Graduate Thesis Or Dissertation
 

Characterization of dairy leuconostocs and method to use Leuconostoc mesenteroides ssp. cremoris to improve milk fermentations

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  • Dairy leuconostocs are frequently used as aroma-producing bacteria in mesophilic cultures used as starters for milk fermentations. Sixty Leuconostoc strains from different culture collections were studied taxonomically for species identification. Based on morphological, physiological and biochemical tests, twenty of these strains were typed as Leuconostoc mesenteroides ssp. cremoris (Leu. cremoris). The objectives of this study were to acquire additional information on phenotypic features of Leu. cremoris strains, such as diacetyl reductase activity and citrate utilization in milk, and to select strains most useful for producing flavor compounds, primarily diacetyl, in dairy fermentations. Great strain variability within Leu. cremoris species was observed. Diacetyl reductase specific activity, indirectly measured by the rate of NADH oxidation, ranged from 0 to 1603 U/mg of cell protein. Most of the strains utilized citrate under neutral conditions, pH 6.5, without concomitant production of diacetyl or acetoin. Addition of citric acid after preincubation (24 hr, 28°C) resulted in significant production of diacetyl (2.8-75.3 genie and acetoin (162.1-764.4 p.g/ml) by pure Leu. cremoris cultures during further incubation (18 hr, 28°C). Simple and direct gas liquid chromatographic analysis without prior processing was applied to quantify acetaldehyde, ethanol, diacetyl, acetoin and acetic acid in milk fermented with Leu. cremoris and Lactococcus lactis ssp. cremoris (Lc. cremoris). Leu. cremoris 91404 was selected as an aroma producer in preparation of experimental buttermilk based on the following characteristics: low diacetyl reductase activity; citrate utilization and high diacetyl production under acidic conditions; growth characteristics and compatibility with Lactococcus strains. However, no diacetyl was detected in buttermilk made in the traditional commercial manner. Fortification of ripened buttermilk with sodium citrate resulted in significant increase in diacetyl and acetoin production during buttermilk storage (5°C for as long as two weeks). Surplus of citrate, low pH (pH 4.5-4.7), sufficient number of active non-growing aroma producers, air incorporation during curd breaking and low temperature storage facilitated citrate metabolism toward production and conservation of flavor during two weeks of storage. Incorporation of a ropy Lc. cremoris strain in starter culture significantly improved the texture and appearance of experimental buttermilk. Survey of commercial buttermilks available in the Corvallis, OR market showed wide variation in concentrations of volatile compounds and in organoleptic characteristics. Manufacture of experimental buttermilk under different conditions revealed that simple modifications in the traditional manufacturing procedure, involving starter composition and delayed citrate fortification, would yield refreshing product with clean, aromatic, thick and carbonated properties.
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