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The recipe for Potato Puree in "Modernist Cooking at Home" calls for precooking the potatoes in water at 65 degrees Celsius for 35 minutes. The bag should hold 500 g potatoes and 1 liter water. Since the recipe says to scale the water at 200% this means that if I am to use 1 kg of potates I need 4 liter of water.

However, trying to do this for only 35 minutes did not give a satisfactory result. Looking back, this is not surprising since the added size of the bag means it needs more time in the sous vide.

What is the right way to scale up the recepie? Should I use two bags? If that is the case, the scaling factor of the water makes little sense. How about 750 grams of potatoes, or 1.5 kg of potatoes?

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I have not read this book, but I note that 1 liter of water is 200% of 500 g of potatos; could that be what is meant? –  SAJ14SAJ Aug 31 '13 at 15:19
    
@SAJ14SAJ I believe the interpretation of the scaling factor is correct, but the question still remains on what is the best way to scale this recepie in relation to time and the number of bags etc. –  HaskellElephant Aug 31 '13 at 15:29
    
Well you could always do the recipe twice, independently, at the same time. That kind of shows the scaling interpretation may be a little odd. –  SAJ14SAJ Aug 31 '13 at 17:00
    
@SAJ14SAJ Yes, I could do that, and then it seems reasonable to use 2:1 amount of water to potatoes described in the recipe. I guess the question is about the what should be done with 750 grams and so on. I was hoping someone had some experience on precooking potatoes for potato puree in this way and tips on timing etc. I guess in a more general setting one could say: "How do I scale Starch Gelatinization in potatoes with a sous vide?". –  HaskellElephant Aug 31 '13 at 17:19
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See Jefromi's answer below; it is almost certainly as I was guessing using the same technique as baker's percentage, but with potato rather than flour as the baseline 100% ingredient. So 750 g of potato would go to 1.5 l (1500 g) of water. In most sous-vide recipes there is enough slush that the number of bags should not make that much difference. You can always divide into 2 if it is more convenient. –  SAJ14SAJ Aug 31 '13 at 17:22
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2 Answers

Assuming it says 100% for the potatoes, I don't think you've got the scaling right. The recipe format picks a baseline ingredient, then gives the quantities for other ingredients as percentages of that ingredient. So if it says potatoes 100%, water 200%, then for 1kg of potatoes you need 2kg (2L) of water. See the detailed explanation from the source. The point here is to make things really easy, not to make you do awkward extra math.

As for the bags, you should definitely use two bags. If you do have two identical bags the size of the original one, your circulator is still keeping the water at the right temperature, and you've put in more water as directed by the recipe, it should take close to the original time. If you put it all in one bag, it'll take longer, and exactly how much longer is pretty hard to guess, since it depends on exactly how you pack it into the bag.

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I feel embarrassed now =D. Thanks for clearing that up for me. –  HaskellElephant Aug 31 '13 at 22:24
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I think the issue you're referring to is that more potatoes require more time to cook to the consistency achieved by following the original recipe.

This of course (like many other cooking issues) is a thermo dynamics problem. As long as you make sure your potato pieces are small enough (I make about 1" think slices) then add twice as much water (that's what they mean by 200%, 1.5Kg of potatoes -> 3L water).

In a sous vide, it would be better to have two bags separate as opposed to one big one. The name of the game is to allow the heat to penetrate. The rate of cooking something twice as thick is not twice as long (it follows heat diffusion formula). By separating the bags you allow the circulator to replenish the warm bath around the food.

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