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Physicochemical, thermal properties and microstructure of six varieties of taro (Colocasia esculenta L. Schott) flours and starches
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Journal of Food Engineering 86 (2008) 294–305 www.elsevier.com/locate/jfoodeng
Physicochemical, thermal properties and microstructure of six varieties of taro (Colocasia esculenta L. Schott) flours and starches Aboubakar a, Y.N. Njintang b,*, J. Scher c, C.M.F. Mbofung a a ENSAI, University of Ngaoundere, P.O. Box 455, Ngaoundere, Cameroon Faculty of Sciences, University of Ngaoundere, P.O. Box 454, Ngaoundere, Cameroon c Laboratoire de Science et Ge´nie Alimentaires, ENSAIA-INPL. 2, avenue de la Foreˆt de Haye, B.P. 172, 54500 Vandoeuvre-le`s-Nancy, France b
Received 29 November 2006; received in revised form 4 October 2007; accepted 9 October 2007 Available online 17 October 2007
Abstract Taro (Colocasia esculenta L. Schott) flours were prepared from six varieties of taro corms coded as RIN, RIE, CN, CE, KW1 and KW2. Starch yields prepared from the flours varied between 66.5% for KW2 to 86.6% for RIE. The proteins content varied from 2.9% for KW1 to 4.9% for CN in the flours. Taro starch had irregular, polygonal shapes and small granular sizes. A wide variation was observed in the granular diameter of each of the starch samples and between the samples. The amylose contents varied from 14.7 ± 1.64% to 30.85 ± 0.63% as determined by the DSC method. The taro starch contained 0.2–0.6% lipids and 2% ash. The onset gelatinisation temperatures of the taro flours varied from 55.2 to 65.49 °C, whereas those of the starches are between 48.08 ± 2.46 for KW2 and 64.37 ± 2.35 °C for KW1. Retrogradation of the starches and the flours, as measured by their enthalpy changes, appeared to vary significantly between the varieties. The water absorption capacity varied from 240% to 470% and from 60% to 250% for the flours and starches samples, respectively. Taro flours had higher solubility index than their starch counterparts. Among the varieties, RIN and CE starches had the highest solubility whereas KW1 starch had the lowest. The sorption study also revealed the high ability of the taro flours to absorb water compared to their starch components. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Taro; Starch; Flour; Microscopy; Physicochemical properties; Functional properties
1. Introduction Taro (Colocassia esculenta) is a tropical tuber crop largely produced for its underground corms and consumed in tropical areas of the world. Taro has been reported to have 70–80% starch with small granules (Jane et al., 1992). Taro is also rich in gums (mucilage) and up to 9.1% crude taro mucilage has been extracted from taro corms (Hong and Nip, 1990). Because of the small sizes of its starch granules, taro is highly digestible, and as such has been reported to be used for the preparation of infant foods in Hawaii and other Pacific islands (Nip, 1997).
*
Corresponding author. Tel.: +237 99 870979. E-mail address: [email protected] (Y.N. Njintang).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.10.006
Despite its nutritional, industrial and health importance, taro has not gained sufficient research attention to enhance its potential. In addition to this fact, it has a high post harvest loss due to its high moisture content (Agbor-Egbe, 1991). As such taro has a poor position on the food security profile of countries. In fact, it has been estimated that there is an average of 30% loss during storage of these tubers, and that this portion could resolve starvation problems in non-developed countries (Agbor-Egbe, 1991). In order to minimize tuber losses, they must be converted from perishable to non-perishable through food processing operations. Since the transformation into starch or flour will decrease losses after the tubers have been harvested, value added processes such as drying and milling may be useful in order to obtain flours and starches from these tubers. Before consideration is given to taro as potential
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sources of flour and starch to produce foods, it is necessary to characterize their chemical composition, physical, physicochemical and functional properties. According to Nip (1997) taro corms and cormels are potential sources of flour and industrial starch that has not yet been utilized. It is therefore clear that a significant amount of work remains to be done on the functional characteristics of native taro starch if it is ever to become competitive with commercial starches such as corn, wheat and potato. Some chemical and physical properties of taro starches and flours have been reported on several Hawaiian (Jane et al., 1992; Sugimoto et al., 1986) and Cameroonian (Mbofung et al., 2006; Njintang et al., 2007a) varieties. But few have been interested on their functional properties. Tagodoe and Nip (1994) reported the functional properties of raw and precooked taro flours. In addition, Jane et al. (1992) reported the physical, microscopic and thermal properties of some Hawaiian taro flours and starches. Njintang (2003) initiated the study of some functional properties of Cameroonian taro flour. Latter on Mbofung et al. (2006) reported some functional properties of six varieties of Cameroonian taro flour. Njintang and Mbofung (2006) investigated the effect of precooking time and drying temperature on the physicochemical characteristics and in vitro carbohydrate digestibility of taro flour. More recently, Njintang et al. (2007b) investigated the relation between the biochemical characteristics of taro flour and the colour of the paste made from its. Njintang et al. (2007a) studied the functional properties of taro flour and their relationship to creep recovery and sensory characteristics of the paste made from it. In the same vein, Njintang et al. (2007c) evaluated the effect of taro flour reconstitution on the texture profile of achu. In addition, Njintang et al. (in press) evaluated the effect of taro flour addition on the functional and alveographic properties of wheat flour and dough. Despite of increasing interest in taro, studies on the properties of its flour and starch remained scarce. The aim of this study therefore was to obtain knowledge about the physicochemical, microstructure and flow behaviour of starches and flours produced from six varieties of Cameroonian taro corms. 2. Materials and methods 2.1. Sampling and production of flours Six varieties of taro corms and cormels, coded WCN, WCE, RIE, RIN, KW1, KW2, were used for the study. They were freshly harvested at 8–9 months of maturity from family farms in Ngaoundere. The corms and cormels were thoroughly washed with tap water hand peeled and trimmed to remove defective parts. Then the tubers were sliced and dried in air convection (CK-2000) at 50 ± 2 °C. The dried slices were first hammer milled (Culatti polymix, France) to pass through a 500 lm screen. The flours were divided into two parts: one part was stored in polyethylene bags and kept in a refrigerator at 4 °C prior
295
to use while the second part was used for isolation of starch. 2.2. Isolation of taro starch Taro flour (100 g) was steeped in water solution (3 L) at 35 °C for 12 h. The slurry obtained was homogenised for 30 min using a commercial blender (model moulinex, France). The suspension obtained was screened using 150 lm sieve and keep to sediment for 24 h. The crude starch was then collected and washed twice with water, dried for 48 h in an oven set at 50 °C and carefully stored in sealed and dried polyethylene bags until required for analysis. The yield of extraction of starch was evaluated by the gravimetric method of Singh et al. (1989). 2.3. Proximate and mineral analysis Flours of each tuber were analysed for moisture (air oven method), fat (Soxhlet) and ash (incineration method) content, as a percentage (w/w), following AACC standard methods (1990). Total proteins (Nx6.25) was analysed using approved methods of Kjeldahl (AACC, 1990) in semi automatic machine (GEHARDT, Paris, France). Reducing sugars were analysed using the colorimetric method of Fisher and Stein (1961). Phosphorous content of starches and flours, as a percentage of sample weight (w/w), was determined following the spectrophotometric method of AOAC (2000) while minerals (Ca, Na, Mg, K, Mn, Mg, Fe) content, as a percentage of sample weight (w/w), were analysed using an atomic absorption (AA) spectrophotometer. 2.4. Determination of the amylose content and amyloglucosidase hydrolysable starch Amylose content was analysed using a DSC (differential scanning calorimetry) method as described by Mestre et al. (1996). The amylose content was calculated using the equation: Amyloseð%Þ ¼ 100
A DH 1 ; DH 2 S
where A and S were, respectively, the weights of the standard amylose and sample used in the experiment, DH1 and DH2 their respective enthalpy change. 2.5. Differential scanning calorimetry measurements (DSC) DSC thermograms of taro starches and flours were recorded on a Perkin–Elmer model Pyris1 (Perkin–Elmer Corp Norwalk. USA), with heating rate of 5 °C/min and temperature rate range of 25–98 °C. Starch, flour or amylose were dispersed in distilled water (1:3; w/v) in an aluminium pan and hermetically sealed. The instrument was calibrated for temperature and enthalpy measurement with indium, and an empty pan was used as reference. The manufacturers’ software was used to calculate the heat capacity and
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integrate the peaks. The onset and end temperatures of the gelatinisation peaks were determined by the intersection of tangents fitted to the leading and trailing flanks of the peak with the baseline.
mined using the nonlinear power equation category of Sigma plot 8.02 (Chicago, IL, USA) statistical package.
2.6. Colour characterisation
Granule morphology was examined using scanning electron microscope (SEM) microscope. A thin layer of starch granule was mounted on the aluminium specimen holder by double-sided tape. The specimen holder was loaded in a Emitech K550 sputter coater (Emitech, Ashford, UK). It was coated with gold palladium, with a thickness of about 30 nm. Scanning electron microscopy was carried out using a Leica Stereoscan 360 SEM (LEO, Cambridge, UK) operated at an accelerating voltage of 10 kV. Light micrograph of the starch samples colored with diluted iodine in potassium iodide was taken with a Diplan-microscope Model GF (Leitz Wetzlar Germany).
