Pierre Christen

Production of volatile compounds by the edible fungus Rhizopus oryzae during solid state cultivation on tropical agro-industrial substrates

When Rhizopus oryzae was grown on medium containing cassava bagasse plus soybean meal (5:5 w/w), CO2 production was at its highest (200 ml.l–1) while highest volatile metabolite production was with amaranth grain as substrate (282.8 ml.l–1). In the headspace, ethanol was the most abundant compound (more than 80%). Acetaldehyde, 1-propanol, ethyl acetate, ethyl propionate and 3-methyl butanol were also present. CO2 and volatile metabolite productions reached their maxima around 20 h and 36 h, respectively.

Optimization of culture medium for aroma production by Ceratocystis fimbriata

SummaryThe fungusCeratocystis fimbriata is known to produce among others a banana-like aroma. With a fractional factorial experimental design a culture medium was optimized to produce characteristic volatile metabolites of this aroma. HPLC was used to separate and quantify some of the metabolites. This showed the influence of the composition of the basal culture medium. It is not yet possible to correlate these results with sensorial evaluation.

FRUITY AROMA PRODUCTION BY Ceratocystis fimbriata IN SOLID CULTURES FROM AGRO-INDUSTRIAL WASTES

Solid state fermentations were carried out to test the efficacy of Ceratocystis fimbriata to grow on different agro-industrial substrates and aroma production. Seven media were prepared using cassava bagasse, apple pomace, amaranth and soya bean. All the media supported fungal growth. While amaranth medium produced pineapple aroma, media containing cassava bagasse, apple pomace and soya bean produced a strong fruity aroma. The aroma production was growth dependent and the maximum aroma intensity was detected a few hours before or after the maximum respirometric activity. Sixteen compounds were separated by gas cromatography of the components present in the headspace and fifteen of them were identified as acid (1), alcohols (6), aldehyde (1), ketones (2) and esters (5).

Growth and aroma production byCeratocystis fimbriata in various fermentation media

The ability ofCeratocystis fimbriata to generate aroma notes from different carbon and nitrogen sources was studied in liquid culture. The medium that gave the best sensory results produced a strong banana aroma. Other notes such as pineapple, apple, pear and nuts, with varied, intensities were obtained from other culture media. Biomass and metabolite productions are reported. In solid state fermentation, the mould was, grown on wheat bran, sugar cane gagasse and a synthetic resin (Amberlite IRA 900) imbibed with a nutritive solution. Mould respirometry was used as growth indicator. While growth was regular, the aromas were not strong as those obtained in liquid culture.

Feather degradation by Kocuria rosea in submerged culture.

A strain of Kocuria rosea with keratinolytic activity was studied. In batch culture, the optimum temperature for feather degradation, bacterial growth and protease secretion was at 40 °C. A specific growth rate of 0.17 h−1 was attained in basal medium with feathers as fermentation substrate. Under these conditions, after 36 h of incubation, biomass and caseinolytic activity reached 3.2 g/l and 0.15 U/ml, respectively. Extracellular protease secretion was associated with the exponential growth phase. In batch fermentation, feather degradation up to 51% in 72 h was obtained with a conversion yield in biomass of 0.32 g/g. No organic acids were detected in the fermentation broth in significant amount.

Growth of Candida utilis in solid state fermentation.

In this work, the growth of the yeast Candida utilis on different solid substrate (wheat bran) and supports (sugarcane bagasse and Amberlite resin) imbibed with a liquid culture medium was studied. Growth was followed by sugars consumption, carbon dioxide production rate (CDPR) and cell count. The results showed the ability of the yeast to grow on the three solid media with fairly good viability and total dextrose consumption in the case of sugarcane bagasse and Amberlite, and partial consumption of wheat bran sugars. After two or three days of culture, a five hundred fold increase in cell population was observed.

Enzymes and food flavor‐A review

Abstract The use of enzymes in flavor generation in food technology is reviewed. In the first part, important products derived from natural macromolecules present in foods such as fats, proteins, nucleic acids and flavor precursors are discussed in terms of the enzymes involved in the reactions and the relation of the products with flavor. Enzymes that are used to eliminate natural or process induced off‐flavors are also discussed. In the second part, the use of enzymes for the direct synthesis of flavoring compounds is presented.

Effect of oxygen on the ethyl acetate production from continuous ethanol stream byCandida utilis in submerged cultures

Abstract The purpose of this work was to compare ethyl acetate production byC. utilis at different oxygen concentrations in an iron free medium. The study was achieved in a batch stirred tank reactor and gaseous ethanol was fed continuously in the air stream. The acetaldehyde production as well as the cell growth were increased when oxygen level was maintained between 5% and 15% rather than 0% and 2%. It was also found that the ethyl acetate maximum concentration was inferior (5.1 g/l against 8.0 g/l) at higher oxygen levels but with a higher productivity (0.1 g/l.h) in that case. Finally, ethyl acetate and acetaldehyde productions were found to be directly dependent on the growth phase in both cases.

Effect of leucine on aroma volatiles production from Ceratocystis fimbriata grown in liquid culture

Aroma volatiles produced by Ceratocystis fimbriata on a defined liquid synthetic medium with and without the addition of leucine were identified by gas chromatography–mass spectrometry and quantified by gas chromatography-flame ionisation detection in the liquid medium as well as in the headspace. Volatiles were extracted from the liquid by simultaneous steam distillation–solvent extraction. Ceratocystis fimbriata produced a complex set of volatile intermediary metabolites, of which ethanol was the dominant compound (∼92–95% of total volatiles). Low molecular weight esters, alcohols, aldehydes, ketones, alkanes, and carboxylic acids were identified in the liquid broth. Alcohols and esters were the most abundant aroma volatiles. Leucine addition effected further growth and higher volatiles production. In the headspace, ethanol and ethyl acetate accounted for 92% of total volatiles over the synthetic medium and 89% when leucine was added. Aroma perception (fruity and banana) correlated closely with liquid and headspace total volatiles.

Effects of water activity, leucine and thiamine on production of aroma compounds by Ceratocystis fimbriata

Ceratocystis fimbriata was grown in a standard liquid medium to determine the production of aroma compounds as affected by thiamine addition to the inoculum, thiamine or leucine addition to the medium, and the effect of water availability. Ethanol constituted more than half of the total volatiles production in the headspace, followed by ethyl acetate (22.6%), ethyl butyrate (10.8%), isobutanol (7.6%), amyl alcohol (1.6%), isoamyl acetate (1.5%), acetaldehyde (1.2%), ethyl propionate (0.9%), isobutyl acetate (0.4%), diacetyl (0.6%) and isoamyl alcohol (0.3%). Although significant two-way interactions were observed (P < 0.05), production of volatile compounds tended to be higher in inocula prepared with thiamine (T+) than in inocula without thiamine (T−), and in the standard medium with thiamine (SMT) as compared to the standard medium alone (SM) and the SM with leucine (SML). Also, the reduction of water activity (aw) resulted in lower quantities of volatiles being produced.