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Letter Re: More About CO2 for Extending Food Storage Life

Thank you so much for your books and Internet work. I have been storing food using the method of dry ice fumigation with five gallon buckets and mylar bags [as described in “How to Survive the End of the World As We Know It” [1]. and in the “Rawles Gets You Ready” preparedness course [2]]. My family and I enjoy using brown rice quite a bit and I have read many articles explaining the oxidation of the fatty acids which is what causes the brown rice to have a short shelf life. What I can’t seem to find is an answer to whether the replacement of air in the mylar bags with carbon dioxide will slow down or stop the rice from going rancid. I thought that maybe you have run across the science behind this idea. Does the minimal amount of gas exchange from outside the container negate this affect?

I have a couple of other observations regarding this storage method as well. When preparing to package foods I always crank up the wood stove in my house so there isn’t a lot of moisture in the air to get in the food. With that said, I still get quite a lot of ice crystals, that turn into water, forming on the outside of the dry ice chunks while they sublimate. I usually put a piece of paper under the dry ice so the residual water doesn’t make it’s way down into the food and get trapped there when I seal up the container. I am not sure if the moisture is originating from the air in my house or from the food that I am packaging. Either way this got me concerned about the advice, which I have seen on many web sites and in other books, that you can put the dry ice on the bottom the container and put the food on top of it. I would think that this would trap some water at the bottom of the container, which is not a good thing.

My other observation has been that after the dry ice is done sublimating and I seal up the mylar bags with my wife’s hair straightener, within a couple of days I go back and check on the bags and they look like they have been vacuum sealed. Apparently there is some sort of chemical reaction with the carbon dioxide that creates a vacuum in the bag. The question about this would be if there are any long term consequences from this reaction regarding shelf life or food quality. Again, thank you so much for all that you do and feel free to post this if you find it to be helpful. Best Regards, – Jesse in Oregon

JWR Replies: Rancidity is caused by oxidation and hydrolysis. The time required for rancidity to occur varies, with temperature as one of the biggest factors. (The higher the temperature, the quicker the onset of rancidity.) Foods with high oil content are prone to what is called oxidative rancidity. This is where there is a degradation of long-chain fatty acids into various short-chain compounds. One of the byproducts is butyric acid, which creates the distinctive “gone rancid” smell. To make a long story short, to minimize the risk of rancidity, keep rice stored below 60 degrees (the ideal would be just above freezing, but avoid fluctuations in and out of freezing), and at the same time minimize exposure to oxygen. Hence, CO2 packaging inside a mylar barrier bag works quite well.