Changes in resistant starch from two banana cultivars during postharvest storage

Food Chemistry 156 (2014) 319–325

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Changes in resistant starch from two banana cultivars during postharvest storage Juan Wang, Xue Juan Tang, Ping Sheng Chen, Hui Hua Huang ⇑ College of Light Industry and Food Science, South China University of Technology, Wushan Road No. 381, Guangzhou 510641, China

a r t i c l e

i n f o

Article history: Received 5 September 2013 Received in revised form 15 January 2014 Accepted 3 February 2014 Available online 12 February 2014 Keywords: Banana resistant starch Physicochemical properties Ripening stage Starch structure

a b s t r a c t Banana resistant starch samples were extracted and isolated from two banana cultivars (Musa AAA group, Cavendish subgroup and Musa ABB group, Pisang Awak subgroup) at seven ripening stages during postharvest storage. The structures of the resistant starch samples were analysed by light microscopy, polarising microscopy, scanning electron microscopy, X-ray diffraction, and infrared spectroscopy. Physicochemical properties (e.g., water-holding capacity, solubility, swelling power, transparency, starch–iodine absorption spectrum, and Brabender microviscoamylograph profile) were determined. The results revealed significant differences in microstructure and physicochemical characteristics among the banana resistant starch samples during different ripening stages. The results of this study provide valuable information for the potential applications of banana resistant starches. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Resistant starch (RS) is defined by EURESTA (European FLAIR Concerted Action on Resistant Starch) as ‘‘the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals’’ (Asp, 1992). There are four types of RS: (1) physically entrapped, inaccessible starch within whole or partially milled seeds (RS1); (2) native granular starch, consisting of nongelatinised granules (RS2); (3) retrograded starch produced by food processing applications (RS3); and (4) chemically modified starch (RS4; Englyst, Kingman, & Cummings, 1992). RS has physiological effects similar to those of prebiotics and dietary fibre: RS stimulates the growth of beneficial bacteria in the gut (e.g., bifidobacteria) and increases the production of short-chain fatty acids associated with gut immune function and microbiota modulation (Fuentes-Zaragoza et al., 2011; Johnson & Gee, 1996). Additionally, RS protects against several diseases, including type II diabetes, colorectal cancer, and other diet-related chronic diseases (Niba, 2002; Topping & Clifton, 2001). Several studies have focused on the functions of RS. Mutungi, Rost, Onyango, Jaros, and Rohm (2009) studied the crystallinity and the thermal and morphological characteristics of RS3 from debranched cassava starch. Garcia-Rosas et al. (2009) assessed the changes in maize tortilla RS content and structure during storage. Aparicio-Saguilan et al. (2007) successfully prepared ⇑ Corresponding author. Tel.: +86 2087112851. E-mail address: [email protected] (H.H. Huang). http://dx.doi.org/10.1016/j.foodchem.2014.02.012 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.

slow-digestible cookies from RS-rich lintnerised banana starch. In addition, Aparicio-Saguilan, Gutierrez-Meraz, Garcia-Suarez, Tovar, and Bello-Perez (2008) investigated the physicochemical and functional properties of cross-linked banana RS. Ble-Castillo et al. (2008) reported that banana RS flour supplementation reduces body weight and insulin resistance in obese individuals with type II diabetes. Unripe bananas are rich in RS2 (Johnson & Gee, 1996; Niba, 2002). As tropical and subtropical fruits, bananas are mainly planted in tropical and subtropical zones. RS isolated from different banana cultivars may have different properties. Moreover, with post-harvest storage, numerous enzymes transform the starches in these fruits into different sugars. Consequently, the RS content of bananas at different ripening stages may differ. However, few studies have focused on the changes of banana RS throughout storage. This study assessed the changes in the content, physicochemical and structural properties of RS from two banana cultivars (Musa AAA group, Cavendish subgroup and Musa ABB group, Pisang Awak subgroup) at different stages of maturity. 2. Materials and methods 2.1. Materials Two banana cultivars were used in this study: Musa AAA group, Cavendish subgroup and Musa ABB group, Pisang Awak subgroup. These cultivars were planted and sold in Guangdong province, China. Using the criteria reported by SH Pratt Co. (Luton, United

