
BET‐C	5.2 ± 0.1a	81.6 ± 0.04b	4279.0 ± 178.2g	3662.0 ± 185.2e	5770.0 ± 203.6h	617.0 ± 17.0i	2108.0 ± 8.5h
TDR‐N	5.3 ± 0.1a	82.3 ± 0.04b	4420.0 ± 113.1f	3634.0 ± 151.3e	5986.0 ± 7.1g	786.0 ± 38.2f	2352.0 ± 118.4f
LBO‐B	5.0 ± 0.1a	81.2 ± 0.6b	5630.0 ± 208.0b	4383.0 ± 145.7c	6466.0 ± 195.2f	1247.0 ± 62.2b	2083.0 ± 41.0g
KAL‐B	5.3 ± 0.1a	81.1 ± 1.7b	4971.5 ± 531.0e	4135.5 ± 148.0c	7628.0 ± 157.5c	836.0 ± 17.0e	3492.5 ± 59.5b
SD4‐G	5.0 ± 0.2a	81.0 ± 0.0b	4230.5 ± 214.0g	3562.5 ± 160.2e	6278.5 ± 92.6e	668.0 ± 18.0g	2716.0 ± 97.6e
AMD‐C	5.5 ± 0.2a	84.0 ± 1.3a	4114.0 ± 175.2h	3729.0 ± 103.1d	6741.5 ± 258.3d	6741.5 ± 74.2h	3012.5 ± 115.3d
KNE‐C	5.1 ± 0.0a	83.7 ± 0.5a	5686.0 ± 274.0b	4556.0 ± 123.4b	8375.0 ± 292.7a	1115.0 ± 31.5c	3819.0 ± 71.0a

Full text

INTRODUCTION

The yam (Dioscorea spp.) is one of the most important root and tuber crops worldwide. It produces edible starchy storage tubers, which are of cultural, economic, and nutritional importance.1 It plays a major role in the food security and the livelihood systems of about 300 million people in West Africa. Beyond providing nutrition, yams hold cultural significance in West African and Caribbean diets, and are included prominently in major traditional ceremonies.2 Although yam tubers have not been used extensively in industry and have served primarily as a traditional domestic foodstuff, there are numerous traditional applications for this crop, highlighting its potential for further uses. Yams can be consumed in various ways, including in dishes made from raw tubers that are fried, crushed, boiled, or stewed.3

Varieties include white yam (Dioscorea rotundata), yellow yam (D. cayenensis), water yam (D. alata), trifoliate yam (D. dumetorum), aerial yam (D. bulbifera), Chinese yam (D. polystachya), and lesser yam (D. esculenta).1, 3, 4 West Africa produces about 90% of world yam production. Nigeria is the largest and leading single producer, followed by Ghana, Côte d'Ivoire, Benin, and Togo.4 Yam is valued as a food source for its high carbohydrate content, including fiber and starch (60% to 89% dry weight), which provide 300 million people in the tropics with about 200 dietary calories per person per day.5 This also highlights its suitability for commercial processing, as yams are inexpensive and readily available.

Starch is one of the most abundant polysaccharides and serves as a valuable source of carbohydrates for humans, contributing to dietary supplementation.6 When starch is heated with water, it readily forms hydrocolloids, which are essential raw materials in both food and industrial processing.7 Gelatinization and starch retrogradation are two critical factors that influence quality, nutritional value, and shelf life of starchy hydrocolloids.7 Previous studies have shown that the chemical composition and structure of starch impact both gelatinization and retrogradation, resulting in starch with different functional properties.8, 9 The ongoing research effort by the International Institute of Tropical Agriculture (IITA) and the Bill Gates Foundation to improve yam varieties in Africa for potential industrial application has led to the availability of selected varieties used in this research.

Although pasting, biochemical, physicochemical, and functional properties of yam starches have been reported previously by Effah‐Manu et al., Oliveira et al., and Xiao et al.,10, 11, 12 information on microstructure, thermal, and techno‐functional properties of the improved varieties of Dioscorea species is limited. Studying these properties can therefore provide important insights into the potential applications of the starches within the food industry. This may lead to new applications for both domestic and industrial purposes, and the findings could assist researchers in understanding the functional behaviors of the starches in food systems, including their gelling, thickening, and stabilizing properties. This study therefore aimed to clarify the starch composition and its functional attributes by evaluating the chemical, structural, thermal, pasting, and rheological properties of ten improved, commonly consumed yam starches from West Africa.

