DSpace at VNU: Physicochemical characteristics and in vitro digestibility of potato and cassava starches under organic acid and heat-moisture treatments

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Title: Physicochemical Characteristics and in vitro

Digestibility of Potato and Cassava Starches under Organic

Acid and Heat-Moisture Treatments

Author: Pham Van Hung Nguyen Thi Mai Huong Nguyen Thi

Lan Phi Nguyen Ngoc Thanh Tien

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Physicochemical Characteristics and in vitro Digestibility of Potato and

Cassava Starches under Organic Acid and Heat-Moisture Treatments

Pham Van Hung1*, Nguyen Thi Mai Huong2, Nguyen Thi Lan Phi3, Nguyen Ngoc Thanh Tien1

1 School of Biotechnology, International University, Vietnam National University in HoChiMinh

City, Quarter 6, Linh trung Ward, Thu Duc District, HoChiMinh City, Vietnam

2 Institute of Biotechnology and Food Technology, Industrial University of HoChiMinh City, 12

Nguyen Van Bao, Ward 4, Go Vap District, HoChiMinh City, Vietnam

3 Faculty of Chemical Engineering, HoChiMinh City University of Technology, 268 Ly Thuong

Kiet Street, District 10, HoChiMinh City, Vietnam

*Corresponding author: Dr Pham Van Hung, School of Biotechnology, International University, Vietnam National University, Quarter 6, Linh Trung Ward, Thu Duc District, HoChiMinh City, Vietnam Tel: +84-8-37244270 E-mail: pvhung@hcmiu.edu.vn

Abstract

A combination of acid (citric acid or lactic acid) and heat-moisture treatment was used to

modify cassava and potato starches in this study Changes in physicochemical properties and in

vitro digestibility of the treated starches were investigated The cassava starch contained 17.0%

amylose and possessed A-type crystallinity, whereas the potato starch had 27.4% amylose and possessed B-type crystallinity After acid and heat-moisture treatment, the crystalline structure of the cassava starch remained unchanged (A type), while the crystalline structure of the potato starch changed from B type to the C (B + A) type The acid and heat-moisture treatment increased gelatinization temperature, peak and final viscosities of cassava starch but reduced peak and breakdown viscosities of the potato starch After acid and heat-moisture treatment, rapid digestible starch contents of the treated cassava and potato starches were significantly reduced However, resistant starch (RS) contents of the treated starches significantly increased as compared to the native starches Citric acid was found to have high impact on formation of RS in starches The RS contents of cassava and potato starches obtained under the citric acid and heat-moisture treatment were 40.2% and 39.0%, respectively, two times higher than those of the native starches

Keywords: Heat-moisture treatment; Starch digestibility; Citric acid

1 Introduction

Cassava and potato, starch-containing tubers, are main materials for producing starches which have been widely used in both food and non-food industries Cassava starch exhibits the A-type crystalline structure, whereas potato starch has the B-type [1] Cassava starch has lower amylose content, smaller granular size and higher gelatinization temperature than does the potato starch [2] In native form, the difference in enzymatic susceptibility of starches was due to the crystalline structure, granular structure, amylose:amylopectin ratio, amylose chain length and linearization of amylopectin [3] The native cassava starch was found to be more susceptible to α-amylase than the native potato starch resulting in lower amounts of resistant starch found in

the native cassava starch as compared to the native potato starch [2,4] Under heat-moisture treatment, changes such as crystalline structure, starch chain interactions, viscosity, gelatinization parameters and acid and enzyme hydrolysis were found to vary with different starch sources [5-7] The potato starch changed the X-ray pattern from B type to C type under the heat-moisture treatment, whereas the X-ray pattern of cassava starch (A type) remained unchanged [5] The heat-moisture treatment significantly increased gelatinization temperature of the potato starch more than the cassava starch, while the enthalpy of gelatinization of the potato starch significantly decreased as compared to that of cassava starch [5] Although the enzymatic susceptibility of the native potato starch was lower than that of the native cassava starch, the heat-moisture treated potato starch had higher enzymatic susceptibility than the heat-moisture treated cassava starch This may due to the crystallite disruption near the granule surface of heat-moisture treated potato starch which could facilitate the rapid entry of α-amylase into the granule interior [5] The difference in susceptibility of starch to the α-amylase resulted in the different amounts of resistant starch found in the native and heat-moisture treated starches Englyst et al [4] reported that raw potato starch with B-type crystallinity had higher amounts of resistant starch type 2 (RS2), the ungelatinized resistant starch granules, as compared to the native starch having A-type crystallinity However, the RS2 is easily converted to the digestible form when gelatinized Many studies on improvement of thermo-stable resistant starch content from various starch sources have been done [3,8] The formation of thermo-stable resistant starch usually involves partial acid hydrolysis and hydrothermal treatments, heating, retrogradation, extrusion, cooking and chemical modification [8] Hung et al [9] investigated the formation of resistant starch by debranching potato and cassava starches using pullulanase and reported that the resistant starch contents of both debranched cassava and potato starches significantly increased The debranched potato starch contained higher amount of resistant starch than did the debranched cassava starch at the same storage condition. The increase in amounts of resistant starch and slowly digestible starch under heat-moisture treatment has also been reported for cereal, tuber and legume starches such as corn, pea, potato, yam, rice and sweet potato [10-12]

