PREDEHYDRATION STEAMING CHANGES PHYSICOCEIEMICAL PROPERTIES OF UNRIPE BANANA

FLOUR

J.H. MWONGA'

Department of Food Science & Technology Makerere University P. 0. Box 7062

Kampala, Uganda

R.S. RAMTEKE and W. E. EIPESON

Department of Fruit and Vegetable Technology Central Food Technological Research Institute

Mysore, 570 013, India

Accepted for Publication June 21,2000

ABSTRACT

Flour producedfiom steamed and unsteamed unripe bananas was analyzed to determine the effect of steaming on physicochemical properties. Steaming of bananas prior to dehydration slowed dehydration of banana slices, increased water uptake, density and solubility of flour and decreased viscosity, setback, breakdown, discoloration and vitamin C content. Banana flour produced with predehydration steaming gives pastes of low paste bulk density, which is desirable for weaning and supplementary foods.

INTRODUCTION

The World production of banana and plantain is -88.5 million tons (FA0 1997). Despite this enormous production, industrial utilization is extremely limited. There is potential for processing unripe bananas into flour (Suntharalingam and Ravindran 1993; Aked and Kyamuhagire 1996; Sole 1996; Perez-Sira 1997). Difficulty in peeling of unripe bananas and browning during processing can be overcome by steaming (Suntharalingam and Ravindran 1993; Sole 1996). However, information on the effect of steaming of bananas prior to dehydration on the physicochemical properties of unripe banana flour is lacking. Application of heat in production of other starchy products such as cassava starch (Perez-Sira and Gonzalez-Parada

'To whom correspondence should be sent

Journal of Food Processing and Preservation 25 (2001) 35-47. All Rights Reserved. BCopyright ZOO1 by Food & Nutrition Press, Inc.. Trumbull. Connecticut. 35

36 J.H. MUYONGA. RS. RAMTEKE and W.E. EIPESON

1997), taro flour (Tagodoe and Nip 1994), wheat and chickpea flour (Livingstone et al. 1993) lead to alteration of the functional properties of starch such as reduction in paste viscosity, consistency and setback. Wills et al. (1984) reported that pulp of fully mature unripe Musa ucurninatu contains 71.9% moisture, 1.9% protein, 0.1% fat 1.3% sugars, 21.2% starch, 3.2% dietary fiber, 0.4% ash and 0.43% total organic acids. Bananas are therefore high in starch.

Banana starch is composed of big granules of varying shapes. Ling et al. (1982) reported a mode length of 15 pm gelatinization temperature of 70.1-74.6 and amylose content of 19.5% for Musa cavendishii starch granules. Plantain starches are also composed of large granules (10-50 pm) (Perez-Sira 1997). According to Perez-Sira (1997) plantain starch exhibits a gelatinization range of 59.3-95C and when compared to corn starch, gelatinized plantain starch suspensions exhibit higher stability and higher viscosity values at the critieal points of Brabender Visco amylograph pasting. According to Kayisu et al. (1981) banana (Musa sp. Var. valery) starch granules are irregularly shaped, with spheroid and elongated forms predominating. Low gelatinization temperatures, high swelling power, high viscosity and rapid thinning are associated with large starch molecule and weak intermolecular bonding (Whistler and BeMiller 1997). The extent of fragmentation of starch granules affects its pasting behavior and may be influenced by the processing given to the starch. According to Colonna et al. (1984) extrusion causes significant macromolecular degradation of starch while drum drying mainly affects the amylose polymers. In several of processes involving heat and moisture, starch granules quickly swell beyond the reversible point. The highly swollen granules that result are fragile and easily broken by stirring, resulting in a decrease in viscosity (Whistler and BeMiller 1997).

When applied to starch or starchy materials, extrusion, drum drying (Colonna et al. 1984; Doublier et al. 1986; Livingstone et al. 1993; Perez-Sira and Gonzalez-Parada 1997) and blanching (Johnston et al. 1996) lead to changes in pasting properties. Changes in pasting properties of starch due to heat treatment is attributed to pregelatinization (Greenwood and Munro 1979). If coarsely ground, pregelatinized starches can be used without cooking, producing dispersions without lumps.

Our objective was to investigate the effect of predehydration steaming on the physicochemical properties of banana flour. In this study, steaming was folIowed by hot air dehydration and dry milling. Steaming and air drylng are adaptable to developing countries where technologies such as drum and spray drying are unavailable. Predehydration steaming slowed dehydration of banana slices, increased water uptake and solubility of flour and decreased viscosity, setback, breakdown, discoloration and vitamin C content. Reduction in paste viscosity is particularly important in the manufacture of weaning and supplementary foods from starchy foods.