MATERIALS AND METHODS

Materials

Ten commonly consumed yam varieties (Table 1) were collected from the experimental fields of the Africa yam project of the International Institute of Tropical Agriculture (IITA) in the four major producing countries: Benin, Côte d'Ivoire, Ghana, and Nigeria. The yams were harvested 9 months after planting and stored for 3 months at ambient temperature before being studied for their starch properties. Two varieties of D. rotundata, Obiaturugo (Obi‐N) and TDr 89/02665 (TDr‐N), were sourced from the National Root Crops Research Institute (NRCRI) in Umudike, Nigeria (5°N, 7.5°E). The D. rotundata varieties SDr1403003 (SD3‐G) and SDr1403004 (SD4‐G) were collected from the Council for Scientific and Industrial Research (CSIR) – Savanna Agricultural Research Institute in Tamale, Ghana (9°, 25'N, 0°, 59' W). Four yam varieties were collected from the Centre National de Recherche Agronomique (CNRA), located in Bouaké, Côte d'Ivoire (7°, 25'N, 5°, 00' W). There were two varieties of Dioscorea alata, Amadouo and Bêtê‐bêtê – this was an improved variety introduced into Côte d'Ivoire in 1992. Krenglè and Kponan are varieties of D. rotundata. Krenglè is a widely cherished traditional variety, particularly appreciated by the local population. Two D. rotundata landraces, Laboko and Katala, were collected from the experimental field of the Université d'Abomey‐Calavi (UAC), Dassa Centre, Benin.

Starch extraction

Starch was extracted using the wet milling method described by Lee et al.13 The yam flour was washed with deionized water at a flour:water ratio of 1:10 and placed in a shaker for 30 min. Initially, a 0.25% (0.0625 mol L^−1^) NaOH solution was added to the starch precipitate and allowed to settle. The resultant solution was then filtered through 100, 200, and 300 μm mesh sieves (ASTM 140) and left to settle overnight at 4 °C to facilitate starch precipitation. The solution was washed and centrifuged at 3000 × g for 30 min at 10 °C. The starch was dried at 40 °C in an oven for 48 h, ground into a powder, passed through a 100 μm mesh sieve, and used to assess the structural and functional properties of the yam starch.

Measurement of chemical composition of starch

The moisture content was determined by oven drying at 105 °C until a constant weight was achieved.14 Starch quantification was conducted using the megazyme amylose/amylopectin assay procedure with the K‐TSTA commercial kit (Megazyme Ireland International Ltd, Bray, Ireland). The amylose content was assessed using a color test with iodine, using the method outlined by McGrance et al.15 with a potato amylose standard (Sigma‐Aldrich, Modderfontein, Johannesburg, South Africa). Results were derived from a pure amylose standard curve, and the absorbance reading was taken at 610 nm.

Morphological characteristics

The starch granule size was measured using a Litesizer Nano ZS (Anton Paar, New Castle, DE, USA). The samples were mixed at a concentration of 0.02% in distilled water and then subjected to a 150 W probe ultrasound (ultrasonic homogenizer) at 25 °C for 5 min to break them apart. After settling, the samples were analyzed at 25 °C.16 The morphological features of the starches were examined using a scanning electron microscope (SEM) (Zeiss Ultra Plus, Oberkochen, Germany) operating at a high voltage of 10 kV. A suspension of 0.002% was prepared for each sample, and 2.5 μL of this suspension was placed on a carbon grid. A thin layer of approximately 1 nm of Au–Pd conductive coating was applied to the sample before observation.17

Analysis of crystalline structure

The crystalline structure of yam starches was analyzed using an X‐ray diffractometer (XRD) (AXS D8 Advanced, Bruker, Rheinfelden, Germany). The diffractometer used Cu–Kα radiation, scanned 5–45° (2θ) in 0.034° steps with a 92 s count time, and operated at 40 kV. Relative crystallinity was calculated using the method described by Wei et al.18

Analysis of short‐range ordered structure

Fourier transform infrared (FTIR) spectroscopy was used to examine the short‐range ordered structure, particularly its double‐helical order, with an emphasis on changes in the outer regions of the samples. Measurements were performed using an FTIR 630 ATR Diamond (Agilent Inc., Santa Clara, CA, USA), with a spectral resolution of 4 cm^−1^, with 32 scans performed per sample. All spectra underwent baseline correction and were subsequently deconvolved within the range of 1200–800 cm^−1^. To assess the short‐range ordered structure of both native starch and starch nanocrystals, the intensities between 1045 and 995 cm^−1^ were compared (IR1).19