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Kingdom; Soltani et al., 2011), the fruits were divided into seven ripening states: 1-entirely green; 2-green with a trace of yellow; 3-more green than yellow; 4-more yellow than green; 5-yellow with a trace of green; 6-entirely yellow; 7-entirely yellow with brown speckles. Bananas at ripening stage 1 were selected and purchased from a local market. Subsets of these bananas were stored at room temperature for different periods of time to attain ripening stages 2–7. However, only bananas at stages 1–5 were used in this study because RS samples from bananas at stages 6 and 7 were colloid-like substances that were difficult to grind. All chemicals used in this study were of analytical grade. 2.2. Isolation and determination of banana resistant starch (BRS) content Using the method reported by Cheng Yanfeng et al. (2008), bananas were peeled, pulped, and digested with pectinase and amylase to remove pectin, cellulose, protein, and digestible starch. The digested banana pulp was centrifuged at 3 000 rpm for 15 min; the resulting precipitate was dehydrated at 50 °C, ground, and stored at 5 °C. RS content was determined by the method reported by Goni, Garcia-Diz, Manas, & Saura-Calixto (1996). Briefly, the method consisted of the removal of protein and digestible starch, the solubilisation and enzymatic hydrolysis of RS, and the quantification of RS. Human gastric and intestinal conditions (pH and transit time) were simulated. 2.3. Structural observations of BRS 2.3.1. Light microscopy and polarising microscopy BRS samples were dissolved in glycerol (50% concentration) and observed under a microscope (Vanox BHS-2, Olympus Corporation, Japan) using both natural and polarised light. 2.3.2. Scanning electron microscopy (SEM) Particles of BRS powder were scanned, using a S3700N scanning electron microscope (Hitachi, Japan). Samples were fixed on an objective table coated with platinum (10–20 nm thickness). 2.3.3. X-ray diffraction (XRD) Cu Ka radiation was used to scan BRS samples over the 2h = 4– 60° range, with a step interval of 0.04°, a scanning rate of 17.7 s per step, a voltage of 40 kV, and a current of 40 mA. The D8 ADVANCE X-ray diffractometer from Bruker Corporation (Germany) was used for the XRD analyses. 2.3.4. Infrared spectroscopy BRS samples were pressed in KBr. An infrared spectrometer (VECTOR33, Bruker Corporation, Germany) was used to scan the samples from 4000 cm1 to 400 cm1 of the infrared region. 2.4. Physicochemical properties of BRS 2.4.1. Water-holding capacity (WHC) WHC was measured by the method reported by Toyokawa, Rubenthaler, Powers, and Schanus (1989). Briefly, 20 ml of starch suspension (5 g/100 ml) were transferred to centrifuge tubes and heated in a water bath for 15 min at 50 °C, 70 °C, or 90 °C. The tubes were centrifuged at 3,000 rpm for 15 min. The supernatant was discarded; tubes containing sediment were placed at a 45° angle for 10 min to allow water drainage and weighed. WHC was calculated by Eq. (1).

WHC ð%Þ ¼

m2  m1  m0  100% m0

ð1Þ

where m0 is the weight of the starch sample, m1 is the weight of the centrifuge tube, and m2 is the weight of the starch sample and centrifuge tube following water drainage. 2.4.2. Solubility and swelling power Solubility (S) and swelling power (SP) were determined, using the method reported by Aparicio-Saguilán et al. (2005). In this experiment, 20 ml of starch suspension (5 g/100 ml) were transferred to centrifuge tubes and heated in a water bath for 30 min at 50 °C, 70 °C, or 90 °C. After the tubes had cooled to room temperature, they were centrifuged at 3000 rpm for 15 min. The sediment and supernatant were separated; the sediment was dried and weighed. S and SP were calculated using Eqs. (2) and (3), respectively.