STEAMING CHANGES BANANA FLOUR PROPERTIES 37

MATERIALS AND METHODS Banana

Mature unripe bananas (Musa cavendishii), three quarter to fill three quarter maturity based on fullness of the banana fingers (Thompson and Burden 1995), were obtained from a wholesale agent in Mysore, India, within 2 days after harvest. The bananas were processed on the day they were received.

Production of Banana Flour

The bananas were washed and steamed (Krauss Maffei Munchen Steam Kettle; Munich, Germany) at 1 kg/cm2 (100-1 15C) for 7 min. This heat treatment was adequate to eliminate discoloration as detected both by colorimetry (Fig. 1) and visual observation and to sufliciently loosen the banana peel, allowing hand stripping of the peels. The bananas were removed from the steam kettle, cooled under ambient temperature for 30 min and the peels were stripped by hand. The pulp and peels were weighed to determine the pulp to peel ratio. The peeled bananas were sliced using a Stephane machine - UM 40 (Mumbai, India) with a 4 mm slicing disk. The slices were dried in a cross flow hot air drier (Armstrong Smith, PVT Ltd., India), with an air speed of 1.5 ds for 4 h. Four hours were required to lower the moisture content of the banana slices, below 12% on a dry basis (Fig.2). Each tray (80x40 cm) was loaded with 1.5 kg of banana slices and the slices dried at 60-65C The slices were milled using a hammer mill (Cadmach Machinery Co. Pvt., Ltd., Ahmedabad, India) to pass through a 0.685 mm screen.

Unsteamed banana flour was produced by hand peeling raw bananas with stainless steel knives by the traditional method used in Uganda. This method entails removing the terminals and slicing the peels off the sides as close to the pulp as possible. Slicing, drying and milling were conducted as described for steamed banana. The Cadmach mill (Machinery Co. Pvt., Ltd., Ahmedabad, India) and Mikro pulveriser (Ramlek and Co. Ltd., London) with 0.685 mm and 0.25 mm screens, respectively were used to grind the slices. Earlier trials revealed that flour produced from unsteamed banana slices with the 0.685 mm screen using the Cadmach mill was comparable in particle size distribution to flour passing through the 0.25 mm of the Mikro pulveriser (Fig. 3).

Analytical Methods

Moisture and crude protein content of the banana flour were determined by AOAC (1997) methods 4.1.03 and 955.04, respectively. As4 crude fiber, crude fat, starch, total sugars, tannins, vitamin C and titratable acidity were d e t e d e d by methods described by Ranganna (1986). Ash determination was by dry ashing at 450C, crude fat by Soxlet extraction, starch and sugars by the Lane and Eynon method after acid hydrolysis and citric acid inversion, respectively, tannins by

38 J.H. MWONGA. R.S. RAMTEKE and W.E. EIPESON

60

55

;: j 20

4 5 6 7 8 9 1 0 1 1

Steaming Time (Min)

FIG. 1. COLOR (L AND AE) OF DEHYDRATED BANANA SLICES STEAMED AT 1 10-1 15C

titration with potassium pennanganate, and vitamin C by the titrimetric method based on reduction of 2,6-dichlorophenolindophenol.

pH for a filtrate from a 10% (w/v) suspension of the flour in distilled water was determined using a Control Dynamics pH meter (C.D. Instrumentation Pvt. Ltd., India).

Cold water solubility of the flour was determined by method 938.03 AOAC (1997). Rehydration ratio was estimated by determining the quantity of water absorbed (mL) per gram of flour solids in the solubility determination.

Pasting properties were determined by a Rapid Visco Analyzer (RVA,3D, New Port Scientific, Sydney, Australia). A 12% (w/v) suspension of flour in water was prepared. The suspension was heated to 50C rapidly (within 1 min) and further heated to 95C at a rate of 6C per minute. The suspension was maintained at 95C for 5 min and cooled to 50C at a rate of 6Cfmin.

A Brookfield Viscometer (Brookfield Engineering Laboratory, Stoughton, MA) was used to determine viscosity for 7 and 10% gruels prepared by mixing the flours with water and heating to boiling within 10 min. The gruels were cooled under ambient conditions to 50C and the viscosity was determined. During both heating and cooling, the gruels were stirred intermittently. Viscosity determinations were conducted using spindle #4. A shear rate of 60 rpm was used in determination of the viscosity for the 7% and 10% gruels, while variable shear rates (6,12,30 or 60) were used to determine the effect of shear rate on viscosity for 7% gruels. The viscosity readings were taken 60 s after shear by the rotor was started.

