Physicochemical and Functional Properties of Sour Starches from Different Cassava Varieties

Abstract

This article reports our investigation on the effect of cassava varieties on the physicochemical and functional properties of sour starches. There were significant differences (P < 0.05) in the ash, pH, amylose, amylopectin, starch damage, total titratable acidity (TTA), sugar, and starch content but not moisture contents of various cassava sour starches. There were no significant differences (P > 0.05) in Water Absorption Capacity (WAC), swelling power, and solubility index, while significant differences were recorded in Least Gelation Concentration (LGC) and color at 5% level and granule size at P < 0.0001 for cassava sour starches. Peak viscosity values ranged from 333.17RVU (clone 4(2) 1425) to 380.75RVU (clone TME 1). There were significant differences (P < 0.05) in pasting properties except for pasting temperature and breakdown.

Keywords:

• Sour starch
• Fermentation
• Cassava varieties
• Cassava properties

INTRODUCTION

Cassava, Manihot esculenta Crantz is an important staple food in sub-Saharan Africa. Cassava can improve food security and rural incomes in the region as it is tolerant to drought conditions and poor soils and its cultivation does not require much labour.[1] The storage root is a good source of carbohydrate and cassava leaves provide a rich source of protein in the human diet.[2] Starch is one of the most abundant substances in nature, a renewable and almost unlimited resource. Starch is produced from grain or root crops. It is mainly used as food, but is also readily converted chemically, physically, and biologically into many useful products.[3]

Acid modification through fermentation has been proven to be one of the best methods of modifying starches.[4] Cassava sour starch is a traditional product from Latin America, especially Brazil and Colombia.[5] Sour starch is produced by a natural fermentation of wet cassava starch (about 50% moisture), followed by sun drying. Cassava sour starch and its production process are totally unknown on the African continent[5] including Nigeria, the country that grows the most cassava. Dufour[6] reported that cassava variety is a factor, which affects the baking expansion property of sour starch. Genetic factors, environmental conditions during the growth of a plant and fermentation, especially the temperature, constitute the most important factors affecting the physiochemical characteristics of starch granules and the bread-making potential of sour starch.[5,7,8,9]

In Nigeria, non-wheat composite flour, cassava flour, cowpea flour, soybean flour, and cassava starch have been used but not cassava sour starch.[10,11,12] This article, therefore, reports on our findings on the effect of cassava varieties on the physicochemical and functional properties of cassava sour starch.

MATERIALS AND METHODS

Materials

Freshly harvested improved cassava varieties (30572, 4(2) 1425, TME1, 92B/00061, 96/0603, and 96/01632) were obtained from the farms of International Institute of Tropical Agriculture, Ibadan. The cassava plants were about 10 to 12 months old at the time of harvest. Processing started within 60 min of harvesting. All chemicals used were of laboratory grade. Ethanol 99.7%, Sodium hydroxide and Hydrogen Peroxide Solution (30%) was sourced from BDH-Analar England. Perchloric Acid (70%) was obtained from- Riedel-de-haen, England. Sulphuric Acid – GFS, Sigma-Aldrich Laborchemikalein GmbH, Seelze.

Starch Extraction

The traditional Eastern Nigerian methods were used.[13,14] Cassava roots (50 kg) were peeled, washed in water, and grated with a commercial mechanical grater. The resultant pulp was immediately sieved through a screen and suspended in 70 L of water. This separates the fibrous and other coarse root material from the starch pulp. The starch pulp was allowed to ferment for 25 days before decanting. The thick sour starch cake at the bottom of the bowl was pressed to remove water, and then oven-dried at 50°C for 18h. The dried sample was milled and stored in a cool place for analysis.

