Microbial Catabolism of Methionine to Improve Cheddar Cheese Flavor- a Comparative Study of the Relative Contribution by Starter Cultures and Flavor Adjunct Bacteria
Principal Investigator: Dr. Bart C. Weimer, Utah State University
Met metabolism in dairy related bacteria is under investigation. Brevibacteria produce more sulfur containing compounds during their metabolism than do lactic acid bacteria. Additionally, the mechanism by which production of methanethiol occurs is different than lactococci. The enzyme responsible for methanethiol production in brevibacteria was isolated to homogeneity and characterized - methionine gamma-lyase (MGL). Addition of MGL, whole cells of B. linens BL2 (commercially available from GB) with either GDL or L. cremoris S2 demonstrated that MGL and whole cells of BL2 produced significantly more volatile sulfur compounds (VSC) than S2 alone in slurries. L. cremoris S2 produced VSC at levels just above flavor threshold. Addition of MGL or BL2 produced 2 to 5 times more VSC than S2 alone. These increases were associated with the treatments and not contamination from other organisms. The predominant VSC were methanethiol, dimethyldisulfide, and dimethyltrisulfide. As the methanethiol content decreased the dimethyldisulfide content increased. This observation suggested the redox potential was important, but was not measured.
Studies to screen other lactic acid bacteria (LAB) for met utilization indicate LAB produced significantly less VSC than brevibacteria. Whole cells or cell free extracts produce VSC, but wholes cells produce less than cell free extracts, suggesting that met transport may be important in the production rate.
Studies are underway to determine the relative amounts of VSC production from organic and inorganic sulfur sources. Initial work was needed to determine a method for measuring total inorganic sulfate in milk. This is done and results indicate that whole and skim milk contain ~45 mg/L of sulfate. Transport of inorganic sulfur (sulfate) by lactococci indicate that they can significantly reduce the sulfate content during log growth and stationary phase. Work is under way to determine if the cells convert sulfate into organic precursors for VSC production.
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Weimer, B. 1999. Brevibacteria. In Encyclopedia of Food Microbiology, R. K. Robinson et al. (Ed.), Academic Press, London.
Ummadi, M. and B. C. Weimer. 1999. Use of capillary electrophoresis-laser induced fluorescence detection to monitor bacterial growth and amino acid utilization. J. Chrom A. (in press).
Dias, B., and B. C. Weimer. 2001. Detection of a-keto acids with capillary electrophoresis in culture supernatants and cheese. J Chrom A (accepted).
Dias, B., and B. C. Weimer. 1999. Production of volatile sulfur compounds in Cheddar cheese slurries. International Dairy Journal 9:605-611.
Weimer, B., K. Seefeldt, and B. Dias. 1999. Sulfur metabolism in bacteria associated with cheese. Antonie van Leeuwenhoek 76:247-261
Steele, J.L., M.E. Johnson, J.R. Broadbent, and B.C. Weimer. 1998. Starter culture attributes which affect cheese flavor development, pp. 157-170. In, Proc. LACTIC '97 conference, Which strains? For which products?
M. E. Johnson, J. L. Steele, J. Broadbent, and B. C. Weimer. 1998. Manufacture of gouda and flavour development in reduced-fat cheddar cheese. Aust. J. Dairy Tech. 53:67.
Dias, B., and Bart Weimer. 1998. Conversion of methionine to thiols by lactococci, lactobacilli, and brevibacteria. Appl. Environ. Microbiol. 64:3320.
Dias, B., and Bart Weimer. 1998. Purification and characterization of methionine g-lyase from Brevibacterium linens BL2 Appl. Environ. Microbiol. 64:3327.
Broadbent, J.R., M. Strickland, B. Weimer, M.E. Johnson, and J.L. Steele. 1998. Peptide accumulation and bitterness in Cheddar cheese made using single-strain Lactococcus lactis starters with distinct proteinase specificities. J. Dairy Sci. 81:327.
Broadbent, Jeffery R., Charlotte Brennand, Mark E. Johnson, James L. Steele, Marie Strickland, and Bart C. Weimer. 1997. Contributions by starter and selected adjunct bacteria to flavor development in reduced-fat cheddar cheese. Dairy Industry Int. 62:35.
Gao, S., D-H. Oh, J. Broadbent, M. Johnson, B. Weimer, and J. Steele. 1997. Aromatic amino acid catabolism by lactococci. Le Lait 77:371.
Weimer, B. C., C. Brennand, J. Broadbent, J. Jaegi, M. Johnson, F. Milani, J. Steele, and D. Sisson. 1997. Influence of flavor adjunct bacteria on the flavor and texture of 60% reduced fat Cheddar cheese. Le Lait 77:383.
Ben Dias - Ph.D. (completed spring '99)
Kim Seefeldt - Ph.D. terminated due to student quitting
S. Ghosh - M.S. - in progress
Weimer, B. C., 1999. Flavor compounds - bacteria and cheese. Land O'Lakes, Minneapolis, MN.
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Weimer, B. C., 1999. Sulfur metabolism in dairy related bacteria. Institute of Food Technologists annual meeting, Biotechnology Section, Chicago, IL.
Weimer, B. C., 1999. Sulfur metabolism in dairy related bacteria. IFT Biotechnology Section Keynote.
Weimer, B. C., 1999. Sulfur metabolism in bacteria associated with cheese. 6th FEMS International Conference, Keynote speaker. Veldhoven, The Netherlands.
Weimer, B. C., B. Dias, M. Ummadi, M. Stickland, J. Broadbent, M. Johnson, J. Jeaggi, J. Steele, and J. Harper. 1997. Improving Cheddar cheese flavor with the addition of brevibacteria. American Dairy Science Association Annual meeting.
Weimer, B. C., 1997. Strategies for improving cheese flavor. Dairy
Managment, Inc. Conference on Cheese Flavor. Chicago, IL.