Hurricanes have the Saffir/Simpson scale. Wind has the Beaufort scale. Hot peppers have the Scoville scale. Sour apples? Not so much.
Confusion abounds around the topic of sourness. The flavor is related to the pH of the item, but it's not a simple one-to-one correlation. A corrosively powerful acid can taste about as sour as white vinegar. Here's some science background from the vineyards of Robert Mondavi and from the North Carolina State Department of Food Science.
Additionally, there's a well-known* problem people have tasting a difference between sour and bitter flavors in the United Kingdom, New Zealand, Australia, and North America.
*And by well-known, that is, it's well-known among sensory professionals.
Here's one article about it. Evidently, our cuisine doesn't expose us to a great deal of bitter and sour flavors.
Abstract statement: We don't know from sour.
And here's another article about human tastes, because I happen to know you want –– ahem –– an extra sensory-professional notation.
This year, there was undeniable progress: larger apples, happier apples, but alas...not-quite-ripe apples.
As much as we wanted the fruit to be ready to sample, it was not. We picked dozens of apples, trying to continue identifying the varieties.
Sour apples need no explanation, surely.
Nearly any apple has some amount of snap, from the tart, lemony Granny Smith apples to those sweet, crispy Honeycrisps. (Let's take a moment of silence for the vile, corky, leather-skinned abomination that is the Red Delicious, an untart apple. Blech.)
But a green apple plucked warm from the tree?
Lacking a scientific rubric to describe degrees of sourness, we were forced to make up a rating while perambulating the groves.
Some was nearly Edible. Surprise surprise! One side or two of an apple high up on a tree was nearly ripe. Juicy and tart like the first cup of fresh cider.
Some were, we might say, Zippy. Too tart –– still apple-ish, but unpleasant. It's hard to imagine that this fruit will ripen. And Sour is only sour. No amount of wishful thinking can reverse the instantaneous prickling of sweat on the forehead after a nibble.
Really sour. Mouth-shrinkingly sour. A hard bite of fruit that brings on a powerful thirst and the wish to wipe the flavor off one's tongue.
And the most unhappy of fruits, which seemed frankly Inedible. These were dry, pithy samples that that tasted like lime and copper. As sour as a battery. Nasty-sour. Happy-to-let-the-wildlife-eat-it sour.
Another year without a satisfying harvest of apples. Farming is a long game, with a long learning curve, especially for part-timers.
Too late one year, too early the next... one year we'll be on hand to enjoy the alchemy of those last few weeks of sunshine and cool nights turning sour green apples into something wonderful.
PS. Okay, okay, one last dash of extra sensory-perception information: Music and taste. Can you taste music? This from Oxford U.