**Abstract**

Aroma in rice is unique and a superior grain quality trait, varieties especially Basmati and Jasmine-type are fetching a high export price in the International markets. Among the identified volatile aroma compounds, 2AP (2 acetyl-1-pyrroline) is believed to be the distinctive biochemical compound contributing the flavor in rice. Genetically, aroma in rice arises by the phenotypic expression of spontaneous recessive mutations of the *OsBadh2* gene (also known as *fgr*/*badh2* /*osbadh2*/*os2AP* gene) which was mapped on chromosome 8. An 8-bp deletion in the exon 7 of this gene was reported to result in truncation of betaine aldehyde dehydrogenease enzyme whose loss-of-function lead to the accumulation of a major aromatic compound (2AP) in fragrant rice. Among the different sampling methods and analytical techniques for the extraction and quantification of scentedness, simultaneous distillation extraction (SDE) is traditional and normalized, whereas solidphase micro extraction (SPME) and supercritical fluid extraction (SFE) are new, very simple, rapid, efficient and most importantly solvent-free methods. These methods are coupled with Gas Chromatography–Mass Spectrometry (GC–MS), Gas Chromatography-Flame Ionization Detector (GC-FID) and/or Gas chromatography olfactometry (GC-O) and also with sensory evaluation for readily examining 2AP compound found in rice. The major factor affecting the aroma in rice was their genetic makeup. However, the aroma quality may be differed due to different planting, pre-harvest and postharvest handling and storage. For a more extensive elucidation of all effective and fundamental factors contributing to fragrance, it is essential to explore target quantitative trait loci (QTLs) and their inheritance and locations.

**Keywords:** aromatic rice, 2-acetyl-1-pyrroline, *fgr*, *badh2*, evaluation methods, affecting factors

## **1. Introduction**

Rice (*Oryza sativa* L.) is a dietary staple food crop and the grain being consumed by atleast 50 per cent of the world's population [1]. It had a decisive role in food, security and in improving the livelihood of people. With the continuous/marked

improvement in standard of living of the people, the ethnic preference of rice is under conversion which increases the demand for superior fine quality rice. Most of the scented rice are inferior in agronomic performances and highly prone to environmental variations [2] yet it paves much attention for their first-rated aroma. The origin and evolution of the aroma gene betaine aldehyde dehydrogenase (*BADH2*) remains unclear but the haplotype analysis firmly establishes a distinct origin of the *badh2.1* allele within the Japonica varietal group [3] and the centre of origin is considered to be the Himalayan foothills in the Indian subcontinent from where it spreads to various parts of the world [4].

It was reported that rice aroma was controlled by single recessive nuclear gene in rice [5, 6]. The biochemical analysis of rice grain reveals the presence of numerous volatiles in fragrant rice revealing the major aromatic compound, 2-acetyl 1-pyrroline (2AP). Recent advances had also discovered the biochemical pathway for biosynthesis of 2-Acetyl-1-pyrroline (2AP) from different types of amino acids and polyamines [7]. Based on the rice genome sequence information [8], the *OsBadh2* gene (present on chromosome 8) is identified as a candidate gene for aroma, which is the most important aroma gene till now. However, several other genes and locus had been reported to be the contributor for aroma [9, 10].

Rice aroma quality evaluation is quite complex due to the complex interaction of numerous volatile compounds, and many affecting factors during planting and processing. With the rapid advancement in the instrumentation and sampling methods for the isolation and determination of 2AP concentration levels it is possible to analyze compounds even at very low concentrations (ppb levels) [11]. This chapter gives insights into the flavor chemistry, the progresses pertaining to the genetic and molecular understanding of fragrance, various extraction, quantification methods and their interaction with genetic and non-genetic factors in rice.
