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Effect of heat-moisture treatment on rice starch of varying amylose content
Food Chemistry 121 (2010) 358–365
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Effect of heat-moisture treatment on rice starch of varying amylose content Elessandra da Rosa Zavareze a,*, Cátia Regina Storck a, Luis Antonio Suita de Castro b, Manoel Artigas Schirmer a, Alvaro Renato Guerra Dias a a b
Department of Agroindustrial Science and Technology, Federal University of Pelotas, 96010-900 Pelotas, Brazil Laboratory of Electron Microscopy, Embrapa CPA-CT, 96001-970 Pelotas, Brazil
a r t i c l e
i n f o
Article history: Received 16 July 2009 Received in revised form 28 September 2009 Accepted 9 December 2009
Keywords: Crystallinity Heat-moisture treatment Microscopy Rice starch a-Amylase
a b s t r a c t The effect of heat-moisture treatment (HMT) on the properties of rice starches with high-, medium- and low-amylose content was investigated. The starches were adjusted to 15%, 20% and 25% moisture levels, and heated at 110 °C for 1 h. The swelling power, solubility, pasting properties, morphology, enzymatic susceptibility and X-ray crystallinity of the starches were evaluated. HMT reduced the swelling power and solubility of the starches. The strongest effect of HMT occurred on the high-amylose starch; the pasting temperature was increased and the peak viscosity, breakdown, final viscosity and the setback were reduced. HMT increased the starch’s susceptibility to a-amylase and promoted a reduction in the starch relative crystallinity. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Starch is a major constituent of rice and an important structural constituent in many rice products (Sasaki et al., 2009). Rice starch is not as widely used as starches derived from other sources due to the high value of milled rice as food. The small size of rice starch granules and the wide range of amylose content of rice varieties provide opportunities for the development of a rice starch market (Zhong et al., 2009). Rice starch is used as an additive in various foods, industrial products, desserts, bakery products and as a fat mimetic in foods such as ice cream, yoghurt and salad dressings (Puchongkavarin, Varavinit, & Bergthaller, 2005). Rice starch is composed of amylose and amylopectin. The ratio of amylose to amylopectin and the branching properties of the amylopectin molecules of a rice starch can affect the physical, textural and pasting properties of rice during the cooking of rice and rice starch (Champagne, 1996). The starch, in its native form, has limited use in the industry. Physical and chemical modifications are commonly used to produce starches with special properties. Although chemically modified starches are available for industrial purposes, most industries (especially the food and pharmaceutical industries) prefer starches that have not been chemically altered. Therefore, physically modified starch, by use of moisture, heat, shear, or radiation * Corresponding author. Tel./fax: +55 53 32757258. E-mail address: [email protected] (E. da Rosa Zavareze). 0308-8146/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2009.12.036
has gained a wider acceptance, because there is no waste of chemical reagents in the modified starch (Adebowale, Afolabi, & Olu-Owolabi, 2005). Hydrothermal treatments (heat-moisture treatment, HMT; and annealing, ANN) are physical modifications that change the physicochemical properties of starch, without destroying its granule structure. Both annealing and heat-moisture treatments are related processes in which the starch to moisture ratio, the temperature and heating time are critical parameters that need to be controlled (Chung, Liu, & Hoover, 2009). The differences between these two treatments are the amount of water and temperature used. Annealing occurs under a large excess of water (50–60%) and occurs at relatively low temperatures (below the gelatinisation temperature), whilst the HMT is carried out under restricted moisture content (10–30%) and higher temperatures (90–120 °C) (Maache-Rezzoug, Zarguili, Loisel, Queveau, & Buléon, 2008). Regardless of origin of the starch, heat-moisture treatment promotes the increase of the gelatinisation transition temperatures, the widening of the gelatinisation temperature range, a decrease in the granular swelling and amylose leaching and an increase in thermal stability. However, depending on the botanical origin and treatment conditions, changes to the X-ray pattern, the formation of amylose–lipid complexes, the disruption of crystallinity, an increase or decrease in enzyme susceptibility has been shown to occur with HMT (Chung, Hoover, & Liu, 2009). Different food products make different demands on the starches to use in their formulations, depending on the desired properties of
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the food product in production. Physical modifications have been applied to starches to alter their physicochemical properties with a view to meeting the various industrial needs of the starches. Heat-moisture treated starch has important properties for food industries. The heat-moisture treatment has been shown to improve thermal stability and decrease the extent of set-back (Adebowale et al., 2005). Therefore, heat-moisture treated starches could be utilised in the canned and frozen food industries, for their respective advantages. The decrease in granular swelling and amylose leaching, and the increase in heat and shear stability that occur on heat-moisture treatment are all desirable properties for noodle manufacture (Hormdok & Noomhorm, 2007). HMT has also been used to enhance resistant starch levels whilst maintaining granule structure (Brumovsky & Thompson, 2001). Most studies on heat-moisture treatment have involved starches from different sources, such as rice, potato, cassava, wheat, maize, canna and sorghum. However, relatively little work has been done to investigate the effect of heat-moisture treatment on rice starches extracted from cultivars with a wide range of amylose content. The objective of this study was to evaluate the impact of heatmoisture treatment on the swelling power, solubility, pasting properties, morphology, enzymatic susceptibility and X-ray crystallinity of high-, medium- and low-amylose rice starches. 2. Materials and methods 2.1. Materials Rice grains of cultivars IRGA 417 (high-amylose), Sasanishiki (medium-amylose) and Motti (low-amylose) with amylose contents of 32%, 23% and 7%, respectively were used. Rice samples were dehulled, polished and ground in order to obtain rice flour.
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90 °C for 30 min. The gelatinised samples were then cooled to room temperature and centrifuged at 1000g for 20 min. The supernatant was dried at 110 °C to a constant weight to quantify the soluble fraction. The solubility was expressed as the percentage of dried solid weight based on the weight of the dry sample. The swelling power was represented as the ratio of the weight of the wet sediment to the weight of the initial dry sample (deducting the amount of soluble starch). 2.5. Pasting properties The pasting properties of the starch samples (3.0 g, 14% moisture content (wet basis)) were determined by using a Rapid Visco Analyzer (RVA – 4, Newport Scientific, Australia) and profile Standard Analysis 1. Parameters including pasting temperature, peak viscosity, breakdown, final viscosity and setback were recorded. 2.6. Scanning electron microscopy (SEM) Starch granules morphology was examined by the scanning electron microscope, model DSM 940 A (Laboratory of Electron Microscopy – Embrapa CPA-CT, Brazil). Starch samples were initially suspended in ethanol at 1% (w/v). A small quantity of each sample was spread directly on the surface of the stub and dried in an oven at 32 °C for 24 h. Subsequently, all of the samples were coated with gold and examined in the scanning electron microscope under an acceleration voltage of 10 kV, a distance of 31 mm and a magnification of 2000. 2.7. Enzymatic susceptibility to a-amylase
Rice starch was isolated with 0.1% NaOH as described by Wang and Wang (2004). Rice flour was soaked in 0.1% NaOH in a 1:2 (w/ v) ratio for 18 h. Then it was blended, passed through a 63 lm screen and centrifuged at 1200g for 5 min. The soft top layer was carefully removed, and the underlying starch layer was re-slurried. The starch layer was then washed twice with 0.1% NaOH and centrifuged. The starch layer was washed with distilled water and centrifuged. The starch was then re-slurried and neutralised with 1.0 M HCl to a pH of 6.5 and centrifuged. The neutralised starch was washed with distilled water three times and dried at 40 °C until the moisture content of the samples reached about 11%.
