Lactobacillus demonstrate thiol-independent metabolism of methylglyoxal: Implications toward browning prevention in Parmesan cheese

Abstract

Endogenous production of α-dicarbonyls by lactic acid bacteria can influence the quality and consistency of fermented foods and beverages. Methylglyoxal (MG) in Parmesan cheese can contribute toward undesired browning during low temperature ripening and storage conditions, leading to the economic depreciation of affected cheeses. We demonstrate the effects of exogenously added MG on browning and volatile formation using a Parmesan cheese extract (PCE). To determine the influence of Lactobacillus on α-dicarbonyls, strains were screened for their ability to modulate concentrations of MG, glyoxal, and diacetyl in PCE. It was found that a major metabolic pathway of MG in Lactobacillus is a thiol-independent reduction, whereby MG is partially or fully reduced to acetol and 1,2-propanediol, respectively. The majority of lactobacilli grown in PCE accumulated the intermediate acetol, whereas Lactobacillus brevis 367 formed exclusively 1,2-propanediol and Lactobacillus fermentum 14931 formed both metabolites. In addition, we determined the inherent tolerance to bacteriostatic concentrations of MG among lactobacilli grown in rich media. It was found that L. brevis 367 reduces MG exclusively to 1,2-propanediol, which correlates to both its ability to significantly decrease MG concentrations in PCE, as well as its significantly higher tolerance to MG, in comparison to other lactobacilli screened. These findings have broader implications toward lactobacilli as a viable solution for reducing MG-mediated browning of Parmesan cheese.

A metabolic reconstruction of Lactobacillus reuteri JCM 1112 and analysis of its potential as a cell factory

“…From a biotechnological perspective, an interesting branch point of central carbon metabolism is the conversion from methylglyoxal (MG) to 1,2-propanediol (1,2-PDO), which can then be further metabolized into 1-propanol and propanoate. L. reuteri possesses all enzymes needed for these pathways, except methylglyoxal synthase (MGS), the step of the pathway converting dihydroxyacetone phosphate into MG [19, 55]. It has been shown that when MG is added to L. reuteri JCM 1112 cultures or when a heterologous mgs is expressed, all the subsequent metabolites are formed [7].…”

“…Hence, all the genes in these pathways except mgs were included in the reconstruction. For methylglyoxal reductase, mgr , we also identified several aldo/keto reductases as possible homologs, based BLAST comparison to genes identified in [19]. However, verification of these hypothetical activities would need extensive enzyme assays, and it is also likely that this reaction is performed by LAR_RS09730 (Glycerol dehydrogenase) [1, 65], which has been added to the reconstruction for the MGR reaction.…”

“…However, verification of these hypothetical activities would need extensive enzyme assays, and it is also likely that this reaction is performed by LAR_RS09730 (Glycerol dehydrogenase) [1, 65], which has been added to the reconstruction for the MGR reaction. Alternatively, MG might be converted directly to lactate by a glyoxalase [19].…”

New insights into nonenzymatic browning of non‐PDO Italian hard cheese through a metabolomics approach

“…2012; Gandhi et al . 2018, 2019) on browning defects. Our findings suggested the potential contribution of methylglyoxal (likely of microbial origin) to the formation of several Maillard‐related metabolites.…”

“…This reaction proceeds at relatively low temperatures during storage without the involvement of reducing sugars (Gandhi et al . 2018). No detailed information exists about the combination of relative humidity and temperature values that may accelerate this reaction (Rocchetti et al .…”