Investigations on the Laboratory Scale Separation of Mung Bean Starch*

Jian Hua Zhu, Beijing (P. R. China), Norbert U. Haase and Wolfgang Kempf, Detmold (Federal Republic of Germany)

Mung bean starch is used as a high-grade raw material for noodle production in the food industry. Until now the applied separation techniques are often less efficient. Therefore, investigations were carried out on the laboratory scale to give first informations about the development of new separation techniques. The raw materials were flour and whole kernels. In both cases it was possible to pro- duce a pure starch with a protein content less than 0.2% in dry matter. Physical and chemical investigations of the isolated starch pointed to the qualification of mung bean starch in different applica- tion fields, especially in view of the high viscosity.

Untersuchungen zur Abtrennung von Mungbohnensurke im LaboratoriumsmaBstab. Mungbohnenstarke ist in der Lebensmit- telindustrie ein qualitativ hochwertiger Rohstoff fiir die Glasnudel- herstellung. Bislang eingesetzte Techniken der Starkeabtrennung sind oft wenig effizient. Deshalb wurden im Vorfeld der Entwicklung neuer Starkegewinnungsprozesse erste Studien im Laboratoriums- maastab durchgefiihrt. Als Ausgangsmaterial kam sowohl Mehl als auch unbehandeltes Saatgut zum Einsatz. In beiden Rllen war es moglich, hochreine Starke mit einem Rohproteingehalt von unter 0.2% i. TS abzutrennen. Physikalische und chemische Untersuchun- gen der isolierten Starken belegten insgesamt die besondere Eignung der Mungbohnenstarke fix spezielle Anwendungsgebiete, die insbe- sonders aus der hohen Viskositatsausbildung resultieren.

1 Introduction

Today legume starches are of commercial interest. One of the main aspects is the nearly complete utilisation of the raw material, especially the protein and starch fraction. Further more, the starch quality is a special one, differing from those of classical starches. In opposite to peas and faba beans the use of mung beans is already established in technical processes, for example the production of glass noodles. The manufactured noodles are white and smooth, pliable, and have good cooking quality. Thus, mung bean starch is qualified for noodle man- ufacturing (high amylose content, restricted swelling, and a C- type Brabender viscosity) [l, 21. Within the Peoples Republic of China the technology of starch isolation is often a traditional one by using acidified water (lactic acid fermentation) [3]. Mung bean kernels are steeped and wet milled. By biological degradation the proteins can be softened and separated. Last but not least the starch is isolated by settling down, some washing steps, dewatering and drying. The described process of starch isolation is a most simple but also a critical one. The phase of steeping takes a long time (about one to three days) and the biological degradation of the proteins needs a well established regulation. The use of these proteins is not possible. Also a continuation of starch separa- tion is quite complicated. With regard to this, we describe experiments of the laboratory scale to get the mung bean starch separation more efficient by using a wet separation technique. Also some quality aspects of the starch are mentioned.

2 Material and Methods

2.1 Material

Mung beans (Phaseoh aureus Roxb.) of trade-mark have been grinded to a fine flour with a Karmas mill or were wet milled with a Starmix.

2.2 Methods

Chemical analyses of raw material and isolated starches included the determination of water [4], crude protein [S]. ash [6], fat [7], and fibre content [8]. The crude starch content was analysed by a polarimetric method (modified Ewers method; 184.0" specific optical rotation) [9]. The isolation of the starch was carried out with different separation techniques, which are mentioned in a further part of this paper. The protein starch separation was realised by the use of either settling tables [lo] or centrifuges [l l] . Particle size and shape of the starch granules were observed under a scanning electron microscope (10 KV, magnifi- cation 1040X) [12]. Yield and efficiency of the starch separation were calculated after a gravimetric starch determination. The color of the starch (whiteness) was measured by a whitemeter, standard 952 [13]. Brabender viscograms (250 cmg sensitivity cartridge; 75 RPM) were carried out with each 25 g in dry matter (DM) starch and 400 ml bidest. water. The differential thermoanalysis (DTA), using a DSC calorime- ter (Mettler; DSC 20), gave special informations about the gelatiniza- tion behaviour [14]. The degree of syneresis was estimated after a storage of the gelatinizated starch at 4°C and 7 d. Swelling power and solubility were determined according to the method of Leach et al. [15] but by modifiing the starch input (0.1 g) and the total volume (100 ml). The dissolved and swollen starch granules were separated by filtration. The amylose content of the starch was determined by an amperometric iodine titration after fat extraction and dissolving the starch in 1~ KOH for about 90 min [16]. The used value of the specific iodine binding was about 19.0%.

