Citrate utilization by free and immobilized Streptococcus lactis subsp. diacetylactis in continuous culture

Methods for the determination of aroma compounds in dairy products: a comparative study

Several methods used for sample preparation and for the determination of volatile flavour compounds in dairy products were tested. Steam distillation was an advantageous method for isolating volatile substances. The extraction yields for acetaldehyde, ethanol and diacetyl were >90%, and acetoin was partly separated from diacetyl. After steam distillation of the sample, gas-liquid chromatography was found to be a rapid method for the determination of acetaldehyde, ethanol and diacetyl in a wide range of concentrations corresponding to those found in dairy products. However, the lower limits for reproducible measurements were high (250, 1250 and 65 μM respectively). From the same sample, colorimetry was shown to be an accurate method for the detection of low levels of diacetyl and acetoin (12 and 57 μM respectively). Diacetyl interfered in the colorimetrie determination of acetoin. However, the interference was < 10% when the diacetyl: acetoin molar ratio was < 0·033. Dynamic head-space gas chromatography proved to be a sensitive method for acetaldehyde and diacetyl determination when a ‘purge and trap’ injector and a capillary column were used. The ranges suitable for determination lay between 25 and 100 μM for acetaldehyde and between 2·9 and 11·4 μM for diacetyl. Acetoin was analysed by HPLC on a cation-exchange column and detected by refractometry for concentrations ranging from 250 to 4000 μM.

Citrate can partially replace carbon dioxide required for growth of Lactococcus lactis subsp. lactis biovar diacetylactis

Lactococcus lactis subsp. lactis biovar diacetylactis was grown as batch cultures on a chemically defined medium. No growth was observed when the cultures were sparged with pure nitrogen (1.3 l l-1 min-1) whereas the cultures displayed exponential growth in the presence of minute amounts of carbon dioxide (0.035 mol-% of the inlet gas). However, in the former case, the addition of citrate restored growth. This suggested that oxaloacetate required for aspartate biosynthesis can be formed by the carboxylation of pyruvate or by citrate catabolism. When the cultures were heavily sparged with nitrogen (2.6 l l-1 min-1), no growth was observed even in the presence of citrate. This indicated that growth in these conditions was repressed by the absence of carbon dioxide required in some other biosynthetic reaction than in the carboxylation of pyruvate leading to oxaloacetate/aspartate biosynthesis.

Food bioconversions and metabolite production using immobilized cell technology

This review explores recent advances in the use of immobilized cells for the production of metabolites used in the food industry, such as enzymes, amino acids, organic acids, alcohols, aroma compounds, polysaccharides, and pigments. Some food bioconversions such as fermentation of soy sauce and various hydrolysis are also considered. Special emphasis was placed on existing or potential industrial processes. This article also reports the effects of the reactor (configuration and working conditions), the immobilized cell physiological status (growing, nongrowing, or permeabilized), and of the carrier type, configuration, and size on the performance of immobilized cell systems. Compared with free cell fermentation, the main advantage of using immobilized cells is an increase in productivity, particularly in the case of continuous fermentation. For monoenzymatic reactions, nongrowing immobilized cells are often reported to exhibit a higher stability than free or immobilized enzymes.

Immobilized cell technologies for the dairy industry

The potential applications of immobilized cell technology (ICT) to the dairy industry are examined. Immobilization modifies the physiology of cells, and the consequences of ICT on lactose as well as citrate metabolism are reviewed. Immobilization also affects the sensitivity of lactic acid bacteria (LAB) to salt and penicillin. ICT can be used to produce starters for the dairy industry, and aspects of biomass production in beads, continuous cell release from beads, and continuous fermentations with filtration cell recycle are examined. Potential applications of ICT to the dairy industry include acidification of raw milk prior to ultrafiltration, inhibition of psychrotrophic bacteria in raw milk, yogurt production, cheese manufacture, and cream fermentations. Impacts of yeast, bacterial, or bacteriophage contaminations in ICT processes as well as their control are discussed.

Effect of Initial Oxygen Concentration on Diacetyl and Acetoin Production by Lactococcus lactis subsp. lactis biovar diacetylactis

The production of aroma compounds (acetoin and diacetyl) in fresh unripened cheese by Lactococcus lactis subsp. lactis biovar diacetylactis CNRZ 483 was studied at 30°C at different initial oxygen concentrations (0, 21, 50, and 100% of the medium saturation by oxygen). Regardless of the initial O 2 concentration, maximal production of these compounds was reached only after all the citrate was consumed. Diacetyl and acetoin production was 0.01 and 2.4 mM, respectively, at 0% oxygen. Maximum acetoin concentration reached 5.4 mM at 100% oxygen. Diacetyl production was increased by factors of 2, 6, and 18 at initial oxygen concentrations of 21, 50, and 100%, respectively. The diacetyl/acetoin concentration ratio increased linearly with initial oxygen concentration: it was eight times higher at 100% (3.3%) than at 0% oxygen (0.4%). The effect of oxygen on diacetyl and acetoin production was also shown with other lactococci. At 0% oxygen, specific activity of α-acetolactate synthetase (0.15 U/mg) and NADH oxidase (0.04 U/mg) was 3.6 and 5.4 times lower, respectively, than at 100% oxygen. The increasing α-acetolactate synthetase activity in the presence of oxygen would explain the higher production of diacetyl and acetoin. The NADH oxidase activity would replace the role of the lactate dehydrogenase, diacetyl reductase, and acetoin reductase in the reoxidation of NADH, allowing accumulation of these two aroma compounds.