Father David,
Thanks for the answer (and the correction on the use of the word “port” - I ought to have been more careful with my terminology

).
I am still not certain, however, that I understand what it is about most ports that is not sacramentally valid. Is it the higher alcohol content you mentioned?
Yes. Most (not all, but most, at least those that are typically imported to the U.S.) have an alcohol content beyond what the Church permits.
I mentioned this a few posts ago (but it can get lost). Yeast cannot survive in an environment above 18% alcohol (this is a generalization of course, some strains have less tolerance). That means, that when making wine naturally—meaning, just squeeze the grapes and let the juice ferment without adding anything else–the yeast will turn the juice into wine by making sugar into alcohol, but once the alcohol content reaches 18% the yeast dies (because the alcohol kills it) and whatever sugar remains makes the wine sweet. That’s why the Church has a maximum content of 18%. There is no theological significance to the number, it’s simply the number that the Church uses by observing the natural processes.
Port Wine adds additional alcohol to the wine (by various means). If that wine exceeds 18% alcohol, it no longer meets the Church’s standard for valid matter. Most port wines do exceed 18%.
Keep in mind though that this is just one criteria. Someone might make a wine called “port” that uses alcohol from distilled grains or potatoes. That’s why the Church (again, usually this is the local bishop) has to investigate every step of the process of making any particular variety of wine. One vintner might make a “California Port” that’s valid for the altar, while another company likewise makes a “California Port” but that one is not valid matter.
Another criteria is fresh grape juice. Adding fresh grape juice means that the wine is not acceptable for the altar. In the U.S. vintners are permitted to add small amounts of fresh juice to a wine to make it sweeter. They often do this because they want one year’s bottling to be consistent with the previous years’. If it’s a small amount, they can do this
without disclosing anything on the label. That’s why I keep insisting that one cannot simply read the label.
Modern day winemakers have all sorts of techniques that they use to produce good quality wine. In biblical times, they did little more than squeeze the grapes, store the juice, and let nature take over, even though they did have different ways of doing that that produced different wines (like storing the juice in clay vats or storing it above or below ground). Most modern winemakers prefer a different variety of yeast over that which grows naturally on the outside of the grape. That means they must first kill the natural yeast before adding the preferred strain. That’s just one more example of how some process might (I say might) exclude that wine for use at the altar. [As an aside, substituting the yeast is permitted].
Someone who understands the Church’s laws and norms for valid matter must contact the vintner and inquire into every single step of the process before knowing whether or not any particular version of wine is valid for use at the altar.