Judul: Bifurkasi sistem mangsa-pemangsa tipe michaelis-menten dengan tingkat pemanenan konstan
Abstrak: Sistem mangsa-pemangsa telah dikenal luas dan dikembangkan dengan berbagai tipe. Salah satunya adalah tipe Michaelis-Menten. Pada karya ilmiah ini dibahas dinamika sistem mangsapemangsa tipe Michaelis-Menten dengan melibatkan tingkat pemanenan konstan. Diasumsikan bahwa pemanenan hanya dilakukan terus-menerus terhadap mangsa sehingga secara tidak langsung juga berpengaruh pada pemangsa. Dinamika sistem diprediksi dengan menunjukkan adanya bifurkasi. Dalam karya ilmiah ini ditunjukkan adanya dua jenis bifurkasi yaitu bifurkasi titik sadel dan bifurkasi Hopf. Pengaruh tingkat pemanenan terhadap populasi mangsa dan pemangsa diperlihatkan dengan simulasi numerik.
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Judul: Analisis model mangsa pemangsa Michaelis-Menten dengan pemanenan pada populasi mangsa
Abstrak: In this manuscript, the dynamics of prey-predator model of Michaelis-Menten is discussed with harvesting on prey populations. Based on the analysis conducted, it was obtained four steady states with characteristics stable, saddle, unstable depending on the value of parameters used. A computer simulation was also carried out to show its dynamics by varying the parameter values. In order to prevent the extinction of the prey and predator, the harvesting level should be lower than maximum limit. The maximum limit of the harvesting level is a quarter of the prey population. If the harvesting level exceeds a quarter of the prey population, then the system has no steady state and both species will be extinct.
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Judul: Characterization of The Chemical Constituents and Antioxidant Capacity of Green and Oolong Teas (Camellia sinensis) Manufactured in Thailand
Abstrak: Tea in Thailand most manufactured in northern area, especially Chiang Rai, Chiang Mai, Nan, and Mai Hang Son province. The aim of this study was to determine and compare the chemical compositions and antioxidant capacity of 4 important Thai tea products. A total of 13 of green teas of 2 major cultivars, assamicaand cv. oolong no.12 (chin hsuan oolong) and 26 of oolong tea of 2 major varieties, cv. oolong no. 12 and cv. oolong no. 17 (chin shin oolong) were collected from 18 factories in Chiang Rai province in 2011. The total polyphenol content was determined according to International Organization for Standardization (ISO 14502-1). For antioxidant capacity, DPPH radical scavenging activitiy assay and ferric reducing antioxidant power assay were determined. The catechins content and caffeine content were determined using HPLC by ISO method (ISO-14502-2). Results showed that about 95% of the 39 tea samples manufactured in Thailand had moisture content below 7%, 5% of tea samples being above 7%. The total polyphenol content in green tea was found to vary from 11.86 to 21.19%GAE, while in oolong tea was found in a range of10.22 to 17.57%GAE. The caffeine content in green tea samples were in a range of 1.69 to 3.7%, while in oolong tea samples were in a range of 1.61 to 3.26%. The catechin content in green tea samples were in a range of 8.31-14.08%, while in oolong tea samples were in a range of 6.81-12.46%. The antioxidant capacity in green tea samples was 164.01 mmole TE/100 g dry basis, while in oolong tea samples was about 132.73mmole TE/100 g dry basis. This study provided useful information for the tea industry in Thailand and other countries in processing and selecting a good-quality tea.
Keyword: Antioxidant capacity, Camellia sinensis, Green tea, Oolong tea, Thailand