Because the hops are added to your beer after the boil and cool down you benefit from the flavoring and aroma characteristics of the hop without the bittering. This happens because hops that are boiled for an extended period (longer than about 15 minutes) have the oils boiled off and what remains is the bitterness.
By not boiling off the oils (by doing your hop infusion late in the boil or by dry hopping after the cool down of your wort) you get the maximum benefit of the flavoring and aroma characteristics of your hops. Dry hopping does not add any bitterness to your beer because boiling is necessary to convert the alpha acids present in the hops to iso-alpha acids which create the bittering in your beer.
The result is a beer that has a floral essence and an intense flavor. These characteristics are very desirable in beers which are "hoppy" such as IPA's or pale ales.
What about the possibility of adding bacteria to your beer by dry hopping (remember, you are not boiling which does sanitize everything)? When I use a muslin bag and a string to dry hop beer in a carboy I boil the bag and string prior to dry hopping. What about the hops themselves? Beyond that there are several factors which actually inhibit the development of bacteria including that hops themselves do not provide an environment that is conducive to the growth of bacteria plus any bacteria which enter the wort during the primary fermentation stage will be competing with yeast which is aggressively converting the wort into alcohol. Bacteria which enters at the secondary fermentation stage will find a situation in which there is a combination of alcohol together with a low pH level both of which will tend of inhibit the growth of bacteria.
Bacteria caused by dry hopping? It could happen but not on a frequency level which should prevent you from using this method to obtain more flavor and aroma in your beer.
Tomorrow: Determining which hops to use when dry hopping
Wine Tidbit: Going out for your evening meal? Planning on taking a bottle of wine along? Be sure to call ahead and ask about the restaurant's corking policy. Most establishments will charge a nominal fee ($10 or less per bottle).
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