Colour measurements of the starch were carried out using a Chromameter CR210 (Minolta France S.A.S., Carrrie`res-sur-Seine) on the basis of L* a* and b* values. The instrument was calibrated against a standard light yellowcoloured reference tile. A glass cell containing the powdered flour was placed above the light source and covered with a white plate and L*, a* and b* values were recorded. 2.7. Starch particle analysis and pasting properties A Coulter Particle Counter, Model Mastesizer S (Malvern Instruments, Orsay) was used to measure the percent volume of starch particles distributed within selected size ranges for dry powder measurement/loose particle distribution as described by Njintang et al. (2006). Flow behaviour of 2.5% flour and starch samples in distilled water heated at 80 °C for 10 min were investigated at 25 ± 1 °C by a parallel plate rotational rheometer (Stress tech, UK). The paste was allowed to equilibrate for 5 min and was tested using a 1 mm gap. The sample was sheared at a programmed rate linearly increasing from 0 to 200 s1 in 300 s. 2.8. Equilibrium moisture content (EMC) and adsorption isotherm The EMC of the taro starch was determined at 20 °C according to the static gravimetric method of Wolf et al. (1985). The desorption isotherms were determined on samples hydrated in a glass jar over distilled water at a room temperature to approximately 30% dry basis moisture content. Samples of 1 ± 0.02 g were weighed in weighing bottles which were put in hygrostats with seven saturated salt solutions (LiCl, MgCl2, K2CO3, NaBr, NaCl, KCl) used to obtain constant water activities environments between 0.1 and 0.9 (Bell and Labuza, 2000). All the salts used were of reagent grade. At high water activities (aw > 0.70) crystalline thymol was placed in the hygrostats to prevent the microbial spoilage of the flour. The hygrostats were kept in thermostats at 20 ± 0.2 °C. Samples were weighed (balance sensitivity ± 0.0001 g) every three days. Equilibrium was acknowledged when three consecutive weight measurements showed a difference less than 0.001 g. The moisture content of each sample was determined by the oven method (105 °C for 24 h) by means of triplicate measurements. The resulting adsorption curve was tested to follow the Oswin model of adsorption of the general form M (g/g) = Ax[aw/(1 aw)]B where M is the moisture content expressed in g/g dried weight, and aw is the water activity. The constant A (g/g) and C were deter-
2.9. Microstructure of starch granules
2.10. Water absorption capacity and water solubility index of taro flour and starch For the determination of these variables, 0.5 g of starch was suspended in 10 mL of distilled water and incubated with mixing for 15 min in a shaking water-bath (Kottermann, Germany) set at 70 °C and centrifuged at 5000 rpm for 30 min. The pellet was dried at 105 °C for 12 h and the water absorption capacity and the water solubility index evaluated according to Anderson et al. (1969) and Phillips et al. (1988), respectively. 2.11. Infrared spectroscopy of taro flour and starches Fourier transform infrared (FT-IR) spectroscopy was used to evaluate structural differences in the samples. An appropriate amount of starch powder was pressed into a crystal window and the FT-IR spectra were recorded on a Fourier transform infra red spectrometer in absorbance mode from 4000 to 400 cm1, co-adding 32 scans at 4 cm1 resolution (Van Soest et al., 1995). Ten replicates were collected for each sample. The obtained spectra were transferred into a data analysis package. 2.12. Statistical analysis All analyses were carried out in triplicate and data expressed as means ± standard deviation. Analysis of variance was performed to calculate significant differences in treatment means, and Duncan multiple test range was used to separate means using the Statgraphics (1997) statistical software. 3. Results and discussion 3.1. Chemical characteristics of taro flour Proximate compositional profile of the different varieties of taro flour is given in Table 1. As expected the results
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Table 1 Proximate compositional profile of the different varieties of taro flour Parameters
Varieties RIE
RIN
CE
CN
KW1
KW2
Moisture (g/100 g) Proteins (%) Total carbohydrate (%) Starch yield (%) Fats (g/100 g) Red. sugars (g/100 g) Ash (g/100 g) Minerals Ca (mg/100 g) Na (mg/100 g) Mg (mg/100 g) K (mg/100 g) Mn (ppm) Fe (ppm) Zn (ppm) Cu (ppm)
8.2 ± 0.2a 3.1 ± 0.1ab 94.8 86.6 0.50 ± 0.10 13.8 ± 0.1b 1.6 ± 0.35a
9.0 ± 0.4bc 3.4 ± 0.1c 93.9 84.36 1.17 ± 0.32 13.5 ± 0.1a 1.5 ± 0.4a
9.6 ± 0.1c 4.6 ± 0.1d 90.5 82.12 0.53 ± 0.15 19.5 ± 0.4d 4.4 ± 0.3b
8.5 ± 0.4ab 4.9 ± 0.2e 91.0 79.85 0.30 ± 0.07 26.7 ± 0.1e 3.8 ± 0.9b
8.5 ± 0.1ab 2.9 ± 0.1a 95.5 68.8 0.33 ± 0.06 14.6 ± 0.1b 1.3 ± 0.6a
8.8 ± 0.2b 3.3 ± 0.1bc 90.8 66.5 0.37 ± 0.06 15.5 ± 0.2c 5.5 ± 1.1c
25.4 <0.5 61.9 5.2 6.4 41.7 42.8 1.2
55.9 4.8 32.9 27.5 13.1 2.4 0.4 0.4
111 4.6 88.9 59.7 62.4 17.6 1.1 1.1
30.7 4.8 90.4 23.7 130.3 15.0 1.0 1.0
91 <0.5 106.7 3.5 127.3 13.1 1.3 1.3
192 5.6 382 11.4 85.6 21.8 1.8 1.8
Means ± SD; Figures in line followed by different superscripts indicate significantly (p < 0.05) different values determined by Duncan’s Multiple Range Test.
showed that carbohydrates, composed mainly of starch, was the most important chemical component in the flours while the proteins and fats were very limited. The carbohydrate levels (calculated by difference) varied from 90.5% to 95.5% while the starch yield varied from 66.5% to 86.6%. The starch yields obtained in this study are in agreement with that (51–58%) reported by Jane et al. (1992) on Hawaiian taro varieties. In addition, reducing sugars in taro flour were also found at high levels. The CN variety exhibited the highest level (26.7%) while the RIN and RIE varieties exhibited the lowest (13.5–13.8%). The difference in the reducing sugars content might reflect the fragility of the starch within varieties. The table also showed that beside carbohydrates, ash represented another important group of component in taro flour with values ranging from 1.3% (variety RIN) to 5.5% (variety KW2). These values are comparable to those reported for most tubers but are higher compared to that reported for cereal crops (Swinkels, 1985). Among minerals that composed the ash fraction of the taro flours, and irrespective of the variety, Ca and Mg were the most abundant with range values of 25.4–192 mg/100 g flour and 32.9– 382 mg/100 g flour, respectively. The relative high amount of calcium in flours was expected since taro belongs to the family araceae recognised to be rich in oxalate calcium crystals responsible for the irritation of uncooked taro. In this respect, Sira (2000) assigned the high level of ash and Ca in taro flour to oxalate. In the present work, we found that the presence of Ca in taro flours could be responsible of the high ash content. In fact, a positive significant correlation (r > 0.69; p < 0.05) between the ash content and Ca or Mg content was established. The fact that magnesium was correlated to ash content might also suggest its role in oxalate crystals formation, but this need to be investigated. Varieties KW1 and KW2 contained the highest levels of Ca and Mg, whereas RIN variety contained the
lowest levels. These observations are in agreement with findings by Njintang (2003), which reported the absence of oxalate crystals in RIE and RIN variety. Beside the major mineral metals cited above, substantial amount of some oligoelements such as Fe, Mn, Cu and Zn were found in flours. Iron is one of the more abundant oligoelement in taro flour with level varying from 2.4 ppm (RIE variety) to 41.7 ppm (RIN variety). 3.2. Amylose, phosphorus and amyloglucosidase hydrolysable starch in native taro flour and starch The ability of the native taro starch to amyloglucosidase hydrolysis is presented in Table 2. The level of hydrolysable starch varied from 37.75% (variety CN) to 58.43% (variety KW2). A negative correlation (R = 0.32, p < 0.05) was observed between the reducing sugars content and the Table 2 Amylose, phosphorus and amyloglucosidase hydrolysable starch in native taro flour and starch Variety
Sample
Amylose (%)
Hydrolysable starch (%)
Phosphorus (mg/100 g)
RIE
Starch Flour
30.85 ± 0.63 26.05 ± 0.27
43.14 ± 2.54 45.07 ± 1.30
0.83 ± 0.08 1.97 ± 0.02
RIN
Starch Flour
16.65 ± 0.15 14.70 ± 1.64
51.23 ± 1.27 58.87 ± 2.60
0.88 ± 0.08 1.96 ± 0.03
CE
Starch Flour
26.77 ± 2.47 18.53 ± 1.92
54.83 ± 1.27 56.11 ± 3.90
0.76 ± 0.18 1.76 ± 0.09
CN
Starch Flour
28.38 ± 2.86 16.07 ± 1.96
37.75 ± 2.54 46.91 ± 1.30
0.82 ± 0.16 1.66 ± 0.33
KW1
Starch Flour
25.43 ± 0.42 19.40 ± 0.04
56.63 ± 1.27 50.59 ± 1.30
0.77 ± 0.22 1.44 ± 0.09
KW2
Starch Flour
29.21 ± 1.50 24.10 ± 1.56
58.43 ± 1.27 61.63 ± 1.30
1.43 ± 0.27 1.36 ± 0.03
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hydrolysis levels. A similar negative linear correlation (R = 0.45, p < 0.05) was observed between the hydrolysis level and the amylose content of the starch. This result suggested that high levels amylose starch hydrolysed more slowly than low level amylose starch. This was expected in the way that amylose is the starch portion which exist in the form of double helix chain, not easily accessible by enzymes. In concordance with this study, Skrabanja et al. (1999) found that pea flour with high amylose content possessed low starch digestibility while pea flour with low amylose content exhibited high starch digestibility; and this has been reported to be related to the ability of amylose to undergo retrogradation and form resistant starch. The amylose content (Table 2) of all the flour samples were relatively close to those (18–24%) reported by Jane et al. (1992) using the gel permeation chromatography and iodine potentiometric method. Using the iodine colorimetric method, Sira (2000) and Pe´rez et al. (2005) obtained an amylose content of 30.3% and 30.6% respectively. In addition, the amylose content of taro starch were in the range values (21–23%) reported for six cultivars of potatoes tubers (Walter et al., 2000) using the colorimetric and the DSC methods. Phosphorus, which has been reported to be covalently linked to the starch and affected its properties (Takeda et al., 1986), ranged in starches from 0.76 mg/100 g (variety CE) to 1.36 mg/100 g (variety KW2). These values were higher compared to those reported by Pe´rez et al. (2005) for Colocasia esculenta (0.01 mg/100 g), Xanthosoma sagittifolium (0.07 mg/100 g) and Manihot esculenta (0.05 mg/ 100 g). For the starch, it was observed a relative negative linear correlation (r = 0.43; p < 0.05) between the hydrolysable level and the phosphorus content. This indicated that native starch with high level of phosphorus exhibited low digestibility. A possible explanation is that lower phosphorus content in starch granules generally results in higher resistant starch (Liu et al., 2007). The levels of phosphorous in tuber starches is typically less than 500 mg/ 100 g and is usually referred to ash (Thomas and Atwell,
