There's a misunderstanding possibly of what Heston suggested for the British Navy. He suggested to sous vide it and then store it in the freezer. They couldn't just toss it in the cupboard. Sous vide food is not typically shelf stable. It has been pasteurized - not sterilized.
Pasteurization reduces pathogens to a safe level, not to a 0 level. It doesn't typically reduce botulism spores to 0. Once the food is exposed to normal temperatures, any anaerobic pathogens will start to reproduce again.
To sterilize you'll need to pressure cook meat through to 121 C for 2.4 minutes - at which point its rather not tasty. You might be able to sous vide at a lower temp for much longer, but I suspect it'll still be past the point of being tasty. Sterilized meat tastes like your typical potted meat.
The machine
I invited a couple of friends who are keen to try sous vide over during the whole process. All 4 of us agreed. The machine is going back.
My immediate first thought was that you had a bad machine. The Vac-Star Sous Vide Chef has excellent reviews on Vac-Star's website and it looks like you are using it within proper specifications. That would be my first hunch, as nothing else seems particularly out of place. If you are able to come back, since this was over a year ago, it would be curious to see if a different machine solved your problems.
On a side note, I looked up reviews for Sous Vide Supreme vacuum pouches and they seem good, too.
Other things
I looked up for reviews of the I noticed the Beyond Salmon blogger had posted a follow-up experiment with sous-vide steaks that I found had useful data. This summarized it well:
What most sous-vide books and websites tell you is that you can't overcook using the sous-vide method. It all depends on how you define "overcook." The steak will not go above the desired temperature no matter how long you hold it in the water bath (well, dah!), but the longer you hold it, the more juice you lose.
I have noticed this from your post:
I have tried various temperatures and times from 55°C to 60°C and from 40 minutes to 4 hours.
Per her experiment, steak A, B and C turned out the best. They also had the lowest temperatures and cooking times:
Steak A: 121°F (49.5°C) water bath for 50 minutes
Steak B: 126°F (52.2°C) water bath for 50 minutes
Steak C: 131°F (55°C) water bath for 100 minutes
It likely would be a good idea to try lower temperatures, and stick to the lower-end of the time scale. Also from the article:
But if you are using an immersion circulator, you might be tempted to put meat in the water bath whenever it's convenient (in the morning before going to work, during kids' nap, etc.) and then have it ready for dinner. It will surely be very tender, but you'd better have a lot of demi-glace handy because it will be dry.
Hope this helps!
Best Answer
Sirloin steak doesn't have a huge amount of collegen, which is why it's traditionally cooked over high heat for as short a period of time as possible. If I were you I'd be more worried about the meat become unpleasantly mushy if left in the bath for too long. I would just use the Douglas Baldwin tables for bringing the meat up to the required temperature: