I'm embarrassed to say it took me five years to realize this. What classic beverage involves sweet fruits, alcohol, and sugar? Not liqueurs, but wine. Compared to wine, liqueurs are just a flash in the pan, a very recent indulgence in the grand history of alcohols, no more than seven or eight hundred years old. Wine is older than recorded history, and the techniques people use in wine-making today aren't a whole lot different from the techniques used in ancient Greece. We may use bentonite instead of ox blood and egg whites now, but the point is the same, and it hasn't really changed in three thousand years.
So, recently I started to think about the problems I'd encountered in my liqueurs. Mostly, they're concentrated around shelf-life. Nothing I make lasts longer then two or three years before spoiling. The spoilage occurs in a few different ways - some of them just turn rancid, others turn a sickly yellowish brown, others form small, solid clumps, and just for variety, my nectarine and strawberry always turn to jelly. It turns out that enology (the science of making wine) has quite a few lessons to teach about just these sorts of problems.
Why do I care about pectinase? Well, like I said above, I have problems with my nectarine turning to jelly. If you've ever made jelly at home (I haven't, but my mother and my girlfriend do), you'll know that what makes jelly gel is pectin. It's a substance, like gelatin, that forms a fairly solid, elastic mass when given a chance, and some sugar and energy to work with. It takes my liqueurs almost two years to form jelly, but they do it like clockwork. A much more complete and technical description of pectin mechanics and pectinase can be found elsewhere, and probably even on the net. But the sites I once had linked here have long gone away.
What I hope pectinase will accomplish is the complete removal of pectin from my liqueurs. Crystallized pectin is too small a molecule to be filtered out effectively, and even worse, it's partly soluble in water, making it nearly impossible to remove it mechanically. But a little enzymatic push could clear up all my problems entirely. I have not tried it yet, but the variety of fermentation environments that it's viable in is heartening. I've had some of it on hand, but I haven't been working with the right kinds of fruits recently, so the opportunity hasn't presented itself lately. Eventually, though, I'm sure.
The unobvious part of all this is fining. Most people will realize that careful and patient filtering will remove a lot of solids, and most people will see that liqueurs which have aged for many months undisturbed will have a fine mist at the bottom, which they can siphon around. But fining is a subtle, chemical system. Basically, you add a substance that attracts the solids and suspended proteins and such together until they form a clump which falls to the bottom. With some fining agents, this happens electrochemically, with the agent having a positive charge which attracts negatively charged particles such as grape tannin.
There are a number of different substances one can use, though you're far better off getting your fining agents from a serious wine-making store. Don't just grab some clay from your back yard and think you'll do anything but poison yourself. Anyway, there are a number of commercial brands of fining agents, and I can't pretend I know them all. Right now, I'm essentially reading from Winemaking, by Anderson and Anderson.
Historically, a wide and unappetizing variety of agents have been used for fining. As with most French cuisine, I wonder how anyone ever discovered these things. Ox blood, egg whites, milk casein, air bladders from fish, gelatin from horse hooves, seaweed, clay, and earth have been used. In fact, almost any protein will work at least somewhat, by binding to other proteins and forming solid deposits. I don't have any idea how the clays work, but obviously they do.
In modern enology, there are organic and inorganic fining agents. Many of the commercial varieties, like Claro K. C. and Sparkolloid, are mixtures of both organic and inorganic materials. Of the pure inorganic, bentonite is probably the best known. It's an aluminum silicate clay from Wyoming, which is apparently quite popular among Californian wineries. There is also a commercial product called PolyClar, which is (for what it's worth) polyvinylpolypyrrolidone. On the organic side, there is still gelatin, though not from horses hooves any more, and "isinglass," which is still made from the air bladder of the sturgeon. Many fining agents need to be filtered out themselves using a fine filter, which may present more problems. But any good wine-making supply store will be able to tell you a lot more about these substances.
