cooking gnocchi for different durations

Are Gnocchi Cooked When They Float (and Why)?

Some cooking instructions are so common, that you kind of assume them to be true. However, just because most recipes on Google tell you something, doesn’t mean it’s true, unfortunately. There are enough statements that are just copied along and not tested, making it hard to find proper ‘evidence’.

So, when someone asked us whether it is indeed true that gnocchi are cooked when they float, we decided to put it to the test. After cooking a lot of (small) batches of gnocchi, our results might surprise you. Yes, our gnocchi were cooked when they floated, but, was floating the ‘signal’ for them being cooked? Or are we all mixing up correlation and causation?

Introducing gnocchi and the question of floating

Gnocchi are small, light, pillowy “dumplings” or “pasta” (they seem to be grouped in various different ways) made from mostly mashed potatoes. The mashed potatoes are mixed with flour (and maybe some salt and egg) to form a dough. This dough is then formed into small, cylinder-shaped pieces (about 2,5cm long by 1,5cm), often with a grooved surface.

A lot of, if not all, recipes we came across call for cooking these potato ‘puffs’ in boiling water until they float to the top. It could take anywhere from roughly 1 to 5 minutes for the gnocchi to float and thus be cooked. After cooking them in boiling water gnocchi might still be pan-fried, to create a brown crust, it generally doesn’t play a role in ‘cooking’ the gnocchi.

Cooking gnocchi: our reference

In order to test whether the floating of gnocchi indeed means that they’re just cooked, we cooked a lot of gnocchi. In order to make these experiments as reproducible as possible, we decided to use store-bought gnocchi. We’re not gnocchi-making experts, making our own gnocchi would probably lead to so much variation between gnocchi, that it would be hard to compare cooking conditions!

Our gnocchi were from the DeCecco brand and as a reference, we followed the instructions on the pack to cook our gnocchi. The pack told us to cook them for 2 minutes in boiling water. It also mentioned that the gnocchi should have floated to the surface once fully cooked.

To keep the water temperature up high, we only cooked small (that is, 6 little gnocchi at a time) batches of gnocchi in 1 liter (approx. 1 quart) of boiling water. All gnocchi were kept at room temperature before being cooked. Time and time again the standard gnocchi showed a very similar cooking pattern:

  • The gnocchi would sink to the bottom for the first approx. 45 seconds. They’d move around on the bottom, but stay down low.
  • Next, the gnocchi would start to bounce up and down a bit. They might come floating to the top and then sink again. Also, one half of a piece of gnocchi might start to lift up, while the other half remained down at the bottom.
  • Then, at 1 minute and 20-30 seconds, all gnocchi would pop up and float completely. This time point was very consistent, over and over again.

Having set and investigated our reference, it was time to dig deeper: why do these gnocchi float AND does that coincide with them being cooked?

pan fried gnocchi

Hypothesis 1: Gnocchi float because of buoyancy

It is well documented that at some point gnocchi will float to the surface when cooked in a pot of boiling water. Why that happens though is less clear. Our first hypothesis, which was suggested on an online forum, revolves around the role of gas bubbles in boiling water.

When water boils water at the bottom of the pot, where it’s hottest, evaporates and forms a gas bubble. This water vapor bubble then travels through the boiling water to the top, to escape. Others have hypothesized that these bubbles carry the gnocchi to the surface and cause the gnocchi to float.

Our observations

An immediate concern we had with this theory is that if this were to be true, raw gnocchi should float as well, as long as the water boils. However, as we saw when cooking our gnocchi, it very consistently remains at the bottom in the beginning, even if the water is boiling vigorously.

Also, if this were to be true, vigorously boiling water should result in more (or faster) floating gnocchi. However, no matter whether the water was just boiling, or boiling vigorously, the gnocchi floated at the same time.

Lastly, to completely debunk our hypothesis, we turned off the heat under the pot once the gnocchi were boiling. If it was just the bubbles holding the gnocchi up, they should sink almost immediately. But that was not the case. On the contrary, the gnocchi would remain floating for another 2 minutes or so. By that time the number of rising popping bubbles had long been reduced down to zero.

