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Physicochemical properties of processed pulse flour

H. Yun1, 2, G. Rema1, 2 and K.J. Quail1, 2

1BRI Australia Ltd., PO Box 7, North Ryde, NSW 1670 Australia,
2
Grain Foods CRC Ltd., Riverside Corporate Park, 1 Rivett Road, North Ryde, NSW 2113 Australia

INTRODUCTION

The nutritional and functional benefits of pulses have raised their profile within the food industry. However, the utilisation of pulses except soybean in food products has been limited and it is considered that this is largely due to their strong “beany” flavour, and a lack of technical information on their physicochemical properties. For soy, unlike the other pulses, there has been extensive research in the area of flavour and product quality (Mateos et al, 1999). The beany flavour in soy has been identified as hexanal. It is believed that hexanal, an aldehyde, is formed from linoleic acid by lipoxygenase and consequently hydroperoxide lyase. Methods to reduce soy flavour have included heat, enzymic, genetic and chemical modification. Various results have been reported (Kwok and Niranjan, 1995, Kato et al, 1981).

Mungbean is an important pulse produced in Australia. The most popular forms of food application for mungbeans are dhal and sprouts. As mungbean flour and starch show high viscosity and heat stability, it is considered that there is a wide range of potential food applications. In this research, the mungbean was selected to investigate any change of physicochemical properties after successful treatment to reduce grassy and beany flavour using various methods.

MATERIALS AND METHODS

Green mungbean grain was de-hulled and milled into flour on the BRI Australia Pilot Mill. The flour was high in protein content and dietary fibre (Table 1).

Table 1. Chemical composition of mungbean flour (%)

Protein

Fat

Fibre

ADF1

Ash

Ca

Mg

K

25.9

8.2

3.8

7.0

3.6

0.11

0.15

1.10

(1ADF: Acid Detergent Fibre)

Heat processes to reduce the grassy and beany flavour of the mung bean flour included hydrothermal and dry thermal processes. With hydrothermal process, steaming was conducted for 25, 35, 45 and 55min at the maximum rate using a steamer (Curtin, Queensland). Autoclaving was done at 140C, 140kPa for 15min.Three dry heat processes were tested including oven roasting (BRI Oven Simulator) at 125C for 30min. A high air velocity system was used at various initial moisture contents of 9, 13 and 17% at 220C, 230C and 240C for 20min, 22.5min and 25min. Flour samples were processed using a microwave oven (Samsung, MS143HSE) in cooking mode for 10min, 15min and 20min. Mungbean flour was processed with various chemicals including K2CO3 (2.5%), CaCl2 (2.5%), MgCl2 (5%), NaCl (1M), NaHCO3 (0.5%), aqueous ethanol (50%), methylated spirit (>98% ethanol) and ozone. Flour viscosity was measured on a Rapid Visco Analyser (Newport Scientific, RVA 4) using the standard 13min profile. Peak viscosity, final viscosity and peak time were measured and analysed. Flour swelling volume was evaluated by AACC method (56-21). Flour colour was measured using Minolta L*a*b* system (CR310). Gels were made using 3g of flour with 25ml of deionised water. Gel hardness was measured by cutting through 80% of the height using Texture Analyser (TA2, Stable Micro Systems, UK).

Results and discussion

Flour colour

All thermal processes reduced the brightness of mungbean flour depending on processing time and temperature (Figure 1). Microwave oven showed the smallest change in the brightness. Potassium carbonate reduced brightness more significantly than other chemicals used in this study. The effect by potassium carbonate was similar to ethanol. Steaming treatment increased the yellowness of mungbean flour, whereas dry thermal processes did not change the yellowness. It indicated moisture had a significant effect on mungbean colour. The ozone process reduced flour yellowness.

Figure 1. Appearance of mungbean flour after various processes

RVA viscosity

Hydrothermal processes slightly increased peak and final viscosity, whereas dry thermal processes had a significant effect on both peak and final viscosity (Figure 2). It was shown that peak and final viscosity increased up to 230C and with higher temperatures it decreased. With microwave process, peak viscosity increased as process time increased, but final viscosity decreased after 15min of processing time. All chemicals increased peak viscosity, but decreased final viscosity except NaHCO3. Aqueous ethanol (50%) did not change peak and final viscosity, whereas 98% ethanol resulted in a large increase in peak viscosity. Generally all processes reduced peak time. It was noticeable that ozone treatment changed viscosity behaviour. The results indicated that there was an optimum thermal processing condition to achieve a certain range of viscosity.

Figure 2. RVA viscosity results of selected processes

Flour swelling volume

Control mungbean flour showed swelling volume of 23.06ml/g. All processes reduced swelling capacity. Particularly hydrothermal treatments had low swelling volumes (Figure 3).

Figure 3. Flour swelling volume results of selected processes

Gel texture and colour

Mixed results were observed for the gel texture assessment. Steaming increased gel hardness, whereas autoclave processed flour produced a softer gel. Microwaving decreased gel hardness up to 15min of processing time, but the gel processed for 20min showed stronger gel texture. Generally thermal processes decreased the brightness of the gel, whereas most chemicals increased gel brightness with the exception of potassium carbonate. No change was observed in yellowness of the gel by thermal processes (Figure 4).

Figure 4. Gel texture and appearance of selected processes

Conclusion

The results indicated that processing mung flour to reduce the grassy and beany flavour changed its physicochemical properties. Thermal processes reduced the brightness of flour and gel products, whereas some chemicals had a minor effect on appearance of the flour and gel. All processes changed viscosity and swelling properties of mungbean flour. The optimum processing condition needs to be selected to meet the specific requirements for food applications.

Acknowledgement

The authors wish to thank Grain Foods CRC for financial support.

Reference

Mateos, G., Latorre, M. and Lazaro, R. (1999) Processing Soybeans, American Soybean Association

Kwok, K.-C. and Niranjan, K. (1995) International Journal of Food Science and Technology 30, 263-295

Kato, H., Doi, Y., Tsusita, T., Kosai, K., Kamiya, T. and Kurata, T. (1981) Food Chemistry 7, 87-94

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