Will moist food spoil in low pressure environments like a near-vacuum?

Submitted by houstoncouchguy t3_111q9v1 in askscience

Of course we all know that food left in a near-vacuum will dry out and prevent dangerous spoilage from microorganisms. But what if the food was kept moist? If you kept a steak under a thin level of water by some means, would the steak spoil at room temperature in a near-vacuum? If not, what is the highest pressure that moist food like a steak could be kept in without spoilage?

I know that some organisms are able to live in low pressure environments, but would they thrive enough to become toxic?

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athomasflynn t1_j8h7uzh wrote

In a near vacuum water is going to sublimate off of whatever you store in that environment. Some organisms like tardigrades are able to survive exposure to vacuum, but they don't "live" in it and they definitely don't reproduce.

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Indemnity4 t1_j8ki2ym wrote

Freeze-drying or low-pressure treatments do not sterilize food. It only puts the microbes/enzymes to sleep and stops them reproducing. Once water is reintroduced to the food, all the microbes start growing again.

You are describing a process called "wet aging". This is the cryovac products you maybe see at a butcher.

Under "mild" vacuum and while wet, there are a lot of natural enzymes in food that will start to break it down. It starts to break down connective tissue and make the meat more tender, without breaking open cells and changing flavour such as dry aging.

You need very specific conditions of pressure and temperature to retain liquid water.

Near-vacuum pressure and all the solid/liquid water will move from the meat into the atmosphere.

You could potentially change the water to solid ice at low-vacuum, but that is also problematic because ice forms big chunky crystals that tends to damage food when it re-heats.

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houstoncouchguy OP t1_j8kxlpv wrote

Yea, I dry age steaks from time to time and really enjoy the flavor, and it typically starts with a shorter period of wet aging.

That’s what actually lead me down the path; wondering what other aging methods exist that others may not have tried. Freeze drying typically imparts an undesirable flavor to the meat.

But sterilization wouldn’t really be my interest because I imagine I still wouldn’t eat them raw. I don’t mind if a few undesirable bacteria exist in most cases since those would get cooked out. It’s when they thrive that the problems start.

So I wonder if the low pressures would prevent them from thriving, and how high of a pressure I could get to and still prevent them from multiplying dangerously. Water doesn’t start boiling off rapidly until it gets to about 1/30th of an atmosphere. And I imagine there is some wiggle room between the pressure where water boils at room temperature and the pressure at which bacteria still multiply. But I was curious if others knew more.

Googling “what is the lowest pressure that bacteria multiply”, and other variations, hasn’t been productive so far.

I imagine I could create a liquid culture medium and bring it down to 1/25th of an atmospheric pressure for a few days and see if it gets cloudy. If not, I could add 1/100th of an atmosphere until I see signs of life. But someone smarter than me must have done something similar before.

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Indemnity4 t1_j8lpo7d wrote

The optimum temperature to store meat is the coldest it can get without freezing, usually -1.5°C - 7°C.

Fancy words below, but the reason low-pressure makes the bacteria stop growing is they run out of oxygen. The process sucks all the air out of the package, and the protective film is a barrier to oxygen. The residual bacteria eat up all the residual oxygen in the bag, aplus ny dissolved oxygen in the water or tissue. They release CO2 up to about 20%, which inhibits further growth of aerobic bacteria.

At that point, only anaerobic bacteria can grow. Typically, those are only in trace quantities compared to the aerobes, so it's sort of a defacto nice effect rather than some amazing sterilization technique. An example of those are the lactic acid bacteria.

> Soon after packaging, the population of lactic acid bacteria is generally below the limit of detection (10 CFU / g), but it increases during storage (40). Lactic acid bacteria ferment glucose and other substrates that are present in meat. When these substrates are depleted, growth stops, typically when the population reaches 8 log/cm2. The metabolic residues of most lactic acid bacteria are not eliminated, however, and can be identified as slightly acidic or milky tastes.

If you have ever opened a "blown" or even old vacuum pack, that first smell you get is the digestive gases such as H2S. Or maybe you smell a faint trace of vomit or rancid butter for second, that is butyric acid as a byproduct of the lactic acid bacteria that is also flavouring the meat.

Low or ultra low pressure won't change the bacterial growth rates of anaerobic bacteria.

If you somehow could make the package sterile, such as pressure canning or pasturization, you end up with tinned meat. That's an interesting route to explore with incredibly long shelf life. There are interesting Youtube videos of modern people eating WW2 old army ration packs with various amounts of subsequent illness.

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