Homebrewing beer is awesome. You can create a variety of flavors using off the shelf mixes, home made recipes, and copies of famous beers, but why not take it up a notch and create flavors that are off the beaten path and unexpected? How? Let's start by adding some sweet fruit flavors to your favorite homebrew mixes.
The kind of fruit, and how you add it, can vary quite a bit depending on your desired end result. Fresh fruit gives a more natural flavor, often times less sweet than canned fruit, because nothing has been altered during the processing and canning of the fruit. If local fruits are used, often times you'll end up with a one of a kind brew just because other regions of the globe don't share the same fruits as your local farms.
There are a few drawbacks to using fresh fruit however. Most fruits are seasonal, so if you find something you really like, you may have to wait months before you can brew it again, and that's never a good thing. Prep time is another issue. Washing, pitting, skinning, peeling, cutting up, mashing, etc. etc. all eat up precious brewing time, and often times takes up so much time you'll give up before you even get started. Lastly, all fresh fruits contain natural yeasts, sugars, and bacteria, none of it harmful, but definitely unpredictable at times, so the batch will require extra attention during the fermenting stages to make sure everything is working out just right. They can also add unwanted flavors and odors to the beer, which in technical terms, just stinks.
More predictable options are fruit concentrates, purees, or fresh juices. A few benefits here: 1. they aren't season, so you can brew the same type of beer all year long, 2. time, simply open a can or extract, add to your brew, and you're done. No muss no fuss, and 3. these products are packaged sterile so there are no extra yeasts or bacteria to change the flavor, smell, or color of your brews. If you are concerned about contamination, then this is the way to go.
When using concentrates, purees, or juices, keep in mind that you should only use products that contain 100% fruit with no additives. The additives can effect the brewing process, and we don't want that. On the upside, these methods often included added sugar, which won't harm the batch at all, however it may raise the alcohol levels slightly, but is that really a bad thing? It's best to avoid concentrates that contain citric acids as a preservative, as they can change the color and odor of the beer, and actually interfere with the yeast, and really throw the brewing times off in one direction or the other. Whatever you do, I do not suggest using jams or jelly, they work, but they often time cloud the beer due to the pectins added.
So, what's the magic amount to add to your brew? That's completely up to you, and part of the enjoyment of homebrewing. You can justify brewing multiple batches one right after another in the name of science and taste. The only way to really figure out what works for you is to experiment, and try different methods, with different fruits, and in different quantities. It's all very scientific. Brew the beer with the fruit, bottle the beer, taste the beer, make adjustments, and start the next batch with different amounts of fruit. As a general rule of thumb, you can start with 1/2 a pound of fresh fruit for every gallon brewed. Most extracts will give you an approximate measurement on the container.
One of the coolest things about fruit flavored beers is their unique color. In order to get the fullest color from your beer, you should store your fruit beer after bottling in as cold a state as possible. This helps to clarify the beer, and bring out the colors and aromas you're looking for. You should store the beer this way for a month if you have the time and space, to get the cleanest beer possible, and the best blend of the fruit you've added in, and the original beer mix flavors.
In the end, even beer that is an experiment is usually pretty good, if not truly unique. I've blended fruits with brew mixes with very unexpected results that were some of my favorites. The lesson here is there is no wrong way to flavor your beer for the most part, and the trial and error will not only get you to a place where you feel comfortable brewing flavored beers, but it will also help you to become a more involved, hands on brewmaster in your own home.