STEAMING CHANGES BANANA FLOUR PROPERTIES 39 350

250

E 0- 200

f 150 I

s 3

100

50

+Steamed

- - Not Steamed

0 0 1 2 3 4 5 6 7

Time (Hours)

FIG. 2. DRYING CURVES FOR STEAMED AND UNSTEAMED BANANA SLICES

Loose and tapped density of the flour was determined by taking the weight of flour filling a 100 mL flask, without compacting and after compacting by repeatedly knocking the flask on the table until decrease in volume ceased (-5 min). Particle size distribution was determined by the method described by Melcoin and deMonredon (1996). A set of 5 standard (105, 125, 150, 180 or 212 pm) circular woven test sieves (Prem Engineering Coop, Model Town, Delhi, India) were arranged in ascending mesh size with the largest mesh size sieve topmost. Flour (100 g) was put on the top sieve and the alternative shaker ( W S Modem Scientific Instruments, Bombay, India) was run for 15 min. Flour retained by the individual sieves was recovered and weighed.

The banana color (L, a, b and AE) was determined using a Shimadzu color measuring instrument (MPC-3 100, Shimadzu Corp., Japan).

Statistical Analysis

The moisture content of slices during dehydration, color of slices and flour chemical composition were compared using t-test. Means for other attributes were compared by two way ANOVA and means separated by LSD. Data for all parameters are triplicates.

40 J.H. MWONGA, R.S. RAMTEKE and W.E. EIPESON

70

60

50

8 c 40

$ 30 rY

20

10

0

0 C .- I

Pan 105 125 150 180 212 Size (Microns)

Steamed (0.685mm mill mesh)

(0.685mm mill mesh)

0 Steamed (0.25mm mill mesh)

=Not steamed

FIG. 3. PARTICLE SIZE DISTRIBUTION OF STEAMED AND UNSTEAMED UNRIPE BANANA FLOUR

RESULTS AND DISCUSSION

The pulp to peel weight ratio was higher (pc0.05) for steamed hand stripped bananas than for knife peeled banana (Table 1). The lower pulp to peel ratio for knife bananas was due to loss of pulp along with peels during peeling. The process of peeling bananas with knives is very time consuming, labor intensive and generally not amenable to large scale processing.

Steamed banana slices dried at a slower rate than unsteamed banana slices (Fig. 2). The slower rate of dehydration observed for steamed banana slices may be attributed to improved water retention by gelatinized starch (Whistler and BeMiller 1997). Steaming therefore increases the energy requirements for dehydration.

Chemical Composition

The composition of the banana pulp and flour are presented in Table 1 and Table 2, respectively. Because of low sugar content, caramelkation, hygroscopicity and slow d y n g are not significant problems in dehydration of unripe banana slices. Steamed banana flour is higher in tannins than the unsteamed banana (Table 2). The difference in tannins content may be attributed to loss of tannins in the latex. The latex oozes out of pulp during and after peeling of the unsteamed bananas, but is retained in steamed bananas.

STEAMING CHANGES BANANA FLOUR PROPERTIES 41

TABLE 1 . PROPERTIES OF UNRIPE BANANA PULP

Property Mean f 6 Pulp to Peel Ratio

Steam Peeled 2.38 i 0.08 Hand Peeled 1.08 f 0.08

Moisture Content(%) 76.25 f 1.06 Starch (YO) 75.76 f 3.46 Total Sugars (%) 7.39 f 0.75 Titratable Acid ( x l 02)(Yo), 0.48 f 0.14 PH 5.58 f 0.19

All constituents except moisture on dry basis. n=3

* As Citric acid

TABLE 2. CHEMICAL COMPOSITION OF UNRIPE BANANA FLOUR

Steamed Not Steamed Moisture (YO) 9.31 f 0.38 9.29 f 0.05 Crude Protein (%) 1.74 f 0.06 1.51 f 0.01 Crude Fat (%) 0.30 L- 0.01 0.32 f 0.03 Crude Fiber (%) 0.99 f 0.02 0.95 f 0.04 Ash (%) 2.99 k 0.01 2.69 f 0.05 Starch (%) 70.64 k 1.79 75.90 L- 1.01 Total Sugars (%) 12.35 f 0.13 9.41 f 0.39 Vit C (mgll00g) 6.18 L- 0.46 13.05 f 0.72 Tannins (%) (x lo2) 0.78 f 0.22 0.43 L- 0.01 Mean fa. All Constituents except moisture on dry basis. n = 3