S ð%Þ ¼

A  100% W

SP ð%Þ ¼

D  100% Wð1  SÞ

ð2Þ

ð3Þ

where A is the weight of dry dissolved solids in the supernatant, W is the weight of the sample, and D is the weight of the sediment. 2.4.3. Transparency An aqueous starch solution was preparing by mixing 1.0 g of starch with 99.0 g of water. This solution was heated in a boiling water bath for 15 min under continuous stirring and subsequently cooled to room temperature. The transparency of the resulting starch paste was detected at 620 nm (UV-1800 spectrophotometer, Shimadzu Co., Japan). Distilled water was used as a blank control, which was considered to have a transparency of 100%. 2.4.4. Starch–iodine absorption spectra Spectra of iodine-bound starch samples were determined, using the method reported by Klucinec and Thompson (1998). BRS (50 mg) was dispersed into 10.0 ml of DMSO containing 10% of 6.0 M urea. Subsequently, 2.0 ml of the dispersed solution, 25 ml of distilled water, and 1.0 ml of I2-KI (2.0 mg I2/ml and 20.0 mg KI/ml) were pipetted into a 50 ml volumetric flask and mixed. The mixed solution was brought to a volume of 50 ml with distilled water. Control solutions were prepared without BRS. A UV–visible spectrophotometer (UV-1800, Shimadzu Co., Japan) was used to scan each sample from 500 to 800 nm; kmax for each sample was defined as the wavelength that resulted in the highest absorbance value. 2.4.5. Pasting properties A microviscoamylograph (Visgraph-E, Brabender Instruments, Inc., Germany) was used to determine the viscosity profiles (in Brabender units, BU) of the starch samples. Dispersions of BRS (6%, dry basis) were transferred to the microviscoamylograph and subjected to thorough agitation. The dispersion was brought to an initial temperature of 30 °C and subsequently to 95 °C at a rate of 1.5 °C/min. The temperature of the dispersion was maintained at 95 °C for 30 min; subsequently, the dispersion was cooled to 50 °C at a rate of 1.5 °C/min and maintained at 50 °C for 30 min (Aparicio-Saguilán et al., 2005). 2.5. Statistical analyses Data were analysed by the SPSS statistical software package, v19.0 (IBM company). Data were expressed as means ± standard deviation. One-way analysis of variance (ANOVA) was used to compare the different BRS samples, Levene’s test was used to assess homogeneity of variances, and the Bonferroni test was used for multiple comparisons. Statistical significance was set at

J. Wang et al. / Food Chemistry 156 (2014) 319–325

P < 0.05. Values followed by the same letter in the same row or column are not significantly different (P < 0.05) in the tables. 3. Results and discussion 3.1. Changes in BRS content during ripening BRS content gradually decreased during storage. Cavendish BRS content decreased rapidly during the first four ripening stages but decreased slowly during the final three ripening stages. In contrast, Pisang Awak BRS content decreased slowly during the initial three ripening stages but decreased rapidly during the final four ripening stages. At the same ripening stage, Pisang Awak bananas consistently had higher BRS content than had Cavendish bananas. There were significant differences in BRS content between the two banana cultivars, a result that may be attributed to differences in enzymatic reactions that convert starches into sugars. The reaction rate of Cavendish bananas was possibly faster, thereby contributing to a rapid reduction in BRS content. 3.2. Structural changes in BRS during ripening 3.2.1. Light microscopy and polarising microscopy Most starch particles in the Cavendish cultivar were oval in shape, whereas others were spindly. In contrast, starch granules in the Pisang Awak cultivar were generally round in shape, while others were triangular (Figs. 1a and b). In both cultivars, the edges of the starch particles were completely intact and sharply defined during the first ripening stage. A subset of starch particles degraded at subsequent ripening stages; this phenomenon might be attributed to the enzymatic hydrolysis of banana starches. Starch particles were observed by polarising microscopy (Figs. 1c and d). Maltese crosses were evident in these particles under polarised light, with certain particles exhibiting cross patterns and other particles displaying X-shaped patterns. The points of these crosses were located at the top and end of the starch particles. In the Cavendish cultivar, the Maltese crosses became weak at ripening stages 4 and 5; in the Pisang Awak cultivar, the crosses became weak at ripening stage 5. This result revealed that there were differences in the crystalline structures of RS between the two cultivars. 3.2.2. SEM Fig. 1e and f shows the microscopic appearance of BRS from Cavendish and Pisang Awak. Most starch particles had smooth surfaces during the initial ripening stages, whereas enzymatic effects caused starch particle surfaces to become rough and wrinkled during post-harvest ripening. At ripening stage 5, there were more degraded and broken starch granules in the Cavendish cultivar than in the Pisang Awak cultivar, suggesting that maturation processes occurred more quickly in Cavendish bananas than in Pisang Awak bananas. This result revealed that enzymatic hydrolysis in Cavendish bananas was faster than that in Pisang Awak bananas. 3.2.3. XRD XRD analyses revealed three major diffraction peaks for Cavendish BRS and four major diffraction peaks for Pisang Awak BRS. A reduction in the diffraction peaks during ripening indicates a loss in the crystallinity of starch structures. The crystallinities are summarised in Table 1. In the Cavendish cultivar, the relative degree of crystallinity of RS decreased very slowly from ripening stages 1–2 and diminished rapidly during ripening stages 3 and 4; the differences were significant. In the Pisang Awak cultivar, the relative degree of crystallinity of RS decreased from ripening stages 1–4; the differences were significant. In both cultivars, the crystallinity of