Measurement of Physicochemical Properties

The moisture content was determined according to the method of AOAC.[15] The samples were dried at 105°C for 3h using the preset oven (Fisher Scientific Isotemp Oven, model 655F, Chicago, USA). The method described by AOAC[15] was employed for ash content determination. The crucibles containing the pre-weighed samples were placed in a heated furnace (Fisher Isotemp Muffle Furnace, model 186A, USA) at 600°C for 6 h after which they were cooled to room temperature in desiccators and weighed. The pH of 10% starch slurry was determined using an Orion, model 720-pH meter.[15] The total titratable acidity of 10% slurry of the sample was also determined as described by AOAC method.[15]

The amylose content was determined by the iodine binding method described by Williams et al.[16] The absorbance was read at 620 nm on the spectrophotometer (Milton Roy Spectronic 601, USA) The starch and sugar Contents was determined by the method of Dubois et al.[17] The absorbance was read with a spectrophotometer (Milton Roy Spectronic 601, USA) at 490nm. The degree of starch damage was measured by an aqueous extractability method of McDermott.[18,19] The absorbance was read at 600 nm using Milton Roy Spectronic 601, USA.

The water absorption capacity (WAC) was determined using the method described by Sosulski.[20] The tube with its content was agitated on a Flask Gallenkamp shaker for 2 min and centrifuged at 4000 rpm for 20 min on a SORVALL GLC-1 centrifuge (Model 06470, USA). The method described by Coffman and Garcia[21] was used for the determination of least gelation concentration (LGC). The test tubes containing the suspensions were heated in a boiling water bath (THELCO, model 83, USA) for 1 h. The tubes and contents were cooled rapidly under running cold water, and then cooled further for 2 h at 4°C. Solubility and Swelling power of the starch at 80°C were measured by the method of Leach et al.[22] The color was determined using color meter (Color Tec PCMTM Color Tec associates, Inc., 28 Center STREET, Clinton, NJ 08809).[23,24]

Granule size and shape of granules were determined by the method of Zobel.[25] The average granule size and starch structure were determined with the aid of an Ordinary microscope (Laborlux S, Leitz wetzlar Germany 513558) and a Computerized microscope (OLYMPUS DP 50,BX51, JAPAN), respectively. Pasting properties were determined with a Rapid Visco Analyzer 3 C (RVA, Newport Scientific PTY Ltd, Sydney).[26,27,28] The data obtained were subjected to Analysis of Variance (ANOVA) using SAS statistical Package. Means were separated using Duncan's Multiple Range Test (DMRT). The values of physicochemical, functional and pasting properties were correlated using Pearson's Correlation Matrix.

RESULTS AND DISCUSSION

The results of the effect of cassava varieties on the chemical properties of cassava sour starch are shown in Table 1. The moisture content ranged from 6.27% for 96/0603 to 7.57 % for 4(2) 1425. The lower moisture content recorded by all the samples is an indication of long shelf stability. It has been reported that low moisture confers higher shelf life to starches. Material containing more than 12% moisture has less storage stability than material with lower moisture content. For this reason, a water content of 10% is generally specified.[29,30] Ash content ranged from 0.10% for 96/0603 to 0.38% for 30572. All the samples complied with the regulatory standard of not more than 1.5% ash content[31] specified by Standard Organization of Nigeria (SON). pH ranged from 3.35 for 4(2) 1425 to 4.32 for 92B/00061; all samples fall within the acidic region. The difference in the acidic content of the samples might be owing to varietal effect, fermentation, and environment among many factors.[7,8]

Table 1 Physicochemical properties of cassava sour starch

Variety	Ash %	Moisture content %	pH	Amylose %	Sugar %	Starch %	Starch damage %	Amylopectin %	Titratable acidity %
30572	0.38a	6.81a	4.29a	20.41b	0.94c	77.18c	1.52a	79.59d	2.52a
4(2)1425	0.17cd	7.57a	3.35c	20.35c	1.17b	81.15bc	1.27b	79.65c	2.34ab
92B/00061	0.35ab	6.56a	4.32a	20.17d	1.31ab	80.08bc	1.32b	79.84b	2.41a
96/01632	0.26bc	6.91a	4.04b	20.17d	1.33ab	85.23a	1.32b	79.84b	1.94b
96/0603	0.10d	6.27a	4.02b	20.01e	1.43a	82.24ab	1.37b	79.99a	1.46c
TME1	0.30ab	6.76a	3.97b	20.47a	0.87c	83.41a	1.37b	79.53e	2.61a
Mean	0.26	6.81	4.00	20.26	1.17	81.55	1.36	79.74	2.21
R-Square	0.95	0.51	0.99	0.99	0.96	0.88	0.91	0.99	0.93
CV	13.49	9.16	0.99	0.09	5.74	1.85	2.87	0.02	7.93
Std Dev	9.16	0.60	0.34	0.17	0.22	2.93	0.09	0.17	0.43
Range	0.10–0.38	6.27–7.57	3.35–4.32	20.01–20.47	0.87–1.43	77.18–85.23	1.27–1.52	79.53–79.99	1.46–2.61
P of clone	∗∗	NS	∗∗∗	∗∗∗	∗∗	∗	∗	∗∗∗	∗∗
Each value represent mean of three replicates. Mean values having the same superscript within column are not significantly different at 5% confidence level. ∗P < 0.05; ∗∗P < 0.01; ∗∗∗P < 0.0001; and NS: Not Significant.

