Really dumb sous vide question
Generally when a recipe says "using a 70 ℃ bath cook sous vide for 30 min", does this imply start the timer when the food is put in, or when the contents reach an internal temperature of 70 ℃?
I'm inclined to guess the straightforward 30 min, because the food is supposed to be sealed which prevents measuring the temperature. Please correct if mistaken!
Start timing when the water returns to temp after dropping the food bag. What are you cooking that takes just 30 minutes?
Yes on the water temp, not the internal temp, which as you point out can't be measured.
What are you cooking for only 30 minutes at 158 degrees - seem too hot and too short.
It's 100 g egg whites, plus sugar, to be foamed into meringue afterwards. From modernist cuisine. If it fails I can always do it the regular way...
It's actually both - here's the science: the food item can only get to the temperature of the water bath. So whether it's in 70°C for 1 hour or 8 hours it will never get above that temp. But it takes time to reach the water temperature, which is why the concern about 30 mins being too short. Even for eggs the shortest cooking time I've seen is 45 mins: http://norecipes.com/blog/onsen-tamag...
There are many things you might cook for less than 45 minutes - just bear in mind that you're not necessarily pasteurizing your food, and it may not be a good candidate for refrigerating and storing after cooking. Most of the multi-hour cooking times you see for sous vide recipes are aiming to pasteurize the food in the bag and often to break down and tenderize food via longer cooking, as you would with short ribs, for example.
In many cases, the principle is very similar to cooking a steak medium rare via conventional methods - you cook it long enough to kill germs on the outside (though some recipes notably don't even do that) and long enough for the internal temperature to rise to the desired point - without bothering to hold the food at that temp for a while as many recipes do. In some cases, you might even deliberately pull the food from the bath before the center reaches the temp of the bath - you might see this referred to in some technical discussions as cooking with a 'delta T' - 'T' referring to temperature, and 'delta' meaning a difference, as in the difference in temp between the water bath and your desired final temperature.
As for eggs, the main reason for cooking times of an hour or more is that an egg is round and relatively thick at its middle, slowing heat distribution. The OP is cooking a meringue in a bag, which should cook through much more quickly due to the much thinner shape. No harm would come from cooking it longer though.
If you get a chance, read some of the recipes from Thomas Keller's book, 'Under Pressure.' You'll find quite a few that call for cooking times of 30 minutes or less.