resistant starch, physicochemical and structural ... · resistant starch, physicochemical and...

Kasetsart J. (Nat. Sci.) 46 : 461 - 472 (2012)

Department of Nutrition and Health, Institute of Food Research and Product Development, Kasetsart University, Bangkok 10900, Thailand* Corresponding author, e-mail: [email protected]

Received date : 07/12/11 Accepted date : 18/04/12

INTRODUCTION

Bananas (Musa spp.) are one of the most important tropical fruits and are consumed worldwide by people of all age groups. The nutritional and functional properties of bananas are known to provide good health; nutritionally, bananas contain available carbohydrates which

provide energy, vitamins B and C, and signifi cant amounts of potassium and magnesium. A substantial percentage of the starch in bananas consists of resistant starch (RS), which has the potential to provide signifi cant health benefi ts similar to those derived from dietary fi ber (Aurore et al., 2009). Due to the high solid content of 40 to 70%, bananas are of interest for processing into

Resistant Starch, Physicochemical and Structural Properties ofBananas from Different Cultivars with an Effect of Ripening

and Processing

Nednapis Vatanasuchart*, Boonma Niyomwit and Wassana Narasri

ABSTRACT

The objective was to determine the functionalities of ‘Kluai Hom’, ‘Kluai Khai’, ‘Kluai Lebmuenang’ and ‘Kluai Namwa’ from the green and the green-to-yellow stages. Starch, fl our and fl ake samples were prepared and determined for resistant starch (RS), physicochemical and structural properties. Starch extraction showed the highest yield of 13.5% for Kluai Namwa and all starches from both stages had a very high RS. The starch of Kluai Namwa contained the highest RS of 77.3±1.1 to 81.1±0.4 g per 100g, while the drum-dried bananas had almost no RS. The rapid viscosity analyzer profi les showed a signifi cantly higher peak, fi nal and setback viscosities of the starches than the fl ours and the fl akes from both stages. Kluai Lebmuenang starches showed the highest peak (315.1 ± 0.7 to 318.5 ± 1.5 RVU) and fi nal (308.4 ± 2.3 to 375.5 ± 1.9 RVU) viscosities. The differential scanning calorimeter results showed that the starches from green bananas had similar thermal enthalpies from 17.1 ± 0.3 to 17.9 ± 0.2 J.g-1, while Kluai Khai in the green-to-yellow stage had the lowest enthalpy of 16.0 ± 0.1 J.g-1. Kluai Hom showed signifi cantly higher gelatinization temperatures than the others, as infl uenced by molecules of amylose and the more confi ned structure of the amylopectin crystallinity. The lowest gelatinization temperature of Kluai Lebmuenang was accounted for by its lowest relative crystallinity revealed by the X-ray diffractograms and all bananas exhibited typical B-crystallites. The electron micrographs of the starches showed irregularly shapes with elongated and oval forms and the surface looked quite smooth, but a slight blister was found only with the Kluai Hom starch. Keywords: resistant starch, physicochemical, structural properties, bananas, ripening

Kasetsart J. (Nat. Sci.) 46(3)462

fl our and starch suitable for making processed health food products. Since bananas are under-used for industrial processing, banana starch properties should be elucidated to increase banana utilization. At present, a healthy choice of functional food products is of increasing interest to consumers. RS types play a major role in the health food industry because their properties are similar to soluble and insoluble dietary fibers in the gastrointestinal tract. Furthermore, RS shows some resistance to human digestive enzymes; its slow rate of digestibility together with the slow release of glucose results in reduced energy intake by the intestinal cells, which is evident from the low glycemic index of the non-digested starch. This can help to improve glucose regulation in diabetics and facilitate weight control for the obese (Faisant et al., 1995). The non-digested starch in the large intestine is fermented by colonic microflora, producing short-chain fatty acids that encourage the growth of benefi cial bacteria, indicating a prebiotic functionality. This may lead to healthier colon cells and help prevent the development of colon cancer. In addition, a diet high in RS can reduce blood cholesterol and triglyceride levels, because of the higher excretion rates of cholesterol and bile acids (Higgins et al., 2004). Overall, increasing the RS content in the diet has the potential to provide several substantial health benefi ts and added value to food products. RS is defi ned as the sum of the starch and the products of starch degradation not absorbed in the small intestine of a healthy individual (Sajilata et al., 2006). There are four types of RS. Type I represents physically inaccessible starch which is locked in the plant cell walls of some foodstuffs, such as partially milled grains, seeds and legumes. Type II is characterized by native granular starches found in food containing uncooked starch, such as bananas, raw potatoes and beans. The RS contents in reference banana fl our samples, determined by three laboratories, averaged 52.1% (dry matter),