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RS decreased slowly from ripening stages 4–5; however, the differences were significant. Crystallinity, which represents the degree of structural order, has significant effects on hardness, density, transparency, and diffusion (Oxford dictionary of science., 1999). In the two banana cultivars, RS crystallinity was not reduced at the same rate, suggesting that the crystallization behaviour in starches is possibly different among banana varieties. 3.2.4. Infrared spectroscopy Similar IR spectra of RS were obtained at all five ripening stages for Cavendish and Pisang Awak, indicating that the characteristic functional groups of RS were not affected by the maturation process (Fig. 2). The broad band at 3400 cm1 is caused by OH groups, whereas the band at 2800 cm1 is generated by CH2 groups (Garcia-Rosas et al., 2009). The peak at 1600 cm1 results from carboxylate ion (COO) stretching vibrations in carboxylate groups. The IR bands at 1300 cm1 and 1022 cm1 are produced by C–O–H bending and C–O–H bending vibrations, respectively. The skeletal modes of the pyranose ring generate the peak at 620 cm1. 3.3. Physicochemical properties of BRS 3.3.1. WHC The WHC of BRS at 50 °C, 70 °C, and 90 °C is shown in Table 2. In general, higher WHC was obtained at higher temperatures. In the Cavendish cultivar, higher WHC were observed during ripening stage 4. In the Pisang Awak cultivar, BRS had the highest WHC during ripening stage 5 at 50 °C and 70 °C and during ripening stage 3 at 90 °C. Therefore, different maturity levels of Cavendish and Pisang Awak bananas should be chosen for BRS extraction and applications in accordance with the different temperature-dependent characteristics of banana fruits and relevant WHC requirements. 3.3.2. Solubility and swelling power In Cavendish, RS solubility increased with ripening (Table 2). Therefore, at 50 °C, 70 °C, and 90 °C, higher BRS solubility was observed at ripening stage 5 than at the first four ripening stages. In Pisang Awak, BRS solubility at 50 °C was higher at ripening stages 4 and 5 than at ripening stages 1–3. However, complete BRS solubility was observed at ripening stages 5 and 3 at 70 °C and 90 °C, respectively. In Cavendish, there were no significant differences in BRS swelling power among the five ripening stages. In Pisang Awak, BRS had the highest swelling power during the first ripening stage. 3.3.3. Transparency In both cultivars, BRS transparency gradually decreased during storage (Table 2). During ripening stage 5, transparency declined. At ripening stages 1 and 2, higher BRS transparency was observed for the Cavendish cultivar than for the Pisang Awak cultivar; however, the opposite was observed at ripening stages 3–5. 3.3.4. Starch–iodine absorption spectra It has been reported that the maximum absorption wavelengths are lower for amylopectin-iodine solutions than for amylose-iodine solutions (Baldwin, Bear, & Rundle, 1944). Klucinec and Thompson (1998), who focused on different fractions of high-amylose maize starches, concluded that the maximum absorption wavelengths of amylose ranges from 643 to 655 nm but that the maximum absorption wavelengths of amylopectin ranges from 559 to 583 nm. The starch–iodine absorption spectra of BRS are shown in Fig. 3. The maximum absorption wavelengths (kmax) of Cavendish BRS ranged from 560 to 580 nm, whereas the maximum absorption wavelengths of Pisang Awak BRS ranged from 570 to 610 nm, indicating that Cavendish BRS contains more amylopectin

