Monitoring lactic acid production during milk fermentation by in situ quantitative proton nuclear magnetic resonance spectroscopy

Abstract

When fermenting milk, lactic bacteria convert part of α- and β-lactoses into d- and l- lactic acids, causing a pH decrease responsible for casein coagulation. Lactic acid monitoring during fermentation is essential for the control of dairy gel textural and organoleptic properties, and is a way to evaluate strain efficiency. Currently, titrations are used to follow the quantity of acids formed during jellification of milk but they are not specific to lactic acid. An analytical method without the use of any reagent was investigated to quantify lactic acid during milk fermentation: in situ quantitative proton nuclear magnetic resonance spectroscopy. Two methods using in situ quantitative proton nuclear magnetic resonance spectroscopy were compared: (1) d- and l-lactic acids content determination, using the resonance of their methyl protons, showing an increase from 2.06 ± 0.02 to 8.16 ± 0.74 g/L during 240 min of fermentation; and (2) the determination of the α- and β-lactoses content, decreasing from 42.68 ± 0.02 to 30.76 ± 1.75 g/L for the same fermentation duration. The ratio between the molar concentrations of produced lactic acids and consumed lactoses enabled cross-validation, as the value (2.02 ± 0.18) is consistent with lactic acid bacteria metabolism.

Applications of NMR in Dairy Research

“…The method was relatively rapid (6–18 min) and, beyond product differentiation, suggests that the technique may be of use in studying, at a more fundamental level, the processes involved in cheese making. The formation of dairy gels (e.g., yoghurt) has been investigated by NMR, and an increase in lactate was observed in real time as the microbes fermented the milk, suggesting that the technique may prove useful in the evaluation of new bacterial strains for dairy products [19]. These authors have also demonstrated the utility of NMR for monitoring the changes in liquid fat content in dairy gels, again demonstrating the utility of NMR in understanding complex processes in food processing and production [20].…”

Active Packaging Based on Coupled Nylon/PE Pouches Filled with Active Nano-Hybrid: Effect on the Shelf Life of Fresh Milk

“…The pH decreases as function of time and, consequently the acidity index increases. It is well known that during milk fermentation lactic bacteria convert part of α- and β-lactoses into D- and L- lactic acids, leading to a pH decrease and, as a consequence, determining the coagulation of caseins [ 19 ]. Milk stored in the active packaging displays a drop in pH less pronounced than the one evidenced in the milk stored in pouches with no active filler.…”

An NMR Metabolomics Approach to Investigate Factors Affecting the Yoghurt Fermentation Process and Quality

“…Lactate shows an increase in concentration during fermentation with a faster slope after 4 h and a further increase in concentration from 8 to 24 h. ST is the strain that displays the lowest increase in lactate and the highest is observed for LB and MX in HT milk. Lactate is also produced through pyruvate [ 14 , 22 , 27 ], thus, the decrease in pyruvate for LB and MX can also be explained by lactate production.…”