But enough of what I don't know. What I someday hope to learn is that fining home-made liqueurs is perfectly viable, and will both clarify and help preserve my hard work. Like everything on this page, I haven't done it yet, but I expect to some day. Like the pectinase above, I'd probably toss in the fining agent two weeks before aging is complete, so I could filter out the fining agents and their precipitates in my normal filtration phase.
Well, it turns out that all this time I didn't know about pressure-fed filtration systems common throughout wine-making. What you have are round industrial filter pads an eighth of an inch thick (or half a centimeter) and eight inches across (twenty centimeters), through which you push your substance with either a hand pump or an electric one. It's intended for wine, which generally has very fine particles, and very few of them, but it can be pressed into the service of liqueurs, too, no pun intended. Really.
Basically, the pads are about a dollar or two each, and must be used in pairs, so you don't want to have to do too many runs. You need to filter your materials by hand fairly well first, but you can have real faith that when they come out of the pump filter, they will be pretty darned clear. I would expect to filter through the standard cheese cloth and jelly bag once or twice, and through paper towels twice. That's usually a fast enough process that I won't give up early. Then, instead of going on to coffee filters as I do occasionally now, dump the lot into the pump filter and let it run.
However, I'm aware that many liqueurs get their heart and sould from fine suspensions of fruit material. Using serious filters will be a ginger and cautious process, as one tries to figure out the proper balance to use between bad suspensions and essential ones. Each liqueur will be different.
Liqueurs are very different from wines in this regard, though. While liqueurs are certainly susceptible to oxidation over long periods of time, air is no immediate threat to them. Most living organisms aren't any threat either, being unable to survive a 40-proof environment. Remember, alcohol is fundamentally poisonous. In some sense, liqueurs are their own preservative. However, there is still something to be said for using sulphite when appropriate.
Sulphite is at the very least a popular sterilization substance. Wine makers routinely wash all their equipment and especially their carboys and bottles with a moderate sulphite solution. As I say elsewhere, it never hurts to have really, really clean materials.
Liqueurs are still susceptible to oxidation, but it takes a long time. Oxidation can be thought of as the process that turns fruit brown. Oxygen helps break the complex sugars and other neat chemicals down into smaller component parts. Once the skin of a fruit is punctured, oxygen gets in easily and starts breaking the area surrounding the puncture to brown mush. Essentially, the same thing is happening in liqueurs. Over time, and especially with bottles that aren't really air-tight, enough oxygen gets in the bottles to start turning the liqueurs into brown mush. Now, I'm still researching just what sulphite can and can't do, but I'm hoping it'll help combat oxidation in the long run. Ascorbic acid has also been suggested as a possible chemical additive to help combat oxidation.
Anyway, I'm unlikely to use sulphite myself, because I have at least one good friend who's allergic to it. And I'm really hoping that better bottles and filtration will make oxidation a non-issue for me. But if you suffer from early spoilage, you might well want to look into sulphites and other natural antioxidants.
Yeast is the most persnickety part of wine. It has to be coddled with the right percent of sugar in solution and the right temperature and acidity ranges, and it has to be kept well clear of oxygen. And when the wine is done, you've got to go to some great length to get rid of the yeasts so they won't make any more trouble. With liqueurs, acidity is completely unimportant, sugar is added just to taste, air is no immediate threat, and temperature is much more flexible. Naturally, the cooler the mixture is the longer it has to steep, and it's possible to have it too warm, but there's nothing that requires you to keep it right at 60, for instance. You just dump all your stuff together, let it sit for a while, filter it out, sweeten it, age it and bottle.
What does all this mean? Well, first and foremost it means that most of the troubleshooting advice you'll get from wine-making isn't really applicable. The big problems you encounter in wine-making involve starting, maintaining and stopping the fermentation. Liqueurs, by contrast, are amazingly easy, being in a sense just glorified drink mixing. But there are areas of overlap, like filtration, clarification, bottling and sterilization. I recommend heartily becoming conversant in wine-making techniques, if for no better reason than being able to recognize when those techniques will benefit your own efforts, and when they won't.