Rice storage using dry ice to purge oxygen

Posted: June 6, 2012 in World

I’ve been wanting to build up a long term store of food but face a few issues here. One issue is the weather – hot and humid most of the year which makes keeping foods cool and free of molds and fungi a challenge. Another issue is pests – anything like rice, beans or pasta gets infested with weevils after just a couple of months even if the package remains sealed.

Much of the information I have read online talks about refrigeration or freezing for long term storage of grains. That is not practical or cost effective for large amounts. I read some studies that compared preservation by refrigeration and by dessication. Good preservation results were obtained with super dessication and no need for costly refrigeration (equipment and power). Elsewhere there is quite a bit of discussion about the importance of eliminating oxygen from the storage container to inhibit molds, fungi and pests. Apparently there is a tradeoff here – if you put an oxygen absorber and a dessicant into the same storage container they battle each other. At least that’s what I read, not sure if it is accurate.

Regardless, I haven’t been able to find a local source of either oxygen absorbers or a suitable dessicant. Instead I have gone with using dry ice to purge the storage containers of oxygen by filling them with carbon dioxide as the dry ice sublimates. The method is easy and I was able to locate a local source of dry ice. One kilogram of dry ice costs only 25 baht which is about US$0.80 at the time of this writing.

The storage containers I am using are five liter plastic water bottles made of PETE. This is the very common clear semi-rigid plastic used for water and soft drink bottles. PETE is a pretty good oxygen barrier, although it isn’t as good as some other materials. Glass containers with air tight lids and containers made of other materials are difficult to find locally and are quite expensive. These five liter water bottles are readily available and inexpensive so if they perform well enough then they will remain my primary choice.

So I did my first batch. Rice is available in five kilogram bags which fill almost completely one of the five liter bottles. The process is simply to drop some chunks of dry ice into the bottle. Then using a funnel pour in the rice. The cap is placed on but not tightened so that the air in the bottle can escape as the dry ice sublimates and expands. Being heavier than the other gases in air, the carbon dioxide fills the bottle from the bottom and pushes the air out the top. It’s not perfect, for sure, and some air and oxygen remain inside, but the atmosphere in the bottle is mostly carbon dioxide and very low in oxygen so is very inhospitable to pests.

You need to watch the bottles and try to determine when the dry ice has completely sublimated so you can tighten the caps to prevent air from going back into the bottles. You can’t tell by looking at it because you can’t see the dry ice buried in the rice. So what I did was tighten the caps for a few moments and then look for signs of pressure building like some slight bulging of the bottle, then loosen the caps again and hear the gases escape. I checked like this now and then and at about two hours it seemed like the process was complete so I tightened the caps and stored them in a dark and relatively cool location.

Now comes an interesting effect. After the bottles have set for a few days a partial vacuum forms inside. Here is a picture of one of the bottles.

Rice storage usin dry ice

You can see that the top of the bottle is slightly deformed due to low pressure inside. When I first filled the bottle with rice there was a small airspace at the top. Now it has comletely shrunken and squeezed in the sides of the bottle. This seems odd and sort of counterintuitive. After all, the sublimated dry ice filled the container with cold carbon dioxide. After capping it tightly I would expect it to expand a bit as it warmed to room temperature. So what is going on here?

I found an answer offered on one website (unfortunately did not bookmark it and cannot find it now). A commenter on that site said she is a molecular biologist who does research involving absorbtion of gases and that she has observed that many grains slowly absorb carbon dioxide. She said that in a sealed contained containing carbon dioxide she would expect to see this effect of pressure reduction inside the container.

Cool! It’s cool because this gives a very visual indication that the container is still air tight and has not allowed any air to leak in. How long it will stay this way I don’t know but I will monitor it. If I get any containers that appear to have leaked then I will simply use those and replace them with new ones.

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Comments
  1. […] a year and a half ago I posted about long term storage of rice using dry ice for purging oxygen in the container. I recently decided to open one of the containers and see how […]

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