# Estimation of Free Fatty Acids in Stored Paddy Rice Using Multiple-Kernel Support Vector Regression
## Materials and Methods
### Preliminary Analysis  
During the storage process, the granary was a large time lag system, and the grain pile can form a \"cold core\" (Figure 3); this phenomenon was caused by the cycle of convection currents when the ambient air outside the granary was warm and the grain was cold. The grain pile can form a \"warm core\" in winter when the grain core was warm, and the outside edges was colder. To research the effects of temperature distribution on FFA, we selected a tall granary with a paddy rice pile that was 59 m in length, 19 m wide, and 6 m high (No. 6 Warehouse at Grain Reserve Depot of Shenyang, Liaoning, China). We took samples every three months from the end of warehousing (October 2017). An electric suction sampler was used to carry out sampling at four heights of 0, 1.9, 3.8, 5.7 m from the grain surface. Eighteen locations distributed evenly were sampled in each layer. Approximately 5 kg stored grain was sampled in each location, and each sample was poured from the sampler and individually bagged. The average moisture of the upper layer (UL), second layer (SL), third layer (TL), and lower layer (LL) of the stored grain was then be obtained, as shown in Figure 4.  
Figure 1: Locations of selected grain reserve depot in northeast China. Figure 3: Schematic diagram of “cold core” in granary. Figure 2: The structure of a digital wireless grain condition monitoring system. The width of the granary was generally 18–36 m and the length was 36–60 m. The headspace above the grain surface was not less than 1.8 m. The wall material was a reinforced concrete structure and the roof was made of a heat preserving material. In the granary, the temperature sensors layout was as follows: The distance between the rows and columns of the horizontal temperature measuring cables should be no more than 5 m; the distance between the vertical sensors should be no more than 2 m; and the distance from the cables to the grain surface, granary bottom, and granary wall should be within 0.3 m to 0.5 m.  
Table 1 shows the average temperature of four layers at each three-month interval. As the depth of the grain layer increased, the average temperature reduced. Figure 4 and Table 1 show that during the storage period, the FFA in stored grain at the UL, SL, and TL increased at a rate significantly higher than that of the LL. In particular, the FFA in stored grain of the UL increased by 20.4%, while that of the LL increased by 11.3%. Furthermore, the greater the distance from the grain surface, the slower the increase of FFA in stored grain. This indicates that the storage conditions are better in the lower layers of the grain pile, that is, the lower the storage temperature, the better the conditions for long-term storage of grain. Figure 5 shows the change in average moisture of the grain in each layer during storage. The reason for the sudden drop of moisture of stored grain in the third month was that aeration was carried out in the second month after the end of warehousing. On the whole, the moisture content in all layers decreased during storage, while the moisture in the UL and LL decreased faster than that in the SL and TL, because the UL was in direct contact with the ambient air in the headspace of the granary, and the LL was closest to the ventilation equipment. Table 1 and Figure 5 show that the LL is the region with low temperature and low moisture, while the UL is the region with high temperature and low moisture. This results in significant differences of fatty acid changes in each layer. Therefore, moisture content is related to FFA accumulation.