How does sous vide cooking time affect moisture (final weight) of meat?

A Practical Guide to Sous Vide Cooking, Effects of Heat on Meat describes what causes moisture loss:

The water-holding capacity of whole muscle meat is governed by the shrinking and swelling of myofibrils. Around 80% of the water in muscle meat is held within the myofibrils between the thick (myosin) and thin (actin) filaments. Between 105°F and 140°F (40°C and 60°C), the muscle fibers shrink transversely and widen the gap between fibers. Then, above 140°F–150°F (60°C–65°C) the muscle fibers shrink longitudinally and cause substantial water loss; the extent of this contraction increases with temperature.

140°F seems to be a critical tipping point, so I'd make sure to cook significantly below that to be safe.

It's the maximum temperature that the protein experiences that determines the moisture loss, not the time duration.

What time does do however is affect the texture of the meat ibid, Tough Meat:

At lower temperatures (120°F/50°C to 150°F/ 65°C), Bouton and Harris (1981) found that tough cuts of beef (from animals 0–4 years old) were the most tender when cooked to between 131°F and 140°F (55°C and 60°C). Cooking the beef for 24 hours at these temperatures significantly increased its tenderness (with shear forces decreasing 26%–72% compared to 1 hour of cooking). This tenderizing is caused by weakening of connective tissue and proteolytic enzymes decreasing myofibrillar tensile strength. Indeed, collagen begins to dissolve into gelatin above 122°F to 131°F (50°C to 55°C) (Neklyudov, 2003; This, 2006). Moreover, the sarcoplasmic protein enzyme collagenase remains active below 140°F (60°C) and can significantly tenderize the meat if held for more than 6 hours (Tornberg, 2005). This is why beef chuck roast cooked in a 131°F–140°F (55°C–60°C) water bath for 24–48 hours has the texture of filet mignon.

Over time, the texture of the meat will change. There are ranges of several hours where no noticeable change occurs though, which is what makes sous vide so forgiving.

Long slow cooking is good for tough cuts of meat, but for naturally tender cuts, cooking for too long will make the meat feel mushy (but still not dry it out).

Update: my answer to curing - Are non-brisket cuts suitable for corned beef? - Seasoned Advice provides a practical example of this with two identical corned beef roasts: 10 hours at high temperature lost 36% weight, while 36 hours at a lower temperature lost only 28%.

Imagine cooking one steak at 140F for 1 hour and another for 5 hours. Will the 5 hour steak be drier and weigh less?

Briefly: Yes, it will be drier, and yes, it will weigh less. And no, 140F isn't the tipping point, though higher temperatures definitely will dry things out faster. Moisture loss will increase with increased sous vide time even at 131F (the minimum temp I'd recommend for long cooking). It likely will happen at even lower temperatures too.

For one set of experiments in this regard, see these two posts, which compare moisture loss with different duration, different temperatures, and also a comparison with a reverse sear/oven method.

To summarize the relevant bit:

How long is too long?

It depends on the thickness of meat of course, but here is some data for 1.25-1.5 inch steaks:

131F for 100 minutes -- 6.1% juice release during serving

131F for 3.5 hours -- 1.93% juice release during serving

131F for 5 hours -- 1.6% juice release during serving

Those results in juice release also mirror observed weight loss in the meat. The 100-minute steak was 90% of its original (before sous vide) weight when pulled from the bag, while the 5-hour steak was 84% of its original weight when pulled from the bag.

I've observed similar patterns in my own sous vide cooking. While there's a certain minimum time needed to bring a thick steak up to temperature and get the juices moving, going past that time will just result in a drier steak and less juiciness. The effect is much more pronounced (in my experience) with ground beef, which seems to dry out quite a bit even at low temperatures with prolonged sous vide cooking even at low temperatures (likely due to the greatly increased surface area that can release juices).

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I had hoped that Serious Eats would weigh in on this, but interestingly they don't. In that link, they note that the meat changes texture with long cooking, but claim that a 24-hour steak "is still plenty juicy (a steak cooked 24 hours loses barely any more moisture than a steak cooked for one hour)" but there are no accompanying stats to explain that claim or how much "barely any more moisture" might be. I've personally noticed that steaks seem to get drier with more liquid in the bag with long cooks, so now I don't tend to go past an hour or so myself unless it's a very thick cut or I deliberately want to focus on tenderness over juiciness.

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EDIT: For those who want more than a random blogger's word on this, or who question the metrics in that post for how "juicy" a steak might seem to be when you eat it, here's a scientific paper showing that perceived juiciness decreases with increased sous vide time (even at low temperatures). As that paper has a paywall, there's a summary of the findings in this blog post.

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EDIT2: In response to a comment that the sample size was not large enough in the blog post, here's an entire master's thesis on cooking sous vide, which shows that "cook loss" (i.e., moisture loss during cooking) increases with sous vide time. One of the studies (n=14 samples) in that thesis shows a statistically significant difference in longer cook times at 55C (131F): after two hours, cook loss was about 14%, whereas at 10 hours and 30 hours, it increased to about 19.5%. The second study (n=40 samples) compares shorter times and various temperatures and does not show a statistically significant (p < 0.5) trend at 55C for 2, 4, 6, and 8 hour cooks. But the graph still indicates an overall trend toward greater moisture loss with greater time past 2 hours even at 55C. For higher temperatures (60C/140F or above), the increased cook time definitely increases moisture loss significantly, from about 14% at 2 hours to about 22% at 8 hours at 60C/140F. At 70C (158F), the effect of time is even greater, increasing moisture loss from about 20% at 2 hours to about 32% after 8 hours. The literature review toward the beginning of the thesis also lists several other published studies that have shown similar trends.

I also would note that cook loss only indicates that less moisture is present in the meat. Perceived "juiciness" is a somewhat different thing, as that depends on free moisture that can be easily released during eating and chewing. Which is why I also linked a study in my first edit above about the perception of juiciness (which declines with increased sous vide time), along with the actual decrease in moisture in the meat.