Citronellol is a fragrance ingredient used in decorative cosmetics and fine fragrances. It has many applications in flavoring, extracts, and food and drug manufacturing. Its use worldwide is greater than 1,000 metric tons per annum. Previous studies concerning the biotransformation of (L)-citronellal to (L)-citronellol using free and immobilized cells of Rhodotorula minuta have been reported. The objective of this research was to study the pathways involved during biotransformation of commercial citral by a free cell method (FCM) and an immobilized cell method (ICM) using Saccharomyces cerevisiae. Biotransformation of citral by FCM and ICM are important methods to convert cheap and plentiful organic compounds into more useful ones, and rely on the ability of plant cell cultures to specifically produce secondary metabolites.The culture preparation was done using different microbial methods and incubation periods to obtain the maximum number of S. cerevisiae cells for citral biotransformation. It was found that using either method, citral was converted to citronellol in high yield. The biotransformation products were identified by their theoretical study (TS), Fourier-transform infrared spectroscopy (FT-IR), ultraviolet visible (UV) spectrometry, gas chromatography (GC), and gas chromatography/mass spectroscopy (GC-MS). Comparison of the two methods showed ICM was more effective as it produced one major product, citronellol (48.5%), while FCM produced more products, yielding citronellol at only 24.2% and 27.2% over a period of 6 and 15 days, respectively.The literature provides several examples of biotransformations of citronellal. The biotransformation of (L)-citronellal to (L)-citronellol by free and immobilized Hodotorula minuta has been reported [1]. The bioconversion of geranyl and neryl acetate by Aspergillus niger has also has been described [2,3]. One report showed that the main reaction of a liquefied method using A. niger was hydrolysis of a terpene to 8-hydroxy derivatives during Z-hydroxylation [4]. Using a sporulated surface culture method, we found that geraniol was converted to linalool and partially oxidized to citral [5]. Microbial transformation of geraniol and nerol by a sporulated surface culture method using five A. niger strains and three Penicillium strains were compared with a submerged liquid method [6]. Several groups have used different fungi to bioconvert citral [7][8][9][10][11][12]. In this article, biotransformation of the pure terpene aldehyde citral is carried out by a sporulated surface culture method, and the pathways involved in this biotransformation are investigated. Microbial transformation of citral by a sporulated surface culture method grown on 50 mL of medium in conical flasks was monitored over 3 weeks. The suspension was extracted with Et 2 O three consecutive times and directly analyzed by GC and GC-MS. p-Mentha-8-en-3-ol Red. O H O OH S. cerevisae Citral OH Scheme 1

Esmaeili A., Rohany S., Safaiyan S., Amir Z.S. (2011): Microbial transformation of citral by Aspergillus niger-PTCC 5011 and study of the pathways involved. Czech J. Food Sci., 29: 610-615.Citronellol and hydroxy citronellal are fragrance ingredients used in decorative cosmetics and fine fragrances. They have many applications in flavourings, extracts, foods, and drugs manufacturing. Their use worldwide is in the range exceeding 1000 metric tons per annum. Previous studies investigated the biotransformation of citral by the free cell method and immobilised cell method with Saccharomyces cerevisae. In this research, the microbial transformation of citral by the sporulated surface cultures method with Aspergillus niger-PTCC 5011 was studied. In experiments, A. niger was inoculated on solid media in conical flasks. The transformation took place for the periods of time of 6 days and 15 days. The results were analysed by the theoretical study, Fourier transform infrared spectroscopy, gas chromatography, and gas chromatography-mass spectroscopy. The major compounds produced in 6 days were acetone (26.2%) and hydroxy citronellal (37.0%), while the major compounds after the 15-day period were acetone (15.0%) and citronellol (36.0%). As citronellol has numerous commercial applications, the 15-day biotransformation period was found to be preferable.