Should you add fruit to a beer? Well, fruit in beer isn't just a recent fad, in fact it is probably how most fermented grain beverages started. While there may be a lot of debate to what came first, wine or beer, from the research I have done, it seems like early feremnters put whatever items could ferment together into a fruit/grain mixture that would get them drunk. Fast forward several thousand years, and the German/Belgian/France area of Europe started adding fruit mixes to their (now well defined) beer in the early 1900. These were called Radlermass or just Radler, and then in England changed to Shandy. These were typically 50/50 blends of citrus juices mixed with a light beer style. These big fruity beers can stretch the definition of what most beer lovers call "beer", but they are a nice drink to have in the hot summer months.
You don't have to go to a 50/50 mix however. Both homebrewers and craft brewers alike, love to add subtle fruit flavors to their base beer styles, and when done correctly, can be pleasing to even the most staunch beer geek. So, how do you do it?
There are a couple things to consider when adding fruit to beer:
Simple is probably better. You don't want to muddy up the waters too much. While you can find complex mixtures that work well, having a simple beer style for the base will let the fruit flavor show up better and brighter in your beer. Also, too many fruits can leave people confused about what they are drinking. You might like Bananas, Strawberries, and Cantaloupe on a fruit plate, but those might not mix well in a beer.
Pasteurize your fruit. Picking up apples from your backyard, crushing them and throwing them in your fermenter might feel like a throwback to simpler times and more "natural", but there is wild yeast, mold, and bacteria all over the fruit even when washed and throwing them into a sugary environment could create off flavors, or spoil your beer. Mash up your fruit and heat it to 160 degrees for 10 minutes to kill the unwanted microbes. Don't boil it though, this can kill some of the fruit flavor and coagulate proteins that can leave your beer hazy. Using a pre-packaged puree can let you skip the pasteurizing process, since it has already been done for you.
You can put fruit in your beer at any time during the process. The mash, the boil, the primary fermentation, or secondary. As a rule-of-thumb however, the later on in the brewing process any flavoring is added, the brighter that flavor will be during consumption, so there are some things to consider. If you want to simultaneously make your beer and pasteurize your fruit, you can add your fruit to the end of your boil (flameout, while cooling). If you don't typically do secondary, you can add pasteurized fruit to the end of your primary (it will ferment again for a bit and push out the oxygen that was introduced when adding the fruit). If you want to add it to secondary, you can add the pasteurized fruit to a sanitized bag and add it there too. The bag will help with clarity before you bottle or keg.
How much fruit to add is a popular question, however this is very subjective. What is a lot to one person may be too subtle for another. I would say that 2-5lb of pureed fruit is most common in a 5 gallon batch of beer, but remember, that the base style of beer matters too. While 2lbs of raspberries might be almost too much for cream ale, it may barely register as a fruit beer in an imperial stout. See the AHA chart for a good jumping off point.
Extract vs. actual fruit? Chances are, if you have had a commercially made fruit beer, there was no actual chunks of fruit used in the process. Most of the time a "natural" fruit extract is used. These can be quite good, and leads to a brightness or fruitiness that you might not be able to achieve with real fruit. Unlike actual fruit which contain high amounts of sugar, Extracts don't ferment, so there is an advantage to adding them right before you bottle or keg, so you can get the flavor level you want in real time. In my opinion there is a flavor base, or a backbone, that brewers get when using real fruit, that just can't be achieved with extract only. Conversely there is a brightness that just can't be achieved when using only fruit. Some of the best beers I have tried used a secret ratio of both... the secret is... it's up to you.