Brian, as someone who's opened a lot of sewer lines, I can definitely say that kitchen sink lines are by far the worst; toilet lines rarely will have much odor at all, but kitchen lines can knock you backwards with one whiff. This is in part because much more water is run down toilet lines, thus keeping it cleaner. Kitchen sinks drains, with or without disposals will often have a greasy build-up on the sidewalls, and black, rotting food deposits lingering in the line.
To minimize these, and their smells, you must flush the lines with more water each time you send grease and food waste down the pipes. If you must run grease down the drain, do so with cold water; I know this sounds backwards, but by using cold, grease congeals before it can stick to pipes as easily. Once a week, filling the sink bowls with cold water, then draining them, will help a lot to keep your sink drains running freely and more odor-free.
The orange & lemon suggested by Loveday will help, too, but like a room deodorizing spray, it will fade & the smell will return.
You could also have a blocked vent as has been suggested; try filling the sinks and draining, (both sides at once, if there are 2 bowls) and listen to see if you can hear gurgling or "popping" noise once the sinks have gone down. If they drain slowly and then gurgle, you might need to check the vent. If they just drain slowly, you may need to snake the sink drains to remove large build-ups.
It could be one or both, but being -40° (I hope you were exaggerating!) I'd stick with the indoor methods first! Good Luck