Conclusion: Hypothesis 1 is Incorrect

What happens when cooking gnocchi?

Clearly, more is going on than just gas bubbles pushing up a piece of gnocchi. To better understand, we need to have a look at what happens when you’re cooking gnocchi. Gnocchi change during cooking and these changes clearly need to be taken into account somehow when explaining the floating vs. non-floating phenomena.

The Ingredients

Starch & Moisture

First up, the components at play in gnocchi. As mentioned at the start, gnocchi are made out of mashed potatoes, wheat flour, and maybe some egg. In order to make those mashed potatoes, they have been cooked on forehand, to soften them. The other ingredients though are all raw when incorporated into the dough.

Both mashed potatoes and flour contain a large amount of starch. Mashed potatoes also contain a lot of water. Also, because the potatoes have been mashed, a lot of the individual potato cells have been broken down or torn apart. This releases a lot of the components of the potato into the gnocchi dough.

Role of flour

The flour in gnocchi is what helps keep the gnocchi together. Flour itself is dry, so it increases the thickness of the gnocchi dough. Also, flour, thanks to the starch and gluten, is great at holding different ingredients together (very much its role in cookies as well!). If you’d just cook some mashed potatoes rolled into a ball it would fall apart easily. Adding the flour will help the potato to stay together.

The Process

After putting together the dough, the gnocchi are cooked in boiling water. This is where several transformations will occur, a lot of which are similar to those happening when cooking potatoes.

First of all, the gnocchi heat up. This starts on the outside, which becomes hot almost instantaneously. The heat then penetrates into the center. Since gnocchi are quite small this will happen rather quickly. We’ll assume that this happens within the first minute or so.

Starch gelatinization

Because of this increase in temperature the starch in both the potato as well as the flour will absorb more water. As a result, the starch swells and might even burst in a process called gelatinization. This is one of the main transformations that also happens when you cook pasta for instance. The starch will hold onto a lot more water now than at the start (and it’s why starch is so commonly used to thicken soups and sauces).

Protein denaturation

Both the gluten proteins in flour as well as the proteins in egg (if you’re using egg) will change when heated. The heat will cause the proteins to unravel and denature. This will help ‘set’ the gnocchi.

Density decrease

The heat also causes some of the moisture within the gnocchi to evaporate and form a gas bubble. However, these gas bubbles have no place to go. They are locked in by the rest of the gnocchi dough. As such, they might expand a little, but not much more. This expansion can decrease the density of the gnocchi, much like a cake that expands in the oven.

Overcooking

At some point, these transformations are all ‘completed’ enough. However, if you then continue to cook the gnocchi, it can actually start to break down again. The starches can break down because of the extended heating time, as well as a lot of the other minor large molecules in gnocchi. This results in the light puffy piece of dough to break apart.

well cook and overcooked gnocchi
Left: well cooked gnocchi ; Right: overcooked gnocchi, it has lost its definition and has started to disintegrate.

When are gnocchi cooked?

So we know what happens when cooking gnocchi, but when is a piece of gnocchi perfectly cooked? Is there even a single point of time where it is cooked well? Unfortunately, this isn’t easy to define and will ultimately also depend on the composition of the gnocchi itself.

Generally said though, all the starch will have to be cooked through (so gelatinized). Generally that means the whole gnocchi needs to be soft and pillowy. Also, the proteins in the flour as well as those in the egg (if you’re using eggs) will need to be cooked.

If you’ve got a laboratory available, you can analyze whether all the starch has been cooked using specific laboratory equipment (e.g. a DSC). However, for most consumers, we’ll likely just want to know when the gnocchi is good to eat.

Seeing how hard it is to define when exactly a gnocchi is ‘cooked’, especially using conventional kitchen tools, it even becomes hard to define whether gnocchi is cooked right when it floats! Keep this in mind when we’re evaluating our next set of hypotheses.

cooking gnocchi for different durations

Hypothesis 2: Gnocchi are ‘perfectly’ cooked at a specific cooking time

To test when gnocchi are cooked and whether consumers can taste the difference. We executed a next experiment. We cooked our gnocchi in boiling water with increments of 30 seconds. Our reference was a raw, clearly uncooked gnocchi. This gnocchi was hard, could not be squeezed, was tough and not a pleasure to bite through.

