Extraction, Identification and Quantification Methods of Rice Aroma Compounds with Emphasis on 2-Acetyl-1-Pyrroline (2-AP) and Its Relationship with Rice Quality: A Comprehensive Review

The 2-acetyl-1-pyrroline (2-AP) is the distinctive aroma compound in rice and many other foods such as popcorn, wheat, and rye bread crusts. Its amount in rice is very minute and varies from 0.05 to 0.34 ppm. Several methods are available for identifying and quantifying the 2-AP in rice. Nevertheless, an extensive method is lacking to extract, identify and quantify 2-AP, which needs attention immediately. The aim of this paper was to have an extensive review to highlight the updated and upgraded knowledge on traditional and recent extraction methods with advanced analytical techniques and instrumentation for isolation, characterization, and quantification of the 2-AP compound from different types of rice samples. In addition, there has also been a discussion of the chemistry, formation, and synthesis of 2-AP in rice and its connection with rice quality.Abbreviations: 1P: ∆1-pyrroline; P5CS: ∆1-pyrroline 5-carboxylate synthase; TMP: 2,4,6-Trimethylpyrridine; 2-AP: 2-acetyl-1-pyrroline; AB-ald: 4-aminobutyraldehyde; AEDA: Aroma extract dilution analysis; ASD: Atmospheric steam distillation; CCU: Capillary column ultra; CO2: Carbon dioxide; CW/DVB: Carbowax/divinylbenzene; CW/TPR: Carbowax/templated resin; CAR: Carboxen; CAR/PDMS: Carboxen/polydimethylsiloxane; CF–SPME: Cold-fiber SPME; DF Detection frequencies; DVB: Divinylbenzene; DVB/CAR/PDMS: Divinylbenzene/carboxen/polydimethylsiloxane; FID: Flame ionization; GC: Gas chromatography; GC-FID: Gas chromatography-flame ionization detector; GC-MS: Gas chromatography-mass spectrometry; GC-O: Gas chromatography-olfactometry; HAS: Headspace analysis; HSSE: Headspace sorptive extraction; KEaC: Kernel elongation after cooking; KDML-105: Khao Dawk Mali 105; LNA: Likens-Nickerson apparatus; LOD: Limits of detection; LOQ: Limits of quantification; MgSO4: Magnesium sulfate; MSD: Mass selective detector; MS: Mass spectrometry; NPD: Nitrogen-phosphorus; OTH: Odor threshold; ppb: Parts per billion; PA: Polyacrylate; PDMS: Polydimethylsiloxane; PDMS/DVB: Polydimethylsiloxane/divinylbenzene; PCI-IT: Positive chemical ionization-ion trap-tandem; PTM: Purge and trap method; RT: Retention time; SIM: Selected ion monitoring; SDE: Simultaneous distillation extraction; Na2SO4: Sodium sulfate; SPME: Solid-phase microextraction; SEfbDI: Solvent extraction followed by direct injection; SIDA: Stable isotope dilution assay; SC-CO2: Supercritical CO2; SFE: Supercritical fluid extraction; SCFs: Supercritical fluids; GABA: γ-aminobutyric acid