while lentil fl our had 8.2% (Goni et al., 1996). Type III RS is made up of retrograded starch or crystalline non-granular starch, like the starch found in cooked potatoes, bread crust, cornfl akes, and retrograded high-amylose maize starch. Type IV refers to specifi c chemically and thermally modifi ed or repolymerized starches (Englyst et al.,1992, 1996). As starchy foods are a main source of energy in the diets of Thai people, a healthier choice of starchy foods that still provide benefi cial functionalities for sustaining good health should be recommended. Therefore, it is of interest to acquire new knowledge about the functionality of different cultivars of bananas grown in Thailand and their suitability for processing into health foods. The objective of this research was to determine the effects of ripening (the green and green-to-yellow stages) and processing on the RS contents and physicochemical properties of the starch, fl our and fl ake obtained from different banana cultivars; the structural properties of banana starch granules were also examined.

MATERIALS AND METHODS

Edible unripe or green bananas from four cultivars, aged 90 to 120 d (about 15 bunches of each) were obtained from banana plantations. Different species of diploid and triploid genome groups were ‘Kluai Hom’ (AAA), ‘Kluai Khai’ (AA), ‘Kluai Lebmuenang’ (AA) and ‘Kluai Namwa’ (ABB) as shown in Figure 1 (Achavachajchuan and Maksumpun, 2007). Sampling for bananas at the green stage was undertaken on the day of harvest and the remaining bananas were ripened to the almost ripe or green-to-yellow stage before sampling again.

Preparation of banana samples For banana fl our preparation, the bananas were peeled to obtain about 2 kg and sliced into 1 mm-thick pieces, spread evenly on a stainless

Kasetsart J. (Nat. Sci.) 46(3) 463

steel tray, dried in a hot-air oven at 50 °C for 8 h, and then milled and passed through a 100-mesh sieve before analysis. Banana starch was prepared according to a water-alkaline extraction process adopted from Zhang et al. (2005) as presented in Figure 2. For banana fl ake preparation, 2 kg of banana slices were homogenized with deionized water and then dried using a drum drier at 180 °C. The dried samples were milled and passed through a 100-mesh sieve before analysis.

Determination of resistant starch Banana samples (100 mg) were hydrolyzed using 4.0 mL of pancreatic α-amylase (10 mg.mL-1) containing amyloglucosidase (3 U.mL-1) for 16 h at 37 °C and the RS recovered

from centrifugation at 3,000 rpm for 10 min. The supernatants were collected for digestible starch determination. RS was quantitatively hydrolyzed to glucose with 0.1 mL amyloglucosidase (3,300 U.mL-1). Absorbance of D-glucose was measured by glucose oxidase/peroxidase reagent at 510 nm and the RS content of duplicate analysis on a dry weight basis (db) was calculated (McCleary et al., 2002).

Pasting properties Using a rapid viscosity analyzer (RVA; 4D; Newport Scientifi c; NSW, Australia), 10% starch and fl our suspensions were prepared in 25 g distilled water. The determination was started at 50 °C, heated to 95 °C, maintained for 2.5 min, and

Figure 1 Four banana cultivars used in the study.