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J. Wang et al. / Food Chemistry 156 (2014) 319–325

1

2

3

4

5

(a) Optical microscopy images (×200) of Musa AAA Cavendish BRS

1

2

3

4

5

(b) Optical microscopy images (×200) of Musa ABB Pisang Awak BRS

1

2

3

4

5

(c) Polarised light microscopy images (×200) of Musa AAA Cavendish BRS

1

2

3

4

5

(d) Polarised light microscopy images (×200) of Musa ABB Pisang Awak BRS

1

2

3

4

5

(e) Scanning electron microscopy images (×500) of Musa AAA Cavendish BRS

1

2

3

4

5

(f) Scanning electron microscopy images (×500) of Musa ABB Pisang Awak BRS Fig. 1. Optical microscopy, polarised light microscopy and scanning electron microscopy images of resistant starch samples isolated from bananas at different ripening stages. (The numbers 1–5 indicate the ripening stage of each banana sample.)

and less amylose than does Pisang Awak BRS. The maximum absorption wavelengths of Cavendish BRS at ripening stages 1–5 were similar at approximately 570 nm. However, the maximum absorption wavelengths of Pisang Awak BRS shifted as bananas matured through ripening stages 1–5. In particular, for ripening

stages 1, 2, 3, 4 and 5, the kmax of Pisang Awak BRS samples occurred at 585, 600, 605, 573, and 595 nm, respectively, suggesting that the amylopectin content of Pisang Awak BRS decreased quickly prior to ripening stage 3, causing kmax to shift to wavelengths near the maximum absorption wavelength of amylose. It

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J. Wang et al. / Food Chemistry 156 (2014) 319–325 Table 1 The relative degrees of crystallinity (%) of banana resistant starch samples.* Banana variety

Ripe stage

Musa AAA Cavendish Musa ABB Pisang Awak

1

2

3

4

5

33.8 ± 0.4a 43.1 ± 0.6a

33.6 ± 0.3a 40.6 ± 0.4b

33.1 ± 0.2b 36.9 ± 0.3c

24.4 ± 0.6c 33.5 ± 0.3d

23.5 ± 0.4d 32.9 ± 0.2e

Values followed by the same letter in the same row are not significantly different (P < 0.05). * Mean of three replicates ± standard error.

Fig. 2. Infrared spectra of resistant starch samples isolated from bananas at different ripening stages. (The numbers 1–5 indicate the ripening stage of each banana sample.)

Table 2 The water-holding capacity, solubility, swelling power, and transparency of resistant starch samples isolated from bananas at different ripening stages.* Banana variety

Water-holding capacity (%) Musa AAA Cavendish

Musa ABB Pisang Awak

Solubility (%) Musa AAA Cavendish

Musa ABB Pisang Awak

Swelling power (%) Musa AAA Cavendish

Musa ABB Pisang Awak

Transparency (%) Musa AAA Cavendish Musa ABB Pisang Awak

Temperature (°C)

Ripe stage 1

2

3

4

5

50 70 90 50 70 90

1.131 ± 0.016d 1.093 ± 0.014e 2.513 ± 0.012d 1.219 ± 0.013b 1.274 ± 0.011b 3.213 ± 0.014c

1.197 ± 0.013c 1.201 ± 0.010d 2.606 ± 0.012c 1.130 ± 0.016c 1.273 ± 0.019b 3.161 ± 0.020c

1.395 ± 0.015b 1.459 ± 0.016c 3.006 ± 0.010b 1.130 ± 0.015c 1.296 ± 0.017b 3.675 ± 0.019a

1.598 ± 0.012a 1.816 ± 0.013a 3.716 ± 0.013a 1.159 ± 0.021c 1.306 ± 0.015ab 3.543 ± 0.023b

