SUMMARYWe have investigated the effect of culture temperature on the growth ofLactococcus lactissubsp.lactisbiovardiacetilactis, on its acidifying power, on diacetyl and acetoin production, and also on the activities of the principal enzymes involved in the synthesis of these two compounds. The rates of growth and lactic acid production decreased by a factor of ∼ 2 when the temperature decreased from 30 to 18°C. At 18°C, the maximal concentration of diacetyl (0·3mM) was 1·7 times that at 30°C, while that of acetoin was unchanged (5·2 mM). These results are explained by the behaviour of the principal enzymes involved in pyruvate metabolism. The activities of lactic dehydrogenase and acetolactate synthase varied little for culture temperatures between 18 and 30·C. However, the activity of NADH oxidase for a culture temperature of 18°C was 3·7 times that for 30°C, while that of diacetyl reductase for 30°C was 2·7 times that for 18°C. The net effect of temperature on these two activities was an increase in diacetyl production at lower temperature.

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 02 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,o respectively. The diacetylacetoin concentration ratio increased linearly with initial oxygen concentration: it was eight times higher at 100% (3.3%) than at O%o oxygen (0.4%). The effect of oxygen on diacetyl and acetoin production was also shown with other lactococci. At 01% oxygen, specific activity of a-acetolactate synthetase (0.15 U/mg) and NADH oxidase (0.04 U/mg) was 3.6 and 5.4 times lower, respectively, than at 100o oxygen. The increasing cv-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.

SummarySeveral 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.

N Bassit et al subsp lactis biovardiacetylactis was studied. The results were described by the methodology of response surface. Linear, squared and simple interaction effects were estimated after an orthogonal polynomial reparametrisation. The effect of temperature was higher than the effect of oxygen for acidification only. The opposite effect was observed for diacetyl and acetoin production and consequently for the ratio diacetyVacetoin (DIA). Optima were found on the limits of the experimental design. Diacetyl concentration and DIA ratio reached a maximum value at urc and 100% of oxygen concentration (0.3 mmoVl and 5.9%, respectively). Acetoin concentration was maximum (5.3 mmoVl) at 26"C and 100% of oxygen concentration. On the other hand, the maximum acidification rate reached its maximum value (0.23 unit pHlh) at 3CJ'Cand 0% of oxygen concentration. For the four variables tested, there was a significant interaction between temperature and initial oxygen concentration. diacetyl 1acetoin 1acidification 1 Lactococcus lactis subsp lactis

No abstract