Comparison of physicochemical properties of oca (Oxalis tuberosa), potato, and maize starches

Journal Pre-proof Comparison of physicochemical properties of oca (Oxalis tuberosa), potato, and maize starches

Fan Zhu, Rongbin Cui PII:

S0141-8130(19)39110-X

DOI:

https://doi.org/10.1016/j.ijbiomac.2020.01.028

Reference:

BIOMAC 14332

To appear in:

International Journal of Biological Macromolecules

Received date:

8 November 2019

Revised date:

28 December 2019

Accepted date:

4 January 2020

Please cite this article as: F. Zhu and R. Cui, Comparison of physicochemical properties of oca (Oxalis tuberosa), potato, and maize starches, International Journal of Biological Macromolecules(2020), https://doi.org/10.1016/j.ijbiomac.2020.01.028

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© 2020 Published by Elsevier.

Journal Pre-proof

Comparison of physicochemical properties of oca (Oxalis tuberosa), potato, and maize starches Fan Zhu*, Rongbin Cui School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand

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* Correspondence, email: [email protected]; Tel: +64 9 923 5997

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Abstract Oca (Oxalis tuberosa) represents a novel source of starch. Starches from the tubers of two commercial oca varieties were studied for various physicochemical properties. One normal potato starch and one normal maize starch were used for comparison. Oca starches showed lower gelatinization temperatures compared to both potato and maize starches. The pasting,

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flow, and gelation behaviors of oca starches were intermediate between potato and maize starches. Oca starch pastes were more viscous than maize starch paste and more elastic than

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potato starch paste. The differences in the properties could be largely due to lower amylose

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content, intermediate contents of phosphate groups (~400 ppm) and granule size (34.6 μm) of

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oca starches as compared to the other two starches. The internal unit chains of amylopectin

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such as the amount of fingerprint B-chains and length of B-chains could also partially explain the different physicochemical properties among oca, potato and maize starches.

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Keywords: New Zealand yam; pasting property; starch gelatinization; novel starch source;

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rheological property; structure-function relationship

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Journal Pre-proof 1. Introduction Oca (Oxalis tuberosa) of the Oxalidaceae (wood sorrel) family is a tuber crop native to the Andes. It was developed with some other tuber crops such as ulluco (Ullucus tuberosus), mashua (Tropaeolum tuberosum), and potato (Solanum tuberosum) in the Andean region [1, 2]. Oca is also commonly called New Zealand yam, though it is not a member of the true yams (Dioscorea spp.). The crop becomes popular in some other parts of the world such as

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New Zealand where it is an important table vegetable [3]. The tubers are narrow and long. They usually range from 25−150 mm long and 25 mm wide

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[3]. The tubers often have deep eyes and are colored mostly in yellow, red, pink, and orange

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(Supplementary Figure 1) [3]. The nutritional value of oca is considered high. Starch is the

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major component of the tuber (e.g., ~60%, dry weight (dw)). The energy content of oca is

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higher than the true yams (Dioscorea spp.), and is similar to potato. Oca tuber is a source of protein and dietary fiber. It contains a significant amount of fructooligosaccharides [4]. Oca

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contains more iron, calcium, ascorbic acid, and riboflavin than some major food crops such as potato, maize and rice [5]. Yellow-orange colored oca tuber contains carotenoids, whereas

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the red-pink colored oca tuber has anthocyanins. A range of other polyphenols were found in oca tubers, including derivatives of vanillic, caffeic, and cinnamic acids as well as flavones and flavan-3-ols [6]. Oca tuber has a moderate level of soluble oxalates which is an antinutrient [7]. Pulsed electric fields (PEF) processing was found to significantly decrease the oxalate content of oca tubers [8]. Because of the nutritional composition, oca tuber has shown biological activities such as antioxidant activity and preventing digestive ailments (e.g., diarrhea and constipation) [4]. Efforts have been made to develop oca as a sustainable food crop. The major component of oca tuber is starch [9]. The starch properties can affect the quality of oca based foods.

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Journal Pre-proof Starch from diverse plants is found as granules. The sizes of the granules range from 1 to 100 μm, depending on the plants [10]. The two major biopolymers of starch are the amylose (mostly linear) and the amylopectin (branched). In most of the starches, amylopectin is the major component (e.g., ~70 to 80%). Minor components such as lipids, phosphates, and proteins may be found in some starches, depending on the plant source [10]. The chemical composition, molecular structure, and granular architecture determine starch physicochemical properties [11, 12]. In recent years, the role of amylopectin internal structure in determining

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the physicochemical properties of starch becomes more focused [11, 13]. A previous study

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systematically measured the molecular structure of starches from two oca varieties with

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commercial significance [9]. The amylose contents of oca starches (~21%) were lower than

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that of both potato and maize starches. The molecular structure of oca amylopectins was much closer to that of potato amylopectin than to that of maize amylopectin. Oca starch has

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B-type polymorph and the granule size was 34.5 μm (volume moment mean) [9]. The

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structural information of oca starch provides a basis to study the structure-function relationships of starch. However, this aspect is much under-studied for oca starch.