Analysis of pasting properties

The pasting properties of starch were estimated using a rapid visco analyzer (RVA) (RVA 4500, Perten, Macquarie Park, Australia), following the method described by Oliveira et al.11 with minor modifications. The moisture content was adjusted to 14%, and 3 g of yam starch was added to 25 g of distilled water in an RVA canister. The pasting cycle lasted 12 min, during which the starch slurry was held at 50 °C for 1 min, heated from 50 °C to 95 °C at 12 °C min⁻¹, and then cooled to 50 °C at the same rate. Pasting time, pasting temperature (PT), peak viscosity (PV), trough viscosity (TV), breakdown viscosity (BV = PV – TV), final viscosity (FV), and setback viscosity (SB = FV – PV) were recorded. Viscosity values are reported in centipoises (cP).

Analysis of thermal properties

The thermal properties of the starch were measured using a differential scanning calorimetry (DSC) analyzer (DSC Q100, TA Instruments, New Castle, DE, USA), equipped with a manual liquid nitrogen cooling system. Starch (2.5 mg) was mixed with 7.5 μL of deionized water in an aluminium pan, sealed, and left for 12 h at room temperature to standardize moisture. The starch samples were subsequently heated from 25 °C to 125 °C at 10 °C per min while a nitrogen atmosphere was maintained at a flow rate of 40 mL min⁻¹.20 The onset (To), peak (Tm), endset temperatures (Te), and enthalpy (*∆*H) values were determined using the instrument's software, TA TRIOS.

Measurement of swelling power and solubility

The swelling power and solubility of starches were measured using the method described by Guo et al.16 Briefly, a starch slurry (2% w/v) was heated in a shaking water bath (Labcon, Krugersdorp, South Africa) at 95 °C for 30 min and then cooled to room temperature. The suspension was centrifuged (Eppendorf 5702R, Hamburg, Germany) at 4000 × g for 15 min at 25 °C, after which the supernatant was collected in a pre‐weighed aluminium dish, and then dried at 105 °C for 2 h. The swelling power and solubility of the starches were subsequently calculated as follows:

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Textural properties of starch gels

The yam starch‐based hydrogels were produced by suspending 5 g of starch in 50 mL of distilled water, which was gelatinized at 90 °C for 30 min. The resulting gel was then poured into a mold measuring 20 × 10 mm in diameter and height. It was kept at room temperature (20 °C ± 2 °C) for 6 h to facilitate slow retrogradation before undergoing five consecutive freeze–thaw cycles. This process included freezing the gels at −18 °C for 20 h and thawing them at 20 ± 2 °C. The hydrogels underwent freeze‐drying for a duration of 48 h with the double compression test method used in analyzing the texture profile of the gels (25 g) in a texture analyzer (EZ‐SX, Shimadzu, Kyoto, Japan). A compression platen PP75 was used with an activation force of 10 N, a pretest speed of 1 m s^−1^, a test speed of 5 mm s^−1^, and a post‐test speed of 5 m s^−1^.20

Analysis of rheological properties of starch gels

A dynamic oscillatory test was performed to find the linear viscoelastic region (LVR) of yam starches using a rheometer (Anton Paar MCR 102, New Castle, DE, USA). Starch suspensions (5% w/v) were prepared in centrifuge tubes with screw caps, heated in a water bath at 95 °C for 15 min. The gel was allowed to stand overnight, and a 25 mm diameter plate–plate geometry with a 1 mm zero‐gap was used to perform a strain amplitude sweep (1 Hz) from 0.1% to 1000% at 25 °C. The edges of the gel samples were coated with a thin layer of paraffin oil to prevent drying, and the gel samples were equilibrated for 5 min. The frequency amplitude sweeps test yielded data on storage or elastic modulus (G′), loss, or viscous modulus (G″), phase angle, and complex modulus, which were analyzed using the software associated with the equipment.

Statistical analysis

Samples were prepared in duplicates and analyses were conducted in triplicates. All experiments were carried out in triplicate and reported as mean plus or minus standard deviation. All data were analyzed using analysis of variance (ANOVA), and means were compared using the Fisher least significant difference (LSD) test (P < 0.05).

