Since the best extraction conditions for total phenolic content,
total monomeric anthocyanin contents and antioxidant activities
from FRAP assay and DPPH assay were not agreeable, the optimum
condition was selected from the shortest total extraction time
which gave an acceptable amount of total phenolics and total
monomeric anthocyanins, but yielded the highest antioxidant
activities. That system was the extraction with 70 ml acetone/
100 ml acetone-water mixture at pH 6.8 for 4 h total extractiontime. Total phenolic content of the crude extract obtained from this
condition was 1878 mg gallic acid/g flour, which was within the
range of total phenolics in rice, 540–3130 mg gallic acid/g grain,
reviewed by Fardet, Rock, and Re´me´ sy (2008). Meanwhile, total
monomeric anthocyanin content of the crude extract, 288 mg cyanidin
equivalent/g flour, was much lower than total anthocyanin
content in black rice, 3276 mg/g grain, as reported by Abdel-Aal and
Hucl (2003). Genetic variation of the rice samples could result in
different color intensity of the rice bran. Besides, extraction solvent
could influence the measurable amount of anthocyanins. Awika
et al. (2004) reported that the crude extracts of black sorghum grain
prepared with acidified methanol (1 g HCl/100 ml solvent) contained
monomeric anthocyanin nearly twice as much as the
extracts from aqueous acetone (70 ml acetone/100 ml solvent).
Anthocyanin content of black rice reported in the study of AbdelAal
and Hucl (2003) was also determined from the acidified
methanolic extracts (1.0 mol/l HCl in methanol). Lu and Foo (2001)
investigated the reaction between acetone and anthocyanin which
yielded pyrano-anthocyanins. This reaction, which could occur at
room temperature during extraction step, might further reduce the
measurable amount of monomeric anthocyanin in our study.
The crude extract from the optimum condition was further used
to study its application in fish oil enriched mayonnaise as antioxidant
and colorant. Solid yield of the freeze-dried extract from this
optimum extraction condition was 2.16 g/100 g rice flour. Extraction
of antioxidants directly from the rice bran would help increase
the yield of dried extract. However, the whole grain pigmented rice
is regularly consumed. Hulled black glutinous rice is much abundant
in local market than the isolated bran. Therefore, the flour
prepared from the whole grain was selected as a raw material in
this fundamental study.