Ask John: Exploding Straws

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Published on: June 17, 2016

Ask John

John F. Hasler

Don’t ask John! Sometimes that is undoubtedly the best option. More than 20 years ago, my then high-school-aged daughter gave me a T-shirt with the caption “To save time, let’s just assume that I know everything.” I still wear it, sometimes even in public. My daughter, who now is an attorney, has often disagreed with my opinions! Most of you will probably not be surprised by that.

Matt, the editor of A Closer Look, asked me to submit a copy of what I recently wrote as a response to a question submitted to the CETA Tech Talk program. Actually, I submitted two responses to Tech Talk on the issue of exploding straws. In my first submission, written in haste and poorly considered, I included the comment “When straws freeze, a vacuum forms inside any air columns as the temperature goes down.” That is really not true. Aqueous fluids, such as cryoprotectant media, expand in volume about 9% when they freeze. Although air does decrease in pressure as it cools, the volume of air in either semen or embryo straws is not large enough to overcome the pressure exerted by the increased volume of the ice.

If a straw is filled completely with liquid and then frozen, as the medium freezes and expands into ice, the straw will very likely blow out one end. Mel DeJarnette at Select Sires very helpfully explained to me that Select Sires does not completely fill straws with semen, but leaves about 1/2 inch of airspace beyond the semen column before sealing. Then, when the straws freeze, the air is compressed and is only about 1/4 inch long after the ice has expanded. On the other extreme, if a straw was sealed with only air in it, it would definitely form at least some degree of vacuum as it cooled.

I do not know where the balance in pressure equilibrium is between the ratio of columns of medium and the total volume of air in most embryo straws. My guess is that unless quite a bit of air is present, there is still a positive pressure when the straw freezes and the medium expands. So, most embryo straws probably do not actually develop a negative pressure or vacuum as they freeze.

What probably happens most often is that if there is a tiny leak and the straw is then stored in liquid nitrogen, there is an equilibration between the straw interior and the liquid nitrogen around the straw and slowly a small amount of nitrogen diffuses in. Then, upon rapid warming, only a small amount of nitrogen need be present and it expands to nitrogen gas at 700 times the volume of the liquid and, POOF, either one end blows out or the straw actually explodes.

The solution to this, of course, is to effectively seal both ends of straws, which includes making sure that the PVC powder at the “wick and powder” end is thoroughly saturated by the first column drawn into the straw.

An effective solution to thawing straws that are suspected of not being thoroughly sealed is to store them overnight in a liquid nitrogen vapor tank with the straws stored above the level of any liquid nitrogen. This will allow any pockets of liquid nitrogen in the straws to diffuse into vapor and reduce the chances of straws blowing up.

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