Wet hopping or fresh hopping a beer is when freshly picked\undried hops are added to a beer at some point of the brewing, fermenting, or conditioning process. These hops are typically added to the beer within a day or two of being picked to maximize the unique flavors extracted from a freshly picked hop. A fresh or wet hop is typically less predictable than a dried hop and will usually impart a lower amount of bitterness than the same weight of dry hops due to the additional moisture weight in the wet hop.
Below is a photo of some cascade hops nearly ready to be picked and used for fresh hopping\wet hopping.
Cascade hops ready to be used for fresh hopping or wet hopping.
Diacetyl is a naturally occurring compound formed during fermentation and has a perceived butter or butterscotch like flavor that is undesirable in most beers. It is important to note that other ingredients used in beer production, such as caramelized grains, may impart a somewhat similar flavor, and the two should not be confused.
Although Diacetyl is tolerated or even expected in some beer styles, recent research has shown that at very high levels and under the right conditions, it can be toxic. The good news is that diacetyl is typically only present at very low levels in beer and can be greatly minimized if certain protocols are followed.
One way to help reduce diacetyl in your beer is to clean and sanitize equipment properly because certain undesirable bacterias produce diacetyl. Another way to reduce the diacetyl in your beer is to pitch a sufficient quantity of healthy yeast and conduct a full fermentation and conditioning cycle prior to cold crashing or kegging your beer.
Yeast is responsible for creating and removing diacetyl at different stages of the fermentation process. Yeast creates diacetyl early in the fermentation process and breaks it down towards the end. If you did not pitch enough healthy yeast to complete the fermentation, placed your yeast in stasis, or destroyed your yeast before the end of fermentation, then you may end up with higher then desired levels of diacetyl.
If you happen to be fermenting a lager, a process known as a diacetyl rest (which requires a fermentation temperature increase) may also be helpful in reducing diacetyl in your beer.
Cold crashing or cold filtering is a common method used to clarify beer. When a beer is cold crashed, it is chilled down to approximately 35 F and left for several days to several weeks. During that time, yeast and other solids tend to clump together and fall to the bottom of the fermenter or holding tank. The clarified beer is then racked above the layer of sediment and potentially ran through a filter if additional clarification is desired. Cold crashing or filtering is not appropriate for some beer such as a hefeweizen or certain Belgian ales where a yeasty flavor or hazy beer is desired.