1999). Thus, a positive linear correlation (r = 0.65; p < 0.05) was observed between the phosphorous level and the ash content in our flour samples. 3.3. Colour and sorption characteristics of taro starches The L*, a* and b* characteristics of the starch sample are grouped in Table 3. The colour characteristics are all in the same order of values: L* (83–94), a* (1.5–5.7) and b* (3.0– 13.8). The result showed that the flour were white (high value of L) less red (low a* value) and less yellow (low b* value). It has been hypothesised that the variation in b* value among samples may be attributed to the amount of carbohydrate and proteins content due to their role in development of non enzymatic browning (Jamin and Flores, 1998). In the present study, no significant correlation was observed between the colour coordinate and neither proteins nor reducing sugars levels of the starch sample. However, it is worth to mention the positive correlation between the proteins or carbohydrate and a* (r = 0.44) or b* (r = 0.24), showing that the composition of the flour has some effect on its colour. The ability of taro starch and flour to adsorb water is presented in Fig. 1. The figure known as moisture isotherm describes the variations in equilibrium moisture content with water activity (aw) at 20 °C. The equilibrium was reached after 3–4 days for the different starch and flour samples. The figures indicated that variation in the moisture content of flours and starches depends on the relative humidity (RH) of the atmosphere in which they have been stored. If the RH decreases, the starches reject moisture; if the RH increases, they absorb humidity (Swinkels, 1985). The shape of the curves is sigmoid, showing one inflection point, which has been reported to characterise materials with high sugars content (Maskan and Go¨gu¨s, 1997). Such material samples present typical curve with three different regions (Chungcharoen and Lund, 1987): region 1, corresponding to aw < 0.2, which relates to adsorption of monomolecular film of water, region B for aw range of 0.22–0.7
Table 3 Colour and adsorption characteristics of taro flours and starches Variety
Sample
Oswin constant B (101)
Oswin constant A (102 g g1)
Colour characteristics L*
a*
b*
RIE
Starch Flour
6.38 ± 0.68 8.49 ± 1.38
3.99 ± 0.51 3.16 ± 0.86
94.9 ± 0.3 87.7 ± 6.0
1.5 ± 0.1 1.45 ± 0.14
3.0 ± 0.25 6.98 ± 0.31
RIN
Starch Flour
7.44 ± 1.00 6.20 ± 0.80
3.18 ± 0.61 5.19 ± 0.76
93.9± 0.5 93.63 ± 0.41
2.1 ± 0.1 2.20 ± 0.07
4.7 ± 0.34 8.86 ± 0.11
CE
Starch Flour
11.05 ± 1.78 6.55 ± 1.63
1.93 ± 0.71 5.44 ± 1.60
88.7 ± 0.3 92.24 ± 0.59
5.1 ± 0.1 1.92 ± 0.02
9.1 ± 0.1d 9.36 ± 0.13
CN
Starch Flour
10.63 ± 1.16 9.19 ± 0.89
2.20 ± 0.52 3.10 ± 0.55
90. 8 ± 0.7 93.36 ± 0.47
3.5 ± 0.2 1.92 ± 0.04
7.2 ± 0.5c 9.31 ± 0.16
KW1
Starch Flour
9.76 ± 0.90 10.21 ± 0.85
2.69 ± 0.49 2.41 ± 0.41
93.5 ± 0.5 88.30 ± 0.42
2.5 ± 0.2 4.20 ± 0.10
5.2 ± 0.3b 11.85 ± 0.16
KW2
Starch Flour
5.75 ± 0.62 6.30 ± 0.80
3.18 ± 0.36 5.09 ± 0.75
83.2 ± 0.5 84.40 ± 40
5.7 ± 0.2 5.67 ± 0.23
13.8 ± 0.2e 13.91 ± 0.30
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Equilibrium moisture content (g/g)
0.30 RIN CN RIE KW2 KW1 CE
0.25
0.20
0.15
0.10
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Water activity
Equilibrium moisture content (g/g)
0.25
0.20
RIN CN RIE KW2 KW1 CE
0.15
0.10
299
In general, the equilibrium moisture content increases rapidly at low water activity aw (0–0.13), and then rises slowly between 0.15 and 0.7, followed by a steep rise above 0.7. The variation of equilibrium moisture with aw was shown to follow the Oswin model (r2 > 0.95; p < 0.01). The constants A from the model shown in Table 3 varied between varieties reflecting the difference in their adsorption. In general, the parameter A was generally higher for the flours than for the starch, suggesting an important contribution of non starch compounds on the adsorption of the flour. Reverse observations were made on the constant B and in this respect a negative correlation (r2 = 0.69; p < 0.01) was shown to exist between A and B. Values of A ranged from 0.019 (variety CE) to 0.039 (variety RIE) for starches and from 0.024 (variety KW1) to 0.054 (variety CE) for flours. For the B values, ranges were 0.57–1.10 for starches and 0.62–01.02 for flours. The A and B values obtained in this study were in the range but systematically higher (for A) and lower (for B) than that reported for corn (A = 0.0787, B = 0.32) (Al-Muhtaseb et al., 2004) and potatoes starches (A = 0.069, B = 0.39) (Peng et al., 2007). 3.4. Differential scanning calorimetry (DSC)
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Water activity
Fig. 1. Sorption isotherm of taro flours (a) and starches (b).
corresponding to adsorption of additional layers over this monolayer, and region C for aw range = 0.7–0.99 corresponding to condensation of water in the pores of the material followed by dissolution of soluble material (Benado and Rizvi, 1985).
The thermal transition of the six taro starches were detected by DSC. All the starch samples exhibited a thermogram with a single endothermic transition typical of the gelatinisation process (Biliaderis and Seneviratne, 1992; Iturriaga et al., 2004). The DSC results are presented in Table 4. No significant difference was observed on the peak gelatinisation temperature and the onset gelatinisation temperature of the starches and the flours. This was not the case in literature where Jane et al. (1992) observed in their studies that the gelatinisation temperatures of the flours were higher than those of the starches. However, a significant difference (p < 0.001) was observed on the enthalpy (DHG) with starches generally exhibiting higher
Table 4 Gelatinization profile (°C) taro flours and starches Variety
Samples
DSC parameters T0
Tp
Tc
DH (J/g)
RIN
Flour Starch
60.93 ± 2.36 63.41 ± 2.38
64.77 ± 2.77 67.24 ± 3.21
66.42 ± 1.27 70.27 ± 3.61
8.43 ± 0.21 11.04 ± 0.86
RIE
Flour Starch
55.20 ± 1.2 54.93 ± 2.64
61.27 ± 0.71 56.48 ± 1.76
64.96 ± 0.90 60.78 ± 2.51
14.95 ± 0.91 16.93 ± 1.34
CE
Flour Starch
57.47 ± 0.97 57.85 ± 3.14
62.78 ± 2.89 74.54 ± 2.20
67.05 ± 2.67 68.81 ± 3.5
11.41 ± 0.89 15.37 ± 0.82
CN
Flour Starch
56.80 ± 1.57 62.91 ± 2.82
59.42 ± 0.61 64.29 ± 3.41
61.78 ± 0.87 66.4 ± 2.25
9.22 ± 0.87 15.19 ± 2.21
KW1
Flour Starch
65.49 ± 1.53 64.37 ± 2.35
69.75 ± 2.34 67.64 ± 1.67
73.33 ± 3.13 69.53 ± 2.6
11.13 ± 0.01 14.59 ± 0.24
KW2
Flour Starch
56.14 ± 1.23 48.08 ± 2.46
58.99 ± 1.11 54.40 ± 3.16
61.24 ± 3.12 57.91 ± 2.50
13.36 ± 0.66 16.75 ± 0.84
T0, onset gelatinisation temperature; Tp, peak gelatinisation temperature; Tc, end temperature of gelatinisation.
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values (mean 14.99 J/g) and flours having lower values (mean 11.42 J/g). These results are in agreement with findings by Ahmed et al. (2008) who reported a higher heat of gelatinisation (DHG) of 16.2 J/g for rice starch and a lower heat of gelatinisation to whole rice flour (4.81 J/g). This clearly show that the gelatinisation profile of flours may be influenced not only by the starch composition (amylose to amylopectin ratio), but also by others such as the granular architecture (crystalline to amorphous ratio), the molecular structure of amylopectin (extent of branching, unit chain length) (Gunaratne and Hoover, 2002). Furthermore, high amylose starches with longer average chain have been reported to exhibit higher transition temperatures (Jane et al., 1992). In all cases, a significant effect (p < 0.05) of variety was observed on the temperatures and the enthalpy of gelatinisation. The peak temperature of gelatinisation varied between 55.56 and 68.67 °C with CN, KW2 and RIE exhibiting lower values and RIN, KW1 and CE exhibiting higher values. DHG values varied from 8.43 J/g (variety RIN) to 14.95 J/g (variety RIE) for flours and from 11.04 J/g (variety RIE) to 16.93 J/g (variety RIE) for starches. The gelatinisation profiles of starches shown in Table 4 are quite similar to those reported by Jane et al.
(1992) for taro flour and starch. However, the initial, middle and end gelatinisation temperatures were lower than those reported by Sira (2000) either for the flours or for the starches. In addition, Sira (2000) reported a significant correlation between Tp and amylose content of taro starch. Similar trend was observed in our study even though the correlation was not significant (r = 0.37). 3.5. Microstructure and particle size of taro starch Scanning electron micrograph showed that taro starch granules had polygonal and irregular shapes of diameter lower than 5 lm (Fig. 2). The particle size distribution obtained from the master sizer (Fig. 3) exhibited a monomodal distribution with a mean diameter of 3–20 lm, higher than that reported by other authors on taro starch which were 1–5 lm (Jane et al., 1992; Sugimoto et al., 1986). The results obtained suggested that the starch granules are associated as cluster or compounds since extraction has not been done in alkaline solution which is known to dissociate them. Microscopic observations of the starch samples confirmed this association. Meaningful observations of the particle size give diameters of each of the starch samples varying from 1 to 5 lm, in agreement
Fig. 2. Scanning electron microscopy (SEM) of starch granules isolated from taro flours varieties KW1 (a), RIE (b), RIN (c), CN (d), CE (e).
