MONITORING OF FERMENTATION BY INFRARED SPECTROMETRY - ALCOHOLIC AND LACTIC FERMENTATIONS

Infrared spectrometry offers a great potential for quantitative analysis of substrates and products in fermentation media. Like this, a rapid measurement of lactose, galactose and lactic acid during lactic fermentation and sugars (glucose and fructose), glycerol and ethanol during alcoholic fermentation will constitute a major progress for the monitoring and the control of these processes. The samples, taken manually during the fermentations, permitted the determination of the interesting spectral ranges for the measurement of these compounds. These ranges, characterized by significant evolutions of absorbance, are situated in the middle infrared zone: between 1100 and 1040 cm-1 for lactose and galactose, between 1593 and 1515 cm-1 for lactate, between 1206 and 1026 cm-1 for molecules of the alcoholic fermentation and between 3000 and 1026 cm-1 for ethanol. Calibration is realized by the partial least-squares method from representative samples of the studied concentrations. The regression coefficients of the correlation between the concentrations measured by infrared spectrometry and liquid chromatography are better than 0.99. The maximum standard errors are 0.46, 0.57 and 0.95 g l-1 respectively for lactose, galactose and lactic acid; 2.5, 0.09 and 0.98 g l-1, respectively for the sugars, glycerol and ethanol.