From there on the gnocchi were cooked for either 30, 60, 90, 120, 150 or 180 seconds. Right after cooking they were placed in cool water to prevent further cooking on the countertop.

Observations

The 30 second sample, which wasn’t floating yet, was clearly still undercooked. The outside had become soft, but the inside was still hard and tough. However, the 60, 90 and 120 second samples were all very similar, despite the fact that the 60 second sample wasn’t even floating yet! All were soft and bouncy and would we have mixed them together, we would not have been able to tell a difference.

Interestingly, starting at 150 seconds, our gnocchi did start to become a little more sticky and crumbly. However, differences were still minor. In other words, had we been served any of the 60-180s gnocchi samples, we would have judged them all as being ‘cooked’!

So yes, gnocchi can float when they’re cooked, but, they don’t have to float, nor should they ‘just’ float or should they float for a specific amount of time. There’s quite a range in which these gnocchi are cooked.

Conclusion: Hypothesis 2 is Incorrect

There’s some leeway when cooking gnocchi, and there doesn’t seem to be a very specific time for gnocchi to be ‘cooked’. That already debunks a big part of our initial question, do gnocchi float when they’re cooked. Yes, they can float, and yes, floating gnocchi surely aren’t undercooked. But, a floating gnocchi is not a requirement for being cooked!

Why would something float?

We now know what happens while gnocchi get cooked and we know that there’s likely not one specific cooking time. But we still want to know why they float and why that does seem to be some sort of assurance against undercooking them!

Next up, let’s have a quick look at why something would even float. Knowing why something floats is crucial when we’re trying to understand whether cooked gnocchi float.

Floating, or its reverse: sinking, is governed by density. The density of a food is defined by the weight of a specific volume of that product (density = mass/volume). If two components take up the same volume but have a different mass, their density is different. The component with the higher mass for the same volume has a higher density than the lighter product.

Feathers vs Lead: Keep in mind that you need to evaluate both the mass and volume to describe density. A trick question is whether you’d prefer carrying 100kg (or pounds for that matter) of bird feathers or 100kg of lead. Of course, both are the same weight, however, those feathers would take up a lot more space than the lead. The feathers have a considerably lower density.

Roughly said, when the density of your food is heavier than that of water, it will sink (as is the case for cocoa powder dispersed in milk). On the contrary, if the density is lower than that of water, it will float. Knowing this, it’s time to go back to our gnocchi experiments.

Hypothesis 3: A cooked gnocchi has a lower density than that of water

It has become clear that for gnocchi to float during cooking, the density of the gnocchi has to go down. But does that density go down permanently? Does a cooked gnocchi simply have a lower density than a raw gnocchi?

To put this to the test we tested several variables:

  1. We placed a hot, cooked, floating gnocchi in cool (room temperature) water. If the density of the gnocchi had indeed permanently decreased significantly, it would still float.
  2. We took a cooked gnocchi that had cooled down to room temperature and placed it back in boiling water, assuming it would float right from the get go!

Observations

When we moved our floating hot gnocchi from the boiling water into water at room temperature it would sink almost immediately. Clearly, the gnocchi hasn’t become that much less dense due to structural changes.

A side note on the density of water

The density of water changes with temperature. Boiling water does have a lower density (0,95 kg/l) than water at room temperature (0,998 kg/l). It is possible that our gnocchi did decrease in density enough to float in boiling water, but not enough to float in room temperature. However, seeing the results of the next experiment, we don’t think that’s the case.

Re-cooking – A real surprise!

Next up, we took some freshly cooked gnocchi that had had a chance to cool back down to room temperature (approx. 20 minutes) and placed them back in the pot of boiling water. We’d assume that they’d either float immediately (they were cooked and were still good to eat), or at the very least, float considerably faster than the raw gnocchi.