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There has been increasing interest to explore unconventional starches from underutilized plant species. These “novel” starches may have special properties with potential to complement chemically modified and/or industrially important starches for various applications. Oca starch represents a type of unconventional starches. So far, only a few studies have been focused on some properties of oca starch sourced from South America [1, 2, 14-17]. Limited aspects of physicochemical and functional properties were studied in those studies. The results of oca starch were sometimes hard to interpret because of the lack of molecular structural information. Furthermore, it was not clear on how the properties of oca starch may be compared to the industrially important and most studied starches such as potato and maize starches.

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Journal Pre-proof It has been suggested that different techniques should be used to study various physicochemical properties of root and tuber starches [18]. For example, starch gelatinization is a rather complicated process and different aspect of this process can be studied using different analytical instruments [19]. The aim of this study was to measure swelling, rheological and thermal properties of oca starches from two oca cultivars with commercial significance in New Zealand. Various techniques were used to study the properties of these starches. One normal maize starch and one normal potato starch were used for comparison.

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The chemical composition and molecular structure of these starches were reported previously

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[9]. In particular, the amylose composition, granular, crystalline, and amylopectin internal

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molecular structure of these starches were highlighted [9]. The structural and compositional

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properties are summarized in this report for increased readability (Supplementary Table 1). The molecular basis responsible for the obtained physicochemical properties of these starches

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was discussed in the context of structure/composition-property relationships.

2. Materials and methods

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2.1. Materials

Two types of oca tubers (red and yellow skins) were obtained from Countdown Supermarket (Auckland, New Zealand) (Supplementary Figure 1). The two types of cultivars were called “Red” and “Apricot Delight” (AD). The oca crops were grown in Manawatu, New Zealand during the winter (June to October) of 2016. A maize starch (Melojel) from Ingredion (Auckland, New Zealand) was used for comparison. Red skinned tubers of a commercial potato variety (Van Rosa) were obtained from Countdown Supermarket (Auckland, New Zealand). All the chemicals used were of analytical grade. 2.2. Starch extraction

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Journal Pre-proof Fresh oca and potato tubers (~ 2 kg for each) were washed, peeled, and cut into small cubes before grinding in a blender (The Waring Commercial, New Hartford, NY, USA) with iced water for 4 min. The homogenates were filtered through three layers of mesh (140 µm, 50 µm, and 30 µm) to remove the fibrous material. The filtrates were collected and the starch was allowed to settle at 4 oC for 24 h. The supernatants were decanted and the yellowish layer on the top of the starchy cake was scraped. The washing procedures were repeated three times. The resulting starch cakes were dried in an air-forced oven (40 oC). The dried starch was

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ground to powder and sealed in plastic bags. The starch yields for Red and AD oca varieties

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were 4.50 and 3.66 % (fresh weight), respectively. The starch purities of the isolated starches

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and the Melojel starch were over 98% based on a total starch content kit (Megazyme,

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Wicklow, Ireland).

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2.3. Swelling power, water solubility index, and amylose leaching Swelling power (SP, g/g) and water solubility index (WSI, %) of starch were studied using

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the method of Li and Yeh [20]. Briefly, starch (150 mg, dry basis (db)) was mixed with 10

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mL water in capped centrifuge tubes. The tubes were heated (85 oC, 30 min) with frequent shaking. Then, the tubes were cooled and centrifuged (3,000 × g, 30 min). The supernatants were poured into an aluminum dish before drying to a constant weight (W1). The remaining material adhered to the tube wall was weighed (Ws). The SP and WSI were calculated [20]. Amylose leaching (AML) of starch was measured using the method of Gunaratne and Hoover [21]. Briefly, starch (20 mg, db) was weighed in centrifuge tubes and water (10 mL) was added. The capped tubes were heated at 85°C with frequent shaking at about 2 min interval for 30 min. The tubes were then cooled to room temperature and centrifuged (3,000 × g, 30 min). The supernatants (1 mL) were withdrawn and the apparent amylose content was measured using an iodine binding-spectrophotometry based method. AML was calculated as the percentage of amylose leached from starch. 6

Journal Pre-proof 2.4. Differential scanning calorimetry (DSC) Starch gelatinization was measured using a DSC (Q1000 Series, TA instruments, New Castle, Delaware, USA). Starch (2.5 mg, db) was mixed with water (7.5 µL) in aluminum crucibles. An empty crucible was used as reference. The sealed crucibles were heated from 15 to 90 °C at 10 °C/min. Onset (To), peak (Tp), conclusion (Tc) temperatures, and enthalpy change (ΔH) were obtained.

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After gelatinization, the samples were immediately stored at 4 °C for 4 weeks before rescanning for retrogradation test. The heating procedure was the same as the gelatinization

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analysis above. The enthalpy change of retrograded starch melting (ΔHr) was measured. The

2.5. Rheological analysis

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2.5.1. Dynamic oscillatory property

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retrogradation ratio (RE %) was obtained by ΔHr/ΔH × 100.