Weigh four parts of 500 g sliced (1 mm) and peeled banana

Macerate each part with 2 L of 0.05 N NaOH solution at high speed for 1 min

Screen the homogenate through doubled sheets of muslin, then

collect the filtrates in a container and let stand for 2 h

Decant the supernatant of dark materials and add 8 L of water,

then stir thoroughly and let stand for 2 h

Again decant the supernatant and screen the filtrates through 120 μm nylon

Add deionized water, then stir thoroughly and let stand overnight

Decant the supernatant and collect the starch portion, then add 8 L of deionized water,

stir thoroughly and let stand for 2 h

Decant the clear supernatant and transfer into a beaker, then

add 1 L of deionized water and stir thoroughly

Transfer the starch suspension into six centrifuge tubes

Centrifuge at 8,000 rpm for 15 min and collect the sediments

Dry using hot air at 40 °C for 12 h and sieve the banana starch through 100 mesh

Determinations

Figure 2 Process for banana starch isolation (adopted from Zhang et al., 2005)


then subsequently cooled to 50 °C and held for 2 min; the total running time for each sample was 13 min. Duplicate measurement of pasting viscosities was performed (Sriroth and Piyachomkwan, 2003).

Thermal properties Using a differential scanning calorimeter (DSC; Pyris I; Perkin-Elmer; Waltham, MA, USA) equipped with a cooling system, according to a method adapted from Vatanasuchart et al. (2005), each starch sample (3 mg, dried starch basis) was weighed in an aluminum DSC pan, and deionized water was added to obtain a 30% starch suspension. The cover was carefully put on and hermetically sealed. Weights of the sealed pans before and after determination were recorded to check for water leakage due to improper sealing. Each sample pan was placed carefully in the DSC and then heated at 10 °C.min-1 from a temperature of 30 to 120 °C. An empty pan was used as a reference, and the instrument was calibrated using an indium control. Endothermal curves exhibiting onset, peak and end temperatures (°C) and melting enthalpy (J.g-1 of the sample weight, db) of duplicate samples were recorded.

Structural properties Wide-angle X-ray diffraction patterns of the banana starch were examined using a JDX 3530 ( JEOL Ltd; Yokyo, Japan) operated at 30 kV and 30 mA and generating monochromatic Cu-Kα radiation of 1.542 Å. Diffractograms were obtained from 4 to 30° (2θ) at a scanning speed of 4°.min-1. The degree of relative crystallinity was calculated from the ratio of the diffraction peak area to the total diffraction area (Shamai et al., 2003). The microscopic observation of starch granules was performed using scanning electron microscopy (SEM; JSM 5600LV; JEOL Ltd; Tokyo, Japan) at 1,000× magnifi cation.

Statistical analysis The SPSS for Windows program, (version 10; SPSS Inc., Chicago, IL, USA) was used to analyze the results obtained from duplicate determination. The mean ± standard deviation for each treatment was calculated and the analysis of variance and Duncan’s multiple range test were used for comparing differences of the mean values at P < 0.05.

RESULTS AND DISCUSSION

Yields of starch, fl our and fl ake processing Using the nine stages of ripening classifi cation by the color index of banana (Aurore et al., 2009), the present research was conducted using different cultivars of Kluai Hom (AAA), Kluai Khai (AA), Kluai Lebmuenang (AA) and Kluai Namwa (ABB) at ripening stage 1 (green) and 3 (more green than yellow). Table 1 shows the yields of starch, fl our and fl ake processed from different banana cultivars. Extraction of starches from Kluai Namwa and Kluai Khai at the green stage showed a very high yield of 13.4 to 13.5 % (wet weight basis), while the lowest yield of 7.5 % was from Kluai Lebmuenang. It was found that the starch content decreased with ripening, particularly for Kluai Lebmuenang with only 1.8% starch recovery during the green-to-yellow stage. For the fl our samples dried using hot air, Kluai Hom showed the highest yield (wet weight basis) of 33.3% at the green stage and 30.7% at the green-to-yellow stage, whereas the highest yield of banana fl ake prepared by drum drier was from Kluai Namwa with 32.0% at the green stage and 34.5% at the green-to-yellow stage. Starch is the principal component of green banana which undergoes important changes during ripening. Generally, the starch content drops to less than 1% at the end of the climacteric period while sugars accumulate to more than 10% of the fresh weight of the fruit (Zhang et


al., 2005). Both α- and β-amylases participate in starch hydrolysis; the conversion of starch to sucrose during ripening involves enzymes such as α-glucosidase and sucrose synthetase (Zhang et al., 2005). In the present study, the contents of starch and fl our from different bananas tended to decrease with ripening, but not for the fl ake samples Nimsung et al. (2007) studied extraction of banana starches and fl ours from two banana cultivars—namely, Kluai Hom Tong (AAA) and