Partial acid hydrolysis of starches prior to heat-moisture treatment was also found to improve resistant starch yield [13,14] Chung et al [10] reported that the increase in thermo-stable resistant starch was because of the survival of some interactions, formed during hydrothermal treatments, after gelatinization This partly restricted accessibility of the starch chain to the hydrolyzing enzyme Recently, organic acids such as acetic acid, lactic acid and citric acid have been used to partially hydrolyze starch prior to heat-moisture treatment to improve the thermo-stable slowly digestible starch (SDS) and resistant starch (RS) of several kinds of starches such

as rice starch [15] and sweet potato and yam starches [16] More various short chains, forming different crystallites that have different melting temperatures, were produced after the acid and heat-moisture treatment resulting in significant increase in SDS and RS contents in the treated starches as compared to the native starches [15,16] In addition, the physicochemical properties

of the acid and heat treated starches such as swelling power, cold-water solubility, viscosity and gel forming ability were also changed [15,16] Until now, little research on resistant starch formation using a combination of organic acid and heat-moisture treatments has been reported for tuber starches, especially for cassava and potato starches Therefore, the objective of this study is to investigate the impact of a combination of organic acid (citric acid or lactic acid) and

heat-moisture treatment on physicochemical properties and in vitro digestibility of potato and

cassava starches

2 Materials and Methods

2.1 Materials

Two kinds of fresh tubers, cassava (Manihot esculenta) and potato (Solanum tuberosum)

grown in the southern part of Vietnam were used in this study and their starches were directly

isolated from the fresh cassava roots and potato tubers

Alpha-amylase from A oryzae (~ 30 U/mg, product #10065), amyloglucosidase from A

niger (≥ 300 U/mL, product #A7095) and cellulase from A niger (~0.8 U/mg, product #22178)

were purchased from Sigma-Aldrich Co (St Louis, MO, US) Other chemicals were purchased from Merck Co (Darmstadt, Germany)

2.2 Starch isolation

The starch was isolated according to the method of Lawal [17] with minor modification The raw cassava roots and potato tubers were washed with tap water to remove any type of contamination and ground using a Waring blender (7015N, Waring® Commercial, USA) Then the slurry was kept in a solution of 4.5% NaHSO3 for 30 min and passed through the sieves (0.232 and 0.105 mm in aperture size) Resultant starches were washed thoroughly in clean water for three times to remove the contaminant substances Finally, the starch sediment was recovered

by centrifugation at 1500 ×g for 20 min and dried in an oven at 40 oC to 10–11% moisture

2.3 Determination of chemical composition of the isolated starches

After isolation, the chemical composition of starches was analyzed to check their purity Amylose content of starch was determined according to the method previously descried by Hung

et al [2] The chemical composition of the isolated starches was determined using the AACC Approved methods [18]: Protein (Method 46-10), lipid (Method 30-10), Ash (Method 08-01) Total starch was calculated as follows: total starch (%, in dry basis (db)) = 100% - protein content (%, db) – lipid content (%, db) – ash (%, db)

2.4 Organic acid and heat-moisture treatment of starch

A combination of acid and heat-moisture treatment was carried out as previously reported by Hung et al [16] The isolated potato and cassava starches (100 g) were dispersed in a measured volume of each acid solution (0.2M lactic acid or 0.2M citric acid) to reach the moisture content

of sample of 30% in a screw capped bottle Then the bottle was equilibrated at room temperature for 24 h before heating at 110 ºC for 8 h After heat-moisture treatment, the starch samples were neutralized with 1M sodium hydroxide, washed thoroughly with distilled water and recovered by

centrifuging at 10,000×g for 30 min The treated starches were then dried at 40 oC for 24 h until the moisture content was less than 10%

