In Situ Infrared Spectroscopic Monitoring of Lactic Acid Fermentation Rates during Yogurt Formation

Abstract

To obtain the first quantitative time scales for yogurt formation, attenuated total reflection infrared spectroscopic monitoring was conducted on the lactic acid fermentation reactions of milk every 20 seconds for 2 hours at 32.5-50°C. Changes with time in the ratios of the infrared band area to that of proteins (amide III) could be fitted by first-order reactions. The temperature dependencies of the obtained rate constants gave activation energy values of Ea=38 and 31 kJ.mol^-1 and frequency factor values of A=1.2×10³ s^-1 (ln A=7.1) and A=7.0×10¹ s^-1 (ln A=4.2), for the decrease in sugar content and the increase in acid content, respectively. These chemical reaction rates during lactic acid fermentation were slower than the gelation rates of yogurt reported in the literature, and are considered to be the rate-determining steps of yogurt formation. Therefore, the time scales of these reactions are expected to provide the first indicator of the time periods for yogurt formation.