RESULTS AND DISCUSSION

Chemical composition of various yam starches

The starch moisture content showed a significant effect (P < 0.05), with values ranging between 8.4% and 12.7% (Table 2). The moisture content of sample BET‐C exhibited the highest value at 12.7%, whereas both samples TDR‐N and KNE‐C had the lowest value at 8.4%. This variation among the starches could be attributed to varietal differences. The values are within the range reported for similar yam starches by Tanimola et al.9 The amylose content also showed a significant effect (P < 0.05), with values ranging from 21.4% to 42.7% (Table 2). Krenglè yam starch (KNE‐C) exhibited the highest amylose content (42.7%), followed by Kponan yam starch (KPO‐C) with 39.7%, but Laboko yam starch (LBO‐B) recorded the lowest value at 21.4%. The apparent amylose content (AAM) values were comparable to the findings of Lu et al.21 and Arueya and Ojesanmi22 but exceeded the values reported by Effah‐Manu et al.10 Based on the classification of starch into waxy starch (0% to 5%), very low‐amylose starch (5% to 12%), low‐amylose starch (12% to 20%), and high amylose starch (25% to 33%),23 all yam starches in this study were high amylose starches.

Granule shape, size, and morphology of various yam starches

Figure 1 illustrates starch granules extracted from yam tubers, showing a diverse range of shapes and sizes. The granules were flat, round, elliptical, or triangular, with a few irregular forms and smooth surfaces. These characteristics are comparable to yam starches reported by Lu et al.21 and sweet potato starches reported by Li et al.24 The average particle size of yam starches ranged from 14.8 μm (SD3‑G) to 24.1 μm (BET‑C) (Table 2), smaller than the sizes reported by Otegbayo et al.1 but consistent with the 19.2–28.4 μm observed by Zou et al.14 Yam variety significantly influenced both average particle size and functional properties of the starch, which is in agreement with Oliveira et al.11

Scanning electron micrographs (SEM) of yam starch varieties.

Crystallinity properties of various yam starches

Figure 2 shows the diffraction patterns of yam starches. Yam starches had four main peaks at 5.7°, 15°, 17°, and 23° (2θ) in the diffraction spectra, which are typical of C‐type crystallinity. This result is comparable with the starch structure observed in most yam species, as previously reported.12, 25 C‐type starch comprises both A‐ and B‐allomorphs within the granules. The study demonstrated that the varieties did not alter the crystal structure, despite varying levels of crystallinity. The relative crystallinity ranged from 32.49% for OBI‐N to 40.76% for KPO‐C, which was higher than the values reported by Shao et al.23 but comparable with the values reported by Lu et al.21 Studies have shown that differences in crystallinity levels can be linked to the amount of amylose in the larger branches, which is connected to the non‐crystal areas of granules and the shorter chains of amylopectin.9, 23 The length of the branched chains of the amylose in the present study could therefore explain the inverse relationship between crystallinity and amylose in yam starch.

X‐ray diffraction spectra of yam starch varieties.

Fourier transform infrared spectrum

The FTIR spectra of the starches showed similar peak intensities (Fig. 3). The characteristic peaks of the ten yam starch varieties did not show a significant shift; the spectra recorded in this study displayed strong bands at 3427 cm^−1^, 2925 cm^−1^, 1642 cm^−1^, 1159 cm^−1^, 1087 cm^−1^, and 995 cm^−1^.

Fourier transform infrared (FTIR) spectra of yam starch varieties.

The resonance peak at 3427 cm^−1^ might be due to the stretching vibration of the hydroxyl groups, which are abundant in starch. The peaks around 2925 cm^−1^ might be from the stretching vibrations of C—H bonds in the methylene and methine groups of the glucose units. A peak observed around 1642 cm^−1^ is often attributed to the bending vibration of water molecules within the starch structure. The other peaks, at 1159 cm^−1^, 1087 cm^−1^, and 995 cm^−1^, fall within the fingerprint region of the FTIR spectrum. These results are consistent with those reported by Oliveira et al.11 and Xiao et al.12 The ratio at 1045/1022 cm^−1^ (IR1) is the ratio of ordered crystalline structure to amorphous carbohydrate structure, and the absorbance ratio of 1022/995 cm^−1^ (IR2) reflects the ratio of amorphous to ordered carbohydrate structure in the starch outer region.19 The IR1 ratio of the yam starches ranged from 1.16 (SD3‐G) to 1.27 (SD4‐G), and the IR2 ranged from 0.724 (SD4‐G) to 0.824 (KNE‐C) (Table 3). Higher IR1 values indicate a greater degree of order.26 These results suggest that all yam starches exhibit a high degree of short‐range order on the granule surface.