Kluai Namwa (ABB). The starches isolated using 0.05 N NaOH had a yield of 29.15 and 31.12% and the fl ours were 56.71 and 54.11% for Kluai Hom Tong and Kluai Namwa, respectively. The result indicated a higher yield of the fl our contents than in the present study, but the contents of starch in dry weight were similar.

Resistant starch composition The results from Table 2 show that the

Table 1 Yields of starch, fl our and fl ake processing and moisture content of fresh pulp from different banana cultivars (wet weight basis).

Banana cultivar Kluai Hom Kluai Khai Kluai Lebmuenang Kluai Namwa% Starch recovery Green (g) 9.6 (12.67) 1 13.4 (13.14) 7.5 (11.70) 13.5 (14.36) Green-to-yellow (g) 10.0 (14.74) 5.2 (10.63) 1.8 (13.32) 8.5 (14.59)% Flour recovery Green (g) 33.3 (9.04) 24.5 (7.58) 31.0 (8.32) 26.0 (9.04) Green-to-yellow (g) 30.7 (8.75) 7.0 (5.89) 24.3 (7.74) 17.0 (8.37)% Flake recovery Green (g) 23.0 (5.51) 21.5 (6.74) 31.3 (6.50) 32.0 (5.38) Green-to-yellow (g) 26.8 (5.34) 29.3 (2.95) 29.8 (6.44) 34.5 (4.49)1 = % Moisture content of each banana sample shown in parenthesis.%Moisture content of fresh pulp of Kluai Hom, Kluai Khai, Kluai Lebmuenang and Kluai Namwa were 69.2, 69.95, 68.29 and 64.34 for green stage, and 72.2, 68.11, 72.55 and 65.56 for green to yellow stage, respectively.

Sample

Table 2 Resistant starch composition (g per100 g, dry basis) of bananas with an effect of ripening and processing.

Banana cultivar Kluai Hom Kluai Khai Kluai Lebmuenang Kluai NamwaStarch Green 71.6 ± 2.2 a 65.2 ± 0.2 a 74.6 ± 0.4 a 81.1 ± 0.4 a

Green-to-yellow 71.0 ± 2.6 a 57.9 ± 1.4 b 74.8 ± 4.3 a 77.3 ± 1.1 a

Flour, hot-air dried Green 54.0 ± 0.4 b 51.8 ± 0.0 c 54.7 ± 1.2 b 58.1 ± 3.5 b

Green-to-yellow 48.4 ± 0.8 c 21.6 ± 0.2 d 33.8 ± 0.5 c 45.4 ± 1.3 e

Flake, drum dried Green 1.2 ± 0.0 d 1.3 ± 0.5 e 1.2 ± 0.0 d 1.1 ± 0.0 d

Green-to-yellow 1.2 ± 0.1 d 0.9 ± 0.2 e 1.4 ± 0.0 d 1.0 ± 0.2 dValues are the average of duplicate determinations. a-d = Mean values in a column not sharing a common lowercase superscript letter are signifi cantly different at P < 0.05 by analysis of variance and Duncan’s multiple range test.