Dynamic oscillatory properties of starch were measured using the method of Li and Zhu [22].

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Briefly, starch suspension (20% solids) was well shaked before loading on the bottom

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platform of a Physica MCR 301 rheometer (Anton Paar, Graz, Austria) coupled with a parallel metal probe (25 mm in diameter). The probe edge was covered with a sunflower oil layer to prevent water evaporation. The gap size, amplitude gamma, and frequency were set at 1000 mm, 2%, and 1 Hz, respectively. The starch sample was heated from 40 to 90 °C and then cooled from 90 to 25 °C at 2 °C/min. The starch paste were then held at 25 °C for 5 min to achieve equilibrium before a frequency sweep (0.1 to 40 Hz). The obtained parameters were storage modulus (G′), loss modulus (G′′), and loss tangent (tanδ) (= G′′/G′). 2.5.2. Pasting property The starch pasting property was measured using the method of Zhu and Xie [13]. Briefly, starch suspension (9% solids) was heated from 50 to 90 °C before cooling from 90 to 50 °C in a Physica MCR 301 rheometer equipped with a starch cell (Anton Paar, Graz, Austria). 7

Journal Pre-proof The obtained parameters were pasting temperature (Ptemp), peak viscosity (PV), hot paste viscosity (HPV), and cool paste viscosity (CPV). Breakdown (BD = PV − HPV) and setback (SB = CPV − HPV) viscosities were calculated. 2.5.3. Gel textural property The starch paste from the pasting analysis above was kept in the capped glass canister at 4 °C for 24 h before testing. Texture of the starch gel was measured under Texture Profile Analysis mode (TPA) using a TA–XT plus Texture Analyzer coupled with a cylindrical

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probe (diameter is 5 mm) (Stable Micro Systems Ltd., Godalming, UK). The distance moved

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(ADH), and gumminess (GUM) were obtained.

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with the probe was 15 mm at 0.5 mm/s. Hardness (HD), cohesiveness (COH), adhesiveness

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2.5.4. Flow property

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The flow behaviors of starch paste were measured using the method of Li and Zhu [22]. Briefly, starch suspension (5% solid content) was heated at 90 °C for 30 min before

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transferring onto the bottom platform of a Physica MCR 301 rheometer (Anton Paar, Graz,

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Austria) coupled with a parallel metal probe (25 mm in diameter). The paste was equilibrated -1

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at 25 °C for 2 min before shearing from 0.1 to 1000 s and then back to 0.1 s . The data was modelled using power law and Herschel-Bulkley equations. Consistency coefficient (K) (Pa·sn), flow behaviour index (n) (dimensionless), and yield stress (σ0) (Pa) were obtained. 2.6. Data analysis The measurements were done in triplicate. The data analysis was done using one-way ANOVA and Duncan Post Hoc Multiple Comparison of means at p < 0.05 on a SPSS software (Version 22.0, IBM, Armonk, New York, USA). The results were present as mean ± standard deviation.

3. Results and discussion 8

Journal Pre-proof 3.1. Swelling power (SP), water solubility index (WSI), and amylose leaching (AML) of oca, potato, and maize starches The SP of Red oca starch (37.1 g/g) was seen to be lower than that of AD oca starch (41.6 g/g) (Table 1). The WSI and AML of the two oca starches at 85 oC were similar to each other. SP, WSI, and AML of oca starches were higher than those of both maize and potato starches. Granular swelling is primarily affected by structural integrity. This structural integrity is

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affected mostly by the starch chain interactions within the amorphous and crystalline region and also by the arrangements of the starch molecules inside the crystalline lamellae [11]. The

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high SP, WSI, and AML of oca starch suggested that oca starch had a high granular integrity.

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Oca amylopectin had less fingerprint B-chains (Bfp) and more of the majority of B-chains

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(BSm) than potato amylopectin (Supplementary Table 1). Such an internal unit chain

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composition may suggest that the arrangements of crystallites in oca starch granules are suitable for water penetration and swelling. The lipids complexed with amylose in maize

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starch may inhibit granular swelling. Although both oca and potato starches had a B-type polymorphic pattern, they had significantly different SP and WSI. Vamadevan et al. [23]

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recently showed that B-type starches could be divided into two types, “growth ring” structure in potato starch and “granular slices” in yam starch. Thus, oca and potato starches might belong to two distinct structural types within B-type starches. 3.2. Differential scanning calorimetry (DSC) of oca, potato, and maize starches The gelatinization parameters of the two oca starches measured by DSC were similar (Table 2). For example, peak temperature (Tp) for Red and AD oca starches were 59.4 and 59.8 oC, respectively, whereas the enthalpy change (ΔH) were 12.9 and 11.7 J/g, respectively. The gelatinization temperatures of the two starches were lower than both potato (e.g., Tp = 65.0 oC) and maize (e.g., Tp = 73.1 oC) starches. ΔH of oca starch was lower than that of potato starch (ΔH = 16.6 J/g) and was similar to that of maize starch (ΔH = 11.5 J/g). These results of oca 9

Journal Pre-proof starch were seen to be different from those of previous studies on oca starch gelatinization (1 oca genotype used in the previous studies) [1, 2, 14, 15]. The differences could be attributed to the differences in oca genetics and growing environment.

