Goodness, I’ve never answered two questions at the same time before! I decided to put the two of you together since your questions were so similar. There are some things that apply to you both, and then I have some specifics for each of you.
I love your ideas about freezing grapes for later use and I’m sure that commercial wineries would love to do the same, only, alas, they are prevented by their larger scale and that you probably couldn’t find freezers big enough. Three cheers for the benefits of being a small-scale micro-vintner like you. Absolutely you may freeze grapes as you like, then bring them out for adding to batches later in the season. Freeze all you have space for, and then thaw, crush, and ferment at your leisure, when you want.
Your idea reminds me of one of the ways we used to keep the cellar busy (and some fermenters full) almost year-round when I worked at Randall Grahm’s Bonny Doon Vineyard. We made a very popular raspberry dessert wine called Framboise, which essentially was a sweetened fruit and brandy infusion. Did we do it when raspberries were at their peak of ripeness, just before the grape harvest season? Heck no. We froze all those raspberries up in Oregon and Washington, then shipped them down to the winery in Santa Cruz, California when we felt like making a batch, usually February, March, or April. That way, we kept the tanks, pumps, and hoses clear for the crazy grape harvest August through November and were able to use the equipment in the off-season.
You do want to avoid freezer burn and oxygen ingress and keep a few things in mind. Here are a couple of tips:
- For red grapes, destem before freezing.
- Depending on volume, destem into 1-gallon (3.8-L) freezer bags, label with variety and pick date and store with all air excluded (vacuum seal if possible).
- Larger amounts of destemmed grapes can be stored in a topped-up plastic pail with lid in a chest freezer.
- Wrap or seal packages of frozen grapes tightly to exclude air. Press air out of plastic bags and lay plastic wrap over the surface of grapes stored in pails.
- Thaw completely before use. Cold grapes don’t do well starting fermentation. Grapes need to be at least 55 °F (13 °C), preferably 60 °F (16 °C) before pitching yeast.
- Use the grapes within a year. Storing anything too long in the freezer will degrade quality.
If you want to just save a few ripe clusters and add them a week or two later when the rest of the grapes ripen and are ready, just be sure you measure the sugar level (degrees Brix) of the frozen batch so you have an idea of when you’d like to add them to the other grapes. As I mentioned above, be sure that you have your frozen grapes up to a decent, warm-enough temperature before you combine them with the other grapes, otherwise it’ll have a distinct chilling effect (sorry, I had to write that) on your fermentation.
Similarly, do be aware that any microbes will be depressed if not outright killed by the freezing process. This could be a good thing, if you’ve got microbes you want to discourage, but it could be a problem if you’re relying entirely on your indigenous yeast population to carry your fermentation.
Especially when using all frozen grapes, be sure to feed your must or juice adequately to help make up for nutrients that might be degraded during the freezing process. It’s best to read manufacturer’s suggestions for the recommended dosage rate. Happy Fermenting!