Sugar content ranged from 0.87% for TME1 to 1.43% for 96/0603. Starch content ranged from 77.18% for 30572 to 85.23% for 96/01632 while the degree of starch damage and TTA ranged from1.27% for 4(2) 1425 to1.52% for 30572 and 1.46% for 96/0603 to 2.61 for TME1 (Table 1). As fermentation increases, the sugar level and pH decrease, while starch content and percentage TTA increase.[24]

Amylose and amylopectin ranged from 20.01% for 96/0603 to 20.47% for TME1 and 79.53% for TME1 to 79.99% for 96/0603 (fermented) and 17.84% for 96/0603 to 26.44% for TME1 and 73.56 for TME1 to 82.16 for 96/0603 (unfermented). High amylose starches are useful in the confectionery industry where candy pieces require a stabilizer to supply individual piece shape and integrity. One of the major uses of starch is to impart viscosity to food.[29] Ash and Amylose obtained were in agreement with the findings of Akanbi et al.[32] and Numfor et al.[33] It is probable that the apparent increase in amylose content in the fermented starch is caused by intensification of the blue colour by linear fractions, resulting from enzyme/acid hydrolysis of amylopectin at the amorphous regions of the starch granule during fermentation.[8]

There were significant differences in ash, pH, amylose, amylopectin, starch damage, Total Titratable Acidity, sugar, and starch content while no significant difference was observed in moisture content at 5% level. Correlation is significant at 1% level between TTA and amylose, ash and colour, amylopectin, and sugar, while at 5% level between TTA and ash. Negative correlation exists between amylose and amylopectin at P < 0.01.

The results of effect of varieties on the functional properties of sour cassava starch are shown in Table 2. The value of water absorption capacity (WAC) ranged from 82.42% for 96/01632 to 92.32% for 30572. High water absorption capacity is attributed to the loose structure of the starch polymer while a low value indicates the compactness of the molecular structure.[34] Swelling power value ranged from 11.53% for 92B/00061 to 13.46% for 4(2) 1425. The swelling power of the samples reflects the extent of associative forces within the flour, therefore, the higher the swelling power, the higher the associative forces.[35] The lower swelling power of starch was probably caused by damage to the granules during fermentation.[35–37]

Table 2 Functional properties and granule morphology of cassava sour starch

Varieties	Water absorption capacity (%)	Swelling power (%)	Solubility (%)	Least gelation conc (%)	Color (%)	Granule size (μm)	Granule shape
30572	92.32a	12.28a	13.95a	8.00a	91.03ab	15.00b	Round
4(2) 1425	83.56a	13.46a	13.20a	6.00b	90.21bc	14.18c	Round
92B/00061	84.73a	11.53a	15.84a	5.00bc	90.95ab	15.00b	Round
96/01632	82.42a	13.45a	11.47a	4.00c	90.88ab	18.83a	Round
96/0603	82.48a	13.11a	13.17a	4.00c	89.33c	14.18c	Round
TME1	82.86a	11.92a	10.87a	5.00bc	91.35a	14.18c	Round
Mean	84.73	12.62	13.08	5.33	90.62	15.14	—
R-Square	0.45	0.73	0.66	0.90	0.88	1.00	—
CV	7.23	6.48	17.29	13.69	0.44	0.00	—
Std Dev	5.55	1.05	2.62	1.56	0.78	1.54	—
Range	82.42–92.32	11.53–13.46	10.87–15.84	4.00–8.00	89.33–91.35	14.18–18.83	—
P of clone	NS	NS	NS	∗	∗	∗∗∗	—
Each value represent mean of three replicates. Mean values having the same superscript within column are not significantly different at 5% confidence level. ∗P < 0.05; ∗∗P < 0.01; ∗∗∗P < 0.0001; and NS: Not Significant.