The differences in the

gelatinization properties among the 3 different types of starches may be explained by a range of factors. The internal chain length of oca amylopectin (ICL = 6.0 and 5.7 glucosyl residues) was shorter than that of potato amylopectin (ICL = 6.4 glucosyl residues) (Supplementary Table 1). A shorter ICL tends to destabilize the ordered arrangements of double helices in

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starch granules, decreasing the thermal stability [24]. Maize starch with the shorter ICL had

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higher gelatinization temperatures than oca and potato starches. This suggested that other

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factors such as amylose content, the presence of minor nonstarch components, and building

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block structure of amylopectin significantly played roles in the thermal property [11, 12]. ΔH is related to the disruption of double helices in starch during heating. A shorter external chain

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length (ECL) contributes to a lower ΔH of gelatinization [25]. The ECL of Red oca

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amylopectin (13.8 glucosyl residues) was close to that of maize and potato amylopectins (Supplementary Table 1). The AD oca amylopectin had longer ECL (14.6 glucosyl residues)

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than that of the other three amylopectins. However, potato starch had the highest ΔH among the starches. Therefore, ECL was not the sole factor determining the ΔH. Other factors, including amylose and minor components (e.g., phosphorus-containing compounds and lipids) affect the starch gelatinization to different extents [12]. Maize starch had more lipids than potato and oca starches [2, 26]. Oca starch had less phosphorus than potato starch (Supplementary Table 1). Increased phosphorus content in starch may cause an increase in gelatinization temperatures [27, 28]. Lipids in the form of amylose inclusion complexes reinforce the interactions of starch molecules in the granules, increasing the stability [12]. The amylopectin recrystallization property was measured (Table 2). The melting peak temperature (Tp) of retrograded maize starch (Tp = 53.7 oC) was somewhat lower than that of

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Journal Pre-proof the other starches. AD oca starch had higher retrogradation ratio (RE = 47%) than the other starches. A shorter ICL of amylopectin leads to less organized packing of crystals from recrystallization and retrogradation [29]. Indeed, maize amylopectin had shorter ICL than the other amylopectins (Supplementary Table 1). A longer ECL of amylopectins has been related to a higher ∆H of melting retrograded starch [29]. Indeed, AD oca amylopectin had the longest ECL among the amylopectins (Supplementary Table 1). The presence of lipids as amylose inclusion complexes could increase the retrogradation rate of starch. The repulsive

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force between charged phosphate-monoesters in starch may decrease the interactions of

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starch molecules, decreasing the retrotrogradation [12]. The phosphate-monoesters in oca and

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potato starches may decrease their retrogradation. However, AD oca starch showed higher

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RE than maize starch with a less amount of phosphate-monoesters. This suggested that the

in starch.

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effect of longer ECL in increasing the RE was stronger than that of the phosphate-monoesters

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3.3.Dynamic oscillatory property of oca, potato, and maize starches Little differences in dynamic rheological properties during heating, cooling, and frequency

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sweep were obtained between the two oca starches (Table 3). TG'max of oca starches (60.8 oC) were lower than that of both potato (67.8 oC) and maize (76.9 oC) starches. This comparative pattern agreed with the DSC results (Table 2). G'max of oca starches (2685 and 2830 Pa) were close to that of potato starch (2895 Pa) but were less than that of maize starch (4643 Pa). G'90C of oca starches (1285 and 1315 Pa) were lower than both maize (3013 Pa) and potato (1720 Pa) starches. tan δG'max (0.13) and tan δ90C (0.094) of oca starches were lower than those of potato starch, but were higher than those of maize starch. During cooling, G'25C of oca starches (2900 and 3045 Pa) were lower than that of maize starch (8490 Pa), but were higher than that of potato starch (2400 Pa). Oca starches had higher tan δ25C (0.05) than maize starch (0.03) but had lower tan δ25C than that of potato starch (0.093). For frequency sweep, oca 11

Journal Pre-proof starches had lower G'40Hz (3460 and 3660 Pa) than maize starch (7570 Pa) and higher values than potato starch (2710 Pa). tan δ40Hz of oca starches (0.20) were larger than that of maize starch (0.10) but less than that of potato starch (0.27). For the frequency sweep, G' of oca starch paste was seen to be less resistant to the sweep than maize starch, whereas G'' was less susceptible than the maize starch. The lower G' of oca and potato starches than maize starch during heating, cooling, and frequency sweep could be partially attributed to the restricted granular swelling, lower water solubility and AML, and smaller granular size of maize starch

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(Table 1, Supplementary Table 1).