Sample


starch samples had signifi cantly higher RS than the fl our and the fl ake samples when different banana cultivars were observed. The starches from the green stage had a high content of RS, similar to the green-to-yellow stage. Among the different cultivars, the starches of Kluai Namwa from both stages contained the highest RS (77.3 ± 1.1 to 81.1 ± 0.4 g per 100g), and the fl ours of Kluai Namwa at the green stage contained the highest RS of 58.1 ± 3.5 g per 100g. The starches and fl ours of Kluai Khai from both stages showed a lower RS than the other samples. The fl ake RS contents of all samples were the lowest being close to 1%. The lower RS contents of bananas following the drum drying was due to the high heating process for starch gelatinization followed by rapid drying, so that structural depolymerization of the resistant starch molecules resulted. This fi nding related to a study by Wang et al. (2007) on corn starch processing by extrusion with high temperature and short time, where the RS content yielded only 6.23%, which was referred to as type-III RS. On the other hand, studies by Englyst et al. (1992) and Goni et al. (1996) on RS analysis of native or raw banana fl ours (referred to as type-II RS) showed values of 51.3 to 53.1%. Also, a study done by Tribess et al. (2009) determined a high RS content in banana fl our, reporting values between 40.9 and 58.5%, similar to the finding (from 47.3 to 57.2%) by Faisant et al. (1995), whereas Rodriguez-Ambriz et al. (2008) found a lower RS content of 30.4%. Several reports indicated that the contents of resistant starch and amylose were positively correlated and affected starch digestibility (Eerlingen and Delcour, 1995; Zhang et al., 2005; Sajilata et al., 2006). Vatanasuchart et al. (2009) found a high content of apparent amylose in the banana fl ours from the common (from 24.7 to 37.1%) and indigenous (from 29.9 to 35.9%) cultivars, whereas the fi ndings of Nimsung et al. (2007) indicated lower amylose contents in banana starch isolated from Kluai Khai and Kluai Hom.

Pasting properties RVA determination indicated that the starches had a significantly higher peak, breakdown, fi nal and setback viscosities than the fl our and the fl ake samples obtained from both ripening stages (Table 3). Nimsung et al. (2007) found that proximate compositions (protein, fat and ash) of fl ours from Kluai Khai, Kluai Hom and Kluai Namwa were higher than those of starches and these components obstructed swelling of the starch granules during pasting, whereas the starch of Kluai Lebmuenang obtained from both stages showed the highest peak (315.1 ± 0.7 to 318.5 ± 1.5 RVU) and fi nal (308.4 ± 2.3 to 375.5 ± 1.9 RVU) viscosities when compared among the banana cultivars. This indicates that the starch molecules from Kluai Lebmuenang could more readily take up the water than the other bananas, so hydration of amorphous areas contributes to intergranular stress resulting in the high viscosities. Therefore, the peak viscosity could be related to the swelling power of the starch granules during heating, while the fi nal viscosity was related to the retrogradation ability of the starch polymers after cooling (Sriroth and Piyachomkwan, 2003). The study by da Moto et al. (2000) on the pasting behavior of banana fl ours from eight cultivars showed higher peak viscosities (383 to 718 RVU) and lower pasting temperatures (49.8 to 51.8 °C) when compared to the present study, where the pasting temperature of the starch from Kluai Lebmuenang was 76.8 ± 0.0 °C for the green stage and 77.2 ± 0.6 °C for the green-to-yellow stage, which were both lower than that of the other starches. In contrast, Kluai Hom starch had the highest pasting temperature of 80.8 °C in both stages. Furthermore, the effect of ripening on pasting temperature was not evident with the starch, fl our and fl ake samples from different bananas, while the fl akes had the lowest pasting temperature. Moreover, pasting viscosities for the fl our and fl ake samples showed a decrease with ripening,


whereas ripening seemed to have no effect on the pasting viscosities of the starches. In particular, the peak viscosities for the starches of Kluai Hom and Kluai Lebmuenang obtained from the green stage were not signifi cantly different from those of the green-to-yellow stage. This fi nding suggested that the lower pasting viscosity of the fl our during ripening might involve starch conversion to sugars and a change in other chemical components as well as non-starch polysaccharides, as was shown in a study by Prabha and Bhagyalakshmi (1998). Likewise, da Moto et al. (2000) showed that banana fl ours from eight varieties contained