The vitamin C content of flour &om unsteamed bananas was more than double that of flour from steamed bananas (Table 1). The difference in vitamin C content is attributable to thermal destruction during steaming. Vitamin C content of flour from steamed and unsteamed bananas was lower than concentrations of 1.69 mg/g fresh weight in “Bhimkol”, a seedless banana from Assam, India (Barthakur and Arnold 1990) and 17.5 mg/g in unripe plantain (Ukhun and Ukpebor 1991). Suntharalingam and Ravindran (1993) reported vitamin C content of 25.0 and 24.7 mg/g of flour when s d u m metabisulfite (SMS) dip was used prior to dehydration of Alukehel and Monthan bananas. Without SMS however, vitamin C was 9.2 mg/g and 10.2 mg/g respectively, for the two cultivars. Flour from steamed bananas contained more total sugars and less starch than flours from unsteamed bananas. This difference may be attributed to partial starch hydrolysis during steaming.

42 J.H. MUYONGA, R.S. RAMTEKE and W.E. EIPESON

Particle Size Distribution

Steamed banana flour produced using the 0.685 mm mesh contained a higher percentage of the large size granules than flour from unsteamed bananas with the same screen (Fig. 3). The particle size arithmetic means (Melcoiu and deMonredon 1996) for the unsteamed (0.685 m), steamed (0.685 mm) and steamed (0.25 111111) flour were 110.7, 181.3 and 104 pm, respectively. Apparently steaming increases the proportion of large size particles.

14000

12000

10000 v) ' ). 8000

6000

c .- v)

:: 5

4000

2000

0 t I

0 12 24 36 48 60

mesh)

mill mesh) +No1 Sleamed (0 685rnm

+Steamed (0 25mm mill

Shear Rate (1/S) FIG. 4. VISCOSITY OF STEAMED AND UNSTEAMED UNRIPE BANANA FLOUR GRUELS

Color, Density, Rehydration and Solubility

Steaming of banana prior to dehydration affects the color (L, a, b and AE), solubility, rehydration ratio and density (Table 3) of banana flour. Flour from steamed banana exhibited higher rehydration than flour from unsteamed bananas. Flour is often used to enhance water uptake or retention e.g. as fillers in emulsion meat products. Flour from steamed banana exhibited higher luminance (L) and AE, lower green hue (a) and higher yellow (b) values compared to flour from unsteamed bananas. The lower L and AE values for unsteamed banana flour may be a result of browning of the bananas after peeling. Steaming inactivates the phenolases responsible for promoting browning in bananas. The higher b values for steamed banana flour are consistent with earlier reports of yellowing of unripe banana pulp during heating (Mowlah et al. 1981).

STEAMING CHANGES BANANA FLOUR PROPERTIES 43

TABLE 3. PHYSICOCHEMICAL PROPERTIES OF UNRIPE BANANA FLOUR

Property Steamed Unsteained Steamed (0.685)' (0.685) ' (0.25) I

PH 5.49 f 0.06 5.73 f 0.26 5.61 f 0 09 Titratabie Acidity' (x102) 0.41 f 0.07 0.65 k 0.00 0.41 _+ 0.21 Color L 74.32 + 6.21 77.00 ?r 9.44 82.86 * 5.90 a 0.63 _+ 0.06 0.56 _+ 0.16 0.60 k 0.19 b 15.01 k 0.07 12.25 f 4.71 13.64 + 0.64 AE 29.80 ?r 6.21 25.92 f 10.7 21.77 k 5.94

Solubility (%) 2.78 0.12 9.68 f 0.60 3.20 + 0.42 Rehydration Ratio 5.13 k 0.18 3.12 f 0.05 4.19 f 0.03 Density (mgfml)

Loose 0.68 f 0.01 0.45 f 0.00 0.70 f 0.01 Tapped 0.87 f 0.02 0.66 f 0.01 0.95 f 0.00

Gruel Viscosity(cPs) # (x 1 0.2)

7 % 1.45 ? 0.07 17.75 k 3.54 72.50 t 0.49 10% 16.50 ? 0.07 24.70 0.63 21.15 k 2.62

*As citric acid. #At 60 rpm using spindle No. 4. Mean f 6, n=3. 'Values are diameters for mesh used in milling of flour.

