Variations in the Volatile Flavour Compounds Composition of Selected Nerica Rice Varieties Grown in Different Agro-Ecological Zones in Lake Victoria Basin, Kenya

Abstract

Rice (Oryza sativa L.) is a staple food consumed worldwide. Some rice cultivars are preferred over others due to their distinctive aroma which contributes to consumer acceptability and increased economic importance. New Rice for Africa (NERICA) is an inter-specific high yielding upland rice variety, which is a promising crop for addressing food insecurity in Africa. NERICA 1, 4 and 10 were released to farmers in Kenya for cultivation in 2009. Although studies have shown variations in NERICA yield due to environmental factors such as soil and weather patterns, there is no information on their influence on volatile flavor compounds (VFCs) composition, hence aroma quality. Some NERICA varieties are said to be aromatic but the constituents contributing to the aroma have not been identified. Indeed, it is not documented if different varieties produce varying aroma complexes. Nitrogen (N) fertilizers are applied to increase crop yields, usually accompanied by changes in composition of plant secondary metabolites. Despite changes in physiological and chemical parameters of plants with environment and N-rates, it is not documented how NERICA varieties vary in growth parameters in Lake Victoria basin. The objective of this study was to investigate agronomic performance and composition of organic volatile compounds of cooked NERICA 1, 4 and 10 varieties and their variations due to location of production and nitrogen fertilizer rates. The experiments were set up at KALRO farms in Kibos, Oyani, and Maseno University Botanic garden in a split plot design with three NERICA varieties (sub plots) and four nitrogen fertilizer levels (20, 60, 100 and 140 KgN/ha as main plots) replicated three times. Field data obtained included plant height, number of tillers and leaf chlorophyll content. Grains were harvested at maturity, sun-dried for seven days, de-hulled and milled. The volatiles flavour compounds were extracted using Licken-Nickerson distillation method; quantified using Gas Chromatography with ethyl decanoate as an internal standard and identified by Gas Chromatography Mass Spectrometry (GC-MS). Agronomic performance was influenced by varietal differences and geographical location of production. Significant (p≤0.05) differences were observed in leaf chlorophyll content of the NERICA varieties at different location of studies, while tillering ability did not vary. Plant height numerically increased with increased N-rates with NERICA 4 cultivated in Kibos being significantly (p≤0.05) taller than those from Maseno and Oyani. This was partly due to favourable soil parameters at Kibos. A total of 110, 100, and 100 VFCs were detected in NERICA 1, 4 and 10, respectively. NERICA 1 was superior to NERICA 4 and 10 in terms of VFCs concentration. The classes of compounds detected were green leaf volatiles, terpenes and aromatic compounds. The main aroma compound 2-acetyl-1-pyrolline (2-AP) was detected in NERICA 1 samples from Oyani and Maseno at 60 KgN/ha and 100 KgN/ha, respectively. Kibos site at N-rate 140 KgN/ha was most suitable for growth parameters, while NERICA 4 had highest growth parameters at all sites. NERICA 1 is aromatic possibly due to the presence of 2AP, while NERICAs 4 and 10 lacked 2AP hence non-aromatic. Maximum VFCs occur at N-rates between 60 and 100 KgN/ha. The key aroma compound was detected at Maseno and Oyani sites. NERICA 4 is recommended for cultivation at Kibos site under N-rates 140 KgN/ha for maximum growth parameters while aromatic rice should be cultivated at Maseno and Oyani within N-rates 60 to 100 KgN/ha for increased quality of rice.