The differences in dynamic oscillatory rheological properties between oca and potato starches

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might be due to the different composition, granule size and integrity, and molecule structure.

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The higher amount of phosphate groups in potato starch may contribute to better water

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penetration during heating and cooling, leading to more viscous component and a higher tan δ. The differences of the starches in the susceptibility to frequency sweep may also be partially

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due to the structural and mechanical properties of the granule remnant after gelatinization.

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This aspect should be better studied for oca starch. Li and Zhu [22] showed that the amylose content of quinoa starch positively correlated with the G' of the pastes during frequency sweep. In this study, G'40Hz of oca starch with a lower amount of amylose was higher than that of potato starch. Therefore, multiple factors affect the dynamic rheology of the starches. Three previous studies also measured the dynamic rheological properties of oca starch obtained from South America [2, 14, 16]. However, direct comparison in the results between this study and the previous studies was impossible because of the different experimental conditions and instrumental settings used. 3.4. Pasting property of oca, potato, and maize starches Similar pasting properties of the two oca starches were obtained (Table 4). Oca starches had lower Ptemp than potato and maize starches. The results were in agreement with those of the 12

Journal Pre-proof DSC and dynamic rheological analysis as described in sections 3.2 and 3.3. Oca starches had lower PV, HPV, BD, and SB than potato starch. Oca and potato starches had higher viscosities than maize starch during the pasting event. A similar comparative pattern in pasting properties between oca and potato starches was obtained previously by Cruz et al [2]. Starch pasting properties are determined by various factors including amylose content, granular morphology, architecture and integrity, amylopectin structure and the composition of non-starch components such as lipids and phosphate groups [11, 12]. The relatively high

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viscosity displayed by potato and oca starches could be partially attributed to the presence of

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phosphate groups. The amylose-lipid inclusion complexes in maize starch could restrict the

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granular swelling of maize starch as discussed in section 3.1. Compared to the structure of the

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oca amylopectins, the longer ICL and the higher amounts of longer B-chains (B3-chain) of potato starch may also lead to more stability of the granules for swelling, leading to a higher

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viscosity during the pasting event (Supplementary Table 1) [24]. A previous study showed

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that PV, HPV, BD, and CPV negatively correlated with the amount of amylopectin Bfp-chains among different sweetpotato starches [30]. However, in this study, oca starch had lower

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amounts of amylopectin Bfp-chains and lower pasting viscosity than potato starch (Supplementary Table 1). The phosphorus and amylose contents of oca starches were lower than those of potato starch. Oca starch had smaller mean size of the granules than that of potato starch (Supplementary Table 1). It has been reported that root and tuber starches with a higher phosphorus content and a larger granule size resulted in higher PV and BD [31, 32]. However, Zaidul et al. [32] also revealed that PV and BD decreased in starch with more amylose. These results showed that some influencing factors (e.g., minor chemical components) dominate over some other factors (e.g., amylose content) in determining specific physicochemical properties of different starches. 3.5. Gel textural property of oca, potato, and maize starches 13

Journal Pre-proof The gel textural properties of the Red and AD oca starches were similar (Table 4). The gel hardness of oca starches were intermediate between that of potato (68 g) and maize (41 g) starches. Similar comparative patterns between the three types of starches were obtained for adhesiveness, cohesiveness, and gumminess. The gelation of the pasted starch under the experimental conditions is mostly due to the starch retrogradation (amylose re-association and partial amylopectin recrystallization) [33]. The higher hardness, cohesiveness, gumminess, and lower adhesiveness of potato starch than oca starch could be partially due to

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more amylose molecules, a longer ICL, and more long unit chains of amylopectin

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(Supplementary Table 1) [12, 29]. Phosphate groups in starch can inhibit the interactions and

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re-association of starch molecules, whereas lipids in the form of amylose inclusion

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complexes can facilitate starch retrogradation [12]. Even though oca and potato starches had higher phosphorous contents than maize starch (Supplementary Table 1), it is evident that the

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importance of amylopectin structure and amylose content over-weighed that of the minor

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components in the gelation properties of these starches. It is interesting to observe that, overall, the pasting properties of oca starches were closer to those of potato starch, while the

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gel textural properties of oca starches were closer to those of maize starch (Table 4). These results suggested that oca starch may replace either potato or maize starches to achieve similar properties in specific applications. 3.6. Flow property of oca, potato, and maize starches The steady flow data of the starches were fitted using the Herschel-Bulkley and power law equations. The coefficients of determination were high (R2 > 0.98) (Table 5). It was seen that the Herschel-Bulkley equation gave a somewhat better fitting of the data than the power law equation with higher R2. Li and Zhu [22] also showed that the Herschel-Bulkley equation best described the flow behaviors of quinoa starch pastes among different mathematical models. Furthermore, the comparative patterns of the results obtained from the two equations among 14

Journal Pre-proof the different starches were similar. Thus, the results derived from the Herschel-Bulkley model are discussed in this study. Increasing the shearing rate to 1000 s-1 largely disrupted the starch paste network. As a result, the consistency coefficients (K) of the starches during the process of increasing shearing rate were higher than that during the process of decreasing shearing rate. The two oca starches had rather similar flow behaviors during the processes of changing shear rate (Table 5). The yield stresses (σ0) of oca starches were less than that of potato starch paste in the course of increasing shearing rate. Compared to that of maize starch,

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the σ0 of oca starches were higher in the course of increasing and decreasing shearing rate.

