What is the best method for preparing different fruits for fruit additions into secondary fermentors?
There are issues of sanitation and good fruit to wort contact, via chopping, slicing, mincing or puree.
Whenever possible I like to use Oregon Fruit Purees. These purees come in large cans that have been flash pasturized already. The fruit is also in a puree format so there is not additional prep and fruit to wort contact is superior to slicing and dicing the fruit. Also for seeded fruits like strawberries and blueberries, much of the seed material has been removed which makes for a cleaner fermentor with less stuff to settle out.
When I can't find a puree that I am looking for I'll sanitize fruit at home myself with mild heat. If you boil fruit you stand the chance of release a significant amount of pectin from the fruit which can lead to excess haze. Not a big deal with certain styles, but if you want to make a clean and clear american wheat style fruit beer the haze can be annoying.
My process is to usually macerate my washed fruit with a little DME, maybe 1:10th of the total weight of the fruit to be added. I'll slowly add heat to the mixture until it gets to ~165F stirring as it heats up. Holding that temp for 15minutes has done a great job of eliminating any noticable bacterial or wild yeast contaminations in the beer in secondary.
Lastly, to help battle any pectin that is released I use Pectic Enzyme or pectinase. This is an enzyme the digests the pectin that does get into solution, thereby preventing the haze caused by pectin. Pectic Enzyme is available at most homebrewshops and certainly findable on-line.
Chop it up.
Let warm up.
Add to secondary.
There are issues with natural yeast and bateria in the fruit. Freezing it will kill most of these. The low pH of the beer, and alchohol will help to prevent the rest from taking a hold in the beer. Make sure to account for the extra water the fruit will add. With large fruit like pears I cut it in strips, stick it in a bag and freeze it. With berries I freeze them, then run them though a food processer. The freezing will also break up the cell walls of the fruit allowing you to extract more flavor.
Freezing doesn't kill the bacteria. It will kill some of it, but not all. However, if you're adding it to the secondary, that's about as good as you're going to get.
I just made a Pomegranate Tripel, and added the poms in JUST after the boil. If you boil the fruit too many pectins come out, but just afterward and the water will still be hot enough to clean the fruit off. Came out real strong, and the sweet/bitterness of the poms offset the MASSIVE spike in ABV.
If you want to use fresh fruit you'll need to freeze it and then thaw before using. Not only does freezing kill of a lot of things you wouldn't want in your beer, it also helps break down the cell walls so it's more easily incorporated. When to add it is a whole other question and depends on the specific fruit and how much flavor you're looking for. Some fruits do well added to the secondary (generally those with "extreme" flavors - very mild or very intense) and some do well added right at the end of the boil. As a general rule you do not want to boil the fruit because it will create a pectin haze. For my chocolate chili porter I add the chilies to the secondary. Fresh chilies get frozen and thawed and dried chilies get some boiling water poured over them for a quick sanitizing soak before adding to the secondary. Then I taste test every day or two and in less than a week it's ready to bottle. The cherry beer and cranberry mead I made recently both had the fruit added at the end of the boil. The fruit was frozen, thawed somewhat, and then added at the end so it brought the temp down to around 160, which will pasteurize the fruit without setting the pectin. The cherries (beer) remained in the primary for about 2 weeks before I racked to the secondary and the cranberries (mead) were in a week or two longer.
I froze pitted cherries in Ziploc bags until I was ready to use them. Then I thawed them, and heated water to ~190 degrees and immersed them for 20 minutes. Over 185 degrees to sanitize the cherries, but under 195 to avoid melting the Ziploc bags.
After I cooled them, I racked onto the cherries in a paint strainer mesh bag. Still in secondary so no results yet.