Starches were submitted to enzymatic hydrolysis according to the method described by Serrano and Franco (2005). Starch (10 g, d.b.) was suspended in 48.5 ml of phosphate buffer (0.2 M, pH 6.0) and hydrolysed with 1 ml of a solution (0.25% w/v) of bacterial a-amylase (A6380) from Sigma, with an activity of 2150 units/mg of solid. A solution of sodium azide (0.5 ml, 10% w/v) was added to prevent bacterial growth. Starch dispersions were incubated in an orbital shaker bath (Dubnoff, Nova Ética, model 304/D, Brazil) (120 rpm) at 37 °C. After the incubation period, the suspensions were acidified to a pH of 3.0 with 0.1 N HCl; they were then stirred for 15 min to inactivate the enzymes, neutralised with 0.1 N NaOH and centrifuged at 1000g for 30 min. Hydrolysis kinetics were determined by the amount of reducing sugars (Somogyi, 1945) that were present in the supernatant, which were withdrawn from individual vials after 2, 6, 12, 18, 24, 30, 36, 42 and 48 h of incubation. Controls without the enzyme were also prepared.
2.3. Heat-moisture treatment (HMT)
2.8. X-ray diffraction
The heat-moisture treatment of rice starches was conducted on samples with moisture levels adjusted to 15%, 20% and 25% (HMT 15%, HMT 20% and HMT 25%, respectively) and equilibrated at 4 °C for 4 days. Samples were then placed in sealed glass tubes and autoclaved at 110 °C for 1.0 h (Hormdok & Noomhorm, 2007). The treated samples were subsequently dried at 40 °C until the moisture content of the samples reached about 11% and were then ground.
X-ray diffractograms of the starches were obtained with an Xray diffractometer (XRD-6000, Shimadzu, Brazil). The scanning region of the diffraction ranged from 5° to 30°, with a target voltage of 30 kV, a current of 30 mA and a scan speed of 1°/min. The relative crystallinity (RC) of the starch granules was calculated as described by Rabek (1980) by following the equation RC (%) = (Ac/ (Ac + Aa)) 100, where Ac is the crystalline area and Aa is the amorphous area on the X-ray diffractograms.
2.4. Swelling power and solubility
2.9. Statistical analysis
The swelling power and solubility of the starches were determined as described by Leach, McCowen, and Schoch (1959). Samples (1.0 g) were mixed with 50 ml of distilled water in centrifugal tubes. The suspensions were heated at 60, 70, 80 and
Analytical determinations for the samples were done in triplicate and standard deviations were reported. A comparison of the means was ascertained by Tukey’s test, to 5% level of significance using an analysis of the variance (ANOVA).
2.2. Rice starch isolation
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Fig. 1. Effect of heat-moisture treatment (HMT) on the swelling power (a) and solubility (b) of high-amylose starch (a), swelling power (c) and solubility (d) of mediumamylose starch and swelling power (e) and solubility (f) of low-amylose starch.
3. Results and discussion 3.1. Swelling power and solubility Swelling power and solubility curves of high-, medium- and low-amylose rice starches are presented in Fig. 1. The swelling power of the native and HMT starches increased as a result of increasing the assay temperature (Fig. 1a and c), due to starch gelatinisation. Starch gelatinisation involves the collapse of the starch granule manifested in irreversible changes in properties such as granular swelling, native crystallite melting, loss of birefringence and starch solubilisation (Collado & Corke, 2003). When the temperature of a starch suspension is higher than the gelatinisation temperature, hydrogen bonds are broken and water molecules
can then penetrate into the granules and hydrate free hydroxyl groups making it swell (Limberger, Silva, Emanuelli, Comarela, & Patias, 2008). However, for rice starch with a low-amylose content (Fig. 1e), the swelling power reached its maximum at 70 °C; temperatures above this only increased the soluble fraction (Fig. 1f). The swelling power of the low-amylose starch is higher than that of the medium- and high-amylose starches, which was also observed by Sasaki and Matsuki (1998), who found an inverse correlation between amylose content and swelling power of wheat starch. The swelling power of HMT starches was reduced with rising moisture content in the treatment as compared to the native starches. This phenomenon was observed with temperatures above 70 and 80 °C for high amylose content starches.
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E. da Rosa Zavareze et al. / Food Chemistry 121 (2010) 358–365 Table 1 Pasting properties of native and heat-moisture treated rice starches. Propertiesa
Amylose content
Native
HMT 15%
HMT 20%
HMT 25%
bD
bC
bB
Pasting temperature (°C)
High Medium Low
82.0 ± 0.27 85.9 ± 0.15aC 61.4 ± 0.43cD
83.8 ± 0.35 85.5 ± 0.53aC 62.5 ± 0.18cC
85.7 ± 0.35 87.2 ± 0.20aB 65.6 ± 0.00cB
88.1 ± 0.02bA 88.9 ± 0.03bA 66.9 ± 0.05aA
Peak viscosity (RVU)
High Medium Low
243.3 ± 2.37cA 244.5 ± 0.87bA 332.8 ± 1.38aB
244.3 ± 1.37bA 243.0 ± 1.00bA 320.5 ± 2.83aC
228.3 ± 3.00cB 243.3 ± 1.34bA 310.8 ± 1.00aD
195.1 ± 0.88cC 244.2 ± 3.00bA 365.8 ± 1.75aA
Breakdown (RVU)
High Medium Low
23.3 ± 0.92cA 61.1 ± 1.69bA 149.8 ± 1.59aB
14.3 ± 0.75cC 43.8 ± 0.15bB 119.3 ± 1.63aC
11.9 ± 0.40cD 18.1 ± 0.50bC 120.9 ± 0.38aC
17.1 ± 0.75bB 15.3 ± 0.68bD 191.8 ± 1.25aA
Final viscosity (RVU)
High Medium Low
363.3 ± 1.75aA 328.9 ± 1.34bA 202.6 ± 0.84cC
317.3 ± 2.08bB 326.1 ± 1.50aAB 224.6 ± 3.13cA
280.7 ± 1.34bC 323.5 ± 0.71aB 213.6 ± 2.05cB
221.7 ± 1.13bD 298.4 ± 2.59aC 199.9 ± 0.25cC
Setback (RVU)
High Medium Low
143.2 ± 2.64aA 143.7 ± 1.43aA 19.6 ± 2.13bB
87.3 ± 4.21bB 126.9 ± 0.65aB 23.4 ± 1.91cAB
64.2 ± 2.07bC 98.3 ± 2.54aC 23.7 ± 1.41cAB
43.6 ± 0.50bD 69.6 ± 1.09aD 26.0 ± 0.25cA
a Different lowercase letters in the same column for each property, and different uppercase letters in the same row, differ statistically (p 6 0.05). Results are the means of three determinations ± the standard deviation. RVU: Rapid Visco Unit.
