Production of 2-acetyl-1-pyrroline, a major component of aromatic rice flavors, by Acremonium nigricans

Heterocyclic compound, 2-acetyl-1-pyrroline (ACPY) which is a major component contributing to popcorn-like aroma in aromatic rice varieties was tried to be produced by microorganisms. A large number of microbial strains including bacteria, yeasts, and fungi from type cultures and also many natural sources were used in the screening with nineteen screening media. The stability of the product was investigated. Optimization of medium compositions and physical culture conditions were done in shake flask cultivations in order to study the effect of nutritional variables and to optimize the product yields. The production in bioreactor was also investigated in a 21 of bioreactor. Sensory evaluation of dilute solution of ACPY was carried out with trained and untrained panels. As a result, a fungus strains of Acremonium nigricans was found to produce about 2.08 mg/l ACPY. The aroma compound contributing to aromatic rice flavor was isolated from the culture broth of A. nigricans by acid and base fractionation and identified by GLC and GLC-MS analysis. The mass spectrum of the peak eluted at 7.920 min for the basic fraction of the culture broth was compared with the peak of ACPY from pandan leaf extracts. The results revealed that both peak had same retention time and showed essentially similar mass spectra. The optimum temperature for the microbial growth and ACPY production was 28 deg C. Inoculum size 2.5*10**(9) soires/L with initial pH at about 4.5-5.5 was found to be optimum for the compund production. For sensory evaluation on odor description it can be seen that pandan-like and aromatic rice were the terms mostly used by the panelists. More than 80 percent of the panelists liked the odor of ACPY while only 10 percent denoted neither like nor dislike. Odor threshold of ACPY in water solution was determined as 0.15 ppb and 0.40 ppb by trained and untrained panels respectively.