* Publication no. 5820 of the Federal Research Center for Cereal and Potato Processing, P.O. Box 23, D-4930 Detmold, Federal Republic of Germany. Fig. 1. Composition of mung beans.

starch/stgrke 42 (1990) Nr. 1, S. 1-4 6 VCH Verlagsgesellschaft mbH, D-6940 Weinheim. 1990 0038-9056/90/0101-01$02.50/0 1

3 Results

3.1 Raw material

The analyses of the main components (Fig. 1) are in good agreement with those of other legume species. The crude protein content was about 26% in dry matter (DM) and the starch content about 50% DM. The fat content was rather low (0.7% DM). The high amount (13% DM) of non specified and detected components, e. g. carbohydrates with a low degree of polymerization (DP), was remarkable.

3.2 Starch separation

3.2.1 First experiments

Starch isolation was carried out according to Haase et al. [14] using settling tables and centrifuges. The material (flour or kernels resp.) was steeped either in dist. water, or in sulphur dioxide solution (0.2%) or in lye (0.05 N NaOH). Further details are listed in Tab. 1 and 2 . Regarding to the analyses of the isolated starches (Fig. 2) the crude protein content of the starches was in relation to the separation technique. Usually, centrifugation resulted in lower protein values than the raffination with settling tables. The lowest level was detected by wet milling without significance to one of the separation techniques (Variants S6 and C6). Also the whiteness of the starch was very high at S6 and C6. In any case the best starch yield was reached by raffination with settling tables (Variants S1 to S6).

Table 1. Variants of Starch Separation by Settling Tables.

Variant Work

S1 (flour, water)

S2 (flour, SOz)

S3 (flour, lye) settling tables

50 g flour were steeped with 200 ml steeping agents at room temperature for 1 h; sieving with 160 and 56 pm sieves; concentration of the passage by centrifugation; raffination by

S4 (kernels, water)

S5 (kernels, SO2)

S6 (kernels, lye)

50 g kernels were steeped with 250 ml steep- ing agents at room temperature for 16 h; grinding in a Starmix for 3 min; sieving with a 160 pm sieve; the residue was grinded two times again in a Starmix for 3 min; sieving of all the passages through a 56 pm sieve; concentration and raffination see above

Table 2. Variants of Starch Separation by Centrifugation.

Variant Work

C1 (flour, water) 50 g flour was steeped at room temp. for 1 h ; centrifugation (2000 X g); decanting; the

C2 (flour, SO2) first layer rejecting; after stirring up with water and a second centrifugation repetition

C3 (flour, lye) three times with water, two times with 70% ethanol and two times with water; at least sieving the precipitate through a 56 pm sieve

C4 (kernels, water)

C5 (kernels, SO*)

C6 (kernels, lye)

50 g kernels were steeped at room temp. for 16 h ; grinding in a Starmix for 3 min; sieving with a 160 pm sieve; the residue was grinded two times again; sieving; all the passages were sieved through a 56 pm sieve; for further preparation see above

Fig. 2 First experiments of starch separation; quality aspects of the starches S1 to S6 (settling tables) and C1 to C6 (centrifuge).

3.2.2 Main experiments

Within the main experiments the results of the first experiments (Section 3.2.1) were considered. Therefore, steeping with sulphur dioxide was not carried out. The separation gave an opportunity for settling tables. Beside this a protein sedimenta- tion has been observed closed to small starch granules. But after a second sedimentation step this phenomenon did not exist any more. A schedule of the whole starch separation procedure is given in Table 3. With regard to the crude protein content (0.1% DM) the isolated starch was mostly pure, whereas the other criterions (whiteness, yield and recovery rate) gave no important differences (Fig. 3).

Table 3. Separation at the Main Experiments.

Variant Work

M1 (flour, water)

M2 (flour, lye) sieve

like S1 resp. S3 (Table l ) , but raffination two times, including sieving through a 56 pm

M3 (kernels, water)

M4 (kernels, lye)

like S4 resp. S6 (Table 1); for further prepa- ration see above

Table 4. Gelatinization Range of the Starches M1 to M4, Detected by DSC.

