Graphene oxide drying

Question

Suppose you have a food that is naturally liquidy, such as curry. You like the flavor but need it less "wet". The problem with drying it is that the aromas also get lost to an extent. However, graphene oxide is selectively permeable to water. Is it practical to dry your fruit, sauce, or other foodstuff in a container covered by a sheet of graphene oxide, letting the water out but keeping the aromas in?

The first question is whether you can buy a sheet of graphene oxide (or more like a lid with a filter made of it) for a realistic price, that's food safe. To address the theoretical aspect, the chemistry literature would be the place to look. It's not clear from your link how permeable grpahene oxide is to water *vapour* - maybe you'd actually need to put the food in a bag made of GO. On the whole freeze drying seems far easier — Chris H, Jan 30 '22 at 08:36
@ChrisH: I think it was water vapor. Won't freeze-drying also drive off volatiles? What's to stop i.e. fruit ketones from evaporating along with the sublimating ice? — Kevin Kostlan, Jan 30 '22 at 08:47
Some will, but you have to get in to vapour pressures as a function of temperature. That's why freeze drying is supposed to be better than dehydrating withheat — Chris H, Jan 30 '22 at 12:34
How dry are you wanting it? I doubt that a few extra minutes boiling would significantly lose flavour - otherwise many slow-cooked dishes would be entirely flavourless. — bob1, Jan 30 '22 at 20:15
@bob1: I find most soups and stews taste watery with weak flavors, while "dry curry" has nice strong flavor (don't know how it's made though). — Kevin Kostlan, Jan 30 '22 at 20:27
Dry curries are made using a minimum of liquid and boiling off any excess. Remembering that most people don't have access to such technology as freeze-driers or anything beyond a heat-source and a pot. There are plenty of "dry" curry recipes out there. — bob1, Jan 30 '22 at 21:40
@ChrisH: The ratio of vapor pressures matters, and the ratio tends to grow somewhat at low temperatures. Since most flavors are less volatile than water it would help. But ice will exclude organics, concentrating them and thus raising their chemical potential quite substantially. So it's unclear which effect wins. I believe the main advantage is that freeze-drying helps preserve the structure of the food. — Kevin Kostlan, Jan 30 '22 at 22:32

score 1 · Answer 1 · answered Jan 30 '22 at 11:50

On first glance, this doesn't sound too promising to me.

First, let's assume that the graphene oxide really only leaves pure water through and nothing else (acting like a filtering membrane). Then, you would end up with the water outside the box - and the flavor compounds in the box, but not within the food where you want them! When you open the box to eat your food, they will dissipate too, together with the air from the box.

Second, I am not sure how likely this "perfect filtration" is. I am not a physicist and might have misunderstood the paper you linked, but I was unable to find a place where it compares the passing of pure water through graphene as opposed to passing a water mixture or a water solution. In fact, it appears to be a theoretical work that models the way water passes through graphene only. So I have no idea of the context of the statement "selectively permeable to water" - maybe it doesn't select as strongly as you hope.

Third, how do you propose to dry the food when it is snugly packed in a GO bag? The standard process of drying food makes use of the concentration gradient of moisture between the food and the surrounding air. Now that there is no surrounding air anymore, you will have to somehow actively transport the water out of it through the GO, and that will be difficult, even if you apply some force. And since the GO channels are so small, this will be extremely difficult. For comparison, try putting something in a physical filter with holes too small for it to freely pass through. For example, cook some crème anglaise and try passing it through a coffee filter. Gravity won't be sufficient for it to pass, but even if you try exerting pressure, you will destroy the filter before it goes through.

And fourth, even if the two first points are somehow solved I don't think you will be able to keep the food as tasty as it was. When you bite into fresh fruit, you enjoy the spilling of flavor-carrying cytoplasm onto your tongue (and I am oversimplifying here by ignoring the effect o,f texture on pleasure). If you have a "dry collection" of flavor-carrying molecules somewhere inside the food, you won't necessarily get the same experience when they land in your mouth, if they are not bound to a fluid the way they were bound originally. Add to that the chemical changes which will happen as a result of the drying process itself, and you will get a very different outcome than that of a fresh fruit.

In the end, this doesn't sound completely impossible, but it seems that the complexity of implementation will be huge (certainly not "a container covered by a sheet of GO"), and the resulting improvement might turn out to be very slight. We will have to wait and see if somebody develops an application for it. And frankly, I hope that food producers would instead invest their money in growing food that has enough aroma in the first place.

1. Would the tiny amount of headspace really hold *so much* flavor? 2. *This* is the most likely problem. I remember GO expands when it absorbs water vapor, so it may let through vapors dissolved in a mostly water liquid. 3. A thin enough sheet was found experimentally (different paper?) to not impede the evaporation of water; a water activity gradient gets set up within the sheet. 4. Yes raisins will still be raisins, of course. Hopefully with a stronger smell, and less musky/more fruity. 5. Yes, caring less about mass or calories and more about phytochemicals is a good idea. — Kevin Kostlan, Jan 30 '22 at 22:20
@KevinKostlan 1. the most important part here is that a mental model where the flavor stays "in the food" is wrong. The headspace will have space for a ton of aromatic molecules, but indeed not all will escape into it, since the concentration matters. 2. yes, we have to wait for experimental observations, it's exciting to think what may come out. 3. My intuition is that there may be a problem here. Fruit doesn't dry on its own, it needs to be either exposed to warm moving air, or to partial vacuum. When protected by the GO sheet, why would the water move out quickly enough? 4 and 5, agreed. — rumtscho, Feb 01 '22 at 19:08

score 1 · Answer 2 · answered Feb 01 '22 at 18:19

Great idea. GO can be engineered with many custom properties. I think for your application (getting excess water out of a curry) it would be better to remove liquid water using pressure, rather than letting water vaporize. This is the process used in reverse osmosis (RO) water desalinators.

Conventional RO uses very high pressure to force water through membranes. The same process is potentially much more efficient with GO. from Wikipedia:

In 2013 Lockheed Martin announced their Perforene graphene filter. Lockheed claims the filter reduces the energy costs of reverse osmosis desalination by 99%. Lockheed claimed that the filter was 500 times thinner than the best filter then on the market, one thousand times stronger and requires 1% of the pressure.

It would be like a molecular-sized version of separating curds from whey. If the membrane can retain salts, it would certainly retain larger flavor molecules.

GO is potentially much more efficient than freeze-drying. And RO can use much less energy than vaporizing water.

Graphene oxide drying

2 Answers2