The tolerance of brewing-related microorganisms to isomerized hop products and the effect on beer stability and quality

The growth inhibitions of four commercially available isomerized hop products, in which dihydro iso-α-acids, tetrahydro iso-α-acids, and hexahydro iso-α-acids are derived from natural iso-alpha-acids, against brewing-related microorganisms were studied. The tests were carried out in three media: liquid (broth, pH 6–7), solid (agar, pH 6–7) and beer (pH 4.2). The results indicate that Staphylococcus aureus (S. aureus) ZJ007 (G+) was the most sensitive strain to any hop product at the lowest concentration of 0.1 g/L in liquid and 0.5 g/L in solid. However, in liquid media, hexahydro iso-alpha-acids showed significant antibacterial potency on E. coli DH5a and Lactococcus species AB133 at 0.5 g/L (equating to 50 mg/L concentration). Interestingly, the growths of E. coli DH5α (G−) and wild Saccharomyces cerevisiae BY101 (wild yeast), respectively, in the solid media were inhibited by hexahydro and tetrahydro iso-alpha-acids (equating to 250–500 mg/L) and dihydro and iso-alpha-acids (750–1500 mg/L), and no effect on Saccharomyces cerevisiae (DM303). By the same means, the growths of Lactococcus species AB133 (possible hop resistant, G+) and Lactococcus lactis NZ9000 (hop-resistant G+, tolerated to iso-alpha-acids) were inhibited by hexahydro > tetrahydro iso-alpha-acids (equating to 500 mg/L) and not affected by dihydro and iso-alpha-acids. Overall, the hexahydro iso-alpha-acids are the most effective bacteriostat. The microscopy did not detect any strain in the inoculated and hopped beer (≥ 50 mg/L or BU) precipitates after fermentation. The GC–MS analysis showed insignificant aroma difference compared to the control beer (DM303 yeast only). For the implications, four isomerized hop products are more soluble and offer higher concentration in any neutral medium (pH 6–7) than their undissociated forms (pH ≤ 4). In the milder pH range, their antibacterial behavior is hexahydro > dihydro > tetrahydro ~ iso-alpha-acids.