Variant Gelatinization Temp. ("C) Begin Max. End

M1 (flour, water) 60.7 74.4 89.9

M3 (kernels, water) 60.7 75.6 92.4 M4 (kernels, lye) 62.4 74.7 97.8

M2 (flour, lye) 62.9 74.8 97.9

2 starcwstarke 42 (1990) Nr. 1, S . 1-4

Fig. 5. 400 ml water).

Brabender-viscograms of the starches M1 to M4 (25 g DM in

Fig. 3 , isolated starches M1 to M4.

Main experiments of starch separation: quality aspects of the

3.3 Quality aspects of mung bean starch

Scanning electron microscopy of the mung bean starch showed smooth granules with kidney like shape. The longest diameter was between 7 and 27 pm, the smallest one between 6 and 15 pm (Fig. 4). The gelatinization range (Tab. 4) gave some information about the kind of steeping. So the initial gelatinization temperature was a little bit higher (+2"C) by steeping in alkaline medium. But in both cases the maximum was about 75°C. Brabender viscograms are pointing out the typical behaviour of legume starches (C-type starch), especially a stable phase of the hot starch paste (Fig. 5). There was no significant difference between water and alkaline treatment. In any case the input of flour was resulting in higher Brabender Units (+300 BU) than using kernels. The values for solubility (Fig. 6) and swelling power (Fig. 7) of the starches M1 to M4 were increasing mostly above the maximum gelatinization range, but without view of one of the treatments ( M l ot M4). The determination of starch retrogradation was carried out by estimation of the syneresis (Fig. 8). As a rule the use of kernels as a starting material gave higher values than flour input.

Fig. 4. Scanning electron microscopy of isolated mung bean starch (MI).

60

+ 50 v, 0 40 \ 3 30

20

10

n 50 70 90

Temp. ( C )

Fig. 6 . temperature.

Solubility (%) of the starches MI to M4 as a function of

40 T

30

6: 20

10

n 50 70 90

Temp. ( C )

Fig. 7. Swelling power (g water/g starch) of the starches M1 to M4 as a function of temperature

M2 M3 Variant

.- 1 M4

-Fig. 8. +4"C.

Syneresis of the starches M1 to M4 after storage of 7 days at

starchlstarke 42 (1990) Nr. 1, S . 1-4 3

4 Discussion

First aim of these experiments was the separation of a pure starch from mung beans. All the tested variants have clearly fullfilled this handicap as shown by the protein contents of the starches. According to the paper of Haase et al. [14] our experiments also gave a good separation efficiency with settling tables. The crude protein content of the best variant was very low (0.1% DM) in comparison with other published values for mung beans and other legume species, e.g. [ l l , 141. Despite of intensitive washing the efficiency of starch separation was about 80%. The pretreatment of the raw material is a very important aspect according to the industrial scale starch separation. Within our experiments the distinction between dry and wet milling didn’t gave clear results. Dry milling resulted in lower yield and whiteness but in higher crude protein content. On the other hand the Brabender viscosity was higher by dry milling - relevant for using in special fields of application. But in any case there is a possibility to show a way of starch separation on the laboratory scale. The tested variants allow a starch isolation with only some aids. The purity of the starch was very high concerning the crude protein content, and the other parameters of starch quality were in good agreement with published values [ l l , 17, 18, 19, 201.

5 Outlook

The efficiency of starch separation from mung beans is often less efficient. In front of the development ofnew processes it is important to get results in the laboratory scale first. As our analyses clearly show, all our experiments were successful. Now we work out a transfer to the semitechnical scale. If only the quality criterions of mung bean starch are of interest (choosen of varieties ; screening processes) the described pro- cedures will allow a starch separation on low quantities but with a high quality status.

Acknowledgements

This work was supported in part by a German Government scholarship to J . H. Zhu. Scanning electron microscopy was carried out by Dr. D. Meyer, Institute for Milling Technology. We wish to thank Mrs. D. Kampmeier and Mrs. W. Senf for technical assistance.

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Addresses of authors: Dip1.-Ing. J . H. Zhu, Research Center for Cereals and Oilseeds of the Trade Ministry, Bai Wan Zhuang Str. 11, Beijing, PR China. Dr. N. U. Hause*) and Dir. and Prof. W. Kempf, Federal Research Center for Cereal ‘and Potato Processing, P.O. Box 23, D-4930 Detmold, Federal Republic of Germany.

*) All correspondence.

(Received: September 19,1989)

starcldstarke 42 (1990) Nr. 1, S. 1-4 4