2.5 Scanning electron microscopy (SEM)

Morphology of native and treated starch granules was observed using a Hitachi scanning electron microscope (SEM) apparatus (model S-800, Tokyo, Japan) at an accelerating potential

of 10 kV [2] The starch granules were firstly dispersed in 95% ethanol and then sprinkled on the surface of an aluminum stub The aluminum stub containing sample was coated with Pt/Pd and the microphotographs of starch granules were taken using the Hitachi SEM apparatus

2.6 Determination of X- ray crystallization of starch

X-ray diffraction analysis was performed using an X-ray diffractometer (Rigaku Co., Ltd, Rint-2000 type, Tokyo, Japan) operated at 40 kV and 80 mA Diffractograms were obtained from 2o 2θ to 35o 2θ with a scanning speed of 8o/min and scanning step of 0.02o [2]

2.7 Determination of thermal characteristics of starch

Thermal characteristics of the native and treated starches were determined using a differential scanning calorimeter (DSC) (DSC-60, Shimadzu, Japan) according to the method of Hung et al [2] A sealed aluminum vessel containing 3 mg and 10 mL of distilled water was heated from 30 to 100 oC at a rate of 10 oC/min The initial, peak and conclusion temperatures and transition enthalpy were automatically calculated using a TA-60WS program (Shimadzu Co., Kyoto, Japan) An empty vessel was used as a reference

2.8 Determination of pasting properties of starch

Pasting properties of the native and treated starches were measured using a Rapid Visco Analyser (RVA-TecMaster, Newport Scientific Pty Ltd, Warriewood, Australia) Starch (2.00 g) was suspended in 25 mL of water, corrected for 14% moisture, by jogging 10 times with a

paddle The procedure used was: 50 oC for 1 min, heating to 95 oC at 12 oC/min, maintained for 2.5 min, and cooling to 50 oC at 12 oC/min Peak temperature, peak and final viscosities, breakdown, and setback were recorded Viscosity was expressed in RVU

2.9 In vitro starch digestibility assay

In vitro starch digestibility assay based on the method of Englyst et al [4] was used to

determine percentages of digestive starch fractions including rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) of the native and treated potato and cassava starches Starch (0.3 g, db) was mixed with 20 mL of sodium acetate buffer (pH 6.0) and then boiled for 30 min in a water bath The sample was equilibrated at 37 oC for 15 min and then

an enzyme solution (5 mL) of α-amylase (1,400 U/mL) and amyloglucosidase (13 AGU/mL) was added The starch solution was incubated with shaking at 37 oC for 120 min The total glucose concentrations of the 20 min-digested and 120 min-digested hydrolysates (G20 and G120, respectively) were determined using the phenol-sulfuric acid method The remaining residue was intensively hydrolyzed with 7M KOH and then with amyloglucosidase (50 AGU/mL) The final hydrolysate was then determined for total glucose concentration (TG) The values obtained for

G20, G120 and TG were used to calculate for RDS, SDS and RS as follows

RDS = G20 × 0.9 SDS = (G 120 – G 20 ) × 0.9

RS = (TG – G 120 ) × 0.9

2.10 Statistical analysis

All tests were performed at least in duplicate Analysis of variance (ANOVA) was performed

using Duncan’s multiple-range test to compare treatment means at P < 0.05 using SPSS software

(SPSS Inc., USA)

3 Results and Discussion

3.1 Chemical composition of isolated starches

The chemical composition (protein, lipid, ash and carbohydrate) of the native potato and cassava starches isolated from raw potato tubers and cassava roots are given in Table 1 The protein content of the native potato and cassava starches were 0.9% and 0.8%, respectively Lipid and ash contents in the isolated starches were also low As a result, purity of the isolated potato and cassava starches was high with 98.2% and 98.6% starch, respectively

Table 1 also shows that the amylose content of the potato starch was 27.4%, significantly higher than that of the cassava starch (17.0%) The potato starch also had higher blue value and degree of polymerization as compared to those of cassava starch The results in this study are consistent with the previous studies as reviewed by Hoover [1], who reported that potato and cassava starches had 25.4% and 18.6-23.6% amylose, respectively Hoover [1] also reviewed that the amylose of tuber and root starches had number degree of polymerization of 1540-8025, while the average chain length of amylopectin of tuber and root starches were 19-44

