Eric,
At the start, you take a free run juice sample, this is your reference point. As you increase pressure, at each new point, you take another sample and taste it next to the reference one. You cut when you feel that what is being squeezed out is at your threshold of tannins/“bitterness” taste preference (we all have different ones, of course).
Personally, I am with Ed and John, what I do is pretty much free run juice and although I know I am “wasting” some juice, its a style preference. We see about 130-140 gallons per ton, but as some already mentioned it also depends on the vintage characteristics: berry size, thickness of skins, etc. It also, to some degree, depends on the press being used as well.
Interesting thread indeed, and it’s pulled in some true ‘experts’ . . . I like it!
I am NOT an expert as it relates to this, but this is what I’ve experienced thus far:
I work with both larger scale bladder presses and a 5 ton Hypac Basket Press. I’ll address the Basket Press to start with:
We use this for all small lot pressings - oftentimes taking box ferments and pressing them separately, but over each other. We taste press cuts, but in all honesty, rarely EVER separate the fractions . . . this puppy does press ‘gently’ and rarely leads to overextracted, ‘bitter’ fractions . . .
Now onto the bladder press:
With whites, we ALWAYS separate press fractions - too much phenolic pick up in later fractions / pH shoots up too high / acid levels drop too low. We’ll ‘treat’ this separate press fraction a little more aggressively and either blend a portion of it back in later or blend it into a larger blend . . .
With reds, we also taste press fractions on a regular basis, but just as with the bladder press, we’re amazed at how even ‘hard press’ fractions are NOT overtly bitter/phenolic/harsh . . . .
Just my $.02 . . . and I am certainly NOT Ed K!
And with regards to yields, we normally get anywhere from 145 - 175 gallons / ton from our basket press . . .
Cheers!