| Abstract | Many studies provide the information on the storability of milled rice as affected by storage conditions after harvest but limited studies have been done on the deterioration of aroma in Khao Dawk Mali-105 (KDML-105) variety during long term storage. Storage limitations and the resulting effects on cooking, eating, and aroma quality should be given as information to consumers. This information however, is limited for KDML-105. Therefore, this study determined the storability of KDML-105 as affected by product packages, storage temperatures, and storage duration in order to develop quality standards which can be used as recommendations for domestic and exporting markets. The KDML-105 obtained from the Northeastern part of Thailand was packed in polyethylene (PE) without the removal of air (normal condition) and laminated films (PE/nylon) with the removal of air (vacuum condition). The packed milled rice was stored at 3 selected temperatures: 15±2°C, ambient, and 36±2°C. Quality studies on stored samples were done every 42 days (one storage period) with 5 replications. The study was terminated when the rice was stored for 7 periods (294 days). Quality determinations were aroma (in term of 2 acetyl-l-pyrroline), sensory evaluation, chemical and physical tests. A method to determine 2-acetyl-l-pyrroline (AP), the major aromatic compound was done in parallel to this study. The AP compound which could be found in the volatile oil of pandan leaf (Pandanus amaryllifolius), was used as an useful "reference mixture" in gas chromatography (GC) analysis where the AP eluted at a retention time (RT) of 5 minutes. Results indicated that the AP was not found in nonaromatic rice and occurred in low concentration in the aged KDML-105. The study, therefore confirms the importance of the AP as a key compound contributing to the pandan-like aroma in KDML-105. In order to evaluate sensory quality characteristics such as aroma, flavor, off-aroma, off-flavor, color, glossiness, cohesiveness, and texture of cooked rice, panelists were trained and selected based on their perception and capability to sense these characteristics. Panelists selected by a means of elimination were able to give better sensory evaluation results than the unselected panelists. Sensory evaluation showed that quality characteristics were preserved when the milled rice was stored under cold temperature. The quality rapidly deteriorated when the milled rice was stored under high temperature, whereas the rate of deterioration occurred gradually when the rice was stored under ambient temperature. Polyethylene, or laminated bags did not influence the deterioration of color, glossiness, cohesiveness, and texture of the stored milled rice. However, these types of packaging affected the deterioration of aroma and flavor. Vacuum packing maintained aroma and flavor intensity, and the degradation of these characteristics were slower than when the rice was packed in PE bags. The signs of unpleasant smell and flavor were observed to appear for the rice packed in PE bags and stored for longer period under high or ambient temperature. Although exposed grains may have higher deterioration of the physical and chemical properties but since the emphasis was on small packing systems, exposed grains was not analyzed. Chemical tests on the rice samples stored under different conditions included the AP content, amylose content, gel consistency, and alkali spreading values. The deterioration of AP increased rapidly when the rice was stored under high temperature and packed in PE bags. Under ambient temperature, the differences in the intensity of AP appeared to be minimal with either PE or laminated bags, but increased differences were observed when storage was prolonged. For long term storage, packing rice in a vacuum bags and storing under cold temperature was considered to be the best storage condition to maintain the AP content. The amylose content and alkali spreading values were not affected by storage temperatures or packing types. For the stored KDML-105 during 7 storage periods, the amylose content was found to be between 15-18% whereas the alkali spreading values were scored as 7 which correspond to "low gelatinization temperature" (< 70°C). Gel consistency showed a close relationship with the hardness of cooked rice as measured by the Instron. During prolonged storage, milled rice stored under cold temperature, cooked softer than the rice stored under high or ambient temperature. The storage temperatures were more effective on the physical characteristics than packing materials. A rapid decrease in the whiteness of milled rice occurred when the rice was stored under high temperature when compared with rice stored under ambient temperature. The rice stored under cold temperature maintained whiteness and hardness of cooked rice during the 7 storage periods. Under ambient temperature, the hardness increased more rapidly during the 5 storage periods and was maintained through the end of storage. The largest changes in hardness during 7 storage periods was observed for milled rice stored under high temperature. The maximum elongation for cooked KDML-105, was observed during storage upto the fourth period after which the ratio became constant. The sensory evaluations for aroma, color, and texture of cooked rice were found to be closely related to the AP content, color of milled rice, and the hardness of cooked rice measured objectively. Relationships between subjective and objective tests were obtained to predict changes in aroma, color, and texture of cooked rice from the milled rice stored under different storage conditions during the 7 storage periods. The storability of the quality characteristics in KDML-105 is based on the traditional or usual storage conditions. Under high storage temperature and normal packing, the rice was graded as "superior" within the early storage period (1st. period or 42 days). Under the same, the quality rapidly deteriorated until it was rejected by consumers after 4 storage periods (168 days). However, under high temperature, when vacuum packed, the shelf life of KDML-105 in terms of sensory quality was prolonged to 6 periods (252 days), and the rice was still graded as "good" quality. After the sixth periods, the rice was rejected and graded as "poor" quality. Under ambient temperature with vacuum and normal packing (RV and RP), the quality deterioration was slower than when stored under high temperature and packed in the same materials. Under RP and RV conditions, the quality was graded as "superior" or "best" within 2 and 3 periods (84 and 126 days), respectively. Under cold and either normal or vacuum packing conditions, the rice was graded as "superior" to "best" during the fifth and sixth periods (210 and 252 days) respectively. Unacceptable characteristics or "poor" quality were not observed during 7 storage periods, under cold or ambient storage temperature. Based on the study, vacuum packing methodology is recommended to millers or exporters rather than a cold storage facility. However, improvement in the vacuum system needs to be studied. Using good quality plastic material and placing the vacuumed bag in a paper box is recommended to overcome the disadvantages of a vacuum package system. Storage limitations for retention of aroma as well as sensory quality in KDML-105 are also recommended to be printed on the package. The shelf-life duration recommended of stored KDML-105 was based on the quality changes of milled rice stored under ambient temperature, since this is the most common storage condition at retail markets or during domestic storage for consumption. |