Aboubakar et al. / Journal of Food Engineering 86 (2008) 294–305 8
301
3.0
2.5
KW1 KW2 CE
2.0
CN NIE RIN
Absorbance
Percentage (%)
6
4
2
1.5
1.0
0
0.5
0
200
400
600
800
1000
0.0 4000
1200
3000
1000
0
1000
0
4
10
8
3
6
Absorbance
Percentage (%)
2000
Wavenumber (cm-1)
Particle size (µm)
4
CE KW2 RIE RIN KW1 CN
2
1
2
0 0
0
20
40
60
80
100
120
140
-1 4000
3000
Fig. 3. Particle size distribution of the taro flours (a) and starches (b).
with the granule size of taro starch reported in the literature. No significant differences were observed on the particle size of the different varieties although compounds granules were more pronounced in the CN and KW1 samples. The size distribution association as individual (simple) or clusters (compound) and composition (lipid, moisture, protein and minerals content) reflect the different origins of starches (Tester and Karkalas, 2002). 3.6. Fourier transformed infrared To get information on the possible difference in starch granules structures between taro starch samples, their FT-IR were recorded. The spectra obtained for the six samples were similar based on the form, but differed in the intensity of the major peaks as shown in Fig. 4. The spectra showed high absorption at the wave numbers 574, 1020– 1026, 1056, 1151, 1365, 1631, 2922 and 3400 cm1 confirming the carbohydrate nature of the samples. According to Van Soest et al. (1995), the bands at 1047 and 1022 cm1 describe the crystalline and amorphous properties of starch, respectively, and their ratios indicate the degree of starch order (Sevenou et al., 2002; Van Soest et al., 1995). In this study, such bands really appeared but the one at 1047 cm1 was so broad that we used the one at
2000
Wavenumber (cm-1)
Particle size (µm)
Fig. 4. Infra red spectroscopy of the different taro flours (a) and starches (b).
1056 cm1. In processing the FT-IR data, we extracted the intensity of the bands and the resulting values, grouped in Table 5 showed that the crystallinity and amorphous indexes of the starch ranged, respectively, from 1.17 (KW2) to 2.63 (RIN) and 1.27 to 3.51. The values of the ratio which varied from 0.75 (RIE) to 0.93 (KW2 variety) seem to suggest in general that taro starch is more amorphous than crystalline at the granule surface. It is worth mentioning that RIE variety is more crystalline than the CE, KW1 and KW2 varieties. The values of the ratio obtained in the present work are not quite different from Table 5 Crystallinity (I1022), amorphousness (I1056) and order index (I1056/I1022) of taro flour and starch samples FT-IR characteristics
Sample
Crystalline index
Varieties RIE
CE
CN
RIN
KW1
KW2
Flour Starch
2.7 3.5
1.9 1.4
1.8 1.7
2.1 2.0
1.7 1.5
2.0 1.3
Amorphous index
Flour Starch
2.3 2.6
1.6 1.3
1.5 1.3
1.8 1.7
1.5 1.5
1.7 1.2
Starch granule order
Flour Starch
0.82 0.75
0.84 0.89
0.87 0.81
0.84 0.86
0.91 0.95
0.87 0.93
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0.8 reported by Olkku and Rha (1978) and Van Soest et al. (1995) for cowpea and potatoes starches, respectively. Comparatively, the values calculated for the starch samples were not significantly different from their corresponding flours justifying that starch is the most important component of taro flour. 3.7. Water absorption capacity (WAC) and water solubility index (WSI) The swelling capacities of the starches are presented in Fig. 5. In general, increase in temperature led to significant increase in WAC up to a maximum situated at temperature 60–70 °C from which they decreased. Above 50 °C little changes on WAC occurred, while beyond water absorption was most conspicuous with the RIE, KW2 and CN being highly prone to swelling. The slow swelling pattern of the starches and flours at relatively low temperatures is confirmed by the report of Agunbiade and Longe (1999). The swelling behaviour of the starches may be affected by the structural arrangements of their amylose and amylopectin (Schoch and Mayfield, 1956). Thus it is most probable that the granular arrangements of RIE, KW2 and CN are less compact and, therefore, may have higher intermo-
lecular areas than the other three varieties. When gelatinisation temperature was reached, the loose of associative hydrogen in the starch granules quickly resulted by hydration, resulting in swelling of the granules (Biliaderis and Seneviratne, 1992). From the results shown in Fig. 1 it can be seen that RIE, KW2 and CN starches and flours held slightly more water than RIN, KW1 and CE varieties (10.06 g H2O g1) and that WAC of taro flours were systematically higher than that of their starch counterpart. These results suggested that the other non-starch components of the flours such as mucilage contributed highly to the absorption of the flour. The variation in WAC of the starches could be due to the difference in the degree of engagement of hydroxyl groups to form hydrogen and covalent bonds between starch chains (Hoover and Sosulski, 1986). Whatever the case the similar trend observed for the water absorption capacity for the flours and starch, confirmed by significant correlation observed (r = 0.87; p < 0.001) suggested that starch play a major role in the binding capacity of starch. The increase in water absorption capacity has always been associated with the increase in the amylose leaching and solubility, and lost of starch crystalline structure (Gunaratne and Hoover, 2002). This was the case in the
36 500
34 450
Water absorption capacity (gH2O/100g flour)
Water solubility index (g/100g flour)
32 30 28 26 24 RIE RIN CE CN KW1 KW2
22 20 18
400
350
RIE RIN CE CN KW1 KW2
300
250
16 0
20
40
60
80
200
100
0
Soaking temperature (°C)
20
40
60
80
100
Soaking temperature (°C)
40 300
RIE RIN CE CN KW1 KW2
250
Water absorption capacity (gH2O/100g flour)
Water solubility index (g/100g starch)
35
30
25
20
15
200
150 RIE RIN CE CN KW1 KW2
100
50
0
20
40
608
80
100
Soaking temperature (°C)
Fig. 5. Water solubility index of the different varieties of taro flour (a) and paste (b).
0
20
40
60
80
100
Soaking temperature (°C)
Fig. 6. Water absorption capacity of different varieties of taro flour and starch.
Aboubakar et al. / Journal of Food Engineering 86 (2008) 294–305
303
Table 6 Flow behaviour, K, and consistency index, n, of different varieties of taro flours and starches Parameters
Sample
Varieties RIE
RIN
CE
CN
KW1
KW2
K (Pa sn)
Flour Starch
19.6 ± 0.7 10.6 ± 0.3
14.8 ± 0.4 4.20 ± 0.13
1.0 ± 0.00 0.06 ± 0.01
1.6 ± 0.05 8.78 ± 0.2
9.9 ± 0.3 10.44 ± 3.97
9.9 ± 0.3 52.3 ± 0.5
n
Flour Starch
0.23 ± 0.01 0.38 ± 0.01
0.16 ± 0.00 0.42 ± 0.01
0.86 ± 0.05 0.35 ± 0.01
0.61 ± 0.01 0.17 ± 0.01
0.39 ± 0.01 0.13 ± 0.05
0.39 ± 0.01 0.30 ± 0.01
present work where a significant correlation was observed between the water absorption and the water solubility, either for the flour samples (r = 0.37, p < 0.01) or for the starch samples (r = 0.56, p < 0.001). The variations in WSI of flour and starches are presented in Fig. 6. CE, CN, KW1 and KW2 flours and starches exhibited very low solubility at 20 °C and formed only a temporary suspension when stirred in water. According to Eliasson and Gudmundsson (1996), the low solubility of starches at low temperatures could be attributed to the semi-crystalline structure of the starch granules and the hydrogen bonds formed between hydroxyl groups in the starch molecules. As the temperature increased, the solubility increased due to the disruption of starch granules and exposure of hydrophilic groups. As expected the maximum solubility (35%) reached by the different samples at higher temperature reflects the amylose contents of the samples. Indeed amylose and consequently starch appear as the major constituents of the flours that solubilise during heating. In this respect, a significant correlation was observed between the solubility index of the flours and starches (r = 0.76, p < 0.001). 3.8. Flow properties The flow properties of taro starches and flours dispersions were studied in the shear stress range 100–500 Pa and the results are presented in Table 6. All the tested flour and starch systems behaved to a very good approximation (r2 = 0.96–1.00) as power law liquid over this range of shear rates. The good fit of power law model confirmed the linearity of the curves for these starches. The starch and flour paste showed pseudoplastic behaviour (shear thinning) and the flow behaviour index (n) was observed to be least for RIN flour (0.16) and KW1 starch (0.13) and highest for CE flour (0.86) and RIE starch (0.38). However, the consistency indices (K) of CE flour (1.0 Pa sn), and starch (0.06 Pa sn) were shown to be least while the RIE flour (19.6 Pa sn) and KW2 starch (52.3 Pa sn) were highest. Viscosity development of a paste has been mainly related to the water binding capacities of the dry ingredients (Dogan et al., 2005). According to these authors, flour which binds the maximum amount of water exhibits a high viscosity. In this respect, a significant correlation was found between the consistency index K and the water absorption capacity of the flour (r = 0.90, p = 0.01). In addition, high
value of consistency index K has been associated to flour possessing high solubility (r = 0.78, p < 0.05) in concordance with study by Dogan et al. (2005) reported on soy flour and rice. According to these authors, in a solution containing flour with less water absorption capacity, there is more free water available to facilitate the movement of particles and as such, low viscosity value is obtained. 4. Conclusion The results obtained showed some differences on the structure and properties of the varieties of taro flours and starches. The study revealed that taro flours are mainly made of carbohydrates and particularly starch. The granule size of the starch (1–5 lm) varied significantly within each variety but not significantly between them. While no significant difference exist on the gelatinisation temperatures of the flours and starches, the initial and the peak temperatures of gelatinisation of the taro starches vary significantly among varieties with general averages of 55 and 65 °C, respectively. The gelatinisation enthalpies are systematically higher for starches that for flours with RIE exhibiting the highest value (14.9 for flour and 16.9 for flour) and RIN the lowest (8.43 for flour and 11.04 for starch). The amorphous index, the water absorption capacity, and the water solubility index of the taro starch equally vary significantly with varieties. Taro flours possessing higher water absorption capacity and higher water solubility index give gels with higher consistency index. Acknowledgements The authors thank with greatness the financial support from the Cooperation Francßaise which helps the first author to carry out the study in the Laboratoire de Sciences et Genie Alimentaire de Nancy, INPL, France. Part of this study was also financed by the International Foundation for Science (IFS, Stockholm, Sweden) and the Committee on the Scientific and Technological Cooperation of the Organisation of Islamic Conference (COMSTECH, Islamabad, Pakistan) through a grant to Yanou Nicolas Njintang. References AACC (1990). American Association of Cereal Chemists. Approved methods of the AACC (7th ed.). St. Paul, MN.