1.569 ± 0.016a 1.700 ± 0.011b 3.034 ± 0.014b 1.432 ± 0.020a 1.345 ± 0.016a 2.190 ± 0.013d

50 70 90 50 70 90

1.50 ± 0.04e 1.69 ± 0.05e 5.31 ± 0.05e 1.30 ± 0.04d 1.50 ± 0.07d 6.99 ± 0.06c

2.19 ± 0.06 2.79 ± 0.07 5.84 ± 0.08 1.90 ± 0.03 1.90 ± 0.04 6.99 ± 0.06

2.54 ± 0.08c 3.77 ± 0.05c 6.86 ± 0.05c 2.57 ± 0.05b 3.56 ± 0.06b 9.70 ± 0.07a

3.34 ± 0.08b 4.82 ± 0.05b 7.14 ± 0.07b 3.30 ± 0.08a 3.40 ± 0.05b 8.40 ± 0.06b

6.22 ± 0.06a 6.93 ± 0.07a 9.24 ± 0.05a 3.19 ± 0.08a 4.49 ± 0.10a 6.98 ± 0.04c

50 70 90 50 70 90

88.32 ± 3.47 87.76 ± 3.27 85.13 ± 4.18 96.98 ± 2.13a 100.86 ± 2.67a 98.37 ± 2.16a

86.34 ± 3.01 89.00 ± 3.19 88.36 ± 2.96 91.98 ± 1.72ab 91.91 ± 2.19b 93.19 ± 1.70ab

84.86 ± 3.88 86.58 ± 4.35 86.77 ± 3.13 88.87 ± 1.81b 88.86 ± 1.92b 86.95 ± 1.89b

83.54 ± 3.61 84.36 ± 3.42 84.54 ± 3.82 91.25 ± 2.51ab 91.80 ± 2.22b 92.74 ± 2.14ab

84.17 ± 3.32 86.06 ± 3.50 85.16 ± 3.78 89.46 ± 2.07b 89.80 ± 2.64b 89.34 ± 2.49b

25 25

2.893 ± 0.017a 2.727 ± 0.021a

2.701 ± 0.014b 2.559 ± 0.023b

2.17 ± 0.036c 2.377 ± 0.026c

1.096 ± 0.039d 2.271 ± 0.031d

0.832 ± 0.013e 1.506 ± 0.017e

d d d c c c

Values followed by the same letter in the same row are not significantly different (P < 0.05). * Mean of three replicates ± standard error.

is likely that amylose was rapidly degraded between ripening stages 3 and 4, causing kmax to shift to 573 nm. At ripening stage 5, both amylopectin and amylose contents continued to decrease in the Cavendish and Pisang Awak cultivars, as indicated by a reduction in the starch–iodine absorption peaks with increasing banana fruit maturity.

3.3.5. Pasting properties The pasting properties of BRS were measured by a Brabender microviscoamylograph; the results are shown in Table 3. The initial gelatinisation temperatures of Cavendish BRS were similar during ripening stages 1–3, whereas the gelatinisation temperatures of Pisang Awak BRS increased as banana fruits matured. In Cavendish

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Fig. 3. Starch–iodine absorption spectra of resistant starch samples isolated from bananas at different ripening stages. (The numbers 1–5 indicate the ripening stage of each banana sample.) Table 3 Banana resistant starch viscosity parameters, as measured by the Brabender microviscoamylograph .* Banana variety

Ripe stage

A(°C)

B(BU)

C(BU)

D(BU)

E(BU)

F(BU)

B-D(BU)

E-D(BU)

E-B(BU)

