Yep, that was pinto. And very informative/helpful.
Preface: I keep a little journal on certain subjects (this being one of them) and over time I update it as I gain more experience/knowledge. Recognizing up front that everything I learned on this comes from others..including here..the info herein is not "new", but perhaps a slightly different way of looking at the same thing. This is how I made sense of everything and put it together. Perhaps it will make sense to someone else too? Or perhaps it will just confuse things all the more
Cuts:
On an average, I keep about 1/3 of what I collect as "hearts/middles" whatever you want to call it. I have found this to be consistent regardless of the volume of the starting mash/wash I use. I'm aware others apparently keep more than this on average. Purely a judgment call on my part but may have something to do with overall volume of run (ie., how stretched out the transitions are).
On an average potstill run, I find that the solventy/sweet heads "slowly" fade to reach maybe 1 or 2 jars of clean middle. Then immediately it starts transitioning to tails (slowly). Its like a bell curve with long ramps on each side up from heads and down from middle through tails (with the middle being narrow). So, if you are "only" keeping those jars directly in the middle that have no hint of heads at all, and no hint of tails....you aren't going to be keeping very much. Effectively, everything will be smeared together to some degree or another as it transitions into and out of the absolutely middle. And the job is to keep that which will blend together nicely. I generally find that if I'm making it for whitedog drinking, I keep it much cleaner than if I'm going to put it on oak. Completely clean middle on oak tastes like oaked vodka. You need some flavor to make it work..so it's good to keep in mind. The trick is..if you go "too" far in what you keep, no amount of oaking is going to clean it up.
Heads:
In my experience, the heads phase is fairly consistent, but tricky. When you first smell what comes out (like finger nail polish) that solventy smell lasts for a while..and fades linearly as it goes. The problem is, late heads are "sweet" too. The chemical taste/bite is usually buried in it as it has faded. This is where you need to be careful. A good airing and diluting it with water usually brings out any "chemical" tastes that may be buried in it and makes it easier to spot. The trade off here, is that late heads are sweet and full of flavor congeners, but they also have a "bite" to them and can be "hot" or slightly chemical tasting. This is the hardest cut to make in my opinion, as the next jar is always a little cleaner as it goes. I usually try to just find the absolute middle "first", then backtrack into the heads. I "always" make heads cut after airing and using water dilution.
Middle
What I'm calling "middle" or hearts..are ONLY those jars that are completely clean, slightly sweet, with no trace of heads or tail transitions in them. I believe, based on numbers, what some people call "middle" or hearts would include a good portion of transitional heads and tails that are drinkable. But, for the sake of learning, this is the way I look at it because this is the way you are going to encounter them. On a 5-10 gal potstill, with no reflux or doubler, there are never that many of these. In my experience, increasing volumes of wash/mash will yield more and longer transitions to and from this absolute middle point, but the absolute middle is always small (as a percentage of the total collected). These few middle jars are always collected so that's not the main point to make about them. The point here is that finding them can also help you find where heads "end" and tails "begin". This is another reason I define them this way.
Tails
It's the tails that are more involved as it goes through 3 different distinct phases. As you go from the sweetness and sharpness of heads into a milder short middle, the first distinct thing you'll notice that changes is the
flavor. You'll need to "taste" to find this.
1st phase: Early tails are still sweet. But, this distinctly "different" flavor begins to emerge. It's faint at first...and has a kind of "bourbony" taste that's not objectionable. There are any number of different ways to describe this taste, you'll find your own, but the important part is that you recognize the "emergence" of this taste. While the solventy tastes/smells of heads "fade out" leaving a clean middle, this is a taste that more "emerges" or "fades in" and it's a distinct change in direction.
The 2nd phase of tails is where this initial new taste becomes stronger, more objectional and the distillate begins to loose its sweetness and become more bitter. A smell/odor begins to emerge or fade in that is somewhat like dirty socks, or a moldy kind of smell. In my experience, it is at this point, that a jar or 2 can become very oily, slick in feel and "really" burn and grab at your throat and make your stomach kick if you taste it. It's like the rather pleasant bourbony taste becomes burned and bitter and the distillate tastes like it's been sitting in a dirty mop bucket. Add to that a chemical taste and you won't have any problem knowing to leave this alone.
The 3rd phase of tails is where everything is headed to water. The distillate becomes much more watery and the burned/bourbony (some say cardboard or wet paper) taste actually becomes kind of "grainy" and less objectionable. If you want to accentuate the "grain" taste of your drink, this is something you can add a little back in. However, you have to be careful as it can also carry the left over "dirty sock" smell and a moldy vibe to it. If you use too much, its something you can't get rid of even with a long oaking.
The above descriptions are what I've come to know. Everybody has their own way of describing any of the various parts of it. But, if you line up all your jars end to end, and go through them sequentially, and keep these general points in mind, you'll not only learn what the difference is between heads/hearts and tails, but you'll have a better understanding of how to blend them. I didn't include abv percentages, or temps, or etc...because I've found that they vary and are dependent upon what kind of still you have and what you are running in it. I've had "tails" at 68% before on my hybrid still.
Because of the nature of heads and how they fade to the middle, I've always found this cut the harder one to make. I usually try to collect in equal amounts to try and follow the transition along. But, if I miss, this is the one I usually miss. Sometimes airing out will fix it some, sometimes it doesn't. Just depends on how bad I missed it. Adding some transtional heads in will boost abv/bite/heat and overall sweetness. The tails cut is much easier for me to find..given that it's an"emergence" of a new taste that is fairly obvious if you pay close attention. For my whitedog, I generally stop as soon as I detect the emergence of this taste and find that after airing, its just right. If I'm making UJSM, bourbon, or brandy, I generally go further...so long as its still sweet. This adds body, mouthfeel, depth of flavor and really adds to the nose so long as the oaking can take care of it. If you go too far, the taste will be a little moldy/muddy and it will have an "off-nose" that's kind of musty and smells of dirty socks. So, I generally loosen up my cuts if I'm going to oak something. It takes a min of 3 weeks on oak cubes before it starts to loose any skunkiness from tails and really starts to open up.