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Journal of Food Engineering 86 (2008) 294–305 www.elsevier.com/locate/jfoodeng
Physicochemical, thermal properties and microstructure of six varieties of taro (Colocasia esculenta L. Schott) flours and starches Aboubakar a, Y.N. Njintang b,*, J. Scher c, C.M.F. Mbofung a a ENSAI, University of Ngaoundere, P.O. Box 455, Ngaoundere, Cameroon Faculty of Sciences, University of Ngaoundere, P.O. Box 454, Ngaoundere, Cameroon c Laboratoire de Science et Ge´nie Alimentaires, ENSAIA-INPL. 2, avenue de la Foreˆt de Haye, B.P. 172, 54500 Vandoeuvre-le`s-Nancy, France b
Received 29 November 2006; received in revised form 4 October 2007; accepted 9 October 2007 Available online 17 October 2007
Abstract Taro (Colocasia esculenta L. Schott) flours were prepared from six varieties of taro corms coded as RIN, RIE, CN, CE, KW1 and KW2. Starch yields prepared from the flours varied between 66.5% for KW2 to 86.6% for RIE. The proteins content varied from 2.9% for KW1 to 4.9% for CN in the flours. Taro starch had irregular, polygonal shapes and small granular sizes. A wide variation was observed in the granular diameter of each of the starch samples and between the samples. The amylose contents varied from 14.7 ± 1.64% to 30.85 ± 0.63% as determined by the DSC method. The taro starch contained 0.2–0.6% lipids and 2% ash. The onset gelatinisation temperatures of the taro flours varied from 55.2 to 65.49 °C, whereas those of the starches are between 48.08 ± 2.46 for KW2 and 64.37 ± 2.35 °C for KW1. Retrogradation of the starches and the flours, as measured by their enthalpy changes, appeared to vary significantly between the varieties. The water absorption capacity varied from 240% to 470% and from 60% to 250% for the flours and starches samples, respectively. Taro flours had higher solubility index than their starch counterparts. Among the varieties, RIN and CE starches had the highest solubility whereas KW1 starch had the lowest. The sorption study also revealed the high ability of the taro flours to absorb water compared to their starch components. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Taro; Starch; Flour; Microscopy; Physicochemical properties; Functional properties
1. Introduction Taro (Colocassia esculenta) is a tropical tuber crop largely produced for its underground corms and consumed in tropical areas of the world. Taro has been reported to have 70–80% starch with small granules (Jane et al., 1992). Taro is also rich in gums (mucilage) and up to 9.1% crude taro mucilage has been extracted from taro corms (Hong and Nip, 1990). Because of the small sizes of its starch granules, taro is highly digestible, and as such has been reported to be used for the preparation of infant foods in Hawaii and other Pacific islands (Nip, 1997).
*
Corresponding author. Tel.: +237 99 870979. E-mail address: [email protected] (Y.N. Njintang).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.10.006
Despite its nutritional, industrial and health importance, taro has not gained sufficient research attention to enhance its potential. In addition to this fact, it has a high post harvest loss due to its high moisture content (Agbor-Egbe, 1991). As such taro has a poor position on the food security profile of countries. In fact, it has been estimated that there is an average of 30% loss during storage of these tubers, and that this portion could resolve starvation problems in non-developed countries (Agbor-Egbe, 1991). In order to minimize tuber losses, they must be converted from perishable to non-perishable through food processing operations. Since the transformation into starch or flour will decrease losses after the tubers have been harvested, value added processes such as drying and milling may be useful in order to obtain flours and starches from these tubers. Before consideration is given to taro as potential
Aboubakar et al. / Journal of Food Engineering 86 (2008) 294–305
sources of flour and starch to produce foods, it is necessary to characterize their chemical composition, physical, physicochemical and functional properties. According to Nip (1997) taro corms and cormels are potential sources of flour and industrial starch that has not yet been utilized. It is therefore clear that a significant amount of work remains to be done on the functional characteristics of native taro starch if it is ever to become competitive with commercial starches such as corn, wheat and potato. Some chemical and physical properties of taro starches and flours have been reported on several Hawaiian (Jane et al., 1992; Sugimoto et al., 1986) and Cameroonian (Mbofung et al., 2006; Njintang et al., 2007a) varieties. But few have been interested on their functional properties. Tagodoe and Nip (1994) reported the functional properties of raw and precooked taro flours. In addition, Jane et al. (1992) reported the physical, microscopic and thermal properties of some Hawaiian taro flours and starches. Njintang (2003) initiated the study of some functional properties of Cameroonian taro flour. Latter on Mbofung et al. (2006) reported some functional properties of six varieties of Cameroonian taro flour. Njintang and Mbofung (2006) investigated the effect of precooking time and drying temperature on the physicochemical characteristics and in vitro carbohydrate digestibility of taro flour. More recently, Njintang et al. (2007b) investigated the relation between the biochemical characteristics of taro flour and the colour of the paste made from its. Njintang et al. (2007a) studied the functional properties of taro flour and their relationship to creep recovery and sensory characteristics of the paste made from it. In the same vein, Njintang et al. (2007c) evaluated the effect of taro flour reconstitution on the texture profile of achu. In addition, Njintang et al. (in press) evaluated the effect of taro flour addition on the functional and alveographic properties of wheat flour and dough. Despite of increasing interest in taro, studies on the properties of its flour and starch remained scarce. The aim of this study therefore was to obtain knowledge about the physicochemical, microstructure and flow behaviour of starches and flours produced from six varieties of Cameroonian taro corms. 2. Materials and methods 2.1. Sampling and production of flours Six varieties of taro corms and cormels, coded WCN, WCE, RIE, RIN, KW1, KW2, were used for the study. They were freshly harvested at 8–9 months of maturity from family farms in Ngaoundere. The corms and cormels were thoroughly washed with tap water hand peeled and trimmed to remove defective parts. Then the tubers were sliced and dried in air convection (CK-2000) at 50 ± 2 °C. The dried slices were first hammer milled (Culatti polymix, France) to pass through a 500 lm screen. The flours were divided into two parts: one part was stored in polyethylene bags and kept in a refrigerator at 4 °C prior
295
to use while the second part was used for isolation of starch. 2.2. Isolation of taro starch Taro flour (100 g) was steeped in water solution (3 L) at 35 °C for 12 h. The slurry obtained was homogenised for 30 min using a commercial blender (model moulinex, France). The suspension obtained was screened using 150 lm sieve and keep to sediment for 24 h. The crude starch was then collected and washed twice with water, dried for 48 h in an oven set at 50 °C and carefully stored in sealed and dried polyethylene bags until required for analysis. The yield of extraction of starch was evaluated by the gravimetric method of Singh et al. (1989). 2.3. Proximate and mineral analysis Flours of each tuber were analysed for moisture (air oven method), fat (Soxhlet) and ash (incineration method) content, as a percentage (w/w), following AACC standard methods (1990). Total proteins (Nx6.25) was analysed using approved methods of Kjeldahl (AACC, 1990) in semi automatic machine (GEHARDT, Paris, France). Reducing sugars were analysed using the colorimetric method of Fisher and Stein (1961). Phosphorous content of starches and flours, as a percentage of sample weight (w/w), was determined following the spectrophotometric method of AOAC (2000) while minerals (Ca, Na, Mg, K, Mn, Mg, Fe) content, as a percentage of sample weight (w/w), were analysed using an atomic absorption (AA) spectrophotometer. 2.4. Determination of the amylose content and amyloglucosidase hydrolysable starch Amylose content was analysed using a DSC (differential scanning calorimetry) method as described by Mestre et al. (1996). The amylose content was calculated using the equation: Amyloseð%Þ ¼ 100
A DH 1 ; DH 2 S
where A and S were, respectively, the weights of the standard amylose and sample used in the experiment, DH1 and DH2 their respective enthalpy change. 2.5. Differential scanning calorimetry measurements (DSC) DSC thermograms of taro starches and flours were recorded on a Perkin–Elmer model Pyris1 (Perkin–Elmer Corp Norwalk. USA), with heating rate of 5 °C/min and temperature rate range of 25–98 °C. Starch, flour or amylose were dispersed in distilled water (1:3; w/v) in an aluminium pan and hermetically sealed. The instrument was calibrated for temperature and enthalpy measurement with indium, and an empty pan was used as reference. The manufacturers’ software was used to calculate the heat capacity and
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integrate the peaks. The onset and end temperatures of the gelatinisation peaks were determined by the intersection of tangents fitted to the leading and trailing flanks of the peak with the baseline.
mined using the nonlinear power equation category of Sigma plot 8.02 (Chicago, IL, USA) statistical package.
2.6. Colour characterisation
Granule morphology was examined using scanning electron microscope (SEM) microscope. A thin layer of starch granule was mounted on the aluminium specimen holder by double-sided tape. The specimen holder was loaded in a Emitech K550 sputter coater (Emitech, Ashford, UK). It was coated with gold palladium, with a thickness of about 30 nm. Scanning electron microscopy was carried out using a Leica Stereoscan 360 SEM (LEO, Cambridge, UK) operated at an accelerating voltage of 10 kV. Light micrograph of the starch samples colored with diluted iodine in potassium iodide was taken with a Diplan-microscope Model GF (Leitz Wetzlar Germany).
