There is a long held debate on whether Nespresso is real espresso or not, and how does the coffee produced by a Nespresso machine compare with what a proper espresso machine can produce. Some have chimed in on whether Nespresso constitutes real espresso before, so there's no need 'to repeat that here. You can read about whether Nespresso is actually espresso at Nespresso is real espresso if you are more interested in that and come back here later.
What I am more interested in discussing here instead is how actually Nespresso coffee tastes in comparison with an espresso from an espresso machine and other brewing methods. There's a lot of discussion that is mostly superficial, dealing with sensory parameters and not with actual measurable ones. I do agree that in the end how a coffee tastes like is the most important aspect of them all, but it is not beneficial to the discussion to simply assert "I like Nespresso" or the contrary. It only tells your particular opinion, fully respectable as it is.
I would like to know more in depth how the extraction of coffee made with a Nespresso machine compares with the extraction coming from an espresso machine, or other similar methods. From that we could assert how well the Nespresso extraction method works, and how better or worse is compared to others.
Nespresso themselves have made a quick comparison, which again is mostly superficial, and you may say also skewed in favor of their pods. I'd like to see a 3rd party scientific study instead. Much more objectively and data-driven conducted.
Of course, no single study can give us an objective truth as the factors at play are innumerable. Multiple studies will be necessary to make a good Nespresso vs espresso comparison. Few have been made, if at all. Nespresso isn't usually involved in these studies that tend to compare manual brewing methods like pour overs, French presses and Aeropress, or more classical methods like filter coffee machines, Moka pots and fully or semi automatic espresso machines.
Insight from a Swiss study
Luckily a study from 2013 analysed 9 different extraction methods, including a Nespresso machine. Some very interesting facts and data can be extrapolated from it (and I did it so you don't have to read it).
The study is by Petracco, M., et al. “Comparison of Nine Common Coffee Extraction Methods: Instrumental and Sensory Analysis.” _European Food Research and Technology_.
In it, 9 different extractions of coffee were compared: two espresso machines, one semi and another fully automatic (Dalla Corte Evolution 20.03 and Schaerer Coffee Celebration BC, respectively), with both an espresso and a lungo size cups, a Nespresso CitiZ, a Bialetti Moka pot, a Bodum French Press, a Bayreuth coffee maker and filter coffee with a Hapag Aarau a140. The analysis comprised measurements of headspace analysis, acidity, titratable acidity, fatty acids content, total solids, refractive indices, chlorogenic acids content and caffeine extraction.
A long list of technical names that are measurements of the components of ground coffee that generally get extracted in our cups when brewing it. Different methods have different capabilities of extracting, generating vastly diverse results in taste.
Regarding the actual taste profile of the coffee, the study implemented 7 panelists to do a blind taste over 2 different days. Coffee was served at 60-69°C both times, and the sensory parameters analysed were: crema (consistency and thickness), visual (mostly color and visual structure of the coffee before drinking it), texture/body, aftersensation (what remains in the mouth after a sip of the coffee). Last but certainly not least, the taste profile was judged and rated. Important to specify that the study considered a coffee with balanced acidity, quite fruity, with a good amount of roastiness and coffee-like bitterness to taste "better". If you have different tastes, you would have probably rated the 9 coffee much differently.
The coffee used was for every method a Guatemalan Antigua La Ceiba. Of course, but for the Nespresso pods, as there's no such type of pod available. An old Arpeggio capsule was used instead.
The results are clear: Nespresso is really, really close to an espresso
The images themselves are clear (DE and SE are the espresso machines, NE is Nespresso, Bia is the Moka pot, DL and SL are the lungo made with the espresso machines, Bo is the French Press, KK is the Bayreuth coffee maker and F is filter coffee):
The above graphics show how a sample of 10ml of each coffee brew does in terms of extractions. Nespresso is constantly the closest to the level of extractions of the two espresso machines. Notable exceptions are the fatty acids (much lower in Nespresso).
The results are similar when measuring the same elements when extracting a full cup of coffee:
Again, Nespresso is always slightly below the level of extraction of the two espresso machines. The longer brews (DL, SL, Bo and KK) show unsurprisingly that they can extract more elements than a simple espresso brew, whether this was done with a true espresso machine or a Nespresso machine. This was to be expected as by using more water to coffee we both increase the quantity of it that goes through the grounds and the extraction time. It would have been interesting to have tested a lungo capsule as well and see how it would have compared here, but that was not done.
A blind test of the aromatic and taste profile was conducted by the study as well. A profile of each brewing method and type is given below:
Following the more quantitative results, also the sensory tests show how a Nespresso machine is the closest to the espresso ones. The taste profile of a coffee brew with a Nespresso pod is the most similar to the one brewed with an espresso machine. Nespresso has even more crema than a classic espresso, but of a less fine texture. More frothed than what an espresso machine can produce, which is closer to a "creamy" state. You can also expect to have the most roasty and bitter flavors in a Nespresso or espresso coffee compared to the other tested methods. Similarly, both Nespresso and the two espresso machines produce the most intense aftertastes, along with the Moka pot.
What does this all mean? In poorer words, you can expect a Nespresso cup of coffee to taste the most similar to an espresso at a café. Which is what Nespresso aimed all along, bringing the espresso experience and taste of a café at home.
Thus yes, Nespresso is really close to a "true" espresso, made in a café with a proper espresso machine. Not identical, but really close. This not only in terms of quantity (how much of the aromatics and solids from the coffee grounds are extracted) but also in terms of quality (how actually the coffee tastes like). It is correct to say that an espresso machine brews "better", as in more extraction and a somewhat more intense taste; it is not correct to say that Nespresso is much far from a true espresso experience, though.
I of course would love to see more studies like the one I just cited. Especially some conducted with a couple of different capsules, and that would measure Nespresso against other popular brewing methods. A comparison of Nespresso coffee against Aeropress and a good pour over like the V60 would be extremely interesting. But that is outside of what Nespresso tries to be, which is a good espresso at home. As far as the data tells us, Nespresso is perfectly comparable to a good espresso.