They present a few problems from a brewing perspective so it’s not a trivial task. Fortunately, one of the guys on my MSc course wrote his dissertation on fruit beers so I got a copy of that and I’ve done some reading around and I think I have worked out how to manage it in a practical way on our scale.
Leaving aside basic questions like the recipe formulation (What fruit to use, how much to use? whole or crushed? fresh or frozen? pureed fruit? or even fruit extracts? What should the base beer be like? etc) there are a number of specific technical issues to consider.
There is the question of when to add the fruit to the beer. At its simplest, the fruit can be added either before the boil or after the boil. Before the boil, it can be added to the mash tun which helpfully filters the fruit pulp out, but risks not much fruit character making it through to the finished beer or added to the kettle whirlpool before cooling the wort. Because the wort is still piping hot at this stage, there is no danger of infection, but apparently the high temperature also lends the fruit a cooked flavour, which isn’t always desired. Also, there’s the fact that flavour compounds are usually fairly volatile and can be driven off by the steam.
I think we should add fruit after fermentation at the conditioning stage. After fermentation, the volatiles won’t be purged out of the beer by the CO2. Also, the beer will also be mildly alcoholic which discourages infection as well as being relatively nutrient poor because the yeast have already eaten the good stuff.
Infection is a big danger because there is no easy way to sanitise the fruit before we add it to the conditioning tanks (heat will damage the fruit, acid/chemical sanitisers wouldn’t be very effective because the fruit affords too many hiding places for the bacteria and anything that would work like sulphides we’d have to declare on the label). I think the best thing to do is simply to keep the contact time very short (less than a week) and then filter the beer to remove all the yeast and bacteria. That should ensure a reasonable degree of stability in the product.
The other tricky thing is how to calculate the ABV. Adding fruit to the beer naturally also adds fermentable sugars, so they have to be taken into account in calculating the ABV. Fortunately, I think there’s a fairly simple procedure for doing this. I will simply measure the initial gravity of the base beer and then its final gravity after fermentation and before I add the fruit. From that I can calculate an ABV. Then I will add fruit and measure the gravity again. The gravity should increase due to the increase in fermentable material. Finally I will measure it after filtering and calculate a new ABV from the change of gravity over conditioning and then add the two ABV values together.
There are a couple of further process issues such as how to handle the fruit in the conditioning tank and how to ensure the fruit doesn’t clog the filter, but I think I can get solve these too.
The funny thing about fruit beers is that they aren’t really sweet and fruity. If you want a sweet fruity tasting beer, then you want one made with lots of crystal malt for the sweetness and fruity flavoured hops like simcoe (oranges), cascade (grapefruit), centennial (lemon). The sweetness goes because the yeast converts the sugar to alcohol. Fruit beers tend to taste tart and dry. The trick is to achieve this without producing something mouth puckering. I’m not sure whether I can quite pull it off. I think I’ll use sweet and fruity hops anyway to guard against astringency.
Still, it may not actually happen. The boss looked a bit taken aback when I said these fruit beers are usually >12% fruit by weight, which would make them pretty expensive to produce.