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The K of oca starches were intermediate of that of potato and maize starches during both

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processes. The flow behavior indexes (n) of oca starches were less than that of maize starch

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but larger than that of potato starch during the process of increasing shearing rate. The n of oca starch became higher compared to that of maize starch during the process of decreasing

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shearing rate. Cruz et al. [2] compared the flow properties of oca starch and potato starch

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pastes. According to their results, the former had a lower K and higher n during the process of increasing shear rate, which agreed with the results of the present study. Li and Zhu [22]

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showed that σ0 of quinoa starch paste positively correlated with the granule size. Indeed, σ0 of oca, potato, and maize starch pastes during increasing shear rate positively followed the order of their granule size. The higher σ0 and K of oca and potato starches compared to maize starch may also be attributed to the longer chain length of amylopectins (CLap) (Supplementary Table 1). Longer chains better entangled with each other to form a stronger network of starch chains. The lower K and σ0 of oca starches than potato starch may be due to the shorter ICL, less amylose, and smaller granule size (Supplementary Table 1). It has been noted that the differences in the flow properties of maize starch paste (K, σ0, and n) of both increasing and decreasing shearing rates were much smaller than those of oca and potato starches (Table 5). This suggested that maize starch paste was more resistant to shear-thinning than the other two

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Journal Pre-proof starches. The differences may be largely due to the structural remnants of the starch granules after gelatinization, which remains to be studied. 3.7.Comparison between oca starch and other starches So far, a very limited number of reports have been published on different physicochemical aspects of oca starch sourced from South America [1, 2, 14-16]. Santacruz et al. [15, 16] showed that oca starch (1 genotype) had lower temperatures and ∆H of gelatinization than

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starches from Arracacha xanthorriza and Canna edulis (1 genotype for each species). At 89 o

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C, solubility, swelling power, and amylose leaching of oca starch were larger than those of

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Arracacha xanthorriza starch and were less than those of Canna edulis. Oca starch paste showed more changes in the rheological properties during storage than Arracacha

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xanthorriza starch [15, 16]. One study showed that oca starch (1 oca genotype used) had a

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similar peak temperature of gelatinization and ∆H to a normal maize starch (1 maize genotype) [14]. Another study showed that oca starch (1 genotype) had higher swelling

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power and solubility and lower paste opacity than olluco (Ullucus tuberosus) and mashua

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(Tropaeolum tuberosum) starches (1 genotype for each starch type) [1]. The gelatinization temperatures and ∆H of oca and mashua starches were similar, whereas the viscosity of oca starch during pasting event was less than that of both olluco and mashua starches [1]. Cruz et al. [2] showed that oca starch (1 genotype) had similar swelling factor to that of potato and olluco starches. The gelatinization temperatures and pasting viscosity of oca starch were similar or different from potato starch, depending on the potato genotype [2]. Unfortunately, direct comparison of the results between these previous studies and the current report were impossible because of the differences in the experimental conditions and instrumental settings.

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Journal Pre-proof A major goal of exploring nonconventional and underutilized starches is to measure their degrees of differences and similarity against those well-known and well-studied starches. Industrially important starches such as maize and potato starches should be used in the studies for direct comparison as this is the case of the present study. The comparative results showed that oca starch had some special properties compared to some industrially important starches (e.g., maize and potato starches). Even though oca and potato starches have B-type polymorph, significant differences were obtained between these two types of starches as

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shown in sections 3.1 to 3.6 above. Oca starch was easy to gelatinize and its paste had

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relatively high viscosity. Compared to potato starch paste, oca starch paste had more

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elasticity. The differences in the physicochemical properties among oca, maize, and potato

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starches could be largely correlated with the differences in the structure and composition as

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4. Conclusions

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described in the sections above.

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The physicochemical properties of the two oca starches were similar. Oca starches had higher SP, WSI, and AML than potato and maize starches. Oca starches gelatinized at lower temperatures compared to potato and maize starches. The pasting, flow, and gelation behaviors of oca starches were intermediate between potato and maize starches. Oca starch pastes had more elasticity than potato starch paste. The differences in physicochemical properties of the oca starches compared to potato and maize starches could be due to these intrinsic factors, including the lower amylose content, intermediate contents of phosphate groups and granule size, intermediate lengths of internal unit chains of amylopectin, and intermediate amounts of longer internal B-chains, and less fingerprint B-chains.

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Journal Pre-proof This study may be fundamentally useful in developing oca starch as a novel starch source. The results of this study will support the development of oca as a sustainable crop. Future studies may continue with genetic resources of oca for starch quality. Structure-property relationships of oca starch components remain to be subsequently measured using a range of different oca genotypes. Chemical and physical modifications of oca starch should be done in

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comparison with industrially important starches.

