Researchers identified 196 lipid molecules across 56 japonica rice cultivars, including fatty acid esters of hydroxy medium-chain fatty acids not previously reported in rice.
The study, published in Food Research International, examined how grain pigmentation and cultivation practices relate to lipid composition. Although lipids make up a small proportion of rice, they influence its nutritional properties, flavor, storage stability, and grain quality.
Researchers analyzed brown, red, green, and black cultivars obtained from farms across Japan and an online marketplace. They extracted lipids using a modified Bligh-Dyer method with internal standards before performing untargeted lipidomic profiling using liquid chromatography coupled with high-resolution Orbitrap mass spectrometry. MS-DIAL software supported lipid identification and relative quantification.
The analysis annotated 196 molecules from five major lipid classes. Multivariate analysis identified lipid patterns associated with grain pigmentation. Brown rice differed slightly from black, red, and green cultivars, although the samples remained broadly similar. Organic and conventional cultivation were also associated with differences in fatty acyls and glycerophospholipids.
A key analytical finding was the detection of fatty acid esters of hydroxy medium-chain fatty acids, or FAHMFAs. The researchers confirmed these structures using multistage tandem mass spectrometry, chemical derivatization, and comparison with synthesized standards. FAHMFAs were most abundant in brown and green rice.
The team also characterized N-acyl-lysophosphatidylethanolamines, known as LNAPEs, which were enriched in black rice. Both lipid groups have been associated with biological activity in previous research, but their detection in rice does not demonstrate a health effect in people.
Black and green cultivars generally had higher ratios of polyunsaturated to saturated fatty acids and a more balanced ratio of omega-6 to omega-3 fatty acids. These results could help researchers compare rice varieties and identify traits for further nutritional or breeding studies.
The researchers also used laboratory digestion assays to measure starch breakdown in selected cultivars. Black rice had the lowest estimated glycemic index, while black and green rice showed slower starch digestion than typical white rice. Because these tests were conducted in vitro, they cannot predict an individual’s blood glucose response or establish clinical benefits.
The study demonstrates the value of combining untargeted high-resolution lipidomics with more specific structural confirmation. Authentic standards, chemical derivatization, and tandem mass spectrometry helped distinguish newly identified structures from preliminary database matches.
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