IC4R003-Metabolomics-2007-18081248

Project Title

Characterization of Volatile Aroma Compounds in Cooked Black Rice

The Background of This Project

Figure. Relative proportion of the primary classes of volatile compounds emanating from a black (Geomjeong-ssal) and white (Jungilpum) cultivar. Vertical bars with different letters are significantly different (p < 0.05): (1) aromatics, (2) nitrogen-containing compounds, (3) aliphatic alcohols, (4) aliphatic aldehydes, (5) aliphatic/alicyclic ketones, and (6) terpenoids.

• Black rice is popular in Asian countries where it is often mixed with white rice prior to cooking to enhance the flavor, color, and nutritional value. It is intensely colored because of anthocyanins (e.g., cyanidin 3-glucoside and peonidin 3-glucoside) found in the surface cells of the grain (3). It has a number of nutritional advantages over common rice, such as higher protein, total essential amino acids, vitamin B1, and minerals (Fe, Zn, Mn, and P) (5), the latter of which varies with cultivar and production location.
• Black rice has a relatively intense flavor that is distinctly different from other types of aromatic rice. Flavor is considered the single most critical quality trait in rice affecting consumer preference (7). Over 300 volatile compounds have been identified from various cultivars of aromatic and nonaromatic rice (8). The volatiles identified vary with the degree of milling (9), isolation technique (e.g., Tenax trapping, simultaneous distillation-extraction, solvents, steam-distillation, and solid phase microextraction) (10–14), cooking method (10, 15), and storage duration (16). Among the volatiles identified, there are a relatively small number of odor active compounds. For example, 2-acetyl-1-pyrroline (2-AP) has a very low odor threshold and is considered to be critical to the flavor of aromatic rice (10). Although aromatic types assessed to date contain 2-AP, they have very different aromas, indicating that other compounds contribute to their respective flavors. In that the aroma chemistry of black rice has yet to be characterized, the objectives were to identify the volatile compounds in a cooked black rice cultivar and to characterize the key odor active compounds versus a typical nonaromatic white rice cultivar.
• The objectives of this research were to study volatile profiles of cooked black rice and to characterize the odor-active compounds.

Plant Materials

• To investigate volatile compounds of cooked black rice, a dehulled black rice cultivar (Geomjeong-ssal Brand Premium Korean black rice, 1.84 kg package, Korean Farm Inc., Santa Fe Springs, CA) was purchased locally in Georgia. The black rice was not milled because the pigmentation is present in the surface cells. A milled common white rice (Jungilpum Brand Premium Korean nonaromatic rice, 9.07 kg, AOFW Co., Doraville, GA) was also purchased in a local supermarket to characterize the key odor active compounds that distinguish black rice from a typical nonaromatic white rice cultivar. Samples were sealed in glass and held at -20 °C until analysis.

Research Findings

• Thirty-five volatile compounds emanating from cooked black rice, collected using a dynamic headspace system with a Tenax trap, were identified and quantified by GC-MS (Table 1). There were 10 aromatic, 4 nitrogen-containing, 6 alcohol, 10 aldehyde, 3 ketone, and 2 terpenoid compounds, the relative proportions of which are displayed in Figure 1. The relative proportion of the main classes of volatiles in 100% black rice was significantly different from that in 100% white rice. The relative proportion of aromatic and nitrogen-containing compounds in black rice was significantly greater than in white rice, whereas white rice had higher relative proportions of alcohols, aldehydes, ketones, and terpenoids. Aldehydes (51.7%) and aromatics (28.4%) quantitatively represented the highest percentage of the total volatiles emanating from black rice (Figure 1). Aldehydes, identified in decreasing order of their relative proportion, were hexanal, nonanal, octanal, heptanal, (E)-2-octenal, decanal, (E)-2-nonenal, (E,E)-2,4-decadienal, (E)-2-hexenal, and (E)-2-decenal.

Labs working on this Project

• University of Georgia, Plant Sciences Bldg., Athens, Georgia 30602-7273
• National Institute of Crop Science, Rural Development Administration, Suwon 441-857, Republic of Korea

Corresponding Author

STANLEY J.KAYS (E-mail:kaysstan@uga.edu)