Appropriate set of pre- and post-harvest treatments for obtaining high levels of aromas and quality in fragrant rice (Oryza sativa L.)

• Autor(a) principal: Goufo Piebiep
• Data de Publicação: 2010
• Idioma: eng
• Título da fonte: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Texto Completo: http://hdl.handle.net/10348/8936
• topic: aromatic rices; 2-acetyl-1-pyrroline; quality; planting density; harvest date; growth regulators; storage conditions

Aromatic rices (Oryza sativa L.) constitute a special group of rice accessions well known for their aroma and/or superfine grain quality. For a successful development of aromatic rices, research regarding factors affecting the quality of the aroma is of economic interest to rice growers and processors. Therefore, this study was conducted as a preliminary step towards the rice quality improvement of two aromatic rice cultivars growing in South China, namely Guixiangzhan and Peizaruanxiang. The fragrance potential of two rice cultivars was investigated using headspace SPME and static headspace in conjunction with GC-MS, and under optimal conditions for the identification and quantification of the intensely popcorn-like smelling compound 2-acetyl-1-pyrroline (2-AP). About a 5-fold difference of 2-AP levels were observed among the two rices with Guixiangzhan having the highest content (3.86 µg/g) comparable to that obtained with Thai KDML 105 rice. Other compounds instead of 2-AP were assumed to contribute to the characteristic aroma of Peizaruanxiang. The two cultivars were subjected to four pre-harvest treatments (planting densities of 16, 19, 22, 28, and 37 hills/m2 , harvesting times of 10, 20, 30, 40, and 50 days after heading, ripening temperatures during the early and the late seasons, application of growth regulators consisting of gibberellic acid, paclobutrazol, 3-indole acetic acid and a mixture of paclobutrazol, proline, and zinc chloride) and three postharvest treatments (storage times of 3 and 6 months, storage temperatures of -4, 8, 20, and 30 ºC, milling degree of 85%). Results were also discussed in terms of antioxidants enzymes activities (peroxidase, superoxide dismutase, proline oxidase), yield attributes (number of panicles per hill, number of spikelets per panicle, grain-filling percentage, paddy yield, 1000-grain weight), milling quality (milled rice rate, whole rice rate, head rice rate), grain appearance (% area with chalkiness, grain vitreosity), malondialdehyde, proline, total soluble proteins, amylose and protein contents of rice samples associated with differing pre-harvest regimes. Highest 2-AP concentrations were obtained for Guixiangzhan and Peizaruanxiang with the lowest planting density of 16 hills/m2 (3.73; 0.69 µg/g), the earliest harvesting time of 10 days after heading (5.24; 0.72 µg/g), a low ripening temperature of 25 ºC (7.12; 2.42 ng/g), the shortest storage time of 3 months (2.40; 0.45 µg/g) and the coolest temperature of -4 ºC (3.42; 0.49 µg/g). After milling, 2-AP content decreased up to 1.5-fold in both Guixiangzhan and Peizaruanxiang. All treatments with growth regulators, although improved grain yield and quality, and enhanced the capacity of Peizaruanxiang and Guixiangzhan to scavenge and control the production of damaging species of active oxygen, resulted in reduced aroma content that negatively affected overall flavor in a smelling evaluation. Decreases ranged from 9 to 24% compared to the control (2.40; 0.41 µg/g). These findings indicate that manipulating pre and postharvest treatments can greatly improve the specific attributes of the domestically produced aromatic rices. It is assumed from the study that keeping aromatic rice under refrigeration, milling it at a low degree and consuming it within six months would be a practical way to preserve its desirable character as monitored by changes in the levels of 2-AP. Results from our investigation also show that altering sowing dates to allow the critical stages of seed maturation to coincide with favorable field environments (cool and dry segment of the year) as well as planting at low density and early harvesting could improve aroma content and other seed qualities. It is also recommended that the aroma quality should be carefully monitored if growth regulators are to be used. With that in mind, China could effectively increase its share of the domestic market of fragrant rices and even tap into the international market