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Declarations

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The authors declare that they do not have any conflict of interest. This study did not receive

[1]

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Table 1 Swelling power, water solubility index, and amylose leaching of oca, potato, and maize starches

Sample Swelling power (g/g)Water solubility index

Amylose leaching (%)

(%) 37.1 ± 0.7 c

60.6 ± 1.3 a

25.3 ± 0.6 a

AD

41.6 ± 0.4 b

61.0 ± 1.3 a

23.5 ± 1.6 a

Potato

45.5 ± 1.4 a

27.3 ± 1.2 b

Maize

12.9 ± 0.1 d

10.0 ± 0.2 c

19.1 ± 1.1 b 17.1 ± 0.9 b

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Red

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Red and AD represent the two oca starches; the measurements were done at 85 oC; values in

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the same column with the different letters differ significantly (p < 0.05)

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Journal Pre-proof Table 2 Gelatinization and retrogradation properties of oca, potato, and maize starches measured by DSC Sample

Gelatinization

Retrogradation

To (°C)

Tp (°C)

Tc (°C)

ΔH (J/g)

Tp (°C)

RE %

Red

54.7 ± 0.3 c

59.4 ± 0.2 d

65.1 ± 0.1 c

12.9 ± 0.0 b

54.4 ± 0.4 ab

33 ± 3 b

AD

55.2 ± 0.2 c

59.8 ± 0.0 c

65.6 ± 0.2 c

11.7 ± 0.1 c

55.8 ± 0.5 a

47 ± 4 a

Potato

61.1 ±0.2 b

65.0 ± 0.2 b

70.4 ± 0.2 b

16.6 ± 0.5 a

55.7 ± 1.2 a

31 ± 1 b

Maize

68.8 ± 0.0 a

73.1 ± 0.1 a

78.1 ± 0.2 a

11.5 ± 0.0 c

53.7 ± 0.2 b

40 ± 5 ab

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Red and AD represent the two oca starches; To: Onset temperature (°C); Tp: Peak temperature (°C); Tc: Conclusion temperature (°C); ΔH:

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Enthalpy change of gelatinization (J/g); RE: Retrogradation ratio (ΔHr/ΔH); values in the same column with the different letters differ

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significantly (p < 0.05)

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Journal Pre-proof Table 3 Dynamic rheological properties of oca, potato, and maize starches Sample

Heating (40 → 90 °C)

Cooling (90 → 25 °C)

TG'max (°C)

G'max (Pa)

tan δG'max

G'90C (Pa)

tan δ90C

G'25C (Pa)

Red

60.8 ± 0.0 c

2685 ± 134 b

0.13 ± 0.01 b

1285 ± 78 c

0.093 ± 0.005 b

2900 ± 0 b

AD

60.8 ± 0.0 c

2830 ± 85 b

0.13 ± 0.00 b

1315 ± 21 c

0.095 ± 0.001 b

3045 ± 64 b

Potato

67.8 ± 0.0 b

2895 ± 64 b

0.23 ± 0.01 a

1720 ± 0 b

0.156 ± 0.003 a

2400 ± 0 c

Maize

76.9 ± 0.0 a

4643 ± 180 a

0.11 ± 0.00 c

3013 ± 156 a

0.079 ± 0.001 c

8490 ± 740 a

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G'40Hz (Pa)

tan δ40Hz

0.050 ± 0.002 b

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3460 ± 354 b

0.20 ± 0.02 b

0.049 ± 0.001 b

3660 ± 170 b

0.19 ± 0.02 b

0.093 ±0.002 a

2710 ± 14 b

0.27 ± 0.02 a

0.030 ± 0.000 b

7570 ± 928 a

0.10 ± 0.01 c

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tan δ25C

Frequency sweep (0.1 → 40 Hz)

Red and AD represent the two oca starches; TG'max: Temperature where G' reaches the maximum during heating; G'max: Maximum storage

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modulus during heating; tan δG'max: Loss tangent when G'max is reached; G'90C: Storage modulus at 90 °C during heating; tan δ90C: Loss tangent at

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90 °C during heating (dimensionless); G'25C: Storage modulus at 25 °C; tan δ25C: Loss tangent at 25 °C (dimensionless); G'40Hz: Storage modulus value at 40 Hz; tan δ40Hz: Loss tangent at 40 Hz (dimensionless); the solid content of the sample was 20 %; values in the same column with

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different letters differ significantly (p < 0.05)

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Journal Pre-proof Table 4 Pasting and gel textural properties of oca, potato, and maize starches Sample

Pasting property Ptemp (°C)

Gel textural property

PV (Pa·s)

HPV (Pa·s)

BD (Pa·s)

CPV (Pa·s)

SB (Pa·s)