The swelling power may be ordered by the treatments’ conditions: HMT 15% > HMT 20% > HMT 25%. These results are in agreement with Olayinka, Adebowale, and Olu-Owolabi (2008) in a study of HMT in sorghum starch, and Adebowale and Lawal (2002) in a study of Bambarra groundnut starch. Hormdok and Noomhorm (2007) reported that the HMT (20% moisture/92.5 °C/ 30 min) of rice starch (27% amylose) reduced the swelling power from 14.11 g/g of native starch to 10.29 g/g. Several authors have observed a reduction in the swelling power of the HMT in potato, cassava (Gunaratne & Hoover, 2002), rice (Hormdok & Noomhorm, 2007), sorghum (Olayinka et al., 2008) and maize starches (Chung, Liu, et al., 2009). The reduction in swelling power following hydrothermal modification has been attributed to internal rearrangement of the starch granules, which causes further interaction amongst the starch functional groups (Hoover & Manuel, 1996), making it form more ordered double helical amylopectin side chain clusters. This accounts for the increased starch crystallinity (Adebowale & Lawal, 2002). The highest solubility for all of the starches was obtained at 90 °C, where most of the granules were gelatinised or swollen (Fig. 1b, d and f). The HMT of high- and medium-amylose rice starches (Fig. 1b and d), at temperatures of 60, 70 and 80 °C presented no significant change in the solubility as compared to their native starch. However, at 90 °C, a reduction in the solubility was observed with an increase in the moisture content. There was a significant reduction in the solubility of the HMT low-amylose starch as compared to the native starch at 80 and 90 °C (Fig. 1f). These results are in agreement with Adebowale and Lawal (2002), who studied the effect of HMT on the solubility of starch at 60, 70, 80 and 90 °C, and observed a reduction in solubility at all temperatures. However, Hormdok and Noomhorm (2007) found no significant difference in the solubility of the HMT rice starch as compared with its native form. The decrease in the solubility of HMT starch indicates that there was a strengthening of the bonds, with an increase in the interactions amongst amylose and amylopectin molecules, impeding them from leaching out of the granules. 3.2. Pasting properties The HMT promoted intense changes in the starches, significantly altering their pasting profile. Table 1 shows the pasting properties of high-, medium- and low-amylose rice starch. The
pasting temperature of rice starch showed a significant increase (p 6 0.05) with an increase in the moisture content of the hydrothermal treatment (Table 1). The peak viscosity of the high-amylose starch for the HMT 20% and HMT 25% was reduced as compared to the native starch. For medium-amylose starch, the hydrothermal treatment did not affect the peak viscosity. For the low-amylose rice starch, the hydrothermal treatment reduced the peak viscosity for the HMT 15% and HMT 20% and increased in the HMT 25% (Table 1) in relation to the native starch. Hormdok and Noomhorm (2007) attributed the reduction in peak viscosity of the heat-moisture-treated rice starch to its restricted swelling capacity. The high- and medium-amylose starches had their breakdown reduced by the HMT. The low-amylose starch showed a reduction in its breakdown value for HMT 15% and HMT 20% starches, and an increase for HMT 25% starches as compared to the native starch (p 6 0.05). The reduction of the breakdown caused by HMT shows that starches are more stable during continued heating and shearing, which is in agreement with Adebowale et al. (2005), Hormdok and Noomhorm (2007), Olayinka et al. (2008) and Watcharatewinkul, Puttanlek, Rungsardthong, and Uttapap (2009). The final viscosity for the high-amylose starch showed a significant reduction (p 6 0.05) with an increase in the moisture content of the hydrothermal treatment as related to the native starch. For the medium-amylose starch, the reduction of the final viscosity happened only at HMT 25%. However, for the low-amylose starch, there was an increase in the final viscosity of HMT 15% and HMT 20% starches. There was a reduction in the setback for high- and mediumamylose HMT starches with an increase in the moisture content of the treatment. Lan et al. (2008) have shown that the setback is influenced by the amount of leached amylose, the granule size and the presence of rigid non-fragmented swollen granules. Chung, Liu, et al. (2009) found that HMT reduces the leached amylose in the starch granules and that this reduction is more significant in starches with high levels of amylose. This may explain the fact that hydrothermally treated starches cause an increasing reduction in the setback in higher amylose content starches because HMT promotes additional interactions between amylose–amylose and/or amylopectin–amylopectin chains which reduce leached amylose content and lower the setback. According to Watcharatewinkul et al. (2009), the HMT (15%, 18%, 20%, 22% and 25% moisture/100 °C/16 h) of the canna starch altered pasting profiles, resulting in an increased in the pasting
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temperature and a decrease in the peak viscosity, final viscosity and breakdown. These authors described that the changes in the pasting properties of the heat-moisture treated starches is due to the associations amongst the chains in the amorphous region of the granule and the changes in crystallinity during hydrothermal treatment. 3.3. Granule morphology by SEM The HMT at 25% moisture slightly affected the format and degree of agglomeration of the high- and medium-amylose starch granules (Fig. 2b and d), making the granules more aggregated and the surface of the granules more irregular, as compared with the native starches (Fig. 2a and c). The HMT 25% low-amylose starch (Fig. 2f) showed signs of the loss of physical integrity with
distension of the granular surface, characteristic of a partial gelatinisation, as compared with the low-amylose native starch (Fig. 2e). 3.4. Enzymatic hydrolysis with a-amylase Fig. 3a–c show the concentration of reducing sugars produced during the hydrolysis of high-, medium- and low-amylose rice starches treated by HMT, respectively. The hydrolysis of the native starches by a-amylase is inversely related to the amylose content (Fig. 3). Franco and Ciacco (1997) found a higher percentage of hydrolysis for waxy maize starch (56%) as compared with normal starch (39%) and Tester, Qi, and Karkalas (2006) also reported the same behaviour of the hydrolysis of native starches by a-amylase. Zhang and Oates (1999) described
Fig. 2. Scanning electron micrographs of high-amylose native starch (a); high-amylose – HMT 25% (b); medium-amylose native starch (c); medium-amylose – HMT 25% (d); low-amylose native starch (e); and low-amylose – HMT 25% (f).
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High amylose
1
2 3 4
5
Native
2 3 1
5
Intensity
4 2 3
HMT 15%
1 4
5
HMT 20% 1
2 3
4
5
HMT 25%
5
10
15
20
25
30
Diffraction angle (2θ)
(a)
Medium amylose
1
2 3 5
4
Native
2 3
1
Intensity
4
5
2 3
1
HMT 15% 4
5
4
5
HMT 20%
2 3
1
HMT 25%
5
10
(b)
15
20
25
30
Diffraction angle (2θ) Low amylose
1
2
3
23
5
5 2 3
Intensity
Native
4
1 4
1
HMT 15% 5
4 1
HMT 20%
2 3 5 4
HMT 25%
5
(c)
10
15
20
25
30
Diffraction angle (2θ)
Fig. 4. X-ray diffractograms of native and HMT rice starches with high- (a), medium- (b) and low- (c) amylose content. Fig. 3. Reducing sugar content of high- (a), medium- (b) and low-amylose (c) rice starches treated by HMT as a function of hydrolysis incubation time.
