I get a lot of questions--hundreds and hundreds of them--but fortunately I have a great big pile of answers. Today's question comes from Rod from Missoula--Hi Rod! He writes:
"The label on my kit says it contains concentrated grape juice along with the other ingredients. How is concentration done to grape juice? Do you boil it? Doesn't that hurt the quality of the wine? Why do you use concentrate anyway?"
Those are most excellent questions, and get right to the heart of what a wine kit is: it's a way of keeping juice stable and fresh throughout the year so you can make wine any time, rather than just for a two-week period during harvest.
Fresh juices, made only from grapes that are crushed and pressed, and maybe treated with a little sulfite have a couple of drawbacks. First, unless you freeze them rock solid, they will always start fermenting within a few days of harvest, from the load of indigenous yeast they bring in from the vineyard.
If you pasteurize the juice, it's technically no longer 'fresh', but it will stop fermentation. What it won't stop is enzymatic and oxidative browning--single-strength juice will begin to brown pretty much right away, and will be over the hill in less than a month on the shelf unless frozen solid.
So how do we preserve juice? By adding concentrate to it. Concentrate is just grape juice with a portion of the water removed. When you take the water out, you're left with a solution high in sugar and acid, and very importantly, one with a very low pH that is heavily buffered--that is to say, even when you rehydrate a kit to full volume, adding back all of the water taken out of the concentrate, the pH will still be much lower than the original juice.
It's also so heavily buffered that when you blend a portion of the concentrate back into fresh juice, the overall pH plummets, which not only inhibits the growth of bacteria, yeast and spoilage organisms, it also prevents enzymatic browning and makes the antioxidant property of sulphite vastly more effective. Concentrate is its own preservative, and allows us to make and ship kits all year long, so you can make them all year long.
Vacuum distillation is the most common method of making concentrate. There are a couple of other way, like reverse osmosis or spinning cone, but they're usually much more expensive than distilling.
Vacuum distillation works like the opposite of pressure cooking: when you raise the pressure in a tank the boiling point of water goes up. When you lower the pressure, water boils at less than 120°F/49°C.
That temperature is low enough to prevent browning and caramelisation as the water boils off as vapor, leaving behind concentrated juice. Because some of the more delicate aromatic compounds get boiled off even at the lower temperature, we use a fractional distillation rig on the concentrator to recover them, and they're returned to the concentrate after processing, ensuring that it has all the goodness and flavor you expect from a Master Vintner kit.