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Roasting beef

 
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smudges dad



Joined: 08 Jan 2015
Posts: 1
Location: Scotland

PostPosted: Thu Jan 08, 2015 9:58 am    Post subject: Roasting beef Reply with quote

This is a bit of an engineering question I posted on another forum and someone suggested I try here for the answer. I think it requires a heat flow formula to solve, but hopefully there will be some expertise on here to help. I've been assuming it would work in degC, but would Kelvin be more appropriate (or even Rankine)? I assume Delisle is right out:). Hopefully the question makes sense.

I like to slow roast beef such as sirloin or topside with an oven temperature around 100 degrees so I get a much more even cook. I also like my beef rare, so I tend to stop cooking when the temperature in the middle reaches 45 degrees, then let it rest so the temperature equalises. I use a thermometer to tell me when it reaches the right temperature and while it rests I do all the veggies, but this means the timing of the meal is a bit up in the air. The meat will usually start off at room temperature (20 degC) but occasionally out of the fridge (5 degC).

Assuming the meat is a cylinder, is there a formula to calculate cooking time based on oven temperature, starting temperature of the core , final temperature of the core and radius of the cylinder? Does the conductivity of different cuts of meat vary much and how should it be included in the calculation?
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Dilbert



Joined: 19 Oct 2007
Posts: 1175
Location: central PA

PostPosted: Thu Jan 08, 2015 5:57 pm    Post subject: Reply with quote

in theory, this is a simple exercise. you can see the basics here:
http://b.web.umkc.edu/beckerb/publications/journals/ht_coeffs.pdf

in practice however it becomes quite more complicated - simply because "beef" is neither uniform nor homogenous. the amount of fat / marbling / bone / (other tissue) will all impact the coefficient.

and, intuitively it is my suspicion that these multiple coefficients change as the meat cooks. for example, once the protein begins to set or the fats begin to render, the coefficient likely changes. which means setting up an integral equation.

in the end, one winds up doing it 'experimentally'
as long as the roast is longer than its "diameter" I've found 30 minutes per inch of diameter at 235'F/113'C to be a reliable guideline - for 10 lbs/4.5kg and up.

if you have a probe type thermometer, stick it in the meat and graph the center temp every 30 minutes - you'll get a very good picture of how it is doing and whether you need to increase or decrease the roasting temp to hit your timing mark.
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