The solubility index ranged from 10.87% for TME1 to 15.84% for 92B/00061. The higher value of the solubility index for 92B/00061 showed that its sour starch would be more soluble in water than the other samples.[7,8] The swelling power of the samples reflects the extent of associative forces within the flour, therefore, the higher the swelling power, the higher the associative forces. Hwang and Kokini[36] observed that side branches function to prevent intermolecular association of carbohydrate polymers. As such, water molecules can more readily penetrate the intermolecular spaces, resulting in enhanced solubility. High amylose content, as well as the presence of stronger or greater numbers of intermolecular bonds, reduces swelling.[37]

The Least gelation concentration (LGC) ranged from 4.00% for 96/0603 to 8.00% for 30572; color ranged from 89.33% for 96/0603 to 91.35% for TME1. Physical properties of starch pastes and gels depend on the concentration of the granules, and the amount of amylose and amylopectin leached from the granules during heating. Viscosity also depends on the shape and swelling power of the granules, entanglement between amylose-amylopectin, granule-granule, amylose-granule, and amylopectin-granule interaction.[38] A Cassava variety with the least LGC means that a small amount of starch is needed to form a gel and this is useful in food industry. There were no significant differences in WAC, swelling power and solubility index, while significant differences were observed in LGC and color at 5% level and granule size at P < 0.0001. Correlation exists between color and ash at 1% level.

Granule size value ranged from 14.18 μm for 96/0603 to 18.33 μm for 96/01632 (Table 2). All the samples have round shape with indentation on one side which is the characteristic of cassava starch (1–7).[7,8] However, there some form of little erosion for sour starches from some cassava CMD roots. Starch granules exhibit distinct variations in size and shape, form and position of the hilum, brilliance of the interference cross under polarized light as well as the ratio of the two constituent biopolymers.[39–42] It has been reported that sour cassava starch surface shows some indentations and slight evidence of pitting. This is similar to what was obtained in this study. It seems that there is not only an acid modification but also an enzymic attack as indicated by the pitting of the granules, which apparently is not as severe as that described by Cardenas and debuckles.[43] The crystalline structure of starch was not changed by the fermentation and drying processes whereas the behaviour of starch polysaccharides in water was dramatically reduced.[44]

Figure 1 Flow chart for production of sour starch.[13]

Figure 2 Structure of fermented starch from cassava TMS 30572.

Figure 3 Structure of fermented starch from cassava 92B/00061.

Figure 4 Starch structure of fermented cassava TME1.

Figure 5 Starch structure of fermented cassava TMS 4(2) 1425.

Figure 6 Starch structure of fermented cassava 96/00603.

Figure 7 Starch structure of fermented cassava 96/01632.