Thermal properties of various yam starches

Figure 4 and Table 3 report the thermal properties of yam starches. Sample SD3‐G had the lowest onset and end set gelatinization temperature (71 and 86.3 °C) and sample KAL‐B had the highest onset gelatinization temperature (79 and 99.1 °C). The peak temperature (Tp) of yam starches revolves around 80 and 88 °C, while the gelatinization temperature range (ΔT) and gelatinization enthalpy (ΔH) temperatures ranged from 14.9 to 19.4 °C, and 10.7 to 15.7 J g^−1^ respectively. The yam starches showed a single endothermic curve comparable to those reported by Li et al.24 and Siroha et al.27 Several studies28, 29 have reported a similar wide gelatinization temperature range with a single gelatinization peak in water for a C‐type starch. These disparities can be attributed to the differences in starch particle size, amylose content, and amylopectin chain length.30 These gelatinization data further confirm the influence of varieties on the structure of the yam starches.

Differential scanning calorimetry (DSC) curves of yam starch varieties.

Pasting properties of yam starches

Table 4 shows the pasting characteristics of the yam starches. There were significant differences among the starch varieties. The yam starches displayed two distinct groups of peak viscosities (Fig. 5), with values ranging from 4114 to 5820.5 cP. The pasting temperatures of the yam starches ranged from 77.6 °C (KPO‐C) to 84 °C (AMD‐C), which is comparable with the range of 75.37–84.37 °C for nine yam starches reported by Zou et al.14 but higher than the 78.7–66.7 °C range reported for yam starches by Magallanes‐Cruz et al.31 The SD3‐G sample exhibited the highest peak viscosity value of 5820.5 cP, indicating a strong swelling ability among the starches. Peak viscosity is an indication of the sample's ability to form a sticky gel after pasting and cooling operations.19 The variation in peak viscosities among the starches could be attributed to their particle size, suggesting that SD3‐G could retain more water than other starches due to its larger surface area.

Pasting properties of yam starch varieties.

Breakdown viscosity (BD) is a critical parameter evaluated during the pasting or gelatinization of starch‐containing foods, reflecting the stability of the starch paste under heat and mechanical stress.16 In this study, the breakdown viscosity values of yam starches varied significantly (at P < 0.05), ranging from 510.5 cP (OBI‐N) to 6741.5 cP (AMD‐C). A high breakdown viscosity indicates that the yam starches degrade rapidly during the gelatinization process, making them suitable for foods with soft textures. This range is higher than the breakdown viscosity values (870.33 cP to 2672.33 cP) reported by Zou et al.14 Trough (T) viscosities ranged from 3562.5 cP to 4610.5 cP, with OBI‐N, SD3‐G, KPO‐C, LBO‐B, KAL‐B, and KNE‐C exhibiting trough values above 4000 cP and final viscosities exceeding 5000 cP. The OBI‐N starch sample demonstrated the largest trough viscosity at 4610.5 cP, and KNE‐C recorded the highest final viscosity at 8375.0 cP, indicating a strong ability to form paste and gel after pasting. In contrast, the lowest trough viscosity values among the yam starch group (SD4‐G, AMD‐C, BET‐C, and TDR‐N) indicate that these starch pastes struggled to create a stable gel after cooking.

The setback (SB) viscosity varied from 1094.5 cP (KPO‐C) to 3819 cP (KNE‐C). Setback is associated with the gelling and retrogradation capacity of starch paste.24 The lower value observed in KPO‐C yam starch may be attributed to a reduced amount of amylose that leaches and recrystallizes. This measurement also suggests that it has potential as a raw material in the formulation of weaning foods.32

Swelling power and water solubility of yam starches

The swelling power and solubility of yam starches in water at 95 °C ranged from 9.2 g g^−1^ for SD3‐G to 12.2 g g^−1^ for SD4‐G and from 7.6% for SD3‐G to 22% for SD4‐G, respectively (Fig. 6(A)). The swelling power of starch indicates the extent of water absorption by starch granules.23 The lower swelling value observed in SD3‐G may be attributed to its high amylose content, which maintains the integrity of the swollen particles and may inhibit swelling.32 The results are consistent with the results reported by Otegbayo et al.1 The differences in swelling and solubility among the yam starches may be due to variations in particle size and amylose content, even though all the starches exhibit similar crystal structures.