Although pregelatinized starch is expected to exhibit higher solubility in cold water than unmodified starch (Whstler and BeMiller 1997), this was not observed in this study. The anomaly was probably due to starch retrogradation and different extents of particle disintegration during milling. The higher rehydration (high water uptake) observed for steamed banana flour than for unsteamed banana flour, however, is in conformity with the fact that gelatinized starch absorbs more water than ungelatinized starch (Whistler and BeMiller 1997; Doublier et d. 1986). The fine (0.25 mm) steamed banana flour absorbed less water than the coarse milled steamed banana flour, probably because of agglomeration of starch granules (Whistler and BeMiller 1997). Steamed banana flour also exhibited higher densities (loose and tapped) than raw banana flour. Tagogoe and Nip (1994) also reported increase of flour density as a result of heat treatment of taro prior to drying.

Rheologieal Properties

Pasting properties and viscosity of gruels made from flours hgh in starch can to a large extent be attributed to the starch in the flours. Flour particle size influences the viscosity of pastes made from starchy flours (Kikafunda et af. 1997; Whistler and BeMiller 1997). High paste viscosity may be desirable in flours used as thickeners. In the manufacture of foods aimed at supplying a substantial amount of nutrients such as weaning and supplementary foods, it is desirable to include

44 J.H. MUYONGA, R.S. RAMTEKE and W.E. EIPESON

materials that do not form highly viscous pastes at low solids concentrations. Gruels prepared fiom flour fiom steamed bananas exhibited lower viscosities than gruels fiom unsteamed banana flour with the same flour concentrations (Table 3). Steaming of bananas prior to dehydration is therefore advantageous in production of flours for use in weaning and supplementary foods but is disadvantageous in production of thickeners. Gruels from the frne milled (0.25 mm) flour exhibited higher viscosity than those fiom coarse flour at both 7% and 10% solids. At 10% solids concentration, the steamed banana fine flour tended to form lumps in both hot and cold water. This tendency is associated with finely milled pregelatinized starch (Whistler and BeMiller 1997). The coarse milled flour mixed easily to form a paste in hot water and may be suitable for use as an instant flour. The viscosity of banana flour gruels reduced with shear rate (Fig. 4). The gruels, exhibited pseudoplastic flow regardless of the flow production procedure.

The lower gelatinization temperature for the flour produced from steamed banana compared to unsteamed banana flour (Table 4) suggests that steaming causes pregelatinization of starch in the flour. The lower peak viscosity of gruels fiom steamed banana flour than unsteamed banana flour gruels is also attributable to pregelatinization. Based on the lower setback, breakdown and consistency of pastes made fiom steamed banana compared to unsteamed banana flour, it can be concluded that steamed banana flour yields more stable and easier to cook pastes (Smith 1964). Paste stability is desirable since the tendency for starchy pastes to thicken upon cooling or to thin upon further heating is disadvantageous in most applications.

TABLE 4. PASTING PROPERTIES OF UNRIPE BANANA

Property Steamed Unsteamed Steamed (0.685)’ (0.685)’ (0.25)’

Gelatinization Temperature ( “C) 50.85 75.35 49.4

Viscosity at 95 C 203 172 343 Peak Viscosity (P) 226 452 380

Viscosity at 95 C after 5 min 131 140 196 holding (H) Viscosity at 50 C (C) 216 358 306 Breakdown (P-H) 95 312 184 Consistency (c-H) 85 218 104 Setback (C-P) 10 96 74

Viscosities in Rapid Visco Analyzex Units (RVU) ‘Values are diameters for mesh used in milling of flour

STEAMING CHANGES BANANA FLOUR PROPERTIES 45

CONCLUSION

Steaming of unripe bananas prior to dehydration may be a useful pretreatment in production of unripe banana flour for use in weaning and supplementary foods since low paste bulk (low viscosity in relations to solids concentration) ensures higher intake of solids (and therefore nutrients) for a given gruel volume consumed. In products required to exhibit high viscosity or water uptake e.g. thickeners, predehydration steaming is disadvantageous since it reduces water uptake and paste viscosity. In absence of machinery for mechanical peeling of unripe banana, predehydration steaming provides a simple and inexpensive way of facilitating peeling of unripe banana for processing.

ACKNOWLEDGMENTS

JHM acknowledges The United Nations University for providing a research fellowship. The authors thank Dr. V. Prakash of CFTRI, Mysore for his advice.

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