Colour measurements of the starch were carried out using a Chromameter CR210 (Minolta France S.A.S., Carrrie`res-sur-Seine) on the basis of L* a* and b* values. The instrument was calibrated against a standard light yellowcoloured reference tile. A glass cell containing the powdered flour was placed above the light source and covered with a white plate and L*, a* and b* values were recorded. 2.7. Starch particle analysis and pasting properties A Coulter Particle Counter, Model Mastesizer S (Malvern Instruments, Orsay) was used to measure the percent volume of starch particles distributed within selected size ranges for dry powder measurement/loose particle distribution as described by Njintang et al. (2006). Flow behaviour of 2.5% flour and starch samples in distilled water heated at 80 °C for 10 min were investigated at 25 ± 1 °C by a parallel plate rotational rheometer (Stress tech, UK). The paste was allowed to equilibrate for 5 min and was tested using a 1 mm gap. The sample was sheared at a programmed rate linearly increasing from 0 to 200 s1 in 300 s. 2.8. Equilibrium moisture content (EMC) and adsorption isotherm The EMC of the taro starch was determined at 20 °C according to the static gravimetric method of Wolf et al. (1985). The desorption isotherms were determined on samples hydrated in a glass jar over distilled water at a room temperature to approximately 30% dry basis moisture content. Samples of 1 ± 0.02 g were weighed in weighing bottles which were put in hygrostats with seven saturated salt solutions (LiCl, MgCl2, K2CO3, NaBr, NaCl, KCl) used to obtain constant water activities environments between 0.1 and 0.9 (Bell and Labuza, 2000). All the salts used were of reagent grade. At high water activities (aw > 0.70) crystalline thymol was placed in the hygrostats to prevent the microbial spoilage of the flour. The hygrostats were kept in thermostats at 20 ± 0.2 °C. Samples were weighed (balance sensitivity ± 0.0001 g) every three days. Equilibrium was acknowledged when three consecutive weight measurements showed a difference less than 0.001 g. The moisture content of each sample was determined by the oven method (105 °C for 24 h) by means of triplicate measurements. The resulting adsorption curve was tested to follow the Oswin model of adsorption of the general form M (g/g) = Ax[aw/(1 aw)]B where M is the moisture content expressed in g/g dried weight, and aw is the water activity. The constant A (g/g) and C were deter-
2.9. Microstructure of starch granules
2.10. Water absorption capacity and water solubility index of taro flour and starch For the determination of these variables, 0.5 g of starch was suspended in 10 mL of distilled water and incubated with mixing for 15 min in a shaking water-bath (Kottermann, Germany) set at 70 °C and centrifuged at 5000 rpm for 30 min. The pellet was dried at 105 °C for 12 h and the water absorption capacity and the water solubility index evaluated according to Anderson et al. (1969) and Phillips et al. (1988), respectively. 2.11. Infrared spectroscopy of taro flour and starches Fourier transform infrared (FT-IR) spectroscopy was used to evaluate structural differences in the samples. An appropriate amount of starch powder was pressed into a crystal window and the FT-IR spectra were recorded on a Fourier transform infra red spectrometer in absorbance mode from 4000 to 400 cm1, co-adding 32 scans at 4 cm1 resolution (Van Soest et al., 1995). Ten replicates were collected for each sample. The obtained spectra were transferred into a data analysis package. 2.12. Statistical analysis All analyses were carried out in triplicate and data expressed as means ± standard deviation. Analysis of variance was performed to calculate significant differences in treatment means, and Duncan multiple test range was used to separate means using the Statgraphics (1997) statistical software. 3. Results and discussion 3.1. Chemical characteristics of taro flour Proximate compositional profile of the different varieties of taro flour is given in Table 1. As expected the results
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297
Table 1 Proximate compositional profile of the different varieties of taro flour Parameters
Varieties RIE
RIN
CE
CN
KW1
KW2
Moisture (g/100 g) Proteins (%) Total carbohydrate (%) Starch yield (%) Fats (g/100 g) Red. sugars (g/100 g) Ash (g/100 g) Minerals Ca (mg/100 g) Na (mg/100 g) Mg (mg/100 g) K (mg/100 g) Mn (ppm) Fe (ppm) Zn (ppm) Cu (ppm)
8.2 ± 0.2a 3.1 ± 0.1ab 94.8 86.6 0.50 ± 0.10 13.8 ± 0.1b 1.6 ± 0.35a
9.0 ± 0.4bc 3.4 ± 0.1c 93.9 84.36 1.17 ± 0.32 13.5 ± 0.1a 1.5 ± 0.4a
9.6 ± 0.1c 4.6 ± 0.1d 90.5 82.12 0.53 ± 0.15 19.5 ± 0.4d 4.4 ± 0.3b
8.5 ± 0.4ab 4.9 ± 0.2e 91.0 79.85 0.30 ± 0.07 26.7 ± 0.1e 3.8 ± 0.9b
8.5 ± 0.1ab 2.9 ± 0.1a 95.5 68.8 0.33 ± 0.06 14.6 ± 0.1b 1.3 ± 0.6a
8.8 ± 0.2b 3.3 ± 0.1bc 90.8 66.5 0.37 ± 0.06 15.5 ± 0.2c 5.5 ± 1.1c
25.4 <0.5 61.9 5.2 6.4 41.7 42.8 1.2
55.9 4.8 32.9 27.5 13.1 2.4 0.4 0.4
111 4.6 88.9 59.7 62.4 17.6 1.1 1.1
30.7 4.8 90.4 23.7 130.3 15.0 1.0 1.0
91 <0.5 106.7 3.5 127.3 13.1 1.3 1.3
192 5.6 382 11.4 85.6 21.8 1.8 1.8
Means ± SD; Figures in line followed by different superscripts indicate significantly (p < 0.05) different values determined by Duncan’s Multiple Range Test.
showed that carbohydrates, composed mainly of starch, was the most important chemical component in the flours while the proteins and fats were very limited. The carbohydrate levels (calculated by difference) varied from 90.5% to 95.5% while the starch yield varied from 66.5% to 86.6%. The starch yields obtained in this study are in agreement with that (51–58%) reported by Jane et al. (1992) on Hawaiian taro varieties. In addition, reducing sugars in taro flour were also found at high levels. The CN variety exhibited the highest level (26.7%) while the RIN and RIE varieties exhibited the lowest (13.5–13.8%). The difference in the reducing sugars content might reflect the fragility of the starch within varieties. The table also showed that beside carbohydrates, ash represented another important group of component in taro flour with values ranging from 1.3% (variety RIN) to 5.5% (variety KW2). These values are comparable to those reported for most tubers but are higher compared to that reported for cereal crops (Swinkels, 1985). Among minerals that composed the ash fraction of the taro flours, and irrespective of the variety, Ca and Mg were the most abundant with range values of 25.4–192 mg/100 g flour and 32.9– 382 mg/100 g flour, respectively. The relative high amount of calcium in flours was expected since taro belongs to the family araceae recognised to be rich in oxalate calcium crystals responsible for the irritation of uncooked taro. In this respect, Sira (2000) assigned the high level of ash and Ca in taro flour to oxalate. In the present work, we found that the presence of Ca in taro flours could be responsible of the high ash content. In fact, a positive significant correlation (r > 0.69; p < 0.05) between the ash content and Ca or Mg content was established. The fact that magnesium was correlated to ash content might also suggest its role in oxalate crystals formation, but this need to be investigated. Varieties KW1 and KW2 contained the highest levels of Ca and Mg, whereas RIN variety contained the
lowest levels. These observations are in agreement with findings by Njintang (2003), which reported the absence of oxalate crystals in RIE and RIN variety. Beside the major mineral metals cited above, substantial amount of some oligoelements such as Fe, Mn, Cu and Zn were found in flours. Iron is one of the more abundant oligoelement in taro flour with level varying from 2.4 ppm (RIE variety) to 41.7 ppm (RIN variety). 3.2. Amylose, phosphorus and amyloglucosidase hydrolysable starch in native taro flour and starch The ability of the native taro starch to amyloglucosidase hydrolysis is presented in Table 2. The level of hydrolysable starch varied from 37.75% (variety CN) to 58.43% (variety KW2). A negative correlation (R = 0.32, p < 0.05) was observed between the reducing sugars content and the Table 2 Amylose, phosphorus and amyloglucosidase hydrolysable starch in native taro flour and starch Variety
Sample
Amylose (%)
Hydrolysable starch (%)
Phosphorus (mg/100 g)
RIE
Starch Flour
30.85 ± 0.63 26.05 ± 0.27
43.14 ± 2.54 45.07 ± 1.30
0.83 ± 0.08 1.97 ± 0.02
RIN
Starch Flour
16.65 ± 0.15 14.70 ± 1.64
51.23 ± 1.27 58.87 ± 2.60
0.88 ± 0.08 1.96 ± 0.03
CE
Starch Flour
26.77 ± 2.47 18.53 ± 1.92
54.83 ± 1.27 56.11 ± 3.90
0.76 ± 0.18 1.76 ± 0.09
CN
Starch Flour
28.38 ± 2.86 16.07 ± 1.96
37.75 ± 2.54 46.91 ± 1.30
0.82 ± 0.16 1.66 ± 0.33
KW1
Starch Flour
25.43 ± 0.42 19.40 ± 0.04
56.63 ± 1.27 50.59 ± 1.30
0.77 ± 0.22 1.44 ± 0.09
KW2
Starch Flour
29.21 ± 1.50 24.10 ± 1.56
58.43 ± 1.27 61.63 ± 1.30
1.43 ± 0.27 1.36 ± 0.03
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hydrolysis levels. A similar negative linear correlation (R = 0.45, p < 0.05) was observed between the hydrolysis level and the amylose content of the starch. This result suggested that high levels amylose starch hydrolysed more slowly than low level amylose starch. This was expected in the way that amylose is the starch portion which exist in the form of double helix chain, not easily accessible by enzymes. In concordance with this study, Skrabanja et al. (1999) found that pea flour with high amylose content possessed low starch digestibility while pea flour with low amylose content exhibited high starch digestibility; and this has been reported to be related to the ability of amylose to undergo retrogradation and form resistant starch. The amylose content (Table 2) of all the flour samples were relatively close to those (18–24%) reported by Jane et al. (1992) using the gel permeation chromatography and iodine potentiometric method. Using the iodine colorimetric method, Sira (2000) and Pe´rez et al. (2005) obtained an amylose content of 30.3% and 30.6% respectively. In addition, the amylose content of taro starch were in the range values (21–23%) reported for six cultivars of potatoes tubers (Walter et al., 2000) using the colorimetric and the DSC methods. Phosphorus, which has been reported to be covalently linked to the starch and affected its properties (Takeda et al., 1986), ranged in starches from 0.76 mg/100 g (variety CE) to 1.36 mg/100 g (variety KW2). These values were higher compared to those reported by Pe´rez et al. (2005) for Colocasia esculenta (0.01 mg/100 g), Xanthosoma sagittifolium (0.07 mg/100 g) and Manihot esculenta (0.05 mg/ 100 g). For the starch, it was observed a relative negative linear correlation (r = 0.43; p < 0.05) between the hydrolysable level and the phosphorus content. This indicated that native starch with high level of phosphorus exhibited low digestibility. A possible explanation is that lower phosphorus content in starch granules generally results in higher resistant starch (Liu et al., 2007). The levels of phosphorous in tuber starches is typically less than 500 mg/ 100 g and is usually referred to ash (Thomas and Atwell,
