ANTIBACTERIAL ACTIVITY OF HOP BITTER RESINS DERIVED FROM RECOVERED HOPPED WORT

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Abstract
The growth of a thermophilic Bacillus sp. (isolated from mash vessels) in sweet wort was significantly inhibited by the addition of recovered hopped wort (centrate). Addition of 2% (v/v) centrate to cultures reduced the specific growth rate (μ) by 98% and growth was completely inhibited at 5% (v/v) level. The addition of 5.7% (v/v) centrate to concentrated cell suspensions in sweet wort resulted in the rapid logarithmic reduction in the number of culturable cells. Centrate did not exhibit any detectable sporicidal activity nor did it prevent spore germination. However, centrate additions of 5% (v/v) and greater did prevent outgrowth of spores. There was no evidence of centrate induced plasmolysis at any centrate level, measured either spectrophometrically or as potassium efflux from cells, despite the observation that cells exposed to even the lowest centrate levels were non-culturable on solid media. HPLC spectra of hop bitter resins in centrate differed from those of freshly hopped worts: centrate typically had a bitterness value of about 55 BU and contained a greater proportion of oxidised components corresponding to the S-fractions, in addition to low levels of a-acids. The significance of centrate hop resin components in relation to the proposed mechanism of antibacterial activity is briefly discussed.