Flowability Properties of Sweet Potato Flour

Transcription

1 Proceedings of the 214 International Conference on Food Properties (ICFP214) Kuala Lumpur, Malaysia, January 24 26, 214 Flowability Properties of Sweet Potato Flour Abu Bakr Mohd Hanim a, *, Nyuk Ling Chin b, Yus Aniza Yusof b a Malaysian Agricultural Research and Development Institute (MARDI), Malaysia b Universiti Putra Malaysia (UPM), Malaysia Abstract The measurement of powder flowability for a new variety of sweet potato flour, the VitAto, known for its high vitamin A contents was compared with commercial wheat and potato flours. The VitAto flour has large average particle size of μm, lower bulk density of.626 g/cm 3 and cohesion index of mm. The VitAto flour is classified as an easy flowing and stable powder but prone to caking. This study has been carried out to provide innovative information to the sectors involved in handling packing and storage of sweet potato flours. Keywords VitAto; Sweet Potato; Powder; Flowability 1. Introduction In Malaysia, sweet potato (Ipomoea batatas) ranks second among the tuber crops next to tapioca and has been cultivated on small scale since the 17 th century. The Malaysian Agricultural Research and Development Institute (MARDI) has introduced the VitAto, a new variety of sweet potato in June 27, the culmination of a 1 years breeding program aimed to develop a nutritionally more superior sweet potato varieties. The VitAto is rich in beta carotene (266 mg/1g) and vitamin C (1.33 mg/1g), dietary fiber content (2.49 g/1g) and has a low glycemic index of 54 [1]. These good nutritional qualities of VitAto have made it a very good supplement of food for Malaysians. Processing of sweet potato into flour is the most satisfactory method of creating a product that is not only functionally adequate, but also remain for an extended period without spoilage. While most scientific research has been carried out to investigate the technological properties of flour for food applications, there is some knowledge lacking in knowledge about engineering properties of flour which in this case is the sweet potato processed into powder form. The usual methods referred by the factory operators are more to empirical knowledge of flour through indications of powder flowing behavior, granulation properties, handling properties (e.g. sieving characteristics after milling and bridging threshold), powder stability (e.g. caking phenomena during storage) and water absorption abilities of flour [2]. The objective of this work is to measure the flowability of the new variety of sweet potato flour, the VitAto, using Powder Flow Analyzer (TA-XT plus, Stable Micro Systems, Surrey, UK). There are published data concerning powder flowability properties for common wheat flours. Wheat flour particles are anisotropic. They have been considered as relatively free flowing, poorly cohesive powders, which do not cause caking problems. There is no information on the flowability of other flours including the sweet potato flour. The physical and powder properties of sweet potato powder is therefore measured and also compared with commercial wheat and potato flours. This sweet potato powder flowability characterization is needed for its commercial value in the industries. 2. Materials and Methods Tubers of sweet potato, the VitAto were purchased from farmers of the Crop, Vegetable and Farm Cluster Project in Kampung Tanjung Sepat by the Department of Agriculture Malaysia located at Kuala Langat, Selangor, Malaysia. The wheat flour was purchased from a local miller, Johor Bahru Flour Mill Sdn. Bhd. while the potato flour was from Synerchem Food Processing Ind. Sdn. Bhd. The production of VitAto flour followed the methods of Zainun et al. [3]. Grad B tubers, which are in the size range of g/tuber were selected. The tubers were washed with water to remove adhered mud and dirt. Washed tubers were cut into strips using a slicer machine (RG-61, Hallde, Sweden) to increase the surface area of tubers before drying. The sweet potato strips were soaked in.2% sodium metabisulphite for 3 minutes to stop browning reaction and produce fine powder later. The strips were dried in a cabinet drier (M , Sanyo Electric, Japan) at 6 o C until final moisture content below 5%. The dried strips were milled in a universal miller (UM5-SS, Safe World, Malaysia) and sieved