6 to 15.5% total dietary fi bers which could affect pasting behavior. Furthermore, the fi ndings of a lowering pasting viscosity and temperature of the fl akes resulted from degradation of the starch molecules by drum drying. Thermal properties DSC determination of the starches from the different banana cultivars showed similar thermal enthalpies from 17.07 ± 0.3 to 17.88 ± 0.2 J.g-1 during the green stage (Table 4). Also da Moto et al. (2000) found a slight change in the heat of gelatinization of the starches from eight

Table 3 Pasting viscosities (RVU) of different banana samples during ripening. Starch Flour Flake Pasting viscosity Green Green-to- Green Green-to- Green Green-to- yellow yellow yellowKluai Hom Peak 278.0 ± 1.4 a 278.5 ± 0.5 a 236.8 ± 3.2 b 220.2 ± 3.2 c 116.9 ± 1.2 d 83.0 ± 2.4 e

Breakdown 55.3 ± 1.7 a 55.7 ± 2.7 a 31.8 ± 4.2 b 34.1 ± 2.0 b 4.8 ± 0.1 c 1.2 ± 0.2 c

Final 302.9 ± 3.7 a 304.4 ± 7.2 a 277.5 ± 2.6 b 263.2 ± 2.0 c 164.2 ± 4.2 d 116.7 ± 2.1 e

Setback 80.2 ± 0.6 a 81.6 ± 3.9 a 72.4 ± 3.7 b 77.0 ± 0.8 ab 52.1 ± 2.9 c 34.8 ± 0.1 d

Pasting 80.8 ± 0.1 b 80.8 ± 0.0 b 83.2 ± 0.1 a 82.8 ± 0.5 a 50.5 ± 0.0 d 52.8 ± 1.2 c

Temperature (°C) Kluai Khai Peak 283.4 ± 1.2 a 259.1 ± 1.5 b 206.4 ± 1.7 c 166.7 ± 4.2 d 165.3 ± 0.9 d 32.8 ± 0.9 e

Breakdown 79.3 ± 0.8 a 35.2 ± 1.5 b 34.1 ± 2.9 b 26.4 ± 5.7 b 88.0 ± 8.5 a 5.4 ± 0.8 c

Final 294.7 ± 1.8 b 364.0 ± 0.5 a 231.2 ± 4.8 c 189.9 ± 2.9 d 112.4 ± 5.9 e 39.0 ± 2.0 f

Setback 90.6 ± 0.1 b 140.1 ± 0.5 a 58.9 ± 0.2 c 49.6 ± 4.4 d 35.2 ± 3.5 e 11.5 ± 0.3 f

Pasting 79.9 ± 0.0 a 79.6 ± 0.0 a 80.3 ± 0.6 a 80.0 ± 0.0 a 57.4 ± 0.0 b 55.7 ± 1.0 b

Temperature (°C) Kluai Lebmuenang Peak 318.5 ± 1.5 a 315.1 ± 0.7 a 212.0 ± 1.5 b 149.6 ± 2.7 c 104.9 ± 0.9 d 73.5 ± 1.1 e

Breakdown 95.5 ± 0.3 a 58.9 ± 0.6 b 43.5 ± 4.1 c 6.0 ± 0.2 de 8.5 ± 0.1 d 2.7 ± 0.1 e

Final 308.4 ± 2.3 b 375.5 ± 1.9 a 230.3 ± 0.4 c 194.3 ± 1.2 d 147.1 ± 0.6 e 114.0 ± 1.9 f

Setback 85.5 ± 1.1 b 119.3 ± 1.8 a 61.8 ± 2.4 c 52.0 ± 2.3 d 50.7 ± 0.4 d 43.2 ± 0.8 e

Pasting 76.8 ± 0.0 a 77.2 ± 0.6 a 77.6 ± 0.1a 78.3 ± 0.1 a 50.8 ± 0.4 b 53.9 ± 4.0 b

Temperature (°C) Kluai Namwa Peak 265.1 ± 0.4 b 278.8 ± 0.9 a 229.6 ± 0.6 c 187.1 ± 3.0 d 150.5 ± 4.8 e 41.0 ± 0.9 f