Pasting properties of cassava sour starches from different cassava varieties are presented in Table 3. Peak viscosity values ranged from 333.17RVU for 4(2) 1425 to 380.75RVU for TME 1, Peak viscosity indicates the water-binding capacity of the starch or mixture. It is often correlated with final product quality. As the temperature is increased, the starch granules swell and increase the viscosity of the paste until the peak is reached.[40] Ring et al.[38] stated that peak viscosity is closely associated with the degree of starch damage and high starch damage results in high peak viscosity. Trough value ranged from 98.79RVU for 4(2) 1425 to 134.17RVU for 30572. Breakdown value ranged from 201.30RVU for 4(2) 1425 to 228.96RVU for TME1. The higher the breakdown in viscosity, the lower the ability of the sample to withstand heating and shear stress during cooking as well as the lower the amount of cross-linked starch which is more resistant to breakdown.[28,32,45–46] Final viscosity values ranged from 140.33RVU for 4(2) 1425 to 167.75RVU for 30572, this is the most commonly used parameter to determine a particular sample's quality as it indicates the ability of the material to form a gel after cooking. Setback value ranged from 33.58RVU for 30572 to 41.54 RVU for 4(2) 1425. Setback value is the difference between final viscosity and trough in the RVU. The phase of the pasting curve commonly referred to as the setback region is the phase where/ when the mixture is cooled, a re- association between starch molecules occurs to a greater or lesser degree. It therefore affects retro gradation or reordering of the starch molecules.[28] High setback is also associated with syneresis, or weeping, during freeze thaw cycles for example, and replacement starches are commonly used where this presents a quality defect. Peak time is the time at which the peak viscosity occurred in minutes. Its value ranged from 3.60min for 4(2) 1425 to 3.90 min for 92B/00061. Peak time was higher than the values of 2.17 to 3.00 obtained by Amarjeet et al.[41]

Table 3 Pasting properties of cassava sour starch

Varieties	Peak viscosity (RVU)	Trough (RVU)	Break down (RVU)	Final viscosity (RVU)	Set back (RVU)	Peak time (minutes)	Pasting temp. (°C)
30572	360.38b	134.17a	209.96ab	167.75a	33.58b	3.78b	65.13a
4(2) 1425	333.17c	98.79d	201.30b	140.33b	41.54a	3.60c	64.28a
92B/00061	342.29c	123.71b	214.50ab	165.54a	41.84a	3.90a	63.78a
96/01632	334.46c	109.00c	208.84b	149.88b	40.88a	3.77b	64.13a
96/0603	362.96b	132.59a	211.75ab	166.38a	33.79b	3.75b	64.08a
TME1	380.75a	127.92ab	228.96a	163.00a	35.09b	3.74b	64.45a
Mean	352.33	121.03	212.55	158.81	37.78	3.75	64.30
R-Square	0.94	0.98	0.76	0.95	0.87	0.95	0.61
C.V	1.91	2.58	3.51	2.39	5.87	0.81	0.82
Std Dev	18.55	13.65	10.25	10.94	4.12	0.09	0.57
Range	333.17–380.75	98.79–134.17	201.30–228.96	140.33–167.75	33.58–41.84	3.60–3.90	63.78–65.13
P of Clone	∗	∗∗	NS	∗	∗	∗	NS
Each value represent mean of three replicates. Mean values having the same superscript within column are not significantly different at 5% confidence level. ∗P < 0.05; ∗∗P < 0.01; ∗∗∗P < 0.0001; and NS: Not Significant.

Pasting temperature values ranged from 63.76°C for 92B/00061 to 65.13°C for TMS 30572 (Table 3). Pasting temperatures were slightly lower than the 64°C to 69°C obtained by Eggleston et al.[29] for cassava flour, but higher than the 42°C obtained by Numfor et al.[33] A range of pasting temperatures from 52 to 60°C has been reported by Moorthy.[42,46] There were significant differences in peak viscosity, setback, peak time, trough and final viscosity at 5% confidence level. No significant differences were observed in pasting temperature and breakdown. Correlations were significant among trough, peak viscosity, peak time, setback, and final viscosity at P < 0.01 confidence level, while negative correlation exists among setback, trough, and peak viscosity at P < 0.01 level.

CONCLUSION

The results from this work on sour starch showed that varieties have significant effect on the physicochemical (except moisture content), functional (except WAC, Swelling power and solubility), and pasting (except breakdown and pasting temperature) properties. Fermentation has no effect on the shape of the starch, rather it only showed the level of starch damage. Based on the results obtained, sour starches made from cassava possess a unique baking expansion property suitable for enhancing quality of cassava based bakery products, as well as developing new products.

Notes

6. Dufour, D. Etude des Potentialités D'utilisation du Manioc. Collaboration CIRAD—SAR / CIAT. Rapport d'activité. CIAT, Cali, Colombie, 1994, 52.

28. IITA. Operation manual for the series 3 Rapid Visco Analysis using thermocline for windows. Newport Scientific Pty, Ltd, 2001.