2 through a 3 µm mesh. The powder were packed in polyethylene bags and stored in a chiller (LF- 817LD, Aseco, Taiwan) at 5-1 o C until further analysis. The powder bulk density was determined following methods of Wang and Kinsella [4]. Ten grams (1 g) of powders sample were weighed and put into 5 ml measuring cylinder. The sample was packed by gently tapping the cylinder on the bench top from a height of 5 cm. The volume of the sample was recorded. The bulk density was calculated as equation (1). Bulk Density = (g/cm3 ) (1) The particle size distributions of powder sample were measured using a particle size analyzer (Mastersizer 2, Malvern, United Kingdom). Before the measurement, the background reading for water was recorded as the calibration and each sample was added until an obscuration of 18-2% was achieved [5] Flow properties of powders were determined by following methods described in Benkovic and Bauman [6] using a Powder Flow Analyzer (TA-XT plus, Stable Micro Systems, Surrey, UK). The Powder Flow Analyzer is constituted by a cylindrical glass vessel of 12 mm height and 5 mm internal diameter with a rotating specific blade of 48 mm diameter and 1 mm height, which is able to go up and down, in right or left rotation. The flowability properties were evaluated during the displacement in a controlled manner of the rotating blade inside the container, filled with the powder sample. The tests performed were the cohesion test, powder flow speed dependency test (PFSD test) and caking test. For each experiment, a fixed mass of 9 g powder was poured into the cylindrical glass vessel prior for the three tests. The three tests were runs in sequence. The conditioning cycle at the beginning of test is a cycle where the blade moves downwards and upwards through the powder column at a tip speed of 5 mm.s -1 without the reading measurement. Testing cycle starts when the instrument starts to measure the force reading. In the cohesion test, there are two conditioning cycles followed by three testing cycles to measure the cohesion characteristics of the sample. The cohesion coefficient is determined during the decompression phase at 5 mm.s -1 speed while the blade moves upwards through the powder column. Cohesion coefficient is calculated by the Texture Exponent 32 software by integrating the negative areas under the force/distance curve. Cohesion index is defined as the ratio of cohesion coefficient/sample weight. The powder categorization scale based on cohesion index is shown in Table 1. Table 1: Powder categorization scale based on cohesion index [6]. Cohesion index (mm) Flow behavior 19+ Hardened, extremely cohesive Very cohesive Cohesive Easy flowing 11 Free flowing For the PFSD test, there are two conditioning cycles followed by 5 sets of 2 cycles at increasing speeds (1, 2, 5, 1 mm.s -1 ) and the final 2 cycles at 1 mm.s 1. The downward parts of the cycles compact the powder and the upward stroke of the cycle uses a lifting action. Compaction coefficients (g.mm) are determined for each compaction cycle (at successive tip speed of 1, 2, 5 and 1 mm.s -1 ) by integrating positive areas under the force/distance curve using Texture Exponent 32 software. The flow stability index was determined by the ratio of the compaction coefficients measured during the first and last compression phases at 1 mm.s -1. In the caking test, two conditioning cycles starts before the testing cycle. The blade levels the top of the powder and compacts the powder column to a specified force. Compaction cycle is repeated five times and at each cycle, the change in column height is recorded. Cake strength and mean cake strength are calculated as the area under the distance force curve divided by initial sample weight using Texture Exponent 32 software. Figure 1 illustrates the typical diagrams of the cohesion, PSFD and caking tests for powders.

3 2 a 1 Force (g) b Distance (mm) Force (g) Force (g) Figure 1: Force versus distance plots for a (a) cohesion test, (b) PSFD test and (c) caking test for powders. The entire experiment was completed in triplicates. MINITAB Statistical Software (Release 16, Minitab Inc., USA) was used to perform the single factor analysis of variance (ANOVA). Tukey s test was performed to compare differences among the mean values at a confidence level of Results and Discussion c Distance (mm) Distance (mm)

4 The flow properties values of powders are summarized in Table 2. The VitAto flour has the lowest bulk density.626 g/cm 3 and highest mean particle size, d 5 of μm compared to wheat and potato flours. Figure 2 shows the particle size distribution of the three flours have peaks that coincide with their mean particle size, d 5. The wheat and potato flours, each has a single distinct peak at mean diameter of and 44.32μm, respectively while the VitAto flour has bipolar curve shape including an additional peak at a smaller particle size of 1 2 μm besides the μm. This bipolar distribution result is similar with the sweet potato flours reported by Aprianita et al. [5] and wheat flour reported by Landillon et al. [2] where the presence of the smaller peak at a smaller particle size is due to the isolated starch granules that probably arise due to the increase of powder extraction rate during milling. Particle size has been considered as one of the most important physical properties which affect powder flowability. With a bigger particle size, the contact area between particles would be less and this reduces particle compaction and results in a weaker intermolecular forces VitAto Wheat Potato Volume (%) Particle size (μm) Figure 2: Particle size distribution of powders Table 2: Powder characteristics in terms of flow properties, cake strength and mean cake strength values of flours Parameter VitAto Wheat Potato Bulk density (g/cm 3 ).626 ±. c.855 ±. a.756 ±. b Particle size d 5 (μm) ±.89 a ± 3.64 b ±.36 c Cohesion coefficient (g.mm) ± b ± 13.6 a ± a Cohesion index (mm) ±.21 b ±.17 a ± 1.71 ab Flow stability (dimensionless).96 ±.5 ab.94 ±.4 b 1.7 ±.4 a Cake strength (g.mm) ± a ± b ± b Mean cake strength (g) ± 8.32 a 7.81 ± 19.7 b ± 8.29 c All means present the average of three independent replications. Values followed by the different superscript in each row are significantly different (P<.5) From the measure cohesion index in Table 2, the VitAto flour is categorized as easy flowing powders with scales ranging from 11 to 14 (Table 1). The commercial wheat and potato flours are categorized as cohesive powder with higher cohesion index values of and mm respectively. Similar result was reported by Landillon et al. [2] on various wheat based powders where they were categorized as cohesive powders. The cohesion relativity between the three flours are aligned with the particle size analysis where the VitAto flour has a significant larger particle size, d 5 of μm when compared to wheat and potato flours measured at and μm, respectively. Powder with smaller particle

