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Modification of Starches in Rice Flours by Acetylation.

ONANONG NAIVIKUL¹, Eakaphan Keowmaneechai²

Native starches show considerable retrogradation at low temperature. Consequencely, some food products produce undesirable characteristics. Rice and waxy rice starches, therefore, were modified by acetylation at 20, 30, 40 and 50 ^oC and pH 7.5-8.0, 8.0-8.5, 8.5-9.0 and 9.0-9.5 to improve their properties. It was found that the optimum modified condition was 30 ^oC with pH range at 8.0-8.5. The amount of acetic anhydride used for acetylation showed linear relations to the acetyl contents of modified starches. When starches were modified with desired acetyl percentages, 0.6, 1.5 and 2.4 %, the results showed non-significantly difference at 95 %.

The modified starches having acetyl group of 0.6, 1.5 and 2.4 % showed more desirable properties as compared to their native counterparts. It was found that most of them were in granule form, their brightness were increased but yellow color was decreased. Their gelatinization and pasting temperature were decreased, whereas peak viscosity was higher by the increasing of acetyl content. Their solubility and swelling power were also increased. After freeze-thawing, the clarity were slightly changed and the filtrate contents were reduced.

Rice, waxy rice flours and their modified starches were used for making Thai-styled custard and oyster sauce, respectively. Their products were accepted at the highest scores for 2.4 % acetyl group of modified starches after the first freeze-thaw cycle for Thai-styled custard and the ninth freeze-thaw cycles for oyster sauce.

Introduction

Native starches show considerable retrogradation at low temperature. Consequencely, some food products produce undesirable characteristics. The modification could solve some of this characteristics. Such as the chemical modification could provide more stability for freeze-thaw condition by using acetylation reaction of starch.

Materials & Methods

Modified starch in rice and waxy rice flours by using acetylation method (Wurzburg, 1964). The variation of the conditions as follow:

1) Differences temperature (20, 30, 40 and 50 ^oC)

2) Differences pH (7.5-8.0, 8.0-8.5, 8.5-9.0 and 9.0-9.5)

3) Differences acetyl groups (0.6 %, 1.5 % and 2.4 %)

Results & Discussion

The modification of starch in rice and waxy rice flour, using acetylation method were compared between differences temperature (20, 30, 40 and 50 ^oC). The results showed that there were non significantly differences among temperature changes. All modification starch in both rice and waxy rice flour caused the lower initial pasting temperature and higher peak viscosity than the native flour (Jarowenko, 1987). The suitable temperature for both rice and waxy rice flours to be modified should be 30 oC which was similar to room temperature, shown in Figure 1a and Figure 2a, respectively.

The variation of pH (7.5-8.0, 8.0-8.5, 8.5-9.0 and 9.0-9.5) during the acetylated modification of starch in rice and waxy rice flours showed non significantly difference. The suitable pH should be 8.0-8.5 which was slightly alkaline, shown in Figure 1b and Figure 2b, respectively.

The higher of acetyl group in the modified starch in rice and waxy rice flour caused lower initial pasting temperature and higher viscosity (Jarowenko, 1987), shown in Figure 1c and Figure 2c, respectively.

The amount of acetic anhydride used for acetylation showed linear reations to the acetyl contents of modified starch in rice flour (Table 1 and Figure 3).

The starch granules of modified starch in rice and waxy rice flours showed somewhat differences from native starch granules (Figure 4 and 5). The swollen granules dued to the alkaline condition during reaction and the shrinked granules due to the drying process could be seen (Jarowenko, 1987).

The Thai-styled custard made from modified starch in rice flour at different acetyled groups (0.6, 1.5 and 2.4 %) were compared to native rice flour. The taste panel accepted the product from 2.4 % acetyled group modified starch in rice flour after the first freeze-thaw cycle at the highest score (Table 2).

The oyster sauce made from 2.4 % acetyled group modified starch in waxy rice flour after the ninth freeze-thaw cycle was accepted by the taste panel at the highest score as compared to native waxy rice flour and the other two concentrations (0.6 and 1.5 % acetyled group), shown in Table 3.

Conclusions

The modification of rice starch in rice and waxy rice flours could be produced by using acetylation methods. The products made from these modified starch in rice and waxy rice flours were stable for freeze-thaw cycle, depended on the kind of products.

Literature Cited

Jarowenko, W. 1987. Acetylated starch and miscellaneous organic esters. In: Modified Starches: Properties and Uses. Eds. O.B. Wurzburg. CRC Press, Inc., Boca Raton, Folrida. pp. 55-77.

Wurzburg, O.B. 1964. Acetylation. In: Method in Carbohydrate Chemistry. Eds. R.L.Whistler, R.J. Smith, J.N. BeMiller and M.L. wolfrom. Academic Press, Inc., New York. pp. 286-288.

Table 1 Comparison of the acetic anhydride used for reaction and the acetyl contents of modified starch in rice flour.

kind of flour	% acetic anhydride	% acetyl group
Rice	3.00 5.00 7.00 9.00	0.946 1.444 1.986 2.512
Waxy Rice	3.00 5.00 7.00 9.00	0.946 1.517 2.089 2.614

Table 2 Taste panel results for Thai-styled custard made from rice flour and modified starch in rice flour1.

Characteristics Acceptance products from differences percent acetyl group of modified starch in rice flour
0 % 0.6 % 1.5 % 2.4 %
Syneresis 3.60 ^c 4.07 ^c 5.33 ^b 7.13 ^a
Smoothness 4.20 ^b 5.07 ^ab 6.07 ^a 6.13 ^a
Viscosity 4.00 ^b 4.33 ^b 5.93 ^a 6.80 ^a
Spreading 5.80 ^b 5.80 ^b 6.33 ^ab 7.00 ^a
Texture 4.00 ^c 4.27 ^c 5.53 ^b 6.50 ^a
Flavor 4.53 ^b 5.07 ^b 6.20 ^a 6.33 ^a
Total Acceptance 4.17 ^c 4.73 c 5.73 ^b 6.90 ^a

¹ Results of product after the first freeze-thaw cycle from 1-9 Hedonic score,
1 = mostly dislike, 9 = mostly like

Table 3 Taste panel results for Oyster sauce made from waxy rice flour and modified starch in waxy rice flour1.

Characteristics Acceptance products from differences percent acetyl group of modified starch in rice flour
0 % 0.6 % 1.5 % 2.4 %
Clearness 6.47^a 6.80^a 6.87^a 6.53^a
Smoothness 7.20^a 6.73^a 6.93^a 6.80^a
Viscosity 5.73^b 6.67^ab 7.07^a 7.20^a
Flavor 5.47^a 6.27^a 5.47^a 6.20^a
Total Acceptance 5.80^b 6.73^ab 6.67^ab 7.13^a

¹Results of product after the ninth freeze-thaw cycle from 1-9 Hedonic score,
1 = mostly dislike, 9 = mostly like

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