1999). Thus, a positive linear correlation (r = 0.65; p < 0.05) was observed between the phosphorous level and the ash content in our flour samples. 3.3. Colour and sorption characteristics of taro starches The L*, a* and b* characteristics of the starch sample are grouped in Table 3. The colour characteristics are all in the same order of values: L* (83–94), a* (1.5–5.7) and b* (3.0– 13.8). The result showed that the flour were white (high value of L) less red (low a* value) and less yellow (low b* value). It has been hypothesised that the variation in b* value among samples may be attributed to the amount of carbohydrate and proteins content due to their role in development of non enzymatic browning (Jamin and Flores, 1998). In the present study, no significant correlation was observed between the colour coordinate and neither proteins nor reducing sugars levels of the starch sample. However, it is worth to mention the positive correlation between the proteins or carbohydrate and a* (r = 0.44) or b* (r = 0.24), showing that the composition of the flour has some effect on its colour. The ability of taro starch and flour to adsorb water is presented in Fig. 1. The figure known as moisture isotherm describes the variations in equilibrium moisture content with water activity (aw) at 20 °C. The equilibrium was reached after 3–4 days for the different starch and flour samples. The figures indicated that variation in the moisture content of flours and starches depends on the relative humidity (RH) of the atmosphere in which they have been stored. If the RH decreases, the starches reject moisture; if the RH increases, they absorb humidity (Swinkels, 1985). The shape of the curves is sigmoid, showing one inflection point, which has been reported to characterise materials with high sugars content (Maskan and Go¨gu¨s, 1997). Such material samples present typical curve with three different regions (Chungcharoen and Lund, 1987): region 1, corresponding to aw < 0.2, which relates to adsorption of monomolecular film of water, region B for aw range of 0.22–0.7
Table 3 Colour and adsorption characteristics of taro flours and starches Variety
Sample
Oswin constant B (101)
Oswin constant A (102 g g1)
Colour characteristics L*
a*
b*
RIE
Starch Flour
6.38 ± 0.68 8.49 ± 1.38
3.99 ± 0.51 3.16 ± 0.86
94.9 ± 0.3 87.7 ± 6.0
1.5 ± 0.1 1.45 ± 0.14
3.0 ± 0.25 6.98 ± 0.31
RIN
Starch Flour
7.44 ± 1.00 6.20 ± 0.80
3.18 ± 0.61 5.19 ± 0.76
93.9± 0.5 93.63 ± 0.41
2.1 ± 0.1 2.20 ± 0.07
4.7 ± 0.34 8.86 ± 0.11
CE
Starch Flour
11.05 ± 1.78 6.55 ± 1.63
1.93 ± 0.71 5.44 ± 1.60
88.7 ± 0.3 92.24 ± 0.59
5.1 ± 0.1 1.92 ± 0.02
9.1 ± 0.1d 9.36 ± 0.13
CN
Starch Flour
10.63 ± 1.16 9.19 ± 0.89
2.20 ± 0.52 3.10 ± 0.55
90. 8 ± 0.7 93.36 ± 0.47
3.5 ± 0.2 1.92 ± 0.04
7.2 ± 0.5c 9.31 ± 0.16
KW1
Starch Flour
9.76 ± 0.90 10.21 ± 0.85
2.69 ± 0.49 2.41 ± 0.41
93.5 ± 0.5 88.30 ± 0.42
2.5 ± 0.2 4.20 ± 0.10
5.2 ± 0.3b 11.85 ± 0.16
KW2
Starch Flour
5.75 ± 0.62 6.30 ± 0.80
3.18 ± 0.36 5.09 ± 0.75
83.2 ± 0.5 84.40 ± 40
5.7 ± 0.2 5.67 ± 0.23
13.8 ± 0.2e 13.91 ± 0.30
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Equilibrium moisture content (g/g)
0.30 RIN CN RIE KW2 KW1 CE
0.25
0.20
0.15
0.10
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Water activity
Equilibrium moisture content (g/g)
0.25
0.20
RIN CN RIE KW2 KW1 CE
0.15
0.10
299
In general, the equilibrium moisture content increases rapidly at low water activity aw (0–0.13), and then rises slowly between 0.15 and 0.7, followed by a steep rise above 0.7. The variation of equilibrium moisture with aw was shown to follow the Oswin model (r2 > 0.95; p < 0.01). The constants A from the model shown in Table 3 varied between varieties reflecting the difference in their adsorption. In general, the parameter A was generally higher for the flours than for the starch, suggesting an important contribution of non starch compounds on the adsorption of the flour. Reverse observations were made on the constant B and in this respect a negative correlation (r2 = 0.69; p < 0.01) was shown to exist between A and B. Values of A ranged from 0.019 (variety CE) to 0.039 (variety RIE) for starches and from 0.024 (variety KW1) to 0.054 (variety CE) for flours. For the B values, ranges were 0.57–1.10 for starches and 0.62–01.02 for flours. The A and B values obtained in this study were in the range but systematically higher (for A) and lower (for B) than that reported for corn (A = 0.0787, B = 0.32) (Al-Muhtaseb et al., 2004) and potatoes starches (A = 0.069, B = 0.39) (Peng et al., 2007). 3.4. Differential scanning calorimetry (DSC)
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
Water activity
Fig. 1. Sorption isotherm of taro flours (a) and starches (b).
corresponding to adsorption of additional layers over this monolayer, and region C for aw range = 0.7–0.99 corresponding to condensation of water in the pores of the material followed by dissolution of soluble material (Benado and Rizvi, 1985).
The thermal transition of the six taro starches were detected by DSC. All the starch samples exhibited a thermogram with a single endothermic transition typical of the gelatinisation process (Biliaderis and Seneviratne, 1992; Iturriaga et al., 2004). The DSC results are presented in Table 4. No significant difference was observed on the peak gelatinisation temperature and the onset gelatinisation temperature of the starches and the flours. This was not the case in literature where Jane et al. (1992) observed in their studies that the gelatinisation temperatures of the flours were higher than those of the starches. However, a significant difference (p < 0.001) was observed on the enthalpy (DHG) with starches generally exhibiting higher
Table 4 Gelatinization profile (°C) taro flours and starches Variety
Samples
DSC parameters T0
Tp
Tc
DH (J/g)
RIN
Flour Starch
60.93 ± 2.36 63.41 ± 2.38
64.77 ± 2.77 67.24 ± 3.21
66.42 ± 1.27 70.27 ± 3.61
8.43 ± 0.21 11.04 ± 0.86
RIE
Flour Starch
55.20 ± 1.2 54.93 ± 2.64
61.27 ± 0.71 56.48 ± 1.76
64.96 ± 0.90 60.78 ± 2.51
14.95 ± 0.91 16.93 ± 1.34
CE
Flour Starch
57.47 ± 0.97 57.85 ± 3.14
62.78 ± 2.89 74.54 ± 2.20
67.05 ± 2.67 68.81 ± 3.5
11.41 ± 0.89 15.37 ± 0.82
CN
Flour Starch
56.80 ± 1.57 62.91 ± 2.82
59.42 ± 0.61 64.29 ± 3.41
61.78 ± 0.87 66.4 ± 2.25
9.22 ± 0.87 15.19 ± 2.21
KW1
Flour Starch
65.49 ± 1.53 64.37 ± 2.35
69.75 ± 2.34 67.64 ± 1.67
73.33 ± 3.13 69.53 ± 2.6
11.13 ± 0.01 14.59 ± 0.24
KW2
Flour Starch
56.14 ± 1.23 48.08 ± 2.46
58.99 ± 1.11 54.40 ± 3.16
61.24 ± 3.12 57.91 ± 2.50
13.36 ± 0.66 16.75 ± 0.84
T0, onset gelatinisation temperature; Tp, peak gelatinisation temperature; Tc, end temperature of gelatinisation.
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values (mean 14.99 J/g) and flours having lower values (mean 11.42 J/g). These results are in agreement with findings by Ahmed et al. (2008) who reported a higher heat of gelatinisation (DHG) of 16.2 J/g for rice starch and a lower heat of gelatinisation to whole rice flour (4.81 J/g). This clearly show that the gelatinisation profile of flours may be influenced not only by the starch composition (amylose to amylopectin ratio), but also by others such as the granular architecture (crystalline to amorphous ratio), the molecular structure of amylopectin (extent of branching, unit chain length) (Gunaratne and Hoover, 2002). Furthermore, high amylose starches with longer average chain have been reported to exhibit higher transition temperatures (Jane et al., 1992). In all cases, a significant effect (p < 0.05) of variety was observed on the temperatures and the enthalpy of gelatinisation. The peak temperature of gelatinisation varied between 55.56 and 68.67 °C with CN, KW2 and RIE exhibiting lower values and RIN, KW1 and CE exhibiting higher values. DHG values varied from 8.43 J/g (variety RIN) to 14.95 J/g (variety RIE) for flours and from 11.04 J/g (variety RIE) to 16.93 J/g (variety RIE) for starches. The gelatinisation profiles of starches shown in Table 4 are quite similar to those reported by Jane et al.
(1992) for taro flour and starch. However, the initial, middle and end gelatinisation temperatures were lower than those reported by Sira (2000) either for the flours or for the starches. In addition, Sira (2000) reported a significant correlation between Tp and amylose content of taro starch. Similar trend was observed in our study even though the correlation was not significant (r = 0.37). 3.5. Microstructure and particle size of taro starch Scanning electron micrograph showed that taro starch granules had polygonal and irregular shapes of diameter lower than 5 lm (Fig. 2). The particle size distribution obtained from the master sizer (Fig. 3) exhibited a monomodal distribution with a mean diameter of 3–20 lm, higher than that reported by other authors on taro starch which were 1–5 lm (Jane et al., 1992; Sugimoto et al., 1986). The results obtained suggested that the starch granules are associated as cluster or compounds since extraction has not been done in alkaline solution which is known to dissociate them. Microscopic observations of the starch samples confirmed this association. Meaningful observations of the particle size give diameters of each of the starch samples varying from 1 to 5 lm, in agreement
Fig. 2. Scanning electron microscopy (SEM) of starch granules isolated from taro flours varieties KW1 (a), RIE (b), RIN (c), CN (d), CE (e).