Breakdown 65.0 ± 1.0 a 66.2 ± 0.7 a 54.3 ± 2.2 b 50.0 ± 4.4 b 61.1 ± 0.6 a 1.5 ± 0.1 c

Final 294.3 ± 3.6 b 318.0 ± 3.0 a 243.4 ± 1.5 c 199.8 ± 0.0 d 128.0 ± 2.2 e 59.0 ± 1.1 f

Setback 94.1 ± 3.0 b 105.3 ± 1.4 a 68.1 ± 0.1 c 62.6 ± 1.4 d 38.6 ± 3.2 e 19.5 ± 0.2 f

Pasting 78.9 ± 0.6 a 79.2 ± 0.1 a 79.1 ± 0.0 a 77.5 ± 0.0 b 50.4 ± 0.2 c Not detected Temperature (°C)

Values are the average of duplicate determinations. a-f = Mean values in a row not sharing a common lowercase superscript letter are signifi cantly different at P < 0.05 by analysis of variance and Duncan’s multiple range test.


cultivars of bananas grown in Brazil, while in the current study, Kluai Hom from both ripening stages had the highest thermal enthalpy values and Kluai Khai at the stage of green-to-yellow had the lowest enthalpy of 16.00 ± 0.1 J.g-1. When the gelatinization temperatures were determined, the onset, peak and end temperatures for the starches of Kluai Hom obtained from the stages of green and green-to-yellow were 74.4 ± 0.0, 77.5 ± 0.0 and 81.3 ± 0.1 °C, and 74.6 ± 0.0, 77.9 ± 0.1 and 81.9 ± 0.1 °C, respectively. Also, onset, peak and end temperatures for the starches of Kluai Lebmuenang obtained from the green and green-to-yellow stages were 68.7 ± 0.0, 72.3 ± 0.0 and 76.3 ± 0.1 °C, and 69.7 ± 0.0, 73.2 ± 0.0 and 77.0 ± 0.0 °C, respectively. The results indicated that Kluai Lebmuenang had the lowest gelatinization temperatures, while Kluai Hom showed signifi cantly higher gelatinization temperatures than the other bananas at both stages. This would have been infl uenced by molecules of amylose and the more confi ned structure of the amylopectin crystallinity of the Kluai Hom starch, whereas the fi ndings of low gelatinization

temperatures together with the high pasting viscosities observed with the starches of Kluai Lebmuenang indicated the less confi ned structure of the amylopectin crystallinity, so less heating was required for melting the crystalline state. The results clearly showed that the DSC gelatinization temperatures and the RVA pasting temperatures of Kluai Hom and Kluai Lebmuenang provided similar trends; thus this confi rmed the hypothesis regarding the starch granule structure. Also, Shamai et al. (2003) indicated that starch gelatinization temperatures have been related to the structural characteristics of crystallinity.

Structural properties Structural properties were investigated with the banana starch from Kluai Hom, Kluai Khai, Kluai Lebmuenang and Kluai Namwa. With X-ray diffractometric examination, the banana starches exhibited similar typical B-type patterns with a peak at 2-theta at about 5.6º, 15º, 17º and 23º (Figure 3). The peak of 5.6º was considered as a fi ngerprint for the B-type structure, as reported by Gallant (1997). The results showed that the

Table 4 Thermal properties of the banana starches from different cultivars during ripening. Enthalpy Gelatinization temperature (°C) (J.g-1) Onset Peak EndKluai Hom, AAA Green 17.9 ± 0.2 ab 74.4 ± 0.0 a 77.5 ± 0.0 b 81.3 ± 0.1 a

Green-to-yellow 18.6 ± 0.0 a 74.6 ± 0.0 a 77.9 ± 0.1 a 81.9 ± 0.1 a

Kluai Khai, AA Green 17.7 ± 0.1 ab 72.2 ± 0.0 b 75.4 ± 0.0 c 80.2 ± 0.0 b Green-to-yellow 16.0 ± 0.1 c 71.6 ± 0.2 c 75.3 ± 0.0 d 80.3 ± 0.7 b