Red

64.3 ± 0.0 c

13.0 ± 0.2 c

2.16 ± 0.05 b

10.8 ± 0.1 c

4.02 ± 0.05 b

1.86 ± 0.05 b

AD

64.3 ± 0.0 c

13.7 ± 0.0 b

2.15 ± 0.01 b

11.6 ± 0.0 b

3.98 ± 0.05 b

1.83 ± 0.04 b

Potato

67.3 ± 0.0 b

18.2 ± 0.3 a

3.10 ± 0.03 a

15.1 ± 0.3 a

5.93 ± 0.02 a

Maize

78.2 ± 0.0 a

2.7 ± 0.0 d

1.48 ± 0.04 c

1.3 ± 0.0 d

2.93 ± 0.04 c

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Adhesiveness

Cohesiveness

Gumminess (g)

(g·s)

53 ± 13 ab

-297 ± 13 b

0.52 ± 0.02 b

27 ± 6 b

48 ± 10 b

-286 ± 10 b

0.51 ±0.05 b

24 ± 6 b

2.83 ± 0.02 a

68 ± 23 a

-391 ± 35 c

0.62 ± 0.05a

41 ± 11 a

1.45 ± 0.04 c

41 ± 5 b

-246 ± 33 a

0.49 ± 0.06 b

20 ± 3 b

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e

r P

Hardness (g)

Red and AD represent the two oca starches; Ptemp: Pasting temperature; PV: Peak viscosity; HPV: Hot paste viscosity; BD: Breakdown viscosity

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(PV − HPV); CPV: Cool paste viscosity; SB: Setback viscosity (CPV−HPV); Hardness: The maximum force recorded during the first

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compression; Adhesiveness: Negative area under curve between the two compressions; Cohesiveness: Ratio of the positive area under curve during the second compression to that during the first compression (dimensionless); Gumminess: Hardness × cohesiveness; the solid content of the sample was 9 %; values in the same column with different letters differ significantly (p < 0.05)

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Journal Pre-proof Table 5 Steady flow properties of oca, potato, and maize starch pastes Increasing shearing rate (0.1 → 1000 s-1) Power law K (Pa·sn)

Red

77 ±1 b

R2

n

0.26 ± 0.01 0.99 ± 0.00 20 ± 1.2 a b

82 ± 1 b

0.25 ± 0.00 0.99 ± 0.00 13 ± 0.6 b a

0.29 ± 0.01 b 0.99 ± 0.00 a

71 ± 4 b

0.27 ± 0.01 b 0.99 ± 0.00 a

272 ± 24 a 0.19 ± 0.01 c 0.99 ± 0.00 a

a

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2±0c

59 ± 9 b

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133 ± 7 a 0.27 ± 0.01 0.98 ± 0.00 158 ± 17 d b

Maize

R2

a

b Potato

n

0.46 ± 0.02 0.99 ± 0.00 -0.3 ± 0.2 c a

a

2.2 ± 0.2 c 0.46 ± 0.01 a 0.99 ± 0.00 a

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AD

K (Pa·sn)

σ0 (Pa)

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Sample

Herschel-Bulkley

K (Pa·sn)

Red

10 ± 1 b

n

a

AD

11 ± 1 b

23 ± 1 a

Maize

K (Pa·sn)

n

R2

22 ± 0 a

4.7 ± 0.2 b 0.64 ± 0.00 a 0.99 ± 0.00 a

21 ± 2 ab

5.4 ± 0.9 b

a

0.50 ± 0.00 0.99 ± 0.00 ab

σ0 (Pa)

Herschel-Bulkley

a

0.51 ± 0.01 0.99 ± 0.00 a

Potato

R2

0.52 ± 0.01 0.98 ± 0.00

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Sample

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Power law

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Decreasing shearing rate (1000 → 0.1 s-1)

a

0.60 ± 0.07 0.99 ± 0.00 a ab

18 ± 1 b

17 ± 0.3 a

0.55 ± 0.00 0.99 ± 0.00 a ab

1.9 ± 0.2 c 0.48 ± 0.01 0.99 ± 0.00 -0.24 ± 0.2 c 2.1 ± 0.1 c 0.47 ±0.01 b 0.99 ± 0.00 a b

a

Red and AD represent the two oca starches; K: Consistency coefficient; n: Flow behaviour index (dimensionless); R2: Coefficient of determination (dimensionless); σ0: Yield stress; the

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Journal Pre-proof solid content of the sample was 5 %; values in the same column with different letters differ

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significantly (p < 0.05)

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Journal Pre-proof CRediT authorship contribution statement Fan Zhu: Conceptualization, Methodology, Writing - original draft, Writing - Review & Editing, Supervision. Rongbin Cui: Methodology, Investigation, Writing - Review &

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Editing.

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Journal Pre-proof Highlights

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Rheological and thermal properties of oca starch were systematically studied Oca starch has lower gelatinization temperatures than potato and maize starches Oca starch paste has more elastic component than potato starch paste Pasting viscosity of oca starch was intermediate between potato and maize starches Amylopectin internal chain composition partially explained oca starch properties

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    

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