that enzyme susceptibility is influenced by several factors, including the ratio of amylose/amylopectin, the crystalline structure and the size of the particles. Amongst these factors, the crystalline granular structure is the most important. The high-, medium- and low-amylose rice starches treated by the HMT 25% had a higher concentration of reducing sugars at all times during the hydrolysis, as compared to HMT 15% and HMT 20% and their respective native starches. These results are similar to those of Kweon, Haynes, Slade, and Levine (2000), who observed a trend toward increased digestibility of the starches with an increase in the moisture content of the HMT. Lorenz and Kulp (1982) reported that the changes in the HMT probably occur in the amorphous regions of the granules that are more accessible to the hydrolysis. According to Gunaratne and Hoover (2002), the HMT promotes a crystalline disruption and dis-
sociation of double helical structures in the amorphous region. These authors studied the hydrothermal treatment (30% moisture/100 °C/10 h) of taro, cassava and potato starches, and concluded that there was an increase in the enzyme susceptibility of heat-moisture treated starch. The possible disruption of the crystals near the surface of the granule can facilitate the attack of the a-amylase within the granule. The HMT 25% starches had a higher enzymatic susceptibility in the first two hours of digestion as compared to the native starches. As reported by some authors (Jacobs, Eerlingen, Spaepen, Grobet, & Delcour, 1998), the amorphous areas of the starch granules are more rapidly degraded by bacterial and pancreatic a-amylase than the crystalline areas. The rearrangement caused by the hydrothermal treatment facilitated the accessibility of the amorphous areas by the enzyme. This may explain the fact that HMT 25% starches show a high concentration of reducing sugars in the first two hours of hydrolysis as compared to the native starches.
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Table 2 Intensity of the main peaks of X-ray diffractograms and relative crystallinity of native and heat-moisture treated rice starches.
a b
Intensity (CPSa)
RCb (%)
Amylose content
Treatment
1
2
3
4
5
High
Native HMT 15% HMT 20% HMT 25%
1124 1198 1190 1128
1250 1394 1370 1236
1262 1390 1368 1246
926 964 1020 1120
1122 1134 1076 994
37.7 37.0 35.8 32.7
Medium
Native HMT 15% HMT 20% HMT 25%
1266 1256 1220 1154
1478 1384 1382 1288
1424 1406 1440 1348
966 972 998 1110
1102 1102 1100 1048
38.5 38.2 37.5 31.1
Low
Native HMT 15% HMT 20% HMT 25%
1136 1288 1332 1270
1318 1466 1520 1380
1344 1508 1568 1456
742 800 846 924
1194 1274 1276 1190
49.1 49.0 44.1 40.3
CPS: counts per second. RC: relative crystallinity.
3.5. X-ray pattern and crystallinity Rice starches exhibited the typical A-type X-ray patterns of cereal starches (Fig. 4). The intensity of five main peaks and the relative crystallinity (RC) are presented in Table 2. The numbers 1, 2, 3, 4 and 5 represent the highest peaks detected in the X-ray diffractograms (Table 2). The crystallinity of the native starches followed the following order: low-amylose > medium-amylose > high-amylose (Table 2). The higher crystallinity of the low-amylose native starch could be attributed to its higher amylopectin content. The relative crystallinity decreased with an increase in the moisture of the HMT starches, though the diffraction intensity of the peak at 19° (peak 4) of the starches increased and the highamylose starch peaks at 15°, 17°, 17.8° and 23° (peaks 1, 2, 3 and 5, respectively) decreased (Fig. 4). These results suggest a decrease in the crystalline areas due to HMT. This may explain the greater enzymatic susceptibility evidenced by heat-moisture treated starches. Gunaratne and Hoover (2002) also found a reduction in the relative crystallinity in their study of potato and true yam starches treated by HMT. 4. Conclusions The HMT reduces the swelling power and solubility of rice starches. The HMT affects the pasting properties of high-amylose starches more intensely than that of the medium- and low-amylose starches, causing an increase in the pasting temperature and a reduction in the peak viscosity, final viscosity and the setback. The HMT causes a loss of physical integrity of low-amylose starch granules treated at 25% of moisture. The HMT promotes an increase in the enzymatic susceptibility of the starch and a reduction in the relative crystallinity. Therefore, HMT rice starch has a higher interest for application in foods that require a lower swelling, lower viscosity and higher thermal stability. References Adebowale, K. O., Afolabi, T. A., & Olu-Owolabi, B. I. (2005). Hydrothermal treatments of Finger millet (Eleusine coracana) starch. Food Hydrocolloids, 19, 974–983. Adebowale, K. O., & Lawal, O. S. (2002). Effect of annealing and heat moisture conditioning on the physicochemical characteristics of Bambarra groundnut (Voandzeia subterranea) starch. Nahrung–Food, 46, 311–316. Brumovsky, J. O., & Thompson, D. B. (2001). Production of boiling-stable granular resistant starch by partial acid hydrolysis and hydrothermal treatments of highamylose maize starch. Cereal Chemistry, 78, 680–689. Champagne, E. T. (1996). Rice starch composition and characteristics. Cereal Food World, 41, 833–838.
Chung, H. J., Hoover, R., & Liu, Q. (2009). The impact of single and dual hydrothermal modifications on the molecular structure and physicochemical properties of normal corn starch. International Journal of Biological Macromolecules, 44, 203–210. Chung, H. J., Liu, Q., & Hoover, R. (2009). Impact of annealing and heat-moisture treatment on rapidly digestible, slowly digestible and resistant starch levels in native and gelatinized corn, pea and lentil starches. Carbohydrate Polymers, 75, 436–447. Collado, L. S., & Corke, H. (2003). Starch properties and functionalities. In G. Kaletung & K. J. Breslauer (Eds.), Characterization of cereals and flours. Properties, analysis and applications. New York: Marcel Dekker. Franco, C. M. L., & Ciacco, C. F. (1997). Estrutura dos grânulos de amido de milho normal e ceroso. Ciência e Tecnologia de Alimentos, 17, 295–300. Gunaratne, A., & Hoover, R. (2002). Effect of heat-moisture treatment on the structure and physicochemical properties of tuber and root starches. Carbohydrate Polymers, 49, 425–437. Hoover, R., & Manuel, H. (1996). Effect of heat-moisture treatment on the structure and physicochemical properties of normal maize, waxy maize, dull waxy maize and amylomaize V starches. Journal of Cereal Science, 23, 153–162. Hormdok, R., & Noomhorm, A. (2007). Hydrothermal treatments of rice starch for improvement of rice noodle quality. LWT – Food Science and Technology, 40, 1723–1731. Jacobs, H., Eerlingen, R. C., Spaepen, H., Grobet, P. J., & Delcour, J. A. (1998). Impact of annealing on the susceptibility of wheat, potato and pea starches to hydrolysis with pancreatin. Carbohydrate Research, 305, 193–207. Kweon, M., Haynes, A., Slade, L., & Levine, H. (2000). The effect of heat moisture treatments no enzyme digestibility of corn starches. Journal of Thermal Analysis and Calorimetry, 59, 571–586. Lan, H., Hoover, R., Jayakody, L., Liu, Q., Donner, E., Baga, M., et al. (2008). Impact of annealing on the molecular structure and physicochemical properties of normal, waxy and high amylose bread wheat starches. Food Chemistry, 111, 663–675. Leach, H. W., McCowen, L. D., & Schoch, T. J. (1959). Structure of the starch granule. I. Swelling and solubility patterns of various starches. Cereal Chemistry, 36, 534–544. Limberger, V. M., Silva, L. P., Emanuelli, T., Comarela, C. G., & Patias, L. D. (2008). Modificação química e física do amido de quirera de arroz para aproveitamento na indústria de alimentos. Química Nova, 31, 84–88. Lorenz, K., & Kulp, J. (1982). Cereal and root starch modification by heat-moisture treatment. Physico-chemical properties. Starch/Stärke, 34, 50–54. Maache-Rezzoug, Z., Zarguili, I., Loisel, C., Queveau, D., & Buléon, A. (2008). Structural modifications and thermal transitions of standard maize starch after DIC hydrothermal treatment. Carbohydrate Polymers, 74, 802–812. Olayinka, O. O., Adebowale, K. O., & Olu-Owolabi, B. I. (2008). Effect of heatmoisture treatment on physicochemical properties of white sorghum starch. Food Hydrocolloids, 22, 225–230. Puchongkavarin, H., Varavinit, S., & Bergthaller, W. (2005). Comparative study of pilot scale rice starch production by an alkaline and an enzymatic process. Starch/Stärke, 57, 134–144. Rabek, J. F. (1980). Experimental methods in polymer chemistry: Applications of wideangle X-ray diffraction (WAXD) to the study of the structure of polymers. Chichester: Wiley Interscience. Sasaki, T., Kohyama, K., Suzuki, Y., Okamoto, K., Noel, T. R., & Ring, S. G. (2009). Physicochemical characteristics of waxy rice starch influencing the in vitro digestibility of a starch gel. Food Chemistry, 116, 137–142. Sasaki, T., & Matsuki, J. (1998). Effect of wheat starch structure on swelling power. Cereal Chemistry, 75, 525–529. Serrano, P. O., & Franco, C. M. L. (2005). Modificação Hidrotérmica (Annealing) e Hidrólise Enzimática do Amido de Mandioca. Brazilian Journal of Food and Technology, 8, 220–232.