5 size is subject to cohesion and their flowability is more difficult. The rationale behind this reduction in flowability at a smaller particle sizes is due to the increased surface area per unit mass of flour. There is more surface area or surface contacts available for cohesive forces, in particular, and frictional forces to resist flow. Flow stability index value below 1 is associated with granulation effects during the flowability test. Flow stability index above 1 is associated with no-granulation effects during the flowability test [2]. The VitAto and wheat powders have flow stability index value below 1 and are and the values do not differ significantly. The commercial potato flour has flow stability value more than 1, thus granulation effects is minimal in its flowability test. The dependence of the powder towards flow speed and its flow stability was assessed by the PFSD test, which gave significant information on how powder acted when being conveyed. In order to replicate the conditions which prevail during powder transportation, the helical blade compressed and decompressed the powder in a cylinder, during which compaction coefficients were obtained. The terms compression and compaction are related to the ability of a loose powder to form a compacted powder by decreasing its initial volume. The compression matches the idea of die pressing of powders and describes the kinetic of vibratory rearrangement of particles within a certain bulk structure [7]. All of the powders show a dependent flow tip speed with decreasing compaction coefficient as illustrated in Figure 3. A decrease in compaction coefficient with increasing flow speed means that the powders become freer flowing [6]. 8 7 Compaction coefficient (g.mm) VitAto Wheat Potato Tip speed (mm.s -1 ) Figure 3: Compaction coefficients of powders Caking is a deleterious phenomenon by which amorphous food powders are transformed into a sticky undesirable material, resulting in loss of functionality and deterioration of quality. Cake strength depends on number of factors i.e. packing efficiency, particle to particle interactions and moisture content [6]. Table 2 shows that the VitAto powder has the highest cake strength of g.mm and mean cake strength of g when compared to wheat and potato flours. As such, it requires a proper packaging system for storage at the low moisture content conditions. The commercial wheat and potato flours have lower cake strength and mean cake strength suggesting lower caking tendency. 4. Conclusions The VitAto flour is categorized as an easy flowing and stable flour. It has a relatively lower bulk density, bigger particle size and lower cohesiveness when compared to wheat and potato flours. It is

6 however more prone to caking, hence proper packaging system and low moisture content condition during storage and processing are advised. 5. References 1. Mahmood, R., Ibrahim, N., Mohd. Nasir, S., Pin, C.H. and Hamzah, R., 27, Varieti ubi keledek baru (The variety of new sweet potato), Agromedia (Institut Penyelidikan dan Kemajuan Malaysia), Landillon, V., Cassan, D., Morel, M. H. and Cuq, B., 28, Flowability, cohesive, and granulation properties of wheat powders, Journal of Food Engineering 86, Zainun, C., Salma, O. and Hamidah, H., 25, Organoleptic acceptability and nutritional properties of the sweet potato based traditional cakes produced using sweet potato flour, Journal of Tropical Agricultural and Food Science 33, Wang, J. and Kinsella, J., 1976, Functional properties of novel proteins: Alfalfa leaf protein, Journal of Food Science 41, Aprianita, A., Purwandari, U., Watson, B. and Vasiljevic, T., 29, Physico-chemical properties of flours and starches from selected commercial tubers available in Australia, International Food Research Journal 16, Benkovic, M. and Bauman, I., 29, Flow properties of commercial infant formula powders, World Academy of Science, Engineering and Technology 54, Janjatović, D., Benković, M., Srečec, S., Ježek, D., Špoljarić, I. and Bauman, I., 211, Assessment of powder flow characteristics in incoherent soup concentrates, Advanced Powder Technology 23,