Abstract
Pigmented rice is considered as the good source of phytochemicals which contains more phenolic contents and higher antioxidant activities compared with non-pigmented rice. However, those phytochemicals are normally extracted using inorganic solvent, using water was rarely found. This study was aimed to compare extraction methods on antioxidant contents and activities of two types of Thai pigmented rice (black glutinous rice and red non-glutinous rice). Pigmented rice was extracted either conventional method alone or a combination of ultrasonic plus conventional method with different solvents (methanol and hot water). Phenolic contents were analyzed by spectrophotometric assay and high-performance liquid chromatography (HPLC). Antioxidant activities were investigated by radical scavenging capacity (ABTS and DPPH). The results showed that ultrasonic pretreatment significantly enhanced the total phenolic, flavonoid, anthocyanin contents and higher antioxidant activities compared with conventional extraction irrespective of solvents and rice varieties. Ultrasonic methanol (UM) extraction proved to extract significant higher content of all the compounds analyzed in both the rice varieties. In contrast, hot water (HW) extracts exhibited the lowest amount of bioactive compounds. However, there was no significant difference between methanol extraction alone and ultrasonic pretreatment with hot water extract on antioxidant contents for black glutinous rice. The contents of individual anthocyanins and flavonoids (cyanidin 3-O-glucoside, peonidin 3-O-glucoside, and quercetin) were significantly increased with ultrasonic pretreatment compare to the conventional method in black glutinous rice while failed to detect the red rice variety. The black glutinous rice contained significantly higher contents of all analyzed compounds and antioxidant activities than red rice. In conclusions, our results demonstrated that ultrasonic pretreatment significantly enhanced antioxidant extraction with higher activities compared with conventional extraction irrespective of solvents and rice varieties. Furthermore, the efficiency of ultrasonic together with hot water extraction was almost equal to methanol extraction which was the one effective solvent for extracting antioxidant.
Highlights
Rice is considered as the good source of phytochemicals including phenolic acids, flavonoids, anthocyanins, proanthocyanidins, tocopherols, tocotrienols, γ-oryzanol, and phytic acid (Goufo and Trindade, 2014)
(3) Ultrasonic pretreatment followed by hot water extraction (UHW) and (4) methanol extraction (UM) sample was carried out pretreatment with ultrasonic probe for 40 mins, 20 kHz and 40% amplitude with temperature not exceeding 35°C
TPC from four extraction methods, the results showed that the total phenolic contents for black glutinous rice ranged from 419.56 to 568.25 mg gallic acid equivalent (GAE)/100 g grains which were higher than red rice 248.75 – 433 mg GAE/100 g grains (Table 1)
Summary
Rice is considered as the good source of phytochemicals including phenolic acids, flavonoids, anthocyanins, proanthocyanidins, tocopherols, tocotrienols, γ-oryzanol, and phytic acid (Goufo and Trindade, 2014). Previous research has shown that pigmented rice contains higher phenolic content and antioxidant activities, along with greater varieties of health-promoting phytochemicals compared with nonpigmented rice (Chakuton et al, 2012). The black rice is well known to be rich in anthocyanin (Hou et al, 2013), it contains Cyaniding 3-O-glucoside, peonidin 3-O-glucoside, malvidin 3-O-glucoside, pelagonidin 3-O-glucoside and delphinidin 3-Oglucoside (Park et al, 2008). While the major flavonoid pigments in red rice were identified as procyanidins. It has been reported that the composition of phenolic compounds is noticeably different between glutinous and non-glutinous rice grains. In non-pigmented rice, total phenolic in non-glutinous rice was higher than the glutinous variety
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