Aboubakar et al. / Journal of Food Engineering 86 (2008) 294–305 8
301
3.0
2.5
KW1 KW2 CE
2.0
CN NIE RIN
Absorbance
Percentage (%)
6
4
2
1.5
1.0
0
0.5
0
200
400
600
800
1000
0.0 4000
1200
3000
1000
0
1000
0
4
10
8
3
6
Absorbance
Percentage (%)
2000
Wavenumber (cm-1)
Particle size (µm)
4
CE KW2 RIE RIN KW1 CN
2
1
2
0 0
0
20
40
60
80
100
120
140
-1 4000
3000
Fig. 3. Particle size distribution of the taro flours (a) and starches (b).
with the granule size of taro starch reported in the literature. No significant differences were observed on the particle size of the different varieties although compounds granules were more pronounced in the CN and KW1 samples. The size distribution association as individual (simple) or clusters (compound) and composition (lipid, moisture, protein and minerals content) reflect the different origins of starches (Tester and Karkalas, 2002). 3.6. Fourier transformed infrared To get information on the possible difference in starch granules structures between taro starch samples, their FT-IR were recorded. The spectra obtained for the six samples were similar based on the form, but differed in the intensity of the major peaks as shown in Fig. 4. The spectra showed high absorption at the wave numbers 574, 1020– 1026, 1056, 1151, 1365, 1631, 2922 and 3400 cm1 confirming the carbohydrate nature of the samples. According to Van Soest et al. (1995), the bands at 1047 and 1022 cm1 describe the crystalline and amorphous properties of starch, respectively, and their ratios indicate the degree of starch order (Sevenou et al., 2002; Van Soest et al., 1995). In this study, such bands really appeared but the one at 1047 cm1 was so broad that we used the one at
2000
Wavenumber (cm-1)
Particle size (µm)
Fig. 4. Infra red spectroscopy of the different taro flours (a) and starches (b).
1056 cm1. In processing the FT-IR data, we extracted the intensity of the bands and the resulting values, grouped in Table 5 showed that the crystallinity and amorphous indexes of the starch ranged, respectively, from 1.17 (KW2) to 2.63 (RIN) and 1.27 to 3.51. The values of the ratio which varied from 0.75 (RIE) to 0.93 (KW2 variety) seem to suggest in general that taro starch is more amorphous than crystalline at the granule surface. It is worth mentioning that RIE variety is more crystalline than the CE, KW1 and KW2 varieties. The values of the ratio obtained in the present work are not quite different from Table 5 Crystallinity (I1022), amorphousness (I1056) and order index (I1056/I1022) of taro flour and starch samples FT-IR characteristics
Sample
Crystalline index
Varieties RIE
CE
CN
RIN
KW1
KW2
Flour Starch
2.7 3.5
1.9 1.4
1.8 1.7
2.1 2.0
1.7 1.5
2.0 1.3
Amorphous index
Flour Starch
2.3 2.6
1.6 1.3
1.5 1.3
1.8 1.7
1.5 1.5
1.7 1.2
Starch granule order
Flour Starch
0.82 0.75
0.84 0.89
0.87 0.81
0.84 0.86
0.91 0.95
0.87 0.93
302
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0.8 reported by Olkku and Rha (1978) and Van Soest et al. (1995) for cowpea and potatoes starches, respectively. Comparatively, the values calculated for the starch samples were not significantly different from their corresponding flours justifying that starch is the most important component of taro flour. 3.7. Water absorption capacity (WAC) and water solubility index (WSI) The swelling capacities of the starches are presented in Fig. 5. In general, increase in temperature led to significant increase in WAC up to a maximum situated at temperature 60–70 °C from which they decreased. Above 50 °C little changes on WAC occurred, while beyond water absorption was most conspicuous with the RIE, KW2 and CN being highly prone to swelling. The slow swelling pattern of the starches and flours at relatively low temperatures is confirmed by the report of Agunbiade and Longe (1999). The swelling behaviour of the starches may be affected by the structural arrangements of their amylose and amylopectin (Schoch and Mayfield, 1956). Thus it is most probable that the granular arrangements of RIE, KW2 and CN are less compact and, therefore, may have higher intermo-
lecular areas than the other three varieties. When gelatinisation temperature was reached, the loose of associative hydrogen in the starch granules quickly resulted by hydration, resulting in swelling of the granules (Biliaderis and Seneviratne, 1992). From the results shown in Fig. 1 it can be seen that RIE, KW2 and CN starches and flours held slightly more water than RIN, KW1 and CE varieties (10.06 g H2O g1) and that WAC of taro flours were systematically higher than that of their starch counterpart. These results suggested that the other non-starch components of the flours such as mucilage contributed highly to the absorption of the flour. The variation in WAC of the starches could be due to the difference in the degree of engagement of hydroxyl groups to form hydrogen and covalent bonds between starch chains (Hoover and Sosulski, 1986). Whatever the case the similar trend observed for the water absorption capacity for the flours and starch, confirmed by significant correlation observed (r = 0.87; p < 0.001) suggested that starch play a major role in the binding capacity of starch. The increase in water absorption capacity has always been associated with the increase in the amylose leaching and solubility, and lost of starch crystalline structure (Gunaratne and Hoover, 2002). This was the case in the
36 500
34 450
Water absorption capacity (gH2O/100g flour)
Water solubility index (g/100g flour)
32 30 28 26 24 RIE RIN CE CN KW1 KW2
22 20 18
400
350
RIE RIN CE CN KW1 KW2
300
250
16 0
20
40
60
80
200
100
0
Soaking temperature (°C)
20
40
60
80
100
Soaking temperature (°C)
40 300
RIE RIN CE CN KW1 KW2
250
Water absorption capacity (gH2O/100g flour)
Water solubility index (g/100g starch)
35
30
25
20
15
200
150 RIE RIN CE CN KW1 KW2
100
50
0
20
40
608
80
100
Soaking temperature (°C)
Fig. 5. Water solubility index of the different varieties of taro flour (a) and paste (b).
0
20
40
60
80
100
Soaking temperature (°C)
Fig. 6. Water absorption capacity of different varieties of taro flour and starch.
Aboubakar et al. / Journal of Food Engineering 86 (2008) 294–305
303
Table 6 Flow behaviour, K, and consistency index, n, of different varieties of taro flours and starches Parameters
Sample
Varieties RIE
RIN
CE
CN
KW1
KW2
K (Pa sn)
Flour Starch
19.6 ± 0.7 10.6 ± 0.3
14.8 ± 0.4 4.20 ± 0.13
1.0 ± 0.00 0.06 ± 0.01
1.6 ± 0.05 8.78 ± 0.2
9.9 ± 0.3 10.44 ± 3.97
9.9 ± 0.3 52.3 ± 0.5
n
Flour Starch
0.23 ± 0.01 0.38 ± 0.01
0.16 ± 0.00 0.42 ± 0.01
0.86 ± 0.05 0.35 ± 0.01
0.61 ± 0.01 0.17 ± 0.01
0.39 ± 0.01 0.13 ± 0.05
0.39 ± 0.01 0.30 ± 0.01
present work where a significant correlation was observed between the water absorption and the water solubility, either for the flour samples (r = 0.37, p < 0.01) or for the starch samples (r = 0.56, p < 0.001). The variations in WSI of flour and starches are presented in Fig. 6. CE, CN, KW1 and KW2 flours and starches exhibited very low solubility at 20 °C and formed only a temporary suspension when stirred in water. According to Eliasson and Gudmundsson (1996), the low solubility of starches at low temperatures could be attributed to the semi-crystalline structure of the starch granules and the hydrogen bonds formed between hydroxyl groups in the starch molecules. As the temperature increased, the solubility increased due to the disruption of starch granules and exposure of hydrophilic groups. As expected the maximum solubility (35%) reached by the different samples at higher temperature reflects the amylose contents of the samples. Indeed amylose and consequently starch appear as the major constituents of the flours that solubilise during heating. In this respect, a significant correlation was observed between the solubility index of the flours and starches (r = 0.76, p < 0.001). 3.8. Flow properties The flow properties of taro starches and flours dispersions were studied in the shear stress range 100–500 Pa and the results are presented in Table 6. All the tested flour and starch systems behaved to a very good approximation (r2 = 0.96–1.00) as power law liquid over this range of shear rates. The good fit of power law model confirmed the linearity of the curves for these starches. The starch and flour paste showed pseudoplastic behaviour (shear thinning) and the flow behaviour index (n) was observed to be least for RIN flour (0.16) and KW1 starch (0.13) and highest for CE flour (0.86) and RIE starch (0.38). However, the consistency indices (K) of CE flour (1.0 Pa sn), and starch (0.06 Pa sn) were shown to be least while the RIE flour (19.6 Pa sn) and KW2 starch (52.3 Pa sn) were highest. Viscosity development of a paste has been mainly related to the water binding capacities of the dry ingredients (Dogan et al., 2005). According to these authors, flour which binds the maximum amount of water exhibits a high viscosity. In this respect, a significant correlation was found between the consistency index K and the water absorption capacity of the flour (r = 0.90, p = 0.01). In addition, high
value of consistency index K has been associated to flour possessing high solubility (r = 0.78, p < 0.05) in concordance with study by Dogan et al. (2005) reported on soy flour and rice. According to these authors, in a solution containing flour with less water absorption capacity, there is more free water available to facilitate the movement of particles and as such, low viscosity value is obtained. 4. Conclusion The results obtained showed some differences on the structure and properties of the varieties of taro flours and starches. The study revealed that taro flours are mainly made of carbohydrates and particularly starch. The granule size of the starch (1–5 lm) varied significantly within each variety but not significantly between them. While no significant difference exist on the gelatinisation temperatures of the flours and starches, the initial and the peak temperatures of gelatinisation of the taro starches vary significantly among varieties with general averages of 55 and 65 °C, respectively. The gelatinisation enthalpies are systematically higher for starches that for flours with RIE exhibiting the highest value (14.9 for flour and 16.9 for flour) and RIN the lowest (8.43 for flour and 11.04 for starch). The amorphous index, the water absorption capacity, and the water solubility index of the taro starch equally vary significantly with varieties. Taro flours possessing higher water absorption capacity and higher water solubility index give gels with higher consistency index. Acknowledgements The authors thank with greatness the financial support from the Cooperation Francßaise which helps the first author to carry out the study in the Laboratoire de Sciences et Genie Alimentaire de Nancy, INPL, France. Part of this study was also financed by the International Foundation for Science (IFS, Stockholm, Sweden) and the Committee on the Scientific and Technological Cooperation of the Organisation of Islamic Conference (COMSTECH, Islamabad, Pakistan) through a grant to Yanou Nicolas Njintang. References AACC (1990). American Association of Cereal Chemists. Approved methods of the AACC (7th ed.). St. Paul, MN.
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