Graduate Thesis Or Dissertation

 

Metabolism of aroma bacteria as related to diacetyl production Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zw12z8709

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  • Factors influencing diacetyl production by the aroma bacteria Leuconostoc citrovorum and Streptococcus diacetilactis were investigated. When grown in association with lactic streptococci, L. citrovorum strain 91404 decreased in cell numbers from 10⁸ to 10⁶ over two weeks of daily subculturing in sterile non-fat milk incubated at 21°C. The ability to produce diacetyl over this period also was lost by L. citrovorum, but the reduction in cell numbers did not appear responsible because it was also shown that 2.5 X 10⁴ cells were able to produce detectable diacetyl after one transfer. Disk assay procedures did not reveal any antibiotic activity by the lactic streptococci against Leuconostoc. The enrichment of milk with 100 μg/ml sodium citrate, 100 μg/ml sodium pyruvate, 10% yeast extract or 50% pea extract did not enhance diacetyl production by L. citrovorum. The addition of 0.125 ppm MnSO₄ caused enhanced diacetyl production up to 12 hours and enhanced diacetyl reduction from 12 to 24 hours by L. citrovorum grown in association with lactic streptococci. The cell-free extracts prepared from these Mn⁺⁺ supplemented cultures of L. citrovorum had 14 times more diacetyl reductase activity than non-treated cultures. Diacetyl production by S. diacetilactis 18-16 was not enhanced by MnSO₄ and flavor reduction was increased only slightly; diacetyl reductase activity was not affected. Treatment of the milk with 0.03% H₂O₂ and sufficient 0.004% catalase to decompose the H₂O₂ resulted in at least twice as much diacetyl production by S. diacetilactis 18-16; the resulting flavor also was stable. This treatment did not increase cell numbers, decrease diacetyl reductase activity or enhance respiration as measured by manometry studies. Diacetyl plus acetoin was synthesized by the cell-free extracts of L. citrovorum 91404 and S. diacetilactis 18-16 when supplemented with 0.1 μmole acetyl CoA plus 0.1 μmole pyruvate in the presence of catalytic amounts of Mn⁺⁺ and TPP. S. diacetilactis also produced diacetyl plus acetoin from a acetolactic acid plus 0.1 μmole acetyl CoA. The synthetic activity of the extracts was not reduced by MnSO₄ addition or stimulated by H₂O₂-catalase treatment. From the results it was clear that important differences exist between the aroma bacteria with regard to optimum requirements for diacetyl production.
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