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Tech. Q. Master Brew. Assoc. Am., 1990, 27(3), 83-89. English, sp
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The microbiology of dry hopping.

Guinard, J.-X., Woodmansee, R.D., Billovits, M.J., Hanson, L.G., Gutierrez, M.-J., Snider, M.L., Miranda, M.G. and Lewis, M.J.

Abstract
Because the volatile oils which give rise to the distinctive aroma of fresh hops are largely lost during wort boiling, it was formerly usual to add more hops either to the fermenting wort or to the beer at the time of racking in order to enhance the flavour. This practice, known as "dry hopping", has undergone something of a revival in the "traditional" sector of the brewing industry. In order to investigate the possibility of microbiological contamination resulting from dry hopping, cultures were taken from samples of whole and pelleted hops and of fermenting wort which had been dry hopped either at pitching or 3 days thereafter. Although a variety of wild yeasts and bacteria were found on hops and in freshly dry hopped fermenting worts, no extraneous microorganisms were found after the second day after dry hopping. It is therefore concluded that the dry hopping of fermenting wort is microbiologically safe (although no conclusions can be drawn about the dry hopping of beer, which was not studied).
Keywords : beer contamination dry hopping microorganism wort