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Zhang, T., & Oates, C. G. (1999). Relationship between a-amylase degradation and physico-chemical properties of sweet potato starches. Food Chemistry, 65, 157–163. Zhong, F., Li, Y., Ibanz, A. M., Oh, M. H., McKenzie, K. S., & Shoemaker, C. (2009). The effect of rice variety and starch isolation method on the pasting and rheological properties of rice starch pastes. Food Hydrocolloids, 23, 406–414.
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Effect of heat-moisture treatment on rice starch of varying amylose content Elessandra da Rosa Zavareze a,*, Cátia Regina Storck a, Luis Antonio Suita de Castro b, Manoel Artigas Schirmer a, Alvaro Renato Guerra Dias a a b
Department of Agroindustrial Science and Technology, Federal University of Pelotas, 96010-900 Pelotas, Brazil Laboratory of Electron Microscopy, Embrapa CPA-CT, 96001-970 Pelotas, Brazil
a r t i c l e
i n f o
Article history: Received 16 July 2009 Received in revised form 28 September 2009 Accepted 9 December 2009
Keywords: Crystallinity Heat-moisture treatment Microscopy Rice starch a-Amylase
a b s t r a c t The effect of heat-moisture treatment (HMT) on the properties of rice starches with high-, medium- and low-amylose content was investigated. The starches were adjusted to 15%, 20% and 25% moisture levels, and heated at 110 °C for 1 h. The swelling power, solubility, pasting properties, morphology, enzymatic susceptibility and X-ray crystallinity of the starches were evaluated. HMT reduced the swelling power and solubility of the starches. The strongest effect of HMT occurred on the high-amylose starch; the pasting temperature was increased and the peak viscosity, breakdown, final viscosity and the setback were reduced. HMT increased the starch’s susceptibility to a-amylase and promoted a reduction in the starch relative crystallinity. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Starch is a major constituent of rice and an important structural constituent in many rice products (Sasaki et al., 2009). Rice starch is not as widely used as starches derived from other sources due to the high value of milled rice as food. The small size of rice starch granules and the wide range of amylose content of rice varieties provide opportunities for the development of a rice starch market (Zhong et al., 2009). Rice starch is used as an additive in various foods, industrial products, desserts, bakery products and as a fat mimetic in foods such as ice cream, yoghurt and salad dressings (Puchongkavarin, Varavinit, & Bergthaller, 2005). Rice starch is composed of amylose and amylopectin. The ratio of amylose to amylopectin and the branching properties of the amylopectin molecules of a rice starch can affect the physical, textural and pasting properties of rice during the cooking of rice and rice starch (Champagne, 1996). The starch, in its native form, has limited use in the industry. Physical and chemical modifications are commonly used to produce starches with special properties. Although chemically modified starches are available for industrial purposes, most industries (especially the food and pharmaceutical industries) prefer starches that have not been chemically altered. Therefore, physically modified starch, by use of moisture, heat, shear, or radiation * Corresponding author. Tel./fax: +55 53 32757258. E-mail address: [email protected] (E. da Rosa Zavareze). 0308-8146/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2009.12.036
has gained a wider acceptance, because there is no waste of chemical reagents in the modified starch (Adebowale, Afolabi, & Olu-Owolabi, 2005). Hydrothermal treatments (heat-moisture treatment, HMT; and annealing, ANN) are physical modifications that change the physicochemical properties of starch, without destroying its granule structure. Both annealing and heat-moisture treatments are related processes in which the starch to moisture ratio, the temperature and heating time are critical parameters that need to be controlled (Chung, Liu, & Hoover, 2009). The differences between these two treatments are the amount of water and temperature used. Annealing occurs under a large excess of water (50–60%) and occurs at relatively low temperatures (below the gelatinisation temperature), whilst the HMT is carried out under restricted moisture content (10–30%) and higher temperatures (90–120 °C) (Maache-Rezzoug, Zarguili, Loisel, Queveau, & Buléon, 2008). Regardless of origin of the starch, heat-moisture treatment promotes the increase of the gelatinisation transition temperatures, the widening of the gelatinisation temperature range, a decrease in the granular swelling and amylose leaching and an increase in thermal stability. However, depending on the botanical origin and treatment conditions, changes to the X-ray pattern, the formation of amylose–lipid complexes, the disruption of crystallinity, an increase or decrease in enzyme susceptibility has been shown to occur with HMT (Chung, Hoover, & Liu, 2009). Different food products make different demands on the starches to use in their formulations, depending on the desired properties of
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the food product in production. Physical modifications have been applied to starches to alter their physicochemical properties with a view to meeting the various industrial needs of the starches. Heat-moisture treated starch has important properties for food industries. The heat-moisture treatment has been shown to improve thermal stability and decrease the extent of set-back (Adebowale et al., 2005). Therefore, heat-moisture treated starches could be utilised in the canned and frozen food industries, for their respective advantages. The decrease in granular swelling and amylose leaching, and the increase in heat and shear stability that occur on heat-moisture treatment are all desirable properties for noodle manufacture (Hormdok & Noomhorm, 2007). HMT has also been used to enhance resistant starch levels whilst maintaining granule structure (Brumovsky & Thompson, 2001). Most studies on heat-moisture treatment have involved starches from different sources, such as rice, potato, cassava, wheat, maize, canna and sorghum. However, relatively little work has been done to investigate the effect of heat-moisture treatment on rice starches extracted from cultivars with a wide range of amylose content. The objective of this study was to evaluate the impact of heatmoisture treatment on the swelling power, solubility, pasting properties, morphology, enzymatic susceptibility and X-ray crystallinity of high-, medium- and low-amylose rice starches. 2. Materials and methods 2.1. Materials Rice grains of cultivars IRGA 417 (high-amylose), Sasanishiki (medium-amylose) and Motti (low-amylose) with amylose contents of 32%, 23% and 7%, respectively were used. Rice samples were dehulled, polished and ground in order to obtain rice flour.
