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The coffee package I've bought has 'strength rating'. It's 3 out of 5.
I wonder what that 'strength' means? How it is measured? Does it relate to the caffeine amount?
The coffee package is from Germany, and from what I've seen, all coffee sorts have this rating, so it's a kind of standard.
Coming from Germany myself I have seen this rating only on a single brand of coffee. There is no standard for this (even though us Germans seem to like standardizations), but basically strength of the coffee is about the inherent bitterness and caffeine content you get out of it.
The stonger a coffee the more caffeine and also the more bitterness you will usually get.
When people talk about strength they often confuse different things like the flavor intensity, caffeine concentration, roast grade, how full bodied a coffee tastes or how much bitterness it contains.
If someone orders a coffee at a corner bakery and asks for a "strong coffee" they probably mean a high ground coffee to water ratio and a lot of dissolved caffeine. If someone complains in the same bakery about coffee that is too strong they probably mean that it is too bitter.
If a coffee professional is talking about coffee strength however, they are most likely referring to the total amount of dissolved solids (or TDS) which is measured with a refractometer. An espresso ranges somewhere in between 7% and 15% while a filter coffee is somewhere around 1,5%. That means an espresso is 93%-85% water and a filter 98,5%. The rest is what has been extracted from the coffee. Keep in mind that this is not to be confused with extraction. Higher extraction (bitter coffee) doesn't make a strong, but actually a "weak" coffee. Because you are running more water through the coffee the TDS will start to decrease at the point where the next ml of water is extracting less of the solids than the ml before. That means that in terms of flavor "strong" means often the opposite of what people would actually associate with a strong coffee. Holding the dosage constant, a strong espresso (too little water, thus something like a ristretto) would taste kind of sour, salty and unbalanced. A weak espresso (too much water) would taste bitter. The point is, strength refers at first not to flavor but to the TDS which is determined by your coffee preparation and not by the roasting. The same coffee roasted darker is not stronger, it just received more heat.
Now, your package of coffee probably refers with it's "three out of five strength" to the roast grade (probably a medium to dark roast) which is just misleading marketing in my opinion.
Some additional reading:
Analyzing espresso recipe strength and Espresso Recipes: Understanding Yield
It's marketing jargon. In this case, it likely refers to the darkness of the roast, as there is often confusion about difference between darkness of the roast and the strength of a cup of coffee.
In truth, coffee has no "strength" until it is brewed. And then, strength is determined by how many solubles you have dissolved into the water. If you want stronger coffee, use more grounds per measure of water. Over-extracting the coffee will make it stronger, but it will also make it more bitter and less pleasant at the same time. (If you're interested, here is a more detailed explanation of this).
The Starbucks-owned brand "Seattle's Best Coffee" uses numbers on its bagged supermarket coffees, too. I equate it to the stars you can use to order Asian takeout with more or less spiciness. There's no set scale I've ever found, but it's a general way to tell the consumer where on the spectrum each product (roughly) lies.