Kluai Lebmuenang, AA Green 17.8 ± 1.1ab 68.7 ± 0.0 g 72.3 ± 0.0 h 76.3 ± 0.0 e

Green-to-yellow 17.6 ± 0.0 b 69.7 ± 0.0 f 73.2 ± 0.0 g 77.0 ± 0.0 d

Kluai Namwa, ABB Green 17.1 ± 0.3 b 70.9 ± 0.3 e 74.6 ± 0.0 f 79.0 ± 0.4 c

Green-to-yellow 17.6 ± 0.0 b 71.3 ± 0.0 d 74.8 ± 0.0 e 79.2 ± 0.1 cValues are the average of duplicate determinations. a-f = Mean values in a column not sharing a common lowercase superscript letter are signifi cantly different at P < 0.05 by analysis of variance and Duncan’s multiple range test.

Sample


Kluai Hom, Kluai Khai, Kluai Lebmuenang and Kluai Namwa samples had degrees of relative crystallinity of 21.0 to 24.6%, 21.5 to 25.6%, 19.7 to 20.5% and 21.8 to 22.8%, respectively. These fi ndings revealed less relative crystallinity in Kluai Lebmuenang than the other bananas, indicating there were high amorphous portions contained within the starch molecules. Thus, Kluai Lebmuenang had a high peak viscosity, obtained with the RVA, and low gelatinization temperatures, obtained with the DSC, when compared to the other bananas. The scanning electron micrographs of the banana starch granules from fl our cultivars obtained with 1,000× magnifi cation exhibited similar appearance (Figures 4A–4D). The granules were irregularly shaped with elongated and oval forms; some of them appeared to be polyhydrons and had polygons with ridges. The surface of the

granules looked smooth, but a slight blister was found only with the Kluai Hom starch granules which had a more elongate form than oval.

CONCLUSION

The present research showed that yields of starch and fl our processed from different banana cultivars decreased with ripening, due to the conversion of the starch to sucrose. A high content of RS, particularly for the starches was observed, with similar RS levels from both the green and green-to-yellow stages. Among different cultivars obtained from both stages, the starches of Kluai Namwa with ABB genotypes contained the highest RS values of 77.3 ± 1.1 to 81.1 ± 0.4 g per 100g, whereas the fl ours of Kluai Khai gave the lowest content, while all drum-dried samples had almost no RS. The lowering effect on the RS contents

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

0 5 10 15 20 25 30 35

Voltage (mW)

2-theta

(D)

(C)

(B)

(A)

Figure 3 X-ray diffraction patterns of green banana starches from: (A) Kluai Hom; (B) Kluai Lebmuenang; (C) Kluai Khai; and (D) Kluai Namwa.


caused by drum drying was due to the high heating temperature that rapidly gelatinized and dried the samples; thus, a structural depolymerization of resistant starch molecules resulted. The starch of Kluai Lebmuenang showed the highest peak (315.1 ± 0.7 to 318.5 ± 1.5 RVU) and fi nal (308.4 ± 2.3 to 375.5 ± 1.9 RVU) viscosities, as a consequence of showing the lowest peak gelatinization temperatures of 72.3 ± 0.0 to 73.2 ± 0.0 °C by the DSC determination during ripening. However, the starches from different banana cultivars at the green stage showed similar thermal enthalpies from 17.1 ± 0.3 to 17.9 ± 0.2 J.g-1. Kluai Hom from both stages had the highest thermal enthalpy values and Kluai Khai from the green-to-yellow stage had the lowest enthalpy. The X-ray diffractograms of the bananas showed the typical B-type pattern, whereas Kluai Lebmuenang exhibited the lowest

Figure 4 Scanning electron microscope micrographs of green banana starch granules from: (A) Kluai Hom; (B) Kluai Lebmuenang; (C) Kluai Khai; and (D) Kluai Namwa with 1,000× magnifi cation.

degree of relative crystallinity, which was related to the RVA and the DSC results.

ACKNOWLEDGEMENTS

The authors would like to thank the Thailand Research Fund for research funding and the Kasetsart University for constant support.

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