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90 °C for 30 min. The gelatinised samples were then cooled to room temperature and centrifuged at 1000g for 20 min. The supernatant was dried at 110 °C to a constant weight to quantify the soluble fraction. The solubility was expressed as the percentage of dried solid weight based on the weight of the dry sample. The swelling power was represented as the ratio of the weight of the wet sediment to the weight of the initial dry sample (deducting the amount of soluble starch). 2.5. Pasting properties The pasting properties of the starch samples (3.0 g, 14% moisture content (wet basis)) were determined by using a Rapid Visco Analyzer (RVA – 4, Newport Scientific, Australia) and profile Standard Analysis 1. Parameters including pasting temperature, peak viscosity, breakdown, final viscosity and setback were recorded. 2.6. Scanning electron microscopy (SEM) Starch granules morphology was examined by the scanning electron microscope, model DSM 940 A (Laboratory of Electron Microscopy – Embrapa CPA-CT, Brazil). Starch samples were initially suspended in ethanol at 1% (w/v). A small quantity of each sample was spread directly on the surface of the stub and dried in an oven at 32 °C for 24 h. Subsequently, all of the samples were coated with gold and examined in the scanning electron microscope under an acceleration voltage of 10 kV, a distance of 31 mm and a magnification of 2000. 2.7. Enzymatic susceptibility to a-amylase
Rice starch was isolated with 0.1% NaOH as described by Wang and Wang (2004). Rice flour was soaked in 0.1% NaOH in a 1:2 (w/ v) ratio for 18 h. Then it was blended, passed through a 63 lm screen and centrifuged at 1200g for 5 min. The soft top layer was carefully removed, and the underlying starch layer was re-slurried. The starch layer was then washed twice with 0.1% NaOH and centrifuged. The starch layer was washed with distilled water and centrifuged. The starch was then re-slurried and neutralised with 1.0 M HCl to a pH of 6.5 and centrifuged. The neutralised starch was washed with distilled water three times and dried at 40 °C until the moisture content of the samples reached about 11%.
Starches were submitted to enzymatic hydrolysis according to the method described by Serrano and Franco (2005). Starch (10 g, d.b.) was suspended in 48.5 ml of phosphate buffer (0.2 M, pH 6.0) and hydrolysed with 1 ml of a solution (0.25% w/v) of bacterial a-amylase (A6380) from Sigma, with an activity of 2150 units/mg of solid. A solution of sodium azide (0.5 ml, 10% w/v) was added to prevent bacterial growth. Starch dispersions were incubated in an orbital shaker bath (Dubnoff, Nova Ética, model 304/D, Brazil) (120 rpm) at 37 °C. After the incubation period, the suspensions were acidified to a pH of 3.0 with 0.1 N HCl; they were then stirred for 15 min to inactivate the enzymes, neutralised with 0.1 N NaOH and centrifuged at 1000g for 30 min. Hydrolysis kinetics were determined by the amount of reducing sugars (Somogyi, 1945) that were present in the supernatant, which were withdrawn from individual vials after 2, 6, 12, 18, 24, 30, 36, 42 and 48 h of incubation. Controls without the enzyme were also prepared.
2.3. Heat-moisture treatment (HMT)
2.8. X-ray diffraction
The heat-moisture treatment of rice starches was conducted on samples with moisture levels adjusted to 15%, 20% and 25% (HMT 15%, HMT 20% and HMT 25%, respectively) and equilibrated at 4 °C for 4 days. Samples were then placed in sealed glass tubes and autoclaved at 110 °C for 1.0 h (Hormdok & Noomhorm, 2007). The treated samples were subsequently dried at 40 °C until the moisture content of the samples reached about 11% and were then ground.
X-ray diffractograms of the starches were obtained with an Xray diffractometer (XRD-6000, Shimadzu, Brazil). The scanning region of the diffraction ranged from 5° to 30°, with a target voltage of 30 kV, a current of 30 mA and a scan speed of 1°/min. The relative crystallinity (RC) of the starch granules was calculated as described by Rabek (1980) by following the equation RC (%) = (Ac/ (Ac + Aa)) 100, where Ac is the crystalline area and Aa is the amorphous area on the X-ray diffractograms.
2.4. Swelling power and solubility
2.9. Statistical analysis
The swelling power and solubility of the starches were determined as described by Leach, McCowen, and Schoch (1959). Samples (1.0 g) were mixed with 50 ml of distilled water in centrifugal tubes. The suspensions were heated at 60, 70, 80 and
Analytical determinations for the samples were done in triplicate and standard deviations were reported. A comparison of the means was ascertained by Tukey’s test, to 5% level of significance using an analysis of the variance (ANOVA).
2.2. Rice starch isolation
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Fig. 1. Effect of heat-moisture treatment (HMT) on the swelling power (a) and solubility (b) of high-amylose starch (a), swelling power (c) and solubility (d) of mediumamylose starch and swelling power (e) and solubility (f) of low-amylose starch.
3. Results and discussion 3.1. Swelling power and solubility Swelling power and solubility curves of high-, medium- and low-amylose rice starches are presented in Fig. 1. The swelling power of the native and HMT starches increased as a result of increasing the assay temperature (Fig. 1a and c), due to starch gelatinisation. Starch gelatinisation involves the collapse of the starch granule manifested in irreversible changes in properties such as granular swelling, native crystallite melting, loss of birefringence and starch solubilisation (Collado & Corke, 2003). When the temperature of a starch suspension is higher than the gelatinisation temperature, hydrogen bonds are broken and water molecules
can then penetrate into the granules and hydrate free hydroxyl groups making it swell (Limberger, Silva, Emanuelli, Comarela, & Patias, 2008). However, for rice starch with a low-amylose content (Fig. 1e), the swelling power reached its maximum at 70 °C; temperatures above this only increased the soluble fraction (Fig. 1f). The swelling power of the low-amylose starch is higher than that of the medium- and high-amylose starches, which was also observed by Sasaki and Matsuki (1998), who found an inverse correlation between amylose content and swelling power of wheat starch. The swelling power of HMT starches was reduced with rising moisture content in the treatment as compared to the native starches. This phenomenon was observed with temperatures above 70 and 80 °C for high amylose content starches.
