Metabolomics-based screening identifies Aspergillus strains that accumulate bioactive leucine catabolites and fatty acid ethyl esters in koji mold-ripened cheese.
This study highlights the metabolic potential of Aspergillus oryzae strains in cheese production, particularly in the context of flavor compound accumulation.
Where it sits
this study against the rest of the hmg (human menopausal gonadotropin) corpusSummary and findings
This study investigated the metabolic attributes of Aspergillus oryzae strains in koji cheese production. It identified the accumulation of fatty acid ethyl esters and amino acids, particularly focusing on 3-hydroxy-3-methylbutyric acid (HMB) and 3-hydroxy-3-methylglutaric acid (HMG) as metabolites derived from l-leucine. The production of these compounds was strongly correlated with l-leucine concentration.
Abstract
Aspergillus koji molds, which play a principal role in manufacturing fermented foods, have been applied to the production of koji cheese. In this study, Aspergillus oryzae strains and their metabolic attributes were studied using nontargeted metabolomics and identified as starter cultures that can introduce novel chemical characteristics. In volatile compounds, A. oryzae Q was recognized as an uncommon strain following the previously selected strain KC43, and its cheese scarcely increased the levels of straight-chain fatty acids and methyl ketones, sharp cheesy odorants. Strain O can accumulate substantial levels of fatty acid ethyl esters (FAEEs) in koji cheese, indicating its potential contribution to the floral note and masking of sharpness in cheese, as reported in blue cheese. In water-soluble compounds, the tested strains as well as the previously selected strains were commonly characterized by a marked increase in amino acid levels. By contrast, 3-hydroxy-3-methylbutyric acid (HMB) and 3-hydroxy-3-methylglutaric acid (HMG), which were detected as unannotated compounds, were highly responsible for strain Q. Metabolic tracing using a <sup>13</sup>C-labeled substrate identified that HMB and HMG were extracellular metabolites converted from l-leucine, along with the formation of α-ketoisocaproic acid and isovaleric acid. The production quantity of these four compounds was strongly correlated with the initial concentration of l-leucine (R > 0.975). The findings proposed the hypothesis that these compounds derived from their CoA-thioesters in l-leucine degradation pathway required for HMG-CoA and mevalonate synthesis. This study is the first to report HMB and HMG as metabolites produced by A. oryzae and as major compounds in Aspergillus-fermented foods.
Background
The study addresses the role of Aspergillus koji molds in the fermentation of foods, particularly in the production of koji cheese. Prior research has established the importance of these molds in developing unique flavors and chemical characteristics in fermented products. This study is significant as it identifies novel metabolites associated with specific Aspergillus strains, which could enhance our understanding of flavor development in fermented foods.
Methods
The study employed nontargeted metabolomics to analyze various Aspergillus oryzae strains used as starter cultures in koji cheese production. Specific strains were characterized for their metabolic profiles, focusing on volatile and water-soluble compounds. The correlation between metabolite production and l-leucine concentration was assessed using a <sup>13</sup>C-labeled substrate.
Results
The primary endpoint revealed that the production quantity of HMB and HMG was strongly correlated with the initial concentration of l-leucine (R > 0.975). The study found that strain O could accumulate significant levels of fatty acid ethyl esters, contributing to the flavor profile of the cheese. Additionally, strain Q was identified as an uncommon strain with distinct metabolic characteristics.
Interpretation
These findings contribute to the existing literature on the metabolic capabilities of Aspergillus strains in food fermentation. While the correlation between l-leucine concentration and metabolite production is statistically significant, the clinical relevance of these metabolites in terms of flavor enhancement or health benefits remains unclear. Limitations such as the absence of human trials and the focus on specific strains may confound the broader applicability of the results.
Key findings
- Production quantity of HMB and HMG was strongly correlated with the initial concentration of l-leucine (R > 0.975).
- Strain O accumulated substantial levels of fatty acid ethyl esters in koji cheese.
- A. oryzae Q was recognized as an uncommon strain following strain KC43.
Limitations
- Not reported in abstract.
- Focus on specific Aspergillus strains limits generalizability.
- No human trials conducted.