Musa AAA Cavendish

1 2 3 1 2 3

78.8 ± 0.5a 78.4 ± 0.7a 79.1 ± 0.7a 65.5 ± 1.0b 79.9 ± 0.8c 80.0 ± 0.7c

498 ± 35a 469 ± 31a 466 ± 23a 302 ± 26b 270 ± 31b 266 ± 37b

490 ± 36a 467 ± 32a 462 ± 28a 300 ± 26b 263 ± 37b 262 ± 44b

377 ± 40a 311 ± 30b 328 ± 39b 240 ± 35c 209 ± 24c 230 ± 40c

588 ± 33a 501 ± 35b 496 ± 31b 329 ± 22c 292 ± 30c 345 ± 37c

519 ± 26a 445 ± 34b 442 ± 33b 293 ± 20c 272 ± 32c 314 ± 31c

121 158 138 62 61 36

211 190 168 89 83 115

90 32 30 27 22 79

Musa ABB Pisang Awak

BU = Brabender unit. Values followed by the same letter in the same column are not significantly different (P < 0.05). A-initial gelatinization temperature; B-peak viscosity (BU); C-viscosity at 95 °C (BU); D-viscosity after 30 min at 95 °C (BU); E-viscosity at 50 °C (BU); F-viscosity after 30 min at 50 °C (BU); B-D: Breakdown; E-D: Consistency. E-B: Setback. Mean of three replicates ± standard error.

*

BRS, the properties of peak viscosity after gelatinisation (point B), viscosity at 95 °C (point C), viscosity at 50 °C (point E), and viscosity after 30 min at 50 °C (point F) decreased during ripening stages 1–3, whereas the viscosity after 30 min at 95 °C (point D) was not affected. In contrast, in Pisang Awak BRS, only the peak viscosity at point B and the viscosity at 95 °C (point C) decreased during ripening stages 1–3. The viscosity of Cavendish BRS was consistently higher than the viscosity of Pisang Awak BRS at points A–F. Cavendish BRS had the highest peak viscosity (point B) during the first ripening stage. The heat stability of BRS paste determines the viscosity after 30 min of incubation at 95 °C (B-D; breakdown), whereas the cold stability of BRS determines the viscosity after 30 min of incubation at 50 °C (E-D; consistency). The setback refers to the difference between the peak viscosity after gelatinisation and the viscosity at 50 °C (Qian & Kuhn, 1999). In this study, Cavendish BRS had the highest breakdown (B-D) and consistency (E-D) values at ripening stage 2 and the maximum setback (E-B) value at ripening stage 1. Pisang Awak BRS had the highest breakdown value during the first ripening stage and the highest consistency and setback values during ripening stage 3. Moreover, higher breakdown (B-D), consistency (E-D), and setback (E-B) values were obtained from Cavendish BRS than from Pisang Awak BRS, with the exception of the setback (E-B) value at the third ripening stage.

4. Conclusions The results of this study revealed that RS content decreased during postharvest storage. At the same ripening stage, BRS content was consistently higher for the Pisang Awak cultivar than for the Cavendish cultivar. Light microscopy and SEM observations revealed that BRS particles in the two banana cultivars exhibited different microscopic characteristics. Starch particles from Cavendish BRS were oval, whereas starch granules from Pisang Awak BRS were round. The Cavendish banana fruits matured more quickly than did the Pisang

Awak banana fruits. The Maltese crosses of BRS were clear under polarised light at the beginning of storage but became weak during the ripening process. BRS crystallinity gradually decreased during the ripening process. The infrared spectra of Cavendish and Pisang Awak BRS were similar and remained relatively unaffected throughout the maturation of banana fruits, suggesting that the typical functional groups of RS were well maintained at all five examined ripening stages. Differences in WHC, solubility, swelling power, and transparency were observed in BRS from the two cultivars and at different ripening stages. Starch–iodine absorption spectra indicated that Cavendish BRS contained more amylopectin and less amylose than did Pisang Awak BRS. The degradation processes of amylopectin and amylose in the two cultivars were asynchronous. Pasting properties, determined by Brabender microviscoamylograph analyses, revealed differences in the heat stability (breakdown value), cold stability, and setback characteristics of BRS from the two cultivars at ripening stages. In general, viscosity values were higher for Cavendish BRS than for Pisang Awak BRS. These results reveal that appropriate cultivars and ripening stages should be chosen for the preparation of RS products.

Acknowledgments This research was supported by The National Natural Science Foundation of China (Grant No. 31301530), the Fundamental Research Funds for the Central Universities, SCUT(Grant No. 2013ZM0063), the Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (Grant No. LYM10016) and the Bureau of Science and Information Technology of Guangzhou, China (Grant No. 2013J4100056).

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