However, that was not the case. Gnocchi cooked for 90 seconds or less took just as long to float back up again. Even more surprisingly, gnocchi that were cooked for 120 seconds or more took a lot longer to re-float back to the surface! They took about 2 minutes, which is more than 30 seconds longer than the raw samples. We were surprised, to say the least! Clearly, there’s more going on here.

Also, remember our results from hypothesis 1. Even if we left the cooked gnocchi in boiling water, it would sink over time. Since keeping it in hot (but not boiling) water might have caused the starches to break down, breaking up the gnocchi, that by itself wasn’t sufficient evidence to debunk this hypothesis. However, with the additional two tests, it’s clear that the density of gnocchi isn’t permanently changed. Instead, there seems to be a temporary change only!

Heat & gases

It led us to conclude that not necessarily the cooking process of the gnocchi determines whether the gnocchi floats. Instead, it’s more likely that temperature and gas formation are the driving factors. As we discussed before, when the gnocchi heats up, water within the gnocchi evaporates. This water can’t escape the gnocchi, but can expand the soft pillow slightly. If enough gas bubbles within the gnocchi have formed, the density will have gone down enough for the gnocchi to float.

Since this formation of gas requires enough heat, it does somewhat coincide with the cooking of gnocchi. For all the cooking processes to ‘complete’ the gnocchi needs to be hot also. As such, these processes happen simultaneously, but don’t necessarily seem to be causually related. On the contrary, it’s probably a correlation, not a causation!

Conclusion: Hypothesis 3 is Incorrect

Conclusion: A HOT, FRESHLY cooked gnocchi does float

All these experiments have led us to conclude the following:

  • Yes, gnocchi float when they are cooked. However, they may have well been cooked before they floated and will still be cooked just fine if they’ve been floating for a little while.
  • Yes, gnocchi sink again when they are overcooked. Best to get them while they float.
  • No, the cooking processes themselves don’t cause the gnocchi to float. Instead, it’s the heat and the resulting gas formation within the gnocchi that causes them to float (which is why a cooled down cooked gnocchi needs time to float back up again).

Why doesn’t a boiled potato float?

You might by now be wondering why this applies to gnocchi, but not to a boiled potato (or a lot of other vegetables for that matter). Potatoes have a very different structure than gnocchi does. For one thing, they aren’t made of a soft expandable dough. Instead, the cells within a potato can’t really expand. So there aren’t as many options for those gases to be trapped and as a result decrease the density

References

Ashley Leonard, Mechanical Engineering: An Integrated Approach, Scientific e-resources, 2019, p. 23-26, link

Valeria Necchio, How To Make the Absolute Best Gnocchi from Scratch, Oct, 22, 2020, the Kitchn, link

Stack Exchange, Why do gnocchi float when they are cooked?, from May-25, 2018, link

USGS, Water density, link

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2 Comments

    • Hi Franck,

      Thank you for sharing the link and the two main conclusions (unfortunately, I don’t have access to the article, but it seems very relevant to our experiments!).

      I think there’s a truth to both:
      1) Yes, I do think they approach the density of water (they even get a density that’s lower than water) when they cook. But, I don’t think there is a direct relationship with the gelatinization process (seeing as how a slightly cooled down, but still fully cooked gnocchi doesn’t float when you place it in boiling water). I think the formation of hot gas bubbles that are trapped within is more important though. Of course, it might well be that the starches need to gelatinize to better ‘capture’ these gas bubbles, so they might be related in a way.
      2) This is a good point. I hadn’t yet thought of bubbles that stay stuck to the surface of the gnocchi… It might be that a few bubbles stay on the gnocchi, helping it stay afloat even when no new bubbles are formed. This could explain why they remain floating for a while (approx. 2 min) once the heat is turned off, but do sink after a while.

      It seems a lot of different mechanisms all interact with one another, probably all contributing in one way or another (sometimes indirectly!) to whether a gnicchi floats.

      That said, I don’t think there’s enough evidence that floating is a result of the gnocchi being cooked. We found that gnocchi can be cooked perfectly fine before they float.

      Thank you and would love to hear what you (and others) think and find when cooking gnocchi yourselves :-)!

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