Functional and textural properties of yam starches. (A) The swelling powers and water solubilities, (B) Gel strength and syneresis rate of yam starch varieties.

Gel strength and syneresis of yam starches

After pasting, the textural properties of the resultant gel were analyzed and the result showed that the hardness varied significantly among the different starches, with values ranging from 5.74 N KPO‐C to 10.74 N SD4‐G (Fig. 6(B)). This might be due to varietal granule morphology, which inevitably yields noticeably different gel textures. These values were higher than those reported in previous studies33, 34 for corn starch and sorghum starch. The gels are solid and easily shaped, making them suitable as thickeners or gelling agents in food products.35 The SD4‐G sample recorded the highest hardness value, which might be due to its high amylose content, as straight amylose molecules tend to stick together more easily than the highly branched amylopectin molecules, which makes the gel harder.36 The syneresis values for yam starches varied from 26% (KPO‐C) to 51.6% (KNE‐C), as illustrated in Fig. 6(B). Syneresis reflects the retrogradation behavior of cooked starch pastes, which primarily occurs as a result of reorganization of amylose along with the recrystallization of the shorter external chains of amylopectin over time.8 In this study, the yam starches are classified as high amylose starches, a classification that is consistent with their elevated syneresis values.

Rheological properties of yam starches

Stress sweep tests were done to assess yield stress (Fig. 7). The storage modulus (G′) and loss modulus (G″) exhibited a linear viscoelastic response characterized by constant values at shear stress above 100 Pa (Fig. 7(A),(B)). All yam starch gels demonstrated elastic behavior (G′ > G″) up to 70 Pa, which is comparable with the findings of Barua et al.37 This result highlights the significance of amylose in yam starch that leached out during the gelatinization process, forming a network that imparts elasticity to starch gels.38 The phase shift (°) and complex modulus (Fig. 7) illustrate the ratio of elasticity to viscosity in the samples as noted by Moorthy et al.39 Yam starches displayed a phase shift below 20° at a strain of 10% to 15%. These values were lower than 45°, indicating that yam starches possess viscoelastic properties with predominating solid‐like behavior (G′ > G″).40

Oscillatory amplitude sweeps of gels of yam varieties. The storage modulus (A), loss modulus (B), phase angle (C), and complex modulus (D).

Principal component analysis of yam starch varieties

Principal component analysis (PCA) showed that the first three components accounted for 71% of the total variation in the yam starch data (Fig. 8). PC1 explained 32.01% of the total variation and differentiated the BET‐C, TDR‐N, and AMD‐C samples from other varieties on the basis of their high syneresis, setback viscosity, and pasting temperature, which are typical of starches with high amylose content. Variety SD4‐G clustered on the negative side of PC1 and positive side of PC2, indicating greater hardness, solubility, and swelling power, possibly because it had the highest total starch content (85%). KAL‐B, OBI‐N, KNE‐C, and SD3‐G clustered on the positive side of PC1, showing moderate final, trough, and peak viscosity. These patterns indicate a starch with high water‐holding capacity, good paste stability during heating, and a strong tendency to form gels on cooling (high retrogradation). PC1 and PC2 together explained 55% of total variance, and PC3 (not shown) increased the cumulative variance to 71%.

Principal component analysis of yam starch varieties.

CONCLUSION

This study evaluated the microstructural, physicochemical, thermal, and rheological properties of starches derived from ten yam varieties commonly consumed in West Africa. They showed strong gelling ability, forming very firm gel upon cooling, and good thickening qualities, which are suitable for applications requiring significant viscosity. They also demonstrated good heat and shear stability, which is an indication that the paste structure holds up well during the cooking process. The high retrogradation rates suggest that the resulting products might harden or age quickly. Principal component analysis showed that BET‐C, TDR‐N, and AMD‐C were differentiated from other varieties by high syneresis, setback viscosity, and pasting temperature. KAL‐B, OBI‐N, KNE‐C, and SD3‐G differed in their final, trough, and peak viscosity, with the SD4‐G variety showing greater hardness, solubility, and swelling power. The varietal differences markedly influenced the microstructure, physicochemical, thermal and rheological properties of yam starch. This study provided data that can be used to support the selection of yam starch with properties suited to specific applications, thereby contributing to the broader utilization of yam tubers in food and non‐food industries.