## 2 Results and Discussion
### Determination of the amount of \(\gamma\)-nonlactone in freshly cooked rice samples  
Unlike environmental samples, where the target compounds are typically present in trace amounts, analyte concentrations in food samples are variable and dependent on the analytical conditions employed at their extraction and concentration [11, 13]. This is particularly true for compounds found at trace levels. Hence, reports of the amounts of trace levels of endogenous analytes in foods must present all of the experimental details, because meaningful comparisons can be made only between experiments that subject the sample to similar conditions [4]. On the other hand, the sample must reach a temperature sufficient either to liberate bound volatile compounds or to thermally generate the compounds, in order to successfully sample the headspace above freshly cooked rice with modified headspace SPME. The temperature variation in the headspace above the freshly cooked rice sample was reported previously as that at the stage of keeping the cooked rice warm for 30 min [19]. All of the free flavor volatiles, the bound flavor components and the thermally generated compounds could be liberated during rice cooking and then extracted, detected and identified in the headspace above freshly cooked rice at the stage of keeping the cooked rice warm. For this reason, the modified headspace SPME method for the study of the aroma of freshly cooked rice is preferred to other sampling techniques. Determination of the amount of \(\gamma\)-nonalactone in freshly cooked rice samples was carried out using chromatographic peak area counts in total ion current (TIC) chromatograms and no external or internal standards were used [22]. Using peak area counts for quantity comparison does not require  
Figure 1: Mass spectrum observed for the chromatographic peak at an RI of 2023 on a DB Wax column with a strong, sweet, coconut-like aroma in the headspace above freshly cooked Mirukiikuiiu rice (**A**) and mass spectrum of \(\gamma\)-nonlactone (also known as 4-pentyl-butan-4-olide) in the Wiley mass spectral library (31309) (**B**). extensive sample preparation, but the sampling procedure and chromatographic conditions must remain constant for all samples. Table 1 compares the relative amounts (chromatographic peak area counts \(\times\) 10\({}^{6}\)) of \(\gamma\)-nonalactone found in the headspace above four freshly cooked rice samples, which were tested under the same operating conditions. The chromatographic peak areas shown are the means of three determinations with standard deviations. The data in Table 1 are only intended to give some idea of the order of magnitude of the amounts. To obtain more reliable results, a calibration method should be used. Of all components identified in the headspace above the freshly cooked rice samples by modified headspace SPME coupled with GC-O and GC-MS (data not shown), \(\gamma\)-nonalactone was among the components with the most average intensity of aroma for Mirkuikituin, LGC-soft and LGC-katsu, as presented in Table 1. This component can be one of the important odor-active compounds in freshly cooked rice for the three cultivars of non-scented rice. However, this component was not detected in Hieri, a scented rice cultivar (Table 1). In previous studies carried out on rice aroma, \(\gamma\)-nonalactone was not reported as one of the important odorants [7, 8, 10]. [PERSON] _et al_. [9] only listed it as one of the minor odor-active compounds in a cooked \"Indica\" brown rice, with the flavor dilution (FD) factor of 1, maybe because of its low concentration in the rice samples. Moreover, this component was found as one of the flavor volatiles in the cooked rice of a scented Kaorimai cultivar [24]. It was recently reported as one of the volatiles in non-scented rice types such as Nihonbare, Koshihikari and Akitakomachi by our group [20].