INFLUENCE OF DIFFERENT TECHNOLOGICAL TREATMENTS ON PHENOLIC PROFILE AND ANTIOXIDANT ACTIVITY OF PIGMENTED RICE (ORYZA SATIVA L.)

Introduction Pigmented rice varieties are rich in polyphenols, such as flavonoids, anthocyanins and proanthocyanidins, with known antioxidant and anti-inflammatory effects. Although more interesting from the functional point of view, the pigmented varieties require long cooking times compared to non-pigmented ones. As a consequence, different technological treatments, such as milling or parboiling, have been applied to these cereals to reduce the cooking time. These treatments could have positive effects on the nutritional value of rice: for example, parboiling process increases the concentration of tocotrienols, tocopherols, and γ- oryzanol. On the other hand, few studies have evaluated their impact on phenolic compounds. The purpose of this study was the evaluation of the effects of two technological treatments (milling and parboiling) on the phenolic profile and antioxidant capacity of different pigmented rice varieties. Materials and Methods Samples of rice with different pigmentation and subjected to different technological treatments were included in this study and compared with some by-products of rice processing (bran) (Table 1). After the harvest, rice was dehulled to obtained brown rice. Subsequently, the milling procedure could be applied as additional step, obtaining milled rice and bran. The samples were provided by Azienda Agricola Bertolone Eleonora di Bertolone Giovanni (Violet, Orange and Carnaroli) or found in the Italian market (Venere and Ermes). The flours, obtain by grinding samples, were stored under vacuum, in the dark, at 4°C until further analysis. Two extraction methods were applied for the quantification of the free phenolic compounds and total anthocyanins [Catena et al., 2019; Colombo et al., 2021]. Different in vitro methods were applied to evaluate their phenolic profile and the antioxidant activity: 1) the Folin-Ciocalteu assay, for the evaluation of the content of total phenolics; 2) the pH differential method, for the determination of total anthocyanins (applied exclusively to Violet and Venere samples); 3) the DPPH assay, to evaluate the total antioxidant capacity. Moreover, it was applied the use of HPTLC technique (High Performance Thin Layer Chromatography) allowed the characterization of phenolic profile and, after derivatization with DPPH reagent, the evaluation of relative antioxidant capacity. Results and Discussion The content of soluble polyphenol compounds (SPC) ranged between 0.28±0.02 and 5.69±0.62 mg equivalents of gallic acid (GAE)/g; as expected brown, purple and black varieties showed the highest SPC (5.69±0.62 mg GAE/g and 3.42±0.40 mg GAE/g, respectively). In all samples considered the technological treatments of parboiling and milling determined a reduction of SPC. The milling process resulted in reductions of -46%, -70%, and -77% for Carnaroli, Violet, and Orange, respectively. The parboiling process was responsible for a reduction of SPC corresponding to -32% and -74% for Orange and Violet, respectively. Considering that phenolic compounds are mainly concentrated in the outer layers of rice, the bran obtained from the milling process showed a phenol content from 6 to 9 fold higher than that of the corresponding whole rice. In agreement with the values obtained for SPC, the brown, purple and black varieties showed the highest antioxidant capacity (AOA) (1.65±0.11 and 1.07±0.11 mg GAE/g, respectively) followed by the orange and red ones (0.25±0.01 and 0.33±0.11 mg GAE/g) and the unpigmented variety (0.14±0.02 mg GAE/g). According to SPC, the technological treatments (parboiling and milling) negatively influenced AOA, determining a reduction between -32% and -76%. Finally, brown violet rice showed the highest anthocyanin content (2.40±0.19 mg equivalents of cyanidin-3-O-glucoside (CY)/g) when compared to the corresponding processed rice (0.06±0.00 and 0.70±0.01 mg CY/g for parboiled and milled Violet rice, respectively) and Venere sample (1.06±0.06 mg CY/g). Anthocyanins are thermolabile molecules, so that they are negatively affected by parboiling treatment, which is based on heating processes. Our data suggested a good correlation between the content of polyphenols and antioxidant activity (R2>0.98). A good correlation was also observed between the content of total anthocyanins and antioxidant activity (R2>0.99). According to our results, the bran of all varieties included in this study are an important source of soluble polyphenols and anthocyanins, associated with a good antioxidant activity. HPTLC showed AOA that well correlated with the spectrophotometric results and with data from the scientific literature [Min et al., 2014]. Conclusions The analytical approaches applied in this study are suitable for the analysis of the samples included in the study. All the technological processes used, negatively affected the concentration of phenolic compounds and the antioxidant capacity of the samples. Among phenolic compounds, anthocyanins seem to be the compounds mainly affected by the hydrothermal process. The spectrophotometric and chromatographic assays showed that the bran of all varieties was characterized by the highest content of anthocyanin, phenolic compounds and antioxidant activity. References 1. Catena S., Turrini F., Boggia R., Borriello M., Gardella M., Zunin P. Effects of different cooking conditions on the anthocyanin content of a black rice (Oryza sativa L. ‘Violet Nori’). Eur. Food Res. Technol., 2019, 245, 2303–2310. 2. Colombo F., Di Lorenzo C., Petroni K., Silano M., Pilu R., Falletta E., Biella S., Restani P. Pigmented corn varieties as functional ingredients for gluten-free products. Foods 10., 2021. 3. Min B., McClung A., Chen M. Effects of hydrothermal processes on antioxidants in brown, purple and red bran whole grain rice (Oryza sativa L.). Food Chemistry., 2014, 159, 106–115.