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E. da Rosa Zavareze et al. / Food Chemistry 121 (2010) 358–365 Table 1 Pasting properties of native and heat-moisture treated rice starches. Propertiesa
Amylose content
Native
HMT 15%
HMT 20%
HMT 25%
bD
bC
bB
Pasting temperature (°C)
High Medium Low
82.0 ± 0.27 85.9 ± 0.15aC 61.4 ± 0.43cD
83.8 ± 0.35 85.5 ± 0.53aC 62.5 ± 0.18cC
85.7 ± 0.35 87.2 ± 0.20aB 65.6 ± 0.00cB
88.1 ± 0.02bA 88.9 ± 0.03bA 66.9 ± 0.05aA
Peak viscosity (RVU)
High Medium Low
243.3 ± 2.37cA 244.5 ± 0.87bA 332.8 ± 1.38aB
244.3 ± 1.37bA 243.0 ± 1.00bA 320.5 ± 2.83aC
228.3 ± 3.00cB 243.3 ± 1.34bA 310.8 ± 1.00aD
195.1 ± 0.88cC 244.2 ± 3.00bA 365.8 ± 1.75aA
Breakdown (RVU)
High Medium Low
23.3 ± 0.92cA 61.1 ± 1.69bA 149.8 ± 1.59aB
14.3 ± 0.75cC 43.8 ± 0.15bB 119.3 ± 1.63aC
11.9 ± 0.40cD 18.1 ± 0.50bC 120.9 ± 0.38aC
17.1 ± 0.75bB 15.3 ± 0.68bD 191.8 ± 1.25aA
Final viscosity (RVU)
High Medium Low
363.3 ± 1.75aA 328.9 ± 1.34bA 202.6 ± 0.84cC
317.3 ± 2.08bB 326.1 ± 1.50aAB 224.6 ± 3.13cA
280.7 ± 1.34bC 323.5 ± 0.71aB 213.6 ± 2.05cB
221.7 ± 1.13bD 298.4 ± 2.59aC 199.9 ± 0.25cC
Setback (RVU)
High Medium Low
143.2 ± 2.64aA 143.7 ± 1.43aA 19.6 ± 2.13bB
87.3 ± 4.21bB 126.9 ± 0.65aB 23.4 ± 1.91cAB
64.2 ± 2.07bC 98.3 ± 2.54aC 23.7 ± 1.41cAB
43.6 ± 0.50bD 69.6 ± 1.09aD 26.0 ± 0.25cA
a Different lowercase letters in the same column for each property, and different uppercase letters in the same row, differ statistically (p 6 0.05). Results are the means of three determinations ± the standard deviation. RVU: Rapid Visco Unit.
The swelling power may be ordered by the treatments’ conditions: HMT 15% > HMT 20% > HMT 25%. These results are in agreement with Olayinka, Adebowale, and Olu-Owolabi (2008) in a study of HMT in sorghum starch, and Adebowale and Lawal (2002) in a study of Bambarra groundnut starch. Hormdok and Noomhorm (2007) reported that the HMT (20% moisture/92.5 °C/ 30 min) of rice starch (27% amylose) reduced the swelling power from 14.11 g/g of native starch to 10.29 g/g. Several authors have observed a reduction in the swelling power of the HMT in potato, cassava (Gunaratne & Hoover, 2002), rice (Hormdok & Noomhorm, 2007), sorghum (Olayinka et al., 2008) and maize starches (Chung, Liu, et al., 2009). The reduction in swelling power following hydrothermal modification has been attributed to internal rearrangement of the starch granules, which causes further interaction amongst the starch functional groups (Hoover & Manuel, 1996), making it form more ordered double helical amylopectin side chain clusters. This accounts for the increased starch crystallinity (Adebowale & Lawal, 2002). The highest solubility for all of the starches was obtained at 90 °C, where most of the granules were gelatinised or swollen (Fig. 1b, d and f). The HMT of high- and medium-amylose rice starches (Fig. 1b and d), at temperatures of 60, 70 and 80 °C presented no significant change in the solubility as compared to their native starch. However, at 90 °C, a reduction in the solubility was observed with an increase in the moisture content. There was a significant reduction in the solubility of the HMT low-amylose starch as compared to the native starch at 80 and 90 °C (Fig. 1f). These results are in agreement with Adebowale and Lawal (2002), who studied the effect of HMT on the solubility of starch at 60, 70, 80 and 90 °C, and observed a reduction in solubility at all temperatures. However, Hormdok and Noomhorm (2007) found no significant difference in the solubility of the HMT rice starch as compared with its native form. The decrease in the solubility of HMT starch indicates that there was a strengthening of the bonds, with an increase in the interactions amongst amylose and amylopectin molecules, impeding them from leaching out of the granules. 3.2. Pasting properties The HMT promoted intense changes in the starches, significantly altering their pasting profile. Table 1 shows the pasting properties of high-, medium- and low-amylose rice starch. The
pasting temperature of rice starch showed a significant increase (p 6 0.05) with an increase in the moisture content of the hydrothermal treatment (Table 1). The peak viscosity of the high-amylose starch for the HMT 20% and HMT 25% was reduced as compared to the native starch. For medium-amylose starch, the hydrothermal treatment did not affect the peak viscosity. For the low-amylose rice starch, the hydrothermal treatment reduced the peak viscosity for the HMT 15% and HMT 20% and increased in the HMT 25% (Table 1) in relation to the native starch. Hormdok and Noomhorm (2007) attributed the reduction in peak viscosity of the heat-moisture-treated rice starch to its restricted swelling capacity. The high- and medium-amylose starches had their breakdown reduced by the HMT. The low-amylose starch showed a reduction in its breakdown value for HMT 15% and HMT 20% starches, and an increase for HMT 25% starches as compared to the native starch (p 6 0.05). The reduction of the breakdown caused by HMT shows that starches are more stable during continued heating and shearing, which is in agreement with Adebowale et al. (2005), Hormdok and Noomhorm (2007), Olayinka et al. (2008) and Watcharatewinkul, Puttanlek, Rungsardthong, and Uttapap (2009). The final viscosity for the high-amylose starch showed a significant reduction (p 6 0.05) with an increase in the moisture content of the hydrothermal treatment as related to the native starch. For the medium-amylose starch, the reduction of the final viscosity happened only at HMT 25%. However, for the low-amylose starch, there was an increase in the final viscosity of HMT 15% and HMT 20% starches. There was a reduction in the setback for high- and mediumamylose HMT starches with an increase in the moisture content of the treatment. Lan et al. (2008) have shown that the setback is influenced by the amount of leached amylose, the granule size and the presence of rigid non-fragmented swollen granules. Chung, Liu, et al. (2009) found that HMT reduces the leached amylose in the starch granules and that this reduction is more significant in starches with high levels of amylose. This may explain the fact that hydrothermally treated starches cause an increasing reduction in the setback in higher amylose content starches because HMT promotes additional interactions between amylose–amylose and/or amylopectin–amylopectin chains which reduce leached amylose content and lower the setback. According to Watcharatewinkul et al. (2009), the HMT (15%, 18%, 20%, 22% and 25% moisture/100 °C/16 h) of the canna starch altered pasting profiles, resulting in an increased in the pasting
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temperature and a decrease in the peak viscosity, final viscosity and breakdown. These authors described that the changes in the pasting properties of the heat-moisture treated starches is due to the associations amongst the chains in the amorphous region of the granule and the changes in crystallinity during hydrothermal treatment. 3.3. Granule morphology by SEM The HMT at 25% moisture slightly affected the format and degree of agglomeration of the high- and medium-amylose starch granules (Fig. 2b and d), making the granules more aggregated and the surface of the granules more irregular, as compared with the native starches (Fig. 2a and c). The HMT 25% low-amylose starch (Fig. 2f) showed signs of the loss of physical integrity with
distension of the granular surface, characteristic of a partial gelatinisation, as compared with the low-amylose native starch (Fig. 2e). 3.4. Enzymatic hydrolysis with a-amylase Fig. 3a–c show the concentration of reducing sugars produced during the hydrolysis of high-, medium- and low-amylose rice starches treated by HMT, respectively. The hydrolysis of the native starches by a-amylase is inversely related to the amylose content (Fig. 3). Franco and Ciacco (1997) found a higher percentage of hydrolysis for waxy maize starch (56%) as compared with normal starch (39%) and Tester, Qi, and Karkalas (2006) also reported the same behaviour of the hydrolysis of native starches by a-amylase. Zhang and Oates (1999) described
Fig. 2. Scanning electron micrographs of high-amylose native starch (a); high-amylose – HMT 25% (b); medium-amylose native starch (c); medium-amylose – HMT 25% (d); low-amylose native starch (e); and low-amylose – HMT 25% (f).