3.2 Scanning electron microscopy of native and treated starch granules

Morphology of native and treated potato and cassava starches is observed in Fig 1 Cassava starch had both polygonal- and spherical-shaped granules with the size less than 50 µm The potato starch contained both large and small granules, which had spherical to oval shapes The surface of the native cassava and potato starch granules was smooth with no evidence of cracks because of the low level of starch damage during isolation Although the starch granules were not broken under heat-moisture treatment, the surface layers of the heat-moisture treated starch granules were gelatinized and cracked with disc-like depressions on the surface of some granules The formation of cracks on the surface with hollow inside granules of the treated maize and potato starches were also observed by Kawabata et al [19] However, Gunaratne et al [5] reported that the heat-moisture treatment did not alter the size or shape of the starch granules of tuber starches including cassava and potato starches The different results obtained were due to the difference in moisture content and heating temperature used in those studies [20] The results

in Fig 1 indicate that lactic acid had a strong effect on the starch granules by breaking down some of the starch granules into smaller pieces, while the citric acid did not break down the starch granules Citric acid might disrupt the crystalline structure and create intermolecular cross-linking both in the amorphous and crystalline regions when it penetrates the granules through channels and cavities resulting in inhibiting the granule swelling [21] As a result, the combination of acid and heat-moisture treatments changed the structure of starch granules and might affect the physicochemical properties and digestibility of the starches

3.3 X-ray crystallization of native and treated starches

X-ray diffraction patterns of the native and treated cassava and potato starches are shown in Fig 2 The native cassava and potato starches exhibited the A-type and B-type crystalline structures as classified by Zobel [22] After heat-moisture treatment, the crystalline structure of the cassava starch remained unchanged (A type) with the main peaks at the positions of 2a, 2b, 3b, 4a, 4b, 5a, 6a and 7 That of the potato starch changed from B type to the C (B + A) type showing by the peaks of 3b and 6a These results are consistent with previous studies which reported that the heat-moisture treatment changed the X-ray diffraction pattern from B type to C type of both potato and yam starches, while the A type crystalline structure of cassava and sweet potato starches was not changed [5,16] The combination of acid and heat-moisture treatment had a similar influence on crystalline structure as the heat-moisture treatment alone The acid and heat-moisture treated cassava starch had the A-type crystalline structure, while the treated potato starch had the C-type crystallinity for both lactic acid and citric acid treatments Thus, the crystalline structure of the treated starch granules was mostly affected by the rearrangement of intermolecular and double helical structures in the granules during heat-moisture treatment The partial hydrolysis of starches by lactic acid or citric acid did not affect the crystalline structure of starches [16]

3.4 Differential scanning calorimetry of native and treated starches

Thermal characteristics of the native and treated potato and cassava starches determined by differential scanning calorimetry (DSC) are shown in Table 2 The native cassava starch had higher gelatinization temperature (65.16 - 74.73 oC) than did the native potato starch (61.20 - 69.71 oC), whereas the enthalpy of the endothermic peak of the native cassava starch was significantly lower than that of the native potato starch After heat-moisture treatment, the gelatinization temperature of the treated potato and cassava starches increased but the enthalpy

of the treated starches decreased as compared to the native starches Similar results have been also reported by Gunaratne et al [5] The changes in thermal characteristics of starches under heat-moisture treatment could be explained by (a) the reduction of destabilization effect of the amorphous region on crystallite melting and/or (b) dissociating the double helices present in crystalline and in non-crystalline regions of the granule to form new polymer chains in the heat-moisture treated starches [5] The gelatinization temperature and enthalpy of the cassava starch treated with lactic acid (LA-HMT CS) were significantly lower than those of the heat-moisture treated cassava starch (HMT CS), whereas the thermal characteristics of cassava starches with and without citric acid treatment (CA-HMT CS and HMT CS, respectively) were not significantly different In contrast, the potato starch treated with citric acid had significantly lower gelatinization temperature and enthalpy as compared to the heat-moisture treated potato starch, while the potato starch treated with lactic acid did not significantly differ Thus, the lactic acid had a strong impact on the cassava starch by breaking down the starch granule and decreasing gelatinization temperature and enthalpy, while the citric acid attacked both amorphous and crystalline regions of the potato starch to produce more short chain without breaking down the starch granules The low enthalpy of the treated potato starch as compared to the cassava starch was due to the easier disruption of the crystallites formed by double helical chains of B-type starches than those of A-type starches [5]

3.5 Viscosity of native and treated starch pastes