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High amylose
1
2 3 4
5
Native
2 3 1
5
Intensity
4 2 3
HMT 15%
1 4
5
HMT 20% 1
2 3
4
5
HMT 25%
5
10
15
20
25
30
Diffraction angle (2θ)
(a)
Medium amylose
1
2 3 5
4
Native
2 3
1
Intensity
4
5
2 3
1
HMT 15% 4
5
4
5
HMT 20%
2 3
1
HMT 25%
5
10
(b)
15
20
25
30
Diffraction angle (2θ) Low amylose
1
2
3
23
5
5 2 3
Intensity
Native
4
1 4
1
HMT 15% 5
4 1
HMT 20%
2 3 5 4
HMT 25%
5
(c)
10
15
20
25
30
Diffraction angle (2θ)
Fig. 4. X-ray diffractograms of native and HMT rice starches with high- (a), medium- (b) and low- (c) amylose content. Fig. 3. Reducing sugar content of high- (a), medium- (b) and low-amylose (c) rice starches treated by HMT as a function of hydrolysis incubation time.
that enzyme susceptibility is influenced by several factors, including the ratio of amylose/amylopectin, the crystalline structure and the size of the particles. Amongst these factors, the crystalline granular structure is the most important. The high-, medium- and low-amylose rice starches treated by the HMT 25% had a higher concentration of reducing sugars at all times during the hydrolysis, as compared to HMT 15% and HMT 20% and their respective native starches. These results are similar to those of Kweon, Haynes, Slade, and Levine (2000), who observed a trend toward increased digestibility of the starches with an increase in the moisture content of the HMT. Lorenz and Kulp (1982) reported that the changes in the HMT probably occur in the amorphous regions of the granules that are more accessible to the hydrolysis. According to Gunaratne and Hoover (2002), the HMT promotes a crystalline disruption and dis-
sociation of double helical structures in the amorphous region. These authors studied the hydrothermal treatment (30% moisture/100 °C/10 h) of taro, cassava and potato starches, and concluded that there was an increase in the enzyme susceptibility of heat-moisture treated starch. The possible disruption of the crystals near the surface of the granule can facilitate the attack of the a-amylase within the granule. The HMT 25% starches had a higher enzymatic susceptibility in the first two hours of digestion as compared to the native starches. As reported by some authors (Jacobs, Eerlingen, Spaepen, Grobet, & Delcour, 1998), the amorphous areas of the starch granules are more rapidly degraded by bacterial and pancreatic a-amylase than the crystalline areas. The rearrangement caused by the hydrothermal treatment facilitated the accessibility of the amorphous areas by the enzyme. This may explain the fact that HMT 25% starches show a high concentration of reducing sugars in the first two hours of hydrolysis as compared to the native starches.
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Table 2 Intensity of the main peaks of X-ray diffractograms and relative crystallinity of native and heat-moisture treated rice starches.
a b
Intensity (CPSa)
RCb (%)
Amylose content
Treatment
1
2
3
4
5
High
Native HMT 15% HMT 20% HMT 25%
1124 1198 1190 1128
1250 1394 1370 1236
1262 1390 1368 1246
926 964 1020 1120
1122 1134 1076 994
37.7 37.0 35.8 32.7
Medium
Native HMT 15% HMT 20% HMT 25%
1266 1256 1220 1154
1478 1384 1382 1288
1424 1406 1440 1348
966 972 998 1110
1102 1102 1100 1048
38.5 38.2 37.5 31.1
Low
Native HMT 15% HMT 20% HMT 25%
1136 1288 1332 1270
1318 1466 1520 1380
1344 1508 1568 1456
742 800 846 924
1194 1274 1276 1190
49.1 49.0 44.1 40.3
CPS: counts per second. RC: relative crystallinity.
3.5. X-ray pattern and crystallinity Rice starches exhibited the typical A-type X-ray patterns of cereal starches (Fig. 4). The intensity of five main peaks and the relative crystallinity (RC) are presented in Table 2. The numbers 1, 2, 3, 4 and 5 represent the highest peaks detected in the X-ray diffractograms (Table 2). The crystallinity of the native starches followed the following order: low-amylose > medium-amylose > high-amylose (Table 2). The higher crystallinity of the low-amylose native starch could be attributed to its higher amylopectin content. The relative crystallinity decreased with an increase in the moisture of the HMT starches, though the diffraction intensity of the peak at 19° (peak 4) of the starches increased and the highamylose starch peaks at 15°, 17°, 17.8° and 23° (peaks 1, 2, 3 and 5, respectively) decreased (Fig. 4). These results suggest a decrease in the crystalline areas due to HMT. This may explain the greater enzymatic susceptibility evidenced by heat-moisture treated starches. Gunaratne and Hoover (2002) also found a reduction in the relative crystallinity in their study of potato and true yam starches treated by HMT. 4. Conclusions The HMT reduces the swelling power and solubility of rice starches. The HMT affects the pasting properties of high-amylose starches more intensely than that of the medium- and low-amylose starches, causing an increase in the pasting temperature and a reduction in the peak viscosity, final viscosity and the setback. The HMT causes a loss of physical integrity of low-amylose starch granules treated at 25% of moisture. The HMT promotes an increase in the enzymatic susceptibility of the starch and a reduction in the relative crystallinity. Therefore, HMT rice starch has a higher interest for application in foods that require a lower swelling, lower viscosity and higher thermal stability. References Adebowale, K. O., Afolabi, T. A., & Olu-Owolabi, B. I. (2005). Hydrothermal treatments of Finger millet (Eleusine coracana) starch. Food Hydrocolloids, 19, 974–983. Adebowale, K. O., & Lawal, O. S. (2002). Effect of annealing and heat moisture conditioning on the physicochemical characteristics of Bambarra groundnut (Voandzeia subterranea) starch. Nahrung–Food, 46, 311–316. Brumovsky, J. O., & Thompson, D. B. (2001). Production of boiling-stable granular resistant starch by partial acid hydrolysis and hydrothermal treatments of highamylose maize starch. Cereal Chemistry, 78, 680–689. Champagne, E. T. (1996). Rice starch composition and characteristics. Cereal Food World, 41, 833–838.
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